How to use the DRYIT semi-continuous tray dryer
Practical Action
Introduction
What is the difference between batch and semi-continuous drying?
A batch dryer consists of a large box or chamber with internal runners that support a number
of trays. Heated air is blown into the chamber, usually by a kerosene or gas fired heater
blower. In use the trays are loaded with the product and placed in the drying chamber. The
doors are closed and the heater turned on and left running until all the product is dry. Most
people using batch drying operate on a 10-hour drying cycle as this allows two batches a day
to be processed.
In a batch dryer the trays at the bottom, that is nearest the source of hot air, dry first and as
drying proceeds those higher up the stack dry. The trays at the very top do not dry till the end
of the 10-hour period. This gives rise to two main problems.
• Some material, particularly that at the bottom, may over dry or be affected by being
hot for such a long time.
• The air leaving the dryer outlet becomes dryer and dryer and hotter and hotter as the
drying cycle proceeds. In other words, fuel is being wasted.
A semi-continuous dryer attempts to overcome these problems. Again it consists of a drying
chamber and heater but is fitted with a mechanical system that allows the removal of the
bottom tray when it is dry, automatic lowering of the remaining trays and the insertion of a
new tray of raw material at the top of the tray. While this is clearly an advantage over batch
drying. The semi continuous system also has three main disadvantages:
- The cabinet is more expensive and complex
- Semi-continuous drying involves 24-hour operation.
- Labour requirements are higher
We can sum up the relative advantages and disadvantages of the two systems as follows.
Batch
Very simple to build
Low cost
Low labour cost, simply fill and empty
Uses more fuel
Some material may be over dried
Product may deteriorate
Semi Continuous
More expensive and complex to build
Ideally requires 24-hr operation.
Better fuel efficiency
Less chance of affecting the product hence better quality.
Better retention of sensitive components, flavours, colours
Greater throughput per day.
A number of modifications have been developed or suggested by people using dryers in
different countries. This, in fact is one of the most interesting aspects of the drying
programme. Each country using the dryer takes an initial prototype which is then adapted
according to local needs, construction materials, methods of working. Few, if any of the
dryers currently in use are identical.
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