ter flow regulators can be supplied The valves o B4-KH13A Bevel-helical gearboxes B4

Flender/Flender Gear Units/Bevel-helical gearboxes B4

urface area. Drying occurs by effecting vaporization of the liquid by supplying heat to the wet feedstock. As noted earlier, heat may be supplied by convection (direct dryers), by conduc-tion (contact or indirect dryers), radiation, or volumetrically by placing the wet materialin microwave or radio frequency electromagnetic eld. Over 8% of industrial dryersare of the convective type with hot air or direct combustion gases as the drying medium. 1 Mujumdar and Beke Over 9% of the applications involve removal of water. All modes except the dielectric (microwave and radio frequency) supply heat at the boundaries of the drying object sothat the heat must diffuse into the solid primarily by conduction. The liquid must travelto the boundary of the material before it is transported away by the carrier gas (or byapplication of vacuum for nonconvective dryers). Transport of moisture within the solid may occur by any one or more of the follow- ing mechanisms of mass transfer: Liquid diffusion, if the wet solid is at temperature below the boiling point of the liquid Vapor diffusion, if the liquid vaporizes within material Knudsen diffusion, if drying takes place at very low temperatures and pressures (.., in freeze-drying) Surface diffusion (possible although not proved)Hydrostatic pressure differences, when internal vaporization rates exceed the rate of vapor transport through the solid to the surroundings Combinations of the foregoing mechanisms Note that since the physical structure of the drying solid is subject to change during drying, the mechanism of moisture transfer may also change with elapsed time of drying. 1.1 Thermodynamic Properties of AirWater Mixtures and Moist Solids 1.1.1 Psychrometry Most dryers are of the direct (or convective) type. In other words, hot air is used both to supply the heat for evaporation and to carry away the evaporated moisture from the prod-uct. Notable exceptions are freeze and vacuum dryers, which are used almost exclusively for dryi