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Could you wind up this run at 150°F instead of 160°F? If the target MC is 7%, you could use a 150°F dry bulb temperature with a wet bulb depression of 35°F (5.0% EMC at 31% RH) for equalizing, giving you a wet bulb temperature of 115°. That’s within the operating parameters recommended for the compressor (maximum of 120°F or so or the wet bulb), so you’d be ok for equalizing. Next, though, there’s the question of how to condition the lumber. If you plan to raise the conditioning temperature to 160°F the wet bulb would have to be set to a temperature of 149°F (an 11°F wet bulb depression yields an RH of 75% and an EMC of 11%). This is outside of the operating range for the compressor, but it doesn’t make sense to run the compressor to extract moisture at the same time as you’re trying to add humidity anyway. To reach these conditions you might shut off the compressor, monitor your wet bulb, and make adjustments as needed to maintain the RH at 75%–but you’re still going to have to add moisture somehow. Raising the temperature from 150°F to 160°F is going to decrease the ambient RH, not raise it. You won’t be able to make a 31% RH at 150°F increase to 75% RH at 160°F without evaporating more water from the lumber–and that’s the last thing you want to do. Here’s a practical approach to conditioning in a DH kiln: Let the kiln cool down for an hour or so after equalization and then increase the RH to the prescribed EMC for conditioning using mist or steam as available. This can result in rapid moisture uptake and speedy conditioning. Keep an eye on the length of the conditioning period and make sure you don’t add too much moisture back to the wood (see the descriptions below about monitoring this). Let the wood sit for a day or two afterwards to let the excess surface moisture evaporate. USING PRONG TESTS TO DETERMINE DRYING STRESSES AND THE END OF THE CONDITIONING PERIOD. Stress relief has to be frequently monitored during conditioning. Residual drying stresses can be indicated by using a prong test at the end of equalization. For each prong test, a one inch thick piece of wood is cut across a board (like for a moisture section) to make prong test sections to judge the degree of casehardening. Prong test sections could be cut from the sample boards or any other lumber you select from the load being dried; movement of the prongs after cutting indicates residual stresses (assuming the moisture in the prongs is uniformly distributed). The manner of cutting prong test samples varies according to the board thickness as shown in Figure 27 below: 54PDF Image | HARDWOOD DRY KILN OPERATION A MANUAL FOR OPERATORS OF SMALL DRY KILNS
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