Heat riser- square or round?

Hi! Brand new to the forum so I'm a little fearful of asking a stupid question that may have already been answered. Planning my first build next week and have been busy scrounging material. I scored an 8" round 1/4" thick plate steel tube for the riser but its only 2' long. I also found someone who has a 6" ID square tube who will cut it to length for me - also 1/4" plate steel. From reading Ianto's book, the csa of all parts need to be the same or greater than the csa of the burn tunnel. The guy that is offering the square tube did the math and says the square tube actually has more area than the 8" tube. Since I suck at math, and he's an engineer I took his word for it ( its all free anyway!). Is this accurate or will I be creating a bottleneck in my riser? Also, why is it so hard to find powdered clay? I have very little time to devote to this on a daily basis and I need to find the fastest way to insulate this without digging it up. Does anyone know where to find bags of clay powder? I've had no luck. Thanks for any help!

Anyway, metal is doomed

Keep calling building supplies , ask for lincoln 60 fire clay

Thanks for your replies! Just a question, why would heavy, well insulated plate steel be doomed in the riser? Isn't most of the cumbustion in the burn tunnel? Or should I just use brick for the riser instead? I've seen pics of both applications. Any advice would be great! As for the Lincoln 60 fire clay, what type of buildings supply places should I check with? The big box stores ( as always) have no idea what I'm talking about. Thanks for your help!

thomas rubino wrote:Keep calling building supplies , ask for lincoln 60 fire clay

I was really happy to find a source locally...there's a big cement plant just north of town that sells Lincoln 60 for $7 a bag (!). Found them after calling all the usual building supply stores...most of them had no idea what I was talking about. And when I went shopping for Perlite, I ran into the same sort of thing...the people at Home Depot and Lowes didn't even know what I was talking about...the people there were just about useless. No offense to anyone of the people there...there's just soooo much stuff there, nobody seems to know what they have (or don't have). Local hardware store had the big 4 cu ft bags in the back for $20. Go figure.

And for a heat riser, you might want to research how to make a cast one...you could always use that piece of metal for the inner form.

That's a great idea! Cast with refractory cement or mortar? Thanks!

Cast a riser with fireclay , perlite and some refractory. Use a cardboard (sonitube) concrete form as the inner form and a 16 gal grease barrel for your outer form. Inner form burns out with the first fire.

Wow! That's great! Thanks for the advice and picture!

In response to your question about the steel riser lasting, both the burn tunnel and heat riser get freakishly hot, up to 2000 F or more. Any steel, even stainless, will corrode at those temperatures, and much faster than you might think.

Ok, that settles it for me. Thanks for all the help and knowledge from the people on this forum!