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Group Title: Circular Florida Cooperative Extension Service
Title: Improving forced-air cooler performance
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Permanent Link: http://ufdc.ufl.edu/UF00008452/00001
 Material Information
Title: Improving forced-air cooler performance
Series Title: Circular Florida Cooperative Extension Service
Physical Description: 9 p. : ill. ; 28 cm.
Language: English
Creator: Talbot, Michael T ( Michael Thomas ), 1948-
Florida Cooperative Extension Service
Publisher: Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida
Place of Publication: Gainesville
Publication Date: 1992
Subject: Precooling   ( lcsh )
Crops -- Postharvest technology   ( lcsh )
Food -- Cooling   ( lcsh )
Crops -- Quality   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
bibliography   ( marcgt )
non-fiction   ( marcgt )
Bibliography: Includes bibliographical references (p. 9).
Statement of Responsibility: Michael T. Talbot ... et al..
General Note: Cover title.
General Note: "June 1992."
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Bibliographic ID: UF00008452
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: ltqf - AAA6717
ltuf - ALD2897
oclc - 26517263
alephbibnum - 002193083

Table of Contents
    Front Cover
        Front Cover 1
        Front Cover 2
        Page 1
        Page 2
        Page 3
        Page 4
        Page 5
        Page 6
        Page 7
        Page 8
        Page 9
    Back Cover
        Page 10
Full Text

June 1992 Circular AE 108

Improving Forced-Air Cooler Performance

Michael T. Talbot, C. Direlle Baird, Steven A. Sargent and Jeffrey K. Brecht

Florida Cooperative Extension Service
Institute of Food and Agricultural Sciences
University of Florida
John T. Woeste, Dean

Circular AE 108

June 1992

Associate Professor and Professor, Agricultural Engineering Department and Associate Professors, Vegetable Crops Department.

Commercial forced-air precooling
has become an important posthar-
vest procedure in Florida for rapid
cooling and maintaining quality of
several vegetable crops, strawber-
ries, blueberries, and cut flowers.
A detailed description of forced-air
cooling and several different types
(Figures 1 and 2) are described in
several publications 11, 3, 4, 5, 6, 8,
9] (numbers in brackets refer to cited
references). One study [21 discusses
in detail the numerous variables
which affect product cooling rate
and/or overall cost of forced-air cool-
ing systems for fruits and vegeta-
bles. These variables relate to the
product, such as, size, shape, initial
product temperature, desired final
product temperature, and thermal
properties; to the product configura-
tion, such as, product packaging
(bulk or in shipping containers), car-
ton vent area, depth of product load
during cooling; to the precooling sys-
tem, such as, air-flow rate, tempera-
ture, and relative humidity, ambient
temperature; and to economics, such
as, time of operation per year, unit
costs of cooling space, heat exchang-
ers, compressors, and fans, electric
power cost, maintenance cost, labor
cost, and interest rates.

In forced-air cooling (Figure 1),
produce is air-cooled rapidly by a dif-
ference in air pressure on opposite
faces of stacks of vented containers
(pallet boxes, corrugated cartons,
flats, etc.). Fans create the pressure
difference, which is called static-
pressure difference. This pressure
difference forces air through the con-
tainers and product, removing pro-
duce heat. The product is most effi-
ciently cooled when the cooling air
flows around the individual fruits or
vegetables in the containers, rather
than by flowing around the outside
of the containers (as in room cool-
ing). In other words, the cooling me-
dium (cold air) comes into intimate
contact with the product to be cooled.

Figure 1. Forced-air tunnel with portable exhaust fan.

Figure 2. Forced-air cooler with permanent constructed air plenum.

Figure 3. Container and pallet stacking configuration for forced-air cooling tunnel.
This publication discusses meth-
ods to evaluate and improve forced- pi 1
air cooler performance. It is in-
tended to assist those interested in 04'.i- 1
improving the performance of an i
existing forced-air cooler as well as p ,PEBS R.
those planning to install new cm
forced-air cooling systems. The in-
formation is applicable to all types
of forced-air cooling systems.

System performance can be as-
sessed by measurement of static-
pressure drop, air velocity or flow,
and cooling rate during commercial
forced-air cooling. Precoolers can be
made more efficient by several
minimal cost methods and by in- Figure 4. Opening in pallets sides
creased management. These meth- for cooling air bypass.

ods include sealing air-leak areas
to force additional air through
products, improving carton stack-
ing configurations or orientation,
modifying pallet-tunnel length and
width, and proper temperature
monitoring. Methods requiring
more time and cost include im-
provement in carton design, in-
creased fan and cooling capacity.

Seal leaks
The path of least resistance is a
key physical principle when con-
sidering forced-air cooling. Air,
like water, flows from a point of
high pressure to a point of lower
pressure by the path which pre-
sents the least resistance. The
goal for effective forced-air cooling
is to insure that the path of least
resistance is through the product
rather than around the container.

Air bypasses or short circuits
whenever the path of least resis-
tance is not through the product.
Such bypasses include openings
between stacked containers on pal-
lets (Figures 3, 4, and 5), between
adjacent pallets, side pallet entry
holes under pallets, loosely in-
stalled canvas covers or through
holes in the canvas, and at the
junction between pallets and air
plenums or ducts.

IsPI e uareI I

mkU t S'U 10I

dwLUZ Ol

and tops and gaps between adjacent pallets provide paths


-' a I I I
I I I I ,

I I I I t



/ I "

Figure 5. Opening in tops of pallets and misalignment of pallets forming forced-air cooling

Figure 6. Plastic used to seal the pallet opening in the row of pallets on the left.

Recent research on forced-air
cooling of peppers [9] has shown
that the air bypass of the openings
beneath pallets can be blocked
with plastic (Figures 4 and 5), sig-
nificantly reducing cooling time.
This study also clearly illustrates
that a large volume of cooling air
can pass through seemingly small
gaps between adjacent pallets of a

Flexible materials such as plas-
tic, canvas or foam rubber should
be used to seal openings that allow

cooling air to bypass containers
(Figure 6). This material should be
placed such that the fan suction
seals the undesirable openings.
Plywood is used in some strawberry
and other forced-air cooling opera-
tions for blocking large, open areas,
and is not effective in reducing by-
passed air. In the above-mentioned
study [9], sealing pallet openings
with plastic was successful in block-
ing air bypasses while lumber
placed against the sides of the pal-
lets was not effective. Apparently
the rigid structure of the wood

could not properly seal the openings
in the pallets. Lumber and plywood
are not effective unless duct tape or
plastic seals the spaces between the
wood and containers or plenums.
Boards can be effective if 5 cm (2 in)
of polyurethane foam is glued to the
side of the boards placed against
the pallets.

A simple idea for sealing pallets
is to attach purchased or salvaged
plastic or scrap corrugated paper to
cover the top of the pallet and the
pallet side openings not normally
used for the forklift blades. This
can be accomplished inexpensively
by in-house personnel using sta-
plers or tape.

The more resistance through the
product, the more important it is to
seal leakage areas. Therefore, re-
ducing the resistance through the
product (e.g. increasing carton face-
vent area) can reduce the signifi-
cance of air leakage or bypass.

Carton face-vent area
For forced-air cooling the recom-
mendation for minimum carton
vent opening is 5% of the container
surface facing the airflow [2, 6, 9].
Many Florida packinghouses should
increase the openings of the carton
vents to achieve this recommended
value, which reduces the resistance
to airflow through the cartons (Fig-
ure 7). A few larger holes produce
less resistance than several smaller
holes of the same total area.

The percent of vent openings of a
particular carton surface or face can
be calculated by determining the
ratio of the vent openings of a par-
ticular surface to the total area of
that surface (Figures 8 and 9). Only
the surfaces perpendicular to the
air flow should be considered. To
determine the percent vent opening
for a container surface, the area of
each vent should be calculated us-
ing basic geometric equations and
the area of all vents added. Most

* *

SA- *~ --

Figure 7. Several currently used and experimental containers with vents of various sizes,
shapes, and locations.

carton venting is circular or round-
ended slots, which are combinations
of rectangles and semicircle ends.
The total vent area should then be
divided by the total face area of that
surface (length times height) and
finally multiplied by 100.

For many packinghouse opera-
tions, cartons are placed such that
the side vents align with the side
vents of the adjacent carton or the
end vents align with the end vents
of the adjacent carton, for example
standard 1-1/9 bushel containers
stacked eight containers per layer
(Figures 3 and 8). In other applica-
tions, such as 40 x 30 MUM
(Modularization, Unitization, and
Metrification) containers stacked
ten per layer, the stacking patterns
require ends of cartons to align with
the sides of adjacent cartons and the
carton vent locations must be de-
signed accordingly (Figures 9 and
10). If the vent holes on adjacent
cartons do not align, a major disrup-
tion of air flow occurs and should be
avoided. For produce cartons with
slender vertical slots commonly
used on the ends and sides, offset of
the carton by as little as 0.6 cm (1/4

in) results in vent hole misalign-
ment. This is another reason for
selecting large rather than small
vent holes, because the larger
holes would allow partial vent
alignment even when the cartons
are not aligned correctly. How-
ever, wide holes can be partially
blocked by the product more so
than narrow slots. Also, more cor-
rugate flutes are cut with a wider
hole, which reduces the strength of
the carton.

Pallet configurations

Pallet placement
In one type of forced-air cooler,
two rows of pallets are placed end
to end to form each side of a cool-
ing tunnel (Figures 1 and 3). It is
important that the pallets are
placed such that very little gap
exists between the cartons of adja-
cent pallets. Otherwise another
path is created for the cooling air
to bypass the cartons. Velocity
measurements [9] indicated sig-
nificant air flow through such
gaps, particularly when plastic
was installed to seal the side open-
ings of the pallets. For example, a

4.5 cm (2 in) by 1.2 m (4 ft) gap was
noted between the two pallets dur-
ing a test at a packinghouse with
plastic installed. The measured
velocity through the gap was 5 m/s
(970 fpm) and the calculated flow
rate was 0.3 m 3/s (650 cfm). The
total flow through an adjacent car-
ton was 0.01 m3/s (26 cfm). At an-
other packinghouse, a 4.5 cm (2 in)
by 28 cm (11 in) gap was noted be-
tween the two pallets with plastic
installed. The measured velocity
through the gap was 3.5 m/s (680
fpm) and the calculated flow rate
was 0.05 m3/s (104 cfm). The total
flow through an adjacent carton
was 0.006 mV/s (12 cfm).

This study [9] suggests that
additional studies should be con-
ducted to determine how varying
the number of pallets, height of the
cartons on the pallets, tunnel width
(distance between rows of pallets),
and similar factors affect the per-
formance of the air handling and
cooling systems. Limited data indi-
cates that reducing the number of
pallets in the cooling tunnel from
10 to 6 reduced the half cooling
time for the last pallet ( without
plastic) from 1.9 to 0.7 hours.
Therefore, it might be possible to
cool 36 pallets (3 groups of 12 pal-
lets cooled) with tunnels 6 pallets
long in the same length of time as
20 pallets (1 group of 20 pallets
cooled) with tunnels 10 pallets long,
if sufficient refrigeration capacity
is available (Figure 11).

In addition to the side openings
in the two-way pallet, the bottom
layer of cartons at times does not
always completely cover the surface
of the pallet. This leaves openings
between boards on the top of the
pallet, thus allowing air to bypass
the product. Significant air bypass
was noted through these open pal-
let surfaces [9]. Normally the car-
tons are stacked flush with one side
of the pallet, while the other side
exposes the surface of the pallet.


48 in
(120 cm)

% Vent Opening

1.9 3.5*

* Hand Hole Assists
Forced-Air Cooling


Area of Round-ended Slot

.. rhi |=L-d -


....... L=l+d--

Area Slot = Area Rectangle + 2(Area Semi-Circle)
= Area Rectangle + Area Circle
= (L-d)d + rd2/4 = (l)d +T r

For example using the side view above (English units):
Area Slot = [(2 3/8)-(7/16)](7/16) + T (7/16)2
Area Slot = 1.00 in2
Total Vent Area = 4 Area Slot = 4.00 in2

Vent Alignment: Vents
are aligned because
containers are stacked
either side to side
or end to end.

Hand Hole,
- 1 x 3-3/8 in
(2.5 X 8.6 cm)

SVertical Slot,
15/32 x 1-15/16 in


Area of Container Face

T ---------- ----

I....... w .. ... "1

Area Face = Area Rectangle = Width x Height = WH
For example using the side view above (English units):
Area Face = (17 7/8)(12) = 214.5 in2
% Vent Opening Area

Total Vent Area
% Vent Opening 100
Area Face
For example using the side view above (English units):
% Vent Opening = (4.00/214.5)100 = 1.9 %

Figure 8. 1-1/9 bushel pepper container showing dimensions, percent vent openings, and pallet stacking configuration.

Vertical Slot.
7/16 x 2-3/8 in
(1 1 X 6.0 cm)


40 X 30 cm MUM PEPPER C(


15 13/16
i c-

11 7/8 in

47 1/2 .in........
(120 cm)

Vent Alignment: Vents
were designed so holes
align when ends and
sides of containers are
cross-stocked. In this
example some vents
align with the space
between adjacent

plane used for
finding Effective
% Vent Opening

Hand Hole.
1 x 3.5 in
% VENT OPENINGS (2.5 X 8.9 cm)
Individual Container 3 8 in)
8. cm)
15 13/16 in 7.4 6.6(8.8)*
(40cm) Effective 11 7/8 in ij E
5.0 (7.2)* (30cm)
Hand Hole Does Not
Assist Forced Air Cooling
1-3/8 in. wide. 1-15/16 in. long semi-circular ended slot
(3.5 cm wide, 4.9 cm long semi-circular ended slot)

Area of Round-ended Slot

r. r l=L-r
+ ---i-r..

... L=l+r -'

Area Slot = Area Rectangle + Area Semi-Circle
= (L-r)d +1/2(Td2/4) = Id +1/2(rrr2)
For example using the side view above (Metric units):
Area Slot = [(4.9-(3.5)/2)](3.5) +1/2[ r(3.5f/4]
Area Slot = 15.8 cm2
Total Vent Area = 6(Area Slot) = 94.8 cm2

Area of Container Face

------... -

Area Face = Area Rectangle = Width x Height = WH
For example using the side view above (Metric units):
Area Face = (40)(32) = 1280 cm2

% Vent Opening Area
Total Vent Area
% Vent Opening = 100
Area Face
For example using the side view above (Metric units):
% Vent Opening = (94.8/1280)100 = 7.4%
% Effective Vent Opening = (12)(15.8)/(3)(1280)100 = 5.0%

Figure 9. 40 x 30 cm MUM pepper container showing dimensions, percent vent openings, and pallet stacking configuration.

r t

Vent Hole Alignment

t t t

3 15/16
(10 cm)




- .-----.. ..

Figure 10. Top view of 40 x 30 cm MUM container showing vent hole alignment when con-
tainers are cross-stacked (end to side).

The side with the exposed pallet
surface should be placed so that the
exposed surface is inside the forced-
air cooling tunnel. MUM-type con-
tainers can reduce this problem be-
cause they cover 90% or more of the
pallet surface [7].

For efficient precooling, packing-
house personnel must be properly
trained and supervised to insure
that pallets are tightly placed and
aligned when forming the cooling

Static pressure
The importance of sealing the
openings under the pallets is illus-
trated [9] by measurement of the
static pressure drop across the pal-
lets (outside to inside of the cooling
tunnel), at the center of each pallet
and at the end of the tunnel away
from the fan. With plastic added,
the pressure drop was nearly dou-
bled. Since the pressure varies with
the square of the velocity, doubling
the pressure would result in a 40%
increase (square root of 2) in the

The packinghouse manager
should routinely check the static-
pressure drop for each cooling tun-
nel at the beginning of a cooling
cycle to insure proper fan operation.
This is particularly important when
the forced-air precoolers employ
more than one exhaust fan for each
precooling tunnel. Twice during
pressure drop checks at one pack-
inghouse [9], one tunnel's pressure
drop was much less than the others.
A check of the two fans revealed
that only one was operating because
a circuit breaker had tripped on the
other. Visual observation of the two
fans was insufficient to determine
whether one or two fans were oper-
ating since both were turning at the
same apparent speed and some suc-
tion was evident when moving the
canvas. Air was coming in through







I ,- Al I

Initial Product Temperature:
Cooling Air Tenmperature:
Temperature to Achieve Half

1 .9 HOURS




)W -
ON 7



75 F

Figure 11. Example of reduced cooling time by reducing the forced-air cooling tunnel length.

the top of the off fan and pulled
back through the operating fan.
Therefore, little cooling air was
passing through the palletized
product. In addition, these fans
were not readily accessible.

A simple, homemade, U-tube
manometer [9] is adequate for pack-
inghouse personnel to quickly check
the static pressure drop at the end
of the tunnel away from the fan and
immediately correct problems (Fig-
ure 12). An additional approach is
the installation (wired to fan elec-
tric circuit) of easily observed lights
for each fan, which are on while the
fans operate. However, pressure
drop should still be checked periodi-
cally, to insure that air is not by-
passing the product.

Temperature measure-
ment/Cooling schedule

Temperature monitoring
Since the goal of forced-air cool-
ing is to rapidly reduce the tempera-
ture of the cooled product the cooler
operator must have a method of
measuring the temperature of the
product being cooled to determine
when cooling has been completed.
The important management prac-
tice of determining when precooling
has been completed to the desired
temperature has been stressed 181.
Due to daily differences in the ini-
tial product temperature, the cool-
ing time to achieve a desired final
temperature may vary. The cooling
time also varies if the cooling-air
temperature cannot be maintained
during cooling due to inadequate
refrigeration capacity. Another fac-
tor which can change the cooling
rate is the product size, since larger
products cool more slowly than
smaller products. In general, dou-
bling the diameter or thickness re-
quires 4 times as long to cool. A
precooling schedule should be devel-
oped for every precooler and modi-
fied as conditions require [8].




1 1- M E -'A I JRI


Figure 12. Static pressure drop and temperature measurement locations.

Recent cooling tests 11,91 investi-
gated the variation of cooling as a
function of bed effect (the effect due
to the depth of the product) and
height of the carton on the pallet.
The experimental data verified that
a bed effect does occur. The product
in cartons exposed to incoming air,
close to the air entrance cooled
more rapidly than product on the
opposite side, near the air exit from
the pallet into the cooling tunnel.
The cooling rate was found to vary
with height of the containers on the
pallet, although no criterion was
established [91. Additional work is
needed to determine the "best" loca-
tion for sampling temperature.

measuring equipment
Dial-type thermometers are com-
monly used to measure the product
temperature during cooling, and the
pulp temperature samples are often
taken at inadequate locations. These
thermometers have a slow response
time (minutes) and limited accuracy.
Also, it is difficult to precisely locate
the probe-the probe may pass
through product and measure air
temperature inside the carton. The
sampling location suggested above
is inside the cooling tunnel and not
easily reached with a probe-type
thermometer (Figure 12). To over-

come these problems, one or more
thermocouples could be inserted
easily into product in cartons on the
inside of the last pallet before it is
positioned for cooling with the ther-
mocouple leads located outside the
canvas area. An inexpensive hand-
held thermocouple reader could be
used to measure cooling for all the
tunnels. The thermocouple reader
is accurate and has a fast response
time (seconds). The thermocouples
are durable and can be used many
times. With additional investment,
permanent thermocouple leads and
remote data loggers could be used.

Velocity and flow rate
Velocity and flow measurements
are difficult and require expensive
instruments, however the total flow
rate is an important item of infor-
mation. A cheaper and more conve-
nient method is to use the manom-
eter mentioned above to measure
the static pressure across forced-air
cooling fans and then estimate the
flow rate using fan performance
curves supplied by the fan manu-
facturer. The performance curve
estimates the flow rate for various
static pressure operating conditions.

This publication presents several
methods for increasing the efficiency
of forced-air precoolers for fresh pro-
duce. Recommendations for increas-
ing efficiency with minimal cost and
increased management include seal-

ing air bypasses (particularly
through the pallets) to force addi-
tional air through products, im-
proving carton stacking configura-
tions/orientation, and proper tem-
perature monitoring. Modifying
pallet-tunnel length and width also
show promise for improving effi-
ciency. Recommendations which
require more time and cost include
increasing carton vent opening,
increasing fan capacity, and in-
creasing refrigeration capacity.
Of these three, the design of the
carton with particular attention to
the percent vent openings should
be addressed first. Increasing the
fan and refrigeration capacity
should be considered only after all
the above-mentioned changes have
been accomplished.

Literature cited
1. Arifin, B.B. and K.V. Chau.
1988. Cooling of strawberries in
cartons with new vent hole de-
signs. Trans. ASHRAE. Vol. 94
(1): 1415-1426.

2. Baird, C.D., J.J. Gaffney, and
M.T. Talbot. 1988. Design crite-
ria for efficient and cost effective
forced-air cooling systems for
fruits and vegetables. ASHRAE
Transactions 94(1):1434-1454.

3. Gaffney, J.J. and C.D. Baird.
1977. Forced-air cooling of bell
peppers in bulk. Transactions of
the ASAE 20(6):1174-1179.

4. Kader, A.A., R.F. Kasmire, F.G.
Mitchell, M.S. Reid, N.F.
Sommer, and J.F. Thompson.
1992. Postharvest technology of
horticultural crops. California
Coop. Ext. Serv. Pub. 3311.

5. Mitchell, F.G., R. Guillou, and
R.A. Parsons. 1972. Commercial
cooling of fruits and vegetables.
California Agr. Expt. Sta. Man.

6. Parsons, R.A., F.G. Mitchell,
and G. Mayer. 1970. Forced-air
cooling of palletized fresh fruit.
ASAE Paper No. 70-875.

7. Sargent, S.A., J.K. Brecht, and
J.J. Zoellner. 1990. Some physi-
cal characteristics of bell pepper
in relation to capacities of stan-
dard and potential shipping con-
tainers. Proc. Fla. State Hort.
Soc. 103:222-225.

8. Sargent, S.A., M.T. Talbot, and
J.K. Brecht. 1991. Evaluating
precooling methods for vegetable
packinghouse operations. Univ.
FL, IFAS, Vegetable Crops
Department Special Series SS-

9. Talbot, M.T. and C.D. Baird.
1990. Evaluating commercial
forced-air precoolers. ASAE
Paper No. 91-6021.

in cooperation with the United States Department of Agriculture, publishes this information to further the purpose of the May 8 and June 30,1914 Acts of Congress;
and is authorized to provide research, educational information and other services only to individuals and institutions that function without regard to race, color,
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Building 664, University of Florida, Gainesville, Florida 32611. Before publicizing this publication, editors should contact this address to determine availability.
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