The dryers consist of a cabinet containing trays 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 70 ^OC. 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 IT systems the trays are designed to force the air to follow a longer zig-zag route which increases the air/food contact time and thus efficiency.

This system also reduces backpressure, which means that cheaper, smaller fans can be used. There are three basic types of tray dryer cabinets Batch, Semi-Continuous and Cross Flow Dryers. To date it has only worked with the first two systems.

Batch Cabinets are the simplest and cheapest to construct. The cabinet is a simple large wooden box fitted with internal runners to support the trays of food being processed. The trays are loaded into the chamber, the doors closed and heated air is blown through the stack of trays until the entire product is dry. Clearly, as the hot air enters below the bottom tray, this tray will dry first. The last tray to dry is the one at the top of the chamber.

The advantages and disadvantages of this system are:

• simple, low cost chamber

• low labour costs – simply load and then unload

• a tendency to over-dry the lower trays

• low efficiency, in terms of furl consumption, in the later stages of drying when most of the trays are dry.

Semi-Continuous cabinets were developed in order to overcome some of the disadvantages of the batch system. In a semi-continuous cabinet, a lifting mechanism allows all of the trays except the bottom tray to be lifted. It is thus possible to remove the lowest tray as soon as the product is dry. The mechanism then allows all the trays to be lowered (now tray 2 is at the bottom of the stack). This leaves a space at the top of the stack to load a tray of fresh material.

Two types of lifting mechanism are available both of which activate four movable fingers that lift the second tray upwards. One design is operated by a handle, which is pulled downwards. Women have found the other design, developed in Sri Lanka, more suitable for use and here the lifting mechanism is a car screw jack, which, on winding up, lifts the four fingers.

The advantages/disadvantages of this system are:

• over-drying is avoided

• product quality is higher

• fuel efficiency is considerably increased

• a higher daily throughput is possible

• the cabinet is however more expensive to construct

• labour costs are higher due to loading and unloading trays at regular intervals

• in order to maximise output 24 hour working is recommended

Cross Flow Chambers

Although it has not, as yet, developed this system it is considered worth mentioning in this short brief. In this chamber the air is blown, through a series of louvres, directly across the trays and then re- circulated over the heater. In the early stages of drying, when a lot of water is being removed, a high proportion of the air is vented to an exit and replaced by fresh air. As drying proceeds the proportion of vented air is reduced. At the end of the drying cycle no air is Vented.

This system then overcomes the problems associated with batch and semi- continuous cabinets in that:

• labor costs are low as it works like a batch dryer

• all the trays dry at the same rate

• fuel efficiency is maximized.

Cross flow systems are however, technically more complex and require automatic humidity sensors to control the percentage of air vented during the drying cycle.
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