Continuing my discussion on cabinet dryers begun in my last column, in this issue I will look at cabinet dryer loading and unloading, residence time, ancillary systems and potential operating drawbacks.
Cabinet dryers traditionally are batch-type systems. Loading and unloading of trays typically is performed manually and can be labor intensive. Cabinet drying systems have evolved to produce a configuration called a tunnel dryer, where the trolleys are indexed continually through the dryer by means of a floor-mounted drive. This imitates a continuous operation but still requires discontinuity to insert new trolleys with product to be dried and remove trolleys with dried product. Systems can be added to automatically load, stack and empty the trays. Implementing all of these enhancements brings you very close to a continuous conveyor drying system.
Cabinet dryers can dry all types of feeds from liquids and slurries to granules, agglomerates and solids (objects). They are suitable for small volumes and sometimes prove to be economically feasible for loads up to 9,000 lb (~4,000 kg).
The residence time within a cabinet dryer is infinitely controllable. It can accommodate high variances in initial moisture simply by adjusting residence time. It can control the temperature extremely accurately and can offer ramp-and-soak performance for applications that demand it. Most cabinet dryers are controlled by a strategically placed thermocouple linked to a solid-state temperature controller. The instrument modulates the heat source to achieve the desired operating temperature.
For demanding applications, a cabinet dryer can be designed to operate at elevated temperatures up to approximately 1,100oF (~600oC). The trays, internals and even the entire unit can be fabricated from sophisticated materials, including various grades of stainless steel. The entire dryer typically is insulated and clad to reduce heat losses.
These dryers commonly are supplied without any dust collection. The required airflow volume and associated velocity reduce the potential for entraining the product, minimizing the carryover. However, should the product be extremely fine and dusty, a dust collector can be installed to control the emissions.
By far, the biggest weakness of a cabinet dryer (beyond labor intensity) is drying efficiency. Because the product remains stationary and does not present new surfaces to the carrier, the evaporative process is somewhat retarded. This may necessitate raising the product temperature to higher levels than would be required by design as well as extended residence times. Principal problems associated with the design aspects relate to airflow patterns within the unit. If the flow of air and the entrance velocity are not uniform from top to bottom in the drying chamber, a profile is established. Then, there will be cold spots within the cabinet that will cause product in certain areas of the dryer to have higher final residual moistures.
Cabinet dryers are versatile, robust pieces of equipment. They only have one moving component -- the fan -- and, therefore, have low maintenance requirements.