TECHNICAL PAPER # 35
Dr. Agustin F. Venero
1600 Wilson Boulevard, Suite 500
Arlington, Virginia 22209 USA
703/276-1800 . Fax: 703/243-1865
Understanding Evaporative Cooling
Volunteers in Technical Assistance
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The author of this paper, VITA Volunteer Eric Rusten,
in technology and international development, and has worked
Kenya and Nepal. The
reviewers are also VITA volunteers.
Bilecky is partner and president of von Otto and Bilecky, an
engineering, construction, and energy management firm
Washington, D.C. Agustin Venero specializes in research and
development in new energy sources for the OMICRON Technology
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UNDERSTANDING EVAPORATIVE COOLING
by VITA Volunteer Eric Rusten
Cooling through the evaporation of water is an ancient and
method of lowering temperature.
Both plants and animals
use this method to lower their temperatures.
Trees, through the
process of evapotranspiration, for example, remain cooler
People accomplish the same thing when they
perspire. For both
trees and people the underlying scientific
principle is the same: when water evaporates, that is,
from a liquid to a gas, it takes heat energy from the
environment, thus leaving its environment cooler.
We have all experienced the result of evaporative
under a tree on a hot afternoon is much cooler than sitting
either in the direct rays of the sun or in the shade of a
As water from the tree's leaves evaporates, the air
the tree is gently cooled.
Moreover, we have all felt
the cooling effect of perspiration evaporating from our
Finally, some of us may have discovered that water kept in a
canvas bag, porous clay container, or in a canteen with a
cloth cover, is much cooler, especially on a hot day, than
water kept in plain metal or plastic containers.
As the water
evaporates from the surfaces of these containers it draws
away from the containers and the water they hold, as well as
the air around them, thus leaving the water cooler.
Since it is possible to cool trees, water bottles, and
by this process shouldn't it be possible to cool other
such as food and dwellings?
The answer to this question is a
Several systems have been designed to use the
principle of evaporative cooling to keep homes cool and
Also, methods have been developed that reduce the
of foods, such as fruits, vegetables, and dairy products,
far enough to retard spoilage.
Although lowering the temperature of fruits and vegetables
levels that retard spoilage is an important benefit of
cooling, 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 prevent
the drying out of produce, and therefore extends its
In general, evaporative cooling can be used where:
temperatures are high;
humidity is low;
water can be spared for this use; and
air movement is available (from wind or
This paper provides an introduction to the process of
addition, the natural limitations and problems associated
with this process, along with some practical applications
of evaporative cooling are examined.
II. BASIC PRINCIPLES OF EVAPORATION AND EVAPORATIVE COOLING
As noted earlier, evaporation is the process of changing a
into a gas. In this
case liquid water becomes water vapor, and
this gas becomes part of the mixture of gases that compose
air. The change from
the liquid state to a vapor requires the
addition of energy, or heat.
The energy that is added to water to
change it to a vapor comes from the environment, thus
Not all substances need to gain or lose the same amount of
to change from one physical state to another.
For example, it
takes much more heat energy to cause a given amount of water
vaporize than to cause the same amount of alcohol to do so.
Water is unique in that it requires a relatively large
of heat energy to change from a liquid to a gas.
It is this
characteristic that enables evaporating water to lower
the temperature of its environment.
On the other hand, the amount of water vapor that can be
and held by the air is not constant; it depends on two
The first is the temperature (energy level) of the air,
determines the potential of the air to take up and hold
vapor. The second
factor is the availability of water. If
or no water is present, the air will be unable to take up
The measurement of the amount of water vapor present in the
is spoken of as the air's humidity.
There are two ways of
measuring the humidity of the air: (1) absolute humidity and
Absolute humidity is the measurement of the
actual quantity of water (measured in grams) in a given
air (measured in cubic meters or liters).
Relative humidity, the
more common measurement, is the measurement of the water
the air as a percentage of the maximum quantity of water
that the air would be capable of holding at a specific
Air that is fully saturated--that is, contains as much
water vapor as possible--has a relative humidity of 100
while air that has only half as much water vapor as it
could hold at a specific temperature has a relative humidity
The relative humidity varies with the temperature.
As the air
cools (i.e., loses energy), its ability to hold water vapor
decreases, which results in an increase in the relative
This is because the ability of the air to hold water vapor
been reduced by the drop in temperature, but the absolute
(the actual amount of water vapor in the air) has remainde
unchanged. If the
air temperature continues to fall the relative
humidity will approach 100 percent, or complete saturation.
The point at which the air is fully saturated is referred to
the dew point. At
temperatures lower than the dew point, water
vapor condenses out of the air onto cooler surfaces.
DETERMINING RELATIVE HUMIDITY
Before attempting to implement any of the evaporative
systems discussed in Section III of this paper, it is
to determine if environmental conditions, particularly the
humidity, are suitable for the evaporative cooling process.
In some situations it may be possible to use already
data, but where this information is not available it will be
necessary to collect it.
The following materials are needed to determine relative
a thermometer, a small piece of cloth, a small glass or
plastic vial for water, and two pieces of cardboard or some
stiff material (the pieces of cardboard should be longer
thermometer and as wide as half its length).
The procedure to determine relative humidity involves two
First, use the thermometer to determine the temperature of
air; note this down as the dry-bulb temperature (i.e., the
taken with the bulb of the thermometer kept dry).
Second, secure a small piece of cloth to the bulb of the
thermometer with some thread.
The end of the cloth should extend
beyond the tip of the bulb.
Then attach the thermometer to the
piece of cardboard.
Next, attach the small plastic or glass vial
to the cardboard just below the end of the thermometer so
the piece of cloth will fit in the vial.
The cloth covered bulb
of the thermometer should be left exposed to the air.
shows the final set-up of this apparatus.
Now, fill the vial with water so that the cloth and the bulb
be kept wet. Using
the other piece of cardboard, fan the lower
end of the apparatus for 30 to 60 seconds.
At the end of this
time note down this temperature as the wet-bulb temperature
(i.e., the wet-bulb thermometer temperature taken with the
end of the thermometer kept wet).
Repeat the final steps
several more times to ensure accuracy.
Add all of the wet-bulb
temperatures together and calculate the average wet-bulb
Use the dry- and wet-bulb temperatures, and the charts in
determine the relative humidity for more than one time of
day, and for more than one day.
Several calculations over the
middle portions of a day, several times a month should be
to determine if evaporative cooling would be effective in a
Exactly how relative humidity data are
used to determine the effectiveness of evaporative cooling
be discussed later.
FACTORS AFFECTING EVAPORATION
As discussed earlier, evaporation results in cooling of the
or other substances.
As the rate of evaporation increases so
does the rate of cooling.
To make the most effective use of this
technology it is important to understand the factors that
the rate of evaporation, and the relationships that exist
between these factors.
There are four major factors that affect the rate of
Although each of these factors will be discussed
it is important to keep in mind that they usually interact
each other to influence the overall rate of evaporation, and
therefore the rate and extent of cooling.
Factor 1: Relative Humidity
Relative humidity, as mentioned earlier, is the measurement
the amount of water vapor in the air as a percentage of
the maximum quantity that the air is capable of holding at a
When the relative humidity is low, only a
small portion of the total possible quantity of water vapor
the air is capable of holding, is being held.
Under this situation
the air is capable of taking on additional moisture, and if
other conditions are also met, the rate of evaporation will
higher. On the other
hand, when the relative humidity is high,
the rate at which water evaporates will be low, and
less cooling will occur.
Under such conditions of high relative
humidity, evaporative cooling may not be effective.
many areas with high relative humidity, such as the humid
tropics, evaporative cooling can be effective if a dessicant
(e.g., silica gel) is used to remove moisture from the air
before it is cooled.
Factor 2: Air Temperatures
Evaporation, as stated earlier, occurs when water absorbs
energy to change from a liquid to a gas.
Air with a
relatively high temperature will be able to stimulate the
process and also be capable of holding a 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 temperatures, less water vapor can be
held, and less evaporation, and cooling will take place.
Factor 3: Air Movement
Air movement, either natural (i.e., wind) or manmade (i.e.,
a fan), is an important factor that influences the rate of
As water evaporates from a surface it tends to raise
the humidity of the air that is closest to the water's
If this humid air remains in place, the rate of evaporation
start to slow down as humidity rises.
On the other hand, if the
humid air near the water's surface is constantly being moved
and replaced with drier air, the rate of evaporation will
remain constant or increase.
Factor 4: Surface Area
The area of the evaporating surface is another important
that affects the rate of evaporation.
The greater the surface
area from which water can evaporate, the greater the rate of
simple example will demonstrate the importance of
surface area to evaporation.
Consider the following two situations.
(1) one liter of water placed in a narrow glass container
with only about 16 [cm.sup.2] of surface area exposed to the
and (2) another liter of water poured into a large shallow
pan with about 180 [Cm.sup.2] of surface exposed to the
these two situations, which one could be expected to dry up
first, if both where left under the same environmental
Because of the large surface area, the large pan of water
would dry up much sooner than the jar.
Even though each of these factors has its own separate and
effect on the rate of evaporation, when combined, their
impact is much greater.
For example, the first two factors can
be discussed together in terms of wet- and dry-bulb
Under conditions where the difference between the wet- and
temperatures is great, the rate of evaporation will also be
great. The graph in
Figure 2 should help explain this situation.
Curve A traces the change in the air temperature (dry-bulb
over a 24-hour period; Curve B traces the wet-bulb
temperature, also recorded over a 24-hour period.
between the wet- and dry-bulb temperatures is the greatest
the period from 10:00 a.m. to 8:00 p.m.
From this it can be
reasoned that the relative humidity over this period was
This is also the time period with the highest average air
Thus, under these conditions it can be assumed that
the rate of evaporation would be relatively great.
If the two
other factors, air movement and surface area, are applied
the rate of evaporation would show an additional
MAXIMUM COOLING POTENTIAL
The extent to which evaporation can lower the temperature of
container or the air depends upon the difference between the
Theoretically, it is possible to
bring about a change in temperature equal to the difference
these two temperatures.
For example, if the dry- and wet-bulb
temperature were 35[degrees]C and 15[degrees]C respectively,
the maximum drop in
temperature due to evaporative cooling would theoretically
reality, though, while it is not possible to achieve
100 percent of the theoretical maximum temperature drop,
a substantial reduction in temperature is possible.
Depending on the environmental conditions, and the method of
evaporative cooling used, it should be possible to achieve
50 and 80 percent of the theoretical maximum drop in
In the example given above, this would have resulted
in a temperature reduction of between 10 and 16[degrees]C.
III. DESIGN VARIATIONS
There are two general methods of evaporative cooling: direct
evaporative cooling involves the movement of
air past or through a moist material where evaporation, and
therefore cooling, occurs.
This cool moist air is then allowed
to move directly to where it is needed.
In contrast to this
process, indirect evaporative cooling uses some form of heat
exchanger that uses the cool moist air, produced through
cooling, to lower the temperature of drier air.
dry air is then used to cool the environment, and the cool
air is expelled.
In situations where cool dry air is more desirable than cool
moist air, the extra effort or expense involved in building
buying and using a heat exchanger may be justified.
On the other hand,
many situations exist where it will be better to use the
less complex and less costly direct evaporative cooling
Evaporative cooling technology is used to cool rooms, homes,
food, or water. The
method of evaporative cooling used, direct
or indirect, depends on: (1) the specific needs of the
that will be cooled; (2) the availability and cost of
energy; and (3) the amount of money and skill available
to buy or build the cooler.
The following discussion will present specific examples of
both methods of evaporative cooling can be applied.
disadvantages, and limitations of each of these applications
are also examined.
DIRECT EVAPORATIVE COOLING
One of the simplest and most commonly used forms of
cooling is used to cool water.
This system usually uses either a
porous clay container or a watertight canvas bag in which
is stored. These
containers are then either hung or placed so
that the wind will blow past them.
The water in the containers
slowly leaks through the clay or canvas material and
from the surface as warm dry air flows past.
This process of
evaporation slowly cools the water.
Small bottles, bags, or jars of produce, medicine, or dairy
products can be suspended in the water so they can be kept
This method of evaporative cooling is common among street
of South Asia, who use it to cool soda pop and fruit for
This type of evaporative cooler has limited
application. One of
the primary limitations is that the drop in temperature will
generally be only a small fraction of the total temperature
reduction that is possible.
This is primarily due to the large
volume of water that needs to be cooled by a relatively
evaporating surface area.
Secondly, only a small number of items
can be placed in large water containers.
The following section of
this paper outlines some common examples of other
coolers. Before any
of these types of coolers are built or installed,
it is necessary to consider the probable effectiveness
of evaporative cooling in the specific environment and to
the benefits gained against costs incurred.
The following section of this paper outlines some common
of other evaporative coolers.
Outdoor Curtain Cooler
A variation of the simple process described above can be used
cool small outdoor areas (Figure 3).
In its simplest form this
involves the use of a sheet of canvas or some other strong,
absorbent cloth as an evaporating surface.
The upper edge of the
canvas sheet is suspended by ropes that are usually held up
pulleys so that the sheet can be lowered and raised
lower end of the sheet is secured in a trough of water large
enough to permit all of the sheet to fit.
When a cooler environment
is desired the canvas sheet is lowered into the trough of
water so that it becomes soaked with water, after which, it
raised. As hot, and
generally dry, air passes through and around
the moist cloth, evaporation occurs, which in turn cools the
This cool moist air then cools the immediate environment.
Obviously, the size of the area that can be cooled using
method is limited.
Moreover, this cooler can not substantially
lower the air temperature.
Even with these shortcomings, people
who have used these simple coolers have said that they do a
fairly effective job of making the immediate environment
simple nature of this cooler is its primary
advantage. If a more
comfortable outside environment is desired,
but cost is an important consideration, this cooler may be a
Indoor Curtain Cooler
Therather simple device described above can be adapted for
canvas, jute cloth, a coconut husk mat, or some
other absorbent material is used to expose water to moving
For use indoors, such a cooling device requires some form of
energy source, generally electricity, to power a fan to blow
air through the absorbent material.
A small water pump is also
needed to circulate water from a lower trough to an upper
This keeps water continually flowing through the absorbent
material so evaporation can occur.
Coolers of this type are used
extensively in the hot, dry areas of the western United
Figure 4 illustrates one such system used in a small
in New Delhi, India.
During the hottest part of the day the
owner of the restaurant would first start the water pump,
wait for the coconut mat to become soaked with water.
this, the fan would be turned on to force hot dry air
The thickness and density of the mat were
sufficient to slow the speed of the air and permit enough
to cool the air substantially.
This air was, in fact,
cool enough to keep people from sitting close to the cooler
even short periods of time.
Even though this cooler is very effective at cooling room
has several important disadvantages.
First, this system depends
on electricity to power both the water pump and the
the cool air that is blown into the room has a relative
of nearly 100 percent.
In some situations this high level of
humidity may be an undesirable since it may promote the
mold and mildew. The
small restaurant in India that used this
system avoided this problem by having only part of the
covered by a roof.
This allowed the saturated air to quickly
escape outdoors. A
further disadvantage of this method is its
constant consumption of water.
In areas where water is in short
supply, its use for cooling purposes may not be justified.
Despite these disadvantages, this cooler is capable of
indoor area at a fraction of the cost of a commercial
air conditioning system.
Cabinet Produce Coolers
Large amounts of fresh produce and dairy products are lost
spoilage in many tropical and subtropical areas of the
this food could be stored at relatively low temperatures
eaten or sold, much of this waste could be avoided.
For many of
these areas, though, commercial methods of cooling food are
either unavailable or too expensive.
Evaporative cooling may be a
practical alternative for use in tropical and subtropical
There are several types of cabinet coolers that use the
of evaporative cooling to cool stored produce.
Four types of
cabinet coolers are described below, in order of increasing
Type I Cooler
This simple cooler (Figures 5 and 6), which is essentially a
variety of materials ranging from bamboo to sawed
timber. It can
be cylinderal or rectangular in shape.
The cloth covering
(Figure 6) that surrounds the cabinet cooler absorbs water
the troughs at the top of the base.
Eventually the entire cloth
becomes soaked with water, and as the air moves past the wet
cloth, evaporation occurs.
As long as evaporation takes place,
the contents of the cabinet will be kept at a temperature
than that of the environment.
Under certain conditions, this simple cooler may be unable
maintain low temperatures.
For example, if the air is very dry
and the wind very brisk, the drying action may exceed the
action of the cloth, thus preventing it from staying moist.
This in turn will prevent the cooler from achieving and
a temperature much lower than the environment's.
type of cooler requires periodic attention to refill the
troughs, which may be a problem.
The consumption of water may
also pose a problem for areas where water is either scarce
difficult to obtain.
The major advantages of this cooler are its relative
low construction costs, and independence from commercial
Type II Cooler
The Type II cooler was designed to eliminate some of the
associated with the Type I cooler.
The design of the Type II
cooler is much the same as the Type I cooler, except that
walls of the Type II cooler are thicker and the water trough
replaced by containers of water that are positioned on top
The walls can be constructed from a variety of materials as
they meet the following requirements:
(1) the material must
allow air circulation; (2) it must very absorbent and
holding a substantial amount of moisture; and (3) the
itself, or the frame surrounding it, must be strong enough to
support the containers of water that will sit on top of the
cooler. One of the
walls of the cooler also functions as a door.
Inside the cooler, lattice shelves are spaced wide enough
so that there is as little obstruction to the air flow as
Small holes are punched along the outer edge of the bottom
This allows the water to drip slowly down to
the absorbent wall material.
The drip flow should be fast enough
to keep the walls continually moist, but not so fast as to
water to drip out of the bottom of the cooler.
Obtaining the exact
rate of flow requires some experimentation, but with
patience, an optimal flow rate can be achieved.
One such cooler (Figure 7) was built by the author for use
eastern Kenya. Four
"debi tins" (these are rectangular containers,
were originally used to store and transport biscuits)
each with an eight-liter capacity, were used as water
The holes were first punched in the bottom of the
about 0.5 centimters apart, using a nail.
Each hole was then
filled with candle wax which was punctured with a small
The wax allowed for the experimentation necessary to achieve
proper size holes for the optimum rate of water flow.
The absorbent walls of this cooler were made by first
sheets of jute cloth on either side of a rectangular wooden
made from five centimeters by five centimeters lengths of
Next, small mesh chicken wire was tacked over the jute
From a notch cut through the top of the frame, small chunks
(approximately 0.5 centimeters in diameter) of charcoal were
poured into the frame and packed between the sheets of jute
cloth. The chicken
wire helped to keep the walls from bulging.
The combination of jute cloth and charcoal allowed
flow to permit evaporation, while at the same time allowing
wall material to remain soaked with water.
On very hot, dry, and windy days, the four containers of
usually lasted the entire day.
At the end of cooler, less windy
days, the containers would often be found partially filled
water. The remaining
water then be poured into a container and
saved for the next day.
Fruits and vegetables were the primary foods kept in the
but occasionally milk and meat were also stored for short
of time. The
reduction in temperature achieved by this cooler,
along with the high level of humidity, were sufficient to
the storage of most fruits and vegetables for five to ten
and sometimes even longer.
Vegetables that were stored in a
shaded area would usually spoil in only two or three
or meat that was placed in the cooler in the morning would
be fresh in the evening when it was needed for the evening
meal. When not
stored in the cooler, milk and meat would usually
be spoiled by mid-afternoon.
Drinking water was also kept in the
provided a much more satisfying and refreshing
drink than water kept in bottles placed either under trees
On days when there was little or no wind, or when the
was high, the temperature in the cooler was not much less
However, for most situations in eastern
Kenya, this cooler prevented a substantial amount of food
spoiling and provided cool water for drinking.
The Type II cooler requires a some carpentry skill to build
tools such as a saw, hammer, block plane, and chisels.
the author used sawn timber, but it may be possible
to use other materials and achieve a similar degree of
Even though charcoal and jute proved to be very effective
materials for the cooler's walls, similar material could be
Consideration needs to be given to the probable
effectiveness of evaporative cooling for the specific
under question before this cooler is built.
Type III Cooler
This third type of evaporative cooler, often referred to as
Janatha air cooler, was originally designed and built in
using baked clay building blocks called "Hourdis"
block (Figure 8).
These blocks, are stacked together to form a rectangular
Slotted or grilled shelves are arranged
in the cooler and a wooden top and door seal the
cooler is usually built on a cement platform.
The hollow core of
each of the clay building blocks is kept filled with
water slowly seeps through the porous clay walls of the
block, eventually evaporating from the surface, thus cooling
Small holes are often drilled in the sides of
each of the blocks and fitted with short lengths of pipe
connect all of the hollow water-filled blocks together.
of the blocks another short length of pipe is fitted to
outside the cooler.
This pipe is used to drain the cooler
periodically to prevent a buildup of salt and mineral
the pores of the baked clay.
If the cooler is not drained, the
flow of water through the pores of the clay will eventually
A diagram of a completed Type III cooler is illustrated in
Two graduate engineering students at the University of Texas
designed an evaporative cooler similar to the Janatha air
Instead of using baked clay, which is known to have a
low level of porosity (i.e., the ability of water to flow
the small pores present in a material), the students used
made from jute cloth saturated with a very watery cement
Before the cement dries and sets, the dip-molded blocks can
formed into desired shapes.
This process of dip-molding allowed
the experimenters to build large blocks that not only had a
level of porosity, but were also very strong and relatively
light. Using this
technology, the students built a cooler that
used long tubular blocks (Figure 10).
Other experiments with dip-molded blocks indicated that a
block could be shaped directly into the walls of the cooler
(Figure 11). An
experimental U-shaped cooler is shown in Figure 12.
Type IV Cooler
This final type of cooler uses electricity to power both a
fan and in some cases a small water pump.
Essentially, this is a
small version of the indoor curtain cooler described
can either be designed and built to be a permanent structure
it can be made as a portable unit.
If a permanent cooler is
desired, it can be built along the lines of the Type II
Since a fan is used, the rate of air flow can be regulated
achieve an optimum rate.
Moreover, the rate of evaporation and
therefore cooling will be rapid since these systems are not
the mercy of intermittent winds.
There are variations of this
cooling system: (1)
an electrified version of Type II cooler,
and (2) a portable electric cooler.
The efficiency of the Type II cooler can be improved with
addition of a small fan and water pump.
The fan can either be
placed in the door or near the bottom of the cooler.
of the fan draws air through the water-soaked walls of the
at a constant and even rate.
This air, cooled through evaporation,
cools the food and water stored in the cooler.
The containers of water used in the Type II cooler are
with small troughs positioned along the upper and lower
the cooler. The
constant circulation of water ensures that the
(*) A detailed description of dip-molding can be found in
by W. Hutchinson and R. Chuang, Inexpensive Evaporative
for Short-Term Storage of Fruits and Vegetables: A Design
Report (See Bibliography).
absorbent wall material is always soaked with water.
along the bottom of the cooler should be built large enough
hold enough water for a full day's cooling.
The second form of the Type IV cooler is an electric
cooler. One such
portable cooler was designed and built by two
researchers at the University of California.
Even though this
portable evaporative cooler was intended to be used
fruit growers in the Southwestern United States, it should
prove useful to individuals living throughout tropical and
areas of the world.
Basically, this portable cooler is a simplified,
version of the electrified Type II cooler.
As shown in Figure 13,
one wall is a sheet of absorbent material, while the
wall has a fan attached to it.
Small troughs above and below the
wall of absorbent material hold water.
A drainage hose from the
lower trough is connected through a small water pump to the
trough on top of the cooler.
This provides constant circulation
of water through the system.
Boxes of fruits and vegetables are placed around the
cooling unit. The
fan forces cool moist air past the produce in
the boxes. Figure 14
illustrates an example of this set-up.
Slowly, the freshly picked produce will be cooled to
that will promote optimum storage life.
This portable cooler has been designed to prevent produce
spoiling before it is sold or sent to market.
Since this unit
takes up very little space and consumes so little
many fruit and vegetable vendors throughout the tropics may
this cooler a cost-effective method of protecting their
INDIRECT EVAPORATIVE COOLING
The high level of humidity that is produced by direct
cooling may be undesirable for some applications.
cooling attempts to solve this problem by using the cool
moist air produced through evaporation to cool drier
resulting cool dry air is then used to cool the desired
This transfer of coolness is accomplished with the help of
a heat exchanger.
All methods of indirect evaporative cooling require power to
both water pumps and fans.
For this reason, indirect evaporative
cooling will have limited application.
It is primarily used to
cool dwellings and rooms.
In such situations these cooling systems
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 reduction in temperature that can be achieved with
system is not as great as the reduction that can be achieved
conventional mechanical cooling systems.
Basic Characteristics of a Beat Exchanger
Figure 15 is a simplified diagram of a heat exchanger.
exchanger is composed of two sets of alternating channels
which air flows. The
air that passes through the vertical
channels comes in contact with water that is either being
or dripped into the channel.
If this air is warm and dry,
evaporation and cooling will occur.
This cool air then cools the
channel walls, which in turn cools the air that is being
forced through the horizontal set of channels.
Finally, the cool
moist air is directed outside the dwelling, while the cool
air is blown into the room or building that needs to be
Factors That Effect Cooler's Effectiveness
As with direct evaporative cooling, several factors
effectiveness of this cooling system.
Among the most important
are the relative humidity and the temperature of the air
cooled. Low levels
of relative humidity promote rapid evaporation
and, therefore, a greater rate of cooling can be achieved.
The rate of evaporation will also be increased if the air
is relatively high.
Incoming air with a high temperature,
however, will need more cooling than cooler air; therefore,
high temperatures can be both an advantage and a
Two other factors that also affect the rate of cooling are
rate of air flow through the heat exchanger and the
the water that is used in the evaporative cooling
the air is forced through the heat exchanger too quickly,
evaporation will take place, and therefore, little cooling
turbulence within the channels may increase the rate
of evaporation. The
size of the water droplets will also
influence the rate of cooling since it will have a
affect on the rate of evaporation.
If the water droplets are
large, they will have a relatively small total surface area,
compared to their volume, from which molecules of water can
droplets have a greater surface area,
compared to their volume, and therefore, evaporation will
more rapidly. This
will in turn promote rapid cooling.
the temperature of the water being sprayed or dripped into
channels will also affect the efficiency of the cooler.
water is cold, the walls of the heat exchanger will cool
this may also slow down the rate of evaporation
since cool droplets need to absorb more energy before
The design of the heat exchanger will also influence the
which cooling occurs.
For example, small channel spaces will
promote more rapid cooling than larger, more spacious
Moreover, if the heat exchanger is made from a material that
conducts heat efficiently, such as metal, the transfer of
from the wet channels to the dry ones will occur more
Two Examples of Indirect Cooling Systems
There are two types of indirect evaporative cooling
basic difference between these two systems is in the design
their heat exchangers.
In one system, air is circulated through
the heat exchanger in both horizontal and vertical
air forced through the vertical set of
channels will be [used to cool] the air flowing through the
horizontal set of channels.
The air in the horizontal channels
remains dry and will be used to cool the room.
In the second
system, air flows through both sets of channels in the same
direction, but like the first system, the cool dry air is
into the room while the cool moist air is directed outside.
Forcing air through the heat exchanger in two different
directions (Figure 15) has the advantage of being able to
different sources of air.
For example, the air for evaporative
cooling can be taken in from the room, while the air that is
to cool the room can be taken from the outside.
Figures 16 and 17 sketch the basic characteristics of one
and the design of the heat exchanger and cooler can vary
depending upon the materials used and the skill of the
builder. Figure 16
shows the two different air circulation
patterns mentioned earlier.
Figure 17 shows four different views
of a working model of bidirectional cooler.
This type of cooler
uses two blowers to achieve this bidirectional flow of air.
Most heat exchangers are made of metal, but a mass-produced
plastic heat exchanger was used successfully in an indirect
evaporative cooler in India.
No matter what type of heat
exchanger is used, it be important that it be designed and
to take advantage of the various principles that can
influence evaporation and heat transfer.
The primary advantages of indirect evaporative cooling for
increasing the comfort level of rooms are the relatively low
purchase or building cost and the relatively low operation
expense, as compared with conventional air conditioning
Before deciding upon indirect evaporative cooling, though,
important that the necessary environmental conditions,
earlier, be present. The more favorable these conditions
more effective the cooler will operate. One such cooler,
in Baghdad, Iraq, proved to be a practical alternative to
conventional mechanical air conditioners. This cooler
seven times the cooling was a conventional air conditioner,
consuming the same amount of electricity. This greater
was in part due to the 17[degrees] centigrade average
between the wet- and dry-bulb temperatures common in
IV. COMPARING THE ALTERNATIVES
The principal alternatives to evaporative cooling systems
refrigeration and air conditioning. These technologies offer
user a much wider range of application. If electricity,
(including that produced by photovoltaic cells), natural
kerosene are available, commercial refrigeration and air
conditioning systems can be used in any environment
th temperature or relative humidity. This is definitely not
case with evaporative cooling. Moreover, commercial systems
the user to control the amount of cooling desired. Again,
is not possible with most evaporative cooling systems.
advantage of commercial systems is that they usually require
day to day attention than comparative evaporative cooling
systems. However, where electricity or other commercial
sources are either unavailable or very expensive, and the
conditions are favorable, evaporative cooling should be
considered as a viable alternative to these more complex and
costly commercial systems.
Although lowering the temperature of fruits and vegetables
retard spoilage is an important benefit of evaporative
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 prevent the drying
of produce, and therefore extends its shelf life.
The primary advantage of evaporative cooling over cooling
that involve commercial refrigeration is its low cost. For
example, an evaporative cooling system developed in the
States to cool fresh produce was able to produce 14 energy
of cooling while using only one energy unit of electricity.
Commercial refrigeration systems commonly produce only three
energy units of cooling for each energy unit of electricity
consumed. Low operating costs in addition to low purchase or
construction costs substantially reduce the total cost of
One final alternative deserves mention. It is possible to
ice at night, even if the air temperature is above the
freezing point, if certain specific conditions are met. This
cooling and freezing is accomplished though the joint
of radiation and evaporation and could be used to produce
cooling. To be effective, natural freezing requires
of humidity, clear unobscured skies, and little or no
wind. Arid environments usually offer such conditions.
To produce ice this way all that is needed is a large flat
container that has a clear view of the sky and is well
from the ground. Figure 19 shows one such set-up that
produced ice for a reseacher at Purdue University in the
States. This device was placed in a field away from all
and buildings and filled with 2 - 3 centimeters of water. On
nights with temperatures between 4 and 7[degrees]C and with
humidities of 90 - 100 percent about 7.5M of ice would form
the surface of the water. If not collected and stored in an
insulated cooler early in the morning, the ice would quickly
soon after the sun rose. It is possible that enough ice to
food for a 24- to 48- hour period could be produced using
process if a large enough natural freezer was used.
The chief disadvantage of this system is its dependence upon
narrow set of environmental conditions, and a corresponding
lack of reliability. The graphs in Figures 20 and 21 show
wind, air temperature, and relative humidity affect the rate
cooling of this natural freezer. Moreover, if the night is
perfectly clear, the rate of cooling is reduced. This system
also requires the user to wake up before the sun rises to
and store the ice that may have formed during the night. If
little or no ice formed because of poor conditions, the user
would be unable to cool stored food. However, if ice is
only occasionally, this is an inexpensive method of making
V. CHOOSING THE TECHNOLOGY RIGHT FOR YOU
Making a decision on which type of cooling or refrigeration
system to use is not an easy process. It is important to
carefully the cooling needs, weighing them against a range
other factors, before selecting any of the options discussed
this paper. If this is not done, frustration and
may result. <see figure 18>
The following checklist may be useful in choosing a suitable
technology. Since every situation is different, this
may not always apply, but it should be of some help.
1. What are your cooling needs? Cooling different foods
temperatures. Cooling rooms or buildings is different
from cooling food.
2. What is the average relative humidity of the area where
needed? If the
relative humidity is consistently high,
cooling will not be available option, and therefore
needs to be considered. If the relative
humidity is low,
then evaporative cooling may be very effective.
3. How windy is the area where the cooling is needed? If
there is little
wind, evaporative cooling may not be the way
4. Is there a good supply of water where the cooling system
will be used? If
water is readily available, evaporative cooling
may be feasible.
5. Are the materials and skills needed to build the cooler
6. Is electricity available? Is it very costly? If
affordable, then a powered evaporative cooler
may be the best
choice since it offers more freedom and is
effective than passive evaporative cooling systems.
7. Are commercial mechanical cooling or refrigeration
they costly? If commercial systems are available,
and not too
costly, then they may be a better choice of
The design and construction of some of the evaporative
discussed in this paper may require the investment of a
substantial amount of time and money. It may, therefore, be
advantageous to turn the building of the evaporative cooler
a business. In India, for example, a local town builder has
started a business building Janatha air coolers. Before this
done, however, it should be determined if there will be
sufficient demand for such a cooler to warrent setting up a
If only a few individuals want to buy or build evaporative
it may be possible to build the coolers. By buying necessary
parts in volume, and by contracting out for the actual
the group can reduce the cost per cooler. As with all
cooperative efforts, it is important to keep very accurate
of all transactions.
1. "A Village
Food Cooler", AP-Tech Newsletter, July 1980,
Volume 4, No. 1,
2. Akuffo, F.O. and
K.D. Klorbortu, "Experiments on Food
Storage in the
Tropics Using Evaporative Cooling", VITA
3. Dunkle, R.V.,
"A Method of Solar Air Conditioning:, Mechancal
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"Solar Absorption Refrigerators in AIT",
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Term Storage of Fruits and Vegetables: A
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