Superheated drying rate

The drying times may range from days to months. Wood, for example, is dried in hot air kilns from weeks to months, depending on the size of the pieces to be dried and the type of wood species. Superheated steam drying under vacuum conditions enhances drying rates as well as product quality. Only batch dryers are suited for these long drying time requirements. 

More recently, Kast [18] has presented modified correction factors for high mass fluxes in laminar and turbulent flows. Although more complex in form, their effect on the calculated drying rates for superheated steam drying of paper was found by Loo and Mujumdar [19] to be negligible. The simpler forms given by Keey are therefore recommended for practical calculations. 

Products that may melt, undergo glass transitions, or be otherwise damaged at the saturation temperature of steam at the dryer operating pressure cannot clearly be dried in superheated steam even if they contain only surface moisture. Operation at reduced pressure, however, is a feasible option that may also enhance the drying rate. 

Pang, S. and Dakin, M. Drying rates and temperature profile for superheated steam vacuum drying and moist air drying of softwoodlwabesc. Drying Technology 17(6) 1135-1148, 1999. 

The advantages of using superheated steam as the drying medium are well known and noted in the chapter on this subject in this handbook. Among those of major interest in coal drying are no fire or explosion hazard, ability to dry at elevated temperatures, faster drying rates leading to small equipment size, and others (Beeby and Potter, 1984 Wolf et al., 1988 Weiss et al., 1991). 

As described earlier, under some circumstances, it is possible to use superheated vapor instead of traditional dry air for drying. This would be supplied to the dryer at temperatures well above 100°C. By using superheated steam, higher heat transfer rates can be achieved. Clearly, however, there is no effect on the other limitation of the drying rate, the moisture diffusion within the material. 

Superheated steam has heat transfer properties superior to air at the same temperature. Since there is no resistance to diffusion of the evaporated water in its own vapor, the drying rate in the constant rate period is dependent only on the heat transfer rate. The convective heat transfer coefficient (h) between steam and the solid material surface can be estimated using standard correlations for interphase heat transfer. Neglecting sensible heat effects, heat losses, and other modes of heat transfer, the rate at which surface moisture evaporates into steam is given simply by... 

Often it is observed that drying rates in the falling rate period are also higher in superheated steam drying relative to air drying. The mechanisms responsible for this phenomenon are, however, quite different. In superheated... 

Bond J-F. Drying paper by impinging jets of superheated steam Drying rates and thermodynamic cycles. Ph.D. thesis, McGill University, Montreal, 1991. 

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