Kiln Drying Forum
Wood Drying Topics => Dry Kiln Control => Topic started by: SKieling on April 18, 2014, 04:26:20 PM
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I had the opportunity to talk to some folks recently who dried southern yellow pine. The subject of venting came up. They use only a few large vents on each side of a double track kiln. All the double tracks I've seen in the northwest, have several per side. It raises the question of what is better, a few large vents or several smaller ones. For example, would four 24"x60" vents be any more or less effective than ten 24"x24" or twenty 16"x18" ? We dry both DF and HF with conventional schedules, no high temp ones.
Thanks
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Rule of thumb: 0.03 square meter vents per cubic meter for softwood kilns. The total venting capacity is more important than size configuration....even more important is to have absolute control of the venting strategy.
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Think about how even you want your RH to be across the length of your kiln. A few big holes will make it less even than many smaller holes. That's my 2 cents worth anyway. The CDK guys might see it different, as they run with no/minimal vents and no doors on the ends. Their charge sees all of the zones on the way through though.
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MY RULE OF THUMB, BEFORE I RETIRED, WAS ONE 21"X21" VENT PER FAN ON EACH SIDE OF THE DOUBLE
TRACK KILN, SPACED BETWEEN THE FAN SHAFTS. THIS WOULD IDEALLY MEAN 11 VENTS FOR 10 FANS SO
THERE WOULD BE VENTS BETWEEN THE FANS AND THE ENDS OF THE KILN WHERE THE AIR FLOW WOULD
NORMALLY PRESSURIZE.
THIS WORKED FOR DIRECT FIRED, STEAM AND SLOPE GRATE GREEN-FUEL KILNS WITH AIR VELOCITIES OF 750 (OLD KILNS) TO 1700 CFM THROUGH THE LUMBER. YOU WILL ALSO FIND THAT AT SOME POINT THE TEMPS IN THE KILN APPEAR TO STALL IN THE EARLY TO MID CHARGE TIME. THIS IS CAUSED BY PRESSURE IN THE KILN RISING DUE TO INSUFFICIENT VENTING. PRESSURE VENTS WILL SOLVE THIS
PROBLEM ALLOWING YOU TO GET TO CONTROL SET POINT EARLIER.