Kiln Drainage

Wood in a kiln releases water in vapor form. But condensation may occur in the kiln, requiring attention to drainage. April 2, 2015

Question
Does liquid water drain from wood (specifically SYP) as it dries in a conventional high-temperature kiln? If so, how can we estimate the amount to properly prepare a drainage system?

Forum Responses
(Sawing and Drying Forum)
From Contributor V:
No it doesn't, if there is water in your kiln it does not come from the wood. I've seen this with a steam supply system that was poorly designed. The problem was two-fold: The spray line feed pipe came out the bottom of the main steam line, effectively acting as a condensate return line/drainage line off the steam line. The result was that when humidification was required from the steam spray it would spray all the condensate in the steam line into the kiln first and then steam. This problem was multiplied by the second problem. Steam traps where not placed optimally, resulting in excessive condensate throughout the system.


From Contributor Z:
There is a lot of water in wood. In fact, some species of wood are more than half water in terms of their weight when they’re fresh cut. Nyle put out a publication with the following example. Moisture content in lumber is generally expressed as a percentage of the dry weight.

For example, if a fresh cut board weighs five pounds per board foot, then weighs three pounds per board foot after it’s been dried in an oven to 0% moisture content, that means it had two pounds of water in every board foot. Two pounds of water per board foot compared to the lumber’s dry weight of three pounds per board foot is a ratio of 2:3—so the lumber has a moisture content of 2/3, or 67%. That’s similar to oak, for example, which is usually about 68% moisture content when fresh cut. It’s really astounding how much water has to be removed from wood to make it suitable for finished products. Take the example of a truckload of oak. Fresh cut oak weighs about 5.4 pounds per board foot. So a truckload of 8,000 board feet weighs about 43,560 pounds - just under 22 tons. Once you remove enough water to get the oak down to a moisture content of 6%-8%, it weighs about 3.5 pounds per board foot. So that truckload now weighs 28,000 pounds, or about 14 tons. That means that to completely dry a truckload of 8,000 board feet of oak, you have to remove 15,560 pounds of water - almost eight tons, close to 2,000 gallons of water. Some of this water will be vented from you kiln but not nearly all of it. Our plant sits near a breeder stream for trout and we had to plug all of our kiln drains. I definitely notice the water now and I agree with the earlier post that water in a kiln could be related to a poorly designed steam system, but not necessarily. To answer your question yes, liquid water does drain from wood.


From Contributor V:
I have never dried oak, so you could be right? I don't believe the moisture could come out of the timber so quickly that you can't vent it out. If that is the case, the venting capacity of the kiln is not matched the rest of the kilns abilities. I've been involved with many drying kilns drying pine. If the heating capacity, air flow ability and venting capacity is matched, you should not get any condensate in a SYP kiln.


From Gene Wengert, forum technical advisor:
Until you go much over 150 F, you should not get any condensation unless you have a poorly insulated floor or walls. All water from drying lumber leaves the wood as a vapor. If there are any cold areas, then the vapor condenses. That is why insulation of the structure is so important – it prevents condensation of the moisture coming from the wood.

From contributor L:
Gene, during the last of the drying process, with vents closed, where does the water go?

From Gene Wengert, forum technical advisor:
If the kiln were seal perfectly then the vapor would stay and at 100% RH and drying would stop, but most kilns have leaks and use vents or a zdH.

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