Firebox Opening

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Jason B
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Firebox Opening

Postby Jason B » Fri May 31, 2013 8:35 am

Just starting my propane tank build. Is there a reason to use a rectangle firebox to cook chamber opening over a round opening or visa versa?
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k.a.m.
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Re: Firebox Opening

Postby k.a.m. » Fri May 31, 2013 11:55 am

Jason B, I use half moon openings. They allow me to get the maximum height I need in the opening when mating a square firebox up to a cylinder, I also prefer my opening to be at the top of the firebox rather than down some.
I hope this helps. :D
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Re: Firebox Opening

Postby Jason B » Fri May 31, 2013 12:08 pm

Thanks k.a.m. When using these calculators, it asked for round opening. How does the half moon fit into that equation. For instance, let's say my plan calls for a 9" ID opening. Would I only cut out half? on a 9" radius? Or would I cut out have a moon with a 18" radius to get the same volume as a full 9" circle?

BTW: This will be a 250gal reverse flow with a 30W X 26D X 26H firebox and a 6" X36" chimney.
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Re: Firebox Opening

Postby HarvestMoon » Sat Jun 01, 2013 1:07 am

A = Pi times r squared
A = Pi times r times r
r = d divided by 2
d = r plus r = r times 2

d = 9"
r = 4½
A = 63.61725124 sq. inches

Now you want a circle with twice the area that you'll only cut out half of.

r = squared root of ( twice the area divided by Pi )

r = 6.363961031 inches
d = 12.73"
which is lose enough to say 12¾
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Re: Firebox Opening

Postby k.a.m. » Sat Jun 01, 2013 6:29 am

Jason, I ran your numbers and your recommended firebox to cooking chamber opening is 162.24 Sq. in.
A half moon of 20.5" wide x 10.25" tall is 165.03 Sq. in.. This is slightly larger than you need but when you add a 1.5 " to 2" grease bridge at the bottom of the opening your back in your numbers.
I hope this helps. :D
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Re: Firebox Opening

Postby Jason B » Tue Jul 02, 2013 4:40 am

I need some more help here. I really wish I had payed more attention to math when I was in school.

Some dimensions have changed. The firebox is now 30"X24"X24" which is a little undersized. The tank volume is 240 gallons and I am running a 5"X44" exhaust. My area for firebox to CC according to the calcs should be 138.24 square inches.

What width and height would that make my half moon opening?
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Re: Firebox Opening

Postby k.a.m. » Tue Jul 02, 2013 7:07 pm

Jason, a half moon of 19" wide x 9.5" tall will give you and opening of 141.76 Sq.in.This is slightly larger than you need but when you add a 1.5" grease bridge you will be back in your numbers.
I hope this helps. :D
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Re: Firebox Opening

Postby dwm » Tue Jul 23, 2013 4:20 pm

Okay, lets say I am horrible at math as well as reading comprehension... but I can weld and I am building a big smoker... what size opening do I need at the fire box? What size exhaust pipe(s)?

36"x36"x48" fire box
10' x 36" smoker pipe
4'x4'x5' vertical box
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Re: Firebox Opening

Postby k.a.m. » Tue Jul 23, 2013 10:01 pm

dwm, your firebox is way over sized for the cooking chamber if you go by the 1/3 rule.
1/3 of your chamber is 40694.40 Cu. in. Your firebox measures 62208.00 Cu.in.
I ran than your numbers and the firebox to cooking chamber opening would need to be 497.66 Sq.in. A half moon of 36" wide x 18" tall would give you an opening of 508.95 Sq. in. by the time you add a 2" grease bridge you will be back in your numbers.

On a side note if you reduce the firebox to 32" H x 36" W x 36" L (41472 Cu. in.) you would need an opening of 331.78 Sq. in. A half moon of 29.5" wide x 14.75" tall would be about 342.00 Sq.in. Adding a 2" grease bridge will bring you back into your numbers.
I hope this helps. :D
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Re: Firebox Opening

Postby Mark0023 » Sat Jun 13, 2015 11:25 am

New to the forum. But I can't figure out the firebox to cook chamber opening. My tank is a 330 gallons. Cooking chamber is 76230.00 and my firebox size is 24x36x36. Going with the half moon shape. Appreciate any information you guys have. Trying to get all my numbers ready before I start.
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Re: Firebox Opening

Postby k.a.m. » Sat Jun 13, 2015 8:42 pm

Here is your numbers Mark0023.
Link to BBQ Pit Calculator
Based on what you have given the firebox to cooking chamber opening needs to be 248.83 Sq.in.
The opening if you choose a half moon needs to be 25.17" wide x 12.5" tall. If you are going to add a grease bridge which I suggest I would go with 25.5" wide x 12.75" tall. This will allow for a 1.5" grease bridge.
I hope this helps. :D
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Re: Firebox Opening

Postby Lisaac » Wed Jun 29, 2016 9:10 am

So I need to borrow someone else's brain power for a moment. I have a horizontal propane tank 24"x 84" 155lbs the firebox will be 22"x24"x24" what should the 1/2 moon opening be with a 2" grease lip? What would be the highest measurement and what would be the widest? Also the chimney is 5"x 27". Thanks in advance and thanks for passing math.
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Re: Firebox Opening

Postby Damon54 » Sat Sep 03, 2016 12:55 am

K.A.M,

I also have a rather large firebox to cooking chamber ratio.
Here is my question: I have my grate elevated so as not to constantly fight ash on long cooks.
Has my fuel (wood/charcoal) height shrunken my Firebox total sq. inches or not??

Thanks my friend. You are invaluable here.
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k.a.m.
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Re: Firebox Opening

Postby k.a.m. » Sat Sep 03, 2016 6:13 am

Damon54 wrote:K.A.M,

I also have a rather large firebox to cooking chamber ratio.
Here is my question: I have my grate elevated so as not to constantly fight ash on long cooks.
Has my fuel (wood/charcoal) height shrunken my Firebox total sq. inches or not??

Thanks my friend. You are invaluable here.

If I am correct you are asking if the added material in your firebox to create the elevated coal base will decease your total volume of space.
If this is what you're asking and you have a larger than needed firebox no Sir you will be fine. But even if you were slightly undersized I would recommend an elevated coal base. I run slightly over sized fireboxes with elevated baskets with Vee cradles in my offsets and they perform great.
I hope this helps. :D
Always remember slow and steady wins the race.

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Re: Firebox Opening

Postby Damon54 » Thu Sep 08, 2016 8:36 am

K.A.M. ,
To be clear my volume is unchanged. My question: Is the area beneath the elevated coal base still relevant to the total volume calculation?? In my case the air inlets are close to fuel grate level.

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