How to use the DRYIT semi-continuous tray dryer
Practical Action
cotton sock which is soaked in water. This has the effect of creating an atmospheric condition
of air that is at 100% RH ie is fully saturated. Hence the wet bulb temperature line indicates
the temperature of saturated air. These temperatures are given on the psychrometric chart
because in a perfect drying situation the air coming out of a dryer should be at 100% RH.
When a wet bulb thermometer is placed in the same air as a dry bulb thermometer the wet
bulb temperature is always less than the dry bulb temperature, (except when the dry bulb is
in air that is 100% saturated), because evaporation from the water soaked cotton sock
surrounding the thermometer bulb has taken place in order to balance the humidity with the
surrounding air. Evaporation causes cooling because the energy for evaporation has come
from the heat in the air. Hence the air will cool down and will hold less water vapour. So the
air gradually cools until it reaches a temperature where it can absorb no more water vapour
from the cotton sock ie it too has reached saturation. At saturation there is no evaporation.
This is the wet bulb temperature.
The best-known use of these two types of thermometer is for recording the weather. So, for
example, on a very hot dry day there would be a lot of evaporation from the sock and therefore
a lot of cooling of the temperature. The greater the difference in temperature between the wet
and dry bulbs the lower the humidity of the air. Tables are available which give the humidity
of the air from knowing the wet bulb and dry bulb temperatures.
Pickup factor
The psychrometric chart is a theoretical tool for measuring the potential of heated air to pick
up moisture from a material such as food.
In reality the data obtained from the chart hardly ever matches the data obtained from
practical experiments. Hence a 'pick-up factor' is used to convert the data from the
psychrometric chart into more realistic data. Although it is unlikely to be exactly the same as
data from practical experiments it would be much closer.
The pick-up factor ranges from 10-50% of the theoretical figure.
Why is it so low?
The ability of air to pick up moisture depends on a number of factors such as how easily the
moisture is evaporating from the food to be dried and how well the air mixes with the food.
During the initial stages of drying, moisture is evaporating easily from the surface - the
constant rate period. However during the falling rate period the moisture is evaporating less
easily. Hence the pick-up ability during the falling rate period is lower than during the
constant rate period.
Normally the constant rate period is much shorter than falling rate period. This means that in
drying of foodstuffs the average ability of the air to pick-up moisture is going to be low. The
greater the initial moisture content, the longer the constant rate period. Similarly the lower
the desired final moisture content, the longer the falling rate period.
Not only are the initial and final moisture contents important in determining the pick-up
factor but also the way a particular food dries and the design of the dryer affects the pick-up
ability.
How to select a pick-up factor
Generally it can be said that foods that dry easily and quickly such as herbs and some
vegetables will have a higher pick-up factor (50%) than fruits, starchy foods and semi
crystallised fruits (30-10%).
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