Upgraded controls, increased insulation, heat recovery and recirculation, and regular maintenance can help improve the product yield and efficiency of rotary and flash drying systems.

Directly heated rotary dryers consist of an inclined, rotating cylinder with internal flighting. This type of design brings process material into direct contact with the drying medium, either in a cocurrent or countercurrent flow configuration.

Existing production equipment such as dryers often offers great potential for increased capacity and productivity with minimal investment. Drying operations in general are energy intensive and often are operated with little regard for future production requirements, energy efficiency and environmental considerations.

Many of these suggestions will prove useful for increasing the product yield and efficiency of rotating and flash drying systems. However, changes to any process always should be evaluated on an individual basis depending upon the nature of the product and operation.

Among the changes that can yield significant benefits are:
  • Implementing methods to reduce losses.
  • Implementing methods to recover and reuse process heat.
  • Installing additional insulation.
  • Improving temperature and process control schemes, among other control systems.
  • Upgrading critical dryer components such as the burner and controls can help minimize losses and emissions.
  • Conducting routine maintenance inspections and regularly scheduling activities that minimize maintenance downtime and repairs.

Rotary dryers are provided with automatic temperature, pressure and electrical controls, and with burner systems capable of firing natural gas, propane or oil.

Reduce Losses. There are several changes you can utilize to reduce losses, but your specific process parameters will determine whether they are appropriate for your application. First, consider reducing the gas outlet vent temperature. Energy in the vent gas is lost to the atmosphere. Remember, though, that it is always necessary to operate at a temperature high enough to dry the product and avoid condensation in the off-gases.

Second, consider whether increasing the airflow rate may boost the dryer capacity. Remember, though, that lower airflow rates at a given capacity minimize losses in the vent outlet gases and reduce the required fan horsepower.

Third, look at ways to reduce the amount of drying necessary. For example, increase the feed temperature or add a mechanical means for dewatering the dryer feed. Dewatering systems such as centrifugation or filtration are far less energy intensive than drying. Evaporation of the inlet moisture is often the highest load on the dryer system.

Heat Recovery and Recirculation. With seemingly ever-increasing energy costs, it “pays” to get as much out of each BTU as possible. Rather than using your dryer as a once-through heating system, look for ways to recover, recirculate and reuse the energy invested in the drying BTUs. For example, use dryer vent gases for heat recovery in other parts of the plant. Alternately, use waste heat from other plant operations in the dryer circuit. In addition, incorporate vent gas recirculation wherever possible by returning a portion of the dryer vent gases to the burner air heater. This has the dual benefit of reducing the dryer’s energy requirements and lowering the airflow volume at the baghouse.

Indirectly heated rotary dryers provide for heating under oxidizing, reducing or inert atmospheric conditions. With heat transfer indirect and through the cylinder wall, the gas velocities within an indirect dryer are low compared to a direct heat dryer.

Add Insulation. Drying is an energy- and heat-intensive process. Adding insulation to the dryer cylinders, ductwork and associated equipment will reduce energy use. In some cases, additional insulation may even allow for increased production.

Improve Controls. Minimizing process upsets will curtail impacts on both your product quality and production rate. Select and operate process and temperature controls with the goal of maintaining a steady state. For example, consider whether different dryer-control parameters would yield steady-state operation. Controlling dryer operation based on the humidity level rather than temperature can reduce the system airflow and associated heat losses.

Upgrade Critical Equipment. Remember that the correct burner operation greatly impacts overall dryer control. Include the burners in your inspection and maintenance plans. In addition, consider having the burner manufacturer periodically inspect and clean the burners. While the burner manufacturer is on site, review the burner for capacity, burner technology and controls.

Operate the burners as close to stoichiometric air/fuel mixture as possible and reduce secondary air intrusion. Consider using separate monitoring and controls for the inlet air and fuel gas mixture. Perform a cost-benefit analysis to determine the payback if you replace old burners with up-to-date technology. Newer burner technologies can provide improved reliability, increased fuel efficiency and reduced emissions of NOX, CO and CO2.

Minimize Maintenance Downtime and Repairs. Early detection, intervention and repair reduce downtime. Schedule regular maintenance inspections and ensure that accurate, up-to-date records are kept. During routine inspections, examine both the drying system and the associated equipment such as the burners, fans, baghouse, motors and drive systems. To maximize maintenance intervals, consider specifying upgraded materials of construction and process control systems.

For rotary drying equipment, inspect periodically for proper cylinder rotation as well as signs of excessive wear on trunnions, thrust rolls and girth gears. Trunnions and thrust rolls must be “trained” properly to maintain the cylinder in its proper orientation.

Minimize air infiltration, especially in flash dryers, by tightening flanges and making sure all connections and openings are closed. “Tramp” air in the drying system due to infiltration can contribute to heat losses, product temperature fluctuations and process upsets.

Increase Production Capacity. Before you can increase capacity, you must determine the limiting factor. Check the fan capacity, burner, dryer, baghouse, scrubber or cyclone. Also, keep in mind that the issues already mentioned such as controls, burner design and maintenance can directly influence the maximum operating capacity of the system.

To increase production capacity, consider whether you can lower the feed moisture or improve the feed quality by maintaining uniform particle size distribution and lower impurities. Alternatively, you may be able to raise the inlet air temperatures, but be sure not to exceed the limitations of the dryer construction, the dust collector or your product. Finally, consider system modifications, including revised flighting within the rotating cylinder, improved feeder, and upgraded seals at inlet and outlet breechings. The dryer manufacturer will best be able to advise you of the available alternatives.

With rising energy costs, manufacturers are looking for new ways to increase the efficiency of their manufacturing operations. Optimizing dryer operation can improve the product yield and efficiency. However, it is always a good idea to consult with the OEM before making any significant changes to avoid problems with equipment warranties, unexpected damage to equipment and possible undesirable changes in the product quality.