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“SHOCKING THE KILN”. For over 100 years some people have thought that kiln slowdowns are due to pushing the kiln a little too fast while liquid water is evaporating. If the surface dries out while water remains in the center, the pathways used to transport water to the surface become interrupted and ineffective – or so the thinking goes. Drying actually isn’t “interruptible”; kiln slowdowns must happen as the wood dries, even with a very gentle drying schedule. At some point the wet line will recede below the surface no matter what kiln conditions are in use. If the kiln operator is controlling by the wettest sample(s) and not by the average MC, and if the kiln control mechanisms and sample boards have checked out, then, according to the laws of thermodynamics, the lumber must still be drying! “Shocking” the kiln is a procedure that some people use to induce a higher drying rate after a slowdown, typically in the range of 15 to 20% moisture content. This is exactly where we normally expect drying to proceed at a slower rate, but if drying slows at this point some advocate for turning the compressor off, raising the dry bulb to 150°F, and adding water to the kiln (as a fine spray or even as a wash on the floor). The RH and EMC conditions increase according to the amount of water added to the kiln chamber. These high temperature-higher humidity conditions are maintained for a day or so before the dry bulb is lowered again. Practitioners state that the drying rate increases after this type of treatment. Shocking the kiln doesn’t increase the drying rate for the reasons that people think. What’s happening is that the lumber is being heated to a much hotter dry bulb than formerly and, since a higher dry bulb increases the amount of water vaporized, a greater amount of water is removed from the sample boards when the compressor is turned back on. There are good reasons not to add water during drying. Tiemann23 noted that steaming lumber during kiln drying didn’t increase the overall drying rate and in fact sometimes caused checking and warping in oak. Additionally, if a dry lumber surface is rewetted while the interior is wet, it’s possible for the surface swelling stresses to exceed the wood strength and honeycomb will result. This is the reason why it’s important not to increase the EMC during the kiln schedule. All in all, if the drying rate slows down to an unacceptable rate, it’s far better to raise the dry bulb temperature and adjust the kiln schedule as described previously than to add water back into the dry kiln! 23 Tiemann, H.D. 1917. The Kiln Drying of Lumber, 2nd edition. J.B. Lippincott Company, Philadelphia. 318 pages. 98PDF Image | HARDWOOD DRY KILN OPERATION A MANUAL FOR OPERATORS OF SMALL DRY KILNS
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