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AIR FLOW IN KILNS. Baffles are used in dry kilns to direct all of the air energy from your fans through (rather than over and around) the lumber. Whether you make them out of plywood, canvas, or metal, keep your baffles in good condition and repair them as needed, because good baffles will save you money and time. Get them close to the lumber, no more than 4” away. Leave a gap for air to circulate over the top layer of boards in your stacks. The air speed you use might be dictated by your current equipment. I’ve observed the air speed to vary a lot in different types of dry kilns. Some small DH kilns that I’ve checked have had air flow rates in the range of 125–200 feet per minute through stickered lumber, whereas larger kilns have more fans with higher horsepower and their air speeds might be 300 fpm or more. Upgrading your fans may have advantages for some species and moisture contents. Hard and soft maple are difficult to check, for example, so if you’re drying a lot of maple in a 4000 BF DH kiln you might put in a third fan to increase the air flow rate; you could probably go as high as 600 fpm without problems. On the other hand, it’s very easy to check red oak, so you might stay with 200 fpm at high MCs. In larger kilns, you could increase the horsepower of the fan motors and maybe even change the fan blades. Reversing fans are helpful if the air flows over packs of lumber more than twelve feet deep. Variable speed fans can be useful, but they run on 220V with 3-phase current. The motors are smaller than the 110V version with the same horsepower, and are generally cheaper, too. AIR FLOW AND TEMPERATURE READINGS. The air speed in kilns affects not only the drying rate, but also (to a small extent) the wet bulb reading. Perhaps you remember (as I do) being told in high school science class to swing a sling psychrometer as fast as possible to get a reliable reading from the wet bulb thermometer, because faster air speeds drive down the wet bulb reading more than slow air speeds? How does air speed affect the reliability of our wet bulb measurements if our kiln air speeds are a bit on the slow side? I’ve read that the wet bulb readings don’t stabilize until the air speed gets to about 600–800 fpm12, but once the air speed exceeds 100 feet per minute of so the WB readings only change by fractions of degrees as the air speed increases. As a practical matter your WB readings are still perfectly adequate regardless of whether your kiln air speed is slow or fast. I was told years ago that the kiln schedules used in the Dry Kiln Operator’s Manual were developed using kilns that ran at about 300 fpm. Nowadays many hardwood kilns run fan speeds in the range of 200-400 fpm but some small DH kilns have less efficient fan setups, and I have measured some that only have 150 fpm air flow. At this air speed the WB might theoretically read just a little higher than it would at 300 fpm (by less than one-half degree Fahrenheit); my own experiments with a sling psychrometer showed negligible differences at room temperature conditions. Small temperature differences on this scale don’t make any real difference to the EMC in the dry kiln. For a kiln set at 120°F dry bulb and 100°F wet bulb running at 150 fpm, the actual EMC in the kiln would only be about 0.1% EMC or less below what you’re attempting to set. That small difference is obviously nothing to worry about. The differences between 300 fpm WB readings and 600 fpm WB readings are even smaller than the ones I referred to above, so even if you use variable speed fans at their fastest setting you needn’t concern yourself with differences between the EMCs you’re trying to set and the EMCs that actually occur in the dry kiln. Very high air 12 Threlkeld, James L. 1970. Thermal Environmental Engineering, second edition. Chapter 10: The Psychrometer and Humidity Measurement. See the lines in Figure 10.6 for unshielded thermometers. 62PDF Image | HARDWOOD DRY KILN OPERATION A MANUAL FOR OPERATORS OF SMALL DRY KILNS
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