Cast iron RMH burn tube

Hey I know this will sound heretical especially to Paul Wheaton who has a YouTube video saying not to use metal in the burn tube.

But I am a plumber and have access to cast-iron sewer lines and long turn elbows. So I’ve been wondering for my first build whether or not a cast iron burn tube with cast-iron riser would be acceptable.

I have looked all over and have never found anyone who actually made the burn tunnel out of cast-iron versus regular steel. Thanks to any contributors.

Hi Dan.

We love heresy. That's where progress comes from.

I think a lot of people will tell you that the iron will rust out in a hurry. They will be completely right if we're talking about conventional stovepipe. How thick are the pieces you're thinking of using?

My feeling is that whatever the thickness, you will eventually experience metal loss and failure. That said, if it's just a temporary structure surrounded by refractory mortars and clay, and if the metal isn't necessary for anything other than holding the exhaust path open while the refractory dries, or heat-cures over time with the firing of the RMH, what does it matter?

As in, if the masonry structure you're surrounding the metal pieces with is structurally and functionally stable, and enough on its own to not only support itself, but to carry out the function of an RMH, what does it matter if the metal that was once there rusts out, leaving only baked refractory?

-CK

The long turn elbow might not be the better choice,  hard right turns induce desirable turbulence in the flame path.

Uninsulated metal rockets can work,  they just aren't as clean or efficient.

Dan Sprenger wrote: Hey I know this will sound heretical especially to Paul Wheaton who has a YouTube video saying not to use metal in the burn tube.

But I am a plumber and have access to cast-iron sewer lines and long turn elbows. So I’ve been wondering for my first build whether or not a cast iron burn tube with cast-iron riser would be acceptable.

I have looked all over and have never found anyone who actually made the burn tunnel out of cast-iron versus regular steel. Thanks to any contributors.

Hi Dan,   My first rocket was made out of 4" metal square stock that I welded together, surrounded with vermiculite and roughly cobbed together. It was amazing to see flames burning sideways and to hear  the rocket roar with only heat waves coming out of the chimney. To experience all this first hand was the best stepping stone I could never have gotten from a book or video. Fast forward about 6 years with all kinds of improvements suggestions from this forum and obtaining a copies of Ianto Evans book and later on The Rocket Mass heaters builders guide by Erica and Ernie Wisner, my views changed once again.
So for your first rocket, I say go for it! Use what you have and gain your own experiences. However, one big suggestion would be to build and test it outside first in a safe place before even considering putting it into your house.  
As Chris pointed out, the metal is not going to last but it will give your enough confidence to go to the next level whenever and whatever that may be.

I've read a few of the books on building RMH and I have the DVDs
My main idea for the use of castiron in the build because of a few things:

The use of a long turn 90 would make it draft easier.
A 6" round has less friction loss than a 6" square does.
Cast iron is what wood stoves are made of.

I'm a plumbing and and HVAC tech. I live in a small town area so I get to wear lots of hats.

Also should have done more searching on my laptop as opposed to my phone. There are a few threads on this subject out there I failed to find. Thanks all.

Dan, the square elbow at the end of the burn tunnel is there to add turbulence, and increase the mixing. If you try to improve that area, it might well fail. Think of it as a carburetor.


Check this thread to get your head a bit more into fluid mechanics.   http://donkey32.proboards.com/thread/1425/flow-visualization

If your plumbing training has anything to do with European ones.  You most certainly have a grasp of fluid mechanics already.

Other than that. Your cast iron will crack, spall etc, if you insulate it as should a rocket be. It will live longer than steel tubes, since the carbon content is higher. But it will fail. For testing purposes. Cooking outside etc.

Or a heater, if you know what you are doing. I mean, have built few RMH before. You could use it, and make it, so it is easy to replace your cast iron core. But remember. Smokeback in a house, if it fails, could kill your loved ones, as fire back, could burn the place.

https://permies.com/t/52544/metal-burn-tunnel-heat-riser

Hi Dan, this must be one of very few threads that are actually encouraging the use of metal, certainly the majority of post on this forum discourage the use of any form of metal in the combustion areas!
Cast iron is really heavy and differcult to work with, the latest developments in this area use lightweight ceramic products.
As an example, you can use a thin wall 8” steel tube and line it with ceramic fibre blanket. This process takes but minutes to accomplish and is guaranteed  to out perform any metal.
I think that once you spent a few hours reading the forum pages and have a grasp of how important insulation and mixing gasses are, you may be dissuaded and decide not to use cast iron.

Cool idea but the thermal dynamics of the chamber won't be friendly to cast iron. Yes it will work and work well at first but the material will melt when it gets hot enough. Cast would have a nice rough surface for turbulence and is designed to stay rigid while hot up to a melting temp. These heaters can get hotter than that in the burn area. Then the want for a smooth inside to your heat mass would also suggest the cast is not the best choice.
Brad

Aside from the undesirability of a sweeping turn from burn tunnel to riser, and the certainty that the iron will degrade over time (how fast is not certain), the conductivity of iron will make it take longer for the core to come up to full operating temperature, as it will move heat in all directions away from the internal surface.

If you must use cast iron for your first experiments, I think you need to use tees rather than sweep elbows, with the third leg of the tee blocked to keep air flowing in the right direction.

But really, cast iron would find its best use in the mass; if it is smooth enough for water/sewage flow, it should be smooth enough for airflow. Wide sweep elbows would be a benefit in heat exchanger ducting where change of direction is needed.

I used to be a plumbing tech(not a plumber).
Cast iron weighs so much that a full stack was worth the trouble scrapping.
Maybe scrap the cast iron and use the cash for materials and beer?

My turbo diesel truck is 27 yrs old, has done 490,000km and has its original exhaust manifold, which at an educated guess is cast iron. Do rocket stoves get hotter than turbo diesel engines?

Steve Farmer wrote:My turbo diesel truck is 27 yrs old, has done 490,000km and has its original exhaust manifold, which at an educated guess is cast iron. Do rocket stoves get hotter than turbo diesel engines?

Yes absolutely!

An angine is cooled by water or coolant. Actually to survive all the problems of expansion, spalling, etc. The most metal in your engine doesn't go much over 100 celcius.

Satamax Antone wrote:The most metal in your engine doesn't go much over 100 celcius.

I don't dispute that, however *some* of the metal, especially in the cylinder and
exhaust immediately behind the engine gets way above that. The water cooling keeps the average heat of the engine head down but it also means the materials in the engine are dealing with massive variations in temperatures eg fuel air mix combusting at thousands of degrees just a few millimetres away from a water jacket at just below coolant boiling point. Are you sure that my multi layer steel head gasket isn't seeing higher temps than the materials a rocket stove is made from?

Your engine will never get extreme heat like a rocket stove can produce. Consider that iron and steel start to glow red on their own at or just below 1200 degrees F and you have never seen any of the motor glow red. If you did it was right before it exploded. Now I regularly can achieve over 1600 degrees just in the burn area around the burning wood. You can do a test yourself by putting the end of a straitened coat hanger down into your fire and let it soak for a minute. Pull it out and it will be orange. Dull orange is 1400 degreesish. A bright orange and you are 1600 degreesish. You only have to rise another 800 degrees or so to start to burn the steel. I am a blacksmith and I do understand iron and steel at high temps.
Brad

Steve farmer, it's pretty much as Brad explained. I don't think any of the parts go anywhere over 400C° in an engine, for any length of time. It is exposed to temperatures higher than that. But, they get evenized by the cooling.

On the other hand, you have a rocket stove, where burn tunnel and heat riser, are as insulated as can be. So the heat is retained. I have seen often, the clay liners in my stoves's heat riser, get bright orange. Which means it is over 1000C° And it stays in there.

This was the remains of our iron or steel pipe we used in a rocket cook stove at our school. Then we used it as a mould for making straw-clay donuts instead. Then we had other problems with it and are not using it anymore.