Heat Control in a Dehumidifier Kiln

Understanding how supplemental heating and the compressor's heating effect interact in a dehumidifier kiln. February 15, 2009

I am in a discussion with a fellow worker as to the going ons inside a Nyle Kiln. My interpretation is the heat element brings the air up to specified temperature and keeps it there cycling on and off as needed. The compressor removes as much moisture as needed to keep the RH as programmed with both switches being on auto. His understanding is that the compressor generates both heat and pulls moisture, and the heat coils are for initial heat only, suggesting the heat be turned off after reaching temp. The Nyle literature is not real clear.

Forum Responses
(Sawing and Drying Forum)
From Gene Wengert, forum technical advisor:
The heat is supplemental heat and is used (automatically) whenever the air is not at the desired temperature. You do indeed have to use supplemental heat initially. Once running, you will likely have enough heat from the compressor and its activities (it is a heat pump), that additional heat will not be needed. However, if your kiln building is not well insulated or you are running really hot, you may need more heat than the unit provides. If you have enough heat, the supplemental heat will not come on even though the switch is on. The supplemental heat is usually electric, which is expensive. Using propane or other less expensive fuel to "kick start" the process may be an economic advantage.

From contributor G:
If your kiln is well insulated and you use Nyle's overtemp system, an automatic temp fan system, you can set it and forget it. My heaters never come on after 90 degrees. The compressor heat will do the rest, only if you well insulate. I run 3 L200 Nyle kilns, and I love them.

From the original questioner:
Thanks for the replies. We just started ours up the other day to re-dry some fir which started around 18% MC. It is an insulated cargo container and seems real tight. We however did not use supplemental heat, just turned on the Nyle unit and waited till it got to 80 before running the compressor which took several hours. We started at around 67 degrees in the container once loaded but it was a nice day out. We are running at 110 degrees right now and the heat does cycle off and on every so often but then will rise above the set temp slightly before dropping again. We are not using any power vents currently. Is this procedure wrong or come winter will the Nyle unit not get up to temp? I kind of missed the whole supplemental heat thing.

From contributor G:
The Nyle heaters are not like electric furnace heaters. They heat very gently. I set my starting temp at 90 degrees, and after that the heaters never kick on again, only after the lumber has reached 90.

I built my kilns and insulated with 2 layers of foam insulation 2 " thick and a layer of plastic before I put the 1/2 " OSB skin on the inside. I put two heavy coats of aluminum roof coating over that. Five years now and I have no problems. It cost me about 5 cents per bd ft to dry. My kilns will heat to 130 degrees or more by storing the heat from the compressor. I figure the R value of my kilns at about 60. Temperatures exceeding 130 could damage your compressor, unless it's mounted external with ducts (unit outside the kiln).

Check with Nyle before you make any changes. I don't know how well cargo containers are insulated, but probably not near R60. DH kiln drying is different from commercial drying kilns. Its hard to damage lumber in a DH unless you do something really stupid.