1*1

Agriculture
Canada

Handling Agricultural Materials

Storage and Conditioning of
Grain and Forage

1*1 Sr app . 3 ,99,

Library / Bibliotheque, Ottawa K1A0C5

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Handling Agricultural Materials

Storage and Conditioning of

Grain and Forage

Research Branch
Agriculture Canada

Publication 1855/E
1990

©Minister of Supply and Services Canada 1990
Cat. No. A15-1855/1990E
ISBN 0-660-13615-5
Printed 1990

Available in Canada through authorized bookstore agents
and other bookstores or by mail from

Canadian Government Publishing Centre
Supply and Services Canada
Ottawa, Canada K1A0S9

Price is subject to change without notice

Previously published under the title
Canadian Agricultural Materials Handling Manual.
3.1 Grain/Forage Storage and Conditioning.
Agric. Can. Publ. 5002

Also available in French under the title
Manutention de produits agricoles : Entreposage et
conditionnement des grains et fourrages

Canadian Cataloguing in Publication Data

Handling agricultural materials. Storage and
conditioning of grain and forage / [staff
editor, Sharon Rudnitski ; contract editor,
Rhonda Birenbaum ; scientific adviser for
contract research, L. Heslop]. -

(Publication ; 1855/E)

Previously published under title: Canadian
agricultural materials handling manual. 3.1.
Grain/forage storage and conditioning.
Published also in French under title:
Manutention de produits agricoles. Entreposage
et conditionnement des grains et fourrages.
"Prepared by UMA Engineering Ltd. -Foreword.
Includes biographical references.
Cat. no. A15-1855/1990E
ISBN 0-660-13615-5

1. Grain-Storage. 2. Grain-Drying. 3. Forage
plants-Storage. 4. Forage plants-Drying. I.
Rudnitski, Sharon. II. Birenbaum, Rhonda. III.
Heslop, L. IV. UMA Engineering Ltd. V. Canada.
Agriculture Canada. Research Branch. VI.
Series: Publication (Canada. Agriculture
Canada). English ; 1855/E

SB190.H35 1990 633.1'0468 C90-099109-7

Staff editor

Sharon Rudnitski
Research Program Service

Contract editor

Rhonda Birenbaum

Scientific adviser for contract research

L. Heslop

Industry Relations Office

CONTENTS

Foreword 5

INTRODUCTION 7

Maintaining the quality of grain in

storage 7

Controlling rodents, insects, and molds 7

Moisture control 7
Temperature control 7
Chemical control 8

2 EFFECTS OF MOISTURE,
TEMPERATURE, AND TIME ON GRAIN
STORAGE 8

2.1 Moisture and temperature 8

2.2 Equilibrium moisture content 10

2.3 Determining moisture content 10

2.4 Temperature-monitoring devices 11

2.5 Time 12

3 SYSTEMS TO CONTROL MOISTURE
AND TEMPERATURE 12

3.1 Aeration systems 12

3.2 Fan operation 13

3.3 Direction of airflow 13

3.4 Natural-air drying 14

3.5 Design and operation 14

3.6 Airflow rate 14

3.7 Operational aids 14

3.8 General discussion 17

3.9 Heated-air drying 17

3.10 Dryer types 21

3.11 Nonrecirculating batch dryers 21

3.12 Batch bin dryers 21

3.13 Portable batch dryers 22

3.14 Recirculating batch dryers 23

3.15 Continuous-flow dryers 23

3.16 Temperature-control systems 24

3.17 Temperature sensing 25

3.18 Fire prevention 25

3.19 Energy sources for dryers 26

3.20 Low-temperature drying 26

3.21 Combination drying 27

3.22 In-bin cooling 27

3.23 In-bin steeping and cooling
(dryeration) 28

3.24 Managing dryeration systems 28

3.25 In-bin cooling and drying 29

3.26 Managing in-bin cooling and drying
systems 29

3.27 Fan selection and system design 30

3.28 Sample problems 31

3.29 Aeration system parameters 31

3.30 Barley cooled in storage 32

3.31 Natural-air drying of wheat 33

3.32 Corn cooled by dryeration 33

4 SYSTEM SELECTION CRITERIA 34

4.1 Storage 34

4.2 Storage bin size 36

4.3 Drying systems 37

4.4 Dryer loading and unloading systems 39

4.5 Electrical supply 40

4.6 Materials-handling systems 40

4.7 Loading 40

4.8 Unloading 41

5 STORAGE AND CONDITIONING OF
HAY 41

5.1 Drying hay in storage 41

5.2 Safe storage requirements: airflow and static
pressure 43

5.3 Air distribution systems 44

5.4 Drying hay 45

5.5 Field drying 45

5.6 Arranging hay in the dryer 45

5.7 Operating the dryer 45

5.8 Terminating dryer operation 46

5.9 Supplementary heat drying 46

References and further reading 48

Tables

1 Environmental conditions associated with
pests in grain storage 8

2 Moisture content designations for common
species of grain 9

3 Recommended minimum airflow for a storage
bin with a fully perforated floor and a level
grain surface in Manitoba 15

4 Recommended airflow for natural-air drying of
wheat in Edmonton, Alta. 18

5 Recommended airflow for natural-air drying of
wheat in Swift Current, Sask. 18

6 Recommended airflow for natural-air drying of
wheat in London, Ont. 19

7 Predicted minimum airflow for drying canola
in Manitoba 19

HANDLING AGRICULTURAL MATERIALS

8 Effect of corn harvest date and initial moisture
content on airflow rates for low-temperature
drying of corn in Toronto, Ont. 20

9 Maximum drying temperatures 21

10 Impact on fuel consumption and dryer capacity
of various drying methods for corn dried from
25% to 15% moisture content 26

11 Basic design recommendations for various
drying and cooling methods 31

12 Airflow related to dryer area and hay moisture
content 44

13 Fan air velocity and static pressure for drying
air in storage 44

Figures

1 Seasonal moisture migration patterns 8

2 Effect of temperature and moisture content on
the allowable storage time for wheat, oats, and
barley 9

3 Effect of temperature and moisture content on
the allowable storage time for continuously
ventilated canola 9

4 Effect of temperature and moisture content on
the allowable storage time for corn 10

5 Equilibrium moisture content of cereal grains
and oilseeds 11

6 Nonrecirculating batch dryer 22

7 Batch bin dryer 22

8 Recirculating batch bin dryer 23

9 Portable recirculating batch dryer 23

10 Continuous-flow in-bin dryer 23

11 Portable continuous cross-flow dryer 24

12 Continuous parallel-flow portable dryer 24

13 In-bin steeping and cooling (dryeration) 28

14 Resistance of grains and oilseeds to
airflow 32

15 Aeration duct arrangements for flat storage
bins 36

16 Lengthwise duct spacing for rectangular
buildings 36

17 Unlined primary distribution ducts 42

18 Lined rectangular distribution ducts 43

19 Fan installation for sound absorption 47

20 Relationship of temperature and time
to mold formation on high-moisture
hay 47

STORAGE AND CONDITIONING

FOREWORD

Handling Agricultural Materials is produced in complete system may require information from

several parts as a guide to designers of several sections of the manual,
materials-handling systems for farm and

associated industries. Sections deal with selec- This section was prepared by UMA Engineer-

tion and design of specific types of equipment ing Ltd., Winnipeg, Man., for the Canada

for materials handling and processing. Items Committee on Agricultural Engineering

may be required to function independently or Services of the Canadian Agricultural Services

as components of a system. The design of a Coordinating Committee.

HANDLING AGRICULTURAL MATERIALS

INTRODUCTION

The financial viability of farms depends, to a
great extent, on how producers maintain the
quantity and quality of crops — from harvest to
the time the crops are sold to market or fed to
livestock.

Grain that is free from insects and mold is more
valuable than out-of-condition grain. Loss of
dry matter or available nutrients from
improperly stored forage means that greater
production is necessary to achieve the same
quantity of output in terms of meat, milk, or
other products. Canada enjoys relatively
favorable climatic conditions for storing grain
and forage crops. Therefore producers here
find it easier to maintain crop quality than do
farmers in more temperate climates.
Nonetheless, losses in storage can affect the net
income of Canadian producers, so it is
worthwhile to control crop storage conditions.

Storage systems operate under a wide range of
variables including the weather, crop type,
moisture content, product utilization or
marketing scenario, and power availability.
Use this manual to identify these variables in
developing storage facilities.

l.i Maintaining the quality of grain in storage

Deterioration of grain in storage generally
results from infestation by:

• rodents

• insects and mites

• fungus and molds

Control infestations by limiting access to food
and water supplies.

Construct storage buildings to prevent rodent
entry. Use concrete floors and curbs under
buildings and steel kick plates around the
bottom joists of wood-frame buildings. As well,
repair leaking faucets and insulate sweating
pipes to prevent water from collecting in open
pools.

Clean bins between fills to remove the out-of-
condition grain that attracts rodents. Old
grain also harbors insects and mold that may
contaminate the bin. In cleaning the bin, spray
insecticide where insects can hide, for example,
in aeration ducts, wall-to-floor joints, and
panel-interconnection seams.

Clean grain spills outside the bins to discour-
age rodents, insects, and molds from populat-
ing. Cleanliness is especially important in
summer when insects are very mobile and mold
spores are readily transported in the warm

wind. Both pests can contaminate storage
facilities in their vicinities.

1.2 Controlling rodents, insects, and molds

1.3 Moisture control Insects and molds can
survive in dry grain but require moisture to
propagate. Grain that becomes tough or damp
in storage, however, is a prime target for
infestation. Prevent moisture from collecting
in the grain mass and block access of free water
into the bin.

Natural convection within the bin concentrates
moisture within the grain mass. Temperature
gradients between the interior and exterior of
the bin promote convection currents. Mix and
redistribute the grain mass. Use aeration
equipment or turn the grain by unloading the
bin, then by reloading the grain into the same
or a different bin. These actions prevent large
temperature gradients from generating
convection currents. Moisture migration
patterns vary seasonally, as Fig. 1 illustrates.

Good design and maintenance of the granary
prevents free moisture from building up in the
grain mass. Close the inspection and access
hatches, ensure waterproof connections of bins
to their foundations, and provide weather seals
where the bins connect roofs to the walls, to
prevent access of snow.

Other moisture-control strategies involve
using a vapor barrier under concrete bin floors
and installing high-density concrete. Both
these measures prevent moisture from
migrating through the floor. To alleviate a
floor moisture problem that already exists,
insert a granular (nonwicking) material below
the bin floor and raise the bin floor above
adjacent grade.

Natural-air drying of grain can often cause
water to condense on the underside of steel bin
roofs during cool nights. This moisture can run
down the underside of the roof to drip points,
usually around the periphery, causing
moisture to concentrate in the grain. Use relief
air openings in the roof to reduce this
condensation problem. In difficult cases,
supplement this venting with positive
overventilation of the air space above the
grain: exhaust air from the space faster than
the air moves through the grain. Insulating
the roof may also help.

1.4 Temperature control The ability of insects and
molds to propagate depends on grain
temperature. Grain harvested on warm days is
particularly susceptible to insect infestations
and to mold formation, especially when
adequate moisture is available. Reduce the

HANDLING AGRICULTURAL MATERIALS

Warm
air

High moisture
zone

\\\

Cold |^j Cold |kw

Air releases
moisture

Fig. 1. Seasonal moisture migration patterns in (A) spring and summer, (B) fall and winter.

temperature of the grain to below 5°C as
quickly as possible after harvest to prevent
insect and mold growth. This action also
reduces the temperature differential between
the grain mass and the outside air which, in
turn, limits natural convection currents.

Monitor grain temperature frequently to detect
grain spoilage. Dry grain stored at a uniform
temperature below 5°C does not deteriorate in
storage.

1.5 Chemical control Pesticides also control
rodents and insects. Two Agriculture Canada
publications provide detailed information on
selecting and using pesticides (Loschiavo 1976,
Control of rats and mice 1979, Cessna 1988).

Using chemicals to control insects and mold
presents a risk to the health of the user as well
as to children, livestock, and pets. Limit the
use of chemicals to situations where an
infestation has developed and the grain has
begun to lose quality. Use of advice in this
manual, however, should help avoid either
situation.

EFFECTS OF MOISTURE,
TEMPERATURE, AND TIME ON
GRAIN STORAGE

2.1 Moisture and temperature

The optimum conditions for reproduction and
growth vary with each species of insect, mite,
or mold. However, some generalizations can be
made for pests that affect grain in storage.
Table 1 presents the environmental conditions
recommended for stored grain to prevent total
loss (by other than chemical means).

Table 1 Environmental conditions associated
with pests in grain storage

Insects

Mites

Molds

Minimum temperature
for reproduction

17°C

5°C

—

Minimum grain moisture

content for reproduction

dry

dry

tough

Minimum temperature
for activity

8°C

3°C

-8°C

Source: Sinha (1971), Loschiavo (1976).

Similarly, each species of grain exhibits a
different ability to withstand storage
conditions without loss, germination, or mold
growth. Grains achieve stable storage
moisture content when the relative humidity
in the space between the kernels is less than
the threshold humidity level for mold
propagation. Table 2 lists moisture content
designations for several grains.

To enhance the storage life of grain, control
both the grain temperature and the moisture
content. Figs. 2, 3, and 4 illustrate the length
of time that grains at various moisture
contents and temperatures can be stored.

As the temperature rises in heating grain,
however, the rate of mold growth and insect
infestation also increases to cause an
exponential rate of temperature rise. In
addition, storage without airflow causes a
faster rate of decay and temperature rise than
might be expected. Fig. 3 reflects storage of
canola that is continuously ventilated.

STORAGE AND CONDITIONING

Table 2 Moisture content designations for common species of grain

Species

Dry

% of weight

Tough

Damp

Wheat

Barley

Oats

Rye

Flax

Canola

Buckwheat

Corn

Peas

Sunflowers

Mustard

Canary seed

Soybeans

Lentils

Triticale

White beans

14.5
14.8
14.0
14.0
10.5
10.0
16.0
15.5
16.0
9.5
10.5
12.0
14.0
14.0
14.0
18.0

14.6-17.0
14.9-17.0
14.1-17.0
14.1-17.0
10.6-13.5
10.1-12.5
16.1-18.0
15.6-17.5
16.1-18.0
9.6-13.5
10.6-12.5

14.1-16.0
14.1-16.0
14.1-17.0
18.1-21.5

>17.0

>17.0

>17.0

>17.0

>13.5

>12.5

>18.0

17.6-

-21.0

>18.0

13.6-

■17.0

>12.5

16.1-

■18.0

>16.0

>17.0

>21.5

Note: Canola is considered safe for storage over winter at 8.5% moisture content. Other grains are considered safe for storage when
they are dry.

Source: Grain grading handbook for western Canada.

30

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storage \. N. \. N. N.

5

i i i i i i i i i i i

14 16 18 20 22

Moisture content. %

24

Fig. 2. Effect of temperature and moisture content on
the allowable storage time for wheat, oats, and barley.

Localized pockets of excess moisture in
otherwise dry grain can result from insect
infestation, concentrations of immature
kernels, or moisture migration. Activity in
these pockets generates heat in what is
thought to be dry, cool grain.

Whenever possible, reduce the temperature of
the grain mass below 5°C. Alternatively,
reduce the moisture content of the grain mass
to prevent insect or mite infestation, which can
cause the temperature of the grain to rise
(Table 1; Figs. 2, 3, and 4).

10 15

Moisture content, %

20

Fig. 3. Effect of temperature and moisture content on
the allowable storage time for continuously ventilated
canola.

Because insects and mites reproduce rapidly,
cool grain as soon as possible after harvest,
especially on warm days when the ambient
temperature reaches 15°C or more. Grain does
not cool quickly in storage without air flow and
retains the harvest temperature for long

periods.

HANDLING AGRICULTURAL MATERIALS

18

20 22 24 26

Moisture content, %

28

30

2.2

Fig. 4. Effect of temperature and moisture content on
the allowable storage time for corn.

In summary, these factors predispose grain to
spoilage:

• presence of immature kernels

• presence of mature grain having a higher
moisture content than average

• presence of grain containing above-average
quantities of damaged kernels and fine 2.3
material

Immature kernels have a higher moisture
content and a higher heat of respiration than
mature kernels. Prevent pockets of immature
grain from forming in storage bins by turning
the grain. In addition, use a grain spreader
during bin filling and satisfy appropriate
moisture and temperature conditions.

Rewetting the grain, poor drying procedures, or
moisture transfer within the bin can cause
pockets of moist grain to form. Use control
procedures similar to those for dealing with
immature grain pockets.

Insects readily attack damaged kernels and
fine grain because the feedstock inside the
kernels is easily accessible. These damaged
kernels and fines, called dockage, are also more
difficult to cool because of their high airflow
resistance. Turn the grain in storage and use a
grain spreader during filling to distribute the
dockage uniformly. Check the bin-loading
method to avoid creating the same problem
during turning. Filling devices often fail to
distribute particles uniformly. However, if
uniform distribution of this material in the bin
is impossible, clean the grain to remove
excessive dockage.

Equilibrium moisture content The equili-
brium moisture content of grain occurs when
the partial pressure of the water vapor in the
grain equals the partial pressure of the water

vapor in the air. The relative humidity (RH) of
the air at equilibrium with material of a given
moisture content is known as the equilibrium
relative humidity. Grain gives up moisture to
air (i.e., dries) when the relative humidity of
the air is less than the equilibrium relative
humidity. Conversely, grain takes on moisture
if the relative humidity of the air exceeds the
equilibrium relative humidity. The point of
equilibrium varies with temperature and grain
type.

When moving ambient air through grain, the
grain either gives up moisture to the air, or
takes it from the air, depending on the relative
humidity of the air, moisture content of the
grain, and temperature of both air and grain.

Fig. 5 illustrates two important notes about
drying grain. As the grain dries, the drying
rate slows significantly once the vapor
pressures of the air and the grain begin to
equalize. In addition, the ability of ambient air
to dry grain declines as temperatures decrease.
Consequently, the potential for removing
moisture from grain by ambient air relates
directly to the net evaporation in any given
geographical area.

Determining moisture content For quick moist-
ure content determinations, rely on electrical
resistance testers. Use testers that demon-
strate good accuracy for most grains over a
wide range of moisture contents. Remember to
recalibrate the electrical resistance testers,
however, for each type of material.

When using a heated-air grain dryer, allow for
the moisture rebound effect that occurs as the
moisture in the grain restabilizes after a period
of drying. Moisture rebound results because
the moisture tester measures only the moisture
in the surface layers of the kernels. These
layers dry much faster than does the centre of
the kernel. Thus, the tester reflects a low
moisture reading immediately after drying.

Moisture rebound is negligible when only a
small amount of moisture is removed from the
grain. However, rebound frequently reaches
1-1.5% when 10% or more of the moisture in
the grain must be removed.

To accommodate moisture rebound and
accurately measure the moisture content,
allow the grain to reach temperature
equilibrium throughout the kernels before
using the moisture tester.

Another way of overcoming the problem of
moisture rebound is to overdry the grain by
about 1%. Alternatively, before testing the
sample for moisture content, grind it, seal it in
a plastic bag to prevent moisture loss, and
allow it to cool to ambient temperature.

10

STORAGE AND CONDITIONING

20

C
CD

cereals at25°C

O cereals at 10°C

□ oilseeds at 10° C

A oilseeds at 25 °C

60 70

Relative humidity of the air

Fig. 5. Equilibrium moisture content of cereal grains and oilseeds.

80

Several publications from the American
Society of Agricultural Engineers (ASAE) offer
standard methods for determining the
moisture content of grains, seeds, and forages.
In fact, the accuracy of all types of moisture
testers are measured against the ASAE oven
dryers.

2.4 Temperature -monitoring devices Monitoring
temperature is the easiest and most convenient
means of checking the condition of stored
grain. Temperature measurements indicate
whether any heating is taking place or whether
the propagation of insects and mites threatens
the quality of the grain mass.

Minor increases in temperature indicate a need
for closer examination. Maintain good records
of temperatures so that even minor
temperature changes can be detected and
evaluated.

Locate temperature sensors in at least three
places within the grain to adequately reflect
the temperature of the storage mass. Good

locations in the storage bin include the top
centre (0.5-1 m below the surface), bottom
centre (0.5-1 m above the floor), and on the
south side (half-way up the bin about 0.1-0.5 m
from the exterior wall).

Because moisture migrates within the bin, the
top and bottom centre locations are most
susceptible to moisture accumulation from
internal convection. Monitoring the
temperature differential from the outside to
the centre reflects changing temperatures
within the grain mass and provides an early
clue to the presence of moisture within the
grain. Uniform temperature throughout the
storage mass ensures the best storage
conditions.

The most common temperature-sensing devices
include thermocouples, thermistors, and gas-
filled thermometers. Thermocouples and
thermistors are relatively low-cost sensors but
they require high-cost meters to interpret the
signals. Gas-filled thermometers are often low-

HANDLING AGRICULTURAL MATERIALS

11

cost devices but function best as probes or when
in fixed locations.

Use thermocouples or thermistors when
several bins must be monitored on a regular
basis. These devices can remain enclosed in
the bins since they generate temperature
signals to exterior meters.

Equip all bins over 300 m3 with devices to
monitor temperature. However, bins of any
size can benefit from temperature-monitoring
equipment, particularly if the stored grain is
tough or is susceptible to heating, such as
canola.

2.5 Time

Time of storage is the most frequently
underutilized factor in alleviating grain
storage problems.

Design storage systems to handle wide-ranging
demands for time variability. With the
appropriate system, an operator can take
advantage of the time variable and use it to
economic advantage.

Consider the scenario involving a harvest of
No. 2 wheat at 18% moisture content on
5 September. The operator faces these
questions:

• Should the grain be dried? 31

• Should the grain be cooled for possible sale
as No. 2 tough?

• Should the grain be cooled for future sale as
feed wheat?

• Should the grain be cooled for future con-
sumption by the operator's own livestock?

• Should the wheat dry in the field prior to
harvest? In this case it may no longer be
No. 2.

The operator can only answer these questions
with a full knowledge of the following:

• grain stocks available

• feed requirements

• tax position

• market conditions

• risk-taking ability

The design of the storage facility should not
add additional constraints.

SYSTEMS TO CONTROL
MOISTURE AND TEMPERATURE

Air moving through the grain mass controls its
moisture content and temperature. The

necessary quantity, temperature, and
humidity of the air depend on the objectives of
the drying operation.

The four basic systems for drying grain are

• aeration

• natural-air drying

• heated-air drying

• combination drying

Aeration maintains the quality of dry grain
and eliminates temperature and moisture
differences throughout the bin.

Natural-air (or unheated-air) drying dries
grain using ambient air. This process removes
limited quantities of excess moisture.

Heated-air drying dries grain using
supplementary heat. Adding heat to the
drying air increases the vapor pressure
differential between the grain and the air and
reduces drying time.

Combination drying uses aeration, natural-air,
or heated-air drying to dry the grain. Such
methods are normally referred to as dryera-
tion, cooleration, and modified cooleration.
Use them during cooling to remove the last
1-6% of moisture from the grain.

Aeration systems

Use aeration to

• cool dry grain for storage at the lowest
practical temperature

• establish and maintain temperature and
moisture uniformity throughout the grain
mass

Aerating grain in storage requires a minimum
airflow of 1 (L/s)/m3. At this rate, grain cools
uniformly to near ambient temperature within
150-200 h. It cools in half this time with
double the rate of airflow.

Airflow rates above the minimum
recommendations increase storage options.
Aerate at 2-6 (L/s) per cubic metre of grain to
reduce the fan-operation time. At this rate, the
grain cools well and attains uniform
temperature. At the same time, aeration
reduces the risk of moisture accumulating in
the stored tough grain.

The ideal grain temperature is within 5°C of
the average ambient temperature.
Realistically, however, maintain grain below
5°C during winter and rewarm it to 10°C in the
spring.

Monitor the grain temperature in the bin to
determine temperature uniformity within the
grain mass. If temperature differentials
develop, moisture may migrate within the bin

12

STORAGE AND CONDITIONING

and reduce the safe storage time. Adequate fan
operation times ensure uniform temperatures.

Moisture migration within a storage bin causes
liquid to deposit in the top centre of a bin
during winter and the bottom centre during
summer. Monitor these locations frequently.
Since it is relatively inaccessible, the bottom
centre of the bin can be effectively monitored
only by measuring the temperature. Aerating
the bottom of the storage bin, however, makes
it less important to monitor the bottom of the
bin.

Do not expect aeration to dry tough grain,
although under ideal conditions some moisture
may be removed. Avoid overdrying the grain,
which reduces the amount of marketable
product.

Taking advantage of increased airflow rates,
faster cooling, or holding tough grain requires
that the aeration system be properly designed
and that the operator understand the influence
of ambient conditions on the ability of grains to
be stored.

3.2 Fan operation During harvest, the primary
objective is to cool the grain. Operate the fan
continuously while filling a bin and afterward
for as long as the exhaust air temperature
measures 5°C above the maximum daily
temperature. Cooling the grain to below 5°C
generally requires several periods of
continuous fan operation as the ambient
temperature decreases. Ignore the relative
humidity of the ambient air when the grain is
cooling. Continue operating the fan, even
during wet weather, because cooling the grain
is much more important than any rewetting
that may take place. Operate the fan for a day
or two after the wet weather to remove excess
humidity from the storage bin.

The top centre of the bin cools last if air moves
upward through the bin. With a downward
airflow, the bottom centre cools last. Cooling is
complete when the temperature of these
locations reaches 5°C or lower.

During the winter, maintain uniform tempera-
tures by operating the aeration fan 1 or 2 days
when the ambient temperature approaches
that of the grain in storage. Extreme cold
reduces the temperature uniformity in the bin
and increases the tendency for moisture
migration. Winter cold also causes condensa-
tion to occur on grain near the air ducts.

When the fan is not operating, keep a cover
over the aeration fan or duct. This cover

• prevents the grain near the duct from
cooling excessively because of severe winter
conditions

• prevents rain or snow from entering the bin

• reduces the accessibility of the bin to rodents

Rewarm the grain in the spring, especially if
the grain is to be stored through the summer
and if the grain temperature measures 0°C or
less. Begin warming in the spring when
average daily ambient temperature reaches
5°C above the grain temperature. Warm in
several stages, eventually bringing the
temperature to 10°C, and select periods when
the relative humidity is below 70%. Warming
under these conditions reduces the risk of
condensation forming on the grain and
rewetting if the grain absorbs the moisture.

Operate the fan continuously during
rewarming to ensure the temperature remains
uniform in the bin. Condensation and spoilage
may occur within a few days if the fan is shut
off prior to grain achieving uniform
temperature.

During the summer, take advantage of fair
weather when the temperature dips to 15°C or
lower to reestablish temperature uniformity.
Do not run the fan when the ambient
temperature is above the grain temperature,
except to complete the period necessary to
establish temperature uniformity.

3.3 Direction of airflow The decision to direct the
air through the grain — from top to bottom, or
the reverse — depends on several factors. Use
the following information to decide which
method to use in designing airflow drying
systems.

Exhausting air through the bottom of the
storage bin has one important advantage. It
eliminates condensation on the underside of
the roof while cooling warm grain during cold
weather.

On the negative side, however, exhausting air
through the bottom of the bin can cause
moisture to accumulate at the bottom centre of
the bin where accessibility for quality
monitoring is most difficult. To avoid this
situation, fix a temperature sensor in the
spoilage location and periodically remove a
small quantity of grain from the bottom centre
of the bin to monitor for potential spoilage.

Additionally, exhausting air through the
bottom poses a problem since it rewarms cooled
grain at the bottom of the bin if warm grain is
added at the top or if solar heat gain raises the
air temperature under the bin roof during
cooling.

Exhausting air at the top of the storage bin has
four important advantages. It offers easy
access to check the condition of the area most
susceptible to spoilage (the top centre). It also
simplifies temperature measurement to
determine when aeration is complete.

HANDLING AGRICULTURAL MATERIALS

13

Air exhausting at the top of the bin helps in
keeping perforations clean during bin filling,
especially when the fan begins operating prior
to filling. And finally, it prevents the solar
heat gain from the roof and space above the
grain in storage from warming the grain in
storage, a particular advantage during spring
and summer.

As a disadvantage, though, exhaust air exiting
the bin through the top may allow
condensation forming on the bin roof to drip
onto the grain.

3.4 Natural-air drying

Unheated ambient air can dry stored grain
provided the equilibrium relative humidity
corresponding to the moisture content of the
grain exceeds the relative humidity of the
ambient air. The drying rate must surpass the
rate at which spoilage develops. The drying
rate increases with increasing ambient
temperature, but remember, this change in
conditions also promotes spoilage. On the
other hand, the equilibrium moisture content
increases as the grain temperature decreases,
further restricting the drying rate already
reduced by the lower enthalpy of cooler air.

3.5 Design and operation Air forced through
grain in storage first dries the grain it initially
contacts. As it passes through the grain it
takes on moisture until it reaches the
equilibrium relative humidity, which
corresponds to the moisture content of the wet
grain. Subsequently, the air passes through
the remaining grain with essentially no drying
effect.

The depth of grain that correlates with the
moisture uptake by air is known as the drying
zone. The thickness and rate of travel of the
drying zone varies with the airflow rate,
ambient conditions, and the moisture content
of the grain. To prevent spoilage, the drying
zone must pass through the entire grain mass
within the allowable storage time. Natural-air
drying, then, is a race to dry the grain before it
spoils.

Normally, a fan blows the air upward through
the bin so the top layer of grain dries last.
During drying, closely monitor the grain in
this location to detect any signs of heating and
to determine when a drying zone has moved
through. If the grain begins to heat, remove it
to a heated-air dryer as quickly as possible to
reduce the chances of spoilage.

In a natural-air drying system the operator can
control only the rate of airflow. Higher airflow

rates move a wider drying zone through the
grain faster. A drying zone, once developed,
moves through grain as long as the fan
operates. Therefore, operate the fan
continuously until the zone has moved through
the entire grain mass, or until the grain
reaches a temperature for safe storage. Expect
grain nearest the air supply to overdry during
good weather and rewet during adverse
conditions.

Continue operating the fan until either the
grain dries or it reaches a safe storage
temperature (see Figs. 2, 3, and 4). Store cereal
grains and corn at 15-18% moisture content
and at temperatures from 0 to — 5°C. No
further drying is required if the grain is to be
used as feed during the winter. Otherwise
completely dry the grain or corn by operating
the fan continuously during the spring as soon
as ambient temperature permits. Moisture
removal proceeds quickly in the spring when
ambient conditions normally favor drying.

3.6 Airflow rate Select airflow rates depending
on:

• grain type and moisture content

• date of harvest

• normal autumn weather conditions for the
area

Tables 3-8 list airflow rates for a variety of
conditions.

3.7 Operational aids Several factors can influence
the efficiency of grain drying.

First, the concentration of fines and broken
material can have a significant detrimental
effect on airflow. Reduce this effect by
removing some of the grain through the centre
unloading port after filling the storage bin.
Alternatively, use a grain spreader during
filling. Hand leveling the grain surface may
also encourage uniform airflow through the
bin.

Start the fan as soon as there is enough load on
the perforated area of the bin floor to prevent
uplifting. Continue operating the fan until the
grain is dry or cool enough to prevent spoilage.

During storage, monitor the temperature and
moisture content of the grain in at least the top
metre of the bin.

During the winter, operate the fan for 6-8 h if
the ambient temperature rises or following
periods when the ambient temperature exceeds
0°C.

Finally, use the fan in the spring to rewarm dry
grain to 10°C, especially when storing the
grain past the end of June.

14

STORAGE AND CONDITIONING

Table 3 Recommended minimum airflow for a storage bin with a fully perforated floor and a
level grain surface in Manitoba

Crop and

% time

harvest

Dry

dried

date

date

in fall

Initial moisture content (%)

16 17 18 19 20 22 24 26

28

Airflow (L/s-m3)

Seed wheat

15 August

Fall

100

9

9

10

15

27

**

97

6

7

10*

15

27

**

94

6

6

10*

15*

27*

**

90

5

5

10*

15*

27*

**

Spring

6

7

10

15

27

**

% time

97

97

100

100

100

**

dried

in fall

1 September

Fall

100

10

11

12

13

17

40

97

8

10

10

13*

17

40

94

7

9

10

13*

17

40*

90

7

8

9

13*

17

40*

Spring

7

8

8

13

17

40

% time

94

91

88

100

100

100

dried

in fall

15 September

Fall

100

19

23

28

30

32

37

97

17

22

23

24

25

27

94

13

13

16

17

20

27

90

10

12

15

16

17

26

Spring

8

8

8

8

14

26

% time

76

73

52

33

82

88

dried

in fall

1 October

Spring

8

8

8

9

13

23

% time

33

15

3

3

39

61

dried

in fall

15 October

Spring

8

8

8

11

15

24

%time

0

0

0

3

9

30

dried

in fall

Commercial wheat

15 August

Fall

100

9

9

10

10

17

34

97

6

7

8

9

17*

34*

94

5

6

7

9*

17*

34*

90

5

5

6

9*

17*

34*

Spring

6

6

7

9

17

34

% time

97

94

94

97

100

100

dried

in fall

(continued)

HANDLING AGRICULTURAL MATERIALS

15

Table 3 Recommended minimum airflow for a storage bin with a fully perforated floor and a
level grain surface in Manitoba {continued)

Crop and

harvest

date

% time
Dry dried

date in fall 16

Initial moisture content (%)

17 18 19 20 22 24 26 28

Airflow (L/s-m3)

1 September

15 September

1 October

15 October

Barley

15 August

1 September

Fall

100

10

11

12

13

15

21

97

8

10

10

12

13

21

94

7

9

10

11

13*

21

90

7

8

9

10

13*

21*

Spring

7

8

8

8

13

21

% time

94

91

88

76

97

100

dried

in fall

Fall

100

19

23

28

30

32

37

97

17

22

23

24

25

27

94

13

13

16

17

20

27

90

10

12

15

16

17

26

Spring

8

8

8

8

9

18

% time

76

73

52

33

15

88

dried

in fall

Spring

8

8

8

8

8

15

% time

33

15

3

3

0

48

dried

in fall

Spring

8

8

8

8

8

16

% time

3

0

0

0

0

0

dried

in fall

Fall

100

16

16

18

40

97

9

—

12

—

18*

40*

94

8

—

10

—

18*

40*

90

7

—

9

—

18*

40*

Spring

7

—

9

—

18

40

% time

88

—

88

—

100

100

dried

in fall

Fall

100

19

19

_^_

24

30

97

16

—

19

—

19

27

94

14

—

17

—

18

27

90

13

—

16

—

17

27*

Spring

13

—

13

—

13

27

% time

91

—

85

—

85

97

dried

in fall

(continued)

16

STORAGE AND CONDITIONING

Table 3 Recommended minimum airflow for a storage bin with a fully perforated floor and a
level grain surface in Manitoba (concluded)

Crop and

harvest

date

%time
Dry dried —

date in fall 16

17

Initial moisture content (%)

18 19 20 22 24 26

28

Airflow (L/s-m3)

15 September

Fall

100

35

- 45

50

**

97

29

39

45

47

94

24

30

31

37

90

20

25

30

35

Spring

13

13

13

17

% time

73

64

45

55

dried

in fall

1 October

Spring

13

13

13

17

% time

45

24

6

15

dried

in fall

Cornt

15 September
1 October
15 October
1 November

10

—

29

—

117

8

—

22

—

49

8

13

20

25

52

8

—

12

—

20

Lower airflows are possible for this condition, but a risk of spoilage would be introduced. See your local agricultural engineer for

details.

Airflow rates would be excessive for this condition.

Data are based on computerized drying simulations using Winnipeg weather data for the years 1961-1970. The final moisture

content was 15.5%. The airflow rates given would complete drying in the spring following the harvest year.

3.8 General discussion Natural-air drying sys-
tems usually provide 10-30 (L/s) of air for each
cubic metre of grain. This rate is sufficient
to dry tough grain, or at least store it through
the winter. It may even be suitable for damp
grain. However, this sort of drying system
demands attention from a knowledgeable
operator.

Drying systems using natural air usually
require less capital investment and less
materials-handling equipment than do heated-
air dryers. Natural-air drying systems also
consume less energy than any other means of
drying. This fact is especially notable in areas
having a low relative humidity in the autumn
and where only small amounts of moisture are
removed from the grains.

In fact, natural-air systems work particularly
well in areas with low relative humidity during
the autumn. In such locations, though, pay
attention to controlling the costs associated
with overdrying; they can be significant.

In areas where the ambient relative humidity
approximates the equilibrium humidity levels

for dry grain, energy costs escalate rapidly.
Systems in these areas require high airflow
rates to prevent spoilage and extended fan
operation to remove the moisture.

In designing natural-air drying systems use
wide and short bins. For economical opera-
tion, select a fan that maintains static pressure
as low as possible. These fans are particu-
larly necessary in facilities storing small-seed
crops (e.g. flax, canola) because pressure loss
through grain increases considerably with
grain depth and air speed. This configuration
of bin and fan reduces the cost of the bin
marginally; however, it increases the cost of
the foundation and perforated floor.

3.9 Heated-air drying

As grain dries it releases moisture to the air at
a rate dictated by two factors: the difference
between the partial pressure of water vapor
(VPD) between the kernels and the air, and the
permeability of the kernels. Excessive VPD
causes physical damage.

HANDLING AGRICULTURAL MATERIALS

17

Table 4 Recommended airflow for natural-air drying of wheat in Edmonton, Alta.

Harvest
date

Year

Initial moisture content (%)

16

18

20

22

24

2nd worst
Worst

Airflow (L/s-m3)

15 August

—

5.0
6.3

20.0
22.5

35.0
41.3

—

1 September

2nd worst
Worst

—

4.4
4.4

12.5
16.3

28.8
28.8

—

15 September

2nd worst
Worst

2.5
2.5

3.8
4.4

7.5
7.5

17.5
18.8

40.0
41.3

1 October

2nd worst
Worst

—

3.8
4.4

6.3
6.3

10.0
11.3

—

15 October

2nd worst
Worst

—

3.8

4.4

5.6
5.6

8.8
8.8

—

The data in this table are based on simulated drying results for the years 1967-1976.

Table 5 Recommended airflow for natural-air drying of wheat in Swift Current, Sask.

Harvest
date

Year

Initial moisture content (%)

16

18

20

22

24

2nd worst
Worst

Airflow (L/s-m3)

15 August

—

5.0
5.0

13.8
15.0

26.3
35.0

—

1 September

2nd worst
Worst

—

5.0
5.6

11.3
12.5

21.3
25.0

—

15 September

2nd worst
Worst

2.5
3.1

3.8

4.4

8.8
11.3

17.5
18.8

33.8
35.0

1 October

2nd worst
Worst

—

3.8

4.4

7.5
7.5

12.5
15.0

—

15 October

2nd worst
Worst

—

3.8

4.4

6.3
6.3

10.0
11.3

—

The data in this table are based on simulated drying results for the years 1960-1974.

18

STORAGE AND CONDITIONING

Table 6 Recommended airflow for natural-air drying of wheat in London, Ont.

Harvest
date

Year

Initial moisture content (%)

20

22

24

26

28

2nd worst
Worst

Airflow (L/s-m3)

15 September

21.0
26.8

35.0
51.3

69.0
72.3

113.0
128.0

175.0
187.0

1 October

2nd worst
Worst

11.7
15.2

25.6
30.3

49.0
66.5

87.5
128.0

111.0
163.0

15 September

2nd worst
Worst

7.7
11.7

17.5
24.5

39.6
73.5

70.0
128.0

140.0
152.0

1 November

2nd worst
Worst

8.2
9.3

12.8
12.8

21.0
23.3

40.8
42.0

67.5
80.5

15 November

2nd worst
Worst

8.2
8.2

7.7

7.7

15.2
16.3

22.1
26.8

39.6
45.5

The data in this table are based on simulated drying results for the years 1962-1973.

Table 7 Predicted minimum airflow for drying canola in Manitoba

Harvest
date

% time
Dry dried
date in fall

10

Initial moisture content (%)

11

12

13

14

15

Airflow (L/s-m3)

15 August

1 September

15 September

1 October

Fall

100

16

97

13

94

10

90

7

Spring

7

% time

88

dried

in fall

Fall

100

26

97

19

94

13

90

13

Spring

12

% time

85

dried

in fall

Spring

12

% time

55

dried

in fall

Spring

12

% time

24

dried

in fall

16

16

16

17

28

13

13

13

15

28*

10

11

13*

15*

28*

8

9

13*

15*

28*

7

7

13

15

28

85

76

97

97

100

26

26

27

27

30

19

19

19

19

20

17

17

17

17

20*

14

15

16

17

20*

12

12

12

16

20

82

79

70

85

97

12

12

12

12

14

39

39

33

30

36

12

12

12

12

12

24

15

15

9

6

Lower airflows are possible for this condition, but a risk of spoilage would be introduced. See your local agricultural
engineer for details.

HANDLING AGRICULTURAL MATERIALS

19

Table 8 Effect of harvest date, climatic condition, and initial moisture content on airflow
rates for low-temperature drying of corn in Toronto, Ont.

Harvest
date**

Climatic condition (year)

1971

1972

1975

1969

1976

(poor)

(good)

Airflow (L/s-m3)

28.1

23.9

23.5

21.2

12.8

26.1

23.8

21.5

16.4

12.4

18.7

20.8

20.0

19.7

13.1

24.4

27.2

26.6

18.3

12.7

26.1

23.8

21.5

16.5

12.4

42.3*

25.3

28.1

21.8

18.9

46.1*

29.8

47.4*

28.9

21.2

1 October

15 October

1 November

15 November

Initial moisture content (%)t

22

24

26

* These airflow rates may not be feasible unless a high-speed centrifugal fan or shallow depths of grain are used.
** Initial moisture content of 22%.
t Harvest date 15 October.

Source: Mittal and Otten (1982).

During initial drying, the grain is still
relatively cool and takes some time to warm,
because of low thermal conductivity. As the
grain dries, the moisture at or near the surface
of the grain evaporates, which further controls
the warming rate because the kernels lose heat
to evaporation. As the grain approaches dry,
the moisture near the surface depletes so the
rate at which moisture can move from the
centre of the kernel through the hull dictates
the rate at which further drying can occur. The
kernel permeability and the VPD affect how
moisture moves through the grain. Rapid
moisture removal from the surface of the
kernel causes the exterior layers to overdry.

Expect some overdrying in high-temperature
drying; but excessively high temperatures and
accelerated drying cause two compounding
problems:

• brittle hulls with lower-than-normal
moisture permeability

• high vapor pressure within the kernels

Together these problems result in stress crack-
ing of the grain hulls. They also lead to in-
creased damage to the grain during handling.

Tough grain normally dries before the grain
temperature and internal vapor pressure reach
levels likely to cause stress cracking. Damp or
wet grain, on the other hand, is very sensitive
to the drying rate. For wet grain, select a dryer
or drying system that yields a slow drying rate,
particularly for the final drying stages. Such a
system substantially reduces damage to the
grain, as well as limits the possibility for
damage in handling.

Rapid removal of moisture in the final drying
stages requires a great deal of energy. Two
factors account for the increase in energy
demand. First, instead of drying the grain, the
energy increases the sensible heat of the grain.
Second, the slower release of moisture from
grain that is almost dry results in exhaust air
with a low relative humidity. The net effect is
a lower dryer efficiency.

Excessive grain temperature reduces the
ability of the seeds to germinate. Both drying
time and temperature are correlated; however,
the short time involved in heated-air drying
generally reduces its impact on the quality of
the grain.

20

STORAGE AND CONDITIONING

Extremely high temperatures cause the grain
kernels to discolor and disfigure. Controlling
the drying process to prevent these distortions,
however, results in temperatures higher than
required for complete drying. Consequently
this approach proves an undesirable control
strategy.

Refer to the temperatures listed in Table 9 as a
guide to safe air temperatures. However, do
not overlook safe grain temperatures when
planning grain-drying systems.

Grain dryers vary in design and operation.
They provide a wide range of conditions that
affect the safe drying temperature. In
particular, two aspects of dryer design most
strongly influence safe drying temperature:

• temperature uniformity in the supply air

• moisture content uniformity in the grain
being dried

The design of the air supply system, the burner
type, the shape of air plenum, and the mode of
operation each affect the temperature of the
supply air at different locations within a grain
dryer. Temperature variations within a dryer
generally remain constant for each type of
grain, but change significantly from one grain
to another because of changes in airflow rate.

For grain with a nonuniform moisture content,
the maximum temperature that the driest
kernels can be exposed to without damage
represents the safe drying temperature for that
batch. A nonrecirculating batch dryer requires
a lower safe drying temperature than a
recirculating batch dryer. The difference
occurs because in a nonrecirculating dryer the
hottest, driest air continuously contacts the
same kernels first throughout the drying
process. Consequently these kernels overdry
by the time the entire batch is dry.

The temperatures given in Table 9 depend on
drying to not more than 1% below dry (as
designated in Table 2), and on removing not
more than 6% of the moisture in one pass
through a high-speed dryer. To remove more
than 6% moisture on the first pass, the initial
drying temperature can exceed the
temperatures shown in Table 9 because
evaporation cools the grain kernels during this
drying stage. However, as the grain gets close
to being dry, maintain the air temperature at
the levels given in Table 9.

Operate nonrecirculating batch dryers at
5-10°C below the temperatures indicated for
commercial use, particularly when drying
oilseeds. Prolonged exposure to high
temperatures affects the oil quality in oilseed
grains.

Table 9 Maximum drying temperatures

Maximum temperature (°C)

Seed or

Commercial

Crop

malting

use

Feed

Wheat

60

65

80-100

Oats

50

60

80-100

Barley

45

55

80-100

Rye

45

60

80-100

Corn

45

60

90-100

Flax

45

80

80-100

Canola

45

65

—

Peas

45

70

80-100

Mustard

45

60

—

Sunflowers

45

50

—

Buckwheat

45

45

—

For damp grain (see Table 2), air temperatures
20°C above those given (Table 9) can be used
safely in the early stages of drying.

3.10 Dryer types There are three main types of
heated-air dryers:

• nonrecirculating batch dryers, in which the
grain remains static during drying and is
then removed

• recirculating batch dryers, in which the
grain constantly mixes during drying

• continuous flow dryers, in which the grain
feeds into the dryer wet and exits dry on a
continuous basis

Each of these types can supplement round steel
storage bins or exist as single-purpose
machines. Select a dryer that permits drying
in 24 h only the amount of grain normally
harvested in a day.

3.11 Nonrecirculating batch dryers Wet grain is
loaded into a batch dryer and heated air is
passed through it until the average moisture
content reaches the dry point. The grain then
cools and is unloaded. Fig. 6 shows a sectional
view of a nonrecirculating batch dryer.

Since the grain remains stationary during
drying, the grain closest to the hot air plenum
dries before grain located further away.
Achieving a dry average requires fairly severe
overdrying of the interior grain. To avoid grain
damage, reduce the operating temperature of
the dryer during the final drying stages.

3.12 Batch bin dryers A batch bin dryer (Fig. 7)
consists of a perforated floor, one or more fans
and heat sources, plus controls to regulate
temperature. Supplementary equipment often

HANDLING AGRICULTURAL MATERIALS

21

,<£f - :. *_Grain

r \ / v;

Air -"
plenum

i •.

*\ :;:-:-V ■■■; y *

Fig. 6. Nonrecirculating batch dryer.

includes timers to aid in temperature
regulation, a manometer to correlate grain
depth with fan capacity, and a bin spreader to
level the grain and to achieve more uniform
distribution of fines and foreign material.

Additionally, wet and dry grain surge bins
reduce the time and supplementary equipment
required to load and unload the dryer. A
second bin dryer using the same heating device
and fan as the primary dryer can function as a
wet surge bin. An airflow diverter shares the
heated air between the two bins.

The airflow rate used in bin dryers varies
widely. However, most systems operate at
125 (L/s) per cubic metre of grain. Increasing
the airflow, either by increasing the fan
capacity or decreasing the grain depth, dries
the grain faster.

Fan
and
heater

Perforated
drying floor

Fig. 7. Batch bin dryer.

Control condensation on the underside of the
bin roof using techniques similar to those used
for natural-air drying (see section 3.4).

For all batch dryers, consider carefully the
time required to load and unload the dryers
and design a drying system to match the
harvest system. Because of their high
resistance to airflow, small-seeded crops like
canola and flax greatly influence the choice of
dryer and the design of the drying system.

3.13 Portable batch dryers The term portable
describes batch dryers that are not the bin
type. These portable dryers pose design
constraints similar to batch and batch-bin
dryers. Most notably, the grain nearest the
plenum dries long before the rest of the batch.
To avoid this problem, maintain the operating
temperature for portable dryers lower than
dryers that provide a greater degree of mixing.

Portable batch dryers differ from bin dryers in
two important ways. Portable batch dryers are
generally smaller in volume and they have a
higher airflow rate per unit volume than bin
dryers. Together these characteristics reduce
the time the grain must remain in the dryer.
Yet at the same time they decrease fuel
efficiency.

Locate a wet surge bin ahead of the dryer and
use it to top up the grain during drying. As it
dries, the grain shrinks. Air escapes through
the area above the grain at the top of the dryer,
rather than passing uniformly through the
entire grain mass. Avoid this problem by
ensuring the dryer is full for drying.

Batch dryers usually come with high-capacity
unloading conveyors. These conveyors are
designed to minimize unloading times and
increase dryer capacity.

Drying very wet grain in cold weather can be
difficult with a stationary batch dryer. This
situation is particularly relevant to small-
seeded crops, which have a higher resistance to
airflow. As the warm air moves through grain
layers, it picks up moisture and cools very
quickly. If the relative humidity exceeds the
dew point of the grain temperature before
leaving the grain mass, water condenses in
the exterior layers of grain and results in a
nearly impenetrable barrier in the grain
column.

Recirculating batch dryers (see section 3.14)
and rack-type continuous-flow dryers (see
section 3.15) can contend with such adverse
conditions by constantly mixing the grain.
Mixing leads to a more uniform grain
moisture content throughout the grain mass
being dried.

22

STORAGE AND CONDITIONING

3.14 Recirculating batch dryers Recirculating
batch dryers (Figs. 8 and 9) continuously mix
grain during drying. Mixing essentially
eliminates most of the operational constraints
associated with batch dryers. It allows high
operating temperatures, thereby increasing
the drying rate. As well, it removes the
problem of severe overdrying of grain adjacent
to the air plenum. Mixing also reduces airflow
resistance; the grain moves continuously so
larger batches can be dried. However, if the
grain is very wet, the continuous recirculation
may damage it.

Depending on the type of dryer, two sorts of
equipment generally recirculate grain. Bin
dryers use stirring augers; portable dryers use
vertical screws discharging at the top centre of

Perforated
floor

Fig. 8. Recirculating batch bin dryer.

Grain

Fan

the bin. Fig. 9 shows a portable recirculating
batch dryer.

Bin dryers, whether recirculating or not,
generally rely on much lower airflow rates
than other types of dryers. This trait makes
bin dryers more fuel efficient. Yet, because of
the lower airflow rates, the grain remains
longer in the bin.

3.15 Continuous-flow dryers A variety of
equipment allows bin dryers to dry grain on a
continuous-flow basis. The most common — a
tapered sweep auger — removes a layer of grain
from the bottom of the drying bed and
discharges it through a central hopper and
unloading auger (Fig. 10). Generally, hot grain
moves from the dryer to secondary cooling bins,
although variations of this system permit
cooling in the dryer as well.

Portable continuous-flow dryers (Fig. 11) are
generally either cross-flow or rack types.
Grain loads at the top of the dryer and flows
down in columns around the central air supply
plenum to discharge at the bottom. A
temperature sensor, located in the grain
column near the bottom of the drying section,
regulates the flow. The operator correlates the
temperature of the grain with its moisture
content.

The grain cools in the lower portions of the
dryer, just ahead of the discharge. Some dryer
designs permit adjustment of the cooling-to-
heating area to accommodate more readily
conditions such as grain moisture, ambient
temperature, and cooling method.

As in stationary or portable batch dryers, cross-
flow batch dryers rely on an airflow
perpendicular to the grain column. Very little
grain mixes in the grain column so it dries from
the inside out.

Rack — or parallel-flow — dryers (Fig. 12) direct
the airflow through the grain parallel to the
grain column. As a result, the temperature

Hot-grain
transfer auger

Grain spreader^.

Unloading
auger

Fig. 9. Portable recirculating batch dryer.

Fig. 10.

Perforated
drying floor

Continuous-flow in-bin dryer.

HANDLING AGRICULTURAL MATERIALS

23

I.

I

«-4".

<-+

<i — Heated
air

Cooling
air

i.
i.

i .
-v
i.
i .
i.
i. .
i.

i '

i.'

i

i -

A

X

1$

Fig. 11. Portable continuous cross-flow dryer.

Fig. 12. Continuous parallel-flow portable dryer.

remains uniform across the column and
contributes to uniform drying. The mixing
grain that flows around the supply and
discharge ducts also assures uniform drying.
Consequently, the grain can withstand high
drying temperatures in this type of dryer.

Rack dryers use no screens so they are suitable
for both small and large grains.

3.16 Temperature control systems

For all dryers the air temperature varies from
one location in the hot air plenum to another.
This location differs for each grain type
because of the changes in the airflow resistance
for different grains. To prevent excessive
drying, monitor the temperature at several
places in the hot air plenum during the initial
run of the equipment for each grain. This
action allows the operator to locate the
temperature sensor in the hottest area or to
recalibrate the sensor so that it correlates with
the higher temperatures known to exist
elsewhere.

Design compromises made to maintain dryer
portability or economy affect the temperature
uniformity of the air supplied to the grain.

These compromises include burner size and
design, installation arrangement, opportunity
for mixing the air before its entry into the hot
air plenum, and temperature and pressure of
the gas supplied to the burner.

The principles of fluid flow that dictate the use
of transitions between fan discharges and large
ducts are ignored, to some extent, in the design
of many commercially available dryers. In
most dryers the combustion device is mounted
at the entrance to the hot air plenum. Thus,
the heated air cannot adequately mix before it
disperses in the plenum. So select burners to
gain complete combustion of the gas as close as
possible to the burner itself. Ideally, choose
burners that yield complete combustion within
about 1 m of the burner under full fire. This
configuration enhances air mixing.

Duct velocities over 15 m/s allow for maximum
burner output. At high duct velocities the
effective capacity of the burner increases
because of increased turbulence. Some models
of grain dryers operate at duct velocities set to
optimize performance of the burner at full fire.
However, the fast-moving air passes directly
into the much larger dryer plenum making
uniform temperature and airflow throughout

24

STORAGE AND CONDITIONING

the dryer impossible to maintain. Reducing
the operating temperature and mixing the
dried grain counteract the problems associated
with design inefficiency.

Typically, operators set the temperatures on
dryers by manually adjusting modulating
valves to regulate the flow of gas to the burner.
The setting of the valve changes for each grain
type, because of the different rates of airflow
required. However, motorized valves for
automatic adjustment can replace the fairly
tedious, trial-and-error manual efforts.
Motorized valve devices cost little and offer
great time savings. Moreover, they provide
more reliable control than do manual devices.
For instance, motorized valves automatically
allow for normal variation in ambient
temperature during the day and for variation
in tank pressure in propane-fired dryers.

The quality of the gas delivered to the
modulating valves depends on the performance
of the vaporizer being used. A vaporizer
mounted inside the dryer plenum cannot
compensate for the effects of fluctuations in
ambient conditions. The vaporizer simply
transfers heat to the gas to allow it to vaporize
before it reaches the burner.

Choose an adjustable vaporizer to accommo-
date the widely fluctuating range of energy
transfer requirements demanded by the dryer.
For example, various grains require different
dryer operating temperatures. As well,
varying ambient conditions and tank pressures
demand adjustments in the vaporizer
operation.

Configuring a motorized modulator valve with
an internal vaporizer or a manual modulator in
conjunction with an external vaporizer offers
superior temperature control for most dryers.
However, an external vaporizer with a
dedicated regulator for discharge gas
temperature and pressure combined with a
motorized modulator valve provides the best
temperature control.

Ensure the gas supply line size is adequate
enough to permit the necessary heat output
from the burner. Smaller control components
cost less and so initially appear advantageous.
However, operating the system in cold weather
soon reveals inadequate burner output and
reduced effective tank volume in undersized
units.

• low-limit sensors that shut off gas in case of
flame failure

• air plenum sensors (see section 3.16)

The grain-temperature monitor, usually
located in the grain near the exterior of the
column, provides a grain temperature that the
operator uses to estimate moisture content. On
batch dryers, this monitor automatically
terminates the heating cycle and initiates the
cooling or unloading cycle. On some batch
dryers, these monitors supplant timers that
provide the same function. In continuous-flow
dryers, temperature monitors control the
discharge conveyors.

Because of its location, the grain-temperature
monitor senses the temperature of the air
moving past it, as well as the temperature of
the grain. It may also sense solar heat or wind
chill, depending on where the dryer is placed.
Although the direct effects of the sun and wind
negligibly affect the actual performance
variation of a heated-air dryer, these climatic
conditions seriously impair the performance of
the control system. For consistent operation,
shield the temperature monitors from both sun
and wind.

Other electronic devices are available for use
as grain-temperature monitors, but users
report variations. Therefore, until the
technology improves, use alternate devices
cautiously.

Check and calibrate monitors annually, since
reliable performance of the dryer depends on
accurate temperature measurement. Ensure
monitors are mounted in appropriate locations.
The best location varies with both dryer and
crop types. Temperature monitors can be
fragile, so locate them where they are not
likely to be damaged.

Monitor and adjust the dryer based on moisture
and temperature measurements from grain
samples removed from the dryer. Moisture
meters generally do not accurately measure
the moisture content of newly dried grain. So
allow for a moisture rebound of 0-2%,
depending on the drying rate and the initial
moisture content of the grain. The moisture
rebound reflects the average moisture content
through the kernels after the grain is removed
from the dryer. Check the rebound carefully
for each condition to prevent overdrying or
underdrying.

3.17 Temperature sensing

Most single-burner dryers rely on three
temperature sensors:

• grain-temperature monitors

3.18 Fire prevention

Fires may occur while drying any crop with a
direct-fired burner dryer. Excessively high
temperatures in the hot air plenum may cause

HANDLING AGRICULTURAL MATERIALS

25

fires. Most commonly, however, fires occur
because combustible materials such as chaff,
dust, or straw accumulate in the burner area.
The burner ignites these materials as they flow
into the grain columns. To reduce the
possibility of fires, prevent access of airborne
combustible materials to the supply-air fan.

Oilseeds also pose fire risks because they tend
to coat the interior surfaces of grain dryers
with a combustible residue which, if ignited,
burns very well. Clean daily any dryers used
for oilseeds.

Other strategies for fire prevention include
orienting the dryer relative to the prevailing
wind, using a coarse filtration medium or a
large filter to avoid seriously impeding airflow,
and using an intake duct to relocate the intake
away from hazardous material. As well, prior
to drying, clean the grain to remove the fine
materials prone to ignition. These strategies
have been used with varying degrees of
success.

Monitoring the dryer to ensure uniform grain
flow, airflow, and temperature also reduces the
risk of fires. Attention to these factors is
especially important when drying sunflowers.

Sunflowers and canola both dry well with
natural air. Take advantage of warm fall
weather to remove moisture from these grains.
Run the grain through the dryer without the
burner operating. Besides preventing possible
fires, this action also saves fuel and limits wear
and tear on the dryer.

For other crops at risk of ignition during
drying, constant attention is really the only
means of preventing fire.

If a fire starts, close the air supply. This action
may snuff out the fire if it is caught early
enough. Alternatively, use a water-spray or
chemical extinguisher. Keep this fire-fighting
equipment within easy reach of the dryer.

3.19 Energy sources for dryers

Essentially any combustible material or
energy source can be used to dry grain. For
most applications in Canada, however, natural
gas, propane, and electricity are the only
feasible energy sources.

If using nongaseous fuels, production of
combustion by-products requires that a heat
exchanger be installed between the combustion
chamber and the air supply. This
configuration reduces fuel efficiency and
increases the capital cost of the dryer.

Table 10 compares the relative fuel
consumption and dryer capacities with various
drying methods for corn. Notice that high-

temperature drying requires a tremendous
amount of energy. This situation makes
electrical energy feasible only for low-
temperature drying. The cost of installing
electrical power at farm sites and the demand
charges levied by the utility for high-power
requirements also limit the use of electricity.

3.20 Low-temperature drying

Low-temperature drying entails the addition of
a small amount of sensible heat to air flowing
through grain. Normally the grain is situated
in a bin that has a totally perforated floor.

Locating the fan-drive motor in the air stream
achieves a temperature rise of 1-4°C across the
fan. Alternatively, installing an electric or
gas-fired heater in the supply air stream raises
the temperature further and increases the
drying rate. This second option suits areas
where the average ambient relative humidity
in autumn exceeds 75%. In most areas, how-
ever, adding small amounts of supplementary
heat this way is less efficient than high-
temperature drying (Mittal and Otten 1982).

As discussed in section 3.4, the increase in the
rate of grain spoilage due to minor increases in
temperature surpasses the increase in drying
rate. Therefore, adding supplementary heat
requires an increased airflow rate to speed the
drying rate further. Natural-air drying
consumes less energy than low-temperature
drying in areas where the equilibrium relative
humidity of air in contact with the drying grain
exceeds the average relative humidity.

Table 10 Impact on fuel consumption and
dryer capacity of various drying methods
for corn dried from 25% to 15% moisture
content

Propane

per tonne

of corn

Electricity

Relative

at5°C

per tonne

dryer

Method

(L)

(kWh)

capacity

Heated-air

drying

30

4.0

1.0

In-bin cooling

26

3.2

1.35

Dryeration

22

2.8

1.6

In-bin cooling

and drying

12

28.0

3.0

Source: Cloud and Morey (unpublished).

26

STORAGE AND CONDITIONING

Low-temperature drying incurs two significant
costs:

• the cost of energy

• the cost of overdrying the grain

Low-temperature drying uses more energy
than natural-air drying where natural-air
drying can be used effectively. For cost
effectiveness, increase the airflow to achieve a
faster drying rate rather than adding
supplementary heat. Increasing the airflow
rate also reduces the risk of spoilage in storage
and enhances the opportunity to hold tough
grain over the winter for final drying in the
spring.

The cost of overdrying grain is realized in a loss
of saleable material.

3.21 Combination drying

The combination drying sequence includes all
the functions between harvest of the tough, wet
grain through to production of cool, dry grain
suitable for safe storage.

The series of drying functions selected for a
given operation depends on the space available,
the economics of each function, the crops to be
handled and their volumes, and the climatic
conditions in the area.

Field conditioning of grain is the most common
drying option for most grains in western
Canada where the climatic conditions usually
allow rapid field drying of grain before harvest.
The real cost of this approach is paid in years
with unusually wet weather. At these times,
significant grade reduction as well as weight
loss can occur when grain is rewetted by rain or
poor drying. Research into the use of
desiccants to speed field drying rates has
shown some positive initial results.

Farm operators in eastern Canada depend on
dryers for many crops, such as corn, beans, and
sunflowers. As well, dryers serve as backup
support in the harvest of small grains.

Ensure that all grain-handling systems can
accommodate damp grain off the field. A
common technique to handle damp grain
involves a simple device to turn or mix grain in
storage. However, if the entire volume is damp
or if long storage periods are necessary,
turning the grain offers only a short-term
solution.

As a minimum, farm systems require an ability
to move air through the grain in sufficient
volume to prevent spoilage. Depending on the
climatic conditions and crops grown, the air-
moving system may also provide the necessary
drying facilities.

Other applications require high-temperature
dryers, raising the question of how to use the
drying system to best advantage. Consider
these observations in answering this question.

• Relatively safe storage conditions exist
when the grain is both dry and cool.

• Energy costs and the risk of grain damage
are highest when a high-temperature dryer
is used to remove the last few points of
moisture from grain that was initially wet.

• Higher temperatures, better fuel efficiency,
and higher capacity for the dryer are
achieved when the grain is not dried
exclusively in a high-temperature dryer.

• The maximum drying capacity of a high-
temperature dryer is achieved when the
dryer is not expected to both dry and cool the
grain. Instead, add a separate burner to the
cooling section of the system. Or for batch
dryers, eliminate the cooling cycle.

In line with these points, numerous
combinations of methods are available to
complete the drying process and cool the grain
outside the high-temperature dryer. These
methods are described as combination drying.

Combination drying entails the transfer of hot
grain from a high-temperature dryer into
storage for cooling. The methods in
combination drying include

• in-bin cooling

• in-bin steeping and cooling (dryeration)

• in-bin cooling and drying

For in-bin cooling, hot grain with a moisture
content of 15-16.5% is transferred to a long-
term storage unit. Up to 40% of the floor space
in the bin is perforated and air flows through
the bin at a rate of 5-10 (L/s) per cubic metre.

For in-bin steeping and cooling, hot grain with
a moisture content of 16.5-18% is transferred
to a temporary storage unit where the entire
floor is perforated. Air flows through the bin at
a rate of 5-10 (L/s) per cubic metre.

For in-bin cooling and drying, hot grain with a
moisture content of 20-22% is transferred to a
long-term storage unit where the entire floor of
the bin is perforated. Air flows through the bin
at a rate of 5-20 (L/s) per cubic metre (Tables
3-8) for natural-air or low-temperature drying.

3.22 In-bin cooling

Run the cooling fan continuously during and
after the drying until the last of the grain is
cooled to ambient temperature. Cooled grain
does not need to be transferred to a long-term
storage bin. Expect a reduction in grain
moisture content of 1% for most crops and 2%

HANDLING AGRICULTURAL MATERIALS

27

for corn. The system efficiently uses the
sensible heat in the grain to aid cooling.

To avoid condensation on the bin walls, start
the cooling fan as soon as hot grain enters the
bin. A fully perforated floor and sufficient
airflow also help to eliminate condensation
problems. Otherwise, the grain must be
transferred to long-term storage.

Completely cool the grain to 0°C. See section
3.1 for details on cooling.

3.23 In-bin steeping and cooling (dryeration)

Hot grain enters the dryeration bin from the
heated-air dryer. In the bin the grain steeps
without airflow for at least 4-6 h before being
slowly cooled.

The useful capacity of the heated-air dryer
increases when it is not used for cooling. Add a
separate burner to the cooling section of a
continuous-flow dryer system to convert the
dryer column to full heat. In a batch dryer,
eliminate the cooling time entirely.

Immediately after heated-air drying, the
outside of the kernels are drier than the
centres. During the steeping process, moisture
equalizes throughout the kernel. The slow
cooling process that follows the steeping period
removes two to three percentage points of
moisture. The actual amount of moisture
removed while cooling is proportional to the
difference between the initial grain
temperature and the ambient air temperature.
In contrast, little water is removed during
rapid in-dryer cooling.

Because less moisture needs to be removed in
the high-speed dryer with dryeration, more
dryer capacity is realized. Dryeration uses the
heat contained in the grain to remove water
during cooling. As a result, the system
requires less fuel for the heated-air dryer.

Dryeration minimizes the kernel stresses
developed in the final stages of heated-air
drying and cooling. It also reduces stress

cracking and kernel damage. The improved
kernel quality yields grain less susceptible to
damage during subsequent handling
operations.

When using dryeration, increase the drying air
temperature in heated-air dryers since the
grain discharges at a higher moisture content
and resides in the dryer a shorter time than it
would with other methods of combination
drying. Increasing the air temperature
increases dryer capacity and improves the fuel
efficiency of the heated-air dryer. However, if
the drying-air temperature is increased,
carefully monitor the grain quality. Assess the
weight and milling quality and inspect for
germination or stress cracking. This routine
helps ensure satisfactory grain quality is
maintained.

The magnitude of fuel savings and increased
dryer capacity with dryeration depend on
weather conditions, grain temperatures, and
moisture contents. Typically, an energy
savings of 20-40% and a drying capacity
increase of 50-75% result. Larger increases
occur when drying low-moisture grain.

For dryeration cooling, set the airflow rate at
5-10 (Us) per cubic metre of grain. Lower rates
of airflow remove more moisture but increase
the risk of spoilage.

After cooling is completed, the grain is moved
from the dryeration bin to aerated storage for
steeping. During steeping, condensation may
build up in the grain next to the bin wall. To
avoid this problem, do not leave the grain in
the dryeration bins for storage unless the bins
have fully perforated floors. Fig. 13 illustrates
the dryeration process.

3.24 Managing dryeration systems Dryeration
systems require at least one separate cooling
bin. Arrange the dryer and the cooling bins to
facilitate switching grain from one bin to
another and for transferring cooled grain to
storage. Ideally, two or more cooling bins
should be available, each holding a minimum
24-h dryer capacity.

High

temperature

dryer

Wet-grain
surge bin

Cooling
fan

Fig. 13. In-bin steeping and cooling (dryeration).

28

STORAGE AND CONDITIONING

Hot grain enters the cooling bin throughout the
day and steeps with the cooling fans off. After
the first hot grain delivered to the bins has
steeped, the cooling fan is turned on while
additional hot grain enters the bin. Set the
cooling fans to start in the evening if hot grain
is first delivered to the bin in the morning.

On the second morning, the dryer discharge
switches to the second dryeration bin and
cooling is completed in the first dryeration bin.
Finally the grain moves into storage.
Alternate this cooling process from bin to bin
on successive days.

Large-capacity cooling bins can safely
accumulate more grain than dries in one day.
So if a system includes only one cooling bin,
make it large enough to hold several days'
drying capacity. Initially check and test the
specific cooling rate for the fan-and-bin system.
Use a 24-h timer or a 1-2-h percentage timer to
control the operation of the cooling fan. These
controllers allow operators to adjust the
operating time of the fan to any percentage of
the total period.

For dryeration, force the air upward through
the bin so that it is exhausted at the top. In
most cases, the cooling fan operates while the
heated-air dryer continues to deliver hot grain
to the bin. With upward airflow, the heat and
moisture removed from the last grain delivered
in the upper portion of the bin do not affect the
grain in the lower part of the bin. Upward
airflow causes heavy condensation on the roof
and upper portions of the bin walls,
particularly in cold weather. Nonetheless,
when the cooled grain passes from the bin, the
small quantity of wet grain around the walls
blends with the remaining grain in the bin so
that no storage problems result.

Design drying systems for easy management.
In particular, the system requires two essential
maintenance checks. First, operators must
monitor the moisture content of the grain
before and after cooling. Second, they must
regularly measure the temperatures of hot
grain discharged from the dryer and of cooled
grain in the bins.

Provide a probe thermometer to monitor the
cooling progress in the bin. Use cables to
monitor temperatures in bins of 500 m3 or
larger. An indoor-outdoor thermometer with
one bulb located in the top air exhaust helps
follow temperatures as cooling progresses.
Remember, though, a low reading at the top of
the bin does not necessarily mean the bin is
cool; the grain in the top centre of the bin
cools last. Check the grain in the centre of
the bin as well with a probe thermometer. Do
not stop the fan if the bin contains any warm
grain.

Exercise caution when inspecting bins during
cooling of hot grain. The hot, moist air coming
off the grain can be hazardous to breathe. Do
not climb into the bin under these conditions.
Be careful even when looking through the door
into the bin because metal surfaces may be
slippery and protective eyeglasses may fog up,
creating a dual hazard.

3.25 In-bin cooling and drying

In this system, heated-air drying in one facility
precedes in-storage cooling and natural-air or
low-temperature drying in another. The
heated-air drying phase of the two-stage
system removes less moisture compared with
single-stage drying; however, in-bin cooling
and drying requires less propane or natural
gas. Total savings depend on the moisture
content level at which the grain discharges
from the heated-air dryer. On the other hand,
electrical energy requirements for the two-
stage system increase as a result of fan
operation. But overall, in-bin cooling and
drying systems require substantially less
energy than systems using heated-air dryers
for both drying and cooling functions.

The drying capacity of the heated-air dryer
operating in an in-bin cooling and drying
system is significantly increased. Less
moisture is removed in the dryer, as compared
with other drying systems. In a continuous-
flow dryer the cooling section of the dryer can
be equipped with a propane burner to provide
even more drying capacity. In-bin cooling and
drying systems can realize capacity increases
up to 300% for heated-air dryers.

3.26 Managing in-bin cooling and drying
systems Do not exceed suggested moisture
content levels for discharge from the heated-
air dryer to the bin for cooling and drying.
During the first year of operation, reduce the
moisture content to 20% or less in the heated-
air dryer until operators gain experience,
especially at measuring the moisture content of
the warm, moist grain.

The moisture content of warm grain is difficult
to measure accurately. Take grain samples
from the cooled grain to establish the control
parameters for the bin. With the airflow rates
used in natural-air drying, grain normally
cools 1 or 2 h after it reaches the bin.

Start in-storage drying fans as soon as hot
grain enters. This action prevents undesirable
condensation build-up on the walls of the bin.
During in-bin cooling, the hot, moist air
discharged from the grain condenses on the
bin's roof and eaves. Condensation becomes a
problem if water collects, runs along roof
supports, and deposits in one place.

HANDLING AGRICULTURAL MATERIALS

29

The downspout to the bin may be a potential
source of problems because moist air rising in it
cools and condenses, and water returns into the
bin. Close the entrance to the downspout at the
bin to prevent air from traveling up the spout.
Spring-loaded covers that automatically close
when grain stops flowing are available but
they require regular service. Ventilating the
space above the grain with a supplementary
roof-mounted exhaust also controls condensa-
tion but this solution requires additional air
inlets on the underside of the bin roof.

Besides these few special management
procedures, in-bin cooling and drying systems
require essentially the same practices as
natural-air drying and low-temperature drying
systems, including setting airflow rates,
operating fans, and monitoring grain
conditions (see section 3.24).

3.27 Fan selection and system design

Proper component selection and design, proper
installation, and good management assure the
satisfactory performance of a system for cooling
and drying grain. Before designing the system,
know the crop to be stored and the storage bin
dimensions. These data, along with additional
information on specific applications, permit
sizing or selection of fans, ducts, bin vents, and
transitions. It also helps determine the
necessary sizes of perforated floor areas.
References cited at the end of this manual
contain application information.

In particular, establish values for these design
criteria:

• airflow rate

• fan capacity

• static pressure

Set the airflow rate according to the crop and
the bin volume. Use the values listed in
Tables 3-8.

Tables 3-8 illustrate the level of risk taken in
selecting an airflow rate for a particular set of
conditions. For a given crop and moisture
content, the rate of spoilage depends entirely
on climatic conditions.

For example, Table 6 reflects the actual
climatic conditions recorded over 12 years,
from 1962 to 1973. Each year differed from the
others in the rate at which corn dried in
natural air, and the rate at which corn spoiled
in ambient conditions. The designation "2nd
worst year" indicates that grain dried before it
spoiled in all but 1 of the 12 years with an
airflow rate as shown in the table. The
designation "worst year" identifies the airflow

rate required to prevent spoilage in all
12 years.

Tables 3-8 also list data for determining
airflow requirements.

The static pressure against which the fan must
operate, and hence the pressure drop (Ap)
across the grain, depends on the resistance to
airflow, the grain depth (Hg), the effects of
using a grain spreader (Ks), and the effects of
the head loss caused by a perforated area less
than the total floor area of the storage (Kp).

The velocity of the air traveling through the
grain and the grain type affect the resistance to
airflow. The velocity is simply the total airflow
rate (measured in litres per second) divided by
the cross-sectional area of the bin (measured in
metres). Fig. 14 lists the pressure drop (Ap) for
several grains. The data in this figure refer to
airflow resistance for Argentine canola, a large
seed. The resistance to airflow for smaller
varieties of canola is approximately 60% of that
predicted for Argentine canola (Jayas and
Sokhansanj 1985).

The effects of a grain spreader in increasing the
resistance can be expressed as a constant (Ks)
for each grain type.

Grain

Ks

Barley

Corn

Flax

Canola

Sunflowers

Wheat

1.5
2.0
1.0
2.2
1.5
1.3

Sources: Friesen and Huminicki (1986) and Jayas and
Sokhansanj (1985).

A partially perforated floor also increases the
resistance to airflow by a constant amount (Kp)
for each type of grain.

% of floor Kp for wheat, barley, sunflowers,
perforated flax, corn, and canola

100

1.0

40

1.1

25

1.3

15

1.5

The total static pressure of the system (ps) is
represented by this equation:

ps = Ap X Hg X Ks X Kp

30

STORAGE AND CONDITIONING

Size the ducts and transitions to conform with
the following standard principles defined by
the American Society of Heating, Refrigeration
and Air-Conditioning Engineers (ASHRAE).

• The duct velocity should not exceed 7.5 m/s
(7500 (L/s)/m2) to minimize head loss and
noise.

• Transitions from the fan discharge to the
supply duct should not create abrupt
changes in dimension. An expansion of
15-20° is preferred. As well, do not create
restrictions in the cross-sectional area of the
ducts.

Calculate the required duct area in square
metres. Divide the total airflow rate (litres per
second) by 7500 (L/s)/m2.

Size the perforated area required in relation to
the total airflow. The desired face velocity (the
airflow per unit area of grain surface in the
bin) depends on the percentage of perforated
flooring and on the type of grain. The
maximum recommended velocity can be
generalized this way:

• cereals 200 mm/s

• canola

75 mm/s

Note that the units mm/s are equivalent to and
used in place of (L/s)/m2 in most applications.

Size the bin vents to yield an outlet velocity of
less than 5000 (L/s)/m2. This value minimizes
flow restriction. For louvered vents, use the
area of the actual discharge opening
(unrestricted area) to calculate the outlet
velocity.

Positive ventilation of the airspace above the
grain, if provided, should exceed the airflow

through the grain. Provide access for the
excess airflow to prevent roof collapse and size
the ducts for an outlet velocity less than
5000 (L/s)/m2.

3.28 Sample problems

3.29 Aeration system parameters Calculate the
system parameters for a load of wheat to be
aerated in a bin with a diameter of 7.3 m and
an eave height of 6.7 m. Use a grain spreader
to load the bin. Choose the maximum common
airflow rate for aeration from Table 11 and
refer to Fig. 14.

Level depth of grain (Hg)

= 6.7 m

Airflow requirement

= common airflow rate X grain depth

= 2 (L/s)/m3 X 6.7 m

= 13.4(L/s)/m2

Airflow rate

= airflow requirement X grain surface area

= 13.4(L/s)/m2 X n X (7.3/4)m2

= 561 L/s

Minimum recommended perforated floor area
for an aeration system from Table 11 is 15%.
The airflow resistance factor (Kp) for a 15%
perforated floor is 1.5.

Static pressure required (ps)

= Ap X Hg X Kp X K s

= 54Pa/m X 6.7 m X 1.5 X 1.3

= 706 Pa

Table 11 Basic design recommendations for various drying and cooling methods

Process

Percentage
above dry

Common
airflow rates
(L/s-m3)

Minimum
perforated
floor area

(%)

Transfer
for final
storage

Aeration

0

1-2

15

no

Natural-air drying

3-6

10-30

100

no

Dryeration

2-3

5-10

40
100

yes
no

In-storage cooling

1

5-10

40

no

Note: Select the airflow rates and perforated floor areas based on local environmental conditions, initial moisture content, crop value,
and economic constraints.

Source: Friesen and Huminicki (1986).

HANDLING AGRICULTURAL MATERIALS

31

where Ap = pressure drop across the grain

= 54 Pa/m with airflow of
13.4 (L/s)/m2 based on the
data in Fig. 14

H g = bin depth (m)

Kp = airflow resistance factor for a
15% perforated floor

Ka = 1.3 because the design calls
for a spreader

Minimum duct area

= airflow rate X maximum duct velocity

= 561 L/s/ 7500 (L/s)/m2

= 0.075 m2

Minimum bin vent area

= airflow rate X maximum outlet velocity

= 561 L/s/ 5000 (L/s)/m2

= 0.11 m2

3.30 Barley cooled in storage Calculate the system
parameters for a load of barley to be cooled in
storage in a bin with a diameter of 5.8 m and an
eave height of 4.7 m. From Table 11, choose
8 (L/s)/m3 as the common airflow rate for
in-storage cooling.

Level depth of grain (Hg)
= 4.7m

Airflow requirement

= common airflow rate X grain depth

= 8 (L/s)/m3 X 4.7 m

= 37.6 (L/s)/m2

Airflow rate

= airflow requirement X grain surface area

= 37.6 (L/s)/m2 X n X (5.8/4) m2

= 993 L/s

Minimum recommended perforated floor area
for an in-bin cooling system is 40% (Table 11).
The airflow resistance factor (Kp) for a 40%
perforated floor is 1 . 1 .

Static pressure required (ps)

= Ap X Hg X Kp X Ks

= 146 Pa/m X 4.7 m X 1.1 X 1.0

= 755 Pa

where A/> = pressure drop across the grain

= 146 Pa/m for barley with
airflow of 37.6 (L/s)/m2 based
on the data in Fig. 14

200

100
90
80
70

60
50

W 40

O
<

30-

20

10

-

1 | i i l I i i M

t 1 1 1 I 1 i in

?

'

/

/

/

/ ,

/ /

/

/

/

/

/

/ /

' //

/

/

- <Pa

/

/

/

0$>

$

<t

0

V

f

<$

'■'■

yS
jf

7

1 IIIM in'

1 1 1 II II II

--

10

20 30 40 50 60 70 8Q90]0Q

200 300 400 600 800 1000

Pressure drop, Pa/m

Fig. 14. Resistance of grains and oilseeds to airflow.

32

STORAGE AND CONDITIONING

Hg = bin depth (m)

Kp = head loss because the per-
forated area is less than the
total floor area

Ks = 1.0 because a spreader is not
used

Minimum duct area

= 993 L/s/ 7500 (L/s)/m2

= 0.13 m2

Minimum bin vent area

= 993 L/s/ 5000 (L/s)/m2

= 0.20 m2

3.31 Natural-air drying of wheat Calculate the
system parameters for a load of wheat to be
natural-air dried from an initial moisture
content of 18% in a bin with a diameter of 6.7 m
and an eave height of 6.1 m. The wheat was
harvested 15 September in Swift Current,
Sask.

Table 5 indicates a recommended airflow rate
of 4.4 (L/s)/m3 for wheat with a moisture
content of 18%. However, Table 11 suggests a
common airflow rate of 10-30 (L/s)/m3 for
natural-air drying. Base the system design
decisions on an anticipated range of grain
moisture contents, for example 16-20%. So
choose a system airflow rate of 13 (L/s)/m3.

Level depth of grain (Hg)

= 6.1m

Airflow requirement

= common airflow rate X grain depth

= 13.0(L/s)/m3 X 6.1m

= 79.3 (L/s)/m2

Airflow rate

= airflow requirement X grain surface area

= 79.3(L/s)/m2x 35.3 m2

= 2796 L/s

Minimum recommended perforated floor area
for natural-air drying is 100% (Table 11). The
airflow resistance factor (Kp) for a 100%
perforated floor is 1.0.

Static pressure required (ps)

= Ap X H g X Kp X Ks

= 420 Pa/m X 6.1 m X 1.0 X 1.0

= 2562 Pa
where Ap

= pressure drop across the grain

= 420 Pa/m for wheat with
airflow of 79.3 (L/s)/m3 based
on the data in Fig. 14

Hg = bin depth (m)

Kp = head loss because the
perforated area is less than
the total floor area

Ks = 1.0 because a spreader is not
used

Minimum duct area

= airflow rate X maximum duct velocity

= 2796 L/s/ 7500 (L/s)/m2

= 0.37 m2

Minimum bin vent area

= airflow rate X maximum outlet velocity

= 27965 L/s / 5000 (L/s)/m2

= 0.56 m2

3.32 Corn cooled by dryeration Calculate the
system parameters for a load of corn cooled
by dryeration in a bin with a diameter of
7.3 m and an eave height of 6.5 m. Use a
grain spreader in the system and transfer
the corn to final storage following cooling.
Choose a common airflow rate of 10 (L/s)/m3
(Table 11).

Level depth of grain (Hg)

= 6.5 m

Airflow requirement

= common airflow rate X grain depth

= 10 (L/s)/m3 X 6.5 m

= 65 (L/s)/m2

Airflow rate

= airflow requirement X grain surface area

= 65(L/s)/m2 X 41.9 m2

= 2725 L/s

Minimum perforated floor area is 40% for
dryeration when the grain is transferred to
final storage for cooling. The airflow resistance
factor (Kp) for a 40% perforated floor is 1 . 1 .

Static pressure required (ps)

= Ap X H g X Kp X Ks

= 157 Pa/m X 6.5 m X 1.1 X 2.0

= 2245 Pa

where Ap = pressure drop across the grain

= 157 Pa/m for corn with airflow
of 65 (L/s)/m3 based on the
data in Fig. 14

Hg = bin depth (m)

Kp — head loss because the
perforated area is less than
the total floor area

HANDLING AGRICULTURAL MATERIALS

33

Ks = 2.0 because the design calls
for a spreader

Minimum duct area

= airflow rate X maximum duct velocity

= 2725 L/s/ 7500 (L/s)/m2

= 0.36 m2

Minimum bin vent area

= airflow rate X maximum outlet velocity

= 2725 L/s / 5000 (L/s)/m2

= 0.55 m2

SYSTEM SELECTION CRITERIA

Four interdependent units make up grain
storage and conditioning systems:

• storage

• drying

• interface of drying and storage

• materials handling

Several value judgments go into the decisions
involved in selecting system components. The
information presented in the first three parts of
this manual should help in understanding how
those decisions are made. Use this section of
the manual to aid in selecting the actual
components for a complete grain storage and
conditioning system.

4.1 Storage

First establish volume requirements. Indus-
trial facilities normally gauge the storage
volume required according to their strategic
requirements. As a minimum they provide
storage sufficient to maintain stock that
permits uninterrupted plant operation during
inclement weather or holidays. Storage
capacity beyond this permits the industry to
take advantage of cyclical prices or grain
availability.

Commercial facilities generally base their
storage requirements on the anticipated
annual throughput divided by the number of
turnovers of stock required to render the
facility economically viable. Turnovers
usually occur 7.5-10 times per year on new
construction of inland facilities. The gross
volume is divided into the number of bins
required to service the crops and grades
produced in the area. The number of crops and
grades generally vary from 15 to 35. Product-

ivity of the area and reasonable transportation
distances by truck dictate the anticipated
annual throughput. Capacity typically ranges
from 3500 to 5000 t for inland facilities and
from 25 000 to 75 000 t for terminals.

The productivity of farms and the marketing
inclinations of the operators determine farm
storage requirements.

If the grain is marketed through the quota
system or for feeding livestock, the system
must include storage for nearly the entire crop.
Cash crops or crops grown on contract do not
require storage because they are normally
taken directly from field to market.

The crop type and the volume requirements
together set the number of bins required. The
crop type largely dictates the bin depth because
this parameter affects product degradation
during filling. The need to force air through
the bin during storage also constrains bin
depth. Limitations in bin depth then dictate
the required total cross-sectional area of bins.
Finally, the practical horizontal dimension of
the bin along with the number of crop varieties
and grades normally grown in the area
establishes the minimum number of bins.

The smallest number of bins that satisfies the
above criteria represents the most economical
storage possible for a given situation.

Storage facilities are normally constructed of
wood, concrete, or steel. The natural
insulating properties of wood provide the
advantage of reduced moisture and related
problems in storage. Wooden granaries are,
however, relatively expensive for small
volumes and are susceptible to rodents and fire.

For multibin storage facilities larger than
3500 t the decision is whether to use steel or
concrete bins. Smaller concrete bins require
repetitive form work thus increasing initial
construction costs. Concrete's natural thermal
buffering reduces the need for aeration in
storage. Nevertheless, aeration is required
where crops are stored for long periods. Include
temperature-monitoring equipment to detect
the onset of spoilage. However, the advantages
of concrete, particularly for deep bins,
disappear when easily damaged crops are to be
stored.

Farm operations most commonly rely on steel
storage bins because of several characteristics
of steel: low capital cost, quick erection, and
wide range of sizes. Steel bins lack the thermal
buffering and insulating traits of concrete and
wood but lend themselves to modular
development and system expansion.

34

STORAGE AND CONDITIONING

Steel storage bins are normally cylindrical for
maximum structural efficiency in hoop tension.
Square bins are only cost effective where space
is limited, for example, for working bins that
service a secondary process operation. Round
bins over 4 m in diameter are corrugated to
provide sectional stability with a minimum of
steel.

For deep bins, the thickness of the wall
increases from the top to the bottom of the bin
to withstand the increasing load. Shallow bins,
however, rely totally on the sectional stability
of the wall material to resist crushing. As the
bin depth increases, the vertical load exerted
by material friction on the walls also increases
substantially. Because of these large vertical
forces, use stiffeners or mini-columns bolted to
the bin wall to increase the vertical load
capacity. The structural integrity of the bin
also requires adequate fastening of the wall
panels to these columns plus support at the
bottom of the columns. Follow the supplier's
instructions for fastening and supporting the
bin elements.

Although stiffeners significantly increase the
maximum depth of storage bin possible from
light-gauge steel, they add significantly to the
cost. Yet they offer a very cost-effective means
of increasing the long-term reliability of the
storage, thus reducing the total cost of a
storage system. They limit bin shrinkage, or
compression of the corrugated panels over
time. Unstiffened bins commonly suffer from
these problems. Stiffeners also reduce the risk
of nonuniform panel compression, which
results from slightly off-centre loading or
unloading of the bin.

A significantly reduced cost for the materials-
handling equipment particularly illustrates
the savings attributable to use of bin stiffeners.
Because stiffeners allow construction of higher
bins, fewer bins require filling. Likewise, the
system requires fewer conveyors and discharge
equipment. The savings are especially evident
if portable augers are not used to load and
unload the storage bins.

Despite the advantages of deep bins, weigh
their use against the increased resistance to
airflow that the deeper bins produce.

Of critical importance, the bin wall-to-founda-
tion connection affects both the performance of
the bin as a structure and its ability to main-
tain grain quality. To preserve the structure
and to accept the uniform loads transmitted by
the bin wall, this connection must be level and
uniform. Lack of bearing uniformity at this
connection concentrates the vertical loads in
the bin wall and predisposes that area to
crushing. At the extreme, a poorly designed

wall connection causes bin failure. A rigid,
monolithic foundation maximizes bin
longevity.

Fluctuations in temperature and load from
stored grain cause the connection between the
foundation and the bin wall to shift constantly.
Maintain the seal annually to prevent
moisture from migrating into the bottom of the
bin. This seal permits some differential
movement and prevents escape of air supplied
under a fully perforated floor. The pressure of
the air may open the seal or identify a fault in
the bin-to-foundation connection. Inspect the
seal annually prior to filling the bin and again
after the air supply system is started up in bins
with fully perforated floors.

Place the interior floor above the bin-to-
foundation connection to reduce migration of
moisture into the bin. Sloping the foundation
exterior to the bin away from the bin wall also
assists in moisture control.

Various arrangements of ducts and perforated
floors are available as well. Some of the most
common include a single perforated duct across
the bin floor, a cross-shaped duct, a V-shaped
duct arrangement, and a rectangular
perforated area in the centre of the bin. Take
care to provide sufficient cross-sectional area of
duct around the central unloading bin well.

Ducts also provide a conduit for the unloading
screw. Seal around the conveyor at the point of
egress from the bin to prevent undesirable
leakage of air. Refer to section 3.26 for
information on duct sizing. Ensure at least
15% of the bin floor area is perforated in bins
up to 6 m high.

Flat grain-storage bins are more difficult than
conventional round bins to aerate effectively.
The shallow, flat bins allow grain to collect in
piles of various depths. Increased airflow
relieves some of the problems created by poor
air distribution in the shallow bins, so use
airflow rates of 2-3 (L/s) per cubic metre of bin
space. Duct arrangement also enhances
successful aeration of flat storage bins.

Fig. 15 shows some typical duct arrangements
for flat bins. Arrangement A permits only a
portion of the building to be used for storage or
aeration during a protracted unloading period.
Uncovered air ducts are blocked off.
Arrangement B requires that the single air
duct be covered from the beginning to the end
of the aeration process.

Duct arrangements perform well if they are
correctly sized and the ducts are properly
spaced. Keep the longest air path from the duct
to the grain surface no more than 1.5 times the
shortest air path. Fig. 16 shows the number

HANDLING AGRICULTURAL MATERIALS

35

4.2

B

Fig. 15.
bins.

Aeration duct arrangements for flat storage

Two ducts
required

More than
four ducts
required

Four ducts
required

6 8 10 12 14 16 18 20
Total building width, m

For

duct

spacing

For
*-1

1 duct For 2 ducts

= (0.500)W M = (0.377)W

For 3 ducts
I--} = (0.168)tV

For 4 ducts
/_! = (0.145)W
L2 = (0.075)W

Fig. 16. Lengthwise duct spacing for rectangular
buildings.

and spacing of the ducts. For level-filled flat
storage bins, the duct spacing equals the grain
depth.

Equip all storage systems with at least two
bins having totally perforated floors. Use them
for any of the purposes discussed previously in
this manual, or simply for holding problem lots

of grain until remedial measures can be taken,
for example, for holding wet grain until a high-
temperature dryer is available. The flexibility
and safety provided by moving 10-30 (L/s) of
ambient air through each cubic meter of grain
in a perforated bin represents the cheapest
means of avoiding losses in grain quality or
quantity. Use these same storage units, as
well, for natural-air or low-temperature dry-
ing, or to increase dryer capacity through
combination drying.

Storage bin size When selecting the bin size
for a particular operation, keep these factors in
mind:

• the crops normally grown in the area

• the average size of field seeded with a single
crop and the yield normally expected for
each crop

• resistance to airflow demonstrated by the
stored grains

From a materials-handling standpoint, settle
on a single bin size for a particular operation.
Incorporating two or three bin sizes in a single
storage system simply adds complexity.

Consider the susceptibility to damage of the
crops grown in an area. Dicotyledonous crops,
such as peas or beans, cannot tolerate long
drops into deep storage bins. Long drops
damage corn more than wheat. For fragile
crops, install shallow bins with large
diameters; or use pea ladders to reduce the
negative effect of long drops into deep bins.

The average size of a seeded field multiplied by
the average yield of the crop with the smallest
harvest offers an economic compromise by
which to select bin size.

For example, consider wheat growing in fields
averaging 65 ha. The fields provide an average
yieldof2.5t/ha.

Bin size

= 65 ha X 2.5 t/ha X 1.3 m3/t

= 211 m3

Select a bin size of 200 m3.

If the 65-ha fields contained flax instead of
wheat, the harvested crop would only fill half
the 200-m3 bin. And if the crop were corn, two
200-m3 bins would be needed.

Another farm raising soybeans on fields
averaging 20 ha might require a bin of 100 m3.
Two bins would be needed to store corn
harvested from the same field. Alternately two
bin sizes of 100 and 200 m3 could handle both
crops.

The airflow resistance of stored grains dictates
the diameter of the bin. The fan performance

36

STORAGE AND CONDITIONING

characteristics of commercial or light
industrial fans limit the usable bin depth.

As an aid in determining bin sizes, estimate
the air pressure and flow required to aerate the
grain (see section 3.26 and Tables 3-8) and 4,3
understand the characteristics of fans
commercially available.

For example, a wheat farm with fields of 65 ha
yielding 2.5 t/ha requires 200 m3 storage bins.
If the owner wishes to accommodate natural-
air drying of canola, bins of what diameter
should be selected?

From Table 11, the required airflow rate is
20 (L/s) per cubic metre of canola.

Fan delivery

= 20 (L/s)/m3 X 200 m3

= 4000 L/s

For this airflow rate a centrifugal fan
operating at an average 3450 r/min would
produce approximately 1000 Pa of static
pressure. Based on Fig. 14, a grain depth of
2.3 m would yield this airflow:

Airflow

= 20 (L/s)/m3 X 2.3 m

= 46 (L/s)/m2

A pressure drop of 420 Pa/m results (Fig. 14).

Actual pressure loss

= 420 Pa/m X 2.3 m

= 966 Pa

From these calculations, a bin 2.3 m deep offers
the most practical option for drying canola
using the natural-air method.

Perform similar calculations to determine
maximum practical bin depths for any other
operating scenario. For example, a grain depth
of 6 m may be acceptable if the grain was
aerated instead of natural-air dried.
Alternatively, based on the requirement for a
storage volume of 200 m3 in a bin with an eave
height of 2.3 m, select a bin 1 1 m in diameter to
dry canola with natural air or a bin 6.8 m in
diameter to aerate it.

In areas of ambient relative humidity less than
60% and where minimal oilseed crops are
grown, the crops can usually be harvested dry.
Farmers in these areas generally designate a
couple of storage bins for natural-air drying.
Aeration systems probably equip the other
storage bins.

In areas where relative humidity commonly
exceeds 75%, natural-air drying systems offer
little value because of their drying inefficiency,
compared with heated-air drying systems
(Mittal and Otten 1982). Size the storage
systems, then, for the harvest volumes

required and equip the storage bins with
aeration devices to cool the grain and maintain
uniform temperatures.

Drying systems

Two factors figure prominently in the design of
grain-drying systems:

• the reasons for drying

• the operational priorities

Reasons for drying grain relate to the crops
grown, and the geographic and climatic
conditions.

Access to drying facilities is especially
important if an operator grows crops that must
be harvested wet because of biological factors.
For example, farmers normally harvest corn
wet and dry it for sale. The fragility of corn in
storage and the transportation costs of wet corn
make it unsuitable for sale without drying.
Sunflowers, too, are normally dried after
harvest, before sale. In contrast, corn destined
for use as feed can be stored as a high-moisture
grain.

For some crops, drying is discretionary; but
economics justify it. In many cases, shortage
of labor means farmers must harvest
continuously. In such cases the dryer becomes
a normal part of the harvesting operation.
Crops such as canola, peas, and beans, if left to
dry in the field, shell during or before
combining. A loss of yield results. Shelling
losses, particularly with canola, can exceed the
cost of early harvest and drying.

Straight combining of many crops is standard
practice in areas where climatic conditions do
not permit drying in a swath. With straight
combining many farmers reduce both the
number of operations required in the field and
the risk of grade and dry-matter loss in the
swath.

Abnormal climatic conditions can force the
drying of significant percentages of crops to
make them suitable for storage or sale.

The type of drying system appropriate for an
individual operation depends on the level of
need and the frequency of use. Some situations
justify specialty drying equipment, for
example, when high volumes of only one or two
crops are harvested and dryers are used
annually.

Alternatively, operations drying a wide variety
of crops with an even wider variety of moisture
contents and grain quality conditions demand
drying flexibility. Commercial facilities
offering custom drying services fall into this
category. They contend with a wide variety of
crops in various quantities and conditions.

HANDLING AGRICULTURAL MATERIALS

37

Other operations call for simplicity and low
capital cost. The dryer may be required rarely,
or a high-temperature dryer would only
supplement the mainstay of natural-air drying
in storage. Many commercial facilities
maintain equipment to dry grain that they
have purchased. This grain has been tough,
damp, or degraded in storage. Drying facilities
receiving such materials require simple
operations. They normally handle high
volumes of relatively dry grain.

Fuel efficiency sits high on the priority list of
many operators. Energy conservation and rec-
overy systems, and alternate fuel systems,
have yet to demonstrate cost effectiveness. The
capital costs are too high for most drying
facilities that do not operate for long periods,
that is, for more than one crop or commercially.
Industrial applications can recover some
return on investment for heat recovery systems
where the system operates longer than in
normal farm use. Systems offering more
predictable and significant fuel savings include
natural-air drying, especially in areas of low
humidity, and combination drying. As well,
recycling the cooling air from a continuous-
flow dryer can also yield fuel savings of
10-15%.

In selecting a dryer, consider these five factors:

• the dryer's ability to dry grain without
quality loss

• fuel efficiency

• the dryer's ability to accommodate the full
range of crop varieties and moisture
contents expected

• the rate at which the dryer removes
moisture

• the rate at which grain can move through
the dryer

Set the moisture removal rate for the dryer
according to the grain supply expected in a
24-h period. Daily drying capacity should
equal the amount of water contained in the
wettest quantity of grain that the operator is
likely to harvest during that time.

Calculate the quantity of moisture to be
removed this way:

(1 - Mi)
mi X

where q

mi =

Mi =
Mi =

— mi
(1 -Mi)

quantity of water removed
(kg)

mass of grain at final
moisture content (kg)

final moisture content

initial moisture content

For example, an operator harvests corn at a
dry-corn rate of 10 t/h, for 8 h/day. Initially the
harvested corn has a moisture content of 30%.
Calculate the moisture removal rate if the corn
is dried to 14.4% moisture content.

Q =

1000 kg X

(1 - 0.144)

- 1000 kg

(1 - 0.30)

= 223kg/t

This calculation indicates 223 kg of water is
removed from 1 t of corn with an initial
moisture content of 30% and a final moisture
content of 14.4%.

The rate of moisture removal

(8 h/day)

= (223 kg/t) X (10 t/h) X

24 h/day

= 740 kg of water per hour

Manufacturers often specify dryer capacities in
tonnes per hour of "dry-hot" and "dry-cool"
grain at 10% moisture removal. Yellow dent
corn is the most common crop dried in
commercial dryers. Consequently, manu-
facturers of drying systems generally publish
comparative performance data for removal of
10% of the moisture from this crop.

Drying rates vary with grain moisture content,
dryer operating temperature, and grain type.
Dryer capacity also varies with these factors.
However, three additional important issues
enter into the design decisions surrounding
dryer selection.

First, grain moisture content is designated on a
wet basis rather than a dry basis. Therefore,
the quantity of water contained in a tonne of
grain increases with the grain moisture
content; that is, more water is removed in
drying grain to a moisture content of 25% from
30% than in drying the same grain to 15%
moisture from 20%. So design the drying
system to handle the maximum expected rate
of moisture removal.

Second, evaporating the last kilogram of
moisture from dry grain requires more energy
and time than evaporating the first kilogram.
Ensure the drying system can dry the grain to
a moisture content of 14% from an initial 24%.

Finally, two factors significantly influence
dryer capacity:

• the ability to move air through the drying
crop

• the rate at which each crop releases
moisture

Do not select a dryer on the basis of its rated
capacity for a single crop. Consider, also, the
other possible uses for the dryer.

38

STORAGE AND CONDITIONING

An operator drying corn at a rate that removes
223 kg of moisture per tonne wants to know the
average moisture removal rate for the system.

The average amount of moisture removed per
percentage point

= 223kg/t/(30 - 14.4)%/t

= 14.3 kg/%

The average moisture removed over 10
percentage points

= 14.3 kg X 10

= 143 kg/t

Performance data for removing 10% of the
moisture from crops is commonly stated.

The dryer capacity rating required for 10%
moisture removal equals the total moisture to
be removed per hour divided by the moisture
removal rate per tonne across 10 percentage
points

= 740 kg/h / 143 kg/t

= 5.2 t/h of dry grain

Note, however, this dryer capacity rating
varies significantly for different crops.

As mentioned earlier, dryer capacity
represents the average capacity of the dryer.
For continuous-flow dryers, capacity is a
steady-state condition. For batch dryers,
however, capacity also reflects the time in
which no drying is taking place, that is, during
loading, cooling, and emptying.

4.4 Dryer loading and unloading systems

Besides the dryer, a drying system also consists
of:

• a surge bin for loading wet grain

• an unloading system that discharges the dry
grain into storage

Do not overlook either component in designing
a total drying system.

The surge bin holds the wet grain until the
dryer can receive it. In some cases the truck
box serves this function but it makes a
relatively expensive holding bin. Instead, use
a hopper-bottom bin for temporary holding.
Flat-bottom bins, equipped with airflow
sufficient to delay spoilage, also offer
alternative storage capacity.

Size the surge bin in relation to the dryer
capacity and the harvesting rate. In the
example involving corn harvested at 10 t/h and
dried from 30% moisture to 14.4%, the capacity
required is the harvesting rate minus the
actual drying rate. Determine the actual
drying rate by multiplying the average rate of

moisture removal by the dryer capacity and
dividing the result by the actual moisture
removal rate per tonne.

Drying rate

= 143 kg/t X 5.2 t/h / 223 kg/t

= 3.3 t/h

to dry the crop from a moisture content of 30%
to 14.4%.

The surge bin capacity required

= (10 - 3.3) t/h X 8h

= 53.6t

For batch dryers, locate a surge bin directly
above the dryer. This configuration virtually
eliminates loading time and thereby increases
dryer capacity. It also permits topping up the
dryer during drying without having to start a
fully loaded conveyor.

Loading a continuous-flow dryer promptly
initiates the drying operation quickly. It is
also desirable to be able to operate a
continuous-flow dryer in a recirculating batch
mode during start-up, until the grain
approaches dry. Then switch the system to
continuous mode.

Augers or bucket elevators, controlled by level
sensors in the top of the dryer, generally supply
wet grain to a continuous-flow dryer. The
sensors control either the auger filling the
dryer or the slide gate (or conveyor)
discharging grain from the wet surge bin into a
bucket elevator filling the dryer.

Batch dryers are normally designed with high
unloading capacity to maximize their drying
capacity. To accommodate this feature, the
drying system requires high-capacity con-
veyors to transfer the grain into cooling bins or
storage. Alternatively, mount batch dryers
above dry surge bins to use gravity for fast
dumping of the dried grain. This arrangement
increases dryer capacity and reduces the
requirements for conveyor capacity.

Some continuous-flow dryers suffer from
inadequate discharge capacity when drying
grain that requires minimal moisture removal.
The dryer unloads slower than the grain dries.
Check the dryer discharge rate prior to
purchase, especially for applications that
require the dryer be used for a wide range of
activities.

Many operators select the unloading and
transfer conveyors for the dryer according to
their primary requirements. If, for example,
the system has primarily been assembled to
dry corn with an initial moisture content of
25-35%, then a continuous-flow dryer can
discharge the grain relatively slowly.

HANDLING AGRICULTURAL MATERIALS

39

Small conveyors or a pneumatic system offer
the least expensive methods for transferring
grain to several storage bins. However, these
options may lack the capacity to accommodate
other drying operations.

Use a surge bin for dry grain between the main
dryer-unloading conveyor and the distribution
conveyors to minimize the need for auxiliary
transfer equipment to unload the batch dryers.
As well, use a surge bin for dry grain with
continuous-flow dryers that rely on a common
bucket elevator to receive both wet and dry
grain.

Base the size of the surge bin for dry grain on
one of two criteria.

• The volume of the surge bin must at least
equal the volume of the batch dryer.

• The bin capacity must handle the amount of
grain that can be dried by the continuous-
flow dryer during unloading an incoming
truck.

4.5 Electrical supply

The availability of energy to support storage
and drying facilities is frequently a major
design constraint. The operation of fans and
conveyors can rarely be sequenced to reduce
the demand load. As well, annual peak energy
use often lasts only a couple of weeks or
months.

Three-phase power offers a significant cost
reduction and an increase in the starting
torque capability of motors without high
current demands. Servicing a site with three-
phase power, however, may be costly,
particularly when the utility's monthly
demand charges must be paid. Consider using
a standby generator to supplement or operate
the facility during periods of peak loading.
Three other means of reducing demand include
power factor control, peak shaving, and load
shedding. Whatever the choice, though,
eliminating the demand charges remains the
most effective cost-saving measure.

As the relative humidity levels increase to near
75%, the airflow required to prevent spoilage
while drying the grain increases dramatically.
Fans must operate for extended periods, to the
point where the total electrical energy required
to dry the grain with natural air exceeds the 4 7
energy required to dry it in a high-temperature
dryer. Nonetheless, the offsetting advantage
for natural-air drying remains the lack of
capital costs.

Frequently, the choice of drying system pivots
on the cost of electrical energy. Climatic

conditions, crop varieties, and crop yield
strongly affect dryer usage and, therefore,
directly influence energy consumption.
Consequently, those three factors often figure
in rationalizing the approach to grain drying.

4.6 Materials-handling systems

For systems relying primarily on high-
temperature dryers (and not simply using them
as backup to contend with abnormal climatic
conditions), incorporate the dryers as a part of
the storage system and in convenient locations
where they can be easily used. If the drying
system is not easy to use, chances are it will be
underused. An underutilized drying system
fails to generate the expected income benefits
despite the operator incurring significant
capital cost and depreciation on the equipment.

A materials-handling system offers several
benefits:

• reduction in the labor required for harvest

• reduction in quantity losses because of
shelling and wildlife

• reduction in dry matter loss

• reduction in grade losses because of
inclement weather

• increase in the crop volume that can be
harvested by existing equipment

• possibility of earlier harvest, leaving more
time for necessary field operations after
harvest

However, these benefits are frequently
insufficient to defray the cost of a materials-
handling drying system. So operators often
additionally use the system to:

• enhance the timeliness of marketing by
improving accessibility to storage through
inclement weather and snow

• blend similar grains to improve the market
value of a larger percentage of the crop

• support secondary processing and feed
applications such as feed preparation, grain
cleaning, custom storage, or custom drying

The materials-handling equipment for a drying
system basically consists of storage loading and
storage unloading components.

Loading An inclined auger or a bucket
elevator serving a horizontal conveyor above
the storage bins makes up the loading system.
The horizontal conveyor permits single-point
discharge of grain into storage. Generally the
loading system receives grain directly from
trucks, surge bins, or dryers.

40

STORAGE AND CONDITIONING

4.8

If both trucks and continuous-flow dryers serve
a single conveyor, ensure it is either a chain
conveyor or a screw conveyor fitted with
hanger bearings. Partially loaded screw con-
veyors without hanger bearings wear rapidly
and may cause significant grain damage.

Low-capacity pneumatic and chain-drag
conveyors, using augers or bucket elevators,
can also transport grain from continuous-flow
dryers to storage bins. These conveyors offer a
less-costly alternative to augers and elevators;
however, because of their low capacity, their
loading flexibility is limited. This restriction
eliminates the ability to receive grain from a
truck at high capacity. As well, loading
systems built around low-capacity pneumatic
and chain-drag conveyors cannot dry grain
efficiently when only small amounts of
moisture are to be removed, since this
operation requires high flow rates.

High-capacity pneumatic and chain-drag
conveyors are available. However, they are not
cost-effective for farm use.

To clean out the horizontal conveyor, for
example, in seed growing operations, use a
U-trough chain-drag or pneumatic conveyor.
Excessive velocity or head loss (because of
trough elbows or length) in pneumatic systems
may reduce the germination of seed grain.
Velocity may also worsen the stress cracking of
the grain created by inappropriate dryer or
combine operation.

Provide U-trough chain drags with adequate
discharge opening areas to permit complete
emptying at the desired location.
Alternatively, use a low chain speed to
compensate for undersized discharge openings.
Unfortunately, this action simultaneously
reduces capacity. Equipment suppliers should
recommend appropriate discharge
configurations.

Unloading Unloading systems normally
consist of an under-floor or inclined auger fed
by gravity or a bin sweep. A secondary
transport conveyor may consist of either an
inclined auger feeding a truck or a horizontal
collection conveyor servicing several bins.
Hopper-bottomed bins that need no mechanical
equipment for unloading offer a storage
alternative, but they are expensive.

The elevation of the unloading conveyor
dictates the elevation of the secondary
transport conveyor. If the elevation of the
primary unloading conveyor discharge is
inadequate or the foundation settles,
excavation to install the transport conveyor
becomes essential. Remember, though,
excavations present drainage and maintenance

problems, so avoid them if possible. Instead,
elevate the foundation well above the adjacent
grade. This arrangement permits good surface
drainage away from the storage facilities.

The design of the discharge transition between
an overhead filling conveyor and the bin roof
must also accommodate settling of the
foundations. Anticipating differential move-
ment at this location saves costly remedial
repair work.

STORAGE AND CONDITIONING
OF HAY

Hay stores safely when the moisture content
reaches 15% or less. This moisture content
prevents mold growth in the hay if it is used
within 12 months.

In areas where weather conditions permit, dry
the crop in the field before baling. Simply
leave the cut hay exposed to the sun until it has
dried sufficiently to store safely. To speed the
field drying process, crimp or roll the hay at
cutting. These actions break the stems,
increase the release of moisture to the air,
reduce drying time by approximately 50%, and
significantly improve hay quality.

Both the rate and duration of field drying (also
called curing) greatly influence the quality and
quantity of hay harvested. Lengthy curing
periods increase dry matter losses and
diminish the nutritive value and palatability of
the hay. Rain or poor drying weather (such as
cloudy skies, high humidity, or still air) delays
drying.

In the case of legume hay such as alfalfa or
clover, the leaves contain most of the nutrient
value. However, as the hay dries, the leaves
tend to separate from the stem and are readily
lost during harvesting. Losses of 4-5% protein
content can occur. Retaining the feed value of
these crops requires either faster drying or
reliance on a harvesting method that ensures
leaves and stems remain intact.

Hay may be harvested at a moisture content
above 15% and dried in storage rather than
cured in the field. This approach reduces risks
to product quality and dry matter from losses
caused by inclement weather.

5.1 Drying hay in storage

To dry hay in storage, use an air delivery
system designed for uniform distribution of air.
Size the ducts to provide a face velocity of air

HANDLING AGRICULTURAL MATERIALS

41

moving outward from the duct into the hay at a
rate of 260 mm/s. Ensure a duct velocity of
5 m/s or less.

Duct layout normally centres on a primary
distribution duct running the length of the
storage. Lateral distribution ducting may

accompany the main duct. Use a single,
perforated central duct without lateral
passages for storage structures up to 10 m
wide. Add lateral ducting to the primary
supply duct for larger structures. Figs. 17 and
18 illustrate common duct cross sections.

Main duct at side of mow for shallow hay storage

Main duct at centre of mow for deep hay storage

19 x 140

38 x 89

25 mesh for
chopped hay;
Hog wire fencing ,)
for long hay

N ,19 x 140

V

brace

-MUAI

38 x 89 stud

UK

38 x 89 sole

r

JJ

Floor

Fig. 17. Unlined primary distribution ducts. All measurements are expressed in millimetres.

42 STORAGE AND CONDITIONING

5.2 Safe storage requirements: airflow and static
pressure Hay generally enters storage during
the warm summer weather when natural-air
drying is reliable in most areas. Properly
designed natural-air drying systems require no
supplementary heat under these conditions.

In some circumstances, however, the storage
system requires supplementary heat provided
through the use of a heat exchanger:

• when the initial moisture content of the hay
is too high for natural-air drying

• when normal climatic conditions do not
allow rapid moisture removal by natural air
circulation

Compared with heat exchangers, direct-fired
heaters offer a more efficient drying
alternative; but beware of the high fire risk
that accompanies their use.

The rate of hay spoilage increases with air
temperature. Supplementary heat sufficient to
reduce slightly the relative humidity of the
supply air is safer than high-temperature
drying, and the same airflow rate as natural-
air drying may be used.

In most cases, hay must be dried to a moisture
content of less than 20% within 3.5 days of
cutting to prevent visible mold growth.
Determine the quantity of water to be removed
from the hay using a method similar to that for
drying grain.

Recall this equation:

(1 - Mi)

q = mf X — m{

(1 - Mi)

where q = quantity of water removed (kg)

mf = mass of grain at final
moisture content (kg)

Mf = final moisture content

M[ = initial moisture content

See section 4.3 for more details.

The moisture-removal capacity of the air being
blown through the hay limits the drying rate
expected from a natural-air dryer. Other more
controllable factors influencing the drying rate
include airflow rate and uniformity of air
distribution. Decreasing the humidity of the
supply air increases the capability of the
ventilation air to remove moisture.

Select the airflow rate according to the
required rate of moisture removal. The
average temperature and relative humidity
levels in the area during hay drying, in
turn, influence the rate of moisture removal.
Use local climatic data and basic
psychrometrics to calculate the theoretical
airflow rate required. Assume the relative
humidity of the discharge air from the dryers
averages around 85% with constant enthalpy
through the dryer.

Base the airflow rate on the anticipated
moisture content of the hay to be dried
(Table 12). Increase the airflow rate with
increasing initial moisture content.

No consistent information exists that
correlates static pressure with airflow rate and
hay quality, in terms of either genetic or
physical configuration (legumes or grasses;
baled or loose). Set the static pressure for the
system in the range of 125-375 Pa; commonly
250 Pa is used.

L

®(D

1. 19x89 mm slats

spaced 75 mm apart

38 x 89 mm spaced

600 mm on centre

38 x 89 mm plate

19x89 mm slats

Duct lining

38 x 140 mm joists spaced

600 mm on centre

Floor
8. 300 mm air control door
9. 150 mm air control door
10. 19x89 mm blocking
11 . No spacing first 300 mm

Fig. 18. Lined rectangular distribution ducts.
HANDLING AGRICULTURAL MATERIALS

43

Table 12 Airflow related to dryer area and
hay moisture content

Moisture

content

ofhay(%)

Airflow rate

Floor area
(L/sm2)

Amount of hay
(L/s-t)

20-25
25-30
30-35

40

75

125

77
155
260

Several characteristics of hay operations help
explain the variability in static pressure that
can be achieved.

Because the fibers shrink, chopped hay settles
by 10-15% during drying and increases
pressure by an amount roughly equal to the
volume decrease. Baled hay, on the other
hand, does not demonstrate the same effect on
pressure.

Tightly stacked bales of hay do not permit air
to flow through fast enough to dry the crop
before it spoils. Consequently, keep the specific
density of baled hay as low as possible, yet
maintain the integrity of the bale in handling.

Bales stacked on edge have a lower resistance
to airflow than bales stacked on the flat. In the
dryer, place the square bales together on edge.
To prevent short-circuiting airflow, do not
leave spaces between the bales.

The static pressure directly relates to the
velocity of air traveling through the hay and
the depth of hay between the duct and free
airspace. Table 13, adapted from a Hydro
Quebec publication, provides a useful reference

for estimating the air velocity and static
pressure required for a drying system.

5.3 Air distribution systems Design the air
distribution system for a hay dryer to ensure a
uniform airflow rate to the entire dryer area.
Size the duct work to conform with good
engineering practices and with the criteria
given in the previous section. Define both the
minimum cross-sectional area of all ducts and
the minimum net open area of duct surface in
contact with the hay.

Locate a primary air-delivery duct
longitudinally, either in the centre of the dryer
or along one side. A duct located centrally
provides adequate air distribution for dryers up
to 10 m wide. To maintain uniform air
distribution, however, the depth of hay above
the top of the duct must equal the distance from
the duct to the edge of the dryer. Inadequate
depths of hay above the duct causes the air to
move through the hay directly above the duct,
taking the path of least resistance.

Choose any configuration for the primary duct.
Figs. 17 and 18 illustrate the most common
configurations: triangular and rectangular.
Line the ducts to permit manual directional
control of the air. Or leave them unlined for
uniform air discharge over their entire length.

Unlined ducts require that the entire duct be
uniformly covered with hay prior to starting
the flow of air. Lined ducts, on the other hand,
permit effective use of only a portion of the
dryer at one time. Use lined ducts where the
volume to be dried does not fill the dryer or
where only portions of the storage are to be
dried.

Size the lined primary ducts to ensure easy
access by the operator. The operator is then

Table 13 Fan air velocity and static pressure for drying hay in storage

Baled hay*

Chopped hay*

Thickness

Air

Static

Air

Static

of hay

velocity

pressure

velocity

pressure

(m)

(mm/s)

(Pa)

(mm/s)

(Pa)

1.83

76

190

76

250

2.30

76

190

76

250

2.75

76

250

76

250

3.20

76

250

76

250

3.70

76

250

86

310

4.10

86

250

97

310

4.60

97

250

112

375

5.30

112

310

127

375

5.80

122

375

137

440

At 40% moisture content.

44

STORAGE AND CONDITIONING

more likely to properly adjust the doors
controlling air direction.

Lateral ducts connected to a primary supply
duct extend the practical width of a hay dryer.
Lateral ducts can take two forms: a slatted
floor over floor joists, or a series of small lateral
ducts on one or both sides of the primary supply 5.6
duct.

Arrange the duct system so that all air paths
through the hay are equivalent. For example,
the distance from the end of the duct to the side
of the dryer should equal the distance from the
end of the duct to the top of the hay surface
when the dryer is fully loaded.

To ensure uniform air distribution, observe the
following:

• Install airtight dryer floors to prevent the
escape of air intended for the hay. Tongue-
and-groove or concrete flooring, or earth, are
adequately airtight. Cover other floors with
150 um polyethylene before assembling the
duct work.

• Eliminate routes for air escape. Structural
columns in the dryer may permit leakage of
air that is intended to go through the hay.
Reduce this short-circuiting by inserting an
impermeable layer such as plastic,
extending at least 0.6 m from the column in
all directions on the surface of the floor.

• Ensure that the lateral ducting within 0.3 m
of the primary distribution duct is not
perforated. Primary distribution ducts
should not be perforated within 2 m of the
fan.

Check the structural adequacy of the floor of
the hay dryer before its use. Hay with a
moisture content of 35% weighs 25-30% more
than the equivalent volume of field-dried hay.
Dry, baled hay has a density of around
0.13 t/m3. The density of dry, chopped hay is
around 0.08 t/m3.

5.4 Drying hay

A hay-drying operation involves four stages:

• field drying

• arranging hay in the dryer

• operating the dryer

• terminating drying

5.5 Field drying Allow the hay to field dry to a
moisture content of 35% before baling. This
step reduces both the risk of mold growth in the
hay as well as field and harvest losses.

Set the twine tension for wet hay the same as
for dry hay. This string tension maintains bale
integrity while permitting adequate airflow

5.7

through the bales. A bale length of 0.85 m
improves manual handling characteristics and
tightness of hand stacking. Bales 0.6 m long
allow improved stack density and bale
durability when bales are stacked randomly in
the dryer.

Arranging hay in the dryer How the hay is
arranged in the dryer depends on whether the
hay is baled or chopped.

Follow these guidelines for baled hay:

• For central duct systems, cover the duct in
even layers of hay. Gradually work upward
and outward to keep hay depths equal in all
directions.

• In a slatted-floor system, mound the first
layer over the duct and slightly past the end
of the slatted floor. Place each successive
layer on as a blanket, eventually reaching
the edge of the dryer.

• When using a lined, primary distribution
duct with a slatted floor system, start
loading with the doors in the top of the main
duct closed and the bottom doors open.
When the hay is 1-1.2 m over the top of the
floor, open the top doors. When the hay is
3-4 m over the duct, close the bottom doors
leaving the top doors open.

• Pack long bales tightly and on edge with
alternate layers at 90° to each other. This
arrangement reduces the chances of air
short-circuiting. Randomly dump short
bales into the dryer, providing the moisture
content is 25% or less. Plug any gaps in the
packing by stacking some bales by hand.

• Use baled hay to create mini-ducts in a
central lined duct that has no slatted floor.
Space the first two or three bales outward
from the duct with 150-200 mm between
them.

Follow these guidelines for chopped hay:

• Cut chopped hay to 50 mm or longer. This
length prevents excessive density and
resistance to airflow.

• Use a blower or other conveyor to put
chopped hay in place.

• Do not tramp wet hay or concentrate it in
one area of the dryer. Move the discharge
pipe frequently.

• Cover the drying floor to a uniform depth to
encourage even air distribution.

Operating the dryer The drying front moves
through the hay in the direction of airflow.
Hay on the top of the dryer does not dry until
the hay below it has dried. Consequently, pile
only as much wet hay in the dryer as permits
the drying front to pass through in time to

HANDLING AGRICULTURAL MATERIALS

45

prevent mold growth: 3.5 days for hay of 20%
moisture content, 4-6 days for 15% moisture
content.

The depth that wet hay can be stacked in a
dryer varies directly with the dryer airflow and
the climatic conditions for the area.
Experience, therefore, dictates the normal
operation for a given installation. However, as
a general rule, set the initial loading to 1.5 m
deep. Then add 1 m/day to fill the dryer.

The drying front moves through the hay
regardless of fluctuations in ambient air
quality. So operate the fan continuously until
the hay dries. Interrupting fan operation
delays drying and allows the hay to heat. With
increasing hay temperature, the risks of dry
matter losses and fire increase, as does the rate
of mold growth.

5.8 Terminating dryer operation When all the
hay appears dry, turn off the fan for 8-12 h.
Then restart the fan and monitor the exhaust
air. If the exhaust air temperature exceeds the
ambient air temperature, run the fan for
several more days and recheck the air
temperatures. Consider the hay dry after three
such checks reveal no further temperature
rises. Shut down the dryer.

5.9 Supplementary heat drying

Heated-air day dryers are operated in one of
three temperature ranges:

• low-temperature dryers operating at 5-1 0°C
above ambient air temperatures

• medium-temperature dryers operating at
10-200°C above ambient air temperatures

• high-temperature dryers operating
300-800°C above ambient air temperatures

Low-temperature dryers increase drying rate
by increasing the capacity to hold the moisture
of the air blown through the hay. Increasing
the temperature of the supply air reduces the
relative humidity of the air, thus enhancing its
drying potential. Drying hay this way
ultimately requires less air, as compared with
natural-air drying.

Some operators use the heat of respiration of
the hay to provide additional heat to dry the
hay and reduce the fan operating time. This
arrangement entails operating the fan
intermittently and allowing the hay to heat
between periods of fan operation. However, the
heat obtained in this manner costs 4-5% of the

dry matter of the hay being dried, a value
exceeding the energy costs saved. As well, heat
captured this way also poses a safety hazard.
Consequently, intermittent fan operation is not
recommended.

Install fans to minimize noise. As well,
consider solar heat gain on the south side of
buildings. See Fig. 19.

Fig. 20 illustrates the correlation between the
time required for visible mold growth to appear
and hay temperature. Since mold growth
depends on both temperature and moisture,
this curve shifts to the right as the hay dries.

Medium-temperature dryers are much less
common than low-temperature dryers.
Medium-temperature dryers demonstrate high
capital costs in their construction and they
need elaborate firing units. As well, the faster
drying rate demands more intense involvement
from operators because of faster turnover rates
in the dryers. Moreover, should drying not
proceed quickly, the risk of spoilage is high.

Design medium-temperature dryers with an
airflow rate, temperature, and hay depth
combination that allows moisture to leave the
hay fast enough to prevent mold formation.
Only qualified designers should make design
decisions concerning the thermodynamics and
psychrometrics related to medium-
temperature drying.

High-temperature dryers that raise the
temperature of the hay sufficiently high to kill
the microorganisms that cause spoilage require
heat-tolerant construction. These dryers
involve high-cost facilities. Choose them only
where continuous operation permits spreading
the capital cost over many thousands of tonnes
of hay. Commercial dryers normally operate
with supply-air temperatures of 370-750°C for
hay of a moisture content ranging from 40 to
70%. Hay with higher moisture contents need
drying at higher temperatures. Discharge air
temperatures usually measure 65-120°C.

With heated-air drying, the hay retains its
nutritional value better than with other drying
methods. Higher carotene concentrations,
superior palatability, and dry matter conserva-
tion all result from faster curing or drying.
Still, the benefits of speeding the drying pro-
cess with supplementary heat may be less si-
gnificant where objectives other than hay
quality must also be met. Carefully examine
the economics of using supplementary heat be-
fore beginning the detailed design of a system.

46

STORAGE AND CONDITIONING

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Chicken wire
over batt
insulation "~

i

Barn wall

<

2

(&8&5leLX iLilSi fl Ji ft?

Fig. 19. Fan installation for sound absorption.

55

50

45

o

o

40

©

35

i_

30

CO

Q)

25

Q.

E 20

,<i>

F-

15

10

5

i

4-

40 80 120 160
Time, h

Fig. 20. Relationship of temperature and time to mold
formation on high-moisture hay.

Hay Dryer
2 m solid

Primary distribution duct

Hay Dryer

HANDUNG AGRICULTURAL MATERIALS

47

REFERENCES AND FURTHER READING

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Fairbanks, G.E.; Fransen, S.C.; Schrock, M.D. 1981.
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Farm drying of milling wheat. 1986. Newsletter of
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Feldman, M.; Robertson, J. A.; Lievers, K.W. 1980.
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48

STORAGE AND CONDITIONING

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HANDLING AGRICULTURAL MATERIALS

49

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