Vapor Drying of Western Woods

Eleven western woods were vapor dried with the following results. Douglas-fir: Vapor drying at atmospheric pressure and at temperatures from 240? to 260?F. reduced the moisture content of l-inch Douglas-fir from 50 to 10 percent in only 6 hours. There was little warping or checking, but about 15 per cent of the knots were loosened or fell out. Drying at temperatures above 225?F. tended to cause severe honeycombing in Douglas-fir 2-inch or thicker. Vapor-dried Douglas-fir exhibited steep moisture gradients and generally severe casehardening. As was the case with all species, the amount of shrink age which occurred in Douglas-fir lumber during vapor drying did not vary appreciably with that which has been reported for kiln-dried material. Chemonite treated white fir: Temperatures below 250?F. were necessary to dry this material and eliminate honeycombing. A drying time of 27 to 30 hours could be expected to dry white fir to 18 percent moisture content. There was very little checking and only slight collapse. Sugar pine: There were no serious defects in the l-inch vapor-dried material, but in the thicker material there was a darkening of the wood at temperatures above 270?F. The 4-inch pine honeycombed. Ponderosa pine: An alternating cycle, using an effluent vapor temperature of 250?F., reduced the moisture content from 120 to 7 percent in 7 hours. Jack pine: The 7- by 9-inch tie material was vapor dried in 12 hours, and pressure treated in another 6 hours with a net retention of 8.6 lb./cu.ft of a creosote-petroleum mixture. The ties were exceptionally free of surface- and end-checking. Western hemlock: Drying time varied from 12 to 114 hours. High temperatures and low relative humidity produced the greatest amount of internal checking, but with an increase in the humidity, internal checking could be eliminated even with drying temperatures of 265?F. Redwood: Redwood can be vapor dried from an initial moisture content of 140 percent to a 10 percent final moisture content in 90 hours with few drying defects. Temperatures of 250?F. caused severe collapse and honeycombing, but drying at 150* and 180?F eliminated the honeycombing and greatly reduced the amount of collapse. Western larch: The l-inch lumber dried very fast and with few defects. However, the 2-inch and 4-inch lumber honeycombed at temperatures of 250?F. and above. Madrone: Rapid drying of madrone was accomplished with little, degrade. Under a vacuum of 20 inches and at 210?F., this species was vapor dried from 94 Percent moisture content to 12 percent in 17 hours. California black oak: This species reacted very badly to the drying conditions of 20 inches of mercury vacuum and 210?F. effluent vapor temperature. All boards were severely honeycombed and collapsed. Tan oak: Like the madrone, there was some collapse and honeycombing, but no more than has been encountered in kiln-drying green tan oak. At 180?F. effluent temperature under a vacuum of 20 inches it took 30 hours to dry from 80 percent to 10 percent moisture content.

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