Being a strong supporter of the educated decision and a proponent of the saying, "Knowledge is power," this year, I am going to run a series of columns on different types of dryers. The range and selection of dryers is vast; as such, it would be a formidable task to attempt to cover every variety and incarnation. Therefore, I will focus on those types that are in the mainstream and commonly installed.
Each type of drying system or specific dryer has a principle of operation that includes feeding, discharging and airflow patterns. Each is suited to a particular type of feed, and each uses a heat source appropriate for the operation. Some types may require dust collection systems or various levels of control. In a nutshell, each dryer has specific benefits and limitations. Implemented correctly, all have applications for which they are clearly suited. This series of columns will cover the above concerns, giving the reader a more fundamental understanding of the dryer, its application and its operation.
This column, the first of the series, will deal with cabinet dryers. Cabinet dryers are by far the most common type of dryer used in industry. Relatively inexpensive, they are found in every industry drying a myriad of products. These dryers frequently are called ovens. I differentiate between ovens and cabinet dryers based on the fact that a dryer, by definition, removes moisture. By contrast, an oven typically will elevate the temperature of a product to allow for a specific process such as curing or preheating to occur. Dryers require removal of the evolved moisture from the system and usually have relatively large volumes of gas (most typically air) passing through the unit. Ovens do not need this airflow to accomplish the process change; if they do, they will, in some instances, recirculate 100% of the process gas.
Principle of OperationIn a cabinet dryer, the feed is loaded onto trays that, depending on the required quantity of material to be processed and hence dryer size, are loaded onto trolleys or inserted into the drying chamber. The door is closed, sealing the system. The trays may have a solid or perforated bottom, depending on the feed particle size distribution. The carrier (typically, air) is drawn in through an inlet duct or recycle channel into the heating compartment. The air is heated to the required temperature in this region, then distributed from side-to-side (cross-flow) or bottom-to-top (through-the-bed flow) in a circular motion, according to specific drying requirements.
The motive force for the air movement typically is a plug-type fan that acts as both an induced draft and forced draft machine. The fan sucks the air through the heating chamber and blows it across the trays, which act as channels or ducts to direct the air. The fresh air inlet is located on the suction side and the exhaust is located on the pressure side. The inlet and typically exhaust will have dampers on them, enabling the percentage of recycled air to be controlled. This recycle improves the efficiency of the unit operation, conserving energy in the gas stream.
Cabinet dryers come in various sizes and may have multiple fans on larger units. There also may be more than one heating compartment. Units can be electrically heated, have gas or LFO heat sources, or use indirect heat sources such as steam or thermal oil.
Cabinet dryers can dry all types of feeds from liquids and slurries to granules, agglomerates and solids (objects). Next month, I will look at dryer loading and unloading, residence time, ancillary systems and potential operating drawbacks.