ABSTRACT
The Need to Understand Vacuum Technology The previous chapters have showed that the primary and secondary drying processes occur at subatmospheric pressure, Le., generally at pressures lower than the vapor pressure of water at the triple point (4.58 Torr). In Chapter 8, an important relationship was established between the pressure in the chamber (Pc), the product temperature (Tp)' and the shelf-surface temperature (Ts). While an understanding of those relationships proves helpful in defining the necessary parameters for the primary drying process, the rate of sublimation was shown to be directly related to the rate of heat transfer to the product. If the pressure in the chamber was too high, the heat-transfer rate would approach zero, and, for all practical purposes, there would be no sublimation of ice crystals. We understood that the condenser temperature (Te) had to be sufficiently low to condense water vapor and that an increase in Te could result in an increase in the Pc' which, for a given Ts' would cause an increase in Tp. However, for drying to take place, the pressure in the container must exceed the pressure in the chamber. Thus, for the drying process, one should understand not only the flow of gases from a given container but also the nature of the flow of gases from the drying chamber to the condenser.
