Shrinkage of Douglas-Fir, Western Hemlock and Red Alder as Affected By Drying Conditions

For each species, 16 groups of 20 specimens each were dried under constant conditions almost to equilibrium. Temperatures used were from 90 to 230?F with condition for an equilibrium moisture content (EMC) of 6, 9, and 12 percent at each temperature except 230? where only a 6 percent EMC was used. Specimens were then equilibrated at room temperature to an EMC about the same as used in drying, after which all specimens were equilibrated to a nominal. 6 percent EMC and finally they were ovendried. Specimens were weighed and measured in the tangential and radial directions while green and after each subsequent step in the procedure. Analysis of the data showed that moisture content attained at equilibrium is less for specimens dried at high temperature than for specimens dried at low temperature, and the three species were affected to about the same degree. The effect is more pronounced above 150? than it is below this temperature. Shrinkage is also influenced by temperature– the higher the temperature the greater the shrinkage?and the effect is also more pronounced above 150?F. The greater shrinkage at the higher temperature cannot be accounted for on the basis of the lower attained moisture content. Specimens dried with conditions for 12 percent EMC shrunk more than did those dried at 6 or 9 percent. EMC. Shrinkage of the three species were not affected to the same degree. Generally, Douglas-fir was affected the least while red alder was affected by far the most. For example, with Douglas-fir, specimens dried in conditions for 12 percent EMC and equilibrated to that EMC, tangential shrinkage was 4.11 percent when temperature was 90?F and 5.49 percent when temperature was 215?F, an increase of 33 percent. In a similar comparison for western hemlock, increase in shrinkage was 65 percent, and for red alder the increase was 130 percent. Similar or slightly greater increases were noted for radial shrinkage.

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