Troubleshooting Low Humidity During Conditioning

A cold spot or air leak in the kiln can prevent the air from reaching the target humidity for conditioning. June 13, 2014

Question
Our conditioning times are getting excessively long as we are having trouble getting the wet bulb to set point, particularly in one kiln. We can't seem to get it above 150-156. We have plenty of boilers and have checked everything we can think of. We even changed the 1 1/2" spray line thinking maybe it was in some way restricted.

Forum Responses
(Commercial Kiln Drying Forum)
From Contributor D:
Depending on the size of the kiln, the 1-1/2 inch steam line could be adequate, but the control valve may be the bottleneck. A 1-1/2 inch globe type valve will not pass as much volume of steam per hour as the same size ball valve. Also, check the spray line. Often, the holes in the spray line are partially blocked with extractives carried by the steam. They should be drilled to at least 1/2".

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From Contributor O:
You say that the times are getting excessively long so we know that the system did work and the valve is big enough. It could be the blocked ports as mentioned or it could be a valve actuator that is failing.

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From contributor W:
It could be a problem unrelated to the spray line, valve or system. Are you venting to keep the dry bulb in control? Or maybe the kiln insulation is wet and you are condensing water on the cold walls, roof or doors. That is a common problem in poorly insulated kilns because the walls, roof and door become one big dehumidification coil. The problem gets worse as the kilns age and weather gets colder.

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From Gene Wengert, forum technical advisor:
You can have the large steam line and a properly working valve, but if there is a poorly insulated spot in the kiln, then this spot will act like a condenser. If that spot has a temperature of 150 F, then the wet-bulb cannot get any higher than 150 F. Contact your local county extension office or engineering extension at your state university and see if they can come to your kilns with an IR heat gun and then you will see the spots (wall, roof, door) where you have insulation failure. (Sometimes, commercial heating or insulating companies have heat guns too.) In a newer kiln where there is a problem initially, we would also examine the insulation at the floor and the above-ground foundation.

By not getting the WB high enough right away, you will find that stress relief takes longer and may not be 100% effective. Also, long conditioning tends to add moisture to the lumber but not give much stress relief. A short-term fix would be to cool the lumber off briefly just before conditioning and then use 100% steam spray with no heat. This may work even with poor insulation for a while, but it can also be hard on the life of a building.

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From contributor F:
I have had the fortunate pleasure of having the exact same problem a few years ago. The kiln in particular we had our learning curve with is a masonry cross-flow design forklift loaded kiln. We could dry the lumber without any complications, but whenever we tried to condition using a 147 WB, we could only get to about 130. We checked everything! Control valves, controllers, loading practices, piping, plugged holes in spray line, holes at correct angle to mix with the air, vents maintaining the closed position, new clean wetbulb wicks before conditioning, etc.

What turned out to be the problem after racking our brains charge after charge was that the rubber gasket on the bottom of the kiln doors had come off. The doors on our type of kilns have a rubber gasket on the bottom to help seal the kiln up when they are closed. When the doors are in the closed position, they have about a 2" to 3" gap between the bottom of the door frame and the concrete. This gasket had come off over the course of opening and closing them charge after charge and after replacing with new gasket, our headache with that problem went away. Of course, after replacing the gasket, we had to beef up our door anchors to keep from imploding the doors. I hope this is of some help, but always remember, there is not any fun in improving your drying practices unless you encounter some obstacles along the way that helps to increase your knowledge of your kilns and how they operate.