Timber dryer air vents

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Dryer Components - Humidity Control

Humidity control in the timber dryer is accomplished by the Air Vents, also called Dampers, and Humidification system. Humidification is accomplished by steam spray or water misting. For steam humidification, the steam should be of low pressure that is not dry. For dryers heated by steam or thermal oil, fine atomised water spray directed on the heat exchanger is good When the atomised water hits the heat exchanger, it will be vaporised into wet steam.Wet steam is easily absorbed by the timber and is good to spray periodically during drying to relieve drying stress.

For Hot water heating system, water mist humidification is recommended. This is because, atomised water, will mostly end up in a pool of water that is difficult to be absorbed by the circulating air. The enthalpy is not quite enough to vaporise the water for a hot water heated dryer.Humidification is very important when the wood moisture reaches the final moisture content. For wood to dry, the moisture content on the wood surface has much lower than the wood core moisture content. This ratio, known as drying gradient (DG), can be anything from 1.8 to 3.2 depending on the timber species. The higher the DG, the faster the moisture diffusion. Each timber species has a recommended DG obtained from research, This value can only serve as a guide as the same wood species grown in different geographical locations may have different DG. The DG can be different if the wood comes from a juvenile tree or an old tree. Heartwood and Sapwood also have different DGs.

When the wood reaches the final moisture content of say 10%, The wood face will have an MC of less than 5% for a DG of 2. The wood face MC must be equalised to relieve all the drying stress and to have the same MC across the timber. This phase after Drying phase is called the Equalizing phase. The dryer will be humidified again to a humidity level to cause the wood face to absorb and distribute the moisture into the wood to achieve an uniform final moisture content of 10%.

If the dried wood is not equalized or conditioned sufficiently, the wood may deform after cutting or machining. Sometimes due to the need for quick turn around of the dryer, the timber can be conditioned outside for a sufficient length of time.