Offset Firebox: How big is too big?

[dwfisk]

Let me say first, I've always had good luck using the Feldon (and others) calculators to size fireboxes, openings, exhausts, etc. But . . .

I've been asked to build a triditional (not RF) pit out of this giant 42" diameter x 108" long surplus anhydrous amonia tank, about 600 gallons or so. It will have a 42" X 84" main cooking grate and a 26" X 84" upper grate and an insulated firebox.

Now, when I run the calculators and stay within the 42" diameter of the tank I wind up with a 42" cube with 2" of insulation resulting in a 38" cube firebox.

That is not a huge problem but my gut says it is just way to much overkill.For you folks that have monster pits, what do yo uthink?

 

[Bigwoods4x4]

Let me say first, I've always had good luck using the Feldon (and others) calculators to size fireboxes, openings, exhausts, etc. But . . .

I've been asked to build a triditional (not RF) pit out of this giant 42" diameter x 108" long surplus anhydrous amonia tank, about 600 gallons or so. It will have a 42" X 84" main cooking grate and a 26" X 84" upper grate and an insulated firebox.

Now, when I run the calculators and stay within the 42" diameter of the tank I wind up with a 42" cube with 2" of insulation resulting in a 38" cube firebox.

That is not a huge problem but my gut says it is just way to much overkill.For you folks that have monster pits, what do yo uthink?

Going off the advice or Mr. Aaron Franklin (who cooks on big a$$ pits-500 gallon LP tanks usually) I know he typically takes a a 250 gallon LP tank and cuts it in half for his firebox on a 500 gallon LP tank cook chamber.

So he is using approximately 1/4 of the volume of the cook chamber for his firebox on a smoker that's pretty close to yours in terms of volume. Now, that is for an uninsulated box. He states directly in the book that you can go smaller on your firebox if it's insulated, but he doesn't say how much. He builds all of his own pits.

I know most of the calculators run off the the theory that 1/3 of your cook chamber volume should be sufficient for your firebox. With insulation, I would go 1/4. so therefore I would go 34x34x34 inside dimensions on the firebox which would give you 39,304 cu in. Your cook chamber is 150,000 cu in. So therefore you would be at 26% volume with the 34x34x34 firebox. That's what I would go with.

 

[dwfisk]

Going off the advice or Mr. Aaron Franklin (who cooks on big a$$ pits-500 gallon LP tanks usually) I know he typically takes a a 250 gallon LP tank and cuts it in half for his firebox on a 500 gallon LP tank cook chamber.

So he is using approximately 1/4 of the volume of the cook chamber for his firebox on a smoker that's pretty close to yours in terms of volume. Now, that is for an uninsulated box. He states directly in the book that you can go smaller on your firebox if it's insulated, but he doesn't say how much. He builds all of his own pits.

I know most of the calculators run off the the theory that 1/3 of your cook chamber volume should be sufficient for your firebox. With insulation, I would go 1/4. so therefore I would go 34x34x34 inside dimensions on the firebox which would give you 39,304 cu in. Your cook chamber is 150,000 cu in. So therefore you would be at 26% volume with the 34x34x34 firebox. That's what I would go with.

Thanks, big fan of Franklin here; had the chance to go in the back and see his pits and keep his book pretty handy. I've built a couple following his design ques, especially the grate level exhaust and man they do draw great! I have a couple 250 gallon propane tanks back in the boneyard, might use part of one and roll some 1/8 inch plate to make an outside skin over 2-3 inches on insulation?

 

[Bigwoods4x4]

Thanks, big fan of Franklin here; had the chance to go in the back and see his pits and keep his book pretty handy. I've built a couple following his design ques, especially the grate level exhaust and man they do draw great! I have a couple 250 gallon propane tanks back in the boneyard, might use part of one and roll some 1/8 inch plate to make an outside skin over 2-3 inches on insulation?

If I had a few 250's sitting around, that's exactly what I'd do. It will turn out great!

 

[TuscaloosaQ]

Just make sure when you build the insulated box... You still build it to the correct size...don't build it 4 inches small all the way Around... You would actually build it 4 inches bigger.... And run from those insulated doors... We no longer build them... We use 3/4 inch thick plate doors on the firebox.... The insulated doors caused us major issues.... And the gain on the insulated door was minuscule...

 

[dwfisk]

Just make sure when you build the insulated box... You still build it to the correct size...don't build it 4 inches small all the way Around... You would actually build it 4 inches bigger.... And run from those insulated doors... We no longer build them... We use 3/4 inch thick plate doors on the firebox.... The insulated doors caused us major issues.... And the gain on the insulated door was minuscule...

Thanks Paul. Yea, my thought is to use a portion of a 250 gallon, 30 inch diameter X 3/8 inch wall tank and then have a 34-36 inch outer skin rolled to go over the insulation, probably from 1/8 inch plate.

Reading Aaron Franklins book carefully he says he starts with a 1000 gallon propane tank for the cook chamber, then he uses 1/2 a 250 gallon (presumably a 30 inch diameter X 84 inch long standard tank) propane tank then puts a 24 inch pipe inside to get a 3 inch insulating air gap. My tank is about 590 gallons so using 1/2 a 250 gallon X 30 inch diameter and then putting the insulation on the outside of that should work fine. Still going to be damn big, ID something like 30 X 42-48 inches long!

And thanks for the door advice, sure simplifies the build!

 

[Bigwoods4x4]

"We use 3/4 inch thick plate doors on the firebox...."

Holy cow, that's insane (in an awesome way). How would 1/4" hold up do you think? That's what I already have laser cut?

 

[dwfisk]

"We use 3/4 inch thick plate doors on the firebox...."

Holy cow, that's insane (in an awesome way). How would 1/4" hold up do you think? That's what I already have laser cut?

Paul or others might want to answer to, but in my experience 1/4" will work fine but you need add some structure to keep it from warping in the heat. For a square firebox door I like to weld 1/2"X1/2" solid square stock to the inside as kind of a square "window frame" and maybe even an X pattern crossing in the center.

 

[TuscaloosaQ]

"We use 3/4 inch thick plate doors on the firebox...."

Holy cow, that's insane (in an awesome way). How would 1/4" hold up do you think? That's what I already have laser cut?

1/4 inch doors are fine... We only use 3/4 thick on insulated fire boxes.... On the standard 1/4 inch thick doors we put them in a a subframe... Works best for us

 

[TuscaloosaQ]

Paul or others might want to answer to, but in my experience 1/4" will work fine but you need add some structure to keep it from warping in the heat. For a square firebox door I like to weld 1/2"X1/2" solid square stock to the inside as kind of a square "window frame" and maybe even an X pattern crossing in the center.

Gotcha brother can't wait to see this beast...we use to build a bunch of jacketed pipe.. When I was pipe fitting and welding pipe... They ran steam thru it to help keep the product on the pipe side from jelling up

 

[QTEX]

Just make sure when you build the insulated box... You still build it to the correct size...don't build it 4 inches small all the way Around... You would actually build it 4 inches bigger.... And run from those insulated doors... We no longer build them... We use 3/4 inch thick plate doors on the firebox.... The insulated doors caused us major issues.... And the gain on the insulated door was minuscule...

Hi Paul, just saw one of your smokers this past weekend and was also amazed at the thickness of that door, wow! Just curious what type of problems those insulated doors cause, just a home hobby guy with a curious mind?

 

[TuscaloosaQ]

Hi Paul, just saw one of your smokers this past weekend and was also amazed at the thickness of that door, wow! Just curious what type of problems those insulated doors cause, just a home hobby guy with a curious mind?

No matter how we braced them.. And believe me we went to the extreme... They would bow or warp... Could be that we have a fairly big intake in the door... The 3/4 I thick doors have posed no problems..

 

[QTEX]

Thank you sir, really admire your work and craftsmanship.

 

[DownHomeQue]

On my 600 gallon (UnInsulated) Firebox.. I used about 107% of Feldon.. My firebox is 30 tall, 40 deep, 40 wide. it works well.. I also oversized the exhaust from the calculator..

 

[TexasBBQ]

Just make sure when you build the insulated box... You still build it to the correct size...don't build it 4 inches small all the way Around... You would actually build it 4 inches bigger.... And run from those insulated doors... We no longer build them... We use 3/4 inch thick plate doors on the firebox.... The insulated doors caused us major issues.... And the gain on the insulated door was minuscule...

TuscaloosaQ, do y'all still build them at 1/3 the size of the CC and what insulation do you recommend? How much heat seems to transfer to the outside of the box?

Thanks
Texas

 

[Cat797]

Dave, I have always followed Feldon's calculator as well. Your firebox calculates as 1/3 the volume of the cook chamber, so I'm comfortable with that. One thing to keep in mind is that you are better off with a bigger firebox than you need as opposed to one that is too small. If it's too small, you'll struggle to get to and maintain temp.........

Ed

 

[m-fine]

All this talk of Feldon is so damn depressing. The formulas are complete bunk. They happen to work for standard size smokers but a lot of made up formulas would also work because smokers will work with a very large range of sizes and dimensions. What the formulas won’t do is scale sensibly because Feldon’s calculations are based on the wrong inputs.

The physical reality is that you need a firebox that is WAY smaller than you think. A box that is only a couple cubic feet in volume can generate all the BTU’s you need. It will be smaller than ideal to work in, so, make it bigger, realizing that the size of the ideal firebox is not driven by thermodynamics or fluid dynamics, but by human factors like ease of use.

I wouldn’t build one smaller than a two foot cube. That’s 8 cubic feet or at-least 4x as big as you need, but it gives you plenty of room to put in logs of any reasonable shape and size, lots of wall and door space for intake vent placement, and lots of room for the pit master to work a fire. I also wouldn’t go over a 4 foot cube. Bigger is just a waste of steel and even if insulated, it will waste more BTU’s than a smaller box.

 

[DownHomeQue]

Feldons has worked great for me. I have built 3 pits using it. I prefer slightly oversized. How many pits have you built using your theory?

 

[Cat797]

m-fine, I'm not sure how long you have been around this game (BBQ-ing), but I've been around it and this and another forum since about 2008, and Feldon's calculator has been around since before then. I believe he even stated way back then that his calculator was based solely upon observations and not science......

So, maybe you're a thermodynamic engineer, or some other kind of "official" scientist. Maybe his forumlas don't scale to your liking. Maybe they are "bunk". Congratulations! Have a coffee on me........

The dude has probably single-handedly helped more people successfully build smokers following the recommendations of his calculator than anyone else on the planet. He did it for free and doesn't ask anything for something he created on his own time.

Sorry, but IMNSHO, I find it depressing to bad mouth a guy who has helped so many people.......

Ed

 

[TuscaloosaQ]

Feldon calculator has worked for thousands of pits..... I see no issue at all with it...... although we do something a little different.... Nothing at all wrong with feldon in my opinion