Hello !

Ive been working variations of rocket stoves using 2inch black pipe for a burn tube with gravity feed pellet system , ive made alot of progress on the design and i would like to share it with the people at permies ! this project is open source ! check out the videos on my channel and enjoy ! https://www.youtube.com/watch?v=uNJjDkoLo6I

Thanks, Andy, and welcome to Permies!


I gather that most folks have found that steel will disintegrate rather rapidly in an insulated heat riser. Have you found this happening in your stove? Have you burned it many times? Have you attempted to measure the temperature in your heat riser (with a thermocouple, or pottery cones, or some other method)?

Thanks for sharing your work with us!

Mike

Hi Andy,

Thanks for sharing your build. I have an off topic request, could you rename your videos with an easier to follow naming convention? (I'm referring to the "part 1, part 2, part 1" uh, part, of the name.

Just for example-

Pellet Burner 3.0 Part 1: Materials Description

Pellet Burner 3.0 Part 2: Assembly

Pellet Burner 3.0 Part 3: First Burn

Pellet Burner 3.0 Part 4: Improved lighting method

Pellet Burner 3.0 Part 5: Autopsy

When there is more than one "Part 1" it's hard to tell which one is the "first" one.

Please do keep us updated, especially after many hours of running your pellet burner. As mentioned above, the insulated metal combustion area is likely to fail in short order. If it does, show and tell, so not to mislead your viewers into thinking that they can safely follow your design. With loose-fill vermiculite surrounding the combustion area, when the metal fails the vermiculite could fall in and clog the system causing the burn/exhaust to exit the feed tube... Dangerous. Especially if it fails after you've become too familiar/comfortable with its operation and so then leave it to run unattended, say, while you sleep!

Personally, I'm not a fan of anything under 4" and/or using metal for the combustion area being referred to as a "rocket stove". Not that I have any authority to decide such things. It's just how I feel.

im going for compact efficiency lightweight and portable heater that could be used in a deer blind , ice shanty , tiny house . steel doesn't disintegrate as easily as your saying , the melting point is 2,500 degrees F i doubt im reaching 1,500 in the burn chamber . not sure what temp my burn chamber is getting too but the most ive delivered to the exterior of the barrel is 500 degrees so far . its not going to melt that pipe or degrade it anytime soon ....nor the barrel for that matter and its working great ! check out the next video ~ https://www.youtube.com/watch?v=bk0Eulw4M-Y&feature=youtu.be

Andy Man wrote: steel doesn't disintegrate as easily as your saying , the melting point is 2,500 degrees F i doubt im reaching 1,500 in the burn chamber .

I'm afraid it does disintegrate that easily. We're not talking about the steel becoming liquid and falling down into a puddle. That's not what you need to worry about.

What we're talking about is scale, in blacksmith's terms. And that DOES happen under 1,500 F.

Here's the first video I could find for an example.

This guy is working that piece somewhere between 1300 F and 2000 F (you can only guess, because it looks different depending on the light). See the flakes falling off starting around 0:18? That's what you're worried about. You can only peel a flake off the surface of your steel so many times... before you don't have any steel left.

Firebox temperatures won't melt your steel, they'll scale it till there's nothing left.

So is someone using a hydraulic press to squeeze the last bits of heat out of their riser?

It's a bit tricky to compare metal being worked at a forge that is gripped, squeezed, poked and prodded to a stationary metal tube that only comes into contact with hot air/gas. I've been running my 1/4" steel tube for only two months now, but I can't detect any signs of wear, other than, a bit of rust. Wood is never perfectly dry, so there will always be some steam going through with the exhaust. Near the beginning of a burn or at the end, water inside the wood will turn into steam, but the riser temps won't be hot enough and it can condense. The fairly consistent presence of water against the steel is a much more likely candidate for long term deterioration of metal components than is high temperature flaking. But 1/4" steel can handle rust for a long time.

When I made my first rmh out of duct pieces, I melted one of the connectors I used, so I get that all metal isn't created equal. But there are tons of applications where steel is used at temps over 2000 degrees effectively over long periods of time without failing. Wood gasification is an example that is extremely close in activity to how a rocket mass heater works. The metal cylinder of a downdraft gasifier does as complete a burn as any rmh and it's entirely made of steel. Gasifiers are made for long term use, running 24 hrs/day without shutting off and lasting for years. If you're ever curious, check out the folks at All Power Labs (All Power Labs. They're a great group of engineers and fire geeks, all their work is shared on the internet and they only charge if you buy a pre-built gasifier from them. The steel they use isn't cheap, but it's gorgeous and it lasts. I've made friends with them so I go by once a month or so to pick through their garbage bin. That's where I got my steel riser and the stainless steel barrel pieces I'm trying to think of how to use in a project in the picture below.

Here's a link they give to a wiki page about types of steel, temp ratings they operate at and discussion of the main concern of oxidation. I've never once seen anything about flaking or scale, but maybe that's part of using stainless steel? It doesn't react with oxygen the way typical iron or steel does?
http://wiki.gekgasifier.com/w/page/6123779/Metal%20Alloys%20for%20High%20Temp%20Operation

um ...yea and it might also warp time space create a black hole and suck in a few critics too if im lucky , it works ... theres no scales flaking or whatever . ive been adding supplemental heat to my house with this thing for the past week burning sticks and pellets my only problem seems to be smoke back when its really windy and condensation dripping back into the stove through the chimney , got a few ideas on how to fix that . il keep it updated for serious inquiry . thanks !

Andy Man, please check "spalling" on this site. If it doesn't spall, it's not rockety enough. You most certainly will have unburned gases. I have melted a gaz bottle, about 1/8 thick. With a 4 incher.

Rick, i'd say the same about your build, if it doesn't spall, it's not rockety enough!


By the way, metal is doomed!

You've created the perfect lose/lose Satamax, if it spalls or flakes, it will fail, if it doesn't spall, it's not rockety enough. Mine hit 950 degrees tonight on the top of my barrel with a 5" gap, so I'm not too worried about my internal temperatures. If I put in dry wood, my exhaust is crystal clear, so I'm not worried about an incomplete burn. And while mine is only two months old, no spalling or flaking on my steel tube riser and I burn every night for 4-5 hours/night. I get that's a tiny amount of usage compared to years and years, so I'll keep reporting back. But so far, the data don't back up your theory ...

And what were you saying about melting a gaz bottle? You mean a metal gas cannister? Glass bottle? I've been meaning to test it out on some copper pipe. My is too hot now to get a piece of copper deep into the burn tunnel, but tomorrow night, I'll test it out. If I can do some serious damage to a copper pipe (melting temp for copper 1983 degrees), will that satisfy you regarding my internal temperatures?

Rick Frey wrote:My is too hot now to get a piece of copper deep into the burn tunnel, but tomorrow night, I'll test it out. If I can do some serious damage to a copper pipe (melting temp for copper 1983 degrees), will that satisfy you regarding my internal temperatures?

The hottest spot in my experimental heaters has been the bottom of the feed and beginning of the burn tunnel. Measured highest temperature: 1172 C (2140 F), much lower temps than that caused inferior exhaust results, according to my Testo 330 gas analyzer.
One important thing to note: the 1/8" steel duct did stay in one piece as long as there wasn't insulation around it. Adding insulation ramped up the temperature to well above 1000 C, the analyzer results were dramatically better and the steel corroded like mad. Conclusion: when the steel isn't spalling the results will be meager even when it looks good. Satamax is right I would say. For reference: read this webpage

It's not necessary to believe all the above, but in my humble opinion your heater could perform much better.

it's not only the temp.
it's temp + oxygen + turbulence + other aggressive gasses.
you can insert an iron rod 6mm with the wood sticks and measure it after a few days...

Hi everybody!

Rick, that's nice you can hit 950F°

Your heatriser must be 310 stainless, the one which seems to be holding the best. 1/4 should last a while. But it will fail.

Check this vid



We have proof that even stainless fails.


As for the gaz bottle, i mean butane metal canister, 13kg. I had made a cyclonic 4 incher, with the gaz bottle as the expansion chamber and neutral vane mixer. The top got soo hot that it collapsed Under gravity in one spot. That means that i was nearing steel melting. And i can tell you, that's no wossey metal.

After, believe us or not, that's your own choice. But please don't misslead others to believe that steel is good for the heat riser, when you only have two months of burning.

Hope this clears things out.

And please, do post a picture of the bottom Inside of your heat riser. So we can compare to your next one!

So you all got me curious and I took apart my heater today to see how it was looking. (See the picture of the riser below). There is clearly some marking, some bubbly looking protrusions. I could feel them with my hand, and they had discolored the metal of the tube. So at one level, I guess that's a successful test of my heater, that it's getting hot enough inside to do that to the metal on the riser. It was only happening on the first six inches of the riser, same like in the video that Satamax posted. It's obviously really early still, but I'm wondering if some of the problem folks have faced with steel is both the grade and the gauge steel they've been using. This isn't even fancy stainless, just 1/4" tubing, but that thickness leaves a lot of room for marking and not having any serious problem. Again, it's way early, so time will tell.

But, based on all of your dire warnings, and out of curiosity to try out my riser 2.0, I switched risers. The new riser is the same steel tubing (it was originally 50" so I cut it in half). But this piece I put inside a piece of 12" duct and packed 8:1 perlite/clay around it. I had used it a few times on some outdoor test projects, so this will be the first time using it for a longer burn in a traditional set up. My thought is, if you all are right and the steel falls apart, six months from now, a year from now, the perlite/clay will have had time to dry/harden and I'll have a 3" wide cast perlite/clay riser that should work great.

The only other goofy thing about switching risers, now that the insulation makes the riser 12" wide (6" steel tube with 3" of perlite/clay around it with a 12" duct as a form), it barely fits in my barrel. My barrel is 14.5" wide, so I've basically got it touching the riser in the back and the 2.5" space in front near where the exhaust leaves and goes into the ducting system. There's no combustion happening once it hits the top of the barrel, so, since it's all about directing where the exhaust goes, I'm thinking this will work, but we'll have to watch and see for a bit. It did heat up quickly and reached 780 degrees on the top of the barrel on a normal pile of wood, so we'll see how it goes after a while. The one concern, so far with the new riser, there's visible smoke coming out of the chimney. Not sure what's up with that. I'm pretty sure the wood I was burning is dry, I'll test it again in a bit with some wood I know is dry and see how it goes. Temps in the burn area were 1700-1800 degrees (using an infrared thermometer that's not rated for temps that high, so take it with a grain of salt , so I'm not sure what the smoke is from.

Rick,
Is there insulation around the whole of the core, the tunnel and the feed tube as well? Too little room around the riser could cause a flow problem, I' m afraid.

My core is a cast core, following Matt Walker's ingredient recipe and some instructions for a 6" core. The core material (1:1 perlite/clay with fiberglass and refractory cement mixed in) is pretty impressively insulated, even after 6 straight hours of burning, it has yet to even darken the wood box it stills lives in. That same 1:1 perlite clay material forms the feed tube, burn tunnel and bottom of the heat riser (around the first 6"). On top of that, that's where my 6" steel tube riser with the 3" of 8:1 perlite/clay insulation goes. Around that, I have a 30 gallon barrel (14.5" diameter) that redirects the exhaust to the ducting system. With my new riser, there's a 6" gap between the top of the heat riser and the top of the barrel.

From what I understand, after the air has left the riser, it's done combusting, and as long as the flow isn't blocked in any significant way to kill drafting, it doesn't matter much how it gets into the ducting. If the air flow is fine, is there any other concern about not having much space between the exterior of the heat riser and the barrel?

Rick Frey wrote: is there any other concern about not having much space between the exterior of the heat riser and the barrel?

Yep, it's called barrel gap, and has to be more than 1.5 inches and more than CSA, to take into account the boundary layer. And then there is the transition area or plenum. Which is very picky.

And you said it. The bottom of your heat riser is cast, no wonder why your metal heat riser upper part is surviving, usualy the spalling starts at the bottom, in contact with the flames.

I was expecting a 3.0 stove to have a sterling heat engine build into it

So that's another advantage of a poured core, that the first 6" of the riser are the core material as well which then potentially opens up other options for the rest of the riser

Regarding the barrel gap, I get there's supposed to be more air space between the riser and the barrel, but, the question was, if combustion is all done by the top of the riser, and the barrel only serves to direct the exhaust out into the ducting system, then why does it matter how much barrel gap there is? You could bypass the barrel and have the exhaust go straight into the ducting system if it worked for where your mass was located. Or is there some process important for rmh's that happens in the barrel other than directing the exhaust?

you can't directing the exhaust with no space.

There's a 2.5" gap in the front of the barrel where the connection to the ducting happens. The barrel is only 2.5" bigger than my riser insulation, so there's no room in back and very little on the sides (1.25" each side). There's enough room for the air to exit the barrel and enter the ducting, there's been no problem with drafting and the air is making it through the ducts and out the chimney no problem. So if there's enough room to direct the air into the ducting, is there any other purpose the barrel serves?

the air is making it through the ducts and out the chimney no problem

if there is no problem then there is no problem.

Rick Frey wrote:is there any other purpose the barrel serves?

Yep, cooling off the gases, so theses shrink into a smaller volume. You start at ambient temp, heat up o 900C° aproximately, or a smidge more in a finely tuned rocket. hen back down entering the flue at around 200C° to 400C° depending what core you have, 6,8 batch etc! The barrel can only extract a limited amount of heat in a given time. So the more powerfull the rocket core, the hotter the gases entering the flue. The more exposed metal of the barrel, the colder the gases. As some burry up to half or more of that barrel into cob.


Some say that the shrinking of gases contributes to the pump effect of the rocket.

This is not clearly proven as of yet!

metal after 10 days inside a feedtube
steel

In our rocket stove's initial incarnation, we used a thick iron heat riser, insulated with ashes. It only lasted a few months, before the bottom several inches got eaten away and vanished.

We used the remainder as a mold to make a straw clay riser instead. That only lasted a few months too before it broke, but since we have an open top of the barrel and put huge pots on it, it's possible that kindling stuffed down, or just things falling in, broke it. Now we've replaced it with another straw clay riser, and let's see, maybe the third times the charm!

shilo kinarty wrote:metal after 10 days inside a feedtube
steel

thought I would try to post your image here.....pretty impressive!

I have managed to do this to a barrel lately, transforming it into a Pocket rocket, and insulating it, with batt insulation, to burn the paint! Where the feed tube ends up, there's a good eighth of the barrel in that state!