How to use the DRYIT semi-continuous tray dryer
Practical Action
For the medium size tray Dryer, heat output should be 50,000 Btu/hr (15kW) with a fan
delivery of 400 cfm (680m3/hr).
Engineering workshops in Peru are now building their own heater units for the large tray dryer.
This is an important step forward in terms of increasing self-reliance.
It may be possible to find suitable heaters and thermostats in your own country. However if
you are having difficulty with this then please contact Practical Action that will be able to
assist with finding suppliers.
Drying
With the tray Dryer the bottom trays will dry faster than the top trays because they are closer
to the hot air. As the air moves up over each tray in a zig-zag fashion it becomes more humid
and cooler. The air exiting from the Dryer should have very little capacity to dry (see earlier
notes).
The drying time of the product can be estimated from the psychrometric chart. The pick-up
factor is critical. For less dense raw materials such as herbs it could be 50% of the
theoretical value. With dense or sugary or starchy foods such as osmo-dried fruits or potatoes
it could vary from 10 - 30% of the theoretical value. The psychrometric gives an idea of
drying characteristics. Practical tests will give the true characteristics.
Hence the bottom one or two trays will take this estimated, or hopefully determined, time to
dry. In the case of herbs this could be around 3 - 4 hours and for fruit around 8 - 10 hours.
After this time the bottom tray should be removed. It should be checked for dryness before
complete removal.
Checking for dryness
This depends very much on the type of raw material being dried. Most fruits are dried until
they become pliable, leathery and the inside does not appear over moist. Most vegetables and
bananas should be dried until they become brittle. Fruits treated by osmosis will be pliable
when dry and still appear moist.
This is a very rough guide. More information could be gathered from purchasing samples of a
reputable competitor (if applicable). Storage trials are a good indicator of moisture content as
earlier discussed.
Removing and loading the trays
The trays are raised by pulling down on the handle and if one person is operating, held in
place by the catch. The door is then opened. It will be seen that all the trays except the
bottom one have been raised. Then using the hook, never your fingers, the bottom tray can be
checked. If the product has reached the desired dryness the tray can be removed.
The door is then closed and the handle slowly released to let the stack of trays drop down. A
tray of fresh material can then be loaded at the top. The bottom tray should be checked for
dryness. Remove as detailed as above.
If dry then remove as detailed as above
Tray removal interval
Subsequent trays should then be removed at regular intervals. The approximate time of this
interval can be calculated thus:
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