The information presented in the preceding sections gives the reader an idea of how to set about estimating the required product residence time under idealized conditions. The designer must now derive the equipment dimensions required to give this residence time. If there are significant deviations from the idealized conditions, for example nonuniform distribution of air and solids or partial back mixing, the designer must also correct the calculated residence time to allow for these nonidealities.

These aspects of dryer design need a knowledge of particle transport mechanisms and velocities and gas flow patterns. Such aspects are impossible to discuss in general terms, since each dryer type is an individual case having little in common with other dryer types as far as these aspects are concerned. Sometimes they are also very complex and impossible to model theoretically. In such cases, dryer design is based on empirical scale-up from pilot-plant tests using the dryer manufacturer’s previous experience as a guide. In other cases, however, they are not so complex, and at least a preliminary design can be made based on laboratory-scale tests allied to the foregoing theory. These considerations will now be illustrated by reference to some important types of continuous industrial dryers.

A. Band dryers

In band dryers, air is blown in cross flow either up or down through a shallow moving bed of particles [Figure 334.10]. A real dryer will consist of a number of sections in series, each of which may have a different air temperature, humidity, and velocity. The bed moves at a constant speed on a perforated belt from one section to the next, so the particle transport is well defined. The design problem is to calculate the required length of dryer for a given band speed, that is, a given bed thickness, and a given profile of air conditions in the series of sections. Repeated calculations can be made for different band speeds and different profiles to obtain an optimal design.

Each section can be considered as a class IV dryer. A simple laboratory experiment can be performed in which air is blown through a layer of material on a static perforated tray and a drying rate curve can be obtained. This curve can be used for two purposes: