Kiln venting

      Are vents needed in a condenser kiln? March 20, 2001

I have two home-built solar kilns. Each holds approximately 5k bd ft. Neither kiln is vented for air exchange. Should they be?

Forum Responses
I do not understand how you can remove about 2400 pints of water per MBF without vents or another form of water removal system.

Gene Wengert, forum technical advisor

Each kiln is 18ft by 10ft by 18ft high, reasonably airtight, with a double layer of clear plastic on the front. A small fan blows the interior air between the two layers of plastic, where it condenses on the inside of the outer layer, then runs down and drips out the bottom. The sun is free and the fans run for pennies per month.

How exactly does this type of kiln work?

Assume that the temperature of the air outside is 70 F. When the sun shines into the dryer, the air inside is heated to 100 F. This hot air then passes through the load of wet lumber and picks up moisture. The dew point of this heated air will perhaps be 80 F. Now, when this hot, humid air is passed across a cooler surface (such as the solar condenser facing away from the sun so it is at about 70 F), the moisture in the hot humid air will condense until the dew point reaches 70 F.

This is a condenser kiln. The main problem is that it is difficult to achieve the very low MCs that you can get in a kiln with vents. Also, when lumber is soaking wet and dries quickly (so. pine, soft maple, yellow-poplar, etc.), the condenser kiln is slower than a vent kiln. My philosophy is "Why condense if you can vent?"

Should you vent in a dehydration kiln also?

There are times when you can lower the RH in the kiln faster by venting then by condensing. In those cases, venting makes more sense, provided the RH doesn't get too low.

Gene Wengert, forum technical advisor

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