New to steam - I need to build a small-scale kiln chamber

some_wood_guy

Hello, I know almost nothing about steam heat, so I appreciate anyone that can provide even basic knowledge to me about my plan.

I am building a low-production firewood kiln to heat treat (kill bugs) split firewood. They are stacked in 48" x 48" IBC totes. I need to heat the internals of each piece of firewood to 160F for at least 75 mins in order to satisfy the state certification for heat-treated firewood. Along with this comes specially-designed temperature data logging equipment, which I already have in the form of a few K-style thermocouples that are inserted into select pieces of wood in the stacks.

I have tried building mine with an LP-fired infrared heater in a 10ftx10ftx6ft high room, but safe clearances are too tight and the heat is not getting mixed enough even with some fans.

My thought is to try a steam-heat system (LP ideal, or if there are wood-fired options that would be great too, I have plenty of firewood). Inside this 10x10 room, I am picturing a couple, or several, fanned heat exchangers on the walls, the air blowing through the coils and directly into the wood (arranged longitudinally to the fans of course). To really optimize and keep the heat in where the wood is ad avoid a hot ceiling, I could directly cover the IBC totes on top with 2" or better foil faced foam board. I could try to "seal" the tops and sides of this foam board chamber around the totes, and I could create "cold air" return channels at the bottom, so the heat exchanger fans pull any cooler air around from the bottom of the totes.

That seems to be the ideal heat exchange system. At first, I pictured removable heat exchangers, directly hung on the IBC totes in order to encourage all heat to enter the totes, but quickly realized that would require flexible piping to the heat exchangers and I don't know if that is realistic, especially with steam. So, Instead I have thought of using heat exchangers permanently fixed to the walls, and I can very carefully slide the IBC totes in between the heat-exchanger sandwich using fork lift forks.

Next, is the heat source. This post is about steam, but I'd also like to ask about a regular hot water "boiler". Before I turned my eyes to steam, I toyed with the idea of using a wood fired hot water boiler with the heat exchangers, but quickly realized that the max temp for the boiler systems are generally 180F - or what is the max temperature that a wood boiler can heat its water loop? My thinking is that it MAY be theoretically possible for the wood totes to get to 160F, but that might take too long. With a temperature delta of only 20F, I'm thinking that it would take a long time for the internals of the firewood to get to target temp.

So, with striving for a higher temperature delta, I came across steam heating. First, I'm pretty sure I'm talking about low-temperature steam. What is the operating temperature of such systems? Am I looking at 210F, 220F, 240F? I don't need anything higher than that I wouldn't think.
Next, would these small wall-mount boilers be appropriate for me? such as Home Depot's offering of "Rinnai I Series Natural Gas or Liquid Propane Boiler with 90,000 BTU Input"
Also, what is the difference with "condensing" vs non-condensing, and do I need to run additional piping for condensation in the primary heating loops?

I would definitely source professional help when it come to building this out, but this post is more intended to ask you folks if what I need to achieve is going to be effective and efficient with steam heat (or hot water heat, if you know of a way to get higher temperature deltas out of a hot water boiler).

Thank you all for your time in reading my situation and helping.

some_wood_guy

Also, It should be noted that when not used, this building and heating room will be unheated and so, Ideally any system would need antifreeze / or other mitigations for sub-zero temps.

Mad Dog_2

Be careful.  What kind of pressure will this be generating?  Mad Dog 

EBEBRATT-Ed

If you need to heat the wood to 175 that presents all sorts of issues. Any electrical wiring in the room will not stand that temperature ...not without going to extreme methods.

You will probably need to get the room to 200 degrees to have enough delta to get the wood hot enough in a reasonable time. You going to have to locate any equipment outside the room.

You need to calculate the lbs of wood that you will put in the room at one time and the temperature of the room when you start heating the wood.

Also the room dimensions and the room construction

some_wood_guy

EBEBRATT-Ed said:

If you need to heat the wood to 175 that presents all sorts of issues. Any electrical wiring in the room will not stand that temperature ...not without going to extreme methods.

You will probably need to get the room to 200 degrees to have enough delta to get the wood hot enough in a reasonable time. You going to have to locate any equipment outside the room.

You need to calculate the lbs of wood that you will put in the room at one time and the temperature of the room when you start heating the wood.

Also the room dimensions and the room construction

Hi, thanks for those inputs. Yes, I'm aware of the wiring and heat issue, for my existing system I'm already using 12V radiator fans with high-temp FEP teflon wiring. And I will not be putting any boiler in that room.

As for the lb of wood, I estimate 1800-3000lbs of wood at any given session. Sessions should last about 18 hours each, given my current esimates. Starting temperature will be anywhere from 30F to 90F. The room is 10ft x 10ft x6ft, and is well insulated with 2-4 inches of rigid foam board on all walls and ceiling.

I've toyed with the idea of sauna heaters -- there is a company that makes an LP sauna heater (and there are wood fired ones too!). I'd like to avoid that, as 1) I would still need to cycle the heated air with my fans and 2) it will take up floor space that I would ideally be using for my totes.

With hot water/steam heat exchangers, I can put that heat source and fan inches away from the wood where it would be most effective.

some_wood_guy

Mad Dog_2 said:

Be careful.  What kind of pressure will this be generating?  Mad Dog 

Not really sure at all, I'm very new to this.

some_wood_guy

EBEBRATT-Ed said:

If you need to heat the wood to 175 that presents all sorts of issues. Any electrical wiring in the room will not stand that temperature ...not without going to extreme methods.

You will probably need to get the room to 200 degrees to have enough delta to get the wood hot enough in a reasonable time. You going to have to locate any equipment outside the room.

You need to calculate the lbs of wood that you will put in the room at one time and the temperature of the room when you start heating the wood.

Also the room dimensions and the room construction

Thank you for your inputs! Yes, I've already considered the temperature and wiring issues, I currently have 12v radiator fans and high-temp FEP teflon wiring that can handle these temperatures. Yes, all equipment will be located outside of the heating room/chamber.

I estimate 1800-3000lbs of wood in at any given heat treatment session, and the session should last 18 hours at my current estiamte. The room is 10ft x 10ft x6ft tall, and is well insulated with 2-4 inches of rigid foam board on the walls and ceilings. Starting temps would be outdoor ambient temps, probably 30F to 80F or so.

Jamie Hall

Well now... is there some reason you need to create your own kiln? Not that I'm opposed in principle to do it yourself, but if you are aiming for heating to meet either US or even tougher international certification, you are in for a long haul.

Take a look at this web site for some ideas...

https://www.canr.msu.edu/resources/firewood-heating-and-treatment-technologies

And if you know timber producers in your area, talk to them or your agriculture extension people for more resources.

leonz

There are complete firewood kilns available to do this that
are made from surplus shipping containers.

Dry steam firewood kilns need to have space for the cast iron
radiators to work as the radiators have to be on the floor.

If you expect to compete with the imported Estonian kiln dried
firewood bundles your wasting your time.

If you have a contract for dried firewood for a wood fired bakery
or wood fired pizzeria that would be one thing.

Short firewood being 12 inches will dry the fastest and crack the least.

A standard lumber moisture meter is all you need.

The fellow that sells firewood to vermont castings kiln dries the
firewood he sells them with a steam kiln.

The radiators sit on a concrete floor in his wood barn and he uses
a skid loader to move steel bins of stacked firewood and dries the
wood slowly.

A small coal fired steam boiler piped to make dry steam with a
double drop header would work well.

Mad Dog_2

Dude...listen to these wise men. I think you're in over your head.   Just purchase one and move ahead. Leave the design and safety concerns to professionals. Don't wanna see money wasted or bodily harm.  Mad Dog 

WMno57

What is the operating temperature of such systems? Am I looking at 210F, 220F, 240F?

212F, the temperature steam condenses to water.

Next, would these small wall-mount boilers be appropriate for me? such as Home Depot's offering of "Rinnai I Series Natural Gas or Liquid Propane Boiler with 90,000 BTU Input"

Nope, those boilers are for hot water space heating, not steam space heating. So 180F max.

Also, what is the difference with "condensing" vs non-condensing, and do I need to run additional piping for condensation in the primary heating loops?

Condensing is found on some hot water boilers, and NO steam boilers. Condensing HW boilers are called Mod-Cons. Con for condensing.

Also, It should be noted that when not used, this building and heating room will be unheated and so, Ideally any system would need antifreeze / or other mitigations for sub-zero temps.

No antifreeze for steam boilers, only HW boilers.
I get what you are trying to do. I think it would be cheaper to find a used kiln or oven.

hot_rod

You can run hot water boilers at 220-240 by having adequate pressure. 10 -11 psi would get you to 240.
How big of a piece of wood? Anything over 4” in diameter I've found doesn’t air dry completely through.

some_wood_guy

hot_rod said:

You can run hot water boilers at 220-240 by having adequate pressure. 10 -11 psi would get you to 240.
How big of a piece of wood? Anything over 4” in diameter I've found doesn’t air dry completely through.

Thanks for confirming that HW boiler can achieve higher heat. Are most off-the-shelf boilers and boiler installations done assuming some amount of pressure? I guess I had always assumed that the systems were non-pressurized.
Given that I will need antifreeze in the heating loop, would this also allow a higher boiling point and maintain a higher temperature while avoiding actual boiling and the pressures that come with that?

WMno57

Are most off-the-shelf boilers and boiler installations done assuming some amount of pressure?

Pressure - Yes. 240F - No.
But it might still work for your application, if you verify that ALL components of the hydronic system can handle 240F at 30 psi (30 is the standard pressure relief valve).

• Controls
  
• Piping
  
• Fittings
  
• Radiators
  
• Etc.
  

The big box stores mod-cons that you listed are not the best choice for this. An old school cast iron boiler would be more robust.

jumper

Sounds counter intuitive but live steam can season wood.
Maybe the steam has to be superheated?

Bob Harper

I love in America we can generate an 86 page document on firewood: https://firewood.nationalplantboard.org/wp-content/uploads/2022/03/firewood_guidelines_aug2020.pdf
I get the hazard and have dealt with it RV camping.
You could build an autoclave and speed up the process. Since you are not reaching pyrolysis temperatures, the cellulose remains intact. What you are really doing is heating the interstitial water, which, in turn, heats the cells and bugs. The single most important factor in getting heat to the core is length. Wood is merely a bundle of straws. Splitting is way overrated about drying since the outer layers shrivel and dry up quickly inhibiting moisture movement laterally.
If you want to remove water, you need a pressure gradient. That can be vapor diffusion, such as creating a dryer zone connecting to the wet zone, or perhaps a salt solution to draw water by osmosis. Neither would be practical here. The goal here is to use the water to heat the wood. He moves from hot to cold. As you heat the end grain, the cell structure opens slowly but there is a delay with heat movement inwards because the water in the log is buffering heat transfer. Only as the whole log becomes warmed and the heat transfer really take off, which is why this process takes time. If a log is riddled with beetles, there will be tunnels, which insulate or inhibit heat transfer.
The most practical way to me would be the old pressure treating methods that relied on rail cars into tunnels that were sealed then pumped full of very hot CCA preservative. Nasty operation but effective. up

Interesting topic. Keep us posted on what you come up with an its efficacy.

Jamie Hall

As I implied above, the timber and logging industry and the associated ag. departments (where there are any left) have done a tremendous amount of research into how to go about drying and treating timber -- or all shapes and sizes. Anything from the tiny bundles of camp wood wrapped in plastic to tempt the adventurers to railcar loads of timber for shipment. Michigan State U. has a lot of resources, so to a couple of others.

The keys are time and temperature...

leonz

jumper said:

Sounds counter intuitive but live steam can season wood.
Maybe the steam has to be superheated?

=================================================================

AS I mentioned earlier simple dry steam is used successfully to
kiln dry firewood checking it using a hand held moisture meter
that is in contact with the end grain of the firewood.

Superheated steam created by 1,001 degree Fahrenheit water is
not necessary to do this.

mattmia2

Once your fluid temp is above the boiling point of that fluid at atmospheric pressure it becomes a huge safety concern. If water at 200f and 15 psig loses pressure it just leaks as a liquid. If water at say 220f and 15 psig loses pressure the water flashes to steam until it loses enough energy to be below the boiling point. this means the integrity of you piping system is a serious safety consideration.

The same thing goes for operating a steam boiler at any significant pressure.

mattmia2

Also as the temp of your kiln approaches the temp of the water or steam the rate of heat transfer will fall of dramatically and you will need to be able to regulate the fire in the boiler to keep it from overheating when it can't dump much heat in to the kiln.

AdmiralYoda

Regarding the electrical wiring for your fans. The wires you have chosen are built to handle the heat....but keep in mind the ampacity of wire reduces over temperature. A certain gauge wire will handle a certain ampacity at room temperature, but at 200F that ampacity will be much lower.

There are ampacity charts and derating charts out there to show what gauge wire you need at XXX temperature for XXX current.

Regarding your insulation. Good thing having it well insulated but keep in mind foam board insulation needs to be covered by a fire barrier. I don't think the typical go-to drywall would be good in this application due to the heat/cold cycles and the large swings in humidity. Most kilns use ceramic insulation for this reason.

Sizing a heat system comes down to EDR. Every radiator has an EDR rating. Install as many as you want, add up the EDR and size the boiler accordingly. The tricky part is that the boiler will be outdoors (I assume). This adds some complexity as they are not typically designed for this and if they are they are more $$$.

It seems to me that in the long run it would be cheaper to purchase a professional one that meets whatever code is required and is UL listed or whatever certification applies. I wonder what the liability/insurance/risk etc is on a home-made setup like this. Does it need to be inspected and pass some sort of code?

It sounds like you are doing this in order to sell kiln-dried firewood, so I imagine there are some sort of regulations and possibly inspections.

PRR

There was a non-trivial building fire near here, a guy was force-drying wood as a business. His setup looked serious. A volunteer firefighter was driving by and spotted it, else it might have burned the lot.

mattmia2

AdmiralYoda said:

Regarding the electrical wiring for your fans. The wires you have chosen are built to handle the heat....but keep in mind the ampacity of wire reduces over temperature. A certain gauge wire will handle a certain ampacity at room temperature, but at 200F that ampacity will be much lower.

You would have to look to the full electrical code and use 150 c wiring and terminations. the derating comes from the the amount of heat the loss to resistance in the wire heats the insulation. since the wire sizes for 15 a, 20 a, and 30 a circuits come from a declaration in a rule instead of engineering data, there is an exception that allows to you use the engineering data for calculating the ampacity when derating for elevated ambient or multiple conductors in a confined area. the upshot of this is that with the 50 c of headroom and the use of actual engineering data you probably will find you don't need larger wire once you do the math. You will need to find terminations rated for 150c. If your system is 12v you probably will need to base your sizing on voltage drop. Note that this probably isn't a low energy system so that the rules for line voltage wiring apply.

Jamie Hall

Check the insulation type as well -- some wire insulations don't play well with high temperatures...

mattmia2

Jamie Hall said:

Check the insulation type as well -- some wire insulations don't play well with high temperatures...

most don't. common types are 60c, 75 c, or 90c. you would need special types for 150c, usually teflon, silicone, fiberglass, or some combination.