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WHAT SHOULD YOU DO WHEN DRYING APPEARS TO SLOW DOWN OR STOP? If you’re paying attention to your sample boards, you’ll notice that the drying rate slows down as they approach 20%–25% moisture content or thereabouts. Don’t panic. This is perfectly normal. If your wettest sample board is still losing moisture, your kiln is operating correctly. People sometimes get flustered when the average rate of MC change starts to slow down (but you shouldn’t be controlling the kiln based on the average MC of all the sample boards in any event). The slowdown you observe occurs because of the rates of water removal for free water and bound water are different. The wetter boards have more free water initially, so the rate of water loss is faster than for drier boards, which have mostly bound water. The overall daily average MC change might be slight, but the kiln is still drying. TROUBLESHOOTING A DRYING SLOWDOWN. What would you do if you found that your wettest sample board is only losing 0.5%-0.75% per day? If you don’t think your kiln charge is drying as quickly as it should, the first thing to do is to double-check your equipment. It’s possible that your kiln conditions aren’t really what you think they are. 1. Check the wet bulb wick, making sure it’s clean and well-moistened, has good air flow, and is properly connected. 2. Check your dry bulb thermometer and its connections as well; if you have a reliable, calibrated backup hygrometer, use it to verify the readings you’re getting with your primary equipment. 3. Check your fans to be sure they are turning properly; at this moisture level the air flow isn’t as important to your drying rate as at higher MCs, but if the fans or baffles have problems your wet bulb may not be getting enough air flow to respond properly. 4. Verify that your baffles are in place. 5. Take a look at your sample boards if everything else looks good so far: maybe your wettest sample board is in a slow-drying part of your kiln. 6. Consider if you might have made a mistake with the sample boards; could your original MC or calculated dry weight be in error? It’s not uncommon to have a board that looks like an outlier when you only use a few sample boards to control the kiln, but if the kiln starts to slow down and you can’t figure out what’s going on, it would make sense to confirm that the sample board(s) you’re using to monitor the kiln is really at the MC you think it’s supposed to be. POTENTIAL SAMPLE BOARD PROBLEMS. Sometimes sample boards aren’t handled with the attention they require – phone calls, customers, all sorts of things might interrupt you. Occasionally these normal business activities result in sample boards being put aside to await a “better” time, or maybe the way you handle them just gets hurried up a bit. What could go wrong? Besides getting the original or calculated dry weights recorded incorrectly, I’ve observed that sample board handling problems generally fall into just a couple of categories: 1) the board ends don’t get coated thickly enough with B.O.S.S.®. They dry out faster and consequently the boards have a less uniform MC from end to end compared to the rest of the kiln charge; 2) boards sometimes get set aside after they’re cut or in-between weighings once the kiln has been started up. 94PDF Image | HARDWOOD DRY KILN OPERATION A MANUAL FOR OPERATORS OF SMALL DRY KILNS
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