Stove Design - Questions for Combustion Experts

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JRLearned
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Post by JRLearned » Wed. Jun. 04, 2014 10:20 am

If you look at my stove (below), you see I've added a large steel chamber on top of the original burn chamber. This chamber was intended to act as an afterburner for burning wood, as I had a catalytic combustor in there and a bypass damper. This idea worked well enough, but utilimately wood was just too volitile and the creosote was just way out of control, even with dry wood. Also, the catalytic combustor wasn't worth the money. There was only a little gain. Still it was a lot of fun to build. I've removed the guts of this chamber and now I'm left with this large empty section above the fire, below the chimney pipe. I'm trying to determine what I could do with it to the benefit of burning coal. It's an empty canvas for a new design, or could be removed entirely.
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I would like to pose a couple ideas/questions about hand fired coal stove design:

Is there a rule for how much air space you want over the coal bed before entering the chimney pipe? Is it better to have the top of the stove very close to the fire? Very far away from the fire? How does the cubic feet of air space above the fire play into coal combustion?

Is there any benefit of a small thermal mass above the fire, surrounding and insulating some of the stove pipe? If I put a steel plate back into the bottom of the empty chamber and ran a stove pipe up the center, I could fill in around the stove pipe with... rocks, fire brick, refractory cement, etc. The thermal mass would heat up, and I'm wondering if it would help stabilize draft, make it easier to keep a dying fire going? Is there any benefit to it, technically?

Would there be any benefit of an insulated wall around the first section of stove pipe, like a rocket stove effect. Would filling the chamber around the stove pipe with thermal mass, or an insulation, create any turbulent exhaust of coal gases that would increase draft or do anything of benefit?

 
franco b
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Post by franco b » Wed. Jun. 04, 2014 5:47 pm

Because the stove and chimney make up a system you are limited as to how low the stack temperature can go and still draft. Yours looks to be a fireplace so stack temperature on surface of pipe probably wants to be about 180 minimum.

I suspect the drum addition that you fitted will lower stack as low as possible.

Thermal mass is good at absorbing excess temperatures of burning wood gas but not nearly as effective or needed with the even burning of coal.

I would leave the drum bottom in place and block the combuster hole leaving the bypass hole. This will have the effect of increasing the velocity of flue gas through the hole and causing turbulence once through the hole making the heat exchange of the drum more effective.

William once posted that space above the fire should be about 2 times fire pot depth. Most oak stoves are close to this.

 
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nortcan
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Post by nortcan » Wed. Jun. 04, 2014 6:14 pm

Maybe too late about combustor for wood burning but when having a combustor, a special insulated chamber should be placed where hot gases leave the combustor device. To be able to burn the gases, the combustor ignites the gases and the secondary gases burning occurs in that chamber where you get the very high temp. needed.

 
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Lightning
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Post by Lightning » Wed. Jun. 04, 2014 6:25 pm

I like Franco b's opinion.. The only thing I can add is that by having the extended space above the coal bed, it also translates into more surface area for heat transfer. As far as insulating the pipe is concerned, I wouldn't insulate it since it also can serve as more surface area for heat transfer into the living space.. :)

I admire you ingenuity.. Very well played.. :up:


 
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warminmn
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Post by warminmn » Wed. Jun. 04, 2014 7:57 pm

You know, I look at that and here is what I see....

Run piping out either as originally or thru the new top, straight thru it. Then purchase another Chubby door and install it into the top part, matching up with the other door. Put a rack in there and you have a oven! You may have to leave the door open for heat release when not in use and would need a low fire for cooking.

Even just installing another door on top without a use would look nice.

It doesnt answer your question at all but I thought I'd throw it out at you. It may get your brain thinking even harder, or not.

 
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JRLearned
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Post by JRLearned » Wed. Jun. 04, 2014 9:56 pm

nortcan wrote:Maybe too late about combustor for wood burning but when having a combustor, a special insulated chamber should be placed where hot gases leave the combustor device. To be able to burn the gases, the combustor ignites the gases and the secondary gases burning occurs in that chamber where you get the very high temp. needed.
My intention was that whole upper chamber would serve as the chamber post-catalyst, pre-flue. But, perhaps it is too large a chamber. If I tried it again in the fall perhaps I'd section off some chamber within the chamber to get more heat build-up.

I found this on google...
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JRLearned
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Post by JRLearned » Wed. Jun. 04, 2014 9:59 pm

warminmn wrote:Put a rack in there and you have a oven!
One of my other hobbies is BBQ and smoking meats. I've considered buying another chubby for outdoor wood fired smoking!

 
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JRLearned
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Post by JRLearned » Wed. Jun. 04, 2014 10:16 pm

franco b wrote:Because the stove and chimney make up a system you are limited as to how low the stack temperature can go and still draft. Yours looks to be a fireplace so stack temperature on surface of pipe probably wants to be about 180 minimum.

I suspect the drum addition that you fitted will lower stack as low as possible.

Thermal mass is good at absorbing excess temperatures of burning wood gas but not nearly as effective or needed with the even burning of coal.

I would leave the drum bottom in place and block the combuster hole leaving the bypass hole. This will have the effect of increasing the velocity of flue gas through the hole and causing turbulence once through the hole making the heat exchange of the drum more effective.

William once posted that space above the fire should be about 2 times fire pot depth. Most oak stoves are close to this.
Thanks FrancoB. I have been concerned about maximizing draft since I have a chimney cap (slate slab masonry type) and have trouble keeping a fire going good when above low 30's outside. I have a permanent manometer installed and never got much above 3.5, 2.5 being more average. I didn't have the manometer before this upper chamber mod. My fear is it is decreasing draft but hard to measure without removing it, so I'm relying on the theoretical. Good point about stack temps.


 
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Post by blrman07 » Thu. Jun. 05, 2014 10:13 am

It looks like your inadvertently rediscovering a dome type parlor stove which were popular in the mid to late 1800's. That peaked my interest on how did a particular design of stove survive where others ended up in the scrap yard? What was the deciding factor for people doing whatever they had to do to keep a stove running and pitching it? Why did they die out as manufactured stoves vrs the square boxes we see today? Simple economics of manufacturing. It's cheaper to make a plane jane box than and ornate pretty box. Drop the price, mass produce them and you stay in business till someone else figures out how to make the same BTU delivery a little bit cheaper. Some folks have asked what is the secret to designing a stove today that approaches the efficiency of the old BB's.

I have been reading engineering books dating from 1870 to around 1905 and the key appears to be the ratio of the grate to the overall heating surface. Many different styles were tried and chucked but some were kept. The base burner design was an obvious winner due to two factors. Efficiency of coal burning and its pleasing to the eye style. Mom didn't mind the mica door stoves because they were pretty and did the job of heating the parlor where the family hung out and dad liked the efficiency.

What did the best burner formula look like compared to other stoves? The ratio of grate size in the surviving stoves burner to heating surface was 27 to 1. When you added up the surface area of all the cylinder or stove sides plus the back pipes the ratio comes out to around 27 to 1. Each one of the mags and books I have read puts the ratio at 27 sq foot of stove/furnace/boiler heating surface for every 1 square foot of grate regardless of the design, size, or shape of the equipment. You have to include all the heat transfer surface area of everything in the stove that is in contact with products of combustion.

What is the ratio of your unchanged Chubby stove vrs the re-engineered Chubby with the dome on the top? Math was my weak subject so I have to get help from people who have math minds for this one. It looks like all we are doing is rediscovering what stove builders in the 1800's took for granted that everyone knew the rules of thumb for chimney height, stove construction, fire pot ratio's. Kinda makes me feel rather small in brainiac power when I realize how much the industry has forgotten compared to what they think they know now.

It seems from my reading that when that ratio got too far either under or over the 27-1 ratio, things went klafooey.

Check the ratio and tell us what you find.

Rev Larry
New Beginning Church
Ashland Pa.

 
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wsherrick
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Post by wsherrick » Fri. Jun. 06, 2014 12:54 am

Don't worry. Barrel extensions have been done before with good results.

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Round Oak with barrel extension.


 
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Sunny Boy
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Post by Sunny Boy » Fri. Jun. 06, 2014 6:47 am

Yup, they work.

One of my brother-in-laws-to-be got this Round Oak out of a local church. It's all he's used to heat his home for many years.

Not as much barrel extension as some, but I've stood next to it and it really puts out heat. It's about six feet tall to the top of the barrel top that the finial is sitting on.

Paul

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