Tray dryers
Practical Action
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 efficiency. This system also reduces back pressure which
means that cheaper, smaller fans can be used.
The airflow in a typical system is shown in Figure 1. There are three basic types of tray dryer
cabinets; batch, semi-continuous and cross flow dryers. To date Practical Action 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 fuel consumption, in the later stages of drying when most of the trays
are dry.
Schematic Horizontal Section
3m
2m
Air out
4 trays per layer x
12 layers
Ducts form
heater
Swinging
vane
1.8m
Dimensions are
Approximate
Compartment 2
Trays not shown
Compartment 1
Figure 2: A typical double chamber batch dryer
Semi-continuous cabinets were developed by Practical Action 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. The other
design, developed in Sri Lanka, has been found more suitable for use by women 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
2