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Dehumidification Drying

Dehumidification drying is a method of drying in which a heat pump is used to recycle the heat that is normally vented to the atmosphere during normal drying. The essential stages of dehumidification drying are as follows;

  • Hot moist air from the dryer is forced over cold evaporator coils.

  • Moisture condenses and water is drained from the system

  • Heat from the air and the latent heat of condensation causes the refrigerant fluid in the evaporator coils to vaporise.

  • The heated refrigerant is then passed through a compressor where it absorbs further heat.

  • The heated refrigerant is passed back to the condenser where its heat is transferred back to the cold dry air coming back from the refrigerator.

In this manner heat is recycled during drying. Energy inputs to this method of drying include that to the compressor and the fans. An electrical heater is also required to bring the temperature of the system up to at least 25 degrees centigrade. The advantages and disadvantages of dehumidification drying are as follows;

Dehumidifier kiln

Basic principle of the dehumidifier kiln. A dehumidifier unit is essentially an air conditioner installed in reverse (Mills 1991).

Advantages

  • Efficient utilisation of heat

  • Operate at low temperatures and therefore small dryers can be built using simple insulated wood frames with plastic, plywood or metal sheets.

  • Maintenance costs and peak energy requirements are low and up to a kiln capacity of 45-70 cubic metres the initial capital investment for heating equipment is low

  • The system operates at low temperatures, 50 degrees centigrade, drying rates are slower but less degrade is encountered. Consequently mixed species can be dried at the same time.

Disadvantages
  • Drying rates are slow.
  • The system provides no means of conditioning for stress relief.
  • Since the kilns operate at low temperatures they need to be well insulated to prevent heat losses.


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