£av = 20.0



"Evaporation rates for rotary dryers are expressed in lb/ft3 • h.

"Evaporation rates for rotary dryers are expressed in lb/ft3 • h.

ing great flexibility. As a result, the batch-type forced-convection unit certainly finds the widest possible application of any dryer used today.

The majority of designs employ recirculatory air systems incorporating large-volume, low-pressure fans that, with the use of properly insulated enclosures, usually provide thermal efficiencies in the region of50-60%. However, in special applications of this type of dryer that call for total air rejection, this figure is somewhat lower and is largely related to the volume and temperature of the exhaust air. Capital investment is relatively low, as are installation costs. Furthermore, by using the fan systems, both power requirements and operating costs also are minimal. Against these advantages, labor costs can be high.

In such a plant, the drying cycles are extended, with 24—45 hours being quite common in certain cases. This is a direct result of the low evaporative rate, which normally is in the region of 0.15-0.25 lb/ft2 h.

Following the recent trend and interest shown in preforming feedstock and in particular with regard to the design of extruding and tray-filling equipment for dewatered cakes, it is now possible to obtain the maximum benefit of enhanced evaporative rates by using through-air circulation dryers when handling preformed materials. Figure 1 shows how a high-performance dryer can product 1950 pounds of dried material in a 24-h period at a terminal figure of 0.5% moisture when handling a preformed filter cake having an initial moisture content of 58%. The very great improvement in performance can readily be seen from the curve, in which it is clear that the corresponding number of conventional two-truck recirculatory units

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