Calculations for reverse flow smoker with 250 gal propane tank

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mikechenry1

Newbie
Original poster
Jun 4, 2017
7
10
So I have been running the calculations for what I think I have finally decided to do with the 250 Gal propane tank. I plan on cutting off the spherical portion at one end and putting my firebox on that side. My tank measures 5 ft in length not including the other spherical end, and a Inner diameter of 30.5 in. I figured that the cooking chamber will be 50820 cubic inches. I need a fire box that has 16923 cubic inches, fire box opening, area under reverse flow plate, and opening at the end of the tank needs to be 204 sq in which translates to 5.5 in being the height of the opening. I am not sure if I want to build a warming chamber on top of the fire box. If so, how does that effect the numbers for the exhaust stack? 

Im still trying to decide on the dimensions for the fire box as well to match the needed volume 
 
I was playing with your numbers and came up with this I hope it helps. :)

Link to BBQ Pit Calculator

If you plan on having your exhaust exit the warmer always then just a short 6" to 12" exhaust is all that will be needed as the warming chamber will act as an exhaust itself you just need a way for the heat/smoke to exit the warmer.

Personally I would add a damper to shut off the heat/smoke and have an exhaust on the cooking chamber. This way the radiant heat from the warmer is heating the warmer not smoke.

My Hybrid cooker Bahama Mama is set up this way where my warmer runs between 150° and 175° on radiant heat alone or I can open the slide and actually cook in it as well.
 
Mike, evening...   Build the FB as big as you like...  Be sure to make it wider than where it will mate up to the CC...  easier fit up...  ALSO, the cut out for the FB mate up, cut past the welded seam on the tank...  easier fit up ....  When figuring out the FB/CC opening, include the thickness of the top of the FB and thickness of the CC ...  add in some room for a drip ledge, at the bottom of the cut out so grease will not enter the FB..   Big fires can happen...

The tutorial below has made some awesome smokers...   If you have any trouble with it, I'm here...    If fairly easy if you go 1 step at a time...

http://www.smokingmeatforums.com/t/...others-ready-to-use-rev5-6-19-15#post_1264161
 
 
Mike, evening...   Build the FB as big as you like...  Be sure to make it wider than where it will mate up to the CC...  easier fit up...  ALSO, the cut out for the FB mate up, cut past the welded seam on the tank...  easier fit up ....  When figuring out the FB/CC opening, include the thickness of the top of the FB and thickness of the CC ...  add in some room for a drip ledge, at the bottom of the cut out so grease will not enter the FB..   Big fires can happen...

The tutorial below has made some awesome smokers...   If you have any trouble with it, I'm here...    If fairly easy if you go 1 step at a time...

http://www.smokingmeatforums.com/t/...others-ready-to-use-rev5-6-19-15#post_1264161
Ditto what Dave said above. Bigger is better. Make sure your calculations are above the lower vents and the FB grate. I should have made mine larger.

Feldon's was not designed with reverse flow taken into consideration. I believe you'll find most on here use the link Dave mentioned above.

My tank is similar in size to yours and I ask lots of questions. You might want to read over it to get a better understanding of the calculations.

http://www.smokingmeatforums.com/t/171029/new-guy-and-this-will-be-my-1st-reverse-flow-smoker-build
 
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Ditto what Dave said above. Bigger is better. Make sure your calculations are above the lower vents and the FB grate. I should have made mine larger.

Feldon's was not designed with reverse flow taken into consideration. I believe you'll find most on here use the link Dave mentioned above.

My tank is similar in size to yours and I ask lots of questions. You might want to read over it to get a better understanding of the calculations.

http://www.smokingmeatforums.com/t/171029/new-guy-and-this-will-be-my-1st-reverse-flow-smoker-build
Respectfully, this is not an accurate statement. There are too many reverse flow cookers to count that are built to the original calculator that work. I use it on a regular basis building or helping folks build and have never had it fail me. There are many calculators now out there but they are just piggy backed off the original with a few mods so someone can say it is theirs. Do they work yes they do but to say the Feldon calculator was not designed for reverse flow is false. 
 
 
Respectfully, this is not an accurate statement. There are too many reverse flow cookers to count that are built to the original calculator that work. I use it on a regular basis building or helping folks build and have never had it fail me. There are many calculators now out there but they are just piggy backed off the original with a few mods so someone can say it is theirs. Do they work yes they do but to say the Feldon calculator was not designed for reverse flow is false. 
My Bad.....Seems I've had this conversation before a few years back on another forum before I joined the SMF. I questioned Feldons being flawed and was quickly question on where I saw it and who said it. I had started a thread to ask questions about a new build I was planning and was basically told I shouldn't question the advice given, just build it the way I was told. I haven't been back there since.

Feldon's was not designed with reverse flow taken into consideration. But,,,,, I believe you'll find most on here use the link Dave mentioned above.
 
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I remember that post vividly Fred. You slammed me out of nowhere on that post and I responded did it upset me no not really. In fact Paul Shirley came to my defense there as well its all good Fred we all have bad days. I build cookers I answer questions from hands on experience but we went over this in that post. The pit Calculator works as it is if you follow it. As I mentioned to Dave the upper intake is not needed if the cooker is built correctly but if folks want to incorporate it into their cooker that is great. All I know is I have helped more folks build cookers to spec with it than I can remember. I am here to help and learn and if I can help turn the tide on a build to success then I have done my job.

I apologize Fred it was the the eyebrow post you slammed me in. The post you are referring to was your 250 gall. build.
 
Last edited:
Thanks for all the replays so far. I am using the formulas from the link on the reverse flow page. For the fire box side. I plan on cutting the sphere part of the tank off and welding a straight plate there. Make it easier to put a warning chamber on the fire box
 
Sounds like a good plan Mike. :) I cut the dome off my Hybrid for that very reason the fit up is a lot better I also have  a slide damper in mine so I can use it for cooking if I want. I have a 3/8" plate set 1" below the top of my firebox it serves two purposes one being it takes the brunt of the heat on my top and two it helps damper the radiant heat in my warmer. It runs about 150°/175° while I am cooking.

If you have any questions on anything I will be glad to help. :)


This is a pic of the buffer plate.

 
 
I remember that post vividly Fred. You slammed me out of nowhere on that post and I responded did it upset me no not really. In fact Paul Shirley came to my defense there as well its all good Fred we all have bad days. I build cookers I answer questions from hands on experience but we went over this in that post. The pit Calculator works as it is if you follow it. As I mentioned to Dave the upper intake is not needed if the cooker is built correctly but if folks want to incorporate it into their cooker that is great. All I know is I have helped more folks build cookers to spec with it than I can remember. I am here to help and learn and if I can help turn the tide on a build to success then I have done my job.

I apologize Fred it was the the eyebrow post you slammed me in. The post you are referring to was your 250 gall. build.
Eyebrow, yea only after you slammed me for trying to answer the posters question with my suggestion, after all, we were there to help and offer suggestions, just like here, not criticize and tell him he didn't need them.

I think mine turned out quite nice.




 
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