How to use the DRYIT semi-continuous tray dryer
Practical Action
to drying. In fact the warm and humid conditions inside a dryer could be highly suitable to
growth of micro-organisms. Hence drying should be carried out as quickly as possible to get a
dried product, which will not support the growth of micro-organisms. (For most dried foods
between 5 and 10% water). In controllable drying it is possible to consistently achieve
maximum drying rates because there is minimum dependence on less controllable factors
such as climate.
3. The humidity of the air
The humidity of air is the amount of water carried by it. Humidity can either be measured in
absolute terms (kg of water per kg of air) or in relative terms. For example, air at 30%, (of the
maximum potential), humidity. The lower the humidity the more useful the air for drying
purposes. As mentioned above raising the temperature has the effect of lowering the humidity
of air. Passing air of lower humidity over the material to be dried results in a greater rate of
drying.
This table shows the effect of heating air at 30°c and 70% RH and ability to pick up moisture
Temperature °C
30
40
50
60
RH%
70
40
22
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Maximum Potential Weight of
Water that this air can pick
up (kg/kg air)
0.002
0.006
0.009
0.012
4. The quantity of air
Referring back to the analogy of drying clothes, it is most common for clothes to be dried
outdoors. The major reason for this is the good availability of moving air. Drying rates can be
increased when relatively large volumes of fast moving air are used. Fast moving air means
that a greater quantity of dry air is passing over the material to be dried which means that
more water vapour can be picked up by the air and carried away. This in turn means that
water within the material will move to the surface at a greater rate.
In most drying systems the quantity (or volume) of air delivered by the fan connected to the
heater is not variable. If local construction of a heater unit with fan is considered then one
point to be aware of is backpressure. The ability of a fan to work against a backpressure is
quoted in fan specifications as inches water gauge "wg. This is the pressure created within
the dryer when it is fully loaded with material to be dried. The effect is to restrict the flow of
air through the dryer. Consult your engineer that the "wg of the fan is enough. The better
design of fan the better it can overcome the effects of backpressure and maintain a high
airflow rate through the dryer. Hence always use the best quality fan available. A very high
airspeed can always be reduced at low expense using dampers. Fan replacement is more
costly.
Case hardening
Case hardening is a phenomenum of drying which affects the quality and is related to a
combination of the above factors. It is, as the words suggest, the formation of a hard case
around the food pieces. This case or coat prevents further evaporation of water at the surface
of the food. Hence drying stops and what results is a tough dry exterior and a soft, moist
interior. The interior is therefore susceptible to spoilage and the overall quality of the product
is very low.
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