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# Fire box size

Hey Yall,

So I have been getting all my sizes down for my build. I tried a firebox calculator I got from a different forum. The problem is that it is giving out a way larger firebox than my own math did. The area I need heated is 77.5 inches long and aprox 22.5 inches in diameter. I say aprox because its a propane tank with the baffle in the bottom as you know. So it doesnt really fall into the category of cylinder volume. but it sure doesnt fall into the category of cube volume. Im not sure which method of calculation I should use, but I have been using the cylinder method since thats closest.

So my calculations have gone kinda like this:

Volume of area needed to be heated: PiR^2*length so i got 3.14*11.25^2*77.5= aprox 30,799 cubic inches of space that needs to be heated.

So then I take that number and multiply by 30% to get the volume of space needed in the firebox.

30,799*.3=9,239.7 cubic inches

Then I take that and get the cubed root of it to get some physical dimensions. This comes out to: about 20x20x20

However, the calculator i got from the other forum is giving me a volume for the firebox of 10,266.33 cubic inches.

This give me dimensions of a little over 22x22x22.

Now I know this doesnt seem like much, and im kinda guessing that either size will prolly work out ok. But Im wanting to know which size sounds correct to you guys.

Where this really comes into play isn't in construction. It's in material cost. Obviously I want to buy the smallest sheet of steel possible to build everything out of one sheet.

I might have just confused you guys. But, if you have input please let me know. If you need me to explain something better let me know, I know I can be confusing when attempting to explain my self.

Thanks

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The Calculators are based on 1/3 the cook chamber size for the firebox.  You used 30% which is slightly smaller then 33.3333%.  This is where your difference is with the calculator.  If anything err on the slightly larger size.

I knew there was something simple. Im not the best at math, so I knew posting here would help find the error. So based on that I have come up with a diagram that would give me all 6 sides of a 22" cube firebox and a 16x74 inch baffle. Using a 4x10 sheet this will give some scrap, but that's the smallest sheet I can get and fit everything I need. Like I say, I figured it wouldn't be a problem either way, but I went with the larger of the 2 options. I figured if I was gonna have to buy a little bigger sheet anyway I might as well make a larger firebox.

Thanks for the info so much.

Is the rule of thumb for the firebox being 1/3 the volume of the cook chamber a must? I have noticed on alot of commercial made smokers the firebox cant possibly be a third of the size? Just wondering.

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