How to use the DRYIT semi-continuous tray dryer
Practical Action
Some typical fresh and dry moisture levels are shown below:
Raw
Most fruits
Herbs
Green leafy vegetables
Coconut flesh only
c. 85%
c. 90%
85-90%
40%
Dried
15-20%
5%
8%
2-3%
The following example shows how to calculate the drying ratio when the fresh and dry
moisture levels are known.
A green leafy herb with a fresh moisture content of 90% is to be dried to a final moisture of
6%. What is the drying ratio?
100 kg fresh will contain 90kg of water and 10 kg of totally dry matter.
10 kg of totally dry matter (0% moisture) will increase in weight, if its moisture content rises
to 6% by:
10 x 0.06 / 0.94 = 0.64kg
Thus 100 kg of fresh herb will, after drying, yield 10.6 kg of dry product. The drying ratio is
thus:
100 / 10.6 = 9.4: 1
This figure is very useful. If for example 648 kg are fed to the dryer the yield will be:
648 / 9.4 = 69 kg.
It is also useful in calculating when trays of material are dry enough to be removed by:
a) Weighing the empty tray,
b) Adding a known weight of raw material and
c) Checking when the tray reaches a calculated final weight.
In addition this calculation, by using the weight of water removed, has application in
determining dryer capacities (see later)
Fuel consumption
The measurement of fuel consumption is fairly simple. If kerosene fired burners are used, the
tank should be dipped with a marked pole to determine fuel use. Alternatively if burner
specifications are available a graph of fuel consumption at different temperatures can be
prepared as shown in the example below:
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