How to use the DRYIT semi-continuous tray dryer
Practical Action
only very slowly, are having a more significant effect on the overall drying rate. See points C -
D on the drying curves.
Implications
During the constant rate period the key factor is the rate of evaporation at the surface of the
food. This can be maximised by decreasing the humidity (through raising the temperature)
and ensuring a good quantity of air is being supplied. The water in the food can be picked up
easily from the surface and so the rate of movement of water to the surface is high. However
fast drying rates are not required for case hardening susceptible foodstuffs.
During the falling rate period it is the rate of movement of water to the surface which
becomes increasingly more significant than the rate of evaporation. This rate of movement is
dependant on the temperature and the surface area. The greater the temperature the greater
the rate of movement of water to the surface; the greater the surface area the easier it is for
water to reach the surface ie the less it has to travel.
In the case of the tray dryer
In the semi-continuous tray dryer the raw material starts in an environment of fairly warm
moving air. The humidity is actually quite high at the top of the dryer and there may not be
much drying in the top tray. As trays from the bottom are removed and new trays loaded at
the top, the trays gradually move through zones of dryer and warmer moving air until
eventually the raw material is in a zone of low humidity and high temperature. Case hardening
is minimised with this system because drying rates are quite low in the early stages.
Psychrometry and the psychrometric chart
Psychrometry is the study of the properties of air. In other words what is going on when air is
heated up or cooled down. The diagram below is a typical representation of the psychrometric
chart.
It looks complicated but....
What is so useful about this chart is that it enables one to predict for example, how much
water vapour can be removed from a raw material using a particular condition (temperature,
humidity and quantity) of heated air. For example from psychrometry it would be possible to
measure if, in a dry warm part of the world, it was actually worthwhile, in terms of cost and
throughput, to heat the incoming air. It may be just as effective to use ambient air at high
speed. Possibly raising the temperature towards the end of the drying process.
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