Contact (conductive) drying.
The main method of dehydration for the various types of high-moisture (mashed) raw materials is conductive drying. The alternative name for such method is the contact drying.
The main merit of such dehydration method as conductive drying heat is directly transferred to the product through the hot surface of the heating elements. And the heat transfer across the surface is much more efficient than through the air, with lower air conductivity. The main advantage of conductive drying is that it is possible to make the process much more intensive through the rapid transfer of heat from the hot surface to the products, so that the raw materials quickly lose the moisture. From an economic point of view, it is important that energy costs are relatively small compared to other processing methods.
The contact drying typeof is generally used to dry mashed potatoes, vegetables and fruit puree. The output is the product in the form of instant recovery flakes. Dried potatoes have been widely used both in secondary processing and in supplying to the final buyer:
The contact drying process uses the rollers, the heat source for which is mainly vapor. Hot oil is sometimes applied as an agent. Drying may occur under atmospheric pressure or under vacuum.
Contact drying equipment for the mashed potatoes production can be of two types that are characterized by the number of rollers. There are one and two rollers dryers.
- The one-roller dryer consists of the main drying roller. At a certain distance it is touched by spreading thrashers. The thrashers, in turn, gradually increase the thickness of the raw material for future vegetable flakes. At the end of the drying roller cycle there is a device fitted with scraper blades, which scrape the product for further chopping. After the product is chopped, it goes for inspection and packaging.
- Two rollers dryers have two drying rollers. The thickness adjustment required for layer drying is adjusted by changing the gap between the rollers. They revolve in direction to each other. Thus, in roller dryers of this type the prepared raw material in liquid or paste state is imposed by a thin layer on a hot roller surface. The finished product after this drying is flakes or powder. The removal of air with saturated vapor from the dryer is done by a built-in ventilation system.
The air of high humidity is removed from the dryer by a fan. The performance of conductive drying equipment depends on three basic parameters. That is the surface temperature of the working rollers, the thickness of the product layer on the surface and the product's drying time..