Fan settings for forced-air shed drying -- Eastern cottonwood

      Proper settings for hydrostats controlling the operation of fans in forced-air drying of Eastern cottonwood. June 20, 2000

Q.
What would be the optimum settings on hydrostat fan controllers for drying Eastern Cottonwood as quickly and safely as possible in forced-air sheds?

Right now, I'm running constantly and have seen no ring collapse, so I don't see where lower relative humidities (RH) are a real issue. Would I need to establish a maximum RH cut-off point to avoid regaining moisture content (MC) at sunrise, or during rainy weather?

Current airflow is in the 200 foot-per-minute (FPM) range. Location is southern U.S.

The lumber we're putting through these things is mostly 4/4 no. 1 clear and better. (Isn't the lower-grade lumber, cut closer to the heart, more prone to collapse?)

A.
Collapse occurs mainly at the heartwood/sapwood zone. It is caused by too-rapid drying, and requires the wood to also be impermeable (perhaps due to bacteria). As a result, not all cottonwood will collapse.

Suggested fan settings would be "off" at RH under 50 percent and at over 92 percent when the wood is wet. As the wood gets drier, set hydrostats to turn the fans off when the RH is over 88 percent. Though there's no bottom RH limit, having the fans shut off under 50 degrees F is smart, since little drying occurs with them running at this temperature and lower, yet you'd continue to incur a substantial electric expense.
Gene Wengert, forum moderator



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