Heat exchange pipe ..... Only stainless ? Or will steel work?

Hi.  I am just starting to build my first RMH in my passive solar greenhouse in Northern Russia... Yes, that's right, I am a Brit living in Russia

Everyone here thinks I am barking mad...... " A what?"

I have a question about the heat exchange pipe.  I understand that it should be as large a diameter as possible . My question is what type of metal is best.  Stainless steel chimney pipe that I see in various videos is really expensive here.  I can get a steel pipe, 3mm thick, 21cm across for less money.... But is it the right material ? Will it be too thick to effectively transfer the heat to the cob ?  My pipe will be about 27/28 feet long, running in a straight line. My greenhouse is about 650 sq.ft.

Any helpful advice would be greatly appreciated

Thanks in advance
Steve

Standard practice is not "as large as possible" but the same diameter as the combustion core, all the way through the system (except the barrel around the heat riser, and the manifold, which should be more spacious because of the sharp turns).

Stainless steel pipe for the heat exchange duct is very rare, basically when somebody gets it cheap or wants a super system. Any steel stovepipe will work as long as it is the right size.

Thanks Glen.  I am building the whole j-tube from fire bricks so can make it to match the steel pipe ( square riser, round pipe ).   The metal pipe is not a stove pipe as such, just something left lying around a derilict Soviet factory that I dug around in.  But it is thick  .

Here metal chimney pipes are made from stainless steel....  yours are  commonly what?  aluminium?

Stovepipe, expected to be able to stand high heat, even occasionally glowing, is typically black iron of around 24 gauge (0.65mm) I think. Ductwork, expected to stand only warm air, is typically galvanized and thinner (26 gauge, 0.48mm).

Insulated chimney pipe is stainless steel.

Your steel pipe would work fine buried in mass; it would hold more heat than thin ducting.

Steve, your pipe will be fine with a 6" or an 8".

If youy're going over system size, , you're delving into the world of bells and downdraft channels, flow slowers etc.


http://batchrocket.eu/en/building#belltheory

http://www.stove.ru/index.php?lng=1&rs=16

Just when you think that you have it all figured out, you realise that you actually know very little :-)   adding a bell to the end of my systems sounds interesting, thanks very much for bringing it up. So.... delay the start of the build , downloaded Erica and Ernies book, a day of intense reading.....and maybe a new design by tomorrow.  

Steve; You can build your heat channels out of brick as well as pipe.  

Or on the cheap with barrels!

http://redirect.viglink.com/?key=bbb516d91daee20498798694a42dd559&u=http%3A//s65.photobucket.com/albums/h228/mremine/NYC%2520Rocket%2520Stove%2520Build/