first build need some help

Discussion in 'Reverse Flow' started by fire bug, Dec 18, 2012.

  1. fire bug

    fire bug Newbie

    Hi guys new to the forum been thinking about building a smoker fo a while and found this site looks to be very helpful. I got my hands on a 325 gallon propane tank and cut it down to 90 in. its 30in. in diameter i put that info into the pit calculator and came up with a 28x28x28 fire box going to cut a half moon into the fire box to match the tank. The question is the pit calculator said a 21in. diameter half moon 175sq in. when i drew that out on on card board 21 in wide its only about 4 1/2 in.tall that only came up to 70 couple sq in. To get 175 sq in my half moon had to be about 27 1/2 in. wide and 9 in. tall . Did i miss read the calculator or did i not figure my sq inches properly. What i would like to know is how tall and how wide my half moon needs to be in my fire box in inches and how many inches above my fire box opening does the top of my RF plate need to be that would save a lot of headaches on my end not to good with the whole math thing. Thanks in advance!
  2. I didnt do the math, but 27' wide by 9" tall half moon sounds about right for that size smoker, why dont you make the firebox the same width as the cooking chamber ?
  3. I am not sure what pit calculator you are using, but if it's the one at I am puzzled too. Taking your firebox dimensions as input does indeed give a half moon diameter of 21.15 inches from the calculator.

    Crunching the math gives an answer close to a 27.5 inch wide by 9 inch tall half moon. For instance, from we get some nice formulas.

    Assuming a 9 inch half moon height, h, and your cooking chamber radius, R, of 15 inches we would get an circle sector angle, phi, corresponding to

    phi = 2*ARCCOS((15-9)/15) = 2.31855...

    This would give the circular segment (half moon) area, A, corresponding to

    A = R*R*(phi - sin(phi) ) / 2 = 178.5 sq inches

    With a 9 inch half moon height you will indeed get 27.5 inch half moon diameter, c, corresponding to:

    c = 2*R*sin(phi/2) = 27.5

    The FB to cooking chamber opening should be approx 0.008 times the firebox volume. With your firebox dimensions 28*28*28*0.008 = 175.62, which is the same as the calculator. I based my RFS on the 0.008 factor and it works nice.

    Cheers /Wes
  4. jarjarchef

    jarjarchef Master of the Pit OTBS Member

    Dave posted this link on another post. I have not figured it out yet, but you should be able to figure the area of a partial circle. I hope it helps.
  5. fire bug

    fire bug Newbie

    Thanks guys it was feldon that I used that's why I got confused when I started laying it out I definitely feel beter about it now. I could go 30 wide on the fire box just figured it would be easier to keep it square. Would I be beter off to make it the width of the cook chamber? Also about how many inches above the opening should my plate be?
  6. using a tank with the round end, it will be easier to cut the tank if your firebox is as wide or more than the cooking chamber( less angles to cut) 

    Also , I think it looks better, gives it a better blend into the tank.  Give the firebox plenty of height as well, by the time you install the fire grate and leave plenty of space below it for an ash pan, you dont want the flames to be hitting the top of the box. Id just go 30x30x30 and you should have no problems.

    Remember, on the pit calculator, those are minimum requirements, going bigger hurts absolutely nothing. Just dont go smaller. You will be controlling the air and heat from your firebox vents, so focus on making them function good.
  7. fire bug

    fire bug Newbie

    Thanks ribwizzard i'll go with the bigger box. Should i make the top of my firebox opening down about 3 in. from the top of the firebox then just set the rf plate on top of the firebox will that give me enough space under my plate?
  8. fire bug

    fire bug Newbie

    Also ribwizzard i went back and typed the 30x30 firebox dimentions into the feldon calculator to see what would change and the halfmoon hole size increased to over 200 sq. in. Will i be ok with the old hole size or should i increase it? If I get too big with my hole it's gonig to put my rf plate above my center line of the tank and thats where i want my first grate.
  9. It should not have increased the hole size. The cook chamber is the same size and still only needs the same amount of air.

    The top of the firbox is usually just a little higher than 1/3 the height of the tank, I suggest a 2nd plate 1/2  inch from the top of the firebox to help from getting a hot spot at the beginning of the reverse flow plate. So go 3/4" higher than that, draw your line and measure it out. You should be just about right sq. inches.

    With 30 inch cook chamber, bottom of door should be 16" from bottom of tank. so thats putting your rack at 16 inches also, you dont need more than 4" between the bottom of the rack and the reverse flow plate, so that will still give you a half moon of 11'x 30" just about.( my mind is calculating thickness of material in there as well so dont critic my math, just my spelling, ok?)
  10. bob1961

    bob1961 Smoking Fanatic

    if you can get a drawing of the dia of your tank size, say 30" circle....draw a line where you think the top of square fire box might go....then use tracing paper with 1" square lines on it....count all the full 1" squares that cover your opening you have drawn....guess at areas that are half covered with a 1" square and double how many for one side to match/mate the other side....easy basic square inch coverage....
  11. Here is a link to the circle calculator.  It is not too difficult to figure out and it will allow you to determine the size and height of the opening under the RF plate.  Assume that the area under it should be 1.5 times the FB opening and you should be golden and have plenty of room above the plate.  Also I prefer to keep the firebox as low as possible while maintaining the correct opening size, but it is just a preference.

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