i put together a rocket stove, it is not installed yet, the barrel is 14" by 42" high, the riser is 6" diameter x 32" high and 11" diameter around filled with refractory cement., also the whole bottom of barrel is filled with the cement up to the bottom of the 6" exhaust near the bottom of the barrel and over the burn chamber on the opposite end of barrel. I have 2" clearance on the top of the barrel to the tip of the riser. The burn chamber is 5" diameter by 20" long and curves up at 90 degrees to meet the 6" riser. the woodfeed area is 7"diameter by 20" tall. When I lite the wood it tries to burn upward instead of being sucking into the horizontal 5" burn chamber and smokes. sometimes it sucks but mostly not. I have a 90 at the exhaust with a 5 foot chimney just to try it. Any ideas as to why its not rocketing?
Sounds to me like the geometry is wrong. Too tall of a feed tube for the riser, and not enough open space in the larger barrel. I these are the most obvious problem areas to look at. Read Evan's book for the proper geometry. The summary is on page 28, if I recall correctly.
Filling with concrete is not the best idea either, although that may not become an issue until it has been used for a while and the concrete begins to fail under the high heat. Basically as the temperature approaches the boiling point of water, the water inside the concrete must be able to expand and escape, and that usually means breaking the concrete.
EDIT (2nd edit for typo):
Just caught that the "cement" poured in at the bottom of the barrel is also refractory. As already noted, there are no overheating concerns in that case. A small point, would be to observe that Peter VDB has mentioned this manifold/transition area works more efficiently is there is a small height difference between the bottom of the barrel and the outlet; I'm thinking on the order of two inches, but I expect that is a ratio of something.
To know of if the area surround the fire riser is sufficient, you need to calculate the CSA. There must be as a minimum the same CSA of the fire riser as is available to flow down the sides of the barrel, and given there is more surface area, and therefore more friction against those sides, somewhat more would seem to be logical. The same is also true of the area immediately above the top of the fire riser and the top of the barrel.
Which makes sense. If either of these areas are smaller in CSA than the fire riser, there will be a constriction or bottleneck of the air flow.
ok thanks Erik. from what I read as long as the area between the barrel and the riser is more than the area of the riser inside diameter, it should be enough which it is? Also the riser is suppose to be twice the burn chamber length which it is. will try some changes.
Also, the feed tube being so much larger in diameter than the burn tunnel means that inlet air is moving relatively slowly downward, not fast enough to pull heat/smoke that want to rise down with it. It is recommended that the whole system be about the same diameter, with the burn tunnel slightly smaller if there is any difference.
It is also recommended that the burn tunnel be square, not round, with a sharp angle where it meets the riser. This promotes turbulence and fuel/air mixing better than smooth round channels. Better mixing means more efficient and complete combustion, thus higher temperatures and better draft.
You said "refractory cement", so heat resistance is not an issue, but is it insulating refractory or regular? Regular refractory will be more durable in the feed tube and burn tunnel where abrasion is a factor, but it conducts much more heat away from the combustion area, keeping it from getting as hot as it could. In some cases this may be a problem. The heat riser is best made from insulating refractory.
It is also very important that the riser be 3 or 4 times as tall as the feed tube. The original ratio recommendation was 1:2:3 feed tube : burn tunnel : heat riser, but recently a 1:2:4 ratio has been promoted. This gives a bit more room for error in various locations.
Thanks for your help fellos. I am wondering, the square tube vs the round tube for the Burn tunnel, has this been tested or is it just theory? From my understanding, if you can get a vortex in a round I believe it would mix better and flow better than square?
Hi Gino. It is good to see another person crafting their own Rocket Mass Heater (RMH). Since you stated that this is not installed yet, I will assume that there is not a significant amount of horizontal run nor elbows between the chimney and the barrel. Any chance that you have some pictures from construction? These can help us understand a lot about the system you have built. A combination of factors may be working against the draft right now.
- As the others have stated, the feed tube may be too tall. It's starting to compete with the Heat Riser for creating the draft. Now the smoke does not know which path to follow. The 13" diameter is not an advantage here, but probably not a source of trouble either, just the overall height.
- What is your fuel like, hard/soft wood, scraps, damp? J-tubes tend to like many small pieces of wood than a couple really big pieces. Once the burn tunnel is at its hottest larger wod will do better in the feed tube.
- Transition (manifold) of barrel to 6" horizontal run. The first book on RMH's did not cover the importance of making the transition from the barrel to the horizontal run nearly enough. This area needs to be much larger to make for an easy transition of all the gases from around the barres into the horizontal run. Some have built shrouds that open up more of the barrel's diameter, so a shape that is wider and short. Others have used a reducer for the next size larger ducting to bring it down to the size of their horizontal run, which here would be an 8" to 6" piece.
- What is the thickness of insulation around burn tunnel? This is not a big player in your draft issue, but is important for insulating the burn tunnel enough to get the super clean burn the RMH is known for. Granted, this isn't your permanent install.
We can help you get your test unit running. Thanks again for posting and partaking of the collective experience here at Permies!
here are photos, if you look at the feed section, i have a lid and air hole on the back so it should not make a difference how high? Yes I am only using a 5 foot chimney at the moment to try it out. will try some minor stuff to see if we can get it to suck before destroying it.
"if you can get a vortex in a round I believe it would mix better and flow better than square?"
In response to a suggestion to lead from burn tunnel to riser off-center so as to make a vortex, I believe it was Peter van den Berg who said that the single vortex does not mix as well as a double ram's-horn vortex. You will get a nice swirl, but unless there is a disruptor to make beneficial turbulence, it will be a smooth swirl which may not mix that much better than straight flow. The square corners apparently create significant drag/turbulence, bad in the riser for Peter's batch box designs but good in the burn tunnel. This is all from my reading of Peter's and others' results and advice.
Are you saying you take air from a hole at the base of the feed tube? If so, that will not pull air down through the wood, and as the wood gets hot, it will start burning up its length and putting out smoke which will not be sucked back down but leak out of the joint around the lid. I have read about someone who does that successfully, but he has a very small system (4" or less) where the total heat is less and it does not seem to have the energy to creep up.
From what i can see, flue transition area is no good. Needs to be enlarged.
The gaps might be a bit tight.
And the burn tunel is not insulated.
So, no Wonder it doesn't work.
Please buy the book, and forget about your metalworking skills. Don't Watch too much youtube vids either; Most are complete crap. They're very good at bragging about their wonderfull new stove with sooooooo many inventions. But fail to report of any flaws or failures! Which nearly all of thèses have.
I really appreciate your inputs and I thank you all, i do have the book, maybe my understanding is not the best of it but also are willing to experiment to see if it can be improved as there are infinite possibilities of designs. Will continue to play around when I get a chance, have my father in hospital at the moment.
Yes, there are infinite possibilities, but a ton of them have been tried already and found lacking compared to the "standard", which only became standard because it works very well. Experiment all you want, but first build one the standard way with standard materials so you really have something to compare your tests to.
And best of luck to your father.
Location: Southern alps, on the French side of the french /italian border 5000ft elevation
When i say, transition area, you just don't weld a tube to the barrel, but make a bigger diameter "plenum" to transition from the barrel gap, to the flue, as turbulence from the change of direction in the flow creates plenty of drag. Usualy, people say, 3x the CSA of the riser.
Carefull of metal building, it's only good for prototyping.
I hope you father gets better soon. The pictures are very helpful. You have good welding skills! I can see why you chose metal for the prototype. As mentioned above, an insulated burn tunnel and the lower part of the heat riser will degrade metal quickly in a "permanent" installation. But back to our discussion of getting your prototype rocketing!
I understand why your feed tube has the opening increase to 13". It's the shape of the preformed metal that you are re-using. If this larger 13" diameter starts to close to the combustion zone, the slower air movement in the large diameter area will make it easier for the fire to creep up the sticks/fuel. Typically the feed opening will be keep the same CSA as the burn tunnel along it's length. This will keep the air moving and cooling the fuel so the fire stays put in the bottom.
The first picture shows a small hole at the bottom in the smaller diameter area of the feed. I believe you had described this as an air intake. This may not be having the desired affect for your prototype. By having this opening it greatly lessens the flow of air coming down the feed. In fact, it makes it easier for the feed opening to act as a chimney leading to the smoke-back you have seen. Combining this with a tight fitting lid over the end of the feed opening has the potential of actually being dangerous. Let me explain the possibilities. With a tight fitting lid, as the fuel is consumed unburnt gasses and heat start collecting in the feed opening. If the unburnt fuel gets hot enough it could start outgassing. Combine that with enough heat and it only needs a breath of fresh air to ignite, like taking the lid off of the feed opening.
All is not lost. A simple two stage test can check this theory. First, block off the additional air intake at the bottom of the feed so the air will be pulled in past the fuel. You could try a small fire to test just this change. Second, if you have some additional duct/piping in the 5" diameter range, same CSA as burn tunnel, stick it down into the small diameter section of the feed opening. Also limit the length of this test feed tube to 1/2 - 3/4 of the height of the current feed opening.
These steps should change its behavior dramatically, even with the 5 foot test chimney. Try this along with some of the other users suggestions and see if the rocket comes alive for you. I look forward to hearing your results!
The metal plate bellow the heat riser. Did you cut it to have a proper gap? And metallic heat riser, well insulated. That ought to fail one day. You already have a two winters on it? Remove the barrel, and check the state of things.
Beniamino Carella wrote:Hi Satamax, I found the problem.
The gap between the riser and the top barrel was not correct. Only 1 cm..! I had to put away the barrel fot the measurment.
I shall ulpoad images later.
Can you help me? Testing my build and smoke comes out the feeder. Riser is curing, but wanted to test a burn with some newspaper before I weld on the rest of the tank.
All is 6 inch round ducts. The feeder and burn are thick steel. The riser has no metal, it's a mix of 7 parts vermeculite, 7 parts perlite, 4 sand, 2 mortar cement. It's only a day old in this video, so it's not cured, but it should burn properly, I think.
Riser is double the burn and double the feed tube in length, all 6 inch. I read above that the feeder should be 1/3 the riser so I'll shorten it and try again.
Please see picture and help!
Location: Southern alps, on the French side of the french /italian border 5000ft elevation
You have an uninsulated steel burn tunnel, which will never get really hot, and will keep the fire from getting as hot as it needs for full combustion. You need that insulated; but when you insulate the steel, it will soften and corrode at full burning temperatures.
I know lots of Youtube videos show all-steel or mostly-steel combustion cores, but that doesn't mean that they really work as well as all-refractory cores with lots of insulation.
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