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Sterilizing w/ Supplemental heat in a VT Solar Kiln

Started by Everest123, March 17, 2021, 01:53:15 PM

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Everest123

I'm looking to enhance the capabilities of my existing VT design solar kiln to allow me to sterilize wood at the end of the dry cycle.  This will be significantly cheaper than building a separate sterilization unit and save me a lot of time and energy moving the wood to do that in a separate step.  My target temperature between 150-160 for a solid 24 hours if I can manage it, but minimum 12 hours to insure that all of my wood is thoroughly sterlized.

My kiln is just a bit over 1000 cubic feet inside in total volume.  Interior dimensions are 17.5' x 7.5' x 8' (average due to sloped roof-line).  It is extremely well insulated on the walls, but only has a single layer of clear paneling on top (oops, I didn't even think to use two!).  Mid-summer the kiln will hit 145 consistently for 3-4 hours mid-day.  Lots of crispy bugs inside when it does that, but I doubt it's sufficient to kill them inside a slabs.  My kiln is in a field separate from any other buildings / flammables, etc.

I've read a LOT of threads and have decided to avoid propane for safety reasons.  I do have electrical services to the kiln so I can run electrical heaters.  My thinking was to use a @YellowHammer suggestion from 2016 and string 500w halogen bulbs in front of my ventilation fans.  

Questions - Is using halogens the best approach in my situation?  Are there better heating options?  Are there ways to calculate how much heat I will need to get to 160 in average weather conditions?  Is my whole approach a futile waste of time?  :D

I'd appreciate some input from the experts. :)

-Jeff

doc henderson

well..  the problem with a solar kiln is the losses through the glazing.  so when it is cooler at night, and no sun coming in, will be your problem.  Insulation just slows the transfer of heat, but enough that heat produced can raise the temp to what is desired.  you might make it to over 133° in less time than 24 hours, but you would need to monitor the internal temp of the wood/slabs.  the 24 hour thing is to more than achieve the internal temp needed.   most heaters will shut down above a certain temp, so you cannot just go buy an electric heater and expect 150°.  If it was safe to seal it up, and run a propane heater, it would run out of oxygen.  you might be able to put a moving blanket over the glazing at night as removable insulation to hold in more heat.  or have some temp stable Styrofoam to push up between the rafters of your kiln.  this is daily labor to do.  If you had known that you wanted a "hybrid" kiln then you could have made a solar collector separate from the building, so it can be fully insulated.  when the temps outside are falling, and sun going down, the ducting can be shut down.  this adds to the complexity and also prob. the expense.  this is why the halogen bulbs win.  cheap efficient (inefficient lights cause they produce so much heat) relatively safe heaters.  @GeneWengert-WoodDoc 
Timber king 2000, 277c track loader, PJ 32 foot gooseneck, 1976 F700 state dump truck, JD 850 tractor.  2007 Chevy 3500HD dually, home built log splitter 18 horse 28 gpm with 5 inch cylinder and 32 inch split range with conveyor powered by a 12 volt tarp motor

YellowHammer

I agree, you will lose a lot of heat through the single layer of plastic panels.  You can use heat transfer equations to calculate the amount of heat you will lose through the panel, basically you calculate or assume an R value for the panels, and also assume no heat loss due to holes.  I bet you could find a house insulation and heat loss calculator on the web.  

Or just buy a couple lights, plug them in and see what happens.  You'll know pretty fast if you have a chance.  Basically measure the interior temperature until it levels off, and see how close you are to the target temp.  If they don't work, you'll still have a couple nice work lamps.  
YellowHammerisms:

Take steps to save steps.

If it won't roll, its not a log; it's still a tree.  Sawmills cut logs, not trees.

Kiln drying wood: When the cookies are burned, they're burned, and you can't fix them.

Sawing is fun for the first couple million boards.

Be smarter than the sawdust

GeneWengert-WoodDoc

The kiln is not designed to get hot enough for sterilization.  With that heat, wood components will dry out and shrink, nails will pop, cracks might develop, insulation might be affected, etc.  it would be better to build an inexpensive hot box used just for sterilization.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Sod saw


Hi Everest 123  I am in NY state.  Why is this important?  Because you and Yellowhammer are way south where you probably did not have your lake and bay refreeze last nite.  Your days are much warmer and you have more and hotter sun for longer seasons than we do here.

I am an electrician who has built and used a solar kiln and am building another one.  All for my own use here next to one of the Great Lakes where that large body of water keeps the breeze cooler that a few miles inland.

I had the same question as you did. Can I use my solar kiln to kill unwanted bugs, etc?  For what seasons?

My first kiln had single plastic on the roof, foam in the walls and only one inch of foam in the floor as I wanted to keep the cost down.

The temperature in the kiln got to just under 160 degrees (F) on more than one occasion during the summer, yes, with wood in the kiln. So I knew that the temp could be hot enough to kill kritters.  

But, what about night  time when the kiln shut down as the sun set?  How was I going to keep the wood warm enough for the heat to reach the core of the pieces of wood?  I needed to add some heat for those hours so the wood will not cool down requiring it to be reheated the next day.  

Propane was one thought but I put that aside as I thought about an open flame that close to the wood, as well as running out of air in the enclosed kiln causing the fire to go out.

The next thought was electric base board heat as I have done in many homes.  How much is needed?  Instead of doing a heat loss calculation, I  just went ahead and installed some and tried it.  Not enough, so, I added more.  My limit was what the generator would power as my saw mill and kiln are away from the grid.

Yea! it worked.  Almost:     oops   when I installed the heating units I knew that there was a little button built into each heater that would limit how warm each heater got.  That limit switch would not allow the kiln to remain above 125 degrees (F) until the sun came up the next day.  That temperature was not ok with me.  I was not confident that the center of the boards would get warm enough to be sterile, especially with thicker stock in the future.

I contacted some heater manufactures and found one who's limit switch is set at about 170 degrees (F).  Yesterday I placed one of theirs in an insulated box with no thermostat -  hard wired directly to the 215 volt generator and let it run.  The thermometer in that "Hot Box"  got to about 170 degrees (F) and the generator kept running and the box started to cool down.  It dropped to about 140 degrees (F) when the limit switch re-closed and the temperature rose again then dropped again then rose again, and over and over.

My conclusion is that that companies electric base board heating units will not over heat in the event that the thermostat gets stuck but will allow the kiln to reach the required 150 degree (F) temperature to heat the core of the wood in a timely fashion.  With a proper thermostat controlling those base board heaters, the equipment should provide enough safe heat to overcome the heat losses thru the poorly insulated roof.  Remember that the fans must also run overnite to keep the hot air from pooling near the roof and allowing the lower layers of wood to cool down.  

The core of each piece of lumber wants to reach 135 degrees (F) for an hour or more while allowing enough time for that to be accomplished.

How about Gene's valid concern about the environment inside the kiln?  You, the operator of your kiln will need to keep an eye on those heaters, wiring, thermostat, etc.  If you notice that they are starting to rust or fall apart, they must be considered worn out and be replaced.  

An electrical inspection of the completed job will probably relieve the electrician of liability in the event of a fire. Follow the electric code and your insurance company will be less likely to refuse coverage.

I will be adding heaters to my new kiln as I progress thru the construction.  We expect to put on the roof this next week and will post a new thread when it is finished and be able to report on its operation.  Oh, by the way - the roof will be double glazing to help cut down on heat losses at nite.     (don't hold your breath, it may take a while)

With you folks being farther south than we are here, I see no reason that you could not add supplemental heat to your well insulated kiln to overcome the heat loss at night to sterilize your lumber at the end of drying cycles.

Give it a try but do it safely.
LT 40 hyd.          Solar Kiln.          Misc necessary toys.
.
It's extremely easy to make things complicated, but very difficult to keep things simple.
.

GeneWengert-WoodDoc

There seem to be two issues with supplemental heat at the end of the drying cycle for sterilization.

1.  Will the heat damage the kiln components, especially the insulation and fan motors?

2.  How efficient is the kiln, energy-zwise?  Or, what will it cost for fuel?  Consider a box with all six walls insulated with 1" insulation that is kept dry.  For a small chamber, the energy cost is $4 for one day.  If you increase the insulation to 2", you will need half the energy of 1" of $2.  If you use 4", then 25% of one inch or $1.  And 6", 67 cents.

Now take the roof and make it a solar collector with one sheet of plastic, as in the VT kiln.  You will lose more energy out the collector than the other five walls combined, even after subtracting the solar input.  A solar cover with two clear covers will save about half the energy of one sheet, but still will lose more energy than the four walls combined.

The best solution is to build an insulated box, called a kiln, and then put a separate hot air solar collector on top of the roof, at the correct angle with two clear covers.  When the fans are blowing through the collector, heat loss unless the sun is shinning.  So, turn the fans on only when efficient to do so.  When adding supplemental heat, turn the collector off to avoid unnecessary heat loss.

Of course, the box is the same as a DH kiln compartment.  You will soon realize that a DH kiln is more predictable and works in cloudy and cold weather, SO IS POSSIBLY THE WAY TO GO.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Dewy

Quote from: Sod saw on March 20, 2021, 04:33:59 PM

Hi Everest 123  I am in NY state.  Why is this important?  Because you and Yellowhammer are way south where you probably did not have your lake and bay refreeze last nite.  Your days are much warmer and you have more and hotter sun for longer seasons than we do here.

I am an electrician who has built and used a solar kiln and am building another one.  All for my own use here next to one of the Great Lakes where that large body of water keeps the breeze cooler that a few miles inland.

I had the same question as you did. Can I use my solar kiln to kill unwanted bugs, etc?  For what seasons?

My first kiln had single plastic on the roof, foam in the walls and only one inch of foam in the floor as I wanted to keep the cost down.

The temperature in the kiln got to just under 160 degrees (F) on more than one occasion during the summer, yes, with wood in the kiln. So I knew that the temp could be hot enough to kill kritters.  

But, what about night  time when the kiln shut down as the sun set?  How was I going to keep the wood warm enough for the heat to reach the core of the pieces of wood?  I needed to add some heat for those hours so the wood will not cool down requiring it to be reheated the next day.  

Propane was one thought but I put that aside as I thought about an open flame that close to the wood, as well as running out of air in the enclosed kiln causing the fire to go out.

The next thought was electric base board heat as I have done in many homes.  How much is needed?  Instead of doing a heat loss calculation, I  just went ahead and installed some and tried it.  Not enough, so, I added more.  My limit was what the generator would power as my saw mill and kiln are away from the grid.

Yea! it worked.  Almost:     oops   when I installed the heating units I knew that there was a little button built into each heater that would limit how warm each heater got.  That limit switch would not allow the kiln to remain above 125 degrees (F) until the sun came up the next day.  That temperature was not ok with me.  I was not confident that the center of the boards would get warm enough to be sterile, especially with thicker stock in the future.

I contacted some heater manufactures and found one who's limit switch is set at about 170 degrees (F).  Yesterday I placed one of theirs in an insulated box with no thermostat -  hard wired directly to the 215 volt generator and let it run.  The thermometer in that "Hot Box"  got to about 170 degrees (F) and the generator kept running and the box started to cool down.  It dropped to about 140 degrees (F) when the limit switch re-closed and the temperature rose again then dropped again then rose again, and over and over.

My conclusion is that that companies electric base board heating units will not over heat in the event that the thermostat gets stuck but will allow the kiln to reach the required 150 degree (F) temperature to heat the core of the wood in a timely fashion.  With a proper thermostat controlling those base board heaters, the equipment should provide enough safe heat to overcome the heat losses thru the poorly insulated roof.  Remember that the fans must also run overnite to keep the hot air from pooling near the roof and allowing the lower layers of wood to cool down.  

The core of each piece of lumber wants to reach 135 degrees (F) for an hour or more while allowing enough time for that to be accomplished.

How about Gene's valid concern about the environment inside the kiln?  You, the operator of your kiln will need to keep an eye on those heaters, wiring, thermostat, etc.  If you notice that they are starting to rust or fall apart, they must be considered worn out and be replaced.  

An electrical inspection of the completed job will probably relieve the electrician of liability in the event of a fire. Follow the electric code and your insurance company will be less likely to refuse coverage.

I will be adding heaters to my new kiln as I progress thru the construction.  We expect to put on the roof this next week and will post a new thread when it is finished and be able to report on its operation.  Oh, by the way - the roof will be double glazing to help cut down on heat losses at nite.     (don't hold your breath, it may take a while)

With you folks being farther south than we are here, I see no reason that you could not add supplemental heat to your well insulated kiln to overcome the heat loss at night to sterilize your lumber at the end of drying cycles.

Give it a try but do it safely.
Sod saw, Can you provide more information on the heater you are using make / model to reach 170*. Thanks

GeneWengert-WoodDoc

So, let's talk about bugs.  All insects that are in wetter wood (above 25% MC) cannot survive in dry wood.  So, they will dry in normal drying to a low MC at any temperature.  The one insect that we worry about in dry hardwoods is the lyctid powderpost beetle.  How are you going to get this insect into a kiln?  It is virtually impossible, especially if you sweep out the kiln after every load.  If there is no infected wood or bamboo nearby, there is no source for the insect.  So, there is no need to sterilize the wood.

Also, appreciate that kiln sterilization only affects the wood at that moment.  The wood can subsequently become reinfected easily if stored close to infected wood or bamboo.  So, it is often best to sanitize the storage area and the wood used for dunnage as well.  Stickers can convey insects, so sanitizing them after each load, and 4x4 spacers as well, is important.  A small hot box will do that.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Everest123

A lot to think about here. In truth besides one 3" slab I've had no issues with bugs and they one was dried through the winter with kiln temps never getting over 130.  And those burrowers were about dead (desperate for water).  I've seen zero signs of PPBs although I have experienced them in the past. But I burned everything they infested, fumigated several times, and haven't seen anything for 5 years now.  PPBs are incredible. They ate the handles of my hammers!!!!!  

Given that my kiln is adjust constructed and working well (for drying) I think a dedicated hotbox is the best option. Not difficult to build, I can do it myself. Appropriate heaters is another question, but I'll try halogens first with fans and see how they do.  I like the idea of a simple sugar collector on top. Also not hard to build. If done right basically this is another kiln!

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