Coarse crystalline solids form an exceedingly porous filter cake through which large volumes of air can be drawn, still maintaining a comparatively low vacuum in the system. Vacuums of 2 to 2½ in. of mercury, with exhauster capacities of 150 c.f.m. of air at 70°F/ft2 of filter area are not uncommon. By preheating such large columns of air, the material can be discharged in a completely dry state. For any given material, the temperature of the air to the filter varies with volume per square foot of filter area.