Re-Drying A Few Wet Pieces to Speed Production
One way to dry loads of lumber more quickly is to separate out the few still-wet pieces for a second pass through the kiln. June 13, 2014
I have heard recently that some kiln operations are using an in-line moisture meter to detect the wet lumber. As they are drying hardwoods, they set it at 8.5% MC maximum. The kiln operator is supposed to get all the lumber below this value, but he is allowed up to 3% that the meter indicates is wet. When they have enough wet lumber, then they re-dry this lumber. I can see that this is going to reduce kiln time slightly, especially for quartersawn or for a pack that is wet and should not be in the kiln with all the dry lumber or just because. I can see that they must think that shortening the kiln time saves more than the expense of re-drying. As I have never heard of this before, I would like to hear comments.
(Commercial Kiln Drying Forum)
From Gene Wengert, forum technical advisor:
I have seen this done with softwoods from time to time, but I have only seen it with hardwoods twice and both were recent. I have seen a lot of hardwood operations that do produce a little wet lumber (by accident), but it is rare that an in-line meter is used to pull any wet lumber for re-drying. I think that what you describe is a step in the right direction. I do not know if 3% is the magic number (costs are different for each operation), but 3% seems reasonable.
Perhaps equally important in such an operation is to provide the kiln operator with a measure of the variation of MC. Variation and average MC are both important. One measure of variation is the Standard Deviation (SD). A good drying operation will be at SD = 0.6 or less. Incidentally, when re-drying, start the kiln with an EMC that is halfway between the highest MC and the target MC. Example: Highest MC is 15%; target is 7%. Start at 11% EMC, but without steam or water injection. Use the water in the lumber to maintain the EMC. Start at 120 F or maybe 130 F dry-bulb.
From contributor W:
A few years ago one of our customers, a large Eastern White Pine mill, conducted a detailed analysis and test runs to find that right percentage of wets and it worked out to be 3%. Interestingly, the wets would only take a day when recycled through the kiln but added at least three-four days to the kiln cycle. Smaller mills who manually stack and unstack lumber often give pinless meters to the guys unstacking lumber and they catch most of the wets. They don't check every piece but can tell by feel and sight which ones need to be checked.
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