Dry-Bulb and Wet-Bulb Settings and Kiln Control

Tips on temperature and humidity control in kiln-drying operations. March 26, 2007

I am kiln drying hardwood pallets down to under 20%. We are targeting 15% using a time drying schedule in a SII dry kiln. The last load we ran we had the dry bulb temperature set at 165 and the wet bulb set at 155. The kiln reached temperature but the wet bulb would only get to 149 and would not vent. What could cause the wet bulb to not reach temperature?

Forum Responses
(Sawing and Drying Forum)
From contributor R:
I'm guessing you're set up to vent when the wet bulb condition in the kiln is over your setpoint of 155. With 165 dry and 155 wet, you are setting conditions in the kiln around 77% RH, which will give you very slow drying. Decrease your wet bulb drastically, like to 140 or so, so you have a large depression, which will vent off some moisture. Also, why not also increase dry bulb? I run a Kiln-Direct with hardwood pallets at 190 degrees every day, with good results. You need a larger gradient between wet and dry bulb to quickly dry lumber. With pallets, drying defects are not really an issue, so go on and crank it up to dry them fast.

From contributor B:
Actually, if the wet bulb was set at 155 degrees, you can't vent until you actually achieve 155. (I am confused by the response above this.) More likely, you didn't have enough moisture in the kiln to reach 155, either because the wood was too dry and there was not enough water in the wood to raise the wb that high, or because you had the spray turned off. If you are trying to KD and HT the pallets, you might need to use the spray. If you're just trying to HT, you might not. Give us a call at SII and we can help, as needed.

From contributor R:
Didn't mean to cause confusion. Contributor B is right - the machine didn't vent, because the conditions in the kiln never reached your WB setpoint. By lowering the setpoint, the kiln would vent moisture. My overall point was that running a kiln schedule with 10 degrees depression for green hardwood, especially when drying defects are not a concern, is more than a little conservative.

From contributor B:
He confirmed for me he is not using the spray, so I don't think he can achieve the wet bulb setpoint that he wants. He's going to try with the spray on automatic to see how it works. The concern is not just drying the pallets out, but quality as well, so too large a depression may adversely affect the quality.

From Professor Gene Wengert, forum technical advisor:
The wet-bulb temperature cannot exceed the temperature of the coldest spot (wall, floor, roof) in the kiln. So, if the spray is adequate, then if there is a poorly insulated spot, that will limit the WB temperature. This cold spot will condense water and essentially act like a dehumidifier at higher WB temperatures. This limiting effect is common in older kilns.