Hill Stick-Wood Furnace: Upsizing

[ChrisZA]

Dear All

It was with great excitement (and with a building sense of respect) that I stumbled upon and read the paper by Prof Hill describing his work on stick-wood burning furnaces. Equally impressive is Hobartian's (I think it is Mr Hobartian for me!) project described on this forum:

https://www.hearth.com/talk/threads...-based-on-the-design-by-richard-c-hill.68593/

From the above thread I quote Mr Hobartian:

"The figures I quoted in my earlier post appear not to be correct as it is stated in the application that a sixteen inch diameter combustion chamber with a height of three to four feet can be loaded with forty pounds of wood which will burn at the rate of twenty pounds per hour and thus create 100,000 BTU per hour."

This would be just shy of 30 kW.

In the Hill-patent it states that a 16 inch diameter combustion chamber with a height between 3-4 feet and a water jacket height of 30 inches will perform as per the above quote.

I would like to build a 60 kW unit and such is the mysterious ways of nature that I am pretty sure multiplying everything by 2 is just not going to work.

One of the foundational ideas of the Hill design is that the water-jacketed portion of the combustion stack prevents pyrolysis of the wood in the jacket region thus ensuring that not all the wood is consumed but only the bottom section that is located within the refractory-lined combustion chamber. I assume an increase of the vertical height of the combustion chamber with a subsequent increase in air flow rates would increase the output of the furnace. Of course the tunnel diameter and length should also be adjusted. Then again, do you really want to handle lengths of wood of more than 5 to 6 ft?

Gentleman I would appreciate your thoughts, recommendations etc on this topic.

 

[maple1]

My only input : I would think rather than 5 ft long pieces of wood, two pieces 2.5' long on top of each other should work OK?

That likely didn't advance things much...

 

[Boil&Toil]

As far as I recall, there should be NO effect on heat output (rate) from a boiler to Dick's design with an extended wood chamber, due to the water jacket keeping it cool above the firebox - merely an increase in time between firings, BTUs per firing, and inconvenience of lighting, cleaning, and loading without breaking the refractory (don't drop logs down an 8 foot hole onto firebrick - lower them gently...) I think he did lay out a concept for doing that, with a below floor install (I'm not digging into the file cabinet to check right this minute.)

Increasing the diameter and fiddling with the tunnel will have effects, but the tunnel, at least, was a matter of a lot of experimental trial and error, so HOW you adjust the tunnel would probably be a matter of more experimental trial and error on your part. Great for science, not so great for getting the thing built and using it efficiently.

The least-surprises approach to get ~60 KW with a Hill furnace design is to build two, or (possibly) two fireboxes feeding one larger heat exchanger. If rigged below floor with a chainfall for loading wood, you probably could scale length to 8 feet. Probably still more hassle than a 4 footer.

 

[hobbyheater]

Dear All
Gentleman I would appreciate your thoughts, recommendations etc on this topic.

I have done some experimenting with my old 30+ years Jetstream ( I have a new spare never used unit) by increasing combustion air from 99 cubic feet per minute to 120 cubic feet per minute I was able to jump its output from 120,000 BTU'S per hour to around 200,000 BTU'S per hour , the big downside was at this output it burnt to hot and the refractory started to disintegrate. .

 

[ChrisZA]

@ Boil&Toil: I might have been unclear in my original post: I do not simply want to increase the water jacket height but the height of the refractory-lined primary combustion chamber thus exposing a greater area of wood to the combustion zone. Naturally the primary and secondary air inputs would need to be adjusted accordingly. I really like the two-primary combustion chamber approach, it will score points just on looks!

Increasing the air for combustion as hobbyheater suggests is something I will definitely try. However, there will be a limit as only so much of the wood is exposed to combustion so at some stage the excess air will start to decrease the temperature of the combustion products.

I guess the sensible place to start is to build as closely as possible to the original design and then adjust once a working furnace is in place.