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Achieving Sterilization with a Solar Kiln in a Humid Climate

A woodworker struggles to get his lumber pile hot enough to kill beetles. April 15, 2012

First run of my homebuilt solar kiln, and I'm trying to salvage some mesquite from my dad's land. It was milled and air dried from November until May and then put into my kiln. Some borer activity was noticed at this time. After 6 weeks or so of 90-120 degree solar kiln interior temperatures, I think my slabs of mesquite are about as dry as they are going to get here on the coast of South Texas. With mesquite being such a borer magnet, I want to sterilize it. I added heat via 3 x 200 watt light bulbs and additional insulation. Now I hit 145 degrees midday, but still drop to about 120 overnight. I have no way of telling if I hit the magic 130 in the wood interior (2-3" slabs) and for how long. Will this be sufficient to sterilize my lumber and how long will it take under these conditions? 24 hour average temp is about 125 if that helps, and I'm trying to figure out how to post a graph from my temp/rh logger.

Forum Responses
(Sawing and Drying Forum)
From contributor O:
This is just a guess but if your kiln is holding 120 degrees F overnight, then it should be getting the wood to 130 degrees F during the day. I have the first charge in a solar kiln, also, and at daytime temps of 103 degrees, I'm hoping the kiln will sterilize the wood.

From contributor C:
I believe you need 140 for 6 hours for powder post beetle. So maybe you will need to consider species of bug you're trying to kill.

In your kilns, do you still have the vents cracked a bit? If the wood is not oven dry, it will continue to release moisture, which holds the temp down a bit. The drier it gets in the kiln, the hotter it can get. The difference between 15% MC and 10% for me was about 15 degrees. The drier it became, the kiln came up in right behind. What is the MC of your slabs?

From the original questioner:
Once the MC got to 10% I quit checking and focused on sterilizing. I know the higher heat is going to lower MC more, but I wasn't worried about checking anymore.

I can only get to sterilizing temps in the heat of the afternoon. I need to know if this is high and long enough or if I need to find another way to add heat to my kiln. I can leave it here for many weeks if necessary if this will eventually and reliably kill borers and eggs! I'm in a better safe than sorry mode.

From contributor C:
I am not sure what is available for core temp check for home users. I believe ppb needs 4 hours at 160F or 6 hours at 140F. One thought is that you could use a cheapo infrared temp meter, Ryobi brand, and make a cut, then immediately check temp. May not be practical trying to get samples in and out...

I did see when looking for controls for my 300 bft kiln that the hydroponic crowd has all kinds of cool hygrosats and thermostats. Piggy back plug-ins, all easy to incorporate with low budget fans and dehu.

Delmhorst makes temp meters. So, crazy idea, how do you think a beverage thermometer (they go lower than cooking thermos) would do tapped into a pilot hole to core depth? Could be a $10 solution.

From contributor C:
Wow. I just was looking at your graph and realized that the RH never went below 65~70%. Was 10% MC your core reading or 3/8" uninsulated pins?

From Professor Gene Wengert, forum technical advisor:
Use the right hand scale for RH.

If this is for summertime, I am concerned about the heat. Are you using added heat? Do you have two layers of plastic or glass? Are the floor and all walls insulated? Is the floor wooden? How can you keep the nighttime temperature so high? I would expect it to drop to about 70 F or lower, about the same as outside at night, which would be close to the dew point. Overall, the solar kiln would range from 60 to 70 F at night and 120 F or so on a hot summer day. Your graph shows 120 F at night and maybe 140 F maximum in the day.

I am concerned about the high dew point, as the outside must be lower than what you show, so what is keeping the inside so high? The outside is often 60 F or lower in most of the US.

From the original questioner:
Gene, the dewpoint is accurate and very similar to outside air values. I am right on the Texas coast of the Gulf of Mexico, less than 1 mile from the beach, so I am fighting a severe humidity issue from the start. I probably should have built a DH kiln, but tried to go cheap from the outset using what I had laying around.

After 6 months air drying through the winter and spring, under shade but in this humid area, a month in the kiln without additional heat to dry as much as possible, I am adding 600 watts of heat behind the baffle (with incandescent light bulbs). Otherwise, I could barely average 50% rh and 110 degrees peak in the afternoon. I have 1" of styrofoam board insulation, 2x6 frame, and hardiboard sheathing on my kiln. The top is a single layer of clear plastic.

The wood is realistically dry enough for local use (50% rh indoors is typical here) by this point. I just need the borer eggs dead now since mesquite is so tasty to the dang longhorn beetles and my air dry stack showed signs of infestation!

From Professor Gene Wengert, forum technical advisor:
Add a second layer. Spacing does not matter. Floor insulation?

I still cannot believe that you can keep it so hot at night. It should drop to ambient temperature at night as there is no heat but there are heat losses. This would give you about 95% RH or higher. Of course, we do not run the fans when the EMC is so high.

From the original questioner:
Gene, you are missing my comment about heating via light bulbs. I have three 200 watt incandescent bulbs keeping it hotter and driving down the RH. No floor insulation. I'll see what I can do after we deal with this tropical storm bearing down on us. The wind keeps shredding the plastic layers so I gave up trying to keep 2. I'll go back and see if I can get a week's worth for extra heat retention, again, after the storm passes!

From contributor C:
Gene, Thanks for explaining the graph, about insulation and doubling the film.

Seeing as heat rises, other than creating a better moisture barrier, what is the point of insulation? I see people insulating solar kilns, but with about R 0 from plastic, even with two layers, this seems pointless. I would guess the only way to combat this is to have a covering that gets put over the collector at night. I have kept nighttime RH down with a dehu set at 35%. The water doesn't move much at night without the heat, but there is no gain in the MC.

Does a 100W light bulb actually emit 100W of heat?

From Professor Gene Wengert, forum technical advisor:
If the heat rises, then the RH drops and more drying will occur, which raises the RH. Then exhaust some of the moist air, heat the incoming air again, get a lower RH, dry, etc.

Indeed the collector, even with two layers, is the main loss. It is hard to allow solar in and yet avoid heat losses using normal materials that are not expensive. A thermopane window with low E glass is a good idea but expensive, and also likely to break.
We do indeed like the kiln to cool and then the RH will increase, relieving casehardening. But we do not run the fans when the RH is high. In fact, at the end of drying, we would not run the fans until the RH is under 40% RH (8% EMC) in order to get 7% final MC.

Note that in the graph presented, the dew point and the outside temperature are equal or nearly so at night and early morning in almost all of the USA, especially at the coastal region. At this time, there is nearly 100% RH. In the standard solar kiln without extra heat and without a concrete floor, the inside temperature will cool to the dew point as well (no fans on) and this will relieve stresses (casehardening).

So, you can see why I am concerned about how the kiln can stay heated at night to 120 F or so. It is not desired and with only one layer of plastic, it should be cooling quickly, assuming he is using the semi greenhouse design that almost everyone uses.

From Professor Gene Wengert, forum technical advisor:
Use rigid plastic for the outer layer. Even corrugated fiberglass is okay. The inner can be a solar stabilized film. Polyethylene or visqueen will get brittle from sunlight quickly. Overall, you are losing too much heat from the structure and so drying is slow, RH is too high, etc.

From contributor C:
Thank you. I am curious. In older posts discussing case hardening and solar kilning, I recall you stating that "cyclical higher nighttime RH is thought to relieve stress." Lately in some posts it seems you are more on the "it does relieve stress" side of the fence. Has there been more research in this area?

About the graph, with heat in the kiln, the dew point would not be accurate if all the dew point info was coming from inside the kiln. Not sure if my cheapo weather station reads it from the console or the remote which is in the kiln. So dew point info could be skewed.

Nighttime interior kiln readings for me seem like they are also skewed because the wood itself is like a sponge and will soak up the humidity as fast as it comes in and keeps the RH down inside the box. That is another reason I had decided I need some way of controlling nighttime RH. But only did so after the load had bottomed out at 12%. Like you said, that will be the EMC because of the night cycle. Then it became necessary to heat nightly somehow to see 10%.

From Professor Gene Wengert, forum technical advisor:
We know that wood dried green from the saw in a solar kiln will not have casehardening stress at the end of drying. It is strongly believed, but no scientific studies have been done to measure, that the high RH at night is what relieves the casehardening probably the same day it was created. We see this same effect with long air drying times. The high RH from time to time relieves some or most of the stress, so AD lumber has little or no casehardening. (A test for casehardening done when the lumber is put into the kiln will show stress, but this is because of the moisture gradient. The stress test prongs must be done with no moisture gradient.)

The dew point should be fairly constant and the same as outside when the kiln is vented a bit and when at low MCs. I think your DP is accurate. The RH agrees with the readings too. Try a bit more venting at night to then get more heating and a low RH during the day.

From the original questioner:
Keep in mind, this is wood that has already been air dried 6 months under cover and in this kiln (without extra heat) for 2 months. I am in the final stages and trying to sterilize.

I guess my thought process is that my wood should be past the point of significant casehardening, so the continuous heat should be okay?

From Professor Gene Wengert, forum technical advisor:
Yes, no casehardening development at this point.

From contributor C:
How did you generate your graph?

From the original questioner:
The graph displayed is from MS Excel. I use a Lascar Electronics EL-USB-2+ temp and rh logger from Allied Electric for data. Its software exports the data directly to an excel file. I copy and paste as required to generate a continuous log of my kiln conditions on a 5 minute logging basis. I take the last 7 days into a separate chart, and that is what was displayed.

To get the graphs, I have to do a print screen of the chart and paste the resulting screen snapshot into a photo editing program. After cropping out the extra border stuff, I save as .GIF file and voila - it's portable and reasonably small. It is probably possible to do a direct print to file as well but I haven't figured that out since I do it so rarely.