New smoker build

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kippy

Newbie
Original poster
Jun 15, 2024
10
3
So Getting ready to start my build with an 80 gallon air tank. Will have a rectangular firebox. When using the Feldon calculator it tells me I should have 51.75 Square inches of area open for the segment opening with a 5.75 radius so I cut the opening accordingly. Using a segment area calculator with those same numbers I come up with almost 75 square inches. Any ideas which is correct? The tank is 20” diameter and Feldon shows the cook chamber size as 18480 cu in. BBQ calculator.com recommends 57 sq in throat opening. So if I have cut the opening too big can I reduce the opening by adding a grease bridge at the bottom so grease won’t leak back in the firebox ? Any advice would be greatly appreciated
 
Not sure what bump means
Just a post to take it back to the top of the page. Sometimes if the forum is real busy a persons post asking a question will get moved down the list and not seen. Its just a courtesy thing trying to help you out.

Jim
 
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Can't help you out with your question though. Have you tried the search function here? Or went to the sub threads on smoker builds? Know there's been alot of threads about this. Otherwise, maybe Joel seenred seenred can help you out... he's built a couple smokers

Ryan
 
So Getting ready to start my build with an 80 gallon air tank. Will have a rectangular firebox. When using the Feldon calculator it tells me I should have 51.75 Square inches of area open for the segment opening with a 5.75 radius so I cut the opening accordingly. Using a segment area calculator with those same numbers I come up with almost 75 square inches. Any ideas which is correct? The tank is 20” diameter and Feldon shows the cook chamber size as 18480 cu in. BBQ calculator.com recommends 57 sq in throat opening. So if I have cut the opening too big can I reduce the opening by adding a grease bridge at the bottom so grease won’t leak back in the firebox ? Any advice would be greatly appreciated

Hello Kippy and welcome! I apologize for not responding sooner - I was out of town all weekend and just got back last night.

The online calculator I like is this one:


I find it simple and easy to understand for a guy who's a little "math challenged" LOL.

Using that calculator, and plugging in an 80 gallon tank, gives me a recommended throat opening of 51.84 sq. inches...so the number you got from from Feldon seems close to me. I'm assuming you cut your throat in the shape of a half-circle?

Like I said, I'm pretty math challenged, so keep that in mind...but I don't come up with that 75 sq. inch number when I calculate your area. Double check me - and I apologize if I'm wrong...but here's what I'm getting:

Using the formula for the area of a circle:

area = (pi) * (radius squared)

So the area a full circle with a radius of 5.75 would be 103.8 sq. inches. So a half-circle with the same radius would have an area of 51.91 sq. inches...which is pretty close to the recommended area Feldon gave you. I'm still unsure where the 75 sq. in. number came from?

Does any of that help? If not, just holler back and I'll try to help you take another look at it.

Again - I apologize if my math is incorrect...or if I'm sharing information you already knew.

Red
 
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Just a post to take it back to the top of the page. Sometimes if the forum is real busy a persons post asking a question will get moved down the list and not seen. Its just a courtesy thing trying to help you out.

Jim
Thanks
Hello Kippy and welcome! I apologize for not responding sooner - I was out of town all weekend and just got back last night.

The online calculator I like is this one:


I find it simple and easy to understand for a guy who's a little "math challenged" LOL.

Using that calculator, and plugging in an 80 gallon tank, gives me a recommended throat opening of 51.84 sq. inches...so the number you got from from Feldon seems close to me. I'm assuming you cut your throat in the shape of a half-circle?

Like I said, I'm pretty math challenged, so keep that in mind...but I don't come up with that 75 sq. inch number when I calculate your area. Double check me - and I apologize if I'm wrong...but here's what I'm getting:

Using the formula for the area of a circle:

area = (pi) * (radius squared)

So the area a full circle with a radius of 5.75 would be 103.8 sq. inches. So a half-circle with the same radius would have an area of 51.91 sq. inches...which is pretty close to the recommended area Feldon gave you. I'm still unsure where the 75 sq. in. number came from?

Does any of that help? If not, just holler back and I'll try to help you take another look at it.

Again - I apologize if my math is incorrect...or if I'm sharing information you already knew.

Red
 
Thanks Red
So I cut my tank opening with a 5.75 “ segment height and the flat cut across the opening measures 18” wide on my 20” tank which makes me think I cut it too big. I plugged those numbers into an Omni circle area calculator and came up with the 77 square inches. If you agree how would I correct that. I have not yet built the fire box but it will be square and not round. I still have 4” between the cook grate being in the center and the reverse flow plate long up with the top of the fire box
 
Thanks Red
So I cut my tank opening with a 5.75 “ segment height and the flat cut across the opening measures 18” wide on my 20” tank which makes me think I cut it too big. I plugged those numbers into an Omni circle area calculator and came up with the 77 square inches. If you agree how would I correct that. I have not yet built the fire box but it will be square and not round. I still have 4” between the cook grate being in the center and the reverse flow plate long up with the top of the fire box
 
Attached are some pics of the cut in the tank for the fire box
 

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Thanks Red
So I cut my tank opening with a 5.75 “ segment height and the flat cut across the opening measures 18” wide on my 20” tank which makes me think I cut it too big. I plugged those numbers into an Omni circle area calculator and came up with the 77 square inches. If you agree how would I correct that. I have not yet built the fire box but it will be square and not round. I still have 4” between the cook grate being in the center and the reverse flow plate long up with the top of the fire box

Attached are some pics of the cut in the tank for the fire box


Ahh...OK the pics give me a better idea of what you're dealing with. I was calculating the area of a flat, 2-dimensional end - like if you cut the bell-end of your tank off square and flat...which is what I did on my reverse flow build. Then I welded on an end cap. Made the calculations easier since everything was 2-dimensional.

Here's a couple pics to show you what I mean:

img25.JPG


img47.jpg



I'll assume your calculations are correct, and that your throat opening is indeed too big. IMO, you can fix that one of 2 ways.

First, you could simply make the opening in the firebox come out to the recommended area of about 52 sq. inches. Once it's welded in place on the tank, the larger opening is covered, and the effective airflow in the throat is correct. Only thing is, since I didn't do my RF build that way, I don't know how that extra metal in the end of the cook chamber might affect air flow...if I'm making sense.

Second, you could simply cut the remainder of the bell end off that tank like I did mine, and make if flat and square. This would make your overall length a little smaller, but wouldn't really change the usable cooking area by much. With a flat end, the area calculations become much easier to figure. Only thing is, it would require a piece of plate to make the end cap. But if your buying some plate to build your firebox out of, you could have enough steel left for that.


I'm not sure if any of that makes sense. If not, holler back and we'll figure it out.

And BTW, if you're interested, here's the link to the RF build I'm talking about. Maybe there are some ideas there that might help.


Red
 
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Ahh...OK the pics give me a better idea of what you're dealing with. I was calculating the area of a flat, 2-dimensional end - like if you cut the bell-end of your tank off square and flat...which is what I did on my reverse flow build. Then I welded on an end cap. Made the calculations easier since everything was 2-dimensional.

Here's a couple pics to show you what I mean:

View attachment 699099

View attachment 699100


I'll assume your calculations are correct, and that your throat opening is indeed too big. IMO, you can fix that one of 2 ways.

First, you could simply make the opening in the firebox come out to the recommended area of about 52 sq. inches. Once it's welded in place on the tank, the larger opening is covered, and the effective airflow in the throat is correct. Only thing is, since I didn't do my RF build that way, I don't know how that extra metal in the end of the cook chamber might affect air flow...if I'm making sense.

Second, you could simply cut the remainder of the bell end off that tank like I did mine, and make if flat and square. This would make your overall length a little smaller, but wouldn't really change the usable cooking area by much. With a flat end, the area calculations become much easier to figure. Only thing is, it would require a piece of plate to make the end cap. But if your buying some plate to build your firebox out of, you could have enough steel left for that.


I'm not sure if any of that makes sense. If not, holler back and we'll figure it out.

And BTW, if you're interested, here's the link to the RF build I'm talking about. Maybe there are some ideas there that might help.


Red
Thanks again. That makes sense to me. I’ll probably have to make the firebox opening smaller as I don’t have room to cut off the dome. If you see the pics I had already scored out the CC door. I’d like to stick to the segment
 
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Thanks again. That makes sense to me. I’ll probably have to make the firebox opening smaller as I don’t have room to cut off the dome. If you see the pics I had already scored out the CC door. I’d like to stick to the segment
Height which gives me 4” between reverse flow plate and cook grate. So that being said looks like I’ll have to shrink the opening inward. Thanks again for your insight and BTW your smoker build looks awesome ! Hope it’s working well for you
 
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Height which gives me 4” between reverse flow plate and cook grate. So that being said looks like I’ll have to shrink the opening inward. Thanks again for your insight and BTW your smoker build looks awesome ! Hope it’s working well for you

Sounds like a plan! Looking forward to watching this build!

And thanks for the comment! I actually sold that pit to another forum member. Ryan Brokenhandle Brokenhandle is cooking on it now, and seems very happy with it.

Red
 
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Sorry I missed this...

daveomak daveomak has a calculator that I used for the tractor... It has what your looking for as far as what size the cut out is... Punch your numbers in and see what you come up with... He has a good tutorial for the different shapes of openings...

 
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