Concept for DIY Furnace

[Snail]

I was inspired by Hobartian’s construction of a Dick Hill Stick Furnace, discussed in this forum.
www.hearth.com/talk/threads/my-progress-in-building-a-wood-fired-boiler-based-on-the-design-by-richard-c-hill.68593/
I have been trying to come up with a design for something similar but easier to build, as my skills are nowhere near to his. I think I have come up with something possible that just might work. However, I have not seen anything like it anywhere else; it’s a bit of a mongrel, so I’d really appreciate some expert input.

The construction of the Rocket Mass Heaters, as described at www.rocketstoves.com using cob, seems a bit simpler but a lot of work and, I suspect there is a knack to it. The internal chimney effect seems a great idea and I am hoping the draft from that mechanism can allow a simpler forced draft mechanism for my design.

As I am not too sure about cob and pumice is readily available here, I was thinking of building mostly in pumice concrete, with high alumina cement. Has anyone experience of this material? It is a bit soft but I am trying for a design where everything comes apart rapidly to allow cleaning and repair, so I’d simply replace any worn parts. If I do a careful job on the moulds, spares would be cheap. Also, as I am not in a very cold climate, the furnace won’t get huge demands placed upon it.

Has anyone got any figures for R-Values for different types of refractory and high temperature insulation? What total R-Value should I be aiming for?

I am going to build outside, next to the wood shelter, so I can make it much larger than the Dick Hill design, in physical size that is, not in power.

The stick feed chamber and the primary combustion chamber will be similar in size to Hobartian’s. The horizontal tube will also be similar. However, the tube will then pass into a large hot updraft chamber. The idea is that this will act as both a long-duration hot combustion zone and a hot insulated chimney. The flow is then passed down over a flue gas to water heat exchanger. Passing the cooling gas downward adds to the chimney effect and also means a thermosyphon can be in counter-current flow.

Because I have the space, I was considering making a real meal of the time delay and the chimney effect, by making the riser chamber about 150cm high and expanding the flow to 40 to 50cm in diameter. Given that the whole chamber is well-insulated, how much do I need to worry about stagnation in corners? Is there an issue with getting interrupted combustion in these corners? One reason for making it rise so far is to give me room for a simpler heat exchanger.

If corners etc are not too much of an issue, I was considering mounting the heat exchanger inside the basic hot chamber, at one end. It would be constructed of very thick spigotted rings of insulating refractory, stacked to form a riser tube nearly the whole height of the chamber. Flue gas will pass down the tube and exit though the bottom of the chamber. Concentric to and inside the refractory tube, will be a large diameter steel pipe, which passes through the roof of the hot chamber and is suspended from it. This pipe is blanked-off at the bottom. The cold water will enter through a smaller concentric pipe, which leads down most of the length of the outer pipe. The flow then passes up the inside of the hot outer pipe. (The inner pipe should be insulated. Can anyone suggest a suitable insulating material? It needs to be quite thick so the annulus is small enough for good turbulent flow. It has to tolerate flowinf near-boiling water as well.)

The heat exchanger should be reasonably simple to build and also easy to dismantle and clean. The contact area could be comparable to Hobartian’s, if I can find a piece of pipe 35cm in diameter that is.

I am hoping the Rocket stove -type internal chimney will allow me to do away with the need for a draft inducer at the end of the flue system. I am thinking of introducing forced, pre-heated secondary air into the primary combustion chamber end of the horizontal tube, in such a way that it induces the primary air flow into the primary chamber, as well as helping to force the combined wood gas and secondary air into the horizontal tube.

 

[Hobartian]

Hi Snail,

I was not able to find the information about Jet Mass heaters with the link supplied.

To make things easier for viewers could you illustrate your ideas.

In my case I chose the design of Prof Hill because the boiler had actually been built and tested. The development of a new design can be a long and exhausting process and a real money pit.

In my view one principle that needs careful attention is that all burning must be completed before the exhaust gases are exposed to the cool surfaces of the fire tubes. Having the fire tubes too close to the fire substantially reducing efficiency because the temperature of the fire is reduced. A cold fire produces creosote.

Prof. Hill experimented with a number of different configurations with the idea of using only one fan but settled on the two fan design.

 

[Snail]

Hi Hobartian,

Thanks for pointing out the bad link, it's fixed now.

I'm no artist but here is a picture.

I know I'm looking at trying something new. Hopefully the costs can be low enough so that if it doesn't work I won't have lost too much. And it might be fun!

My vertical hot chamber will be quite a bit larger than yours, so providing the flow doesn't just tunnel though the middle, I should have good retention time at high temperature. The rocket stoves apparently burn reasonably cleanly with a shorter time delay. However I want a lot of flexibility to burn rubbishy wood, so I am being a bit over the top with this.

 

[Hobartian]

Hi Snail,

The illustration gives me a much clearer picture of your proposed design. However, unfortunately I don't think there will be any simple answers to your design questions.

I have taken an extract from a paper of Prof. Richard C. Hill where he discussed the problems with burning wood and the building of his wood stick boiler.



In the last paragraph of this extract Prof Hill says "Given this complexity, the only design approach is to cut and try."

I think you have a long road ahead of you but as you said there could be a lot of fun on the way. History shows that many innovators have faced massive skepticism in trying to introduce new designs and concepts.

I guess if you can visualize how your design will work and can't find any reason why it should not work I would go for it.

 

[tom in maine]

If you look at Dick's original paper, that design can be upscaled with reasonable success.
The Rocket stove design you posted will have issues with spilling smoke without a decent (read that as big!) induced draft fan on the exhaust.
Rocket stoves basically follow the parameters that Dick discovered and noted in the page that Ian has posted.
His original work had a forced draft fan for turbulence and primary air and the induced draft to keep smoke out of your face and house.

 

[Snail]

Hi Tom,
Thanks for your comment. Could you possibly expand a little more? I'm puzzled at why my design would be worse than a straight rocket mass heater when it uses the same basic chimney design and the RMSs don't have any sort of fan at all. They operate purely on the differential chimney effect of the combustion temperatures in the riser and the downward flowing cooling air in their gas to gas heat exchangers. I was intending to further add to the RMS-type draft, by adding forced secondary air from jets in the start of the horizontal tunnel, the idea being that this will induce flow through the primary burn chamber, force the mixed gases down the tunnel and cause very turbulent mixing. I also read somewhere that it was best to limit turbulence in the primary chamber and this could achieve that.

By the way, I'm not trying to upscale the Hill design regarding output. It's only that, as it's going to be an outside installation, I could use of the lack of space constraints to make something a lot bulkier but, because of the greater time between the start of secondary combustion and the heat exchanger, possibly even cleaner-burning.

Hi Hobartian,
I have tried to keep the 3 Ts, Time, Temperature and Turbulence uppermost in my mind. I think that the Time aspect should be answered by the much larger volume in the system, compared with the Dick Hill original. Temperature will depend on my supplying enough insulation and getting the right lambda. In order to get the turbulence, I was hoping that blasting the secondary air through small jets would provide enough??

I've also been thinking about designing against cracking. Cracking is the result of the inside faces being much hotter than the outside ones, resulting in the inside being placed in compression, as it expands, and thus putting the outside in tension. If the inner refractory was made very thin and it was then confined by a layer of compacted pumice, without cement, the stresses should be a lot less. Of course repair could be a messy business if things are let go too long.

 

[tom in maine]

The issue I see is the amount of pressure drop that the refractory zone before the chimney has.
The exhaust is going to slow down and lose flow up the chimney.
If the draft is good enough, probably not an issue, but on that semi-cool rainy day in the autumn, starting is going to be a pisser.

There are enough threads here about people spilling smoke, especially on startup to be conservative and integrate an ID fan.
You can certainly try it without one, but what I see in your plans is going to probably warrant one.

 

[Snail]

Hi Tom,

I'm not sure if I understand you correctly. Did you mean the external chimney in my diagram or the internal riser/heat-exchanger "rocket chimney"? If the former, remember that in the rocket stoves, the draft is driven by the density difference between the combustion-temperatures in the internal riser and the much cooler gas in the down-flow condenser. Many rockets have no conventional chimney at all, they simply exhaust sideways through the wall. So the distance between exterior chimney and primary chamber isn't relevant. If the latter, don't be mislead by my non-existent CAD drafting skills and tools! I will try not to make the horizontal flue any longer than those for a Rocket stove, as laid down in the books. Surely that would work? There seems to be a reasonable proportion of successes with rockets placed in the living rooms, if the books are followed. Then in addition, I hope to be adding draft, as shown in this second diagram. (Mind you, I'm not so sure what sort of pipe will take the heat, maybe the pipe entry into the hot ducting should be a lot shorter than I've shown.) NOTE second diagram is simplified from the first, it's only intended to show the secondary air proposal.

As I am building outside, some back-smoke at start up only wouldn't be the disaster it would be inside, not that smoke is ever good.

I would probably not be brave enough to omit a draft inducer if I was making this for inside. On the other hand, I am not even brave enough to build anything like this inside, on my first attempt.

 

[tom in maine]

As Ian (and Dick Hill) said, you just have to cut and try. We (Dick and I) did that a lot and ate a lot of smoke along the way. The first drawing you showed has the exhaust going up and then down to get out of the unit.
This will be difficult to start up without an ID fan.
Smart to do it outside the house for testing.

 

[Snail]

HI Tom,

I still mean to have the flue gases going up and then down.

All Rocket Mass Heater stoves have the flue gases going up and then down. They work like this:

The going up leg is massively insulated, so it is all at nearly combustion temperature, say 2000deg F. The downward length is the heat exchanger, so the bottom is 400F or cooler. So the average temperature going down is maybe 1200F. The difference between the legs up and down is therefore about 800F, which is what drives the draft. Compare that with a conventional chimney, where the flow is driven by the temperature difference between the outside air and the after the heat exchanger temperature, say 400F-60F=340F. So on those figures the Rocket interior chimneys would be about twice as effective per unit height as an ordinary chimney. In fact, as the Rocket risers are often built inside a 55 US gallon drum, which limits the height of the chimney to less than 3 feet, and as they seem to work fine, the enhanced effectiveness is possibly even greater than that, I just conservatively guessed at the combustion temperature and they do seem to aim at very low flue exit temperatures, I've not yet read enough to find full details. Also, the cooling effect is highly non-linear, so the true average temperature in the downward leg would be considerably cooler than just the average between top and bottom of the flue.

In some cases, it is claimed, the flue gases out of a RMH are so cool that that a conventional vertical flue is not only not necessary but not even desirable! I must say that seems hard to credit. Why don't their flues rot out in that case? There may be a bit of over-claiming by some in the RMH community. However, the efficacy of the flue system is pretty well-established as far as I have been able to ascertain. There is one design that actually uses two rise/fall parings in a couple of drums in series, in order to extract the maximum of heat.

Start-up can be a problem in many chimney systems. My stove in my living room being a truly horrible example. Ever had to open every door and window in the winter because of a failed start-up? Thank heavens I don't live in a really cold climate! I hoped that the induction from the forced secondary air would be enough to kick-start it. I think that I'd better build a real quick and dirty low-durability design to test the system, given your experienced doubts on the matter.

Do you have comment on how I can go about testing my design? No visible smoke is a fairly low standard. Would monitoring the combustion temperature be a good guide on how effective the combustion is? Obviously a flue gas analyser would be ideal, but I can't see my getting access to one of those at reasonable cost. Presumably, maximising combustion temperature would give the optimum lambda for a given system? If I fiddle with secondary/primary balance to do that, what is the minimum combustion temp that I should be achieving to be assured that the system is clean and reasonably efficient? Do you have any advice on thermometers for both the combustion and heat exchanger exit flue temperatures?

Still scatching my head on durable construction. Hobartian's experience has given me pause.

 

[tom in maine]

I would start with looking at Dick's entire paper. And then blending in what you are interested in doing with Rocket stove design.
Rocket stoves as a backpacking thing or third world device are not necessarily as germane as Dick's work for heating water.

There are a couple other folks who have done their own designs on Hearth. I would look them over.
There is a reason that you are a bit overwhelmed. It is a lot to "cut and try", but it is fun, especially if you have a house mate who has a sense of humor.

There are cheap wood combustion analyzers on Ebay for about $450. That is a cheap date. A simple flue thermometer and a visual inspection for smoke
is pretty darn good, for a low tech analysis. Inspecting for crud in the flue can tell you how well things are going.

I think there are enough people here who could use a good ID fan, that you should be thinking about it up front.

 

[Snail]

Hi Tom,

I have read Dick's papers and have downloaded his patents. I haven't fully mentally digested them yet, nor a heap of combustion science stuff, I hope to do so, trying is fun too.

My wife is tolerant, but not to the extent I can start on the stove before I finish the wallpapering!

Interesting comment about the divide between the cob-and-rocket-in-a-teepee guys and this forum. Almost a cultural divide. I think that trying to combine their ideas and some from Dick is what I am clumsily trying to do.

There is a naturally good reason for the cultural divide, even though they work well, rockets and their ilk are pretty well untransportable, so will always rely on expensive artisan-type labour or DIY construction. Not easy to commercialise, so less scientific development (There is still some good stuff). But I am bemused at the visceral resistance to electric fans and the like. And their arguments against using a rocket flue to feed a low pressure water heat exchanger, especially if unpressurised and outside, seem a bit overstated. Half of the older wood stoves in this country had an exchanger in the back of the fire-box, not good for combustion cleanliness but not an explosion risk either.

I didn't realise that combustion gas analysers got as cheap as that! Even so, as I am doing this as much for fun as use and the prospects for commercialisation are nil, I think I'll pass on that for now. If I have any success fine, if not, it'll at least have been interesting and I'll have a new wood-shelter.

What type of thermometer would be best for the hot flue, i.e. just below the heat exchanger in Dick's original design? I have American and International units all mixed up in my brain as yet, that's part of the mental digestion I mentioned I have to do, but it seems to me that with dry wood and 50% excess air I would be looking at upwards of 2000F?

 

[tom in maine]

We used to use high temp thermocouples or handheld laser thermometers for combustion zones.

 

[DevilsBrew]

There are additional divides in the RMH world - the traditional RMHers, the rocket stove guys, and the hybrid inventors.

I am interested in your build. Please post pictures when you start.

 

[Snail]

Hi Tom,

I may be able to borrow a potter's thermocouple for a bit. What sort of temperatures were you measuring? Some of my wood is has been 60 years as part of a dry house, so it will certainly be at equilibrium with the local air. Mind you that's pretty humid, so it won't be bone dry now it's been in the woodshed for a while.

Hi Devil,

Thanks for the encouragement. I'm working in Jakarta at the moment, so will be a while before I start. I think that thinking about winter things is partly a way of dealing with the heat here .

Been checking materials. It looks as though unless I incorporate exhaust gas recirculation, the combustion temperatures will rule out natural materials. Pumice, vermiculite and perlite are all plentiful here, pumice ridiculously so, but they all soften or melt at less than 1000C. That could be a problem because my big hot chamber will take a lot of warming up if its made of thick fire brick. Not much use having a super-clean hot burn phase if it takes half the firing time to actually get hot. Need to do some calculations before getting on to the suck it and see phase.

 

[tom in maine]

I never bothered to measure combustion zone temps. I relied on color of the firebrick and what wound up in there being melted.
One evening two glass perfume bottles were in with some papers I put in. I realized they were there as soon as I dropped the papers.
They exploded (with a poof, not a boom) and when I cleaned out the ash, I had some very nice melted glass pieces. We also melted copper
and some aluminum that wound up in with other paper.
The highest temps were with forced draft. Natural draft does not get nearly as hot.

Maybe this is not a great insight, but we can consider the combustion process as being similar to a automobile carburetor.
If it is fuel rich it wastes energy. If it is air lean, it tends to run hot (as long as there is not too much air to pull energy away from the fire).

After using a LOT of different refractory materials, hard firebrick is still a great deal here in the US.

 

[Snail]

I never bothered to measure combustion zone temps. I relied on color of the firebrick and what wound up in there being melted.
One evening two glass perfume bottles were in with some papers I put in. I realized they were there as soon as I dropped the papers.
They exploded (with a poof, not a boom) and when I cleaned out the ash, I had some very nice melted glass pieces. We also melted copper
and some aluminum that wound up in with other paper.
The highest temps were with forced draft. Natural draft does not get nearly as hot.

Maybe this is not a great insight, but we can consider the combustion process as being similar to a automobile carburatot.
If it is fuel rich it wastes energy. If it is air lean, it tends to run hot (as long as there is not too much air to pull energy away from the fire).

After using a LOT of different refractory materials, hard firebrick is still a great deal here in the US.

Hi Tom,

There is no doubt in my mind that you are right, fire bricks are the way to go, in a climate where firing is long and frequent. In that case the fire bricks are likely to still be hot when starting up, much of the time. In my case, the system will usually be starting from bone cold and the number of start-ups per hour of burning will be high. Hence, the penalty of having to reheat a large thermal mass of relatively conductive lining is proportionally a lot higher for me. My desire for a relatively oversized hot zone makes it even worse, because there is way more refractory surface than usual, in proportion to the power of the furnace that is.

 

[Mark Holden]

A couple of ideas to add;
I built a downdraft ceramic kiln once. to induce the fumes to go down before going up through the chimney, I would preheat the chimney at the start of a burn. It doesn't take much heat to get the draft started; some type of closeable bypass port would do the job neatly.

I'm considering incorporating an oil burner into my design. I would use it to start the burn, and possibly use it's blower for the wood burn too.I have 2 of them.
Opinions requested on that one.

 

[peakbagger]

At an industrial plant I used to work for I had to install a backup thermal oxidizer to burn hazardous gases. It would sit cold for weeks and then had to be at 1600 degrees in 15 minutes. It had a ceramic brick combustion chamber with pleated ceramic downstream of the burner. Both held up well. There was a bit of refractory downstream that had to be redone every year. I expect the ceramic is expensive but it sure works well.

A big aspect of any refractory is if its damp. If there is any moisturein it when cold, if heated too quickly the water vapor will vaporized and destroy it

 

[Boil&Toil]

Going back to post number 3, I think you are complicating the water flow needlessly. Bring it in at the bottom and run it straight up.

I have a copy of Dick's paper that my sister brought home for me from Orono (back when it was new), and newspaper clippings. I've been hauling them around for over 30 years and might get my round tuit one of these decades. I was sad to see the commercial version (Madawaska Wood Furnace - was there another liscensee as well?) not make it.

Where you can use it (it won't take physical abuse), ceramic fiber is awesome refractory material. Oddly enough the maple syrup folks are doing a good deal of practical work on where & how it works with wood combustion, though most of them still like to throw a lot of heat away up the stack. Adding a ceramic coating to the hot face has multiple benefits (reflects more heat, keeps ash from building up in the fiber, makes it a touch more durable/cleanable)