UNIVERSITY OF CALIFORNIA

COLLEGE OF AGRICULTURE

AGRICULTURAL EXPERIMENT STATION

BERKELEY, CALIFORNIA

The Smokiness of
Oil-Burning Orchard Heaters

WARREN R. SCHOONOVER AND F. A. BROOKS

BULLETIN 536
August, 1932

UNIVERSITY OF CALIFORNIA PRINTING OFFICE
BERKELEY, CALIFORNIA

CONTENTS

PAGE

Summary 3

The smoke problem 4

The origin of the smoke nuisance 4

Heat, not smoke, effective for frost protec-
tion 5

Need of relief from the smoke nuisance 6

Economic importance of orchard heating 8
Initiation of the investigation of the smoke

problem 8

Outline of the problem 9

Methods used in measuring smoke 10

Methods available 10

Tests in the field at Pomona 11

Laboratory methods developed at Davis.. 12

Smoke units used for comparative results 13

Accuracy of the method 14

List of orchard heaters tested 15

Test results of standard makes of orchard

heaters 17

Effects of incomplete combustion 17

Smoke tests of open-container smudge

pots 18

Smoke tests of short-stack heaters 18

Influence of stack height on smokiness 22

Smoke tests of open-flame heaters 23

Smoke tests of heaters with straight lou-

vered stacks 25

Smoke tests of cone-combustion-chamber

heaters 29

Smoke tests of Hy-Lo 1929 Model orchard

heater 30

Smoke tests of "nondistilling"-type

orchard heaters 31

General discussion of results 33

Heater groups according to smokiness 34

Estimate of the number and smokiness of

heaters in use 36

Operation methods for reducing smoke

output 36

PAGE

Test results with different fuel oils 38

Analyses of oils used in tests of standard

heaters 41

Analyses of oils before aDd after burning 41
Analyses of oils used for testing influence

of oil character on smokiness 41

Method of testing influence of oil character

on smokiness 43

Variations of smokiness not consistent

with oil in different heaters 45

Tests on stacks of new design 46

Requirements of a good orchard heater 47

Interpretation of test results on new stacks 47
Smoke tests of annular-combustion-cham-
ber stacks 48

Smoke tests of enlarged-combustion-
chamber stacks 48

Smoke tests of straight tall stacks 51

Principles of combustion in open-flame

stacks 52

Smoke tests on new open-flame stacks 54

Field measurement of the smokiness of or-
chard heaters 54

Visible records of smokiness 55

Correlation with the light-interception

method 56

Use of felt method for smoke measure-
ments in the field 58

Acknowledgments 58

Appendix A : Description of apparatus and

test methods 59

Appendix B: Correlation between light
interception and weight of smoke par-
ticles 63

Additional studies on correlation of smoke
density and weight 66

THE SMOKINESS OF
OIL-BURNING ORCHARD HEATERS2

WARREN E. SCH00N0VER3 and F. A. BKOOKS*

SUMMARY

In southern California 70,000 acres of orange groves are protected
occasionally against frost damage by burning low-grade fuel oil in
simple sheet-iron orchard heaters and smudge pots. The great quantity
of smoke produced when burning millions of gallons of oil each frosty
night frequently results in a serious smoke nuisance. A study of the
smokiness of oil-burning orchard heaters was made to aid in the abate-
ment of this nuisance.

Visual methods ordinarily used for smoke estimation in industrial
regions are not applicable to a study of the smokiness of orchard heaters
because of operation at night and the prevalence of smoke throughout
the citrus districts. Four other methods were developed permitting an
accurate classification of orchard heaters as to smokiness. Two of these
methods classify according to smoke blackness, and two according to
smoke weight.

Tests were run to determine the smokiness of the heaters in general
use. The tests showed :

1. That the different heaters vary greatly in smokiness.

2. That it is possible to burn ordinary grades of fuel oil in simple,
inexpensive heaters without producing visible amounts of smoke at
normal burning rates.

3. That the smokiness of many types of heaters can be reduced by
proper regulation and frequent cleaning.

4. That the composition of fuel oils available commercially has no
consistent influence on the smokiness of different heaters.

i Keceived for publication July 12, 1932.

2 This project was initiated under the leadership of A. H. Hoffman. He died
soon after the completion of the field trials at Pomona. Mr. C. E. Barbee had
charge of the project for several weeks after the death of Mr. Hoffman and has
since been responsible for the construction and operation of all the testing' apparatus.

3 Extension Specialist in Citriculture, temporarily assigned by the Agricultural
Extension Service to the Experiment Station to continue the investigation.

4 Associate Agricultural Engineer in the Experiment Station, appointed August,
1931.

4 University of California — Experiment Station

5. That laboratory tests run at summer temperatures are a reliable
indication of the relative smokiness of heaters *as operated in the field
during the winter.

Heater manufacturers have availed themselves of the smoke-measur-
ing1 facilities afforded by the laboratory for a study of heater design in
relation to the smoke problem. As a result of these studies new stacks
for use on old heaters have been developed which reduce the smoke out-
put at normal burning rates to invisibility.

Portable apparatus was devised in order to measure the smokiness of
orchard heaters as operated in the field. Accurate and semipermanent
visible records of smokiness were obtained. A method of correlating
field records with laboratory determinations was found which permits
field records to be interpreted quantitatively.

THE SMOKE PROBLEM

The Origin of the Smoke Nuisance. — Experiments begun in Califor-
nia as early as 1896 gave some indication that smoke or steam was effect-
ive in protecting citrus orchards against frost. Smudges of smoldering
wet straw were lighted alongside orchards. Growers soon recognized
that for frost protection heat was at least as important as smoke.

Coal was used for some years but proved unsatisfactory in burners
then available. The first oil heaters employed were open pails which
produced both heat and smoke. The grades of cheap fuel available at
that time would not burn satisfactorily in open containers. However,
the advantages of oil — it is plentiful, cheap, and easily lighted and
extinguished — led to the initial efforts for improvement of orchard
heaters from the standpoint of better control of the burning rate and
ability to burn all of the oil contained in the heater. All of the heaters
then in use smoked badly, but the growers, while realizing the impor-
tance of heat, still believed the smoke to be of some benefit and no im-
provements for better combustion were demanded. The public was
doubtless annoyed by the smoke but recognized the importance of the
citrus industry to the economic welfare of southern California and so
little complaint was made.

The steady growth of orchard heating as a regular practice in cold
locations had established many thousands of smoky heaters in orchards
by 1922. The freeze of that year brought the first realization of the
seriousness of the smoke problem and of course of the value of heating.
At that time attempts to develop smokeless heaters began and encourag-

Bul. 536] Smokiness of Oil-Burning Orchard Heaters 5

ing progress was made, but the rapid growth in number of heaters in
use increased the total smoke output so much that the smoke nuisance
continued to grow.

Heat, Not Smoke, Effective for Frost Protection. — The belief that
the smoke itself helped to conserve the heat in an orchard was widely
accepted because frosts rarely occur when low clouds are present. How-
ever, in 1920 Kimball and Young5 found that the smoke had little if any
value. According to their measurements heavy smoke decreases the rate
of heat loss by radiation about 10 per cent but does not prevent tempera-
tures from reaching a minimum as low as that reached in similar smoke-
free locations.

The radiation frost has peculiar characteristics in that air, cooled
mainly by contact with the ground and other surfaces which radiate
rapidly on clear nights, tends to settle in low areas underpinning the
warmer air, which is relatively lighter. A wind would mix the warmer
air with the cold and decrease the probability of frost, but on calm, clear
nights the air next to the ground will cool rapidly even with no influx
from surrounding hills. This usually creates a "temperature inver-
sion, ' ,6 which fortunately acts as a virtual ceiling ; for the cold ground
air, if uniformly heated a few degrees, will rise only a short distance
before reaching the level of common density, thus limiting the volume
to be warmed.

That successful frost protection depends upon heat generated from
a relatively large number of small fires per acre is now well demon-
strated by ample field experience. Figure 1 shows thermograph records7
obtained in an orchard equipped with open-pail oil heaters. It should be
noted that the temperature dropped rapidly in both the heated and
unheated orchards at 2 :45 a.m. and again at 4 :45 a.m., and that safe
temperatures were maintained by lighting more heaters until eventually
54 per acre were used.

Usually 50 heaters are required for fully protecting an acre of
orange orchard. Figure 28 shows thermograph records indicating that
on a severe night safe temperatures could not be maintained by burn-

5 Kimball, Herbert H., and Floyd D. Young. Smudging as a protection from
frost. U. S. Monthly Weather Eeview 48:461-462. 1920.

6 Young, Floyd D. Nocturnal temperature inversions in Oregon and California.
U. S. Monthly Weather Eeview 49:145. 1921.

7 Young, Floyd D., and C. C. Cate. Damaging temperatures and orchard heat-
ing in the Rogue River Valley, Oregon. U. S. Monthly Weather Review 51:617-631.
1923.

s Young, Floyd D. Notes on the 1922 freeze in southern California. U. S.
Monthly Weather Review 51:584. 1923.

6

University of California — Experiment Station

ing 25 heaters per acre (even though a gallon or more of oil was burned
per heater per hour) , but that with 5,0 heaters burning, the temperature
was raised above the danger point.

Need of Belief from the Smoke Nuisance. — The protection now af-
forded to approximately 70,000 acres of citrus orchards is estimated to
consist of nearly 3,300,000 orchard heaters, of which about 2,900,000 are
oil burning. Sufficient oil for only one filling of these heaters totals
2,500 railway tank carloads.

11 p. m.

8 a.m.

Fig. 1. — Temperature records in a heated orchard and at an outside check
station on the same night. (From Ext. Cir. 40.)

If weather conditions should require general heating in all districts
as much as 15,000,000 gallons of oil might be burned during a single
night. No such conditions have occurred, but the burning of much
smaller quantities frequently has caused the smoke nuisance to become
acute.

Because of the nature of a radiation frost, there is little or no wind
to blow the smoke away. This smoke pall decreases the heat from the
sun and therefore necessitates longer hours of burning. Growers sustain
other losses from inefficient heaters that give off unburned fuel into
the atmosphere and from the added expense of washing smoky fruit.
In extreme cases there are price discounts when washing has been
unsuccessful.

BUL. 536] SMOKINESS OF OlL-BURNING ORCHARD HEATERS

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8 University of California — Experiment Station

The character of the soot is such that closed windows do not prevent
damage to furniture, draperies, clothes, and merchandise. Under these
conditions personal discomfort is, of course, intense. Highway traffic is
endangered and business is interfered with generally.

It is estimated that the oil consumption in orchard heaters for the
last two weeks of December, 1930, was 17,000,000 gallons. On the basis
of data now available it is evident that the carbon content of orchard-
heater smoke discharged into the air during this 14-day period is to be
reckoned in millions of pounds. It was generally acknowledged that the
smoke damage was considerable.

Economic Importance of Orchard Heating. — The citrus industry
brings into California about $100,000,000 a year,9 and this income,
which supports many industries besides the actual growing of the fruit,
is in constant danger of reduction through various hazards. About one-
third of the citrus acreage is in locations cold enough to make orchard
heating an essential orchard practice, and there are few, if any, loca-
tions entirely free from damaging frosts. The efficient protection of the
colder orchards insures a regular supply of fruit so that markets can be
maintained, and also guarantees to consumers fruit of high quality
undamaged by frost.

The number of heaters lighted and their burning rates vary according
to weather conditions, but the average oil consumption will run about 15
to 20 gallons per acre per hour. The cost of heating at present prices of
fuel and labor, varies from about $0.75 to $1.00 per acre per hour
exclusive of interest and depreciation on equipment.

Despite general recognition of the fact that the economic welfare of
the entire state depends in large measure upon protecting the citrus
crop from frost damage, the public demands relief from the unnecessary
smoke produced by orchard heaters.

Initiation of the Investigation of the Smoke Problem. — The Agricul-
tural Experiment Station undertook to measure the smokiness of or-
chard heaters at the urgent request of representatives of the public, fruit
packers, and growers acting principally through the ' ' Orchard Heating
Improvement Committee, ' ' organized by the Los Angeles Chamber of
Commerce, January 9, 1931. L. D. Batchelor, Director of the Citrus
Experiment Station of the University of California at Riverside, empha-
sized the need and urgency of this investigation, which was assigned
priority over other engineering-research projects then under way at the
University Farm.

9 Approximate average of recent years, California Fruit Growers Exchange,
annual reports.

BUL. 536] SMOKINESS OP OlL-BuRNING ORCHARD HEATERS 9

The questions raised by the Committee10 in regard to orchard heaters
were :

1. The value of the smoke emitted by such heaters as a frost pre-

ventive.

2. The relative value, for orchard heating, of a heater that does not

smoke and one that does.

3. If it shall be determined that all such heaters issue more or less

smoke, then the minimum amount of smoke, per heater, neces-
sary for operation.

4. Ways and means of elimination, by entrapment or otherwise, of

the smoke emitted from such heaters. .

On January 30, Mr. A. H. Hoffman, Director L. D. Batchelor, and
Mr. W. R. Schoonover of the College of Agriculture, met with Mr. R. L.
Willits and W. K. Beattie of the Orchard Heating Improvement Com-
mittee and Mr. Floyd D. Young of the United States Weather Bureau.
At this meeting it was agreed that the Agricultural Experiment Station
would undertake the investigation in cooperation with the Committee.
The claim that smoke was of value as a frost preventive was considered
adequately disproved by the experiments of Kimball and Young.11 It
was agreed also that smoke from oil-burning orchard heaters constituted
the main problem because oil is the only fuel obtainable under present
conditions in adequate quantities at a reasonable price, and because
approximately 90 per cent of the heated acreage is equipped with oil-
burning heaters.

The project was restricted to an investigation of the smokiness of oil-
burning orchard heaters and does not include a study of the value of
orchard heating, or the efficiency of heaters. These two subjects have
been reported previously.12

Outline of the Problem, — The problem then was to study the smoke
output of the oil-burning heaters in common use, using the grades of
fuel most readily obtainable, and also, if possible, to determine the

10 Anonymous. Orchard heating regulations considered at mass meeting. Cali-
fornia Citrograph 16:145, 180, 181. 1931.

ii Kimball, Herbert H., and Floyd D. Young. Smudging as a protection from
frost. Monthly Weather Review 48:461-462. 1920.

12 Webber, H. J., et al. A study of the effects of freezes on citrus in California.
California Agr. Exp. Sta. Bui. 304:243-321. 1919. (Out of print.)

Schoonover, Warren E., Robert W. Hodgson, and Floyd D. Young. Orchard
heating in California. California Agr. Exp. Sta. Bui. 398:1-69. 1925. (Out of
print.)

Hoffman, A. H. Laboratory tests of orchard heaters. California Agr. Exp. Sta.
Bui. 442:1-37. 1927. (Out of print.)

Schoonover, Warren R., Robert W. Hodgson, and Floyd D. Young. Frost pro-
tection in California orchards. California Agr. Ext. Cir. 40:1-73. 1930.

10 University of California — Experiment Station

factors influencing smoke output. Large variations in smoke output
were expected because the heaters burn any kind of fuel oil whose pour
point is below 30° F, and, furthermore, at any bowl level, with large
variations of bowl and stack temperatures, with almost no control over
air-fuel ratio, and with secondary combustion sometimes above the
stack. Nevertheless, to control the smoke nuisance a practical method
of smoke measurement was required that would yield significant test
results with all heaters in question.

The subcommittee on research of the Orchard Heating Improvement
Committee agreed to choose the heaters to be tested, furnish used speci-
mens for the tests, supply the required amounts of the proper fuels,
secure a suitable location for field trials, supervise the regulation of
heaters, and formulate a decision as to what might be considered a
reasonable limit of smoke tolerance. The Agricultural Experiment Sta-
tion agreed to supply apparatus for making carbon determinations and
for determining fuel-consumption rates, make oil analyses for compari-
son of fuels used in tests with regular run of fuels used by the growers,
and supply personnel to conduct tests.

The purpose of the work was to. assist in eliminating the smoke
nuisance by :

1. Furnishing data to be used as the basis for legislation.

2. Furnishing information for the guidance of manufacturers in

improving their heaters.

3. Developing subject matter to be used by the Agricultural Exten-

sion Service for guiding growers in better heater operation.

METHODS USED IN MEASURING SMOKE

Methods Available. — Industrial smoke is usually measured by mak-
ing visual comparisons of its blackness with black and white color
standards such as the Ringelmann chart.13 This method is not directly
applicable to orchard-heater tests because it cannot be used at night.
Furthermore, its grading in five steps of '20, 40, 60, 80, and 100 per cent
black is too coarse to register minor variations in orchard-heater smoke,
which are of considerable interest in determining the causes of smoki-
ness. Other methods considered for smoke determinations were :

1. Direct-weight methods such as the collection of a soot sample on

a weighable filter, or the electrical precipitation of smoke

particles.

13 Faust, H. M. Smoke and its prevention. Ohio Engin. Exp. Sta. Cir. 24:12.
1931.

Bul. 536] Smokiness of Oil-Burning Orchard Heaters 11

2. Indirect-weight methods such as the collection of a sample on an

asbestos filter for making carbon determinations by chemical
methods.

3. Measurement of the opaqueness or apparent smoke density of the

products of combustion by a light-interception method.

4. Measurement of blackness by collecting a sample on a white filter

and determining the magnitude of the blackening effect by
photometric methods.
All of these methods were used and are discussed later.

Fig. 3. — Taking smoke sample by aspirator from tip of flame.

Tests in the Field at Pomona. — Mr. Young and the members of the
Committee were of the opinion that the tests should be conducted under
orchard conditions during cold weather. On the basis of experience
available at that time a direct-weight method using paper filters seemed
best adapted to a field study under orchard conditions.

Smoke determinations were made by holding a sampling tube in the
stream of gases discharged by a heater just above the tip of the flame
(see fig. 3) . The attempt to use this simple method to obtain weighable
samples on light paper proved unreliable and the method was aban-
doned for three reasons : (1) the smoke deposit could not be determined
because even when weighings were made in an air-controlled chamber

12 University of California — Experiment Station

with complete reconditioning for moisture content, the final weights of
the filter papers were both greater and less than the original (owing to
change in composition of the paper itself because of exposure to the
flue gases, and to variable loss of oil processed in the paper) ; (2) the
samples were not dependable because it was impossible to draw them
from the exact center of the waving flame tip ; and (3) occasional breezes
disturbed the weighing of fuel consumption and affected stack combus-
tion. These difficulties forced the complete refraining of the project and
the development of new test methods.

Laboratory Methods Developed at Davis. — In order to get away
from the errors in the filter-paper method it was decided to estimate the
smoke on a light-interception basis, which is a standard method of smoke
determination.14 The essential feature of the method is that a beam of
light from a constant source is passed through the smoke stream and the
light which is not intercepted falls upon a light-sensitive photo-electric
cell or a radiation pyrometer. Proper electrical instruments, indicating
the intensity of the light transmitted, measure the relative opaqueness
of the smoke. Then by measuring the total volume of the smoke stream
passing the light, per pound of fuel burned, the relative quantity of
smoke emitted by different heaters can be determined.

This method avoids the previous error in weighing filter papers,
eliminates the sampling error by dealing with the entire smoke stream
discharged from the heater, and avoids errors due to occasional breezes
because of its location indoors. The dilution of the smoke stream with
extra air simultaneously affects both the opaqueness and total volume
of air flow and thus does not influence the determination of relative
smokiness. The values of smokiness obtained with such an apparatus do
not represent actual weight or quantity of smoke no matter how accu-
rate the opaqueness and volume determinations may be. However, for
comparing different heaters the method is adequate — in fact goes one
step beyond the usual basis of smoke-abatement ordinances, which grade
by blackness without the volumetric determination.

If a reasonable correlation could be found between the opaqueness
of smoke and the density of its carbon particles, the light-interception
results might be interpreted in terms of weight within the limits shown
by the correlation. This, of course, was highly desirable, and the neces-
sary auxiliary apparatus for correlation data was added to the light-
interception equipment without sacrificing its advantages over all other
methods in being continuous, quick, and reliable.

14 American Society of Mechanical Engineers. Power test codes; instruments
and apparatus, Part 20, smoke-density determinations. Amer. Soc. Mech. Engin.,
29 W. Thirty-ninth Street, New York City. 1930.

Bul. 536] Smokiness of Oil-Burning Orchard Heaters

13

Figure 4 shows the apparatus used in making the smoke tests. The
orchard heater, at the extreme right, is placed on automatic self-balanc-
ing scales, which measure the loss of weight as the fuel is burned. The
centrifugal blower at the left creates a gentle suction, drawing the
smoke and some surrounding air into the hood, past the light, and
through the main orifice where the rate of flow is measured. Two small
orifices each with an area %0o of the total orifice area divide the total
smoke stream to obtain 1 per cent samples for carbon weight deter-
minations. The sample from one orifice flows through an electric pre-
cipitator which collects the solid particles for direct weighing. The

Fig. 4. — Laboratory apparatus for measuring orchard-heater smoke.

other sample flows through an asbestos filter which retains the solid and
liquid particles for chemical analysis. A felt filter also can be used to
obtain a deposit for comparison of blackness. Detailed description of
this apparatus is given in Appendix A.

Smoke Units Used for Comparative Results. — The correlation be-
tween smoke opaqueness determined by light interception, and carbon
weight determined electrically and chemically is fully discussed in Ap-
pendix B. In early tests with three fuel oils in the standard heaters
there appeared to be a reasonable correlation between the opaqueness-
quantity unit and weight such that one ' ' pound-smoke ' ' unit as defined
below was approximately equivalent to 1 gram of carbon per pound of
fuel burned. However, tests with seven other orchard-heater fuel oils
indicate that the correlation between smoke opaqueness and density of
carbon particles is not generally satisfactory even at high burning rates,
and is unusable at low burning rates.

14 University of California — Experiment Station

The correlation between opaqueness determinations and felt filter
blackness appears to be reasonably satisfactory as shown in figure 35.
Both methods are very fast. The light-interception method has the ad-
vantage of reading directly, continuously, and instantaneously while
the felts offer the advantages of simplicity and semipermanent record.
Each of these methods depends largely on the blackness of the smoke.
Either method gives accurate, comparative results which can be inter-
preted in terms of the other method. If the invisible vaporized carbon
compounds in the smoke are to be measured, another basis, such as the
determination of the weight of carbon particles would be required.
However, this characteristic of the smoke is not of general interest.

It therefore seems best to report the data on the basis of a quantity
unit representing the product of the average smoke-opaqueness units
(apparent density) times the total air flow (volume) during the period
of burning one pound of fuel. This unit has been designated as a
1 l pound-smoke ' ' unit.

Accuracy of the Method, — Since the weight-correlation problem is
avoided by comparing test results in pound-smoke units, the method as
a whole is subject only to the following small errors :

1. Error in weighing the loss of fuel as burning proceeds. For short
^2-pound runs this might amount to 5 per cent, but the average error is
very small owing to the electric contact signals and the operating of
two stop watches by independent observers. The automatic balance
developed later reduced this error to a negligible amount,

2. Error in reading millivoltmeter because the smoke comes in puffs.
This error may amount to 20 per cent in the case of individual readings,
but the average error over the usual 10 readings is small. It is greatest
in percentage with the least-smoky heaters.

3. Error in determining air flow due to the assumption that dry air
is being measured, while what is measured is in fact a mixture of air
with products of combustion, including carbon dioxide, water vapor,
and unburned or partially cracked oil. This error has been carefully
estimated and is of a negligible magnitude.

4. Errors due to instruments and orifices. The instruments and ori-
fices have been calibrated, and the errors are known to be insignificant.

All of these errors are relatively unimportant. The determinations
are of such accuracy that all heaters show distinctive characteristics of
smokiness in spite of large momentary deviations in most heaters from
their average performance.

Figure 5 shows graphically typical results obtained with the present
apparatus when testing a heater of unusually smooth-burning character-

BUL. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS

15

istics. The upper line shows that the burning rate (shown as a 3-minute
moving average) is rarely constant. This irregularity largely results
from the distillation cycle in the heater bowl. Some heaters even show
regularly spaced smoke peaks. The bottom line is the record of opaque-
ness, as observed each minute, corrected for temperature. The abrupt

~o s /o

JO 40 SO 60 70 SO

T/ME AFTEe STARTING reST. Minutes.

Fig. 5. — Continuous smoke test of heater No. 32, Ily-Lo 1929 Model.

return to smokeless readings when the draft was cut down after a high
burning rate shows the sensitivity of the apparatus. The middle line of
smokiness (in pound-smoke units) is derived from the other two by
using the volume of total air flow during the period of burning one
pound of fuel.

LIST OF ORCHARD HEATERS TESTED

Table 1 lists the orchard heaters and stacks tested. Photographs of
many of these are given in the bulletin so that readers may identify
definitely the models used. The figure numbers in which these photo-
graphs appear are given in column 2. Graphs of the smoke-test records
and diagrammatic cross sections of the heaters and stacks will be found
in the figures listed in column 3.

16

University of California — Experiment Station

table 1
List of Heaters. Tested and their Photograph and Ctjr.ve Numbers

No.

Photograph

Smoke test

fig. No.

fig. No.

1

2

3

4

1

18, 21,22

15,28

National Jumbo Cone

2

6

Garbage Pail

3

20

Hy-Lo, Double Stack, square bowl

4

20

12

Hy-Lo, single short stack, round bowl

5

17

Smith-Evans

6

18

17

Kittle

7

22

Citrus, Olsen Stack

8

19

15

National Baby Cone

9

20

8, 24, 25

Citrus Regular

10

Canco 5 gal.

11

7

Dunn

12

19

13

National, Exchange model, 5j-inch stack

13

6

Hamilton Bread Pan with stack

14

13

Wheeling

15

Canco 3 gal.

16

Chinn

17

6

Hamilton Bread Pan

18

Hamilton, oblong with stack and down draft

19

7

Hamilton, square bowl with down draft

20

19

10

National Double Stack

21

17

Bothwell

22

20,21

8

Citrus, 15-inch stack

23

20

12

Hy-Lo Double Stack, round bowl

25

20,22

Hy-Lo, single short stack, square bowl

26

19,22

9,29

Citrus, high stack

27

20

Citrus Gas Flame

29

18

17

Fugit

30

19

11

National Junior Louver, 15 inch

31

18

14

National, Exchange model, 7-inch stack

32

18,21

5,16,24,25

Hy-Lo 1929 Model

34

19

14, 29, 30

National, Exchange model, 6-inch stack

New stocks

52

26

27

Lamco Gyradiant

60

26

27

Hy-Lo Giant

61

28

Hy-Lo Giant Junior

62

26

28

Hy-Lo Drum Stack

63

32

Hy-Lo, straight stacks

75

32

Hy-Lo, tapered stacks

76

32

National Junior Louver, 18 inch, holes spaced spirally

80

26

29

Hinchcliff, 36-inch

81

29

Hinchcliff , 30-inch

90

26

32

National Junior Louver, 18 inch, holes spaced in rows

91

26

32

O'Keefe and Merritt, 6-inch straight stack

91A

31

O'Keefe and Merritt, 7-inch straight stack

100

27

O'Keefe and Merritt Corrugated

148

26

32

Hy-Lo, tapered stack

BUL. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS 17

TEST RESULTS OF STANDARD MAKES OF ORCHARD HEATERS

The heaters were tested in the regular runs over the entire range of
burning rates permitted by the draft regulators. The average burning
rate in the field is from 4 to 5 pounds of fuel an hour. The normal operat-
ing range is from 21/2 to 7 pounds an hour.

Figures 6 to 17 show the detailed results obtained with the various
heaters. The marked points indicated by crosses, triangles, circles, etc.,
show the estimated smokiness in pound-smoke units as calculated from
four to ten light-interception readings taken at 40-second intervals
while burning % pound of fuel at the rate indicated. Development of
the automatic balance permitted a continuous determination of burning
rates, so that smokiness could be determined for each minute as shown
in figures 5 and 30. Where such continuous observations were made, they
are shown on the smoke-test graphs by single checks ( V )• Each curve
indicates the line of trend of smoke production as the burning rate is
varied. The curve shown does not indicate the exact smokiness but
merely the probable center of a band of variable width. It is to be
expected that any single reading might deviate from the line of trend
by as much as 25 per cent, Occasionally a reading may deviate to a
much greater extent from the line of trend because of unstable burning
conditions.

Effects of Incomplete Combustion. — The field trials at Pomona, sup-
plemented by orchard observations and laboratory tests, indicate that
many factors influence the smokiness of an oil burner, but that the only
factor of great importance is the degree to which complete combustion
is approached. If oil is completely burned the products will be carbon
dioxide, water, and very small amounts of ash and sulfur dioxide. Sub-
stantially complete combustion occurs under the following conditions :
(1) oil thoroughly atomized and mixed with the proper amount of air
for combustion; (2) high combustion-chamber temperature to sustain
combustion ; and (3) combustion completed before the gases are chilled.
The best conditions result in a blue smokeless flame. If there is a less
favorable admixture of air with the combustible gases arising from the
bowl of a heater the flame may be yellow and either smokeless or smoky.
A flame is yellow because it contains particles of incandescent carbon
which did not come into contact with enough oxygen for complete com-
bustion at the instant of reaching the ignition temperature. Under
favorable temperature conditions these particles may be consumed be-
fore leaving the flame and there will be practically no smoke. If such a
flame is cooled many unburned carbon particles will escape to form

18 University of California — Experiment Station

smoke. For example, the flame from a kerosene lamp or a candle may be
smokeless, though yellow, but if a cold object is held in the flame it will
become coated with soot or if a puff of cold air disturbs the flame it will
become smoky.

Smoke may be caused also by the cracking of the complex hydro-
carbons at such a distance from the region most favorable to combustion
that carbon atoms can agglomerate to form particles too large to be
burned readily. ' ' Cracking ' ' is the name given to a process extensively
used today for the purpose of breaking up the more complex hydro-
carbons into the simpler ones, either in vapor or liquid phase. Fuel oil
produces simpler hydrocarbons when heated between 525° and 650° F.
A representative type of reaction for such a mixture might well be
C14 H30 + heat -^C7 H10 + C6 H14 + C. During the cracking process,
which occurs when oil vapors are superheated, the larger molecules are
broken down, producing lighter hydrocarbons and leaving a residue of
carbon. In the refinery the process is carried on in a still and the carbon
is left in the still as a hard coke. In a heater partial cracking may take
place if the oil vapors are sufficiently heated to break the molecules while
the air supply is insufficient for complete combustion. Carbon resulting
from cracking appears as soot accumulations in the bowl and stack or
as smoke.

Smoke Tests of Open-Container Smudge Pots. — Figure 6 shows the
performance of what are essentially open containers with no regulation
except control of the size of the fire by sliding covers on or off. The oil
is vaporized and burns at the surface, producing a flickering, smoky
flame. Air is deficient in the mass of flaming gases and temperatures are
favorable for some cracking. No. 13 has a short stack, which, however,
has little effect, because the sliding cover permits free burning from the
surface.

Smoke Tests of Short-Stack Heaters. — The next stage in the develop-
ment of heaters was the addition of drafts for regulating burning rates.
Heaters 10, 15, and 16 fall in this class. In addition they have short
stacks, which also aid in controlling the burning rate. The stacks were
intended to improve the combustion, but these heaters are little if any
less smoky than open containers. The smokiness varied between 16 and
28 and averaged about 24 pound-smoke units.

Figure 7 shows the performance of short-stack heaters of larger
capacity having down-draft tubes designed to concentrate a generating
fire in a hot spot on the oil so as to facilitate burning the entire charge
even though low-grade oil is used. Hamilton down-draft heaters of other

BUL. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS

19

models but similar to No. 19 were tested with results nearly like those
shown on the diagram. The points determining the curves were obtained
at air temperatures of about 90° F, and those indicated by checks at

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Fig. 6. — Smoke tests of open-container smudge pots, heaters 2, 13, and 17.

46° to 50° F. The determinations are too scattered to permit an accu-
rate study of the effect of temperature, but they do show that the classi-
fication of the heater is not changed.

20

University of California — Experiment Station

These heaters are more smoky than open containers. Apparently the
nature of the combustion is such that more of the hydrocarbon gases are
cracked and there is nothing in the construction to cause effective mix-

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Fig. 7. — Smoke tests of short-stack orchard heaters, Nos. 11 and 19.

ing of air with the burning gases. In fact the air is admitted in such a
manner that it probably chills the flame to below the most favorable
combustion temperatures.

BlTL. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS

21

Figure 8 shows the performance of two models of Citrus heaters —
the short-stub stack or Kegular, and the perforated 15-inch stack.
Warm and cold-weather tests are shown. From a smoke standpoint these

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Fig. 8. — Smoke tests of Citrus Kegular and Citrus 15-inch heaters, Nos. 9 and 22.

heaters are little better than ordinary smudge pots. The 15-inch stack
increases the smokiness at low burning rates, probably because of stack
temperatures more likely to cause cracking. It is to be noted that the use
of kerosene reduced the smoke output to less than half the usual amount.

22

University of California — Experiment Station

Influence of Stack Height on Smokiness. — In order to meet the de-
mand for a tall stack and less smoky heater the manufacturers of the
Citrus heater sold a top stack to be put on above the 15-inch stack. This

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Fig. 9. — Influence of stack height on smokiness of Citrus high-stack heater, No. 26.

stack has been sold in 30, 24, and 18-inch lengths. Figure 9 shows the
effect of stack height on this type of heater tested with various lengths of
top stack varying from 12 to 36 inches.

BUL. 536] SMOKINESS OF OlL-BURNING ORCHARD HEATERS 23

The 12 and 15-inch top stacks do not produce enough draft to in-
crease the air intake into the lower stack sufficiently to give improved
combustion. On the contrary they promote cracking. The 18-inch top
stack provides slightly better combustion than the 15-inch bottom stack
alone. The 24-inch top stack improves combustion noticeably at low
burning rates and gives reasonably good results at high rates. However,
there seems to be some instability in the lower range, where a slight
change in conditions causes cracking to take place and the smokiness
may rise to as high as 35 pound-smoke units.

Many other heaters show unstable combustion at certain burning
rates, the most pronounced effect being found in heater No. 7 (Citrus
heater with Olsen Stack) in which the smokiness varied between 6 and
36 pound-smoke units with little change in burning rate and rose to 51
units during one test, Similar performance may be expected from the
Apollo heater, which varies from scarcely visible smoke at 4 pounds an
hour to 75 pound-smoke units at 5 pounds an hour.

The Citrus heater can be made fairly satisfactory by use of a 30-inch
or 36-inch top stack above the 15-inch section. The stronger draft pull
of the high stack draws in sufficient air for practically complete combus-
tion over the burning range to 3 to 6 pounds of fuel per hour. These tall
stacks can be used only on models of recent construction which have
tight-fitting covers. The draft is so strong that burning rates cannot be
controlled with heaters having loose-fitting covers. It is also difficult to
extinguish the fires in these tall-stack modifications of the Citrus heater.

Smoke Tests of Open-Flame Heaters. — The open-flame stack (some-
times called "lazy-flame ' ' stack) is very popular because of ease of light-
ing and regulation, the release of the hot products of combustion near
the ground, and the low cost and depreciation. No stack parts are heated
excessively. In this type the mixing of air and gases takes place in the
stack and most of the combustion occurs at the top of the stack. The
proportion of total heat in radiant-energy form is greater with the open-
flame than with tall-stack heaters. Figure 10 shows the performance of
the National Double Stack heater (No. 20) belonging to the open flame
type. This stack is not very satisfactory in regard to smoke but it offers
interesting possibilities, some of which are shown in the lower curves,
which indicate the effect of introducing extra air at various points.
These experiments with extra air were continued on newer types of
open-flame stacks and will be discussed in another section.

Figure 11 shows the smokiness of a heater (No. 30) with a Junior
Louver stack, an open-flame type, on both round and square bowls.
Tests were made with clean and dirty stacks during warm and cold

24

University of California — Experiment Station

weather. The performances of these stacks on the two types of bowl are
similar and there is little difference between the 15-inch and 18-inch
heights. This type of stack, as well as the one shown in figure 10, admits
just enough air along the stack to burn fairly well at a low rate, but at
average rates and higher it tends to cause cracking, and consequently
smokes.

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Fig. 10. — Smoke tests of National Double Stack open-flame orchard heater, No. 20.

Heater No. 27, the Citrus Gas Flame, was tested clean and dirty and
with and*without the baffle usually supplied with the bowl for use with
this stack. With the baffle this heater will not operate practically in the
field. Without the baffle its smokiness was above 20 pound-smoke units
in tests at all burning rates,

Figure 12 shows the performance of the low-stack or open-flame
models, heaters 4 and 23, manufactured by the Scheu Products Com-
pany. The stacks of these heaters have a great many small holes uni-
formly distributed from the top nearly to the bottom. As combustible
gases rise in the stack, air is admitted through each hole and a small

BlTL. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS

25

tongue of flame shoots from the hole into the stack. The remaining un-
burned gases burn with a smoky flame at the top of the stack. Some of
the smoke is probably caused by cracking within the stack. The per-
formance of this heater is greatly improved by a new bottom collar pro-
viding for air intake near the base of the stack.

Similar results were obtained with square-bowl heaters, No. 3 and
No. 25, using the same stacks. The heaters manufactured by this com-

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Fig. 11. — Smoke tests of National Junior Louver stack heater, No. 30.

pany have had numerous changes in attachments for improving com-
bustion and gas generation within the bowl. Most of these have only a
minor influence on smoke output. Lack of time has prevented a study
of all the possible combinations.

Smoke Tests of Heaters with Straight Louver ed Stacks. — Figures 13
and 14 show results with heaters having tall stacks with louvers in the
lower section. Stack diameters range from 5 inches in No. 14 to 7 inches
in No. 31. The curves represent the characteristics of each stack when
properly cleaned. When the air intakes become partially clogged by

26

University of California — Experiment Station

soot the smokiness increases as shown by the upper curve for heater
No. 34 (fig. 14) . When this record was taken the soot accumulation had
decreased the stack diameter to approximately that of heater No. 14.

A study of the curves and of the heaters in operation indicates cer-
tain causes for the performance as illustrated. In the case of heater
No. 14 the smallest possible gas-generating fire in the bowl results in the

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Fig. 12. — Smoke tests of open -flame heaters, Nos. 4 and 23, manufactured by the
Scheu Products Company.

production of a hydrocarbon-air mixture too rich to burn in the small-
diameter stack except at the topmost of the rows of louvers. The
primary combustion at this point seems to crack part of the gases and
cause a heavy smoke output. As the supply of combustible gases is in-
creased a smaller percentage of the combustion occurs in the stack and
less cracking takes place. There is also an increase in the secondary com-
bustion above the top of the stack. As the heat is increased at this point
some of the elementary carbon formed during the cracking is consumed
and the smoke output decreases as the burning rate is increased. Stacks

BUL. 536] SMOKINESS OP OlL-BuRNING ORCHARD HEATERS

27

of diameter larger than 5 inches admit enough air to permit practically
complete combustion in the louvered section at low burning rates. At
the points where the air enters, the air-fuel mixture is lean enough to

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Fig. 13. — Smoke tests of 5 and S^-inch louvered-stack heaters, Nos. 14 and 12.

burn explosively ; growers call this ' ' louvering. ' ' These small explosions
increase turbulence and cause better mixing of air and gas, which im-
proves the combustion. For each size of stack there is a burning rate at

28

University of California — Experiment Station

which the air-fuel ratio is satisfactory and combustion is apparently
completed in the lower part of the stack. Under these specific conditions
the smoke output is small. The range of burning rates over which air-

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Fig. 14. — Smoke tests of 6 and 7-inch louvered-stack heaters, Nos. 34 and 31.

fuel ratios are reasonably satisfactory increases as the stack diameter is
increased. With all stack diameters the smoke output increases rapidly
as the generation of oil vapor in the bowl is pushed beyond the capacity

BUL. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS 29

of the combustion chamber, reaches a maximum, and then decreases
coincidentally with the development of a secondary blaze at the top of
the stack.

It is unfortunate that heaters of this type cannot be lighted without
opening the drafts far beyond the amount required for normal burning.
This results in excessive smokiness during the warming-up period and
also perhaps later because of soot accumulation while burning at too
high a rate. This same difficulty of lighting is experienced to a greater
or less degree with all heaters of the lean mixture, explosive-fire type.
Possibly this difficulty can be eliminated by the use of wicks to permit
lighting with less draft opening.

It is apparent that heaters with straight louvered stacks are very
erratic. They need careful draft regulation and frequent cleaning of
stacks to give satisfactory performance. The best results are obtained
when "louvering" takes place throughout the entire length of the
louvered stack section. If No. 34 is burned without a top stack, * ' louver-
ing" occurs at burning rates of 1 or 2 pounds an hour and the smokiness
is about 4 pound-smoke units, but at a 3V2-p°und rate it is 36 units.
With the top section on, however, "louvering" occurs at all rates be-
tween 2 and 6 or 7 pounds and good results are obtained as long as the
heater is clean.

The worst over-all performance shown in any of the tests was that of
heater No. 31 without a top stack. Apparently this type of stack per-
mitted the greatest amount of cracking to take place. The smokiness at a
burning rate of 3^2 pounds an hour was as high as 65 pound-smoke units.

Smoke Tests of Cone-Combustion-Chamber Heaters. — Figure 15
shows the smokiness of heaters with louvered cone-shaped combustion
chambers. It is similar to that with the louvered straight stacks. The
National Baby Cone, heater No. 8, has a very narrow range of satisfac-
tory air-fuel ratio. When the air supply becomes too limited there is a
very sudden and steep rise in smoke output ; this occurs at a 3-pound
burning rate. Closing the top three rows of louvers, which can be done
with a hammer, lowers the fire into the larger part of the cone and
considerably improves the performance at usual burning rates. If the
cone is altered in this manner a hot area develops at the top, which may
result in rapid oxidation of the stack at high burning rates. The National
Jumbo Cone, heater No. 1, with larger combustion chamber, is satisfac-
tory over the entire normal operating range as long as it is clean. The
upper curves show the smokiness of the same heater when using a dirty
stack. The latter records were taken after burning about 14 gallons of
fuel without cleaning the stack.

30

University of California — Experiment Station

Smoke Tests of Hy-Lo 1929 Model Orchard Heater. — Figure 16
shows the performance of the Hy-Lo 1929 Model, heater No. 32. This
heater usually gives satisfactory results over a wide range of burning

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Fig. 15. — Smoke tests of cone-combustion-chamber heaters, Nos. 1 and 8.

rates as shown by the line of trend. At times it becomes very smoky,
owing usually to having the fire jump back and burn at the holes in the
base of the burner. When burning normally, combustion is of the explo-
sive, lean-mixture type. This heater is the only one showing significant

BUL. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS

31

differences between warm and cold-weather tests. It was less smoky on

the cold run. Figures 5, 24, and 25 give further test results on this heater.

Smoke Tests of "Nondist Ming "-Type Orchard Heaters.— figure 17

shows the performance of the separate-container or so-called "non-

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Fig. 16.— Smoke tests of Hy-Lo 1929 Model heater, No. 32.

distilling ' ' type of heater. This type differs from the distilling types in
burning fresh oil of constant composition. Kittle, heater No. 6 (gravity
feed), and Fugit, No. 29 (pressure feed) require an oil of higher grade

32

University of California — Experiment Station

than that commonly supplied for other orchard heaters. In both of these
heaters, gas is generated from the oil in a hot burner and combustion is
completed at the point of gas generation without any opportunity for

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Fig. 17. — Smoke tests of "nondistilling ''-type heaters, Nos. 5, 6, 21, and 29.

cracking to take place. Results are very satisfactory under proper ope-
rating conditions. No. 6 becomes smoky if the oil is fed too rapidly for
the capacity of the burner.

BUL. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS 33

Smith-Evans, heater No. 5, and Bothwell, heater No. 21, are similar
in general design. They both drip oil onto a hot plate in a burner which
admits insufficient air. A type of combustion takes place which results
in considerable cracking. It is apparent that in addition to smoke these
heaters discharge some unburned gases, but the quantities are not great
enough to cause more than intermittent blazes at the top of the stack.
It would appear that burner and stack design are more important in
preventing smokiness than the method of supplying oil to the burner.

GENERAL DISCUSSON OF RESULTS

The tests show that there is great variation in the smokiness of or-
chard heaters, the range being from less than 1 pound-smoke unit to
more than 60 at a burning rate of 5 pounds of fuel an hour. An indi-
vidual heater varied from as low as 4 pound-smoke units to as high as
46 at this burning rate, the increase being due to sooting up of the air
passages. Heaters vary considerably in smoke output as the burning
rate is changed, usually producing more smoke as the burning rate is
increased until the secondary combustion at the top of the stack becomes
effective. Also when the burning rate is reduced below the critical point
the smokiness usually increases, partly because the longer time con-
sumed in burning a pound of fuel increases the accumulation of soot
measured.

The tests so far conducted probably represent the best the heaters
can do with the fuel used. The heaters were always cleaned before start-
ing the regular test, filled to an average level with clean oil and placed
level ; covers were made tight, and drafts properly set. Furthermore,
they were under constant observation while burning and were shielded
from breezes. No extended studies have been made of any one heater.
Repeated tests while changing only one variable at a time, such as the
oil level, would probably aid in determining the exact causes of smoke
production in each case.

However, a study of the results indicates the validity of certain con-
clusions as follows :

1. Smokiness is governed to a large extent by the design of the stack.
In several tests the same stack on different bowls with different draft
devices showed substantially the same characteristic.

2. The influence of air temperature on smokiness is slight and it may
be expected that laboratory or summer field tests usually will be com-
parable with winter field tests.

34

University of California — Experiment Station

3. Accumulation of soot in the heaters has no consistent influence on
smokiness except on heaters having tall stacks either with or without
combustion chambers. Soot accumulations in such stacks and combus-
tion chambers greatly increases the smokiness. The smokiness of low-
stack smudge pots and of open-flame heaters is not greatly influenced
by soot. The principal effect is a decrease in burning rate, which may
continue to the point of practically extinguishing the heater. This
effect is particularly pronounced in the Citrus heaters.

4. It is possible to burn a relatively crude fuel in a very simple,
inexpensive orchard heater and keep the smokiness below the level of
ordinary visibility.

Fig. 18. — Standard heaters reasonably free from smoke: A, heater No. 1, Na-
tional Jumbo Cone ; B, No. 6 Kittle ; C, No. 29, Fugit ; D, No. 31, National, Exchang(
model, 7-inch stack; E, No. 32, Hy-Lo 1929 model.

5. Unburned carbon and hydrocarbons given off in the smoke amount
to a direct fuel loss of 16 per cent in some cases and probably would
average 5 per cent for all the heaters now in use. It is not correct to
judge the efficiency of combustion by the blackness of the smoke because
of the presence of invisible combustible carbonaceous matter. However,
with the usual orchard heater the loss of unburned fuel can be con-
sidered below 1 per cent when the smoke output is not visible.

Heater Groups According to Smokiness. — It appears logical to divide
the standard heaters tested into the following four groups :

1. Heaters 1, 6, 29, 31, and 32 (fig. 18), which are reasonably free
from smoke at all burning rates under good operating conditions.

BUL. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS

35

2. Heaters 8, 12, 20, 26, 30, and 34 (fig. 19), which can be operated
with little smoke up to burning rates used in moderately cold weather,
but which may produce excessive smoke under certain conditions.

J

B

E

Fig. 19. — Photographs of standard heaters which can be nearly smokeless at cer-
tain low burning rates : A, heater No. 8, National Baby Cone ; B, No. 12, National
Exchange model, 5%-inch stack; C, No. 20, National Double Stack; D, No. 26, Cit-
rus, high stack; E, No. 30, National Junior Louver, 15-inch; F, No. 34, National
Exchange model, 6-inch stack.

3. Heaters 3, 4, 9, 22, 23, 25, and 27 (fig. 20), which are smoky but
commercially important. Nos. 3, 4, 23, 25, and 27 can be operated so as
to give results similar to those in group 2, but they are erratic and as
burned in the field usually would be much worse than the group 2
heaters.

3L. 11 &

■

A

B

E

G

Fig. 20. — Photographs of standard heaters which are smoky but commercially
important: A, heater No. 3, Hy-Lo Double Stack, square bowl; B, No. 4, Hy-Lo,
single short stack, round bowl; C, No. 9, Citrus Eegular; D, No. 22, Citrus, 15-inch
stack; E, No. 23, Hy-Lo Double Stack, round bowl; F, No. 25, Hy-Lo, single short
stack, square bowl ; G, No. 27, Citrus Gas Flame.

4. Heaters 2, 5, 7, 10, 11, 13, 14, 15, 16, 17, 18, 19, and 21 which are
very smoky but which are mostly of obsolete types.

36 University of California — Experiment Station

Estimate of the Number and Smokiness of Heaters in Use. — The
Fruit Frost Service of the United States Weather Bureau completed in
June, 1932, a survey of the numbers and types of orchard heaters now
being used by citrus growers.15

According to these estimates the 2,900,000 oil-burning heaters may
be classified into distinctive groups and the smoke output estimated
from a study of the test data as follows : about 1,350,000 heaters believed
to be of such a t;Tpe that the smokiness will average around 12 pound-
smoke units at a burning rate of 5 pounds an hour; 50,000 with an
average smokiness of 4 pound-smoke units at the 5-pound burning rate ;
and 1,500,000 with an average smokiness of 28 pound-smoke units.

Of the 1,500,000 heaters averaging 28 pound-smoke units, 500,000
are of such a type that it will be difficult to reduce the smoke output.
Most of these are obsolete heaters of very little value. This figure
includes approximately 55,000 garbage pails, which can be used for
orchard storage of oil. These heaters should be replaced by others hav-
ing a smoke output of 8 pound-smoke units or less over the normal
operating range. The remainder of the smoky heaters consist mainly of
about 200,000 Dunn heaters and about 800,000 Citrus heaters with short
or 15-inch stacks. It is believed that stacks can be put on all of these at
a relatively small expense and the smoke output can be brought below
8 pound-smoke units.

If changes as indicated were made and if the 1,350,000 heaters men-
tioned above as averaging 12 pound-smoke units were cleaned regularly
and adjusted so as to cut the smoke output down to an average of 8, the
total smoke output of the community might be cut to less than half its
present amount.

It should be pointed out that the smoke nuisance will not be elimi-
nated until it is feasible to keep the smoke production of all heaters
below the limit of visibility. This limit is from 3 to 5 pound-smoke units
at a burning rate of 5 pounds an hour. If a pound-smoke unit be con-
sidered equal to 1 gram of smoke carbon, such a limit would still allow
smoke particles to be discharged from an orchard heater at the rate of
0.4 grams a minute at the above average burning rate.

Operation Methods for Reducing Smoke Output. — It is evident that
considerable improvement in smoke production may be obtained by
grower cooperation without the necessity of turning to other fuels or the
purchase of large quantities of new and expensive equipment.

15 Unpublished data furnished to the Orchard Heating Improvement Com-
mittee by Floyd D. Young, Senior Meteorologist, United States Weather Bureau.

Bul. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS

37

The most important suggestions for growers are :

1. Not to burn oil in open pails and other obsolete heaters such as
most of those manufactured prior to 1915. Figure 6 shows the perform-
ance of heaters in this class.

2. Clean soot from stack and drafts, particularly of heaters with
combustion chambers or louvered stacks. Figure 21 shows how drafts
and stacks become clogged. Frequently stacks become sooted up during
the warming-up period.

Fig. 21. — Photographs of soot collection on covers and stacks of orchard heaters:
A, on underside of cover of heater No. 1, National Jumbo Cone; B, in throat of
heater No. 1, National Jumbo Cone; C, in throat of heater No. 22, Citrus 15-inch
stack; D, around thimble in heater No. 32, Hy-Lo 1929 model.

3. Regulate heaters systematically so as to maintain the best com-
bustion rate for each type of heater. Figure 22 shows how smokiness
varies as burning rate is changed. Most heaters smoke more at high
rates.

38

University of California — Experiment Station

4. Study the recommendations of the Fruit Frost Service of the
United States Weather Bureau with regard to temperatures at which
to begin to light heaters. Have an adequate supply of tested and
properly sheltered thermometers. Be careful not to burn more oil than
is necessary to maintain safe temperatures.

Fig. 22. — Photographs of smokiness at different burning rates: A, heater No. 1,
National Jumbo Cone; B, No. 7, Citrus, Olsen Stack; C, No. 26, Citrus, high stack;
D, No. 25, Hy-Lo, single short stack, square bowl.

TEST RESULTS WITH DIFFERENT FUEL OILS

The Fruit Growers Supply Company has found that it is not feasible
to place rigid specifications on the oil used for orchard heating. Growers
have storage facilities for 35,000,000 gallons, which is only a little more
than enough for one filling of the heaters. Therefore they have to buy
on short notice grades of oil normally carried in storage at the refineries.

BUL. 536] SMOKINESS OF OlL-BlTRNING ORCHARD HEATERS

39

The oil formerly used was a straight-run distillate of from 32° to
34° Baume gravity. At present no regular cut is made between Diesel
engine fuel oil of 27°+ and kerosene base of 38° to 40° Baume gravity.
Most growers use the Diesel engine fuel. During rush periods it is the
only satisfactory fuel oil available in large quantities because kerosene
base is not normally carried in storage by the refineries. However, there
is some opportunity to choose between various lots of Diesel fuel. The
specifications ordinarily used are :

1. Sulfur content less than 0.75 per cent (some samples have shown

as high as 3 per cent but the normal content is about 0.25 per
cent).

2. Carbon residue less than 0.50 per cent.

3. Pour point below 15° F (some samples have run from 30° to

40° F pour point) .

TABLE 2
Source of Oils Used in Standard Tests

Oil No.

Description and source

Amount used

1
2
3
4

5

Kittle fuel as used in Los Angeles County (Pomona)

Orchard heater fuel used in Los Angeles County (Pomona)

Orchard heater fuel— 30 gravity— Woodland

Orchard heater fuel — 30 gravity — Cooks Oil Co., Emeryville. Delivered

from Suisun, California

Orchard heater fuel — 35 gravity — Cooks Oil Co., Emeryville. Delivered

Sample
Sample
100 gals.

100 gals.

35 gals.

6

7

Orchard heater fuel — 30 gravity — Cooks Oil Co., Emeryville. Delivered by
Sheldon Oil Co., Suisun

Orchard heater fuel — 30 gravity — Cooks Oil Co. Shipped from Emeryville,
9-9-31

100 gals.
200 gals.

TABLE 3

Analyses of Heater. Oils*

Flash

point

(Cleveland

Open

tester)

Viscosity
(Say bolt

Universal
100° F)

Distillation test

Pour
point

Carbon

residue

(Conradson

method)

Sulfurt

Oil
No.

10 per cent
over at

90 per cent
over at

End-point

(Parr
Sulfur
Bomb)

°F

sec.

°F

°F

op

°F

per cent

per cent

1

149

36

395

495

510

Below 12

0 047

0.62

2

230

43

435

615

627

Below 12

.301

57

3

158

33

360

490

515

Below 12

.020

.26

4

195

39

405

620

632

Below 12

.160

.43

5

222

40

444

616

630

Below 12

.075

.47

6

190

39

400

600

622

Below 12

.283

.49

7

221

43

438

640

652

Below 12

0.028

0 53

* Analyses reported by H. W. Allinger, Division of Chemistry,
t Corrected for NaCl occlusion.

40

University of California — Experiment Station

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3.98
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BUL. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS

41

Analysis of Oils Used in Tests of Standard Heaters. — In testing the
smokiness of standard heaters an average grade of 30° Baume gravity
orchard-heater oil was used. It was not possible to reorder the fuel oil
as required and receive exactly the same kind used in the previous tests.
Table 2 shows the source of the oils used in the standard tests and table 3
shows the analyses. Oils 3, 4, and 6 have been used in most of the tests
except for the Kittle and Fugit heaters in which oil No. 5 was used.
Kerosene was burned in a few tests (see fig. 8) and was shown to produce
considerably less smoke than ordinary orchard-heater oil.

It was noted in connection with the tests of standard heaters that
when return was made to a given burning rate after operating several
hours at various other rates, the smokiness reading was approximately
the same as earlier in the run, indicating that smokiness was not changed
appreciably as a result of changes taking place in the oil as burning
progressed.

Analyses of Oils Before and After Burning. — Specific data on the
changes in two oils after burning are shown in table 4. Four heaters
were used and the oils analyzed before and after burning down to resi-
dues of approximately 0.2 to 0.4 of the original weight. The composition
changes in the oil as a result of burning were marked but were less than
the natural differences occurring between the various fuel oils available
to growers. It is to be noted that the original pour points (-10° F and
+15° F) of the two oils both rose to 35-40° F, and the initial boiling
points rose on the average 36° F and 51° F, respectively. Furthermore,
the percentages of wax and Conradson carbon residue were increased to
a greater value than can be accounted for by the reduction in oil volume
due to burning.

TABLE 5
Source of Oils Used for Testing Influence of Oil Character on Smokiness

Lot

Gravity,

No.

° Baume

Source

20

30

Blended from Standard Oil Co. 27°+ and 32°+, Cooks Oil Co., Emeryville

21

32+

Standard Oil Co., El Segundo

22

27+

Standard Oil Co., El Segundo

23

27+

General Petroleum Corporation

24

27+

Shell Oil Co.

25

30+

St. Helen's Petroleum Corporation

26

27+

Union Oil Co.

Analyses of Oils Used for Testing Influence of Oil Character on
Smokiness. — Six lots of oil were furnished by the Fruit Growers Supply
Company for testing the influence of the chemical and physical char-
acteristics of the fuel oil on smokiness. These were compared with oil

42

University of California — Experiment Station .

TABLE 6
Analyses of Oils Used for Testing- Influence of Oil Character on Smokiness

Flash
point
(Cleveland
Open
tester)

Viscosity

(Say bolt

Universal

100° F)

Engler distillation test*

Pour

point

Carbon
residue

(Conrad-
son

method)

Oil

No.

10 per cent

at

90 per cent
at

End

point

Sulfurf

20t

°F
181

sec.
39

°F
382

°F
580

op
620

Below 12

per cent
0 305

per cent
0 209

21

180

36

380

560

600

Below 12

.027^

.125

22

228

44

450

590

600

Below 12

.0341

.188

23

243

49

485

650

680

30

.375

.151

24

206

37

420

540

550

Below 12

.081

.433

25

245

44

475

630

645

12

.028

.298

26

190

37

410

560

600

Below 12

0.213

0.302f

* On distilling No. 23 the condensed distillate congealed between 80 and 90 per cent, and on No. 25
between 86 and 90 per cent.

t Sulfur determined from oxygen bomb rinsings following calorific valve determinations at Berkeley.
The sulfur on No. 26 is an estimate made from part of the rinsings.

t Analyses made by H. W. Allinger.

1 Carbon residues analyzed by W. B. Dye.

TABLE 7

Vacuum Distillation Range and Heat Values of Orchard-Heater Oils

Nos. 20 to 26

Per cent

No. 20

No. 21

No. 22

No. 23

No. 24

No. 25

No. 26

over

Vapor temperature, °F

(pressure 10

mm mercur:

/ absolute)

Start

108

125

173

185

160

170

156

5

160

160

228

234

194

232

184

10

180

176

244

254

204

250

194

20

212

192

260

284

216

268

206

30

234

208

270

304

226

278

216

40

250

220

279

320

235

290

224

50

258

230

290

338

248

300

234

60

264

246

308

*

257

316

242

70

275

254

328

*

268

336

254

80

312

280

*

424

280

362

276

90

376

322

406

470

302

398

324

95

440

*

444

518

326

434

352

End point

476

416

468

535

365

462

418

Per cent recovery

98

98

Heat values as B. t. u. per lb. (to an accuracy of ±25 B. t. u. per lb.)

000 19,200 18

19,050

19,300

,700

19,100

* Determination uncompleted.

Bul. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS

43

No. 20, which was used for most of the tests run during the winter of
1931-32. Table 5 shows the source of these oils.

These oils were carefully analyzed by the Division of Chemistry,
according to the standard methods of the American Society for Testing
Materials. The vacuum distillation range and heat values were deter-
mined by the College of Engineering. Table 6 shows the results of the
oil analyses. Table 7 (plotted in fig. 23) gives the vacuum-distillation
data and heat values.

1

|
Is

I

eel// S'

<?£*&*

£2

/o

JPO JO 40
0/ST/LL/IT/Off,

50 60 70

Per Cenr Over.

SO 90 /OO

Fig. 23. — Vacuum distillation range of orchard-heater oils 20 to 26 inclusive

(see table 7).

Method of Testing Influence of Oil Character on Smokiness. — Tests
were run using heater No. 9, Citrus Regular, as typical of a smoky heater
and No. 32, Hy-Lo 1929 Model, as typical of a relatively smokeless
heater. These heaters were chosen because in the standard tests their
smokiness had been exceptionally steady, which is of considerable ad-

44

University of California — Experiment Station

vantage in determining small differences between fuels. The procedure
in running the tests was as follows : Heaters were cleaned thoroughly
between tests. Heater bowls were filled uniformly to a level 3 inches

«

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Fig. 24. — Influence of oil character on smokiness of heaters No. 9 and No. 32.

from the top at the start of each test. Each test included four burning
rates. At the beginning of a test the heater was warmed up by burning
at a high rate and then reduced to a rate of 3 pounds an hour or less.
When the burning rate had become steady twenty-five or more readings

BUL. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS

45

of smokiness and weight change were taken at one-minute intervals.
Then the drafts were opened, and after conditions became steady the
tests were repeated for each of the three other burning rates.

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Fig. 25. — Smoke-test observations of heaters No. 9 and No. 32, burning oil No. 21

Variation of Smokiness Not Consistent with Oil in Different Heaters.
— Figure 24 shows the results of the oil studies. The curves are based on
averages of ten consecutive smokiness determinations for each burning
rate. Figure 25 indicates the spread of the individual smokiness deter-

46 University of California — Experiment Station

urinations for oil No. 21. This figure shows results typical of those ob-
tained with the other oils. The scale of burning rate used in figures 24
and 25 is twice that used in reporting the standard tests. The oil test
results indicate some difference between fuels, especially with heater
No. 32, in which the smokiness from the best oil is about half that from
the poorest. This degree of variability is present over the full range of
burning rates but the individual oil curves cross so that there is no con-
sistent variation between any two oils. This difference has little real
value as the worst smoke was scarcely visible from this heater. When
the normal spread of individual determinations is taken into considera-
tion it can scarcely be said that the character of the oil, within the range
shown by the analyses, had a significant influence upon the smokiness of
either of the heaters used in these tests. This is especially true if cross
comparisons are made between the two heaters. Oil No. 20, which gave
the least smoke in heater No. 9 at low burning rates and the most smoke
at high rates, was about average throughout the whole range in heater
No. 32. Oil No. 25, which was the best in heater No. 32, was about
average in No. 9.

Conceivably the divergence of the curves for heater No. 9 at low
burning rates and the departure of several oils from the general per-
formance curve for this heater as shown in figure 8 are partially caused
by different rates of soot accumulation. Observations in the field and
laboratory indicate that new heaters or ones which have been very
thoroughly cleaned do not display their normal smokiness immediately
after lighting. Probably, then, the warming-up period before the low-
burning-rate tests were started caused different degrees of soot forma-
tion with different oils. The only general conclusion which seems jus-
tified is, that the smokiness of a great variety of heaters cannot be mate-
rially reduced by placing more rigid specifications on oil than those now
in use.

TESTS ON STACKS OF NEW DESIGN

As soon as it became apparent that smoke-abatement ordinances
would be adopted in some counties, the manufacturers of heaters began
intensive study of stack design. The tests on standard heaters showed
the need for improvements and in some instances suggested lines of
procedure. The availability of test apparatus enabled manufacturers
to make a systematic study of stack design in relation to smokiness.

No new heaters were studied in the laboratory, for it was felt that
the citrus industry could best be served by aiding the development of
stacks designed primarily for the improvement of heaters already in

BUL. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS 47

use. Manufacturers have made frequent visits to the laboratory to study
stacks. More than sixty different stacks have been tested over the
normal range of burning rates to determine their smokiness, and many
others have received single-point tests to indicate whether or not further
study might be desirable.

Requirements of a Good Orchard Heater. — The problem of develop-
ing simple new heater stacks is not easily solved. Smokelessness is only
one of many requirements all of which a heater must meet as closely as
possible in order to be useful. According to the generally accepted ideas
as to the more important specifications of a heater, it should :16

1. Hold sufficient fuel to burn all night without refueling, even

though 7 pounds or more of oil an hour be burned at times.

2. Be capable of sufficient regulation to give its greatest heat just

before sunrise even though the fuel in the reservoirs, is low by
this time (ordinary burning rates are from 3 to 5 pounds an
hour) .

3. Be able to burn any of the ordinary grades of heating fuels on the

market without smoking and without leaving a heavy residue.

4. Be rain-tight.

5. Deliver the heat and products of combustion near the ground, but

without heating the ground unnecessarily.

6. Be easy to light and regulate by inexperienced labor under all

weather conditions.

7. Be capable of being lighted with the draft opening set as required

for normal burning.

8. Be readily extinguished by merely closing the regulator and cap-

ping the stack.

9. Be arranged for filling and for soot removal without taking off

the stack or cover.

10. Be so designed that if it burns dry the bottom of the heater will

not be damaged.

11. Avoid oil condensation on the stack or cover.

12. Be of reasonable cost.

13. Be made of good material and show small annual depreciation.

14. Be easy to take apart, clean, and store.

Interpretation of Test Results on New Stacks. — All of the standard
heaters tested were known to have reliable burning characteristics in the
field. An observer could also notice differences from usual operation if
for instance a cover happened to be loose. However, in testing a new

16 Schoonover, Warren R., Robert W. Hodgson, and Floyd D. Young. Frost pro-
tection in California orchards. California Agr. Ext. Cir. 40:1-73. 1930.

48

University of California — Experiment Station

design one cannot be familiar with its normal characteristics, and of
course a new stack has not been subject to the ordinary orchard operat-
ing practice. The new stacks have been tested for smokiness only. A
season's use in an orchard might increase the smoke output. Field trials
alone can prove their practicability for general orchard use. However,
the great improvement in test results of most of the new designs in com-
parison with the standard heaters is so encouraging that it appears
advisable to report the results in spite of the limitations.

Not all of the new experimental results can be reported, but the
smokiness of the better stacks developed in each group is shown in figures
27, 28, 29, 31, and 32. These are so much better than most of the standard
heaters that smokiness and burning rate are shown on scales twice as

A

B

T)

G

Fig. 26. — Photographs of new smokeless stacks for orchard heaters: A, No. 52,
Lamco Gyradiant; B, No. 60, Hy-Lo Giant; C, No. 62, Hy-Lo Drum; D, No. 80,
Hinchcliff 36-inch; E, No. 90, National Junior Louver, 18-inch; F, No. 91, O'Keefe
and Merritt, 6-inch straight stack ; G, No. 148, Hy-Lo tapered stack.

large as before. The photographs in figure 26 show a number of the new
stacks. All types have been studied including simple, slip-on stacks for
use with the Citrus heater to stacks much larger than any heretofore in
use and giving complete combustion within the stack over a wide range
of burning rates.

Smoke Tests of Annular-Combustion-Chamber Stacks. — Figure 27
shows the smokiness of stacks which, have large annular or ring-shaped
combustion chambers. In these stacks the gases rising from the bowl
come into contact with excess air at the base of the stack. Conditions in
the burner at the bottom of the stack are favorable for complete and
practically smokeless combustion. Because this involves high tempera-
tures the stack must be made of high-grade material. A large surface
for radiation serves to deliver considerable heat close to the ground.

Smoke Tests of Enlarged-Combustion-Chamber Stacks. — Figure 28
shows the smokiness of three models of Hy-Lo stacks with medium-sized

BlTL. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS

49

s:

I

No.se

oclU No. 60

3 4 S 6

8Vm//VG P/ITE- Pounds per Hour

Fig. 27. — Smoke tests of annular-combustion-chamber stacks, Nos. 52, 60, and 100.

peuM

3 4 S 6 7

3£/GN/ffG P/)7£ - Poarc/s per ftour

Fig. 28. — Smoke tests of enlarged-combustion-chamber stacks, Nos. 1, 61, and 62.

50

University of California — Experiment Station

combustion chambers. Their prototype, the National Jumbo Cone, is
also shown for comparison. These heaters are similar in burning char-
acteristics to the heaters with larger ring-shaped combustion chambers

J 4 S 6 7 0 9

BU&MMG GATE - Po ana's per rfoar.

Fig. 29. — Smoke tests of straight tall stacks on Citrus bowls : stacks 26D, 45,

80, and 81.

and are excellent at burning rates permitting completion of combustion
in the burner or lower part of the stack. The smokiness increases greatly
when the capacity of the burner is exceeded. Stack No. 61A had a

BUL. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS

51

smaller stack diameter at the top than No. 61 and although the burner
was the same the performance was not as good, and this experimental
design was dropped.

These stacks are all of the lean-mixture type requiring warming up
before being adjusted to normal burning rates.

Smoke Tests of Straight Tall Stacks. — A number of attempts have
been made to improve the Citrus heater by equipping it with some sort
of a tall stack. Figure 29 shows the results of such attempts. The best

jo 40 so 60 ro

T/MF AFTES 5T/1£T//VG TEST, Af/nufes.

Fig. 30. — Continuous smoke test of 6-inch Exchange stack on Citrus bowl.

results were obtained with a 36-inch top section placed on the 15-inch
stack of heater No. 26 and with the Hinchcliff stack of 36-inch length.
With these stacks combustion is practically completed in a limited por-
tion of the lower stack much as in the enlarged-combustion-chamber
stacks. The straight stacks lack the large radiating surface of the latter
and develop local hot spots. Field experience will be required to deter-
mine if the stack life is long enough to make them practical.

The 6-inch exchange stack is even more erratic on the Citrus bowl
than on the round bowl with a down-draft tube. Curves show clean and

52

University of California — Experiment Station

dirty runs and checks indicate cold-run smokiness. The peculiar per-
formance of this stack is well illustrated in figure 30, which shows a con-
tinuous record of the cold-weather run with smokiness plotted at minute
intervals. A periodicity in smokiness peaks is indicated at all burning
rates.

o

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&L/eWrtG /2/irf - Povoc/s per //our.

Fig. 31. — Influence of bottom air openings on smokiness of open-flame stack,

No. 91A.

Principles of Combustion in Open-Flame Stacks. — Combustion in a
good open-flame stack should be similar to that in an ordinary Bunsen
burner. That is, sufficient air for a good air-fuel ratio should be mixed
thoroughly with the combustible gases at the base of the stack and the
flame should be at the top. If anything causes the fire to "strike back"

Bul. 536] Smokiness of Oil-Burning Orchard Heaters

53

and burn at the air-mixing holes the gases are cracked and the smokiness
is greatly increased. In a Bunsen burner using gas under pressure the
normal blue flame changes when "striking-back" occurs to a yellow

3 4 S 6 7

BUPN/NG PATE- Pounds per Hoar.

Fig. 32. — Smoke tests of new open-flame stacks on Citrus bowl.

somewhat smoky flame. In a heater burning oil vapors under natural
draft the change is from a yellow smokeless flame to a reddish-colored
very smoky flame.

54 University of California — Experiment Station

Apparently it is relatively easy to design an open-flame stack which
will burn without smoke at a single fixed burning rate, but very difficult
to design one which will be satisfactory over the whole range of burning
rates desired by citrus growers. The problem of air mixing is not simple.
If adequate air is admitted for complete combustion at high burning
rates it will be too much for low burning rates. The mixture then will
be very lean and there will be a tendency for the fire to drop below the
air-intake zone and burn the rich oil vapors in the collar, thus producing
a smokiness like that of the old smudge pots. A lean mixture, further-
more, tends to burn explosively, which may cause the generating fire in
the bowl to blow out. If the air intake is correct for low burning rates
it will be insufficient for higher burning rates and the stack will show
characteristics similar to early models of lazy-flame heaters such as Nos.
4, 20, 23, and 30. Figure 31 shows the effect on smokiness of changing
the bottom air openings,

Smoke Tests on New Open-Flame Stacks. — The manufacturers have
worked constantly to develop successful stacks of this general type, with
results which have sometimes been exceptionally promising. Figure 32
gives data from tests on various experimental models. The repeated
efforts made in the development of stacks of this general type are indi-
cated by the progressive lowering of the smoke curve. The data from all
the tests on new open-flame stacks are now being used by manufacturers
as the basis for developing the models they expect to sell.

The new production models have not been tested, but tests on experi-
mental stacks indicate the possibility of making open-flame stacks the
smoke of which is scarcely visible over burning rates of from 2.5 to 6
pounds of oil an hour. It also seems probable that such stacks can be
both practical and inexpensive.

FIELD MEASUREMENT OF THE SMOKINESS OF ORCHARD HEATERS

During the early winter of 1931 field measurements were made with
the electrical precipitator in connection with a portable smoke-collecting
device mounted oh a truck. The portable apparatus was similar to
the laboratory apparatus in all basic principles with the omission of
the light-interception parts. One per cent of the total smoke discharged
was passed through the precipitator while burning % pound of fuel.
Although dependable results were obtained, the method is not well
adapted to field studies, for it is cumbersome and expensive. Technically
trained operators are required.

BUL. 536] SMOKINESS OP OlL-BuRNING ORCHARD HEATERS

55

Visible Records of Smokiness. — In the course of field, demonstrations
conducted by the Agricultural Extension Service, visible records oi
relative smokiness were made by pulling 1 per cent of the total smoke
stream for 1 minute through a 5-inch square of white filter paper. This
method was very effective in demonstrating smokiness and it appeared
to offer possibilities of development into a simple, accurate, and portable
means of measuring orchard heater smoke.

After experimenting in the laboratory, it was found that satisfac-
tory records could be made on a white filtering felt, such as is used in
some automobile air cleaners. The smoke sample was collected on a felt

Fig. 33. — Felt sampling apparatus. The felt is held in the square frame between
the two rectangular funnels (to the right of the manometers).

9 inches square with %-inch margin, giving an exposure of 56.25 sq. in.
This size was chosen because preliminary studies indicated that this area
was sufficient to permit taking the sample while using the suction
normally developed by the centrifugal fan. (See fig. 33.) A large num-
ber of tests were run to determine the sensitivity of the method and also
the amount of smoke which could be collected on the felt in order to give
good comparisons between all of the heaters to be tested. The whiteness
of the felt, the depth of penetration of the soot, the degree of dispersion
of the carbon, the presence of filterable vapors, etc., influence the black-
ness of the deposit. It finally appeared that a satisfactory record would
be obtained by collecting on this white felt the soot from 0.1 per cent of
the total smoke produced by burning 1 pound of fuel. Such a record
shows a light gray with the good heaters and still is not too black with

56 University of California — Experiment Station

the bad heaters for accurate calibration of the smokiness. For the very
best heaters it is advisable to collect 0.5 per cent and for the worst 0.05
per cent in order to obtain measurable records.

The grayness of these felt samples can be measured electrically by
reflected light with the same equipment used for the light-interception
method in measuring the opaqueness of smoke. However, the standard
color wheel is more commonly used because of its simplicity. The nature
of the felt smoke records and the method of grading them by the color
wheel are illustrated in figure 34. The color wheel used has one disk
covered with black velvet and the other with a piece of the same white
felt material used in making the tests. The areas of black and white
exposed can be varied, and when the wheel is spun to blend the black

Fig. 34. — Photographs of color wheel (stationary and running) used to
measure blackness of felt sample.

and white into a gray, an exact match can be obtained from any smoked
felt sample. The relative smokiness is expressed in the terms of the per-
centage of black required.

Correlation with the Light-Interception Method. — This felt method
provides another quantitative means of evaluating the relative smoki-
ness of orchard heaters. Furthermore, one can compare the results with
tests made with the light-interception method and thus interpret them
in pound-smoke units. The correlation between percentage of blackness
and pound-smoke units is shown in figure 35. The curve is based on
simultaneous determinations of smoke opaqueness and sample collection
by the felt. Opaqueness readings were taken at 5-second intervals while
the smoke was being drawn through the felt and burning rates were
determined on the automatic balance. The curve indicates a very high
degree of correlation between the smokiness as determined from per-
centage of blackness on the color wheel and the smokiness as determined
in pound-smoke units by the light-interception method. This is to
be expected, for both methods depend upon the quantity and black-

Bul. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS

57

ness of the soot. If the accuracy of the color-wheel determinations is
not required, quick and dependable comparisons of the grayness of
the felt smoke records can be made with properly prepared gray color
standards.

According to the correlation shown in figure 35, the very best heaters
would produce felt smoke records less than 30 per cent black, or a smoki-
ness under 3 pound-smoke units. A good heater, that is, one which would
give off smoke scarcely visible to the eye at a 5-pound burning rate,

s

&

\

/

J*1

°

1

Very Best tteoterj

Good tteoters

Far tieoters

Smoky fieoters

8od 1 tieoters

^ c ^

/o

0

1

>» ^

y~

1*

/

t*

S

0

& ^

ntf

1 *

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Jfi*

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§

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£o

o

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*s

^j^-K4 —

JO 40 JO 60

0LACf<rtE55 Of r~£irt Per Cent 3 toe A.

Fig. 35.

-Correlation between blackness of felt sample and quantity of soot
collected.

would produce records from 30 to 45 per cent black or a smokiness of
from 3 to 5 pound-smoke units. The fair heaters burning under favor-
able conditions would produce records from 45 to 60 per cent black, or
a smokiness of from 5 to 10 pound-smoke units. A smoky heater would
produce records from 60 to 75 per cent black, or a smokiness of from 10
to 17 pound-smoke units. The bad heaters would produce records
blacker than 75 per cent, or above 17 pound-smoke units.

The above grades of blackness are only slightly different from the
Ringelmann chart, which has been adopted by many cities as the of-
ficial standard of comparison for the enforcement of smoke abatement
ordinances.17

!7 Anonymous. A digest of smoke ordinances in American cities and Canada.
Power 74:447-482. 1931.

58

University of California — Experiment Station

Use of Felt Method for Smoke Measurements in the Field. — The felt
method appears to meet the requirements for use in the field ; it is simple,
inexpensive, and accurate. All the apparatus required can be mounted
readily on a light truck capable of being driven into orchards while
frost-protection operations are in progress. (See fig. 36.) Inexperi-

Fig. 36. — Field apparatus for measuring orchard-heater smoke.

enced operators can easily be trained to collect dependable records of
the smokiness of orchard heaters. This type of apparatus is also suit-
able for manufacturers who wish to carry on studies leading to heater
improvement.

ACKNOWLEDGMENTS

This project received very effective cooperation from many persons
outside the Experiment Station. Thanks are due particularly to the
Orchard Heating Improvement Committee, under the chairmanship of
Mr. B. R. Holloway, for its personal attention and the equipment
furnished. The California Fruit Growers Exchange has also been gen-
erous in supplying technical information and several lots of fuel oil.
Mr. Floyd D. Young of the United States Weather Bureau rendered
considerable assistance in starting the project and Mr. F. T. Seabern of
Pomona very kindly arranged for the experimental field work to be done
in his orchard.

BUL. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS 59

APPENDIX A: DESCRIPTION OF APPARATUS AND TEST METHODS

Equipment for making smoke-density determinations on the light-
interception basis as developed and built in the laboratories of the Agri-
cultural Experiment Station is shown in figure 4. The work of testing
standard heaters was completed before the apparatus was remodeled to
increase its sensitivity in determining both opacity and burning rate.

Fig. 37. — Automatic self-balancing scales used for determining burning rate.

The details of the original equipment need not be given ; for all of the
results reported were developed on apparatus similar in all essential
details to that described below.

The heater being tested is placed on scales under the hood at the ex-
treme right hand. The scales (fig. 37) are self -balancing ; the beam car-
ries one end of a chain (like a ''Chainomatic" balance) and a mercury
contact which operates an auxiliary device while in the low position to
raise the other end of the chain, thus lightening the beam load. To in-
crease the sensitivity of the scales an electro-magnet in series with an

60

University of California — Experiment Station

ordinary light flasher causes the beam to oscillate regularly. Since the
frequency of the contact period is constant, its duration is what regu-
lates the winding-up device. This duration of the contact period is
governed in turn by the average position of the beam, thus keeping the
scales in perfect balance. A vernier slide attached to the winding-up
mechanism indicates accurately the loss of fuel weight and hence the
burning rate.

Vrho/es*.

tbncentrofea n/a

\-S/Je /vt>es-/

to mi Ih vo/t me/er

Jjus/ol/e //gf>f socket
'Sec/ion of smoke s/ocA

Fig. 38. — Diagram of light-interception apparatus for measuring opaqueness

of smoke.

A centrifugal fan at the extreme left draws the entire smoke stream
with some surrounding air into the hood and up through a 6-inch stack
into a 10-inch horizontal pipe. The sudden change of section and the
right angle turn serve to mix thoroughly the smoke particles and air,
giving a stream of uniform opaqueness under steady burning condi-
tions. The smoke stream is then flattened to 41/4 inches and spread
horizontally to 24 inches where it crosses the path of the light beam.
Small air holes in the 4-inch pipes enclosing the light beam, drilled near
the channel wall allow just enough additional air to seep in to prevent

Fig. 39. — Felt strip used to check uniformity of smoke distribution and
length of light path through the smoke.

the smoke from swirling out into the light tubes. (Glass screens could
not be used to confine the smoke stream because they would become
coated with soot, ) The plan of construction of the light tube is shown in
figure 38. The sharpness of boundary and uniformity of smoke spread
was tested by a felt strip left in the line of light just long enough to
become gray with soot. (See fig. 39.)

For a light source, a standard Balopticon 1,000 watt lamp and its
concave mirror is used. On the far side of the smoke stream a condens-
ing lens focuses the parallel rays on a radiation pyrometer (fig. 40).
The voltage impressed on the source lamp is held constant at the low

BUL. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS

61

value of approximately 65 volts, which produces a pyrometer reading
of 15 millivolts with clear air. This setting is checked before and after
test. The opaqueness of the smoke is indicated by a decrease in the
electromotive force (e.m.f.) of the pyrometer and is measured in con-
centration units.18 Each successive concentration unit represents a re-
duction of 10 per cent of the original light intensity. That is, one con-
centration unit reduces the millivolt reading to 13.500, two units to

Fig. 40. — End view of laboratory apparatus for measuring smoke, showing
radiation pyrometer at the end of the light path (see arrow at the right).

12.150, three units to 10.935, etc. The smoke-stream temperature at the
light beam is noted and the value in concentration units is then increased
by the correction for temperature to a standard-density basis.

From the light, the smoke stream passes into a large drum 18 inches
in diameter through a honeycomb of %-inch tubes 3 inches long and
slowly approaches the orifice plate. Three orifices are provided : one
large central orifice (3-inch diameter) for measuring total flow and two
fractional orifices to obtain 1 per cent samples for carbon collection, the

is Simon, A. W., L. C. Kron, C. H. Watson, and H. Eaymond. A recording dust
concentration meter Eev. Sci. Instruments 2:67-83. 1931.

62 University of California — Experiment Station

first by electric precipitator, and the second by a Buchner funnel for a
chemical analysis or by a felt for color-wheel comparison. The pressure
drop across each small orifice is balanced exactly with the main orifice
by butterfly valves some distance downstream. A definite fraction of
the total flow can be obtained unaffected by the temperature or pressure,
which apply alike to the three orifices.19 Calibration of the main orifice20
was accomplished by using a special elliptically rounded approach
nozzle with a Venturi expanding cone.21 The flow through the frac-
tional orifices was measured in a large displacement tank with a stop
watch by noting the rate of discharge of water, which was regulated to
balance the manometer between the downstream taps of the large and
small orifices. Both the pressure differential and temperature are ob-
served at the orifice for the calculation of air flow.

The radiation pyrometer e.m.f. readings are taken at definite and
frequent intervals so as to give a practically continuous record of rela-
tive smoke density at the point of observation. The average value of
opaqueness, suitably corrected for temperature (expressed as smoke-
concentration units), multiplied by the number of thousand cubic feet
of air (standard conditions) pulled through the stack while burning a
pound of fuel gives a convenient smokiness unit. This quantity unit,
which has been designated heretofore as a "pound-smoke" unit, al-
though not actually weight, is comparable with a mass unit per pound
of fuel burned.

A typical calculation is as follows :

Opaqueness (average of 6 readings) = 13.55 millivolts

= 0.965 concentration units

Thermocouple (average reading at light _ 218 millivolts
path)

= 134° F (calibrated)

Temperature correction to standard air _ 458 + 134

conditions (32° F) ~ 490

Therefore, opaqueness (corrected for

temperature)

= 1.165 concentration units

/458 + 134\
0.965 ( j^ ) concentration units

lo Hodgson, John L. The laws of similarity for orifice and nozzle flows. Amer.
Soc. Mech. Engin. Trans. 51 (FSP) :303-332. 1929.

20 Bean, H. S., E. Buckingham, and P. S. Murphy. Discharge coefficients of
square-edged orifices for measuring the flow of air. Bur. Standards Jour. Research
2:561-658. 1929.

2i Schiller, Ludwig. Hydro- und Aerodynamik. In: Wien, W., and F. Harms.
Handbuch der Experimentalphysik 4(1):581. Akademische Verlagsgesellschaft.
M.B.H., Leipzig, Germany. 1931.

BUL. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS 63

Differential pressure (drop across = ^ incheg water

orifice)
Thermocouple (at orifice) = 2.04 millivolts

Therefore, differential (corrected for
temperature as above)

= 128°F

= 3.95 inches

And the air flow?2 = 127 V 3.95

= 252 cubic feet per minute (standard
conditions)
Change of fuel weight = 0.35 pounds in 6 minutes

Therefore the burning rate = 3.5 pounds per hour

— 17.15 minutes per pound

The total air flow per pound of fuel i^-ir v Oro i • -p +

1 l 17.15 X 252 cubic feet

burned is therefore

= 4.32 thousand cubic feet

Therefore the smokiness = 1.165X 4.32 pound-smoke units

== 5.02 pound-smoke units.

In this way it is possible to calculate, on the basis of fuel consumed,
the relative smokiness of different heaters at different burning rates
without error due to sampling or to admixture of outside air with the
products of combustion.

APPENDIX B: CORRELATION BETWEEN LIGHT INTERCEPTION AND
WEIGHT OF SMOKE PARTICLES

In order to secure data for determining the weight of carbon per
pound of fuel burned corresponding to one pound-smoke unit, a series
of chemical determinations was run. Smoke-density readings on the
millivolt meter and the necessary data for calculating air flow were
taken simultaneously. In making the chemical determinations during
the first part of the investigation gas samples were withdrawn from the
smoke chamber through an M-shaped sampling tube introduced just
downstream from the point of making the opaqueness determinations.
Small holes approximately yiG inch in diameter were drilled in the
sampling tube at right angles to the direction of gas flow. The suction
for removing the gas samples was created by allowing water to flow
from a steel oil drum mounted on scales, The rate at which the water
discharged from the barrel was controlled to collect the gas sample con-

22 127 is the numerical value of the coefficient of discharge and dimensional
factors of the appropriate air flow formulas given in the Bureau of Standard
Journal of Research. (Bean, H. S., E. Buckingham, and P. S. Murphy. Discharge
coefficients of square-edged orifices for measuring the flow of air. Bur. Standards
Jour. Research 2:561-658, 1929.)

64 University of California — Experiment Station

tinuously while burning a known weight of fuel. The volume of gases
(approximately 6 cubic feet) withdrawn from the smoke chamber was
calculated from the weight of the water which ran from the barrel. The
gas volumes were reduced to standard conditions except that the correc-
tion for water vapor was not applied.

Later chemical determinations were based on the 1 per cent sample
taken through one of the small orifices described above.

For making a carbon determination of a sample, the flue gases were
passed through a filter mat of shredded asbestos supported in a 130-mm
Buchner funnel. The weight of smoke absorbed on the filter was not
determined directly ; instead, its carbon content was found by combus-
tion and weighing the carbon dioxide. The Division of Chemistry co-
operated effectively in establishing test procedure, in making all chemi-
cal determinations, and in interpreting the data. Standard analytical
methods were used with additional refinements which made certain that
the variations observed resulted entirely from the test heater and not
from the method of determination. This method gives the weight of
carbon in the absorbed smoke, from which can be calculated the meaning
of 1 pound-smoke unit in terms of weight of carbon per pound of fuel
burned.

The smoke analyses show a mixture of carbon and unburned car-
bonaceous matter. Some of the smoke particles are practically pure
carbon and some are rather oily. This difference in character of the
smoke was determined by taking a second sample on another filter while
maintaining as nearly as possible the same burning conditions. The oily
matter was extracted by ether, the ether driven off, and the remaining
carbon determined as before by combustion.

For the fuel oils used in the tests of standard heaters, figure 41 gives
the results of the chemical determinations compared with pound-smoke
units. It will be seen that the value per unit is not constant but varies
between 0.81 grams and 1.58 grams per unit with an average value of
1.00 grams of carbon in the smoke for each pound-smoke unit. The
greatest dependable value is 24 per cent above the average and the
smallest 19 per cent below. Forty-two determinations were made, repre-
sentative of three different oils and 21 different heaters operated over
a range of burning rates varying between 3.5 and 15.0 pounds of fuel
per hour. The dotted line drawn on figure 41 shows how the value varied
with one oil in one heater over a range of burning rates. The average
correlation value of 1.0 gram per pound-smoke unit applies to a light
path 24 inches long through the smoke stream. For any different length

BUL. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS

65

the value would change in inverse proportion.23 The above data indicate
that the weight of smoke for the three fuel oils used in testing the
standard heaters might be estimated from light-interception measure-
ments if the accuracy of 25 per cent is satisfactory, but further tests
with seven other fuel oils showed much wider variation, especially at
low burning rates.

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Fig. 41. — Correlation between chemical determinations of total carbon
and pound-smoke units.

This variation may be explained as follows :

1. Weight of soot particles of given density varies substantially as
the cube of the linear dimension of the particles, while the area for light
interception varies as the square. Thus the degree of dispersion of the
smoke particles influences the result.

23 Simon, A. W., L. C. Kron, C. H. Watson, and H. Eaymond. A recording dust
concentration meter Ee v. Sci. Instruments 2:67-83. 1931.

66

University of California — Experiment Station

2. The smoke is not pure carbon. Adsorbed oily matter may increase
the weight of individual particles without increasing their size.

3. The smoke comes from some heaters in distinct puffs. This makes
it difficult to get a correct figure for the average number of millivolts
even when reading at intervals of 40 seconds. The filter sample is taken
continuously and represents an average.

4. Relatively transparent smokes seem to contain colorless carbon
compounds, capable of being collected by the asbestos filters.

In the last three cases the felt method should show slightly better
correlation with weight than would the light-interception method.

Fig. 42. — Electrical precipitator and its connections with sampling orifice
and high-tension transformer.

Additional Studies on Correlation of Smoke Density and Weight. —
The additional data mentioned above as bearing on the correlation
between opaqueness and weight were developed in connection with
studies of the influence of oil composition on the smoke output of a
smoky heater and a typically good heater. Seven different fuel oils of
variable characteristics were used. These data were obtained by the
same chemical methods as before, by use of asbestos filters for collecting
samples, and also by use of an electric precipitator to collect smoke
samples which could be weighed directly and later analyzed for carbon,
hydrogen, and ash content. The precipitator was developed with the
cooperation of the Western Precipitation Company of Los Angeles. It

BUL. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS 67

is a laboratory model, alternating current, Cottrell precipitator24 capa-
ble of handling 2 cu. ft. of smoky gases per minute (see fig. 42). The
precipitating potential is 30,000 volts developed by a small transformer
operating on a 110-volt circuit. Smoke is precipitated both on the cen-
tral high voltage electrode and on the glass container, which is sur-
rounded by a wire-mesh grounded electrode.

Precipitation of the smoke particles carried in the hot gases is visu-
ally complete up to a certain load. With very smoky gases some smoke
may be carried over, especially if too long a run is made. The chemical
studies seem to indicate that light smokes contain heavy molecules, prob-
ably compounds of carbon, hydrogen, and oxygen, which intercept very
little light and are not precipitated electrically but which are adsorbed
onto the asbestos filters. However, for average conditions, the direct
smoke weight data correlate fairly well with weights determined chemic-
ally. The precipitated smoke contains about 10 per cent ash, which
would tend to give higher results with this method because ash was not
determined on the filtered smoke. For extremely light or heavy smokes
the correlation between the chemical and electrical methods is not good.

For the average conditions applying to all the tests of the old heaters
supplied by the committee 1 pound-smoke unit may be considered ap-
proximately equal to 1 gram of carbon in the smoke from 1 pound of
fuel. Examination of the weight data obtained during the oil studies
indicates that the correlation between smoke opaqueness and weight is
not satisfactory at average or high burning rates, only a general trend
being evident. At low burning rates, no consistent correlation was
found. It varies with different oils and with different heaters as well
as with the burning rate, as is pointed out in figure 41. One pound-
smoke unit may represent as little as 0.75 grams or as much as 3 grams
of carbon in the smoke per pound of fuel burned. This indicates that
the pound-smoke unit results, if interpreted in terms of grams weight
per pound of fuel burned, should be considered as minimum values.

24 [Anderson, E.] Cottrell processes of electrical precipitation for removing
suspended particles from gases. Leaflet issued by the Western Precipitation Com-
pany, 1016 W. Ninth Street, Los Angeles.

Simon, A. W., and L. C. Kron. Electrical precipitation. Amer. Inst. Elec.
Engin. Paper 32-32. Eev. in: Elec. Engin. 51:93-95. 1932.

27m-9,'32