Preheated Secondary Air

I'm in the process of adding preheated secondary air over the coal bed in my homemade hand-fed stove. I burn hi-volatile bituminous coal, so it's kind of a must if I want to avoid soot, smoke, etc. I'm in the middle of the process, cutting, grinding, welding, etc on the stove. My stove is similar to the Harman Mark III. Pasted below is a drawing of what I'm planning. Grates are red, firebrick is tan, bafffle and heat exchangers are yellow, and the secondary air system is blue. Stove body is not shown. View is from front corner of stove, looking up.

I'm wanting some opinions as to the amount of secondary air required. I know it depends on many factors, like firebox size, firing rate, coal quality, etc. But what do you all think would be a good pipe size? I'm going to pipe air under, beside, and over the coal bed, to allow the air to preheat. The pipe will not be in contact with the coal, but will be very close to it for a total of about 4' of pipe, then be above the coal bed for another 12" or so. If I need even more preheating, I can build a hollow baffle and pipe the air into it too. Then again, the baffle could potentially cool the air after I've smothered the bed with new coal. I also plan to replace the current steel baffle with firebrick, layed directly on top of the upper secondary tubes.

I have initially planned on using two 1" diameter pipes (it's actually a hybrid system of 1" pipe and 1-1/2" square tubing, taking advantage of my firebrick supports which are made of square tubing).

Seems like there would be a delicate balance when burning the volatiles - as long as you have them burning, it might sustain itself, kind of generating it's own heat. But if you loose the secondary burn, it might be tough to restart it, until of course you get the spontaneous "boom" later on. Trying to avoid that though. With that in mind, maybe the bigger the pipe is better - that way the air velocity is slowed, giving more residence time and a higher preheated temperature.

So - any opinions on minumum pipe size? Any other suggestions? Thanks

Steinke in WY

Also, what might be the merits of using tubing vs pipe under the grates? I would think the air in tubing would receive more heat from the grates above, or at least heat up faster. But the pipe would hold heat longer. Maybe there's very little difference, and pipe would be better based on longevity.

Nice design and I think it could work but I keep thinking that once the volatiles are gone you won't want any secondary air whatsoever....would you just plan on shutting off the secondary air after volatiles are gone?
As far as the tubing under the grates, would it be straight just like the pipe in the diagram?

I'm thinking of putting a bimetal thermostat and butterfly valves on the tubes at the back of the stove to shut the secondary air once the volatiles are gone. Might be tricky to get the heat settings correct, as the tubes exit the stove below the coal bed. Old Earth stoves had a thermosat control on them. For now, I'll probably control it manually until I find if the secondary system even works.

I do think that the secondary air will need to be shut at some point though. Maybe by limiting the primary draft (by allowing more secondary draft), I can more slowly meter the volatiles too. Then when the volatiles are gone, The primary could be opened more. Suppose that means a dual thermostat system, or creatively turning the butterfly valves with the bimetal strip (offset butterflies on primary and secondary pipe, when one's mostly open, the other's mostly shut). Or maybe leave primary thermostat control out altogether, and just have the secondary close once volatiles are gone.

The tubing would be the same place as the pipe in the diagram - just thinner walled. I may just try using pipe first - I can always replace it with tubing later.

Is the flue on the top or back?

I think the air is too high in the firebox needs to be nearer the burning

coal bed to work the most efficiently.

The heat exchanger could be of larger tubes with more surface area.

Good luck with the project!!

BigBarney

The flue is at the top of the stove.

So you're thinking that I need more pipe surface area exposed?

I'll look at lowering the tubes some - but if I get much lower, the front "air wash" for my door glass will interfere with loading. Though not shown in the diagram, I plan to use some stainless bars above the front row of firebrick to be a able to bring the coal bed to within 8" of the tubes. I may even remove the firebrick retainers, stack another row of firebrick around the stove (with stainless retainers at intermediate brick corners), and get the coalbed about 12" deep for a longer burn. This would bring the bed to within about 4" of the secondary tubes.

Thinking about putting a top load door on too, if I deepen the coalbed, or I'll have trouble loading it.

Thanks for the input BigBarney.

The tubes on my Mark III aren't all that big ... and there's only 2 of them. That setup should work good. Mine blow plenty of hot air out with the 135cfm fan.

I would use plain A36 steel in the boiler construction,I think most stainless steel

will fatigue crack from the heat and cool cycle.

The closer you get the hot air to the coal you will increase the air temp

even by washing it over the coal.Right at the top of the coals is ideal.

Post a picture of the proposed changes.

BigBarney

This is burning Bituminous coal in my boiler.

SMITTY wrote:The tubes on my Mark III aren't all that big ... and there's only 2 of them. That setup should work good. Mine blow plenty of hot air out with the 135cfm fan.

Smitty - I think you're talking about the heat exchanger tubing that the distribution fan blows through. What I'm refereing to is secondary combusion air tubes to provide preheated air to the coal bed/fire.

I have redrawn it some, adding more firebrick, using firebrick for the baffle above the tubes, and adding one more tube at the back of the firebox. Also shown is a "coal bar" similar to the Harman TLC2000. I could even make the coal bar a little taller to get a deeper bed.
Steinke

Do you think the draft alone will be enough to draw sufficient air through the secondary air tubes?
Will it be constructed so the secondary air pipe with the holes drilled in it can be easily replaced when needed?

Your point about easily replacing the drilled pipe is a good one. I had planned on threading the ends of each pipe, and using a jamb nut (i.e. an electrical nut) on each end to hold it in the holes in the square tubing. When the pipe goes bad, just back the jamb nuts out and remove the pipe, or if nuts are siezed, cut the pipe in two with a sawzall and remove it. Not sure how long it will last, but if I only get one season out of it, it's no big deal. Could use stainless, but replacing mild steel pipe each season would likely be cheaper. Have to see how long it lasts.

And you raise a good question regarding if the draft is sufficient to draw the secondary air through the tubes. I'm not sure - Berlin and others have had success with secondary air, but I don't think their designs had quite so much pipe length to them. Could be that I'm restricting the flow too much and won't get much air. Some wood stoves such as Lopi, Napoleon, and others use some passageways to heat secondary air and draw it in with draft, but again, I don't think they use such long tubes either. I have lots of bends, etc. which could restrict it quite a bit.

I could abandon the idea of puttings tubes under the grates, and also across the stove front and sides, and simply drill holes in the sides or front of the stove about the height of the door top. Then come across into a hollow baffle plate. Might draw more air, but I don't think it would get nearly as hot. Surely would work at high-burn, but when I load with coal, I just think I need the extra time to heat the air so that I can burn the volatiles from the start.

I'll just have to pick an option and find out. I may get a chance to work on it in a few days. Keep the ideas coming though - it might save me some trouble.

Steinke

Does the secondary air need to be preheated? And if so how hot is ideal? Member Doug is burning Bitt. in a Vogelzang pot belly with only a air inlet in the load door. I don't know if it is burning the volitals or simply washing them up the flue. If the gasses are already hot won't the simple addition of extra combustion air work? like when you open the load door after a fresh load and get that woosh.
What about the idea of adding combustion air into a preheated secondary burn chamber/heat exchanger like the Vermont castings vigilant that is rated to burn low to mid vol ital bitt. coal
Or using your existing design with the addition of a secondary preheated burn chamber?
Just some thoughts that have been kicking around in my head, with the idea of doing exactly what you are doing.
Another idea I had was to preheat the secondary air through channels attached to the outside of the stove body before bringing it into the secondary burn chamber.
Last thought that comes to mind is that if the secondary air proves to need less restriction, then bring it in through a double wall back, on the back of the stove, then in through a boxed baffle.
Not trying to change your whole design, just bouncing ideas off you to see what comes back.

Lots of interesting thoughts, I don't have many answers though. From what I've gathered, the air needs to be preheated. I don't think it's enough to just bring it through the load door, as lots of stoves do that, but few are truly successful at burning volatiles until things really get hot, and then you have to really keep things hot to continue to burn them.

I read a really old publication on industrial applications for reducing smoke in stokers, and they were eyeing the 350F to 400F range for preheated air temps. I guessed it would have to be hotter than that, but I'm not sure.

Yes - we all know we can get ignition of the gasses when you open the load door after a fresh load, but it's hard to maintain that situation with my stove and my coal. Things above the fresh coal cool too much after a fresh load, and it takes an hour or so to get the "boom". Lots of soot and smoke occurs during that hour. Hence some bank the coal bed to avoid the boom. But I still wonder even with banking if you do a good job of burning volatiles, or if they just go out the stack.

Pete69 wrote:What about the idea of adding combustion air into a preheated secondary burn chamber/heat exchanger like the Vermont castings vigilant that is rated to burn low to mid vol ital bitt. coal
Or using your existing design with the addition of a secondary preheated burn chamber?

I guess I thought I was trying to do what you are describing, my burn chamber being all the piping I've drawn.

Regarding your statement about passing secondary air through passages on the outside or back of the stove - with all the firebrick in the stove to direct the heat back to the coal bed, it seems that any outer pipes would remain relatively cool compared with pipes "inside the firebrick".

I'll chew on it all awhile yet, and hopefully others will chime in with even more knowledge. Thanks for the replies.

Steinke

You've got me thinking about hollow grates...again. A couple years ago I was thinking of the idea of air flowing through hollow grates keeping them cooler to make them last much longer but didn't think about using the heated air for secondary combustion til now.??
I couldn't think of a way to make it work though because the air flow and temperatures of the grates would be different at different times through the burn cycle so the grates would still need to be able to handle high temperatures when air flow was at a minimum and also the grates would need to move/rotate to remove ashes but not leak air in the wrong place at pivot points. Also I couldn't think of a way to effectively control the air flow in hollow grates either.... I've always thought the ultimate stove would be one that better utilizes the extreme heat from the spot you don't want too much heat...the grates.
Fireplace tube grates (without the fan) give you a basic idea if what I'm thinking.http://www.woodlanddirect.com/Fireplace-Accessori ... lower-Unit
There are liquid cooled grate systems that you might want to read about. http://www.patentstorm.us/patents/6145451/description.html.
Warm morning model 400 has the "four flue" design that is interesting but doesn't have any way to control air flow through these flues. Here is a link to a brochure where you can see these "flues" running up the corners of the firebox.

My experiments with heated secondary air showed that the heat and combustion environment in the firebox very quickly turned regular steel into a very weak crusty husk of it's former self. My tubing went through the coal bed, made a 180* turn and 'blew' down on the top of the fire.. When the amount of volitiles, heated air and fire was correct, the added air made the top of the fire burn like a big blowtorch.. very impressive.. My secondary piping only lasted about a week, but I was burning about 250# of coal a day.. it was a firebox about 22"x24"x12-16". Quite a pile of coal.

But as mentioned, establishing and maintaining the correct mixture of heat, heated secondary air, and volitiles is quite difficult in a hand fed stove. This is where a stoker really 'shines'. You can adjust the feed rate of the fresh [lots of available volitiles] coal, the air and the condition of the fire [heat, depth, and accumulated ash/clinker]. All of these adjustments will keep the volitiles burning, the chimney clean, and the heat output of the stoker stove high.

I believe there will be enough draft to pull a significant amount of air through the secondary tubing, since the secondary tubing will be fairly unrestricted, but the combustion air to the coal bed will be restricted by the combustion air controls. So I think the chimney will be able to pull plenty of air through the secondary piping. The problem will be to get the right 'mix' to burn the volitiles, and to maintain the correct mix..

You should be able to burn off some of the volitiles at least part of the time,, but you will need to keep track of the 'right' time to add fresh coal, you want the fire to be fairly hot, so the secondary air is HOT, and the fresh coal can be quickly heated to give off concentrated volitiles in a combustible mix.

I think you will be trying to become an organic version of a mechanical stoker..

I'm looking forward to the end results of this 'science experiment' !!

If you can find an auto/truck/tractor salvage yard, and find an old cast iron manifold to use in the fire to heat the air, then the other secondary pipeing may last a lot longer since it won't be directly in the coal's ~2000* heat.

Greg L