Backyard 20 gallon RF. 1st build - Advice on too LARGE of Firebox

Discussion in 'Reverse Flow' started by mike dula, Apr 19, 2016.

  1. mike dula

    mike dula Newbie

    Hello all from Mansfield TX. First post but Ive been lingering for some time. Another Newb with yet another RF build :) I know all the reverse flow horizontal build must get old to the pros here, but us newbs have to start somewhere. I just like the project and working with metal, and Ive been itching to build something new...

    A neighbor gave me a 14 x 30 air tank that I thought would be a perfect smoker project. Then I sourced a a 12 gal air tank locally for 20.00. All other costs so far has just been cut off wheels and welding wire. I was told the tank was a 30 gallon, so the 12 gallon in my mind originally was perfect. It wasnt until I measured that the tank ended up being a 14x30 or 20 gallon...Heck, looking at it I knew the tank was way to large.

    Plans are a small efficient RF backyard rib smoker. CC is 14x30, firebox (currently) is 12x25. CC to fire box football opening is a 6.25" diameter. Feldon say my fire box is 186% or 86% too large :) and my opening needs to be 7.59"

    I need some advice. I'm in the beginning stages of my build so I'm not so far off I cant shorten it .According to the calculation I have to make it smaller. With my tools it will be a chore with cutoff wheels to cut 2 perfect circles to make the firebox something more manageable like a 12x15. The plan is to use the end of the firebox as the door to make it easy to scrape out the ashes. Another option is to raise the floor of the firebox, which I'm a bit against because I like the space for larger chunks or small logs.

    What are your I have no option other than shortening the firebox, can this large of size work? 

    Last edited: Apr 19, 2016
  2. mp4s

    mp4s Fire Starter

    Too large of a firebox and you'll have a hard time managing your temps - too much of your energy will be used in the firebox itself. I would cut it shorter and weld a flat piece of steel on the end and cut the door out of that.

    Sent from my SAMSUNG-SM-G870A using Tapatalk
  3. mike dula

    mike dula Newbie

    Thanks. Ive heard others say a larger firebox is not usually an issue. But I can see the energy waste in heating the whole thing. Again the thought for keeping it long was to be able to put some sizable chunks of wood in it. 

    Looks like I'll be shrinking it down, since the football cc to fb opening is already set to about 6" I need to tune the firebox size to match, no? My cook chamber is 4620ci, and FB is 2827ci. this says my FB to CC opening should be 22 sq in. or have a 7.6" chord. 

    To bring it down to 6" I need to shorten the FB from 25" to 16" 

    Does this sound about right? Also, since this is my first smoker expo is creating a RF overkill on something this small? 

    Thanks all
  4. mike dula

    mike dula Newbie

    Not much feedback, i'll keep it rolling. I PM'd Dave and he said the large firebox should be fine. The box is well sealed and should create great heat, and gives me the luxury of extra space will just need to keep the intake well adjusted, that heat has nowhere else to go but through the chamber...This size box will allow for some nice sized sticks. 

    Still looks like a turd but once built will get the paint stripped and wire wheeled and painted. Not sure what color, but not black. 

    Got work on the firebox done tonight. Decided on a homeade handle and heated and hammered the flat stock into a handle. Im happy with how tight and well sealed the FB looks. Tomorrow the FB grate will be built as well as the intakes. Should be getting the RF plate this weekend and the stack and drain mocked up. I found some 2x4 tubing and angle iron in the shed so I should have enough for the legs. 

  5. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    Did you use the "football" shape calculator in my tutorial ???    Just curious...  

  6. mike dula

    mike dula Newbie

    No Dave I didnt not initially. Now im basically measuring to see how off I am. I found this great info all after I cut and welded the opening, didnt even think about the physics involved. .I think I mentioned in my intro I like to do things the hard way :) I found this site, and the match involved after googling larger sized fire boxes on smokers and to see if it would be an little did I know. 

    Heres what I have though. 

    Diameter X Diameter X 0.7854 X Length = Volume in cubic inches / 231 = Volume in gallons
       Measured inside end to end center point of outside dome and diameter 14 x 14 x 0.7854 x 30 = 4618ci or 19.992 gallons

    Volume in cubic inches X 0.004 = FB/CC opening in square inches
       Calculated Volume: 4618 x .004 = 18.472 sq/in

    So using your calculations and a target of 18.472 / 2 = 9.236

    My Chord AB is 7.5"

    Segment Height: Top to bottom of football opening is 3.5" divide that by 2 (for the football half) segment height is 1.75

    Chord AB = 7.5

    Segment Height ED = 1.75

    Gives me a surface area of 9.12

    Full area 9.12 * 2 = 18.24

    Slightly undersized the area your calculations gave me...And, oddly close considering the cut was a visual educated guess.

    These look right to you?


    I see now you are calculating off of the Radius and Segment area. So, these numbers, as close as I wish they we're are likely off. Will need to re-measure more when I get back to the house...
    Last edited: Apr 22, 2016
  7. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    Here you go...   Any questions ???   Beat on the back door....   I don't answer the front door this time of year..  Too many politicians wanting to make me promises they can't keep...  or wanting to smile and be my friend...  

    ================== edit 12/24/14

    If you are planning a round FB and round CC.... Follow this.....

    Volume in cubic inches X 0.004 = FB/CC opening in square inches.... the answer is divided by 2.... then that number is solved for in the circle calculator by manipulating the segment height.. when the radius and segment height box is checked... That will give you the lower and upper halves of the football opening... then double the new segment height number and run the calculator... (if originally it was 4", put in 8").... that's the width of the football, AND the new height of the RF plate... and if you look at the chord AB number, when the 8" segment height was crunched, that is the width of the RF plate....
  8. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    Your tank is 30 x 14..... = 4600 cu. in.

    x.004 = 18.4 sq. in. FB/CC...  under the RF plate and at the end of the RF plate UNLESS you have a football for the FB/CC opening.. We will calculate that later...

    x 0.33 = 1500 cu. in. minimum for the FB...

    4600 x 0.001 = 46 4.6 sq. in. for the air inlets on the FB usually divided up 20% upper air and 80% lower air inlet...  my mistake, great catch...

    4600 x 0.022 for the stack volume = 101 cu. in. / .7854 x 2 x 2 = 32" stack height....

    Circle calculator....  Radius 7"....  segment height ED 2.5"   =  segment area of 18.6 sq. in..

    The numbers in RED are too small for this size smoker...   we will make them bigger...

    Seg. Hgt. of 2.75" and area of 21.4" will work...  for our new calculation of the football...

    21.4 / 2 = 10.7 ..   substituting 1.7" seg. hgt. in the circ. calc. = 10.65 seg. area...  close enough...

    inserting 3.4" for the new seg. hgt. for the football (2 x 1.7) the CHORD AB, the width of the RF plate, is 12" width...

    and the seg. area is 28 sq. in....  That is a good number for good air flow...

    The RF plate should be mounted at the top of the football shape...   3.4" up from the bottom of the CC.... 

    The end of the RF plate should be about 3.5" from the end of the CC.... 

    The exhaust should be about 2.5" - 3" ID and be approx. 34-36" above the CC...
    Last edited: May 3, 2016
  9. mike dula

    mike dula Newbie

    10-4...This is priceless info Dave! I cant thank you enough. I'll get to work making the FB to CC opening a bit larger. Sorry, it took me a second to realize I needed the radius for proper calculations of the CC to FB. I left a little metal on the opening that can be ground off. Tomorrow I'll see if that can get the opening larger...otherwise, might need to cut and re-weld. The price I pay for building without research first. 
  10. mike dula

    mike dula Newbie

    Hopefully this pic helps explain the measurements. Both tank are different in diameter, so they both have a different radius, the top of the football shape will be taller than the bottom because of the smaller radius.With the metal left over from the firebox, I can grind it off, this will give me a chord near 8" and a total segment height of up to 4" if needed but more realistically 3.75.To Open the hole up. 

    Dave I noticed you mentioned a segment height of 2.5? Thats the segment per each half? Giving a total of 5"? ...See pics. I dont recall using 2.5. 

    If im not mistaken, 3.5 total height / 2 = 1.75 Segment, with a 7" radius = a surface area of 11.106 or a total surface area of both halves 22.212

    This is where Im at now, I can open it up to 3.75 (or even 4") total segment height,  so 3.75 / 2 = 1.875 = 12.281 surface area, or total of 24.562


  11. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    That' close enough....  It should work fine.....  I'd go with it....

    There are 4 calculations for the football....  1- the original size of the opening for a regular reverse flow....   2- half of the area for a "temporary" segment height for the football... 3- doubling the 1/2 height area for the football opening... 4- calculating the football width and the AB chord for the RF plate...   

    If you put your tape across the top of the existing opening, do you get something close to 12" width for the RF plate...
  12. mike dula

    mike dula Newbie

    Its almost exactly 12in I bought a 12" plate tonight and it's going to work perfect...
  13. mike dula

    mike dula Newbie

    Dave what is the minimum RF plate thickness for this of small size smoker?? Starting to think what I have might be too thin, but too thick will keep the weight down so I can move it.

    Got this little guy standing on all 4's and built the firebox tray....Although the handle wont work, its completely in the way of being able to pop open the handle and throw in some wood.

    Last edited: Apr 25, 2016
  14. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    3/16" minimum....   For better performance, spend a few bucks and go with 1/4"....
  15. meatbox

    meatbox Newbie

    I like the fire box tray pullout. Nice build! Looking forward to seeing it done
  16. mike dula

    mike dula Newbie

    Thanks! Its slow but getting there. I had to redo the handle tonight. I made it smaller and flipped it to the bottom so that I could more easily throw sticks in it. 

    Thanks Dave, the sheet I have is only 16 gauge. I'll need to source something else. My problem is the metal shops around here are only open during my hours of work so I can never get time to go out there....still trying to locate a stack as well. 
  17. mike dula

    mike dula Newbie

    Not a lot to update on, even though I spent about 5 hours working on it last night after work. I got a huge amount of rust out of the inside. I was able to get a 3/16 plate for the RF plate, and a 3" stack. Got the plate fitted to the curves of the tank last night, built the drain channel under the plate then cut access holes down the plate to allow grease to drain down the angle iron under the plate and down to the large valve. 

    The plate is slightly angle downwards towards the valve. and I didnt "V" the plate but built a dam at the end. Im having hell with my welder, mostly the auto darkening mask is giving me fits, making it hard to see while welding, and the rust. Im weling the plate in fully only because I dont want grease to go down the sides and seep near the heat source. 

    I dont have pics up yet of the grooves cut in the plate for the drainage, and last night I startup up on makng the cooking grate.


    Last edited: Apr 28, 2016
  18. mike dula

    mike dula Newbie

    Starting to look like a smoker. Got the plenum and stack on. Its a 3" 5/16 wall pipe, at 32" tall. 

    Firebox inlets WOULD be done, but ran out of welding gas...Local tractor supply was out. Its 3 - 1.5" holes, two at the bottom, one at top, w adjustable internal sliders. 

    Plenum and stack (dont mock my welds)

    Despite the picture angle, the stack is level....

    Lower intake...ran out of gas before I could finish welding it. 

    A little of the plate and drain...with the slide out rack. 

  19. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    4600 x 0.001 = 46  4.6 sq. in. for the air inlets on the FB usually divided up 20% upper air and 80% lower air inlet...

    Looks like you will be on the short side for air inlets to the FB...   Any particular reason for that ??
    Last edited: May 3, 2016
  20. mike dula

    mike dula Newbie

    4600 x .001 = 4.6 right? going of this number 1.5" diameter intakes * 3 = 4.5. 

    If its "46" sq in. Thats TWICE the size of my FB to CC opening, or, basically leaving the door open. 

    am I off on this? The front plate I left some roof to square them out if needed, but nowhere near 46 sq in. 

    Last edited: May 2, 2016

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