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Recent research has yielded a qualitatively comprehensive picture of the drying process which differs from the classical theory in many details but confirms the general features of the constant-rate period and falling-rate period. Drying experiments including continuous precise measurements of drying rates, moisture distribution, and temperature distribution have been reported. The influence of the nature of the solid on the falling-rate period has been clarified. The existence of a characteristic temperature for the falling-rate period has been observed. Because of its correspondence to the wet-bulb temperature of the constant-rate period, this characteristic temperature has been named the pseudo-wet-bulb temperature. The falling-rate period is not yet amenable to general quantitative analysis although some progress has been made. Further development of drying theory will depend on gaining a better understanding of the mechanisms of simultaneous heat and mass transfer in porous materials. Work on this subject is proceeding, but these mechanisms, and the material properties controlling them, are not sufficiently well defined at present to permit their incorporation into quantitative drying theory.
Assistant professor, Rensselaer Polytechnic Inst., Troy, N. Y.