Message Thread:
supplemental heat for solar kiln to kill bugs
1/9/22
I built a solar kiln back in 2009 following the Virginia Tech design and this has worked great for all these years. I am wanting to add a heat source to ensure that I eliminate bugs in the wood being dried. During the summer months here in Michigan sometimes it can be hard to maintain temperatures above 133 degrees for a sustained period to kill bugs that may be inside the wood. I am not concerned with setting the sap in softwoods because I mostly dry hardwoods like Ash, Cherry, Hickory, Oak, and Walnut in the kiln. I have heard some people using an electric heater to bring temperatures up, also propane heaters but these will add moisture. Another option is a wood stove because I have plenty of firewood to use for fuel. Looking for recommendations.
1/10/22 #3: supplemental heat for solar kiln to ...
Have you ever gone on The Forestry Forum? It gets discussed there quite often. One of the cheapest sources I've seen is a DIY build using an oven element, a thick slab of aluminum, and the oven dial and switch to set the heat. I don't think anyone has ever posted any plans though.
https://forestryforum.com/board/index.php?board=8.0
1/11/22 #4: supplemental heat for solar kiln to ...
I have a Timberking mill and harvest logs off my property, cut them into lumber or slabs, sticker and stack outside to air dry for 6 months to a year, then rotate stock into solar kiln to complete drying. I monitor with my Delmhorst pin meter try to follow every advice that Pro. Wengert or other contributors to this sites informative knowledge base have to offer. I never really had problems with bugs in the past and if I found something it became firewood. Now that I am retired and have decided to sell rough sawn or planned hardwoods I need to consider a heat treating process for sterilizing the wood for bugs. I don't want to spend money on a vacuum or de-humidification kiln that are on the market but rather build something of my own to heat treat the wood. I emailed Pro. Wengert last spring about using a small dehumidifier in my solar kiln but he quickly told me it was not a good idea. Cost to run the unit plus the set-point will not get the temperature needed.
So I am thinking of building a container on the back of my pole barn where I have access to electricity either 120/220 single phase. Reading many forums on this topic I have decided to keep the container separate from the building for insurance reasons in case of fire. I am thinking of a simple wooden box that could hold 200 board feet of lumber to dry in. The plywood would be attached on the exterior of the wooden frame with insulation in the middle air cavity and on the interior use a material called AlucaBond which is a 4mm aluminum composite panel that would hold up to heat and moisture. Reading some of the posts on the forestry forum that RichC provided a link to, I am considering using 500 watt heat lamps on a timer with small muffin fans to circulate the heated air. Any thoughts on this or recommendations on equipment or size? I have read on previous forums for use of insulation that 2 inch foam may not hold up to the heat and if it doesn't what insulation should be used? I plan on making the box to accommodate lumber that is 8 feet in length and width of stack would be 3 feet wide for loading and unloading on my Kabota tractor forks. The proposed dimensions are 4 ft wide x 10 ft long x 4 or 5 ft tall. After reading many posts on forums I have come to the conclusion that my best option is to keep the solar kiln as just that a place to dry the wood, and build a heat treating structure to sterilize wood. Winter time gives one plenty of time to plan, looking for recommendations.
1/14/22 #5: supplemental heat for solar kiln to ...
I built an electric kiln in my wood storage shed. I use it to KD wood that I first AD to below 20%. I can also bring the load up to 140F after it's dry should I need to kill any bugs, though I have rarely needed to do so. All that's needed is a fan or blower to recirculate the air and an oil filled room radiator if the box is insulated well enough. The photo shows the kiln. It's 4 feet high x 4 ft deep and 10 feet long. It holds about 400 bf of stickered wood with enough space at the front and back for the 600 cfm blower to circulate air through the stack. The door and floor is just 2" thick foam. The walls and ceiling have 3-1/2" fiberglass with a vapor barrier over it, then 2" of foam insulation over that.
You can buy these oil filled radiator heaters at Home Depot, etc. for about $50. You want the old type with mechanical controls, not a digitally controlled one because it won't survive at 140F. These heaters can produce about 1500 W and that is more than enough for the kiln size I have even in Winter when the temp. is 0F. It costs about $50/month to operate in Winter, so the cost/bf is really low. The advantage of these heaters is 1) they are cheap, and 2) they are safe. Depending upon the model you pick you may have to bypass a hi temp. cutout thermostat in order to get to 140F.
I started with a dehumidifier in this kiln during the drying process, but soon learned it's not necessary as long as I start with wood below 20%. Instead, I use a simple inlet and outlet vent to allow high humidity air out and drier fresh air into the kiln. After I get down to 7% MC I close the vents and increase the temp to 140F if I need to sterilize the wood. I use digital controllers on the outside of the kiln to control the radiator and another for monitoring the RH inside. I formerly used it to control the dehumidifier until I found it unnecessary.
I've been using the kiln now for 2 years. I've only sterilized loads twice with it, but have had at least 8 drying runs through it w/o issue.
The total cost was less than $500. I wanted to build it on the outside of my shed but this was the cheapest and fastest option.
John
1/14/22 #6: supplemental heat for solar kiln to ...
John
That oil filled radiator can get up to 150 degrees? It takes a lot of heat to get 400 board feet up to 133 in the center of all the stock. How long does your sterilization cycle take?
1/14/22 #7: supplemental heat for solar kiln to ...
I don't know if it can get to 150F but it gets up to 140F w/o a problem, as long as there's no hi temp. cutout. 400 bf of oak weighs about 1400 lbs. The specific heat of oak is about 0.57 BTU/lb-F so to increase the temp from 110F (where I finish the KD cycle) to 140F takes about 24,000 BTU = about 7 KW. The heater can output 1.5 KW. Worst case it takes about 1 KW to compensate for heat loss from the kiln, which leaves 0.5 KW to go into raising the temperature of the wood. So, it takes at least 14 hours to get to wood up to 140F. Of course, it takes longer because the thermal conductivity of wood is not like metal. I could figure it but my guess is 50% more time is plenty, so 14 hours becomes 21 hours. I leave the wood at 140F for a minimum of 24 hours after the kiln reaches 140F, which gives some additional cushion. If I were really worried about bugs, as in I was seeing live ones or visible frass, I would put a temperature probe into the middle of a couple of boards to verify I have 140F for at least 3 hours.
There are other, more industrial, heater options that won't get hot enough to cause a fire. I use the oil filled radiator because it's cheap and it works for my setup. The key to making it possible to use such a low wattage heater is a well insulated chamber.
I hope that helps.
John
1/16/22 #8: supplemental heat for solar kiln to ...
Thanks Rich and John for you input on this. John you mentioned pictures of your kiln? Did you post them?
1/16/22 #9: supplemental heat for solar kiln to ...
Well, I had posted a photo, but it's never a sure thing on this site, so let's try again. The one shot shows the kiln ready to close up. You can see the dehumidifier but like I said earlier, I no longer use it. The oil filled radiator is to the right behind one of the other door panels. You also can see the blower on the far left of the opening. It blows air down the left side and then around the back so it can flow through the stack.
Another photo shows the RH and temperature controllers. Inkbird brand from Amazon. The RH controller is just there now so I know what the RH is inside so I can decide how to adjust the vents. The oil filled radiator is plugged into the temperature controller and controlled from the outside by it.
I follow EBAC's drying schedule for the species and thickness of wood I have in the kiln, as far as possible, but usually only heat to 110F max, although the kiln has no trouble going up to 130 or 140F.
I can report that those PVC stickers don't like going up to 140F unless you want noodles! I'm back to using wood stickers in the kiln, but the PCV ones are great for air drying.
Hope this helps.
John
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1/16/22 #10: supplemental heat for solar kiln to ...
It's going to take an incredibly long time to get the center of a 12" wide board up to 133 degrees when the heat source is only 7 degrees higher.
1/17/22 #11: supplemental heat for solar kiln to ...
If you go through the heatflow calculations, a piece of 2" thick oak, starting at 110 F will take 5.25 hours to get to 135F with the drier temperature at 140F. So if 3 hours at 133F is sufficient to kill any bugs, wait at least 5.25 + 3 = 8.25 hours after the temperature inside the kiln reaches 140F.
That's much less than the 24 hours I wait.
It really does all work.
John
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