How to use the DRYIT semi-continuous tray dryer
Practical Action
How to use the DRYIT semi-continuous tray dryer
Contents
1.) Preface
2.) Introduction
3.) Some economic considerations
4.) Principles of drying
5.) Operation of the tray dryer
6.) Annex 1: Useful data
7.) Annex 2: Fuel consumption
8.) Annex 3: Conventional psychrometric chart
Preface
Some years ago, Practical Action, then known as ITDG, received a letter from a businessman
in Colombia, South America asking if ITDG knew of any low-cost, controllable Dryers suitable
for herbal teas. While he felt sure that there was a market for these products in his country,
as imported brands were widely on sale, he was however unable to convince his bank that it
was possible to produce dried teas able to compete in terms of price, quality and
presentation. His loan request for $40,000 to purchase an imported dryer was thus
rejected.
By good luck ITDG had just prepared the first draft drawings for the construction of what is
now called the DRYIT semi-continuous tray dryer. These were sent to the enquirer and a unit
was built at a cost of $4000. The company was soon able to get into production and now,
the company is still in operation, running two dryers. Imported herbal teas have now almost
been displaced from the market. This short story illustrates the key objective of ITDG's DRYIT
programme and range of small industry dryers that are:
• Low cost
• Controllable
• Can be largely built locally
• Provide an ideal test system to investigate both pre drying (farm production, sanitation
etc) and post drying aspects (packing, presentation, marketing and economics)
Preserving a crop or food by drying it is probably the most common method used worldwide.
A whole range of drying methods are used from the simplest-laying the product out in the
sun, to complicated systems such as spray and freeze-drying. The choice of system used
depends on a number of factors the most important of which are:
• The nature of the product being dried
• The scale of operation
• The cash value being added through drying the local climate
• The demands in terms of quality of buyers
One particularly common method used in small industry is tray drying in which the food is
placed in trays with a mesh bottom through, or over which, heated air is passed. Small tray
Dryers are commercially available and basically consist of a box full of trays and a source of
hot air. Such commercial units, which in general have to be imported, tend to be too
expensive for small businesses in developing countries - a typical half-ton dryer now costs
$85,000.
There are three models of DRYIT dryers the Batch Dryer, the Medium Semi-continuous Tray
Dryer and the Semi-continuous Tray Dryer. These are described in more detail later.
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