A range of technologies is used for food drying which include tray and tunnel dryers, spray, roller and
freeze dryers. With the exception of tray dryers, none of these are appropriate, in terms of cost and
output, for use by small and medium enterprises.
While sun drying on trays or in solar dryers can be considered as tray drying the term is normally
applied to small industrial systems with some form of air heater and a fan to pass air over the product
being dried. While small tray dryers are available from Europe and the USA, where they are used in
pilot plants and Universities, their cost (in excess of £25,000) makes them unaffordable and
uneconomic for producers in developing countries.
In the early 1980’s ITDG (now Practical Action) recognised the need for small, controllable, powered
tray dryers capable of producing high quality products that could be constructed by engineers in
developing countries to a great extent from locally available materials. The required basic
development work was carried out and there are now tray dryers, based on the principles developed
by ITDG, in some eight countries. The greatest up-take of the technology has been in Latin America
where probably over 100 units are now operational. The key point to bear in mind when considering
the local construction of such a dryer is to understand the basic principles involved and adapt them
to local conditions such as the dimensions of local plywood sheet, common stock steel sizes, social
conditions and fuel availability.
The tray dryer family
Practical Action is able to supply information to allow the construction of four types of dryer. In some
cases full drawings and a How to Construct and Use manuals are available together with a case study
book. Much of this information is in English and Spanish.
The dryers consist of a
cabinet containing trays
which is connected to a
source of air heated by
gas, diesel or biomass
such as rice husk. The air
temperature is usually
controlled by a thermostat
which is normally set
between 50 and 70OC. The
air enters the bottom of
the chamber below the
trays and then rises,
through the trays of food
being dried, and exits from
an opening in the top of
the chamber. In the
Direction of Air Flow
Figure 1: Basic parts and airflow pattern in a Practical Action dryer.
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