Making a Drying Kiln from an Insulated Truck Body

This conversation touches on the quantity of heat needed to dry a load of wood, the advantage of using a dehumidifier, and the possible issues with corrosion. April 18, 2015

Question
I just bought a 12'x7'x7' reefer box from a produce deliver truck. This is an insulated truck box and is all aluminum on the inside. I am turning it into a new kiln and was wondering what the best way to heat it would be. My old kiln, a 8'x4'x4' box, was simply heated with one or two 500 watt work lamps and that got me to 100-125 degrees without a problem. This new kiln is much bigger, so I doubt work lamps would be very economical.

Forum Responses
(Sawing and Drying Forum)
From Gene Wengert, forum technical advisor:
This kiln can hold 2000 BF, which means you will need at least four million BTU to dry well-air-dried lumber to 7% MC. A 1500 watt electric heater (like used to heat a bathroom) run for one hour will deliver about 5100 BTU. So, two heaters run for 400 hours (17 days) would be the minimum time. Of course, they would use 3 KWh x 400 hours x$.18 per KWh is $200.



From contributor R:
Gene, if your kiln chamber is insulated well where are all the BTU's going?


From Gene Wengert, forum technical advisor:
It takes energy to evaporate the water. Even with good insulation there will be some heat loss through the walls, floor, roof. When water is evaporated the humidity in the kiln will increase, so we need to vent the moist air and its heat to the outside and then bring in drier, cooler air.


From contributor R:
Why not use de-humidification?


From contributor T:
I have a dehumidifier in mine and it works great.


From Gene Wengert, forum technical advisor:
A DH kiln is a good idea need, and because they are so energy efficient your cost for electricity would be maybe 1/3. Plus, the bathroom heater probably has some safety concerns. Also, with the heaters they would actually shut off when they reach about 85 F, so drying would take much longer than calculated. Industrial heaters would be better, but maybe no safer due to flammable wood dust. The safest would be a furnace, like is used to heat a home that uses an air plenum exchanger so the kiln air does not see a flame or hot surfaces. In any case, make sure your household insurance covers your home if the kiln does catch fire. Often a business in or near a home requires a special policy to protect the home. Kilns do catch fire now and then.


From the original questioner:
I thought about a DH, however, this is a kiln for reclaimed lumber and much of the lumber being loaded into it is from old Whiskey Barrels. These wooden barrel staves still have alcohol soaked in them. When I built my original kiln I noticed that, because of the alcohol, any exposed metal was corroded within a week. I am afraid to know what that will do to a dehumidifier. I may try it anyway! My homeowners insurance has been converted to a commercial policy already, and the kiln is 50' away from the nearest structure.


From Gene Wengert, forum technical advisor:
I suspect that there is not any alcohol in the staves, as alcohol evaporates so easily. Alcohol does not cause corrosion. It is more likely that the corrosion you noted was due to high MC and tannic acid. Commercial DH kilns have a coating on the DH coils to reduce or avoid corrosion of the coils. However, the condensed water is likely to be fairly acidic, so the way you carry this water mixture away is important. You may even have to neutralize it and itís quite easy to do.