Firebox to cook chamber opening shape

[TSied]

So I'm starting my first build. I have a 24" diameter 3/8 thick steel pipe. The cook chamber is going to be 44" with a firebox of 17". I got a 5" piece of pipe for a smokestack. Here is my question, running the numbers it says I need 61.52 sq in for a throat size. Is there any optimal shape of that hole? I mean is there any pros/ cons to a circle, a oval, a half circle, football shape, a trapezoid ect? Or is there a minimum height or length it should be for optimal airflow?

[towtruck]

I would run a half circle. You can find calculators that show you how to measure them in the calculator section of the forum.

Welcome!

[Big T]

Welcome aboard! The shape does not affect performance at all.

[TSied]

I didn't really figure that it would matter much but I haven't seen it talked about anywhere on the forum so figured I'd ask. Now on a half circle shaped hole would you put the long edge toward the bottom side of the cook chamber and the radius up by the cooking grate or the other way around?

I'm looking forward to starting this project. Originally I had an old propane tank that I was going to use but then I cam across this old Florida Power and Light steel pipe that is 3/8" thick so i switched over to using that instead. Figure the steel pipe is over twice the thickness of the propane tank. It's going to be a heavy bastard when I'm done though

[Big T]

If I was using a round FB I would go with a football shaped throat so the I could keep the FB as low as possible on the CC. Also you want the throat to be at the very top of the FB so that you don't trap heat inside the FB.

[Pete Mazz]

[TSied]

Pete,
This was my original idea of how to do it but I noticed something kind of funny that I can't figure out unless I'm missing something easy somewhere. See how you typed in 62 in the spot it says (throat area from the pit calc), and it spits out an overall height of 7 and width of 17 for a calculated throat area of 63.
OK well if you go in and type 78 in the spot it says (throat area from the pit calc) then it still gives you the exact same 7" height and 17" width cut but with a calculated area of 78.
Here's my problem with that, if the dimensions of the cut are the exact same the calculated area can't change.... My guess is that the calculator actually rounds the numbers off to whole numbers. Like if you wanted it to be 63 sq in then the exact measurements are probably more like 6.6" height and 16.6" width and if you wanted it to be 78 it's more like 7.4" height and 17.4" width but the calculator rounds these numbers. I guess I could go through and do the math manually to figure out an exact number. I'm probably being OCD and over analyzing the whole thing and being exact dead nut on the sq inches doesn't matter that much anyway.

[Pete Mazz]

Hmm. Site had changed since I published. Should work again. Thanks for letting me know .

[Pete Mazz]

[Dirtytires]

Thanks for the fix Pete....appreciate all the work you put into it.

[TSied]

ahh yep, so I was right and it was rounding the numbers to whole numbers then. Makes sense. Thanks for the fix. This is probably what I will go with. With my firebox having such a big diameter the bottom of it is going to sit pretty close to the ground if I put it as low as possible on the cook chamber, but thats fine by me. It may look a tad funny but should work well.

[Rodcrafter]

Good work


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