Measuring Air Velocity in a Kiln
Airflow is usually measured exiting the kiln. Here are some details on good practice. June 13, 2014
Question
I have three questions regarding measuring air flow through a kiln. We are currently using an Alnor Velo Jr. to measure air speed; however it’s the placement of the device that we are arguing about. The kiln operator on site says that the device should be held at the entering air side of the lumber in the kiln. I argue that it should be on the leaving air side. I have only been able to find one reference to this at all, in the USDA report Quality Drying of Hardwood Lumber FLP-IMP-GTR-2. In it, it indicates leaving side, but the kiln operator insists that it’s a typo. Could you please help swing the consensus one way or the other? Also, I have read from Dr. Fred Lamb on this site that hardwood drying should have an air flow of about 350f/m. Would that be measured at air-in, or at air-out, or just based on fan volume calculations? Finally, I read that higher EMC readings on the air-leaving side can suggest insufficient air flow. How much variation should there be before having to take corrective action?
Forum Responses
(Commercial Kiln Drying Forum)
From Gene Wengert, forum technical advisor:
We are always measuring leaving air flow. Remember that what goes in must come out. You will see the same instructions in Drying Hardwood Lumber, p.14. It is next to impossible to measure entering air accurately, especially with the instrument that you have. It depends on the species with respect to the air flow; 350 fpm is too high for green oak and for many species of 8/4 and thicker. For maple and other white woods, 350 is likely too low. For air-dried, 350 is ok, but it is not required to be so high, so many operations reduce the fan speed to save energy. With a wet bulb, lower air flow can mean an error in WB temperature. Lower air flow can mean less heat form the steam pipes. This is covered in Drying Hardwood Lumber, p.14 -21. The lower the air flow, the higher the exit EMC above the entering EMC. This is true. Higher air flow means faster drying and for some woods, too fast can mean quality loss. Plus 10% faster can mean nearly twice the energy use.
From contributor D:
I would think that 350 for green oak would be acceptable if the humidity was high enough. Am I wrong?
From contributor O:
The measurement of air flow through lumber is an inexact science at best. I have taken flow readings with ten readings coming up with ten different results in the same kiln. Let's throw in: not a perfect loading, various thickness, various species, various moisture contents and amount of venting changing static pressure. That should be enough to make you feel less than confident with any numbers you may obtain. With dead green white oak you run a fine line between check development from excess airflow and stain/mold/wet pockets from too little air. Comparing hygrometer readings on the exiting side of the loading with wet bulb readings will help. Move the air just enough to keep the dry bulb reading just a bit higher than the wet bulb. That's the best you can do.
From Gene Wengert, forum technical advisor:
When using a portable hygrometer or psychrometer in a kiln you need a temperature measurement of at least 1/2 degree F accuracy. I have seen commercial units in a brass holder with every two degrees marked on the thermometer and they are worthless for kiln drying.
From Contributor L:
Any air flow testing we have done or have been taught has always been done on the exiting air. We have always found a hot wire meter to be more accurate than the fan type. But with either one there can be quite a difference if it is not aligned to the flow properly.
From Contributor K:
Gene, you had mentioned that the hygrometer/psychrometer used should be within .5%. Do you have a recommendation of a reliable instrument? The one used at our facility seems to me inadequate for our drying purposes.
From Gene Wengert, forum technical advisor:
I would be very satisfied with 2% or 3% RH accuracy. It is a moisture meter that should be 0.5% MC. Also, the RH measurements (or EMC or WB/DB) should give about 1% Rh accuracy or maybe 2% RH. To do that, WB/DB needs to be 1/2 degree F accuracy.
