solar kiln extra heat

[Oth]

 

 

 

 

Here's some pics, Doc.

I've done some homework and according to my calculations at 30 F I'll need 3860Whr to keep it at 150 F. If i throw some 2" foam on top of the glazing temporarily I would need 2024Whr. This is assuming 2 ACH, which I felt was conservative. Did calculations 50f also which is 100f off 150 so I can easily multiply that result by say .9 to get 60F results or 1.3 to get 20F.

I read a few of the pieces of published literature I could find out there which all said the 30 minutes at 133 inside the wood. Read more on this forum and the only thing I'm confused by is how 24hrs at 150 became the rule of thumb. I can see that for very thick slabs but it seems like massive overkill for most batches. Is it just belt and suspenders or am I missing something?

[scsmith42]

Wood is an insulator.  When you crank the heat up inside the kiln the exterior of the lumber might reach 140F pretty quickly, but the core may still be below 125.  When I'm finishing a run in my Nyle and the temp has been at 120F for a week or more, then I only need a 13 degree rise in the core oof the boards.  This takes less time versus a "finish off and sterilization cycle" load that I put in for sterilization purposes only.  Typically this is for lumber that has air dried down to EMC.  If the air dried load goes in when it's 70F outside, I need to raise the core by 63 degrees - not just 13.  So it's going to take more time. 

Moisture content plays a role here as well, due to the cooling effect of the evaporation process.

What I've observed is that 6% lumber will reach targeted sterilization temperatures much more quickly than 15% lumber (such as hardwood for woodworking versus pine for structural use).

24 hours at 150 allows plenty of time for the core to reach 133F (presuming it's at the end of a drying cycle and not a fresh air dried load just placed in the kiln), which is why most of us follow that rule.

Per Gene Wengert, the 30 minutes at 133F inside the wood is incorrect.  All you need is for all portions of the wood to reach 133.  Keeping it there for 30 minutes is an extra safety margin, especially considering that kilns may not have consistent airflow totally throughout them and the heat is conveyed through the hot air.

[Oth]

Yes, for sure only trying to sterilize at the end of the drying cycle. So the 24 hrs is if you're putting it in the kiln just to sterilize it. If it's already been in the kiln drying you still do the 24hrs out of habit or are you shortening that period since, as in your example, you only need to make up 13 degrees?

[doc henderson]

Well, if you pull your car in the garage and turn on the forced air heat, you can get the air to 70° in no time.  It will take many hours to get the concrete and cars up to that temp.  Temperature is the result of heat applied to and object.  It depends on density so air heats fast.  think of temperature as heat density in an item.  It will take a while to heat a 5-gallon bucket of water.  in fact, the definition of a BTU is the energy to raise temp of 1 ml of water 1 degree centigrade.  a cord of hedge wood is 32 million BTUs.  all forms of fuel will tell you about the BTUs per unit volume or weight.  the other factor is that the temp increase is linear with the exception of a phase change.  the phase change from solid to liquid, or liquid to gas, requires energy itself, so the change in temp is halted, but the energy is consumed.  So not only do you have to heat the mass of wood but also any remaining water.  this is why it is probably easier to sterilize wood that is relatively dry.  So, a temp of say 160° for 24 hours is presumed to get 4/4 lumber that is stickered and with fans going to at least 133 degrees internal.  You can measure temp and MC of wood that is in a kiln.  but temp is a big probe in theory.  A IR temp gun will tell you the surface temp.

[doc henderson]

how can you measure the core temp?  well, if you cannot the industry standard is 24 hours.  You could correct for the fact that the wood is dry and relatively warm.  but the liability is on the producer.  I will try to find a little home experiment I did with a meatore probe.

[doc henderson]

https://en.wikipedia.org/wiki/British_thermal_unit#:~:text=The%20British%20thermal%20unit%20%28BTU%20or%20Btu%29%20is,part%20of%20the%20United%20States%20customary%20units.%20

https://forestryforum.com/board/index.php?topic=112021.msg1755589#msg1755589

[Oth]

Yea, I understand the physics behind the whole endeavor. I saw your meat probe trial, I think I'll replicate it when it comes time. Thanks for everyone's responses. Enough theory, I'll just play around when the time comes and see the results.

[doc henderson]

sounds good!  nice job on the kiln.  My comments were in response to your Question about time if you are already warm, and I think the answer is we do not know.  but if the wood is for you, you can make that determination.

[Sod saw]

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Oth,    Two thoughts:

There are a number of manufacturers who make thermometers and/or thermostats that have the thermocouple dangling at the end of a small wire (3 or 4 feet long).

If you can protect the brains of that thermometer from rain, mount it outside the kiln for easy reading and run the wire and thermocouple into a small drilled hole in to the center of your sample piece of wood.  This will allow you to get an idea as to the time and temperature ratio as your kiln chamber attempts to further warm up.

You might be able to compare the wood interior (especially with large and thick pieces) temperature with the actual kiln chamber temperature.  I trust that you have many thermometers scattered thru-out the kiln chamber.

Next thought:   A stand alone transformer will do a fine job of changing voltage of Alternating Current (AC).  It can change voltage to increase the output voltage up from the original input voltage.  A transformer can also change the voltage so that it is less than the original voltage.  Transformers are also used in applications where the out put voltage is exactly the same as the input voltage.

Your 12 volt fans that are operating from your solar panels run on Direct Current, not Alternating Current as is in most homes here in USA.  The polarity of the connection from solar panel to fan will determine the direction of the air flow.  If you reverse the polarity of the wires to the fan you will reverse the direction that the air flows thru your stack of wood.

What does that have to do with transformers?  The AC flowing out of the output side (load side) of a transformer reverses polarity with the change of the waveform of the input (supply) voltage.  Here in the USA the 60 cycle sine wave causes the polarity to change quite fast.

Your 12 volt transformer will attempt to have the air flow thru your stack of wood in one direction while the sine wave goes positive then will change polarity (just like you did in the example above where you swapped wires to make the air flow in the other direction).  Then when the cycle goes negative the fan will attempt to change direction and blow the air in the other direction.  Then positive, then negative, then positive, then......  you get the idea.   At 60 cycles per second of current flow the fan will attempt to change direction 120 times each second.  The result is that the fan will hum and maybe burn out (overheat) due to no air flowing past itself because it can not get turning at all.

So what to do?  My previous post talked about a power supply to drive the fans in my kiln.  Look at how many watts each of your fans draw and add those fan wattages together.  Buy a 12 volt DC power supply that operates on your generator voltage that can supply 130 % of that added fan wattage.  Or buy a separate power supply do drive each fan if you have large fans.   The math is easy.

Keep the power supply outside of your kiln chamber where it is not heated.  In a perfect world you will have a small fan built into the power supply itself.

Nice looking kiln!


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[Oth]

DC vs AC hadn't occured to me when I said that, good catch.

I have 2 Sensor push thermometers/hygrometers that live update to an app on my phone which charts out temperature and relative humidity over time. Just put them in a couple days ago to start keeping better records. Went ahead and got a Bluetooth meat thermometer with 2 probes also. Any thermocouple that saved information that I could find was $250+ and the meat thermometer with the extra probe was just a bit over $100 and it can double as an actual meat thermometer.

I've got a guy I can get 400W solar panels from for $100 bucks. I may pick up a couple of those and wire it to some 12v heaters minimally to speed up the process in the winter. If it lets me get to sterilization temps in the summer then that's a bonus. As far as sterilization, I'm leaning towards a lp or diesel heater piped in. Seems like the least fussy option. Whatever I do, I'll collect the data.

[doc henderson]

my probe meator can actually anticipate after a period of time, when the temp should reach the target.  not sure if it works in wood but it looks at external vs internal over time and plots it out.  You can see if it close and know when to expect the internal 133° to be reached.

[Oth]

Doc, I think I'm now on a list of some sort since I just googled the R value of meat to compare it to wood

[Kit B]

I've struggled with the same issues with my solar kiln.  I started making sure everything was as tight and well insulated as possible, then added halogen lights.  Temperature was still shy of 150.  Then I added in a propane heater (with the propane outside), I surrounded this with some metal roofing panels I had to protect the kiln.  This worked, I was able to get to 150 relatively quickly.

Of course, never willing to leave well enough alone, I ran across Scott's post about using a torpedo heater which just seemed an excellent idea.  I liked that it got the heater itself outside the kiln.  So I cut a hole in the kiln, built a platform, etc.  When I hooked it up and did a trial run I realized I had made the hole a little to high in the kiln and was in danger of melting my roof.  So I added 10 ft of 6 inch duct with a series of half inch holes along the top side to try to distribute the heat.  This seems to work, but I still need to add some metal to protect the roof section above the end of the pipe.

Those torpedo heaters kick out A LOT of heat.  If you decide to use that method (which works) be careful not to place the hole into the kiln to high.