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so checks and splits are less likely to develop. Some vacuum drying systems like platen and high frequency are appropriate to the application of a constant top load, which minimizes warp problems during the drying process. According to Rice and Wengert (1987), Virginia Tech in the mid-1980s conducted several runs in a commercial vacuum kiln with heated platens, drying several hardwood and softwood species, from 3⁄4 to 4 in. in thickness. They reported overall quality to be good, with little checking. In a study conducted by Leiker and Adamska (2004) on beech, spruce and maple, the authors reported that surface checks and honeycomb were absent, except when overheating was forced. However a moisture distribution was present and for 50mm samples, the MC was lowest at the center and maximum in the layer immediately below the surface. Consequently, Wengert and Lamb (1982) evaluated several methods for drying 5/4 and 8/4 red oak lumber. They recorded the number of the drying checks in lumber dried by radio frequency vacuum (RFV), cyclic vacuum drying and pre-drying followed by kiln drying. Vacuum drying had substantially fewer checks including end, surface and internal checks. They also found that RFV had less crook (36% less than pre-dried). Simpson (1987) dried red oak of 4/4 and 10/4 thicknesses using a heating blanket vacuum system and reported no surface checking and low (2 to 2.2%) honeycombing. Avramidis et al. (1994) found no internal and surface checks in RFV. In vacuum drying technology oxygen is reduced, which reduces the incidence of chemical discoloration caused by the oxidation of certain natural compounds in the wood (Fortin 1998). Moldrup (1992) established that with vacuum drying, lumber is similar in color after drying to the color before it. A study carried by Welling and Riehl (1999) using Robinia psuedoacacia, determined that using high pressure steam treatment temperatures, ranging from 100 – 140oC, (maximum vessel pressure of 13 bar) can produce the desired brown colors while also reducing case hardening defects as a final conditioning stage of drying. In a study performed by Rice and Wengert (1987), the authors reported very good color for several hardwood and softwood species after been dried with heated platens. 14PDF Image | Impact of Vacuum-Drying on Efficiency of Hardwood Products
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