SOLAR DRYING
Introduction
The heat from the sun coupled with the wind has been used to dry food crops for preservation for
several thousand years. Other crops such as timber need to be dried before they can be used
effectively, in building for instance. This sun-drying has often developed into solar-drying, where
the drying area is in an enclosed ventilated area – often with polythene, acrylic or glass covering -
as a more efficient harnessing of the elements of the drying operation. There are innumerable
designs in use and each has its advantages and disadvantages. However, there are three basic
designs upon which others are based: solar cabinet dryer, tent-dryer, and solar tunnel dryer.
These are discussed below after a brief description of the principles of drying.
Basic principles of drying
Drying depends upon:
• Temperature, humidity and quantity of air used
• Size of the pieces being dried
• Physical structure and composition
• Airflow patterns within the drying system
Heat is not the only factor which is necessary for drying. The condition, quality and amount of
air being passed over and through the pieces to be dried determine the rate of drying. The
amount of moisture contained in the air to be used for drying is important and is referred to as
absolute humidity. The term relative humidity (RH) is more common and is the absolute
humidity divided by the maximum amount of moisture that the air could hold when it is
saturated. RH is expressed as a percentage and fully-saturated air would have an RH of 100%.
This means that it cannot pick up any more moisture. Air containing a certain quantity of water
at a low temperature will, when heated, have a greater capacity to hold more water. The table
below gives an example of air at 29oC with an RH of 90%. Such air, when heated to 50oC will
then have an RH of only 15%. This means that instead of only being able to hold only an extra
0.6 grams of water per kilogram (at 29oC), it is able to hold 24 grams per kilogram. Its capacity
to pick up moisture has been increased because it has been heated.
When placed in a current of heated air, food initially loses moisture from the surface. This is the
constant rate period. As drying proceeds, moisture is then removed from inside the food
material, starting near the outside. Moisture removal becomes more and more difficult as the
moisture has to move further from deep inside the food to the surface. This is the falling-rate
period. Eventually no more moisture can be removed and the food is in equilibrium with the
drying air.
The effect of air temperature upon relative humidity
Air temperature oC
RH%
Amount of water/kg air needed to reach
100% RH (grams)*
29 90 0.6
30 50 7.0
40 28 14.5
50 15 24.0
* ie the potential for the air to pick up moisture (RH = Relative Humidity)
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