Fundamental Study of Vacuum Drying

Abstract

Mechanism of vacuum drying (press. range: 4-300mmHg) was studied, using SHIGARAKI clay as testing material.Drying by radiation heating was conducted, using apparatus A (Fig. 1). Data obtained are summarized in Table 1. Drying curves and characteristic curves of drying are shown in Figs. 2 and 3. When the water content was equal, the ratio of the decreasing drying rates for various drying conditions were found to be equal to the ratio of the constant drying rates. Thickness of the material was observed to have no influence on the decreasing drying rate (Fig. 4), therefore, drying time may be considered to be proportional to the thickness of the material. Water content gradients in the material were measured at various drying times (Fig. 5). Obviously, these curves were different from those under atmospheric pressure.From these facts, we may conclude that the drying of clay in vacuum is typical of the drying caused by the capillary action.Surface evaporation coefficient, kg', defined by Eq. (3) was calculated and shown in Table 1 and Fig. 13.Drying by conduction heating with air leakage was carried out, using apparatus B (Figs. 6 and 7). Data are summarized in Table 2.Drying curves and characteristic curves of drying are shown in Figs. 10 and 11.The air was leaked quantitatively to the vessel and the water vapor pressure in the vessel was measured and calculated precisely.During the constant drying period, the surface evaporation coefficient, kg, defined by Eq. (5), was calculated, and it is shown in Table 2.The relation between kg and total pressure, π, was correlated for various drying conditions, as follows:(6)The values of kg' could be expressed by Eq. (6) within the range of our experiment.Thus, we can design the vacuum chamber dryer and decide the pump capacity, by using Eqs. (4), (5), and (6).