The dryers are made of trays held in a cabinet which is connected to a source of air heated by gas, diesel or bio-mass 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 Practical Action systems the trays are designed to force the air to follow a longer zigzag route which increases the air/food contact time and thus improve its efficiency. This system also reduces back pressure which means that cheaper, smaller fans can be used.