When mechanical dehydrators are used, the product is placed on metal mesh belts in a tunnel, or in a cabinet on trays where controlled, elevated temperatures are used. Heated air is circulated by blowers and the air temperature, relative humidity and air velocity are controlled. Hot air driers of this type are classified as parallel flow, counter flow, direct flow, or cross flow, depending upon the direction of flow of the heated air. In bin, loft and fluidized-bed driers the heated air is blown upwards though the product.