How to use the DRYIT semi-continuous tray dryer
Practical Action
Drying efficiency
The efficiency of drying is an overall measure of how much of the energy supplied to the Dryer
is used to dry the food. It is a different indicator than the pick-up factor. The efficiency of a
Dryer is calculated without using a psychrometric chart because it is based on factors such as
weight of water lost during drying and the amount of heat supplied to the Dryer. These can be
worked out without referring to the chart. A very efficient dryer uses ALL of the energy
supplied to dry the food. A very inefficient dryer dries food more slowly because, for example,
heat is lost from the dryer and air comes out from the dryer still with capacity to remove
water.
Efficiency can be improved by making modifications to the insulation of the Dryer, use of re-
circulated air, by ensuring that as much as the raw material as possible comes into contact
with the air without overloading and by ensuring that the exiting air is as saturated with water
vapour as possible.
Drying as mathematics
In this section the knowledge gained from the previous two sections will be applied to enable
predictions of the drying process to be made.
I. The Psychrometric chart can be used to calculate:
a) the potential of air to pick up moisture from a raw material
b) the potential amount of water that can be picked up over time
The following basic data will be needed:
• Temperature of air entering Dryer
• Temperature of surrounding air
• Humidity of surrounding air
II. The efficiency of the drying process can be calculated from knowing how much water was
actually removed from the raw material and how much heat (or energy) was used to assist that
process.
What is the potential of the heated air to pick up moisture from the material?
Working through the following example:
Surrounding air: Temperature
RH
18°C
60%
The Dryer heats the air to 50 °C
From the psychrometric chart
A dry bulb temperature of 18°C and 60% RH corresponds to point A. Reading across to the
absolute axis (point B) gives a figure of:
0.007kg moisture/kg air.
It is then possible to calculate the maximum possible amount of water vapour in the air at
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