CIRCULAR 400

NOVEMBER 1950

THE PRINCIPLES OF

ORCHARD HEATING

ROBERT A. KEPNER

This circular gives the highlights of find-
ings made during orchard heating tests that
were conducted in citrus orchards in south-
ern California during 5 winters, from 1937
to 1942. The tests were made by the Division
of Agricultural Engineering.

As a result of the tests and the findings,
recommendations are made as to types of
orchard heaters, their placement in the or-
chard, and their burning rates. Specific rec-
ommendations are made for border heating.

Since these studies and recommendations
involve certain known physical laws of heat
transmission, as well as orchard environ-
mental conditions, these too are explained
in a simplified manner in the first few pages.

The Author is Assistant Professor of Agri-
cultural Engineering and Assistant Agricul-
tural Engineer in the Experiment Station,
Igpyi University of California, at Davis

CALIFORNIA AGRICULTURAL EXPERIMENT STATION • THE COLLEGE
OF AGRICULTURE • UNIVERSITY OF CALIFORNIA • BERKELEY

CONDUCTION

i

TRAVELS TO HERE

HEAT APPLIED
HERE

/ '/

SUN HEATS SURFACE OF SOIL

/ I

1 \

OA^O^A,

COVER CROP REDUCES
AMOUNT OF HEAT PUT
INTO SOIL

CONDUCTION WARMS SOIL FOR FOOT
OR SO BENEATH SURFACE, STORING
HEAT FOR USE AT NIGHT.

mz

MOIST COMPACT SOIL
BETTER THAN LOOSE
DRY SOIL

CONVECTION (NATURAL)

STACK GASES RISE AND
MIX WITH ORCHARD AIR

J

J

COOL AIR COMES IN
CONTACT WITH HEATER

U
V

IS WARMED AND RISES

^

rz7

CONVECTION (FORCED)

O

IS WARMED BY CONTACT
WITH HEATER AND CIR-
CULATED THROUGH ORCHARD

COOL AIR IS BLOWN PAST
HEATER BY WIND MACHINE

RADIATION

TOP OF TREE CHILLED BY
RADIATION TO COLD SKY

COLD HERE

FRUIT AND FOLIAGE HERE
"SEEN" AND WARMED DIRECTLY
BY RADIATION FROM HEATER

"DARK" SIDE OF
TREE NOT WARMED
BY RADIATION

HOT HERE

RADIATED HEAT TRAVELS ONLY
IN STRAIGHT LINES TO OBJECTS
WHICH CAN "SEE" THE SOURCE.

For intelligent use of orchard heaters, it is
well to know some fundamentals

Before considering the technical as-
pects of how orchard heaters function,
how effective the various types may be,
and how they should be used, it will be
helpful to briefly review the ways in which
heat may be transferred and the environ-
mental conditions under which orchard
heaters are used.

Heat transfer is accomplished in one
or more of three different ways : by con-
duction, by convection, or by radiation.

Conduction is the transfer of heat
from one part of a solid body to another
part, or from one body to another that is
in physical contact with it.

Thus if one end of a rod is heated, the
other end will be warmed through con-
duction along the rod. Movement of heat
beneath the surface of the soil is by con-
duction. When the sun heats the surface
of the soil, heat is conducted downward
into the top foot or so below the surface
and stored until used at night.

Convection is the transfer of heat
from one point to another within a gas
(such as air) or a liquid, by motion or
mixing.

In natural convection the motion is
entirely the result of differences in density
between the cooler and warmer portions.
Warm air is less dense than cooler air
(i.e., one cubic foot of it weighs less).
Hence it tends to rise as the cooler air
settles to take its place. This principle is
used in a floor furnace in a home.

Forced convection means that the
motion is primarily from an external
source, such as a fan. Wind or a wind
machine produces forced convection by
circulating and mixing air in an orchard.

Radiation is the direct transfer of
heat through space from a hotter body to
a colder body. Radiation is transmitted
without loss through empty space and
with very little loss through dry air.
Other gases, such as carbon dioxide and
water vapor, absorb at least part of the
radiant energy.

A typical example of radiant heat
transfer is from a fireplace in a cold
room. A person standing near the fire
will be warmed on one side by radiation
from the flame and hot bricks, but will
remain cool on the side away from the
fire.

Because radiant heat travels through
space only in straight lines, an object
must "see" the heat source in order to
receive radiation from it directly. A
shield or other object placed between the
heat source and the cool body will reflect
or stop part or all of the radiant heat
transfer.

In an orchard, leaves and fruit which
are exposed to a heater will be warmed
by direct radiation from the heater and
may be a degree or two warmer than the
surrounding air. The opposite side of the
tree, however, will remain cool unless
"seen" by another heater.

Orchard heating requirements are
influenced by a number of factors

Atmospheric radiation. Except for
the rather infrequent invasion of citrus
districts by cold air at low levels from the
polar regions (the so-called freezes),
most orchard heating is needed because
of the loss of heat from the orchard by

radiation to the cold sky. The air in an
orchard becomes chilled mainly by con-
tact with the exposed surfaces of the tree
foliage or the cold ground, which are
losing heat by radiation to the sky.
When the moisture content of the air is

[3]

high, the net rate of radiation loss from
the soil and foliage to the sky is reduced
and orchard conditions tend to be steady
with a slow drop in temperature. On
nights when the air is unusually dry (low
dew point), the radiation loss is large
and orchard temperatures may be ex-
pected to drop rapidly.

Temperature inversion. During the
daytime the sun warms the ground and
other exposed surfaces, which in turn
warm the air that comes in contact with
them. Thus the air near the ground is
warmer than the overhead air during the
daytime, and tends to rise.

During the night, the air near the
ground is cooled (as discussed in the
preceding section) instead of being
warmed. Since this cooled air is more
dense than the warmer air overhead, it
remains close to the ground, becoming
colder as the surface temperature of the
earth continues to fall. Since this is just
the reverse of daytime conditions, it is
known as a temperature inversion.

A large inversion (overhead air con-
siderably warmer than orchard air) is
favored by a warm preceding day and
strong atmospheric radiation during the
night. A condition of large inversion is
often referred to as a "low ceiling" be-
cause the heated air and stack gases from
the heaters rise only a relatively short
distance before encountering warm
enough natural air to restrict their rising.

With a small inversion or "high
ceiling," the heated air must rise higher
before encountering warm overhead air.
Under these conditions much of the con-
vective heat from the heaters is wasted
above the tree tops, and heating is more
difficult than with a large inversion.

The graph on page 6 shows the
amounts of temperature rise obtained at
various heights during actual tests with
a fairly small inversion and with a mod-
erately large inversion. This chart shows
that with a small inversion there is more
heating above the tree tops and less
within the tree zone, than for a large in-

version. Typical air temperatures at vari-
ous heights are also listed.

Cold air drift. Air chilled by contact
with the ground or foliage, being heavier
than warmer air, will slowly flow down-
hill and underrun the warmer air. The
filling of ground depressions by cold air
from neighboring slopes in this manner
increases the frost hazard in the low
spots. Conversely, there is less danger of
frost on the higher slopes.

Even on nearly flat terrain and with a
relatively quiet night, the air drift across
an orchard may have a velocity of 1 to 2
miles per hour at the 20-foot elevation,
with velocities within the tree zone being
perhaps one-third as great. This incom-
ing air must be heated if the orchard is
to be adequately protected. The impor-
tance of border heaters in this regard
will be discussed later.

Thus air drift or air drainage is one of
the major factors affecting the response
from orchard heaters, particularly in iso-
lated orchards. In the so-called "mass
heating" of large areas, the air-drift ef-
fect diminishes except for border or-
chards.

Wind. When the wind velocity be-
comes great enough, the lower layers of
air are warmed by mixing with the
warmer air above. Such a wind (perhaps
only 3 or 4 miles per hour) may greatly
reduce the temperature inversion and
boost orchard temperatures above the
danger point within a period of a few
minutes. If, however, there is little or no
inversion, wind will increase heating re-
quirements by bringing more cold air
into the orchard.

Heat from the soil. In an unheated
area, the ground is the main source of
the heat lost by radiation to the cold sky
during the night. Heat from the sun,
which was stored in the top foot or so of
soil during the daytime, is conducted up
to the cooled ground surface during the
night to replace the heat lost to the sky.

Any practice, therefore, which in-
creases the soil thermal conductivity will

[4]

DAYTIME

AIR NEAR GROUND IS WARMED BY CONTACT
WITH FOLIAGE AND SOIL, RISES AND MIXES
WITH COOLER AIR OVERHEAD

NIGHT
(INVERSION OF DAYTIME CONDITIONS)

TREES AND SOIL SURFACE COOLED
BY RADIATION TO COLD SKY

AIR IN ORCHARD IS COOLED BY CONTACT WITH CHILLED TREES AND

EARTH

—WILL NOT RISE BECAUSE IT IS HEAVIER THAN WARM AIR OVERHEAD

LARGE INVERSION (LOW CEILING)

OVERHEAD AIR CONSIDERABLY
WARMER THAN IN ORCHARD

HEATED AIR RISES ONLY SHORT
DISTANCE BEFORE ENCOUNTERING
WARM NATURAL AIR

SMALL LOSS OF CONVECTIVE HEAT

SMALL INVERSION (HIGH CEILING)

OVERHEAD AIR ONLY FEW DEGREES
WARMER THAN IN ORCHARD

LARGE LOSS OF CONVECTIVE HEAT

Height

Temperature increase due to heaters, in degrees F
Heating effects and temperatures at various heights, based on tests in a 15-acre orchard; aver-
age fuel consumption 33 gals, per acre per hour. The indicated temperatures for various heights
are based on an assumed value of 25° F for the 5-foot elevation at the unheated station.

improve the availability of the soil heat,
as well as increasing the flow of heat into
the soil during the daytime. Covering or
loosening the soil surface is detrimental
to heat transfer, while a high moisture

content is beneficial. However, if a frost
occurs within a few days after an irriga-
tion, evaporative cooling from the wet
soil may more than offset the advantage
of the increased thermal conductivity.

Some heaters were found more effective
than others in small inversions

Heater characteristics. As already
mentioned, the output from heaters in an
orchard is available partly as convective
heat (stack gases and air warmed by
contact with the heaters) and partly as
heat that is radiated from the flame or
from hot metal surfaces.

When the inversion is large, both the
radiant heat and the convective heat are
useful. With a small inversion, however,
most of the convective heat rises above
the tree tops and is wasted, which means
that radiant heat must then be the prin-
cipal means of protection.

The proportion of heater output which

is in the form of radiant energy is dif-
ferent for the various kinds of heaters.
For example, the hot-stack type of heat-
ers used in these tests (Jumbo Cone, Kit-
tle, Return-Stack, and 7-inch Exchange
Stack) had 25 to 30 per cent of their
output available as radiant heat at nor-
mal burning rates. The lazy-flame heaters
tested (Hy-Lo 230A and Riverside Jun-
ior Louvre) had about 20 to 22 per cent
radiant energy, while the generating-
type (Fugit) had only 18 per cent.

Performance in an orchard. The
tests showed that when the inversion was
large, there was little choice between the

[6]

various types of heaters; i.e., the tem-
perature response for a given fuel rate
was about the same regardless of the
kind of heater. When the inversion was
small, however, those heaters which had
the highest radiant output (hot-stack
type) were the most effective. The gen-
erating-type heaters were the least effec-
tive at small inversions, and lazy-flame
heaters were in between these other two
types (see drawing below).

Regardless of the type of heater used,
the amount of fuel required to obtain a
given temperature rise is greater for
small inversions than for large ones. For
example the tests indicated that when the
temperature 60 feet above the ground
was only 4°F. warmer than at 5 feet
(small inversion), the amount of fuel
required for a given response in the cen-
ter portion of a 15-acre orchard would
be 50 per cent greater than with an in-
version of 10°F. (moderate inversion).
Fuel requirements near the edges of an
isolated orchard are greater than in the
center portion (as discussed in a follow-
ing section).

Heater placement. In general, only
about one-third to one-half of the energy
radiated from a heater strikes the trees;
the remainder goes to the sky or to the
ground. Part of the heat radiated to the
ground is useful in heating air that comes
in contact with the warmed soil, but that
which goes to the sky is practically
wasted.

The location of heaters with respect to
the trees has considerable influence upon
the total amount of radiant heat received
by the trees and upon the uniformity of
distribution to the trees. Heater place-
ment, then, is a factor to be considered,
particularly under severe heating condi-
tions (small inversion) where radiant
heating is of prime importance.

In general, the use of a large number
of small fires gives better distribution of
heat (either radiant or convective) than
a fewer number of heaters at higher
burning rates. Assuming a maximum of
one heater per 2 trees, the most uni-
form distribution of radiant heat is ob-
tained when the heaters are placed at the
centers of spaces between trees, in a stag-

Types of heaters tested. Left: hot-stack, or combustion-chamber type; center: lazy-flame type;
right: generator type. The hot-stack heaters produce the most radiant heat and are the most effec-
tive under severe heating conditions.

a a a c
o a & a
i a a a c

a fl 3 fl
a a a a

^

\

^i

ii I

[7]

gered pattern. Each tree then receives
equal amounts of radiation from two
opposite sides. Placing the heaters in the
tree rows reduces the loss of radiant heat
to the sky (more of it strikes the trees),
but the distribution is not so uniform
because the heaters are close to one side
of the trees.

With the above arrangements, if only
every other row of heaters is needed (one
heater per 4 trees), the pattern with

heaters in tree spaces will still result in
equal amounts of radiant heat to all
trees; with the heaters in the tree rows,
the other rows will receive only about
one-third as much as the heated rows.

When the heating requirements are
small and only every fourth row of heat-
ers is used, the coldest rows receive very
little radiant heat with either pattern,
and therefore must be heated almost en-
tirely by convection.

Border heating requires special attention —
and on all sides of the orchard

Extra heaters on all sides. Tests in ample, in two runs with a uniform dis-

a 15-acre heated orchard indicate that tribution of heaters over the entire

the rising stack gases and heated air over 15-acre area (no extras on borders), the

an orchard create an updraft which temperature rise in the trees at the up-

draws in cold air from all sides of the wind edge was only about 40 per cent as

plot, even on the downwind side directly great as in the center part of the orchard,

against the prevailing air drift. For ex- On the downwind edge the response was

FOR OUTSIDE AND PERHAPS FIRST ROW IN

ON ALL SIDES EXCEPT UPWIND SIDE. USE

TWICE AS MANY HEATERS AS USED WITHIN THE PLOT

ALONG UPWIND EDGE,
USE 4 TIMES AS MANY

*lMf

HEATERS AS USED WltHIN^© QQ Q)Q Q) Q) QQ 0 @ Q>®&

THE PLOT (BUT NEVER
MORE THAN 2 HEATERS
PER TREE)

E%&gto8*u*

PREVAILING
AIR DRIFT

3f $' o © o rsfa rata cfe dh> m

&

IN THESE TWO ROWS

USE TWICE AS MANY

HEATERS AS USED

WITHIN THE PLOT x^ x~x~ x—x — x~x— x~ K ~X~K- *~ * X * X

Typical example of border heater spacing when using one heater per 2 trees within the orchard.

x y
ten us

[8]

somewhat greater than at the upwind
edge, but still only 60 per cent as great
as at the center. The response gradually
increased for about 10 to 15 rows in
from the edges, and was about constant
over the remaining center portion of the
orchard.

From these results, it is evident that
extra border heaters are needed on all
sides of an isolated orchard. The impor-
tance of border heating, even in fairly
large orchards, becomes more apparent
when one considers that with 90 trees
per acre, a strip only 5 trees wide around
a 15-acre area or 7 trees wide around 30
acres represents half of the entire or-
chard. Thus since the reduced response
due to the inflow of cold air along the
borders extends in ten rows or more
from the edge on all sides, more than
half of a 30-acre orchard would be af-
fected by border conditions.

During the runs just mentioned, the
rate of fuel consumption was about 33
gallons per hour per acre. In other tests
with considerably lower fuel inputs (less
heaters per acre and lower burning
rates), there was proportionately less
difference between the heating effects
near the borders and in the center of the
orchard, presumably because less cold
air was drawn into the orchard by the
reduced updraft.

The following general considerations
should be kept in mind in regard to bor-
der heaters:

High radiant output is especially
desirable from border heaters because
the outside trees may then be heated
directly by radiation without the need
for warming all of the inflowing cold air
before it enters the orchard. If a grower
has more than one kind of heater, those
giving the highest percentage of radiant
heat (hot-stack or combustion-chamber
type) should be used for border heating.

Burning rates. Do not operate bor-
der heaters at excessive burning rates;
use larger numbers of heaters at normal
burning rates. In the center portion of

the orchard, burning rates should be
kept as low as possible to minimize the
amount of cold air drawn in by the up-
draft over the orchard.

Distribution of heaters. Border
heaters should not be concentrated on
the outside of the orchard, but should be
distributed over the first 2 or 3 rows in
from the edge. Extra border heaters are
usually needed on all sides of an isolated
orchard, but with greatest numbers on
the upwind side.

The tests did not include enough large-
plot runs to actually check the best ar-
rangement of border heaters. However,
for the usual conditions with reasonably
flat terrain and low wind velocity, the
following spacings are recommended for
isolated orchards (see drawing on page
8).

On the outside of the orchard along
the upwind border, use 4 times as many
heaters per tree as are used within the
orchard. For the first 2 rows in from
the edge, use twice as many as are used
within the* orchard. However, never use
more than 2 heaters per tree on the
outside nor less than one heater per 2
trees on the outside and the first row in.

Along all other borders, use twice as
many heaters per tree on the outside and
perhaps for the first row in, as are used
within the orchard. Never use less than
one heater per 2 trees on the outside.

Large-scale heating. In the so-
called "mass heating" of large areas, the
border effects are confined mainly to the
orchards on the outer fringes of the
heated district. If an orchard well within
a heated district does not have other
heated areas reasonably close, it will
probably need some extra border heaters.

Wind machines, when used in com-
bination with uniformly distributed heat-
ers, help to give a response greater than
the sum of the normal responses when
either heaters or wind machines are used
alone. Within the zone of disturbance,
the air mixing caused by the wind ma-
chines tends to make the convective heat

[9]

from the heaters more useful than when must do the entire job and high radiant
heaters alone are used. However, beyond output is fully as important as for border
the zone of influence (such as in the heaters in an orchard where wind ma-
corners of a square orchard) the heaters chines are not used.

SUGGESTED READING

The following is a partial list of reading material which covers some of the aspects of orchard
heating in more detail.

Schoonover, Warren R., F. A. Brooks, and H. B. Walker. Protection of Orchards against Frost. Cali-
fornia Agric. Ext. Cir. Ill : 1-70. 1939.

Discusses the need for heating and economic considerations (but based on 1937 costs). Includes
a general discussion of methods of adding heat to an orchard, general orchard heating practices,
and heating equipment.

Kepner, Robert A. Operation of Orchard Heaters. California Agric. Exp. Sta. Bui. 643: 1-32. 1940.
Presents observations, test results and recommendations regarding the field operation of various
kinds of heaters without regard to their relative effectiveness in the orchard.

Brooks, F. A. Action of Wind Machines in Frost Protection. American Fruit Grower, March 1947.
Describes typical wind-machine installations and discusses conditions under which they may be
expected to provide some protection from frost. (Note typographical error: brake horsepower
should read 70 instead of 90.)

Brooks, F. A., D. G. Yerg and R. A. Kepner. Wind Machines Tested. Citrus Leaves, Vol. 28, No. 12,
December 1948.

Presents results of tests and observations of wind machines used without heaters, during the
winter of 1947-48. (See also California Agriculture, Vol. 2, No. 12, December 1948.)

Leonard, A. S., and Robert A. Kepner. Return-Stack Orchard Heater. California Agriculture,
Vol. 4, No. 6, June 1950.

Discusses design principles which make the Return-Stack heater virtually smokeless, and presents
results of general field experience with this heater during a 10-year period.

Brooks, F. A., D. G. Rhoades, and H. B. Schultz. Frost Protection for Citrus. California Agriculture,
Vol. 4, No. 9, September 1950.

Reports preliminary tests to determine the response from a combination of wind machines and
distributed heaters.

Kepner, Robert A. Orchard Heater Smoke Lessened. California Agriculture, Vol. 4, No. 10, Oc-
tober 1950.

Gives recommendations for proper operating and maintenance procedures to minimize the
smokiness of the better types of orchard heaters.

[10]

In order that the information in our publications may be more intelligible it is sometimes neces-
sary to use trade names of products or equipment rather than complicated descriptive or chemical
identifications. In so doing it is unavoidable in some cases that similar products which are on the
market under other trade names may not be cited. No endorsement of named products is intended
nor is criticism implied of similar products which are not mentioned.

20m-ll,'50(2930)WP

•Ess

AGRICULTURE

• • . Contains brief, easy-to-read progress reports
of agricultural research, and is published monthly
by the University of California College of Agricul-
ture, Agricultural Experiment Station.

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