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Wood drying under vacuum

Raw timber is treated to prevent decay in unprotected environments. In order to do this, the wood is loaded into large, air-tight vessels. Once loaded, vacuum pumps are used to draw vacuum on the tank to extract water vapor from the wood. Chromated copper arsenate (CCA) is then injected into the tank. The pumps are then used as compressors to pressurize the tank and to impregnate the wood with the desired concentration of CCA.

The various vacuum drying processes are divided into two categories: the continuous vacuum drying process, in which the vacuum is maintained continuously throughout the drying process and the discontinuous vacuum drying process, in which phases of convection heating at atmospheric pressure alternate with phases under vacuum. The continuous vacuum drying kilns include hot platen ,superheated-steam, air-steam mixture and high-frequency heated kilns. This article deals primarily with the continuous vacuum class of dry kilns.

Principles of the vacuum drying process

Vacuum kilns dry lumber in an airtight chamber maintaining an air pressure lower than normal atmospheric pressure. The boiling point of water is reduced by these conditions, which increase the rate of evaporation at the surface and results in temperature and total pressure gradients favorable to the flow of humidity through the thickness of the wood.

Wood–drying in oxygen-free or oxygen-reduced environments also has other advantages. In particular, the discoloration of wood is partly or completely blocked.

Main advantages of vacuum drying

The operating characteristics of the vacuum drying process result in significant advantages in terms of productivity, quality and energy consumption.

Productivity: Vacuum drying is faster than conventional processes because operating at a vacuum lowers the boiling point of water, resulting in more rapid surface evaporation.

Quality: Vacuum drying results in an environment that is favorable to the quality of dried products. Because the boiling point in a vacuum is lower, vacuum drying involves relatively low temperatures. Wood is stronger at lower temperatures and, therefore checks and splits are less likely to develop.

Vacuum drying in an oxygen-free or oxygen-reduced environment reduces the incidence of chemical discoloration problems caused by the oxidation of certain natural compounds in the wood.

Energy: Shorter drying times and lower temperatures minimize heat loss from vacuum drying chambers.

Other significant advantages: Vacuum drying offers other significant advantages besides those of productivity, quality and energy savings. They include quicker customer service response (on time delivery), shorter time for material in process and the flexibility to dry several products of varying sizes.

For further information about wood drying using vacuum, please contact Osanak in our applications support department; o.mir@vpcinc.ca

Vacuum Products Canada Inc. www.vpcinc.ca 1-800-269-6030 reachus@vpcinc.ca
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Last Modified Wednesday, March 21, 2012