The food can be either exposed to the sunlight (in direct systems) or heated air is passed over shaded food in indirect systems. Direct systems are used for food such as raisins, grains and coffee where the color change caused by the sun is acceptable, but most foods need indirect systems to protect the colors in the food. Other types of driers use fans to blow the air over the food but this adds to the capital and operating cost and removes the advantages of driers in rural areas which can not operate without electricity.
There are three basic types of drier, each of which has many variations. 1, tent driers (direct), 2, cabinet driers (direct or indirect) and 3, chimney driers (indirect). Each of these types uses natural air circulation although it is possible to fit an electric or wind powered fan to increase the speed of the air.
Tent Solar Dryer – Figure 5
This type consists of a ridge tent framework, covered in clear plastic on the ends and the side facing the sun, and black plastic on the base and the side in shade. A drying rack is placed along the full length of the tent. The bottom edge of the clear plastic is rolled around a pole, which can be raised or lowered to control the flow of air into the drier. Moist air leaves through holes in the top corners of the tent.
The advantages of this type of drier are the low construction costs and simplicity of operation. However, like other types of solar drier, there is relatively poor control over the RH of the air in the drier and so, poor control over drying rates. It is also lightweight and fairly fragile when
moved or in windy conditions.
Cabinet Drier – Figure 6
The basic design is an insulated rectangular box, covered with clear glass or plastic. There are holes in the base and upper parts of the box to allow fresh air to enter and moist air to leave. The inside of the cabinet is painted black to act as a solar collector. In indirect types, a flat plate is painted black and suspended in a insulated frame. Air is heated on both sides of the plate before passing into the drying cabinet. Food is placed on perforated trays within the cabinet and warm air from the collector rises up through the food and leaves through the top. The length of the cabinet is approximately three times the width to prevent shading by the sidewalls.
The sides can be made from board or mud-coated basket work. Larger models can be made from mud, brick or cement. The insulation can be wood shavings, sawdust, coconut fibre, dried grass or leaves, but should be at least 5cm thick to keep the inside temperature high. If insects are a problem, the air holes should be covered with mosquito netting. Drying trays should be made from basket work or plastic mesh. Metal should not be used as it can react with the acids in fruits and some vegetables and cause off-flavors in the food. These type of driers are used for fish, fruit, vegetables, root crops and oilseeds. They have capacities of up to 1 tonne.
This is a modified cabinet drier in which a solar collector of black plastic or burnt husks is covered by clear plastic on a wooden framework. A black plastic chimney heats up the air above the exit to the drier and therefore increases the airflow through the drier.
Artificial (Mechanical) Drier
These use fuel to increase the air temperature, and reduce the RH and fans to increase air speed. They give close control over the drying conditions and hence produce high quality products. They operate independently of the weather and have low labor costs. However, they are more expensive to buy and operate than other types of driers. In some applications, where consistent product quality is essential, it is necessary to use mechanical driers.
Light Bulb Drier
This consists of an electric light bulb inside a wooden box. If electricity is available this is a simple, low cost drier which may be suitable for home preservation. The capacity is very small and it is not likely to be useful for income generation. The bottom of a box is painted black, or covered in soot or black cloth. The sides are covered in shiny material (for example aluminum paint) to reflect the heat onto the black surface. Air circulates by natural convection in a similar way to the solar cabinet drier, but in this case the drier can operate all night as well as all day.
The design is similar to the solar type but in this case the heat is supplied by burning fuel or electricity. If electricity is available, a fan can be used to increase the speed of air moving over the food and therefore increase the rate of drying. To be economical it is likely that this type of drier should be relatively large (1-5 tonnes). These are successfully used for drying herbs, tea and vegetables.
These driers consist of trays or tanks containing a deeper layer of food than those found in cabinet driers.
They have a larger capacity and are often used for grain drying, where the amount of water to be removed is smaller than for example fruits and vegetables, but the quantities involved are larger.
There is considerable scope for the use of small driers that combine the low cost of solar heating with better control of mechanical driers.
source: practicalaction.org, photo from omick.net