How to use the DRYIT semi-continuous tray dryer
Practical Action
= 65-61.75 = 3.25 dry material
With 6% moisture this weighs = (3.25 x 0.06 / 0.94) + 3.25 = 3.46 kg
Total moisture lost
= 65 - 3.5 = 61.5 kg
Thermal Efficiency
= (61.5 x 2.3 / 4.5 x 2.6 x 46) x 100 = 26%
46 is the heating value of diesel (MJ/kg)
2.3 is the latent heat of water at 50°C
2.6 is kg/hr diesel.
A list of useful data is given in annex 1.
Summary of this entire section
The objective in drying is the most amount of dried product at the highest quality and lowest
cost. There are four basic factors that control drying: temperature, humidity, airspeed and
surface area. During drying there are two basic drying rates: constant rate and falling rate.
The above four factors play different roles during each rate.
Drying should take the moisture content of the food below its critical moisture content.
Packaging and storage will help to ensure that the critical moisture content is not exceeded in
an unreasonably short time. Exceeding the critical moisture will increase the risk of the food
spoiling and therefore becoming unsaleable. Different foods have different critical moisture
levels and this is reflected in different requirements for packaging and storage.
Psychrometry is the study of the properties of air. Knowledge of how to use the psychrometric
is very useful for predicting drying times for a particular drying condition. This is however a
theoretical calculation and in practise the potential of the air to pick up moisture during
drying is 30- 50% of the values determined from the psychrometric chart.
Thermal efficiency is the measure of how much heat or energy put into the system was
actually used to dry the food. Factors that govern efficiency of drying: design of Dryer, batch
or semi continuous, condition of exit air, insulation. Additional equipment needed:
Anemometer, thermometer, humidity meter.
Operation of the tray dryer
The flow charts detailed below gives the main steps in processing a range of foodstuffs using
the tray dryer:
Raw material pre-processing stages
1. Raw material
2. Weigh
3. Remove roots\ stalks\ poor material
4. Wash \ clean raw material
5. Peel \ chop \ slice \ dice
6. Weigh & calculate percentage loss
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