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 2-1/2 in. mercury, with exhauster capacities of 150 CFM of air at 70ºF per sq. ft. of filter area, are not uncommon. By preheating such large volumes 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.

SOURCE HEAT - If the air to the filter is heated directly with gas or oil, the products of combustion will be mixed with the air. However, clean air can be delivered to the filter by using indirect heaters. Also, steam coils can be used if comparatively low-temperature air will accomplish the drying. Direct fired heaters are about 95% efficient, while indirect heaters have a thermal efficiency of only about 70%.

CAPACITY - The capacity of the top-feed filter depends on operating conditions and on the characteristics of the materials handled. Cake thickness ranges from 1/2 to 1-1/2 in. and normal drum speeds from 1/2 to 2-1/2 rpm. In general, when a bone-dry cake is to be delivered, capacities range between 100 and 900 lb. per sq. ft. of filter area per hour. Higher capacities are possible if only partial or no drying is required.