My First Reverse Flow Build - 125-Gallon Propane Tank (Plenty of Pics)

Discussion in 'Reverse Flow' started by bluffton smoker, Aug 29, 2012.

  1. Hi All,

    I have begun my first Reverse Flow smoker build. 

    I am using a 125-Gallon propane tank. The tank is 24" diameter and approx 64" long (averaging the domed ends). I took advantage of the calculator and worked out the dimensions pretty well (I think). I traced the fire box to smoke box opening out on paper and overlaid a 1" grid to verify my numbers were correct. I am in the process of cutting the fire box and smoke box end plates. I wanted to throw my design thoughts out there to see what sticks to the wall (so to speak).

    I am not tucking the fire box up under the smoke box because it will limit my access to tend the fire. (I plan to cut the fire box door in the upper side of the fire box as opposed the end because the end is domed and will not be easily accessable to me once mounted in place. I know I will have heat loss, but will the loss be enough to worry about?

    My air inlet will be on the lower left side of the fire box with a handle to easily adjust. My stack will be 6" X 2" rectangle which will give me roughly the same area as a 4" round stack.

    This is my numbers per the calculator: 

    Length of Tank               64”

    Cooker Chamber Size      48” of Tank - 21700 cubic inches

    Firebox Size                    16” of Tank - 7240 cubic inches

    Firebox Intake Vents       1 ½” X 3” X 5 Vents

    Smokebox Opening                   5 3/4” - 60 square inches

    Chimney Pipe Size          2” X 6” @32” above Smoke Box

    RF Plate Opening            90 square inches

    Any and all ideas and critique (constructive, of course) is welcomed.

    Last edited: Oct 3, 2012
  2. Herse what im doing with the 120 gallon tank I had, I added 1 foot to the center to make the chamber taller, but I did tuck the firebox up under it.

  3. Ribwizz,

    I like how you raised the chamber 1 foot. It looks good. I also like the way you tucked the firebox under....  Which gives me an idea. I gotta go out and check it out as I haven't started welding anything together yet. The problem i'm afraid of is that I'll have a poorly designed fire box. BUT if I rotate the fire box 90-degrees, and stand it up with the dome end down.... Just mebbee... I gotta go check it out and see if it will work for me. Might just be the answer... I'll let ya know...
  4. What are you using for a firebox?   A tank?
  5. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    BS, morning....  Your numbers look perfect....  

    Are you having to put wood in the firebox from under the smoker ??  Are you figuring an ash pan to remove the wood ashes ??  It is difficult to picture what the final access will be....   Dave
    Last edited: Aug 30, 2012
  6. My tank is 64" long and 24" diameter. I cut the tank leaving a 48" smoke box and a 16" fire box. I just dropped the firebox down 18. 1/4" and welded in place. This leaves 5 3/4" opening for 60 square inch opening between the 2 chambers.... I haven't posted any pics yet they should be showing up in the next day or so...

    Thanks for checking my numbers. I am glad I didn't just SWAG 'em.

    I am planning on the door to the fire box being on the same side as the smoke box door. It looks like it will be approx 12-13" wide & 16-18" in height following the curve of the tank. It will hinge on the top like the smoke box door will. The wood will load thru this side door, and the ash pan will slide out from under the smoke box. The air intake vents will be under the smoker as well and will be easily adjustable from a standing position. 

    Does any of this make sense??
  7. The pics as promised...

    The tank is only 3/16" thick and my plate is 1/4"...

    I cut the tank with 7" x 1/8" cutoff blades loaded up in a curcular saw. It did a clean job...


    I used the lighter end (fire box) to trace out the 1/4" plate for the endcaps (2)

    I welded the endcap onto the fire box...

    Rolled everything up side down and welded the firebox to the smoke box.

    I didn't weld the end plate on the smoke box yet as I gotta slide the RF Plate in first.

    the pics make the chambers look tilted, but my square says its all good...

    The legs will be next...

    I plan on using 2 1/2" pipe for the legs with 8" iron wheels. 2 of the wheels will be casters. So it will have a bit of mobility...
  8. One sugestion, instead of the door opening to the top, what if you fabricated a square frame out of , lets say 1/4" x 3" falt bar and cut a square out of the round end of the firebox and inserted it for a door frame. Then you could put your firebox vent right in the door. and keep the airflow going straight into the cooking chamber.

    Just a sugestion, but I think you are on your way to building a nice smoker.
  9. sqwib

    sqwib Smoking Guru OTBS Member

    That is a perfect size, the diameter is perfect, fuel consumption will be minimal.

    One thing I want to point out is the inlet from the fire box into the smoke chamber looks a tad small from the picture.

    This is a copy / paste from my blog

    JRodriquez pointed me in the right direction.

    20" DIAMETER TANK 10" X 10" X 3.1358024 divided by 4 = 78.395, actually that does not apply to sectioning the tank in this matter, but thanks to a fellow builder and his CAD program he came up with my numbers actually being, 61.418, so I am good to go.
    Below is an explanation of this from JIRodriguez.

    Figured out where the difference was coming from the formula 3.14xRxR gives you the area of a wedge shaped opening exactly 1/4 of a 20" dia. pipe (like a pizza cut top to bottom & side to side).

    But the shape profile is different when you split the pipe with parallel lines every 5". The two areas closest to the center point have a larger area than the two areas on the outside. So you end up with the two middle sections measuring 95.661 in. sq., and the two outside sections measuring 61.418 in. sq.
  10. Yeah, I thought the hole between the chambers looked a bit small too. I took a piece of plywood and traced out this opening then drew a grid over this tracing and manually counted the little 1" squares... I ended up with 61 of these little 1" squares...  

    I think I'm also gonna stay the course with the fire box door. It will work out best for the location this smoker will live.

    Thanks to all for the input. It is much appreciated and keeps me thinking outside the box, so to speak.
  11. I have the legs on the smoker, but haven't started on the RF Plate yet. As this is a round fire box and a round cook chamber, what is the optimal distance between the main cooking grate and RF Plate? I have about 7 inches to play with. I am building this smoker using the numbers from the calculator, BUT I want to leave room to increase the hole between the chambers if needed. I was thinking of setting the plate @ 4-5 inches below the grate. Any advice??
  12. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    Bluffton, afternoon.....   The area between the RF plate and the bottom of the smoker should be 1.5 times the area of the firebox opening to the smoker....  this will allow for good air/smoke/heat flow through the smoker....  there is friction loss of flow to be considered through the smoker...  that ratio should be considered at the end of the RF plate to allow for the air/smoke/heat to take a 180 deg turn toward the stack...  

  13. Thanks Dave,

    I'll run the RF Plate 4" below the main cooking grate. This will allow room for any future configuration changes as needed.  I laid off the build for the holiday weekend. I should pick it back up on Tuesday... I should have more pics as well... Will keep all updated...
  14. I 'm back... Had to do a little work that pays the bills.... It was a good thing to get away from the build and restart fresh...

    I re-re-remeasured the opening itself between the fire box and cook chamber and found my original sketch that I had been measuring from was off. I am off by 1/2". Which means SQWIB was correct. "IT DID LOOK A TAD SMALL". It was only 50 square inches on a hole that should be 60 square inches by the calculator. 

    Next chore, Cut the fire box loose, grind it down & re-weld. I'm gonna allow plenty of room. Maybe 7" opening, which will be 80 square inches of opening between the two chambers. This will give me 5" between the top of firebox and the Main Grate. The RF Plate should go in no more than 1" above the fire box which will still allow 4" between the RF Plate and the Main Grate.  I will take pics along the way....

    Again, thanks for the input. It make me re-think everything through and catch this problem before it was too late....

    One question though... Is too big a hole between the fire box and cook chamber a bad thing? My calculator calls for a 60 square inch hole. Should I just shoot for a 70 inch hole instead of 80? Any advice is appreciated...

    Thanks again..
  15. I cut the firebox loose and re-cut the opening between the fire box and cook chamber. I allowed 6 1/2" which should give me somewhere between 70 - 75 square inches so I should be good as I only require 60.

    I also got the legs in place.

    I have the cook chamber door marked out. It is 16" high & 36" wide.

    I have 1/2" pipe for the hinges, I gotta gp pick up some 1/2 round bar to complete the hinges. But I will get a start on the doors today.
  16. The door cut on the Cook Chamber did not go too well. I cut the top cut horizontally and down each side about 1 1/2 inches to give me room to get the top door flange welded in place. ( I planned to weld the hinges in place before cutting the door completely loose)...  I tacked the flange every 3 inches or so, which went pretty well. But when I welded it in place it rose on the back side, opening up my flange seal. When I went to heat it up and beat it back in place, there was enough spring in my tank so that I wasn't able to bring the flange back in place. 

    Soooo.....  I cut the door out completely where I can put it on an anvil and convince it to return to it's original shape. I will weld the flange to this door completely and THEN hang it on the hinges properly.

    Here are the pics so far...

    I haven't welded the end on the Cook Chamber yet as this is where I'm gonna insert the RF Plate after my first burn. That should happen after I get both doors cut and swinging properly.

    I will continue tomorrow...
    Last edited: Sep 16, 2012
  17. Had the same problems with my doors, everything perfect until I welded the flange on. I had to use a hyd press and work the doors also.
  18. Yeah, That's what I figgered. I ain't lucky enuff to have a press, tho. I'll just convince em in place with a rosebud and a big a$$ hammer...

    I'm gonna get the Fire Box door cut out and get the flanges all welded up on both doors. Then I'll begin the serious door formin....
  19. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    Bluffton, evening.....  When it comes to welding hinges and flange seals on your smoke..... blind weld them.... no warping..... Dave


    Plug and slot welds (fig. 3-18) are welds made through holes or slots in one member of a lap joint. These welds are used to join that member to the surface of another member that has been exposed through the hole. The hole may or may not be completely filled with weld metal. These types of welds are often used to join face-hardened plates from the backer soft side, to install liner metals inside tanks, or to fill up holes in a plate.
  20. Oh be careful with that rose bud, Heat will do some things you dont want it to do sometimes. And its hard to shrink the meatl once its been heated and streched.

    Another ideal is to use a heavy piece of angle that wont bend, lay it across what you want to bend and use wood blocks and heavy c-clamps to work it into shape.

    Good Sugestion DaveOmark, I just wanted that nice solid bead of weld around my flange and took the risk, ended up being a lot more work, but sure looks preaty.

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