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In general, for most species except white woods like maple or yellow-poplar sapwood, you should try to minimize your kiln drying expenses by getting your wood dry enough from air/shed drying to finish the drying in the kiln within 10 to 14 days. This is difficult to time, so check the moisture content of the lumber in your stacks periodically. In general, well air-dried lumber will have a moisture content of 20 to 30%.16 The amount of time needed to air dry lumber varies with its thickness and the time of year that it gets stacked, but it could take the better part of a year to air dry lumber satisfactorily if it’s stacked in cold weather. Some species (yellow-poplar, for instance) will, of course, dry more quickly than others. In 2015 dollars, you should plan on making at least $25–30/1000 board feet (BF) of lumber in the kiln per day (gross profit). Increasing the number of times you can turn the lumber in your kiln helps your bottom line. More time spent on the air drying yard or shed means less time spent in the dry kiln – and potentially more money in your pocket. Your kiln costs will increase if you start with green lumber and your drying profit per board foot will decrease, though some of that profit loss might get offset by increased quality. You may have higher handling costs if the lumber doesn’t go directly into a kiln. SAMPLE BOARDS. Just as you can’t measure the weights of every pebble in a gravel driveway, you can’t monitor the weight of every board in a pack of lumber. Not only is it impractical, it doesn’t make sense when you can use sampling to get the information you need. That raises a really important question: What exactly is the information that you need? You need to know the range of moisture contents in the lumber throughout the drying cycle! Every board is different in the beginning but, as I showed earlier, the moisture contents will get more similar as drying progresses. Wood is a natural material, so in any charge of lumber we know that: ∗ Different boards will have differing moisture contents; ∗ There will be variability in the densities of the boards, even from a single log; ∗ The moisture contents of the heartwood and sapwood will likely be different; ∗ The drying rates of heartwood and sapwood will be different; ∗ The drying rates of high grade boards will be different from boards with more knots; ∗ The drying rates of flatsawn, riftsawn and quartersawn boards will be different; ∗ Thick boards will dry more slowly than thin boards. Since all of these types of boards might be present in a lumber pack or kiln charge (and sometimes with the added challenge of mixed species in your kiln), it should be obvious that there will be a range of moisture contents and drying rates represented by the boards at the beginning of drying. Your challenge is to get all the boards to the same degree of dryness, and to do it within the specifications your customer requires (perhaps 7% MC plus or minus 1%, for example). 16 Don’t partially air dry lumber (i.e., don’t stop air drying at MCs above 30%), as this can lead to color differences within the wood when it’s kiln dried. In maple, for example, the drier part of the wood will be white when a cross- section is cut, but the wetter core will turn darker when it is dried at the higher temperatures to which it is subjected in a dry kiln. This can create all sorts of problems between the seller and the buyer. 71PDF Image | HARDWOOD DRY KILN OPERATION A MANUAL FOR OPERATORS OF SMALL DRY KILNS
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