Most dryers currently in service, being used as designed or specified, offer the potential to increase output and productivity with a minimal investment. The performance of others, which may not be operating as designed due to a number of conditions, also can be improved using the same techniques. Here are some simple concepts that can be implemented to improve your dryer's performance and potentially increase its output. In addition, if you're in the market for a new dryer, these concepts may allow you to reduce your investment and optimize certain operating conditions of your new equipment.

Increasing the Temperature Differential

The higher the temperature differential (ΔT) across the dryer, the more efficient the operation, the higher the energy transfer, and the greater the productivity of the unit. In many instances, users may have concerns about operating temperatures that are unfounded, and these temperatures can be adjusted without a detrimental effect. Even a small adjustment can result in a much improved yield.

Increasing the temperature differential may increase the inlet temperature or reduce the exhaust temperature -- optimally, it will affect both. Some of the primary concerns regarding increasing the ΔT are:
  • Damaging the product (overheating, discoloring, modifying the particle characteristics, skinning, cracking).
  • Increasing the humidity of the exhaust stream, potentially causing a moisture block.
  • Creating condensation problems related to the exhaust humidity.
  • Causing thermal expansion of the dryer due to the higher temperatures.
  • Exceeding the physical limitations of the materials of construction.
  • Increasing heat losses due to inadequate insulation and leakage.
The process of drying imparts various energies to the feed, including sensible heat and latent heat of vaporization. Sensible heat raises the temperature of the feed and the fabric of the dryer to the operating condition, and no more. Water molecules that evaporate from the product being processed retain the latent heat as they leave the product mass and hence, reduce the energy of the mass. This reduction in energy, in the form of heat, will promote the phenomenon of evaporative cooling and will keep the product mass at a reasonably constant temperature for the bulk of the drying process. Testing often reveals that this temperature is substantially lower than the temperature at which damage would occur to the product.

Similarly, it is preferable to maintain the exhaust above the dewpoint temperature. In many instances, there is a conservatism that is applied to this aspect. Once again, testing the actual condition will provide a potential opportunity.

Increase the Temperature of the Feed. By increasing the temperature of the feed, the required transfer of sensible heat from the dryer to the feed is reduced. The higher the feed temperature, the greater the benefit. This in turn reduces the required energy for the application and allows the additional energy to be used to increase the drying rate.

For example, if the viscosity of the feed must be reduced, it will be fundamental to the operation to heat the fluid, perhaps using a dedicated heating system. This heat will be used effectively again in the process. Consider using surplus heat to preheat the product, or determine whether installing a dedicated system would prove cost effective.

Use the Exhaust Air Effectively. There is a appreciable quantity of energy contained in the exhaust stream. Harnessing this energy to promote drying is a valuable, if somewhat under-utilized, resource. There are two basic approaches to recover this energy:
  • Recycling. Recycling the air within the dryer reduces the sensible requirements to heat the air from its atmospheric condition to the operating condition. Recycling involves redirecting the exhaust air, or a portion thereof, back into the process. Limiting factors for recycling will include saturation of the gas and depletion of the oxygen content of the gas (for direct-fired applications). They can be overcome by controlling the percentage recycle.

  • Recuperation. The use of recuperation to preheat the feed product, inlet air or combustion air offers additional advantages. This same concept also could be used as the source of energy to preheat the product. Recuperators can be air to air, air to solid, or air to liquid units. Some recuperators may be relatively large and will absorb a certain amount of power (from the fans) to overcome losses associated with the equipment.
Reducing the temperature of the exhaust stream to below its dewpoint will provide additional energy but may have certain problems associated with the condensate so formed.