Mist systems and relative humidity

      Mist systems, what they do, and some general information about relative humidty and its effect on wood. 1998.

by Professor Gene Wengert

Q.
Can you pass on some information about the water spray systems I've seen that are used to control humidity?

A.
The purpose of a cold water mist is to increase the amount of water vapor in the air--the humidity. But, in order to convert liquid water to vapor requires energy--roughly 1000 Btu's per pint of water. It doesn't matter if the water is in drops or on a big bucket on the stove--all water takes the same energy to convert to vapor. If the water is warm you will save a very small amount of energy--maybe 50 to 70 Btu's of the 1000 required. So, heated water is not critical. But, one potential problem with a liquid water mist system is that you will have to supply the energy for evaporation from your "normal" heating system. It is just as efficient, therefore, to have a small boiler (sometimes called a pony boiler; I see them in the Grainger catalog for under $1400) to provide the humidity you require.

So long as we are discussing humidity, three additional points:

First, do not humidifiy your plant to a level that is wetter than the customer's home or office. Why? Because, if your plant is humid and the lumber is equally moist, you will solve all the cracking and warp problems in the plant, but you will just postpone these problems until the customer gets the product. It is better to have the problem show up in the plant than in use.

Second, the real cure is to get dry lumber (maximum of 7.0% MC in the winter for most of the U.S.) and keep your plant around 6% EMC (30% relative humidity). And, always double check the MC yourself--don't rely on a value stated in the invoice. Measure the RH with an electronic sensor--not the brass office-style devices.

Third, if the temperature varies in the plant, so will the RH and the EMC. For example, assume that the temperature and RH are 75F and 30%, respectively, which means 6% EMC. If there is a cold spot at 65F, the RH there will be 43% and the EMC will be 8%. If there is a hot spot at 85F, the RH will be 22% and the EMC 4.5%. (As a rule of thumb, a difference between the MC of lumber and the EMC of the air of more than 2% will begin to lead to problems.)

Professor Gene Wengert is Extension Specialist in Wood Processing at the Department of Forestry, University of Wisconsin-Madison.

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