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Air circulation around drying timber is important because the ability of air to absorb water from wood is finite. Once the air surrounding wood becomes saturated evaporation of moisture from wood will stop. It is therefore important to replace the wet air surrounding drying wood with dry air. Even where there is a continuous stream of air passing over the wood the layer of air in immediate contact with the wood will move more slowly and have a higher vapour pressure than the main stream of air. This is known as the 'boundary layer effect'. With increasing air velocity in the main stream this effect decreases and evaporation rates from the wood surface increase, particularly when the air flow is turbulent rather than laminar. An increase in air speed can therefore be regarded as equivalent to a reduction of the humidity barrier near the wood surfaces. Since air entering a stack of drying timber gives up heat and takes up moisture it is cooler and more humid where it leaves the stack of timber than when it enters and the drying rate slower than on the air inlet side of the stack. In kiln drying, this is refered to the Delta T. The faster the air speed and the narrower the stack, the smaller is the difference between the two sides. For this reason fairly high speeds are desirable in a drying kiln particularly where the timber being dried is very wet and permeable. In most modern kilns the uniformity of drying is further improved by reversing the direction of air flow through the kiln at regular intervals using the fans. Moisture pickup by air entering drying stacks also limits the size of stacks. Hence stacks for air drying are usually only 2 metres wide and a 6 metres high. |