Heat Sources for a Dehumidifying Kiln

A discussion of the safety and practicality of various ways to heat air for a dehumidification kiln. November 14, 2009

Question
Does anyone have suggestions as what to use for a heat source for a dehumidification kiln? I need to increase the temperature to approximately 125-130 F to sterilize the wood of any potential insects. The kiln size is approximately 10' x 3.5' x 3.5.

Forum Responses
(Sawing and Drying Forum)
From contributor P:
Try a couple of 100 watt light bulbs. My little kiln is 11 X 4 X 5 and I get 105 or so with one 75 watt bulb.


From Professor Gene Wengert, forum technical advisor:
You can buy safe heaters for dusty environments. I would not rate a light bulb as being extremely safe in a dusty environment, as dust can catch fire at surprisingly low temperatures and light bulbs can get very hot, especially if the fan is not blowing.

From contributor S:
The auto-ignition temp for sawdust is in the 400-500 degree F range... I say go for the light bulb, and keep the kiln box as dust free as possible. People have been using these small kilns for years with light bulbs for extra heat. If you hunt around you can probably find a switch that will shut off the power to the light bulbs if the whole thing gets too hot (temperature cut off switch).

From Professor Gene Wengert, forum technical advisor:
The ignition temperature for wood is 400 - 500, but for dust, it drops to under 300 F. This is why dust presents such a hazard. Dust will begin to char at 200 F.

Here is a quote for a wood MSDS:
Wood dust, from cutting operations, is a strong to severe explosion hazard if a dust "cloud" contacts an ignition source. Partially burned dust is especially hazardous if dispersed in air. 212 deg. F. has been suggested as the upper temperature limit for continuous exposure of wood without risk of ignition. (Wood dust may require a still lower temperature.) White pine flour as a "cloud" in air requires 0.04 j minimum energy for ignition and can produce an explosion pressure of 113 psig maximum.


From contributor B:
I run my bulbs through an adjustable thermostat. They turn off when the temp gets to the setting, be it 90, 110 or 135. You can get them new at Grainger for around 65 or on eBay for 20 - 25 bucks. Try searching for temperature control. The first couple I bought would not go higher than 120. If you want 130, make sure that is what you get.

For another level of safety, I think you can also use a temperature limit switch if you want that would kill the power to everything if the temp goes over the settings. I do not think heating it to 125 will do what you want concerning killing bugs; did not for me. At least 130.


From Professor Gene Wengert, forum technical advisor:
Although I advise against using light bulbs (the bulbs will be hotter than the air), if you do use bulbs, it would be a great idea to have a smoke alarm nearby in case there was an issue. As we have discussed before, if this unit or any kiln unit were to catch fire and burn a house, it is likely that the insurance company would not cover the damage as it was caused by a business venture in a home (unless you get a rider to your policy).

From contributor C:
Gene, I did a search without success on a dust safe heater. Any guidance on finding these? I'm also interested in finding the temperature controller mentioned above. A search on eBay didn't lead to anything that looked quite right.

From the original questioner:
Well... I really wanted to stay away from the light bulb heating method, but that may end up the way I will have to go. Could you suggest any specific heaters that would be safe in this environment?

From Professor Gene Wengert, forum technical advisor:
The safe heating technique is to use a heater that has the heat generated on one side and then a wall or heat exchanger on the other side. The hot air on this second side goes into the kiln. It is just like a gas furnace in a home.

From contributor B:
We have talked about this in the past and I think I remember you suggesting a 2000 watt electric heater. The picture I had in my head for is an electric heater like I remember from childhood. They have what appeared to be metal wires that got red hot from the electricity that provided the heat. To me that did not sound safe. The heater you are explaining in this thread is different than what I was thinking.

From contributor S:
Here's something from the US Gov't OSHA website:

The high temperatures of some heat-producing equipment, such as lamps and lighting fixtures, can ignite flammable atmospheres if they exceed the ignition temperature of the hazardous material. The National Electrical Code requires special marking of heat - producing equipment with temperatures above 100oC (212oF).

So reading this, it sounds like standard light bulbs are not supposed to exceed 100 degrees C. I think a little common sense would go a long way here. The small amount of lumber that goes into one of these small kilns could be pretty well de-dusted in just a few minutes.


From contributor T:
First and foremost, find a thermostat, controller, etc. that will work reliably at 130-140 degrees. Then buy an oil filled electric heater and bypass the on/off switches so it is totally controlled by the thermostat/controller. Also a standard 2-6' long 220v baseboard heater will work with a safe thermostat mentioned above.

Most important is getting a reliable thermostat/controller and having a fan to circulate the air over the heat source and through the wood. I would go with the baseboard electric heater and 140 degree thermostat.


From contributor B:

I would not be comfortable bypassing factory switches on an electric heater that was designed to run at 85 degrees and making it run at 140 degrees. Would all of the components the heater is made of be able to operate safely at 140 degrees?

If anybody reading this is associated with a race team that uses one of those point and shoot thermometers, shoot it at a couple light bulbs and tell us the results. I would like to see the difference between a 75w, 100w, and 150w. I am using 100w bulbs.


From contributor C:
I found a controller that has the right range. Anyone have comments on the Ranco ETC-111000-000 Digital Temperature Controller? Will it handle turning off and on a small heat source? I'm thinking of replacing my small heaters and trying radiant heat wire mats like the ones used in bathroom floors under tile. There should be no ignition source from them, but they may not come up to temperature either. Any comments on that approach?

From contributor N:
I would stay away from the radiant heat. If it is in the floor, the bottom boards will dry a lot faster than the rest. You need indirect heat in the air for even drying.

From contributor S:
The heat controller mentioned above is rated at 15 "resistive amps." Electric heaters are rated in watts. Watts is voltage x amps, so 120v X 15 amps = 1800 watts. Most heaters for home use are about 1500 watts.

Run a good test before you put any wood in there. That is, install the heater and run it all the way up to temperature and leave it for a while, then open up your kiln and inspect everything. I'd prefer to use a baseboard type heater that you can hard wire in. The cords that come on some heaters look kind of light duty to me.


From the original questioner:
I ended up finding a couple of heat attachments that are made for the blowers I am using. They use the heated wires within an enclosed unit and are thermally protected along with the blowers themselves. They seem to produce enough heat and I would guess the thermal shut off would be higher than 130 degree range. It is stated that if they (heating elements) get too hot they will shut off until they cool off enough and then switch back on.

From Professor Gene Wengert, forum technical advisor:
Because dust is so easy to ignite, I think it would be prudent to dust off the lumber before drying (especially from those mills that make very fine sawdust) and also vacuum out the kiln after every use.

From contributor P:
I air dry to 15 - 16 percent under cover, then thickness plane one side to an even thickness. My planer is serviced by a dust/chip removal blower that would suck start a B-52 bomber (or suck a golf ball through a garden hose). This takes care of all of the pesky dust.

My temps still only get to 104 or so when drying using only one 75 watt bulb.

My bedroom furniture made with cherry dried in this manner has moved only slightly in the last two summers, with just some minor adjustment in one drawer with wood-to-wood slides. I also built a beech cabinet a few years ago with the same methods and result. I am in a high humidity area with the relative humidity reaching 60 to 70 percent in the summer (central New York state). Works for me.

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