ABSTRACT

Although the merits of using superheated stem as the drying medium for convective drying were recognized over a century ago, commercial exploitation of this idea did not occur until only about three decades ago. Compared to hot air drying superheated steam drying affords the following advantages: higher drying rates above a certain temperature, lack of oxidative damage of product, elimination of fire and explosion hazards, ability to utilize dryer exhaust which is superheated steam at a lower specific enthalpy, etc. For certain products it also yields a better quality product, e.g., higher mechanical strength in drying of paper made of mechanical pulp, higher porosity of certain food products allowing faster rehydration, whiter and stronger silk thread by drying silk cocoons in steam, better color and lack of internal cracks in drying of woods. On negative side, this technology is more expensive, not suited for products that are temperature sensitive and viable only for large production rates and with feeds of high water content. Also, if the exhaust steam from the dryer is not utilized in the same plant the net energy consumption is comparable to that of an air dryer.