Journal of Microwave Power and Electromagnetic Energy (JMPEE)
TITLE	Microwave Drying of Solids [PDF]
AUTHORS	G.B. Allen 1967 2 3 78
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Abstract The application of high power microwaves to the removal of moisture from solids, though far from a new concept, has until recently been impractical due to lack of suitable microwave generation equipment. From the number of power microwave systems now in operation, it is apparent that this is no longer a serious problem. Following a brief introduction to the drying of solids, an examination of the effects of microwave fields on wet materials is presented. Some basic relationships between power density, frequency, dielectric loss factor and penetration depth are established, and typical driers discussed. Relatively basic empirical heat and lass transfer relations are inspected to check compatibility with microwave drier conditions. Terms to account for the effect of microwave fields are then introduced and prediction of the potential areas pf advantage and disadvantage of microwave power are based on the expressions. The importance of the large positive internal temperature and vapour pressure gradients associated with microwave fields is apparent. Several factors involved in the design of a microwave drying system are subsequently discussed. Some results of the microwave tray drying of a coarse sand and an inorganic hydrate in a 1 kW experimental system are presented. The drying curves are examined in detail and attempts made to provide qualitative explanations for all of the important features. Quantitative checks of several of the previously derived expressions are also made. The economics of microwave drying are outlines, then illustrated by an example comparing air and microwave tray driers ion a typical heat-sensitive powder drying applications. It is concluded that, in spite of many problem areas, sufficient advantages exist to justify the use of microwave power in drying.