Page 198 - Agriculture_Form_Three

Agriculture for Secondary Schools

evaporative cooler is a storage structure made up of one or more porous walls or
surfaces that enable air to pass through. Water is added to the walls or surfaces of the
cooler to keep it moist but not wet. As air flows across the moist walls or surfaces of
the cooler, the air temperature is decreased due to the loss of heat through evaporation
of water. Depending on the relative humidity of the ambient air, the temperature in
evaporative coolers can be lowered by about 5 - 10. Therefore, evaporative coolers
provide an environment which is both lower than ambient temperature and at a
higher level of relative humidity for the storage of fresh produce.

Charcoal cooler is an example of evaporative cooler. This cooler is made of open
timber or wood frame and two-sided wire mesh filled with charcoal. Charcoal is
used because it has pore spaces which absorb and store water inside. The charcoal
should be kept moist throughout. This can be done manually or using an overhead
water tank like in drip irrigation to spray the charcoal; to keep it moist. As warm
dry air flows through the moist charcoal, it draws energy from its surroundings. This
makes water to evaporate into the air giving a cooling effect. The higher the rate of
flow of dry air, the lower the temperature is achieved.
Procedures for constructing a charcoal cooler

Step 1: Identify a suitable location for your cooler. It does not matter whether it is
under the sunshine or a shade. What is most important is that you choose a location
where you do not have to shift the cooler from place to place. This is a permanent
structure.
Step 2: Determine the size of you cooler that you want to construct. The size will
depend on volume of produce you want to store.

Step 3: Erect a double wire mesh boundary with at least a sizeable width within.
Step 4: Pack large charcoal particles in between while filling the gaps with smaller
charcoal particles. Avoid use of charcoal dust because it will pollute your produce.

Step 5: For the base, you may use mud and for the roof, you may use dry grass. Do
not use wire mesh or charcoal for the base or roof construction.

Step 6: Put a door to keep away animals and thieves.
You have to be mindful that you have to pour water on the charcoal in order to keep
it moist throughout. This is important to maintain the cool temperatures within. You
can either do this manually or automatically using constructed structures similar to
those used in drip irrigation. The pouring of water can be done continuously or at
a time depending on weather condition. The higher the temperatures, the more the
water required. Figure 8.14 shows a section of a charcoal evaporative cooler.

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