The main parameter that determines the thermal drying option is the principle of heat injection to the dried product.
The main types of drying used in the drying production of fruit and vegetable are:
In recent years the vacuum drying and its variety, that is sublimation drying, are becoming more popular.
Convective way. Дhis kind of drying differs from others as the heat supply to the particles surface of the dried raw material is caused by the movement of thermal conductor (hot air). Then the hot air is mixed with the moisture emitted from the product. This is the best way to dry products likevegetables, fruits, mushrooms and berries.
As a rule the equipment used for the convective type of drying consists of the following construction units:
— main drying chamber where the product drying process directly takes place;
— heat exchanger where the hot dryer agent (air) is created. The heat exchanger may also be located both inside the drying chamber and separately.
— duct systems that serve to supply the process air and exhaust the spent air.
Depending on the type of energy used, the heat exchangers can be:
The drying equipment of a convective type differs in the way the product is placed for drying (bands, trays, sieves and shelves), as well as the various devices the product is moved. The dehydration process may take place both in atmospheric pressure and under vacuum (vacuum dryers). The processing lines may be fitted with systems with both natural draught and forced circulation of the drying agent.
Depending on the type of product and line performance, there are several types of drying equipment:
– cabinet equipment;
– box equipment;
– band (conveyor) equipment;
– tunnel equipment.
The cabinet and box dryers are the batch operation lines, and the band and tunnel dryers are continuous.
At present, continuous-action band dryers are most in-demand. The benefits of this type of equipment are:
– allows the production process to be fully automated;
– allows to manage key parameters in real time,
– drying is carried out according to the most suitable modes, which apply to each type of raw material separately.
– finished dried products are homogeneous and of high quality.