Continuing my discussion of conveyor dryers, in this issue I will look at dust collection, operational control and conveyor dryer limitations.
Remember, conveyor dryers process at rates that are consistent with their specific applications but are on the lower end of throughput capacities in the assortment of dryer technology. This relatively low rate is a limitation primarily imposed by physical logistics and capital cost. Conveyor dryers principally are through-the-bed dryers although cross-flow and radiant units are used occasionally for specific products. Units can be directly or indirectly heated by using burners (gas, LFO or HFO) or coils (steam, electrical heater banks or thermal oil).
Collecting DustDue to the relatively large particle sizes and the low gas velocities, most conveyor dryers need little by way of dust collection. However, some do, and fabric bag collectors are used most commonly. Fines collection from within the dryer is an important design aspect because many products will have reduced quality if "locked up" product from earlier production is allowed to contaminate current production. Additionally, certain accumulated products will "over dry" and present a fire hazard if not removed from the system.
Operational ControlAlthough simple solid-state controllers with hard-wired logic can be used effectively for these dryers, the use of programmable logic controllers (PLCs) is becoming the standard. Control typically is set up independently for each zone. Energy is modulated on either zone-exhaust temperature, above-the-bed temperature or exhaust-air humidity. In addition, with the advent and commercial availability of optical pyrometers and near-infrared transmitters, it is becoming economically viable to control on product temperature or moisture content.
LimitationsThere are some significant limitations to this technology. The most apparent of these is high temperature operation. As temperature increases, expansion increases. With the integral design of these dryers, the relative expansion of one region to another at elevated temperatures can cause stresses and fractures if not properly taken into account.
The most frequent operational shortfall is related to uneven product characteristics. Poor formation of the bed and an uneven temperature profile across the dryer are the main reasons for this shortfall. If the bed is not established evenly across the belt, the carrier gas will short circuit the material, following the path of least resistance. The areas of the bed that are thinner and, hence, offer lower resistance to the gas will over dry while the thicker sections of the bed will remain relatively wet. If the distribution plenum is not designed to provide an even distribution of hot gas across the width of the belt, the dryer performance will again be adversely affected, producing unevenly dried product.
Heater clogging causes another operational weakness. This is most common for steam units where the coils build up with product, restricting airflow and posing a possible fire hazard. Reducing the gas velocity through the belt, having expansion sections in the dryer housing and designing the ductwork to have velocities below that of saltation will limit this occurrence. Clean-out pans and periodic preventative maintenance are other key methods for controlling this condition. Belt tracking is another area that can prove to be a burden.
On the whole, conveyor dryers fill a unique segment in the drying equipment arena. They are self-contained units and provide a gentle method of processing various products to achieve a narrow range of final moistures.