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Dehumidification technology for drying lumber


October 2023 Dehumidification technology for drying lumber from Sawmilling South Africa

By Henco Viljoen, Timber Soft: Kilns 4 Africa

In kiln drying, lumber is placed in a chamber where airflow, temperature and humidity are controlled to provide as rapid drying as can be tolerated by the lumber without increasing defects. Each kiln type is defined by how the temperature and humidity are controlled.

There are two dehumidification processes: passive and active dehumidification (DH). Passive DH is where moist air meets a surface cold enough to reduce the air temperature to below dew point. We usually experience it as condensation on a cold drink glass, bathroom mirror or car windows.

Active DH is where air is introduced to a surface below the dew point. A refrigeration circuit expands the gas to form the cold side. It compresses the same gas (heat of compression) to reheat the same air. Air is forced over the cold or evaporator coil, where water is removed through condensation. The cooler and dryer air passes over the condenser coil to be reheated.

Alternative to venting

During kiln drying, moisture from green lumber evaporates into the air, increasing the humidity inside the chamber. The lumber can’t continue to dry if the air becomes over-saturated. Therefore, the moisture must be periodically removed from the chamber.

In a conventional kiln, moisture would be expelled through venting. Venting causes a significant heat loss, resulting in a waste of energy to bring the kiln back to temperature.

Instead of venting, active DH kilns use a refrigeration system or heat pump that condenses the excess moisture. The air passes over the cooling coil, and the evaporated water condenses into liquid form and flows down the drain as a stream of cool water. After the moisture is removed and heat is reintroduced to the dry air, it is pumped back into the kiln chamber to repeat the process.

Heat of compression

Unlike conventional kilns, the heater in a DH kiln is only used during the initial warm-up. The energy efficiency of the heat return is such that each time this process occurs, the air leaves the dehumidifier at a higher temperature than when it entered because of the heat generated from running the compressor (heat of compression).


The advantages of DH kilns include:

  • Fewer exhaust vents because vents are only used to cool the dry bulb (DB) temperature in the chamber during normal operation.
  • High-pressure or cold-water spray systems for conditioning. 
  • Depending on the control strategy, dehumidification kilns maintain more consistent DB and wet bulb (WB) temperatures.

Nyle L200

Timber soft: Kilns4Africa has partnered with Nyle Dry Kilns to bring its small drying unit, the L200, to Africa. Nyle is a world authority on kiln drying. These units are perfect for small-scale drying, like running a 6m or 12m container kiln with a small sawmill.

Source: September 2023 | Issue 30 • WoodBiz Africa

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