Downdraft stove concept

I was toying with the idea of how to design a downdraft stove, looking a the design of the Dunsley/Yorkshire stove from England.

I drew one up and attached it as a PDF. Any thoughts, suggestions, criticisms, etc?

Not much detail in the drawing, but I think you can get my drift. Of course firebrick, doors, gaskets, handles, etc would be added, but are not drawn.

I burn WY bituminous coal, and would like to do a better job of buring the volatiles/smoke. I built my own stove patterned off the Harman Mark III - works good, just still have trouble with volatiles.

Steinke

I'm pretty sure this would not work without some type of timer to run air through the coal bed when the furnace was idling. I still don't see any heated, after primary combustion air. Burning off the volitiles requires the volitiles to be released from the Bit coal by heating in the primary fire, the smoke/volitiles then need to be haot enough to burn and have oxygen rich air added. The vlolitiles will then burn like a blow-torch.

The problem I had with burning off the volitiles was finding a material to have in the coal fire to duct fresh air through, then dumping or spreading this heated fresh air over the fire. I used regular mild steel, and it only lasted a week or so in the coal bed.. it just got weak, flakey and fell apart.. the enviroment is very nasty.. Some Stainless steels may work, or a cast iron would be best. But getting a cast iron tube/ distribution head cast to your individual specifications would be very expensive.

Steinke: are you not happy with the stoker furnace?? didn't it eventually start burning cleanly ?

Greg L

I'm going to move this to the Bituminous forum, since it really applies to burning Bit coal..

Greg L

Might be far easier buying a Yorkshire Stove direct from Dunsley.

With air coming from the top down, ash will be at the top of the bed with unburnt coal resting on the grates. What happens when you shake?

I like the design. I think that if you made sure that you had air intakes for both over the fire and under the fire, it may work surprisingly well. I would consider enclosing the fire box and the heat exchanger with an insulated jacket to get more area and more efficient heat transfer for the distribution fan. That way you would have a mini furnace and could get closer clearances to combustibles. Just a thought. I think that you are on the right track though for just a radiant stove. I'd like to see it built. Nice idea.

I don't see how that could possibly work. Draft for the fire needs to come from below the grates, and through the bed of coals for combustion, THEN down draft. Or, am I missing something here? Down draft air for combustion works GREAT for wood fires. If you wanted to down draft air for the coal fire, maybe you could try this; Down draft combustion air through a tube. In other words, Combustion air would follow a tube down through the middle of the coal bed, and exit below the grates. The tube wouldn't melt, due to the cool air flowing through it. Maybe the air would be preheated, which in turn might burn off gasses better. I don't know.........Just throwing out some thoughts.

steinkebunch wrote:I burn WY bituminous coal, and would like to do a better job of buring the volatiles/smoke. I built my own stove patterned off the Harman Mark III - works good, just still have trouble with volatiles.

To burn volatiles you need high heat and air at the right place. They should be burned as gas and before precipitating as bits of carbon or smoke. To accomplish this various downdraft stoves have been designed to force the flue gas down to the hottest part of the stove which is the burning bed, before exiting. A combustion chamber that is hot enough will do a better job without the need for downdraft. The trick is to separate the needs of the combustion chamber from the heat absorbing portion of the stove.

Early Victorian boilers were simply a long tank with a fire under one end. The flue gas traveled the length of the tank heating the water. They were big and not efficient. With the advent of the Scotch Marine design the fire was moved to a combustion chamber in front of the tank and flue gas was directed through a series of tubes that went through the tank. Better combustion and much more compact boiler. Combustion was separated from heat absorption.

In an effort to design a wood stove that would accomplish both a hotter chamber and separate heat exchange I came up with a simple design that worked very well. I did not go further with it because it did not hold enough wood. It might though be just the thing for bit coal which has similar problems burning volatiles as wood.

It is 3 concentric cylinders or any shape you like. The inner cylinder is the combustion chamber which in my stove was stainless steel but in a coal stove would be fire brick all the way to the top. The inner cylinder has slots at the top to act as flue passage to the next cylinder which causes the flue gas to descend on the outside of the inner chamber keeping it very hot. It descends to the bottom and then exits to the outer heat exchange surface of the outer cylinder and out the flue. Standard grate and feed at the top side with provision for secondary air through the feed door. The combustion area should be incandescent.

steinkebunch wrote:just still have trouble with volatiles.

When you say trouble, do you mean just trying for a cleaner burn, limiting smoke or soot? Proper control of secondary (above the coal bed air) will burn volatiles/limit smoke drastically and better yet, preheated secondary air will do an even better job of burning off volatiles.

Franco B is right about heat/air in the right place.
Your kind of on the track of "gasification" with that design and while I think your design might work ok I'm not sure that design would burn cleaner than a stove/furnace using preheated or properly controlled secondary air. I would focus on a design that uses secondary air (preheated) as well design that would concentrate more heat in the firebox to help consume volatiles.
Another thought... how would you keep the grates from overheating? Gasification type furnaces use "hifire" type materials at the bottom where the intense heat is.
Good job on the drawings.

Greg - yes I'm very happy with my stoker, just trying to tweak a stove for the garage.

If I understand downdraft correctly, you are actually burning the coal at the bottom of the coal bed, and since the draft is coming from above, the "new" coal above the lower burning coals is slowly ignited, and the volatiles are forced through the hot coals below it.

I figured to line the firebox with firebrick, but I didn't think the heat exchanger would need lined. Definitely want air intake above the coal (but upstream according to draft direction) But maybe some adjustable air inlets in the ash pan door would be good.

If you look at the website regarding the Dunsley stoves, it might help explain it. (see page 3 of the following pdf)

http://www.dunsleyheat.co.uk/images/yor ... 0stove.pdf

Maybe I'm all wrong on this concept - that's why I'm bouncing it off you all.

---While writing this post - 2 others submitted posts - guess I hit a nerve. I'll have to take time to digest the 2 posts above, but still enjoying the discussion. Not really dying to build another stove, would love to tweak the one I have, just getting ideas. Thanks

Steinke

Another good design is that of the Lopi stove from the 1980s. It used a hollow ceramic baffle above the fire supplying secondary air. The ceramic reflected heat back to the fire and itself became very hot. I think this was the first of the high efficiency wood stoves.

The Vermont Castings Vigilant is designed similar to franco b's diagram, and is rated for burning Bitt. I have as yet to try it, but some say it burns Bitt. well.

steinkebunch wrote:Not really dying to build another stove, would love to tweak the one I have, just getting ideas. Thanks

Would suggest trying a baffle feeding secondary air similar to what is used in the Lopi stove. Could be steel or iron sheathed on the fire side with fire brick.

The Dunsley stove does not claim to be smokeless using Bit coal.

OK - I'd been toying with secondary preheated air for awhile too - I think you guys brought me back to reality. I don't have time to build another stove anyway.

Member "Berlin" has a pretty good concept for a hollow baffle above the fire that receives secondary air to preheat. I think I'll see what I can do there.

Good thoughts.

Steinke

onehotxfirefighter wrote:Might be far easier buying a Yorkshire Stove direct from Dunsley.

How much does one cost?