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< prev - next > Crop processing Drying Tray dryers_KnO 100319 (Printable PDF)
Tray dryers
Practical Action
Double chamber batch dryer
plywood over wood frame
Cabinet 1.8 x 2.0 x 3.0m
28 trays. Useful tray area 1.2m2
Heat source:
indirect diesel heater blower, max 60kW heat
240 kg per 24 hours
a few hours to load and unload
Cost (in UK):
cabinet £500
heater £1500
Economic considerations. During processing any food value is added and the value added per kg
processed must be sufficient to cover all the costs involved such as labour, fuel, rent and
depreciation costs. In general very little value is added to low value foods such as cereals and root
crops. For this reason it is only economic to dry such commodities in very large plants. Considerable
value can however be added to certain fruits, herbs, nuts, spices and vegetables such as mushroom
or asparagus. These are the foods most commonly processed by small and medium drying enterprises
Drying data. Most of the experiences to date have been with drying herbs and fruits. Typical drying
times for herbs in semi-continuous dryer at 55oC are 4 hours prior to the removal of the first tray,
after which a tray of dry herbs can be removed every 15 minutes. Each tray removed is replaced with
a tray containing 5kg of fresh herbs. In the case of sliced fruits however it requires 8 to 10 hours to
dry the first tray. Thereafter, trays can be removed and loaded every 30-45 minutes. Drying data
and production costs for a wider range of products are currently being investigated and Practical
Action can access additional information through its network of development organisations.
Organisations and enterprises seriously interested in developing dried food products may be
interested to know that a test dryer has been designed that can be locally constructed. This allows
drying rate curves for a given food to be quickly and cheaply prepared. Such information is invaluable
to those involved in equipment design and new product development. The unit consists of one foot
box heated (in the same way as the tea-chest dryer) by a domestic hair dryer. The tray of food being
investigated is suspended from an electronic balance reading to 0.1g. The weight of product is then
simply plotted against time as it dries. Drawings are available from Practical Action.