Vacuum drying of porous solids under supercooling

Vacuum drying of sintered glass beads, cemented glass balloons, and baked clay was investigated under supercooling. Glass beads and glass balloons, which were made of a common glass material, were used for comparison. Concerning sintered glass beads, there was no effect of supercooling on drying rates at a pressure of 0.93 kPa. In cemented glass balloons as well as in baked clay, however, a prominent peak of drying rates emerged in drying rate curves and at the same time, a stepwise increase in the surface temperature appeared under supercooling at a pressure of 1.19 kPa; no peak of drying rates emerged in drying rate curves of sintered glass beads. The total amount of latent heat of vaporization and sensible heat generated at the peak of drying rates was 39-59% of the latent heat of solidification. A part of the latent heat of solidification evaporated the water in the sample, producing a peak of drying rates as well as a stepwise increase in the surface temperature of the sample.