Bruce, I'm trying to understand what it is that you are trying to accomplish.
Do you intend to insulate the J-tube? If this design gets up to temp, that metal is going to spall like crazy. However, I wonder how it will draft-- the riser looks terribly short for such a long burn tunnel. Will it vent through a mass? Do you have a sketch of the final intended layout?
-I built into an existing fireplace
-has a 4.5in CSA
-has a cast riser with an iron liner
-burns wood, pellets, or both
-was temporary...I used clay slip...the fireplace has been restored to a Rumford configuration
I wouldn't say that this design is a viable solution for your particular situation, but it might give you a few more ideas.
The feeder is just a couple of pieces of 3in exhaust pipe-- an 18in straight piece and a bent slant tip. I made a couple of cuts on the end of the tip and bent the center piece into a crude peg shape.
The bottom of the pellet feeder is an iron trivet with three 1in bolts as legs.
The peg rests in one of the holes in the trivet. The edge of the feed tube helps control the flow of pellets.
The stove's outstanding draw prevents the feeder from backdrafting.
The barrel is a 7 gallon steel heavy gauge with a ring clamp lid. I picked it up at a local salvage yard.
I'm about to experiment with insulating the feed tube on this one-- I've got some ceramic wool and S/S band clamps that should do the trick. With the pellet feeder, it's not an issue, but when I burn it with just wood, the tremendous heat ignites all of the fuel in the feed tube all at once and causes the system choke itself down with embers.
I haven't used a stove like this for an extended period of time, and this one is only a couple of months old, but I know that the stovepipe doesn't hold up very well at the bottom edge of the feed tube where temps are highest. The barrel, however, is far more durable. Using the pellet feeder exclusively would make a system such as this last much longer-- the feeder setup keeps the fire in the center of the barrel and maintains a consistent burn rate. Although it remains to be seen, I suspect that the feed tube will hold up a bit longer when insulated.
This little stove just has a chimney, not a heat riser, and insulating it would defeat the purpose since a lot of radiant heat is emitted from the chimney itself.
I would not consider ones of these stoves to be as efficient or hold more wood than a "conventional" RMH, but they do produce high enough temps to result in a very clean burn.
Check out this thread to see the small format RMH I built that includes a pellet feeder.
The evidence is clear, look at the chart, insulating bricks insulate. Their rate of thermal conductivity is determined by their composition, not their mass.
You're creating an issue where there is none and hence, creating confusion.
Fire brick, wood stove splits, smelter brick, kiln liner, foundry brick, all have very similar composition... So similar, in fact, that they have been given one category and have been assigned one value on the chart.
You're splitting hairs on a bald head... A red herring... There is no cheese down this tunnel.
The only differentiation between types of brick are "dense" and "insulating". These are obviously two highly generalized categories, but it's safe to assume that all types of fire/kiln/smelter/foundry brick have been lumped into the "insulating" category, regardless of of size, density or application. Therefore, all other types of non-insulating bricks have been categorized as "dense". The values for each are what we would expect-- bricks that are intended to insulate do so, and all the rest do not-- or at least, not as well. These two values also coincide with my qualitative analysis.
The next two lines in the chart are referring to "brickwork", which we can only assume to mean some form of brick and mortar construction, be it a wall, column, etc. The differentiation of these two categories, "common" and "dense", is between the types of work, not the types of brick.
Red brick, old or new, is full of iron oxide and will conduct heat faster than any fire brick. In all of my experimentation with building and burning cores, I found that red brick, no matter what variety, did not preform as well as fire brick. Out was hard to achieve high temps and I had several bricks fracture from the heat stress.
The fact of the matter is that all fire brick is mainly composed of paloma grit which is mostly alumina and silicon dioxide. It's this specific composition that gives fire brick its ability to withstand higher temperatures and conduct heat at a slower rate. The slow transfer rate is essential within an RMH core where higher temperatures are desired. I built two cores using foundry bricks (the "heavy" kind) and not only did they perform beautifully, the bricks showed no sign of fracturing or surface breakdown.
First of all, it only reflects a certain amount of heat back into the system where you need it the most, while it conducts and transfers the rest of it elsewhere. Enough of the right kind of insulation will let you achieve the higher temps, but when you do, the other problem rears its ugly head: reduction and spalling.
Here's a picture I took of the inside of a 4.5 inch system after it been in constant use for 6 weeks...
The feed tube, burn tunnel, and the beginning of the riser are refractory brick, but the rest of the riser is a piece of schedule 40 steel pipe. The riser is insulated with 2 inches of a tightly packed refractory compound made up of fireclay, perlite, silicon dioxide sand, and alumina fibers. You can see the spalling of the steel sticking out like a bunch of steel cornflakes, creating a ton of drag on the system. The stuff that looks the same thing going on on the firebrick, is just excess fireclay slip from the assembly of the core. As you can see in this photo, it knocks off easily and reveals smooth firebrick...
The metal spalling, however, does not come off so easily, and merely continues throughout use. Here's a picture taken from the top of the riser after about 4 months of constant use...
As you can see, the insulation worked quite well, because i was able to get hot enough temps at the top of the riser to blister and spall the steel.
Metal seems like a quicker, sensible option at first, but in the long run, it will fail.
I did the roll-over crimp with a hammer and a pair of channel-locks...tap tap tap, crimp crimp crimp, done!
The reasoning behind the smaller chimney is 1) it's what I had on hand; 2) I built another one of these back in March, but a 6in chimney-- I didn't like small gap between the two holes in the lid... This version is a but more sturdy; 3) when burning, the CSA of the feed tube is greatly reduced due to the presence of the fuel.
Here's a video of a guy who has built something that's almost exactly like what you've drawn...
This video shows some of the limitations of his design
I understand that you want to be able to incorporate the actual smoke from the wood into the baking chamber, so as to impart flavor, but I think the purpose of the "two-barrel" design is to allow for the use of scrap or less desirable wood as fuel, thereby protecting the food from any foul aromas. However, with the "two-barrel" design, you could simply place a few small chips of whatever wood you like in the bottom of the baking chamber (have you tried pecan shells? --fantastic!)
I did make a larger pellet hopper that allowed for a 3hr burn time, which worked rather well as this system proved itself to be more of an 'on-demand' heat source rather than a mass that would deliver heat over an extended period of time. The whole construction held up rather well throughout its use, but i am sad to say that it is no longer in existence-- as you will find out in reading the above post, it was largely an experiment to prove the viability of a sub-6" system and I have converted the living room fireplace to a pseudo-rumford style. I have kept the riser and barrel/manifold and plan to use them to construct a similar system in a future wilipini/greenhouse.
This season's project will consist of a 6" system in the basement with an isolated mass and a forced-air heat exchanger that will be able to deliver heat throughout the house per the demand of the thermostat in the main living area. I am still gathering materials, but construction will begin soon, so be on the lookout for a new thread.
A heat riser of appropriate height, with a consistent CSA, is one of the most essential elements (if not THE most essential element) that is required for a rocket stove or rocket heater to function properly. It is the specific dynamic of expanding hot gases rising in a smooth-walled tube of equivalent diameter that gives you the powerful draft engine that generates the vast majority of draft velocity in any 'rocket' system-- or in just about any other wood burning contraption or construction for that matter. By changing this essential element into a funnel, you eliminate this dynamic. It's a basic principle of fluid dynamics that inverse flow through a funnel (from the smaller end to the larger) will set up turbulent flow. Streamlined flow is most desirable in the riser, while turbulent flow is most desirable in the combustion point. It is the streamlined flow in the riser that provides the high velocity draft at the combustion point, and turbulent flow in the combustion chamber that ensures the most amount of all available fuel is burned in the hottest part of the system. Think of it this way: it's not the fire that's pushing a system, but rather, it's the exhaust of the fire that's pulling.
Ask yourself this: are there any other examples of a chimney, on anything, anywhere, that increases in diameter from bottom to top?
No. If anything, there is a plethora of examples of chimneys, throughout the ages, throughout the world, where the diameter decreases to ensure proper draft.
Lucy Guss wrote: Is your exhaust pipe run completely up the chimney or just the 12+ inches higher than the barrel?
I'd love to make it longer, but 12" is as far as I can extend it without cutting a notch in the iron flue of the fireplace-- it only has a 3" opening
My chimney is a metal chimney due to the nature of the fireplace. Any thoughts on the degree of heat/creosote coming from the exhaust as problematic for this?
The barrel exchanges most of the heat into the room. With mass around the exhaust, I could probably have exhaust temps around 140-150F; right now they're ~200F when the stove is at temp.
I do not see a mantle in your photos. Any thoughts on how hot the tile behind the unit is getting? Though the hearth itself is brick, the facing on it is some funky faux brick with a mantle above.
The house is over 100 years old and that's the original fireplace-- well, what's left of it. It's made of brick and is quite stout since the backside of it protrudes 12" into the bedroom on the other side of the wall. There is a mantle about 24" above the top of the barrel. The fireplace actually has a rectangular opening that is a few inches higher than the tile facade. I have to keep the kettle of water on top as a heatshield for that horrible piece of wood trim that some foolish previous owner DIY'd around the fireplace --so, yes, it get's pretty toasty.
Feng Nie wrote: When the fire die down say when you want to clean the ash out, do you have a lot of smoke coming from the wood feed??
With the reducer improvement, now I don't even have to worry when the pellet feeder runs out because the draft is strong enough to suck the smoke right off the top. But, I clean it out in the morning before I fire it up --hardly any ash. Even then, I have to open the feed tube and let some air draft through it for about 2hrs before it's cool enough to stick my hand in there. I went three days without cleaning it, just to see... burned a total of 40lbs of pellets and about 4cu. ft. of wood... pulled out about 1cup of ash.
Even though this system has been developing under the posted heading 4.5" rocket stove with pellet feeder and ashbox , I felt it necessary to re-post it under this new heading because it has truly evolved into a successfully operational mini rocket mass heater. Plus, for those who are struggling with the need for small footprint/low mass/limited output applications, I think it might be more useful to post something that concisely presents all of the pros and cons of such a system, design parameters, and a clear illustration of what I've put together.
To begin with, I would consider this system experimental, but completely functional. In addition, although my application is somewhat unorthodox, I think there are some aspects that can be generally applied to smaller systems --if not simply encourage others that such a system is possible. However, there were a few definite operational challenges that I suspected during the design and mock-up phases of this project, that have been confirmed during installation and optimizing...
--> The exhaust must have a very limited horizontal run.
--> The exhaust requires a vertical chimney that is at least 12" taller than the top of the barrel.
--> 4" is the lowest limit to which a properly drafting mini-rrmh system can be scaled.
--> The smaller combustion area requires a grate, ash collection, and adjustable bottom draft to prevent occlusion from cinders and maintain proper burn temp.
As far as the limitations on a horizontal run: I would have to say that a lot depends on how tall the vertical chimney is as to how long a horizontal run can go, but not a lot can be expected from a 4" system; plus, you can't extract all of the heat or the gases will stall out when they try to go up. On the whole, you have to sacrifice a little bit on the efficiency side of the equation in order to have enough heat leftover for the vertical chimney to do its job.
Bottom draft is a tricky subject and can easily backfire on you (pun intended ). Sor far, I would have to say that a good rule of thumb is that the total CSA of the combined feed tube and bottom draft openings cannot exceed the mean CSA of the system. As a practical example, this system has a feed tube opening that is 4" x 4.5" and a draft opening that is 4" x 0.5", but they both can't be fully open at the same time. The only times I use the bottom draft is either when I'm using the pellet feeder (which takes up half of the feed tube) or when I'm shutting the stove down-- whereby I close the lid on the feed completely and leave the bottom draft open to burn out whatever is left. I'm definitely getting good temps because this is my grate after 1 week:
This spalling is not an issue as i have designed this to be cheap and easily replaceable. A short piece of HSS 2x4 is easy to come by at the scrapyard... 10 min with the angle grinder... done.
Here's a SketchUp of what's going on... [the fireplace is only represented by the dimensions of its interior]
As you can see from the drawing, what makes this a "mass" system by default is the fact that I've integrated it into the existing fireplace --which happens to be constructed atop a 3'x3'x5' concrete block. Unfortunately, the major detractor to this huge block is that it's in direct contact with the ground-- essentially making it impossible to fully 'charge' it with thermal energy. But, since concrete is so slow at conducting heat, I've found that after a long (6-8hr) burn, I can sort of charge up the top half and get some residual heat, but it's mostly delivered right back into the core of the stove. I have a pile of brick rubble and big pieces of basalt that are supposed to fill in the exhaust side of, as my wife refers to it, “the big brick box” (-- she's not too fond of my design), but I was waiting until I'd worked out all the final bugs with draft.
Here's the list of critical dimensions (in inches):
feed tube opening: 4 x 4.5
bottom draft opening (w/ ashbox in place): 4 x 0.5
burn tunnel: 3 x 4.5
total burn tunnel length (from front of feed tube to back of riser): 13
length of burn tunnel ceiling: 4.5
bottom course of riser flue: 4.5 x 4.5
second course of riser flue: 4.25 x 4.25
third course of riser flue: 4 x 4
riser: 4 (diameter) x 17
total riser height: 27.25
interior barrel dimensions: 14 (dia) x 18.25
top gap: 1.25
manifold opening: (circle segment) 10 (chord) x 4 height
exhaust: 4 (dia)
The original intent of this system was supplemental heat, but so far, when I go toe to toe with the gas furnace, I win. Prime example: got home last night from picking up the wife and kids; my wife went straight to the thermostat and turned it up a little to take the chill off the house; I went over to the other side of the living room and fired up the rocket; then, about halfway through what would be expected as the typical run-time for the gas furnace to bring the temp up 3 degrees, the furnace shuts down-- I was able to heat up the living space of the house faster than the central heating system!
Optimizations after installation consisted mainly of futzing with the exhaust; shortening the horizontal run, removing a 90degree turn, and keeping the smoke path above the negative pressure plane of the system. But the biggest improvement to the draft velocity was installing this reducer plate at the bottom of the burn tunnel:
The bevelled edge catches coals and pellets that fall into the burn tunnel and holds them against the slots on the side of the grate where they are burned completely, but more importantly, the height of the plate reduces the burn tunnel from 4.25" x 4.5" to 3" x 4.5". Decreasing the CSA at the point in the system where the gases are expanding increases their velocity. This is the basic principle behind any rocket engine used for propulsion. So far, this one change has provided the greatest improvement to the system's overall function.
Here's a video of it burning with the new reducer installed:
Now for the bad news.... it can't stay. I promised my wife that this was an experimental installation to prove the viability of such a system and that as soon as the burn season was over, I would disassemble it and rebuild the pseudo-rumford insert that I had mocked up in the fireplace over the holidays (she misses the aesthetic of an open fire). BUT, now that I have proven to her that I can effectively heat the house with a scaled down unit, I have been given the go-ahead to develop a 6" system that can go in the basement and tie into the central heating system. WOOHOO!
I finally got the time to conduct an outside test burn of this entire system's core components and I would dub it to be a definite success. I lit it at 7:15 pm, with the outside temp at ~38F and the clay slip and perlite mortar still wet. It drafted immediately and never looked back. I burned it for over 7 hours without any smokeback or flame up the feed tube. It purred right along on a mix of fuel-- first pellets, then pellets and wood, then I took out the pellet box and burned a mix of oak and pine. It burned nice and clean most of the time, but it took about 3 hrs to cook out all of the moisture before it really took off. The temp on the top of the barrel slowly climbed to ~550F, until it dried out and I switched to wood, then it shot right up to ~825F. I plan to encase the entire exhaust within some type of small mass... I just haven't decided what to use yet. The riser is what I would consider temporary (4"dia. by 17" sched40 steel wrapped with ceramic (alumina) wool-- total outside diameter: ~10"). It's offset with one side in full contact with the inside of the barrel on the feed tube side. Not only is this structurally necessary, it keeps the feed tube side of the barrel cool with it being so close to the fire. The permanent riser will be far more substantial so that when the steel burns out, there will be a refractory conglomerate behind it.
Have you seen this diagram? As far as dimensional ratios, this image has it all in a nutshell.
NOTE: for some reason, the "D" dimension that is mentioned in the formulas, is missing or obscured in the drawing-- it represents the length of the burn tunnel.
your calculations are correct and, yes, you're going to have to adjust the gap down to get things moving along properly.
think about the fluid dynamics at that point: you've got screamin' hot gases and flames that are being 'pushed' up by the draft engine of the system, your heat riser, but you want them to head in the opposite direction. with a gap of 4", you are giving the gases too much room to whirl and whorl before they reach the outer edge of the bell and start heading down. all this fluffing about makes the gases lose their velocity and get, well, confused. the small gap is needed to get your difference engine to actually do the extra work of pulling your gases down. instead of letting them swirl about for a bit, the small gap sort of 'peels' open the flow and makes it spread evenly across the top-- kinda like the end of a trumpet. this makes the gases flow smoothly to the outer edge and start flowing down the sides where they cool off and continue to fall. the small gap helps these falling gases 'pull' the system along.
John Eee wrote: I have a insulated shipping container with aluminum t bars as the floor.
Sounds like a dream come true...
I doubt that the inherent design of a refrigerated shipping container would include the contingency of thermally conducting the heat from a rocket stove directly into the floor. I would go with your original plan of a wood floor and isolate your mass.
So, after burning pellets non-stop for 6hrs without ever slowing down, I took out the pellet feeder and switched over to pallet slats...and that's when she really took-off ! It gladly self-fed full loads of 2' sections without a single flame up or smokeback-- and halfway through the second load the steel riser finally started to glow red hot! I tried to take another video, but my little camera is apparently not sophisticated enough to capture the subtlety of glowing red metal in the dark .
So, after burning pellets non-stop for 6hrs without ever slowing down, I took out the pellet feeder and switched over to pallet slats...and that's when she really took-off ! It gladly self-fed full loads of 2' sections without a single flame up of smokeback-- and halfway through the second load the steel riser finally started to glow red hot! I tried to take another video, but my little camera is apparently not sophisticated enough to capture the subtlety of glowing red metal in the dark .
here's the same 4.5" system, completely re-built, with a new HSS 2x4 grate/sleeve for the ashbox. naturally, the old lid i was using for a grate was getting a bit haggered-- but it was tin-snip thin. still, it was holding up over about a total of 30-40 hrs of burn time.
this new grate works even better with the pellet feeder...nice consistent 'rockety' fire.
this is going to be the core of the small system i'm going to build into the fireplace in the video.
very interesting...I like the simplicity of the idea, if not the rustic antiquity.
however, I wonder how much of the smoke is actually burned off as opposed to how much is 'filtered' by the rocks. if the ones on the top are covered with that much soot, then it would seem that not all of the smoke is actually being consumed by fire. yes, the exhaust cleans up and becomes clear, but with all the carbon deposits on the rocks closest to the top of the riser, I wonder how much CO and methane made it out alive.
still like it, though...you could definitely scale it up into something like a low-brow masonry heater-- big brick box full of rocks with a firebox... say that three times fast;)
Andor Horvath wrote:Interesting design...might want to consider something heavier (oxy/acetylene tank or similar) for the upper radiant "barrel"
While that would give it longevity, and a bit more thermal transfer (although I think surface area would be quicker multiplier-- I've thought of using a piece of 10" stove pipe, but then I would have to have an offset riser and, it begins to get too close to the feed tube opening and, I would possibly have to incorporate a heat shield of some kind, which wouldn't be the end of the world), but I guess one of the running themes with this project is portability so, the lighter the better. Also, as I mentioned before, this is not what I would consider a refined rendering of the intended design-- I'm considering having the junction of the 8" pipe to the cast material be gasketed such that pipe can be removed and replaced when necessary. The structural steel channel that is the ash box/grate for the coal pit, is also removable for replacment when it burns out.
and perhaps you could clarify - is the lower barrel "full' of insulating refractory?
I don't understand the junction between "barrel" and exhaust.
The squeejaw way the elbow exits the barrel? As far as the angle at which the elbow exits, once again, not a perfect drawing. For the most part, it's proportionally correct, but not what I would call positionally correct. Things like...
...the ductwork on the bottom (which I might do with 4" flexible exhaust tubing, the automotive kind, instead of 4 elbows-- not only for a smoother path for the gases, but to be able to line up with the side of the barrel exactly and not have to cut some eccentric opening. I will probably also use a more serviceable joint at the exit of the barrel-- like a class B snap-loc),
...or the way the gases transition from the bottom of the upper "barrel" into the ducting through the lower barrel (this will be much less clumsy than it appears as I will be cutting and bending a 10"x4" 'register box' to create a much better junction),
...or the distance to which the stove pipe "barrel" extends into the cast material (this will be much shorter)
...or the way the feed tube has the just slightest taper towards the opening (this is just the way SketchUp warped the tube when I 'rotated' the end to get the bias-- I'm sure there is a much better way to do this, but it was the intuitive solution at the time. In reality, I'm actually considering making the feed tube portion of the form be shaped on top of a firebrick split so that the opening will have a flat bottom and a round top, kinda like a train tunnel. This will not only give the feed tube greater durability, it will be a better flow transition at the coal pit)
...all of these things need to be hashed out in a new drawing. As before: this is just to get the initial idea across...I'll be posting a revision soon.
Or the T-junction? (which is what I think you are referring to) The tee will have an endcap on the bottom and is intended as a fly-ash/condensate catcher and clean-out.
A heavier section top "barrel" could also be augmented with some added fins or pipes to assist in heat dissipation.
[Now you've got me thinking;)] That's actually an excellent idea to add to this design and I already have visions of cutting aluminium channel dancing through my head. I'm gonna have to find somebody with a drill press and a bandsaw that I can use...
Another approach might be to extend your assembly vertically, imagine another barrel acting as mass/bell under what you have drawn
While that is entirely possible, since I do have two of these old CD water barrels, and each one has a nice recessed lip on the bottom that fits perfectly into the lid of the other for easy stacking...I promised the person that gave me these that if the first stove works, I would make him a second one in the other barrel. I mean, these are pretty cool. They both still have the complete paint-stamped "Civil Defense Department" instructions on the side for how to use it to store and dispense drinking water...or use it as a commode ;P The real cherry-on-the-cake subtle irony with these is that they were manufactured by Rheem!
...yeah, I know...endless possibilities!
True! While scrounging at a local place that has a lot of smaller sized steel barrels, I did see a malt syrup barrel that was the 14"x35" size (22gal?) ...tall and narrow... it would be a bit of a squeeze at the top with a 'V-tube' style rocket, but it might work.
...oh, I'm gonna be spending a lot of time on SketchUp...
I have, and I would have to say that that there are a few other similar 'radiant bell, no mass' designs that I have seen, but another critical point to this project is to have a self-contained, radiant-mass system that will still deliver heat after the fire has gone out. The other plus to this design is give the unit a better 'safety zone' by essentially having the stove be the mass and giving it a slower dissipation rate. It remains to be seen what kind of burn time/heat retention/hot spot dynamics it will have...it may be a smaller system, but I do expect it to burn quite nicely. ...and, not to be a broken record, but: portability.
Nice concept, you've got us all thinking; I'm working on going with smaller CSA's too, hope to post some pics soon
Thanks, Andor. The sharing of ideas always moves things along faster than the mulling of ideas ;). I hope to see your pics soon.
There have been a lot of individuals that have expressed interest in a sub-6" system. Either for space concerns, heat output/BTU, or portability.
I think that such a system can be made to work. I know that there are concerns with such issues as draft velocity, laminar restriction, gas friction, etc. Plus, at what point does the idea of down-scaling the principles of an RMH become a moot venture simply because the reduced output of a smaller system would take longer to 'charge' a thermal battery anyway, thereby requiring the same amount of fuel as a slightly larger system.
Bearing all of this in mind, I still think that such a system has merit and that a lot of these perceived limitations could be designed and tinkered out.
I think the first place to start is to re-think the format a bit. Rather than trying to simply design a smaller version of the 55-gallon, two-ton mass, cobbed-in bench style system, the 4" RMH is going to need a different approach-- one that accommodates for the smaller system's shortcomings by reducing its limitations yet still create a robust system that could effectively heat a <500sq.ft. area, maybe slightly larger.
I've been banging this whole concept around in my head for months now... I've gone through good ideas, bad ideas, different sketches, possible solutions, possible failures, built many little J-tube and L-tube stacks and risers in my back yard and fireplace, ranted on and on about Bernoulli, Heisenberg and Venturi to my eight-year-old while driving him to school... and after all of that, I think I have something worth kicking around the table. I finally took the time to put something together in SketchUp so that I can throw it out here and see if it floats .
So far, this project still seems feasible, and I've amassed almost all of the necessary materials, so the first phases of construction are not far off, but I think open discussion is in order before that takes place. Of course, I will document all of the construction here as this moves forward.
In my experimentation with burning in small draft systems, the major issue I noticed with all of them is ash build-up-- smaller CSAs become occluded quite easily. This convinced me that a smaller system is going to require a grate/ashpit of some kind and an effective ash collector.
I also noticed that it is quite easy to choke down a smaller system while trying to get enough fuel into the feed tube to achieve high temps and a good re-burn in the riser and that some type of draft control and/ or draft induction would be needed to ensure proper draft and maintain higher temps.
Now, the main case brought against a 4" system is that it doesn't have the oomph to drive a downdrafted exhaust through a horizontal bench mass to achieve the same efficiency as a typical 6" system. Well then, here's my solution: quit expecting too much from a 4" system-- decrease its mass load, decrease its horizontal run and give up a bit on the efficiency side of the equation. Look at it this way... if the typical answer to a small footprint/small space/lower output problem is a 'pocket rocket', which is much less efficient than an 'average' RMH (if there is such a thing:)), then there has to be some middle ground.
First off, let me say that I just got my hands on SketchUp, so I'm no master and some of the more subtle details are not hashed out in this model, but this is good enough to get the main ideas across.
I have a 16" x 22" steel civil defence issue water barrel that is the intended container for the project. Everything in the barrel will be cast inside of a mix of fireclay, alumina fibers, sharp sand, and perlite. I've test fired this material and it comes out light and hard. I have no real idea at this point what the final mass will be, but I'd be willing to shoot in the ballpark of <100lbs. The parts that are to be the feed tube, burn tunnel, coal pit, and lower part of the riser, will be made from a cardboard form that, after it is cast into the system, will be burned out before the actual heat riser is installed. The heat riser will be made of alumina/fireclay mache wrapped around a piece of sched.40 steel pipe that may, or may not, burn out, while everything else will be steel 4" duct. You can see that the feed tube and the burn tunnel have been rotated 45 degrees: this is so the system is somewhat self-cleaning. Ashes and embers fall into the coal pit where they are allowed to burn completely before sifting into the ash box which will have a removable drawer for easy cleaning during operation. There will be a cover of some kind on the ash drawer to control the bottom-draft. The radiant exchange will be a section of 8" single wall black stove pipe with an endcap. The exhaust chimney will be a 6' section of double-wall classB gas vent.
Jeremiah wales wrote:After all of the information I have seen. Rocket Mass Heaters seem to work best in very small homes. Even one room Homes. What is created is a Lizard Rock that people put in the one room and are able to keep warm as the most important part of the one room home is the Lizard Rock.
Please dont take it wrong or be offended, Just see so many people focus on laying on the Mass or sleeping on the mass. It works like the Hot Rock I kept in my Snake Tank years ago.
This is where you are misunderstanding...
Just because you see pictures of people sitting or lying on the thermal mass, does not mean that the only way that heat is transferred out of the mass is through conduction. The mass is radiating heat, just as a wood stove's primary method of delivering heat is through radiant energy-- it's just that a wood stove has a limited amount of surface area and operates at very high surface temperature, so there is no way you would take a seat on one.
A thermal mass is also a radiant heat source, just with a larger surface area and a lower, but consistent, temperature. People sit on them simply because they can, not because the have to.