solid fueled grove heaters1

GERBER: SOLID FUELED HEATERS 45

5 feet than under more calm conditions. In any

event, very little protection was obtained at 15

feet with 10-14 mph of wind.

The heaters used were basically a convective

type. They were not designed to produce radi

ant heat. Experience with oil fired stack heaters

has shown that more protection will be obtained

on windy night from heaters which produce radi

ant heat than from heaters which produce most

ly convective heat.

The protection obtained under calm condi

tions compares very favorable with protection

from other devices with about the same heat

output per-acre-per hour. Increasing the heat

output, by increasing the pressure increased the

protection, but only slightly. The system pro

vided more protection at 15 feet on calm nights

than at 5 feet. Labor requirements were cer

tainly less than with most any other system;

however, skilled labor was required for opera

tion of the vaporizer. Vaporizer operation must

be carefully supervised. Even with the careful

supervision during these trials, there was at

least one occasion when the gas pressure in the

tank dropped and heat output and protection

also dropped. Adequate vaporizer performance

can hardly be overemphasized.

Summary

A propane heating system was field tested

for 2 seasons. Aside from vaporizer supervision

it appeared to oprate adequately and few prob

lems were encountered. Protection varied from

3 to 5°on calm nights to 0.5 to 2° on windy

nights. This compared favorably with other

convective heaters.

Acknowledgements

The authors wish to express their gratitude

to Mr. Frank Mirth, and Mr. Fred Hutchinson,

whose cooperation and contributions made these

trials possible.

SOLID FUELED GROVE HEATERS1

J. F. GERBER2

Wood, coke, used rubber tires and charcoal

have been used as solid fuels for grove heating

and cold protection, and they were probably the

oldest fuels used (8, 5). Coke and charcoal

usually were used as fuels in heaters or open

trays, but wood and rubber tires were burned

in open fires (2). Wood as a fuel for cold pro

tection had many desirable qualities. It was

easily stored, easily lit, had a reasonably high

heating value, was locally available and modest

ly priced (8). Coke and charcoal had higher

heating values than wood and burned longer

with less attention, but a heater was required,

and it was difficult to light and refuel the heat

ers. Rubber tires created still new problems;

they trapped rain water in the summer which

served as a breeding ground for mosquitoes;

they produced a heavy, noxious pall of smoke

when burned and they left a residue of wire

from the tire bead in the grove which was a

nuisance.

lFlorida Agricultural Experiment Stations Journal Ser

ies No. 2555.

2Associate Climatologist, University of Florida, Depart

ment of Fruit Crops, Gainesville, Florida 32601.

Most growers who were faced with an an

nual cold protection problem switched to oil

fueled grove heaters for cold protection (1).

These devices have given very good results in

most instances, but they require a capital ex

penditure of from $350 to $450 per acre for

adequate protection. Moreover, labor require

ments to light and refuel the heaters are greater

than can be supplied from the normal grove

crews so that an additional heating crew is re

quired.

Following the disastrous freeze in the Lower

Rio Grande Valley in Texas in 1962 (3), several

of the major oil companies reached the decision

to attempt to design grove heaters which would

burn solid fuels in a burnable container or in

a very low cost container. The objectives of the

design were to reduce or eliminate the capital

cost, eliminate special fuel storage facilities and

in most cases provide cold protection by placing

the heater under the canopy. Most of these de

vices were manufactured and tested first in the

Rio Grande Valley in Texas (9, 6).

The idea was fairly prevalent in the Texas

citrus region that cold protection could only be

46 FLORIDA STATE HORTICULTURAL SOCIETY, 1966

Table 1.—Heaters used in the solid fuel heating tests and some of their characteristics.

Name

Tree-Heet

Tralite

Thermo Pac

Hillpo Block

Conventional

Heaters

Manufacturer

Mobil Chemical Co.

Humble Oil and

Refining Co.

Sonoco Products

Co.

Hillpo, Inc.

Various

Heat Output

Per Hour

BTU1

10,000

20,000

25,000

25,000 2 100,000

100,000

150,000

100,000

150,000

Burning

Time

HRS

if

8-10

8-10

6-8

6-10

Heater

Required

No

Yes

No

No

Yes

Fuel

Material

Petroleum

Coke

Petroleum

Wax

Petroleum

Wax

Asphalt

and rubber

Oil

Package

Poly Bag

Metal Tray

Bituminized

Fibre Con

tainer

None

Values are manufactures specified heat production rates

2Two sizes used, one with 25,000 BTU/hr. and the other 100,000 BTU/hr.

Figure 1.—Solid fueled heaters in the field tests. From left to right, Therntopac in two sizes, Dino-Heat (not tested)

and refill, Tralite and refill, foreground, Tree-Heet and Hillpo block.


Table 2.--Heaters used, test performed, test numbers, and other test data.

Test

No.

2

3

k

5 6

7

8

9 10A

10B

11

12

13 14

15

16

17A

17B

18

19

2A

2B

2C

3

4A

kB

5A

5B

6

Plot

No.

2

2

2

2

2

2

2

2

2

Date

1/11 1/16

1/17 1/18

)/2k

1/25

1/27 1/30

1/31

1/31 2/1

2/3 2/k

2/5 2/6

2/6

2/21

2/21

2/25 2/26

1/23

\/2k

1/24

1/25

1/26

1/26

1/28

1/28

1/29

Heater

Tralite

Tralite

Tree-Heet

Trali te

R. Stack

Tree-Heet

' •

1 '

i i

i i

R. Stack

Tree-Heet ii ii

Hillpo Blk

Tralite

Tralite

Frost Guard

R. Stack

Tree-Heet

Thermo-pac

Tree-Heet

R. Stack

Tree-Heet

Tralite

Units/

tree

3

3

k

k

k

k

8

8

3

\/h

8

8

8

8

k

8

16

\/k

8

8

1

3

3

3

\/k

1

k

2

k

1

8

3

Location

Canopy

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

Middles

X

X

X

X

X

X

X

X

X

X

X

X

X

X

Size of

test

2 acres

2 "

5

5

5

5

5

5

3

i

i

1

1

1 i

i

30 "

9 trees

9 " 5 acres

5 " 9 trees

9 "

9 " 30 acres

1 acre

1 "

5 acres

9 trees

9 "

35 trees

35 "

35 "

35

35

35

35

35

ii

ii

"

ii

35 "

Tree

size

20-301 ii

ii

n

n

it

ii

ii

ii

11

i

i

i

1 i

1

1

1

1

1

1 i

8-10 ii

ii

n

11

" n

n

11

used to blunt the edge of disasters; therefore,

originally only the trunk and main limbs were

considered in cold protection tests (7). Such a

view, however, is not held by the Florida indus

try, which regularly protects not only whole

trees and foliage, but also the fruit.

Methods and Materials

During the winter season of 1965-66, a series

of field heating tests were conducted by the

Department of Fruit Crops of the University of

Florida at Gainesville. These tests were con

ducted in a mature 'Parson Brown* citrus grove

near Orange Lake, Florida, referred to as Plot

1, and in a young grove of mixed varieties on

the campus at Gainesville, referred to as Plot 2.

Several types of heaters were used in the field

testing program. A summary of the heaters

and some of their characteristics is given in

Table 1. A photograph of the devices used in

the test is shown in Fig. 1. Table 2 is a sum

mary of the tests performed, the heaters used,

the number per tree, heater location and other

pertinent data.

Plot 1 was a 40-acre block of which 30 acres

were mature 'Parson Brown' sweet oranges on

sour orange rootstock. The trees were spaced

25 x 25 feet or about 70 per acre and were

between 20 and 30 feet tall. A 10-acre control


area was located in the northeastern quadrant

of the grove. It was planted to navel oranges

in the summer of 1965 and they were quite

small. The test area of 5 acres was immediately

west of the control area. The trees were typical

of large mature orange groves. A plot layout is

shown in Fig. 2.

Plot 1 was heavily instrumented. An 80-foot

tower was erected with wind and temperature

sensors at each 10 feet. An additional air tem

perature was measured at 5 feet. A 50-foot

tower was erected in the check area, but only

air temperatures were measured. Leaf temper

atures were measured on 4 trees at 5, 10, 20

and 30 feet in the test area and on numerous

trees in the check area, but only at one height

since the trees were small. All temperatures

were measured with 22 gauge copper-constantan

thermocouples.

Air temperature probes were shielded by

placing them one inch from the open end of a

horizontal thin-walled aluminum conduit. Leaf

temperatures were measured by attaching ther

mocouples with masking tape to the top surface

of semi-exposed leaves. Both individual thermo

couples and thermopiles were used to measure

the temperatures. They were calibrated by plac

ing the couples in an ice-water bath maintained

at 32° F. Calibration was acceptable when the

couples were within ±0.5° F at the ends of the

cables used to connect to the measuring instru

ments. Wind speed was measured with sensi

tive 3-cup Cassella anemometers. The output of

the thermocouples weree recorded on three 20-

point Leeds and Northrup, Speed-O-Max, Type

G, recording potentiometers equipped with au

tomatic reference junction compensators. A to

tal of 60 temperature points was used, and each

point was recorded once every 80 seconds. Wind

speed was continuously recorded on a 20-channel

Easterline angus event recorder.

Plot 2 had a total area of about 2 acres.

Two small plots of young trees between 8 and

12 feet tall were selected for experimental pur-

r<S \> SMALL

6

Fig. 2 -- Plot layout.

Plot No. 1

Orange Lake

'/3ZO


poses. It was not as intensely instrumented as

Plot 1. Air temperature was measured to 50

feet and leaf temperatures were measured at

only 2 levels. No wind data were obtained. The

output of the thermocouples was recorded with

a data logger manufactured by Non Linear

Systems, Inc.

Each chart was read manually and the data

were copied into tabular paper forms. In excess

of 100,000 bits of data were handled in this

manner. These data were used to produce mean

values for 10 and 30 minute periods. No attempt

was made to determine the variance of the

values, but because of the large number of

individual observations making up each mean

a high degree of precision was obtained.

Protection was calculated by establishing the

difference which existed between the control and

test area prior to the initiation of heating. The

protection obtained was then set equal to the

algebraic sum of the pre-existing differences in

temperature plus or minus the difference obtain

ed after heating.

Results and Discussion

The cold protection obtained during the test

ing is shown in Table 3. This table summarizes

the results for all of the devices and tests. As

can be seen, heating beneath the canopy was

slightly more effective than heating in the rows.

The difference was usually less than 2° F and

usually more than 1° F.

The petroleum coke heaters were more effec

tive in the low levels of the tree than at the

higher levels. Very little protection was pro

vided above 20 feet except when 16 of the units

were used beneath the canopy. This represents

an unrealistically large amount of fuel and the

cost would be prohibitive. It was included for

two reasons. The first was to determine if more

heaters would produce a marked increase in pro

tection and the second was to determine if the

depth of the heating effect could be changed by

increasing the number of heaters and thus the

effective size of the heat source. The heating

effect was more than doubled and substantial

protection was obtained at the 20-foot level.

Crawford (4) had indicated that the depth of

the heating effect was a function of the fire size

among other things (Table 1). This appeared

to be one of the reasons why very little protec

tion was provided by petroleum coke above 20

feet and in some cases 10 feet.

The petroleum waxes appeared to bive better

protection in the 10 to 20 foot region. This was

probably due to the fact that the fire size was

Table 3.— Protection obtained with solid fueled devices for all tests performed at different heights.

Heater

name

Tree-Heet

Tree-Heet

Tree-Heet

Tree-Heet

Tree-Heet

Hillpo Block

Tralite

Tralite

R. Stack

Tree-Heet

Tree-Heet

Tralite

TraKite

Tralite

The rmo Pac

No. heaters

per tree

1

4

8

8

16

1

3

3

1/4

4

8

1

3

3

2

Heater

location

C1 R2

X

X

x

X

X

X

X

X

X

X

X

X

X

M3

X

X

X

X

X

X

5'

3-0

2.2

4.8

2.9

7.5

3.2

3.2

3.0

2.8

2.2

8.6

1.9 8.2

7.6

7.2

Mean Protection

10'

(Large

2.2

1.2

4.0

2.1

8.0

3.4

3.8

2.4

2.3

(Small

1.8

3.2

1.7

5.6

4.0

4.9

20'

1

0

2

1

8

3

3

1

1

trees

.4

.7

.3

.1

.0

.2

.2

• 4

.7

trees

_

-

-

-

-

30'

20-30

0.6

0.5

1.8

0.0

-

-

_

1.4

1.6

8-10

_

-

-

-

-

PROTECTION °F ON LEAVES

Maximum Protection

5' 10'

feet tall)

4.8

2.9

11.3 3.8

9.9

4.7

5.3 4.6

3.5

4.2

1.7

9.1 2.8

10.2

4.4

6.7 4.4

2.7

feet tall)

4.3 12.5

9.2

15.9

3.5

5.7 -

8.2

17.6

20'

2.8

1.0

7.5

2.0

10.2

4.3

5.4

2.2

2.1

_

-

-

-

-

30'

2.1

0.6

5.3

1.1

-

-

-

2.2

2.1

-

-

-

-

-

Minimum

5'

1.8

1.7

1.5 2.0

2.6

1.7

0.8

2.7

1.4

0.5 2.2

-

4.4

-0.8

10'

0.8

0.9

-0.3 1.1

1.9

1.8

1.6

1.5

1.3

0.2

0.1

-

3.6

1.0

Protection

20'

-0.1

0.6

-0.2

0.3 2.4

1.9

1.8

0.9

1.3

-

-

-

-

30'

-0.1

0.4

-0.8

-1.2

-

-

-

0,6

0.9

-

-

-

-

-

1 Canopy 2Rows 3Middle


about twice that of the petroleum coke. Some of

the petroleum wax heaters, "Tralite," could be

placed beneath the canopy, but others such as

"Thermo Pac" could not. However, placement

under the canopy resulted in fire damage to the

young trees from hot wax which spilled and

flared. This damage was sufficiently severe that

some limbs were lost. Spillage and flaring occur

red from several causes. If the heaters were not

exactly level, hot, melted wax ran over the edges

of the containers and flaring occurred. The

walls of the fiber containers would sometimes

partly collapse, and spillage and flaring resulted.

If rain water got into the containers, it collected

beneath the lighter wax. When the wax was lit,

it gradually melted and eventually reached the

boiling point of the water. Steam from the

water ejected a froth, flaming wax steam mix

ture which flared and caused damage.

The "Hillpo" blocks were only tested on one

night. They produced a large hot fire and had

to be used in the middles. They were burned in

pairs. Since the fire size was large (100,000-

150,000 BTU), the depth of the heated layer

extended to the 20-foot level. However, this was

the result of only one night's test and from our

experience, it is unusual to get precisely the

same results twice. This is because of the vari

ability of the meteorological conditions from

night to night. Therefore, the same degree of

certainty cannot be given this test as can be

attributed to the other tests.

It is evident that the cold protection obtained

was closely related to the total heat input per

acre per hour as is shown in Table 4. Heating

beneath the canopy was more effective than heat

ing in the rows and middles, especially with the

petroleum coke heaters.

The mean cold protection obtained with the

solid fueled devices in the large trees is shown

in Fig. 3. The 16 "Tree-Heet" units per tree

are included primarily for comparison purposes.

Several important facts are apparent in the

figure. The petroleum coke blocks provided pro

tection mostly below 10 feet. At 20 feet the

protection was quite small except when 8 units

were used beneath the canopy. The petroleum

wax heaters provided protection over a deeper

layer and when used beneath the canopy were

very effective in the zone from 5 to 20 feet.

The region of maximum protection occurred be

tween 10 and 15 feet with "Tralite," the "Hillpo

Table 4.--Mean protection obtained in lower 20 feet of large trees and heat output.

Mean Protection

°F in the

First 20 Feet

2.2

3.7

7.7

1.4

2.0

3.4

2.3

2.3

3.3

BTU per1 Acre-hour

2,800,000

5,600,000

11,200,000

2,800,000

5,600,000

4,900,000

4,900,000

3,550,000

5,250,000

Location

Canopy n

ii

Rows

Canopy

Rows

Rows

Rows

Heater

Tree-Heet ii n

M n

ii n

ii n

Tralite n

R. Stack

Hillpo Blocks

'Computed.


JO.O

9.0

3.0

1.0

o

2.o

/.o-

UMDER

BLOCK

/6 UA//T3

/o 15 Z5 3 a

Fig. 3 — Protection obtained at 5, 10, 20 and 30 feet in large trees with various heater numbers and

placements.

Table 5.--Time required to light heaters

Heater

Tree-Heet

Tralite

Tree-Heet

Tree-Heet

Tree-Heet

Tralite

Hillpo Blocks

Return Stack

Torch

Conventional

n

"

ii

ii

"

ii

11

Units

per tree

k

3

k

8

8

3

1

1/2

Units

per acre

280

210

280

560

560

210

70

35

Man hours

per acre

0.45

1.5

0.15

0.81

0.43

0.37

0.17

0.25

Heater

location

Canopy

Canopy

Rows & Middles

Canopy

Rows & Middles

Rows & Middles

Middles

Middles


Blocks" and the 16 "Tree-Heet" units. This

change in shape of the temperature profile with

in the heated layer was again probably due to

the higher heat concentration or fire size.

Numerous operational problems were en

countered with the solid fueled heaters. Light

ing and refueling were generally found to be

more troublesome and labor consuming than

with conventional heaters. Generally, the larger

the individual units, the easier the refueling.

This was most apparent with the "Tree-Heet"

and "Tralite." For example, to place 8 units per

tree beneath the canopy in 5 acres required

5,600 individual unit handlings. At even 5 sec

onds per unit, which was rapid, 7.8 man hours

was required; and at 10 seconds per unit, which

is more realistic, over 15 hours or 3 man hours

per acre. Lighting problems were similar. The

labor required in lighting is shown in Table 5.

Most of the solid fuels required specialized

handling and placement. "Tree-Heet" was pack

aged in a polyethylene bag with a special igni

tion layer on the top. This required care to

avoid puncturing the bag, and placing the heat

er in proper position. If the bag was punctured

and water entered, the heater was ruined and

would not burn.

"Tralite" had to be stored in the field in an

inverted position. This was necessary to keep

the water out of the metal tray. When inverted,

the lids which controlled the burning rate usual

ly came off. This meant that just prior to

lighting the heater had to be inverted and level

ed and the lids had to be replaced. On a dark,

cold night beneath the canopy this was very

impractical. Refueling by replacing the wax

cartridge in the heater was difficult because the

wicking material had to be scraped from the

tray, and the lids had to be removed and re

placed. Often the cartridge was not exactly the

right size and the operation was very slow.

The "Thermo Pac" was difficult to light un

less exactly level and water collected in the

folds of the polyethylene bag aggavated the

lighting problem.

In most respects the "Hillpo Block" was the

easiest to use, but it produced a heavy pall of

smoke, and the blocks could not be stacked be

cause they broke apart or deformed under their

stack weight.

Most of the heaters burned well after being

lit. The "Tree-Heet" lit very slowly and prob

ably did not burn correctly. All the manufac

tures have been informed of the results and the

products which will be offered in the future

will certainly have many improvements over

these which were tested.

Conclusions

Solid fueled heaters were tested in a very

intensive program. Generally cold protection

was obtained, but many practical problems re

main. The following conclusions were reached.

1. When 8 units of petroleum coke were used

beneath the canopy, 3° to 5° F of protection

was obtained to a height between 10 and 20 feet.

Very little protection was obtained above 20

feet.

2. Four units of petroleum coke did not give

adequate protection when used either beneath

the canopy or in the rows.

3. When 3 petroleum wax heaters, "Tralite"

were used beneath the canopy, 3° to 4° F of

protection were obtained in large trees (20 to 30

feet) and 5° to 8° F in small trees (8 to 10

feet). Protection was rather uniform to 20 feet.

4. Relatively large fires, 100,000 BTU per

hour or more, produced protection to a greater

depth. The maximum protection occurred near

10 feet and protection at 5 and 20 feet was

nearly the same.

5. Heating beneath the canopy was more

effective than heating in the rows and middles,

but the extra labor required probably makes it

impractical. The added protection was only 1°

to 2° F.

6. The labor requirements for the solid fuels

were generally greater than for conventional

heaters.

7. Many practical problems remain to be

solved. The heaters used have in many cases

been improved since the tests were conducted.

The grower should acquaint himself with the

storage, handling, lighting and refueling prob

lems as well as cost and availability on short

notice from the supplier.

LITERATURE CITED

1. Brooks, F. A. 1959. An Introduction to Physical Microclimatology. Univ. of Cal., Davis, California. 253

pages. 133-161.

2. Camp, A. F. 1931. Grove heating. Proc. Fla. State

Hort. Soc. 44: 207-211.

3. Cooper, W. C, A. Peynado and J. R. Furr. 1962. Effects of 1961-1862 winter freeze on Valencia oranges in Florida, Texas, and California. Proc. Fla. State Hort. Soc.

75: 83-88.

YELENOSKY AND HORANIC: FREEZE PROTECTION 53

4. Crawford, T. V. 1964. Computing the heating re

quirement for frost protection. J. Applied Meteorology. 3: 750-760.

5. Geiger, R. 1950. The Climate Near the Ground.

2nd ed. Translated by M. N. Stewart. Howard University Press. Cambridge, Massachusetts. 482 pages. 403-412.

6. Leyden, R. F., R. A. Hensz and J. E. Fucik. 1965. Under-the-tree heating tests in citrus orchards using petroleum coke fuel blocks. J. Rio Grande Valley Hort. Soc. 19: 54-60.

7. Maxwell, N. and J. Cappenter. 1966. Performance of petroleum wax under-the-tree heaters on citrus during a freeze in the winter garden of Texas. J. Rio Grande Valley Hort So. 20: 31-38.

8. Skinner, B. C. 1917. Orchard heating. Proc. Fla. State Hort. Soc. 30: 83-88.

9. Young, R., N. Maxwell, M. Bailey and W. R. Cow-ley. 1964. Temperatures in and around grapefruit trees as affected by under-the-tree heat sources. J. Rio Grande Valley Hort. Soc. 18: 15-20.

THE EFFECT OF DIFFERENT TYPES OF HEATERS USED SINGLY

AND IN COMBINATION FOR FREEZE PROTECTION

George Yelenosky and George Horanic1

Abstract

We used four different types of heaters,

separately and in combination, to protect citrus

trees less than 6 years old and in areas less

than 1 acre. These heaters included two types

Crops Research Division, Agricultural Research Service, U.S. Department of Agriculture, Orlando.

which burn propane gas and two types of rela

tively new solid-fuel heaters.

Our results indicate that some types of heat

ers can be used together more effectively than

others, depending on conditions and the situa

tion. Propane gas heaters alone provided ade

quate increases in temperature in our study.

Such heaters can be supplemented effectively

with solid-fuel types of heaters. One type of

solid-fuel heater was as effective as propane

Figure 1.—Propane gas I heater.

solid fueled grove heaters1

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