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A Review of Porous Evaporative Cooling for the Preservation
of Fruits and Vegetables.
Isaac F. Odesola, Ph.D.* and Onwuka Onyebuchi, B.Sc.
Department of Mechanical Engineering, University of Ibadan, Ibadan, Nigeria.
E-mail: ifodesola@yahoo.com
ABSTRACT
The review of a porous evaporative cooler for the
preservation of fruits and vegetables are reported
in this paper. The different types of evaporative
cooler designs under review include: pot-in-pot,
cabinet, static, and charcoal cooling chambers.
The gap between them is either filled with a jute,
damp cloth, or sand. Water is linked to the cooler
at the top, thus keeping the chamber cooled. The
result of transient performance of the cooler
revealed a depression in temperature in the
storage chamber. Thus, the evaporative cooler
has prospect for use for short term preservation
of vegetables and fruits soon after harvest.
(Keywords: pot-in-pot, cabinet, static, charcoal cooling
chambers, fruit, vegetable, food preservation)
INTRODUCTION
Cooling through evaporation is an ancient an
effective method of lowering temperature. Both
plants and animals use this method to lower their
temperatures. Trees, through the method of Eva
transpiration, for example, remain cooler than
their environment.
The basic principle relies on cooling by
evaporation. When water evaporates, it draws
energy from its surroundings which produces a
considerable cooling effect. Evaporative cooling
occurs when air that is not too humid passes over
a wet surface: the faster the rate of evaporation,
the greater the cooling. The efficiency of an
evaporative cooler depends on the humidity of the
surrounding air.
Evaporative cooling is dependent on the
conditions of the air and it is necessary to
determine the weather conditions that may be
encountered to properly evaluate the possible
effectiveness of evaporative coolers (Menzer and
Dale, 1960). On the other hand, the amount of
water vapor that can be taken up and held by the
air is not constant: it depends on two factors: the
first is the temperature (energy level) of the air,
which determines the potential of the air to take
up and hold water vapor. The second involves the
availability of water: if little or no water is present
the air will be unable to take up very much.
The operational effectiveness of an evaporative
cooler is made up of a porous material that is fed
with water. Hot dry air is drawn over the material.
The water evaporates into the air raising the
humidity and at the same time reducing the
temperature of the air.
FACTORS AFFECTING EVAPORATION
There are four major factors that impact the rate
of evaporation. It is important to keep in mind that
they usually interact with each other to influence
the overall rate of evaporation, and therefore, the
rate and event of cooling.
Factor 1 - Relative Humidity: This is the amount
of water vapor in the air as a percentage of the
maximum quantity that the air is capable of
holding at a specific temperature. When the
relative humidity is low, only a small portion of the
total possible quantity of water vapor that the air
is capable of holding is being held. Under this
situation, the air is capable of taking on additional
moisture, and if other condition are also met, the
rate of evaporation will be higher. On the other
hand, when the relative humidity is high, the rate
at which water evaporates will be low, and
therefore cooling will be low.
Factor 2 - Air Temperature: Evaporation occurs
when water absorbs sufficient energy to change
from a liquid to gas. Air with a relatively high
temperature will be able to stimulate the
evaporative process and also be capable of
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holding relatively great quantity of water vapor.
Therefore, areas with high temperatures will have
higher rates of evaporation and more cooling will
occur. With lower air temperature, less water
vapor can be held, and less evaporation and
cooling will take place.
Factor 3 - Air Movement: The movement of air,
either natural or artificial, is an important factor
that influences the rate of evaporation. As water
evaporates from a surface it tends to raise the
humidity of the air that is closet to the water
surface. If this humid air remains in place, the rate
of evaporation will start to slow down as humidity
rises. On the other hand, if the humid air and the
water surface is constantly been moved away and
replaced with drier air, the rate of evaporation will
either remain constant or increase.
Factor 4 - Surface Area: The greater the surface
area from which water can evaporate, the greater
the rate of evaporation.
MAXIMUM COOLING POTENTIAL
The extent to which evaporation can lower the
temperature of a container depends on the
difference between the wet bulb and dry bulb
temperatures. Theoretically, it is possible to bring
about a change in temperature equal to the
difference in these two temperatures. For
example, if the dry and wet bulb temperatures
were 35°C and 15°C, respectively, the maximum
drop in temperature due to evaporative cooling
would theoretically be 20°C. In reality, though,
while is not possible to achieve 100 percent of the
theoretical maximum temperature drop, a
substantial reduction in temperature is possible.
METHODS OF EVAPORATIVE COOLING
The two general methods include direct and
indirect evaporative cooling.
Direct Cooling
Direct cooling involves the movement of air past
or through a moist material where evaporation,
and therefore cooling, occurs. This cooled moist
air is then allowed to move directly to where to
where it is needed. In contrast to this process,
indirect cooling uses some form of heat
exchangers that use the cool moist air produced
through evaporative cooling, to lower the
temperature of drier air. This cool dry air is then
used to cool the environment, and the cool moist
air is expelled.
A direct evaporative cooling is a line of constant
wet bulb temperatures. In the course of direct
cooling operation, wet bulb temperature and
enthalpy remains unchanged, dry bulb
temperature reduces while relative humidity and
specific humidity increases (Babarinsa,1986).
Direct evaporative cooling is the most commonly
used form of evaporative cooling used to cool
water. This system usually uses either a porous
clay container or a water tight canvas bag in
which water is stored. These containers are then
either hung or placed so that the wind will blow
past them. The water in the container slowly leaks
through the clay or canvas material and
evaporates from the surface as warm dry air flow
past. This process of evaporation slowly cools the
water.
Limitations: The drop in temperature will
generally be only a small faction of the total
evaporative reduction that is possible. This is
primarily due to the large volume of water that
needs to be cooled by a relatively small
evaporating surface area. Only a small number of
items can be placed in large water containers.
Indirect Evaporative Cooling
The high level of humidity that is produced by
direct evaporative cooling may be undesirable for
some applications. Indirect evaporative cooling
attempts to solve this problem by using the cool
moist air produced through evaporation to cool
drier air. The resulting cool air is then used to cool
the desired environment. This transfer of
coolness is accomplished with the help of a heat
exchanger (Singh and Narayah,1999).
All methods of indirect evaporative cooling require
power to run both water pump and fans. For this
reason, indirect evaporative cooling will have
limited applications. It is primarily used to cool
dwellings and rooms. In such situations these
cooling system are generally less expensive to
buy or build and operate than conventional air
conditioning systems.
On the other hand, indirect evaporative cooling
cannot be used in all environments, and the
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reduction in temperature that can be achieved
with this system is not as great as the reduction
that can be achieved with conventional
mechanical cooling systems. (Babarinsa, 2000).
The primary advantage of indirect evaporative
cooling for increasing the comfort level of rooms
are relatively low purchase or building cost and
the relatively low operation expense, as
compared with convectional air conditioning
systems (Singh and Narayah,1999).
COMPARING ALTERNATIVES
Reduction in the temperature of fruits and
vegetables to retard spoilage is an important
benefit of evaporative cooling, though it is not the
only one. Evaporation not only lowers the air
temperature surrounding the produce, it also
increases the moisture content of the air. This
helps prevents the drying out of produce and
therefore extends its shell life.
The principal alternatives to evaporative cooling
systems are refrigeration and air conditioning.
These technologies offer the user a wider range
of applications. If electricity, natural gas, or
kerosene are available, commercial refrigeration
and air conditioning systems can be used in any
environment regardless of the temperature or
relative humidity. This is definitely not the case
with evaporative cooling. Moreover, commercial
systems allow the user to control the amount of
cooling desired. Again, this is not possible with
most evaporative cooling systems. Another
advantage of commercial systems is that they
usually require less day to day attention than
comparative evaporative cooling systems.
However, where electricity or other commercial
energy sources are either unavailable or very
expensive, and the environmental conditions are
favorable, evaporative cooling should be
considered as a viable alternative to these more
complex and costly commercial systems.
(Babarinsa, 2000).
The primary advantage of evaporative cooling
over cooling methods that involves commercial
refrigeration is its low cost, less than $2. For
example, an evaporative cooling system
developed in the United States to cool fresh
produce was able to produce 14 energy units of
cooling while using only one energy units of
electricity (Hutchisoon, 2000).
Commercial refrigeration system only commonly
produce only three energy unit of cooling for each
energy unit of electricity consumed (Singh and
Narayah,1999).
DESIGN CONSIDERATION/CHOOSING THE
RIGHT TECHNOLOGY
Arriving at a decision on which type of cooling or
refrigeration system to use is not an easy
process. It is important to review carefully the
cooling needs, weighing them against a range of
other factors before making a decision. The
following checklist may be useful in choosing the
right design:
1. What are your cooling needs? Cooling
different foods require different
temperatures.
2. What is the average relative humidity of
the area where cooling is needed? If the
relative humidity is consistently high,
evaporative cooling will not be a viable
option, and therefore another system
needs be considered. If the relative
humidity is low, then evaporative cooling
may be effective.
3. How windy is the area where the cooling
is needed? If there is little wind
evaporative cooling may not be the way
to go.
4. Is there a good supply of water where the
cooling system will be used? If this is
readily available, evaporative cooling may
be feasible.
5. Are the materials and skills needed to
build the cooler available?
6. Are commercial mechanical cooling or
refrigeration systems available? Are they
costly? If commercial systems are
available, and not too costly, then they
may maybe be a better choice of
technology.
PREVIOUS TYPES OF EVAPORATIVE
COOLER DESIGN
Large amounts of fresh produce and dairy
products are lost due to spoilage in many tropical
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and sub-tropical areas of the world. If this food
could be stored at relatively low temperatures
until eaten or sold, much of this waste could be
avoided. Different types of evaporative cooler
have been reported in the literature, some of
which include the following (Anyawu, 1995):
Pot-In-Pot
These are simple designs of evaporative coolers
that can be used in the home. The basic design
consists of a storage pot placed inside a bigger
pot that holds water. The inner pot stores food
that is kept cool. One adaptation on the basic pot
design is the janata cooler, developed by the food
and nutrition board of India (Roy,1985). A storage
pot is placed in an earthenware bowl containing
water. The pot is then covered with a damp cloth
that is dipped into the reservoir of water. Water
drawn up the cloth evaporates keeping the
storage pot cool. The bowl is also placed on wet
sand, to isolate the pot from the ground.
Mohammed Abbah (Longmone, 2003), a teacher
in Nigeria, developed a small scale storage pot-
in-pot system that uses two pots of slightly
different size. The smaller pot is placed inside the
large pot and the space between then is filled with
sand.
In Sudan, the Practical Action and the Women’s
Association for Earthenware Manufacturing have
been experimenting with the storage design of
Mohammed Abbah. The aim of the experiment
was to discover how effective and economical the
Zeer storage is in conserving foods. Zeer is the
Arabic name for the large pots used. The results
are shown in the following table (Longmone,
2003). As a result of the tests, the Women’s
Association for Earthenware Manufacturing
started to produce and market the pots
specifically for food preservation (Longmone,
2003).
Figure 1: Pot-in-Pot (Roy,1985).
STATIC COOLING CHAMBER
The India Agricultural Research Institute develops
a cooling system that can be built in any part of
the country using locally available materials (Roy,
1985).
The basic structure of the chamber can be built
from bricks and river sand, with a cover made
from cane or other plant materials and sacks or
cloth. There must be a nearby source of water.
Construction is fairly simple, first the floor is built
from a single layer of bricks, and then a cavity
wall is constructed of bricks around the outer
edge of the floor with a gap of 75mm (3”) between
the inner wall and the outer wall. This cavity is
then filled with sand. About 400 bricks are needed
to build a chamber of the size shown below. A
covering for the chamber is made with canes
covered in sacking all mounted in a bamboo
frame. The whole structure should be protected
from sunlight by making a roof to provide shade.
Table 1: Vegetable Shelf-Life (Longmone, 2003).
Produce Shelf-life produce without using the
Zeer Shelf-life of produce using the
Zeer
Tomatoes
Guavas
Rocket
Okra
Carrots
2 days
2 days
1 day
4days
4days
20 days
20 days
5 days
17 days
20 days
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After construction of the walls and floor, the sand
in the cavity is thoroughly saturated with water.
Once the chamber is completely wet, a twice daily
sprinkling of water is enough to maintain the
moisture and temperature of the chamber. A
simple automated drip watering system is shown
in Figure 2.
Figure 2: A Static Cooling System (Roy, 1985).
CABINET COOLER
A convenient cabinet is usually encapsulated by
evaporating cool surfaces. In some cases, the
cabinet is constructed from metallic materials with
charcoal placed adjacent to the sides with the
result that heat conduction takes place between
the outer and inner metal container walls and
combine radiative and convective heat transfer
within the storage area. This results in little or no
temperature difference between the evaporative
cooler storage chamber and the ambient air
temperature. In particular, seepage of water is
inhibited by the non-porous container (Raha,
1994).
CHARCOAL COOLER
The charcoal cooler is made from an open timber
frame of approximately 50mm x 25mm (2” x 1”) in
section. The door is made by simply hanging one
side of the frame. The wooden frame is covered
in mesh, inside and out, leaving a 25 mm (1”)
cavity which is filled with pieces of charcoal. The
charcoal is sprayed with water and when wet
provides an evaporative cooling. The frame work
is mounted outside the house on a pole with a
metal cone to deter rats and a good coating of
grease to prevent ants from getting to the food
(Sharma and Rathu, 1991). The top is usually
solid and thatched, with an overhang to deter
flying insects.
All cooling chambers should be placed in a shady
position, and exposure to the wind will help the
cooling effect. Airflow can be artificially created
through the use of a chimney (i.e., using a mini
electric fan or an oil lamp to create airflows
through the chimney) the resulting draft draws
cooler air into the cabinet situated below the
chimney. The butch cooling cabinet uses this
principle to keep its contents cool. Wire mesh
shelves and holes in the bottom of the raised
cabinet ensure the free movement of air passing
over the stored food.
Figure 3: Charcoal Cooler (Sharma and Rathu,
1991).
ADVANTAGES OF EVAPORATIVE COOLER
DESIGN
The main advantage of this unit is that it uses
simple passive cooling features to achieve low
temperatures for the preservation of fruits and
vegetables. It requires no special skill to operate
and therefore is most suitable for rural
application.
Evaporation not only lowers the air temperature
surrounding the produce, it also increases the
moisture content of the air. This helps prevent the
drying out of the produce, and therefore extends
its shelf life (AP-Tech, 1980).
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In general, evaporative cooling can be used
where:
1- Temperatures are high
2- Humidity is low
3- Water can be spared for this use; and
4- Air movement is available (e.g., wind).
CONCLUSION
Evaporative cooler to some extent has influenced
the rate of water loss by reducing the temperature
in the storage chamber and increasing the
relative humidity. Moreover, fruits and vegetables
continue the life process that existed before
harvest. They respire and in doing so use up
oxygen and give up carbon dioxide and generate
heat. Temperature and relative humidity have
been established to be a major factor that causes
the deterioration of foodstuff.
Since significant evaporative colling temperature
depression from the ambient air temperature
always occurs during the times of the day when
cooling is most desired, the cooling condition
achieved are suitable only for the short term
preservation of vegetables and fruits soon after
harvest.
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Newsletter. 1:10-11.
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“Experiments of Food Storage in the Tropics Using
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Energy. 6(2):125-8.
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Evaporative Coolers For Low Cost preservation of
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20. Thompson, J.F. and R.F. Kasmire. 2001. “An
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ABOUT THE AUTHORS
Dr. Isaac F. Odesola, is a Registered Engineer
(COREN) and currently serves as a Lecturer 1, in
the Department of Mechanical Engineering,
University of Ibadan, Ibadan, Nigeria. Dr.
Odesola’s research interests include thermofluids,
process design, energy studies and computer
simulation.
Onwuka Onyebuchi, holds a B.Sc. (Mechanical
Engineering) and currently serves as an
Engineer, Research Assistant at the University of
Ibadan, Ibadan, Nigeria, with research interests in
thermofluids, refrigeration, and energy studies.
SUGGESTED CITATION
Odesola, I.F. and O. Onyebuchi. 2009. “A
Review of Porous Evaporative Cooling for the
Preservation of Fruits and Vegetables”. Pacific
Journal of Science and Technology. 10(2):935-
941.
Pacific Journal of Science and Technology
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