Often a vacuum process covers several of the regions mentioned here. In batch drying the process can, for example (see Fig. 2.74), begin in region A (evacuation of the empty vessel) and then move through regions B, C, and D in steps. Then the course of the process would be as follows:
The choice of the pumping system is decided by the highest vapor partial pressure occurring and the lowest air partial pressure at the inlet.
It will now not have an effect. Instead, it would only be pumped empty by the pumping system with a further drop in vapor pressure.
Roots pumps and fore-vacuum pump (with open gas ballast) alone can now continue pumping. With short-term drying, the separation of the condenser filled with condensed water is particularly important, because the gas ballast pump would continue to pump from the condenser the previously condensed water vapor at the saturation vapor pressure of water.
With longer-term drying processes, it suffices to shut off the condensate collector from the condenser. Then only the remaining condensate film on the cooling tubes can reevaporate. Depending on the size of the gas ballast pump, this re-evaporation ensues in 30 - 60 min.
As previously indicated, the drying of solid substances brings about a series of further problems. It no longer suffices that one simply pumps out a vessel and then waits until the water vapor diffuses from the solid substance. This method is indeed technically possible, but it would intolerably increase the drying time.
It is not a simple technical procedure to keep the drying time as short as possible. Both the water content and the layer thickness of the drying substance are important. Only the principles can be stated here. In case of specific questions, we advise you to contact our experts.
The moisture content E of a material to be dried of which the diffusion coefficient depends on the moisture content (e.g. with plastics) as a function of the drying time t is given in close approximation by the following equation:
E0 where E is the moisture content before drying
q is the temperature dependent coefficient. Thus equation (2.31) serves only for the temperature at which q was determined
K is a factor that depends on the temperature, the water vapor partial pressure in the vicinity of the material, the dimensions, and the properties of the material.
With the aid of this approximate equation, the drying characteristics of many substances can be assessed. If K and q have been determined for various temperatures and water vapor partial pressures, the values for other temperatures are easily interpolated, so that the course of the drying process can be calculated under all operating conditions. With the aid of a similarity transformation, one can further compare the course of drying process of a material with known properties with that of a material with different properties.
Experience has shown that shorter drying times are obtained if the water vapor partial pressure at the surface of the material is relatively high, that is, if the surface of the material to be dried is not yet fully free of moisture. This is possible because the heat conduction between the source of heat and the material is greater at higher pressures and the resistance to diffusion in a moist surface layer is smaller than in a dry one. To fulfill the conditions of a moist surface, the pressure in the drying chamber is controlled. If the necessary relatively high water vapor partial pressure cannot be maintained permanently, the operation of the condenser is temporarily discontinued. The pressure in the chamber then increases and the surface of the material becomes moist again. To reduce the water vapor partial pressure in the vessel in a controlled way, it may be possible to regulate the refrigerant temperature in the condenser. In this way, the condensertemperature attains preset values, and the water vapor partial pressure can be reduced in a controlled manner.