36" side fire box backyard build

Discussion in 'Side Fire Box' started by iridelow, Aug 24, 2015.

  1. iridelow

    iridelow Fire Starter

    Hey guys, I'm new to the forum but not new to fabrication and BBQ/smoking meats.

    I am about to start the build of my new side fire smoker. I do have some questions to start with, and also if you have any tips or tricks, don't hesitate to share them.

    I work at a pipe fabrication shop and we had 2 pieces of 36" pipe that was getting scrapped so I claimed them. It will be 36" OD x .375 wall pipe, 6 ft cooking chamber and 2'-6" fire box.

    My plan is to do a double door setup door on the chamber with 2 slide out racks on each side (only because I wasn't sure if I could get away with a single door and not have any structural issues...I would rather have a single door)

    Fire box will also have a slide out rack for cooking directly over the heat when needed. I plan on putting a flat/warming area on the exterior of the hot box also.


    How big does the hole need to be between the firebox and cooking chamber? 

    Also, when I weld the firebox to the cooking chamber, should the top of the fire box be about at the mid-line of the cooking chamber? Or should it be lower? What is the proper way to locate/place the firebox?

    Can I get away with a single door setup and not have any structural issues?

    I like the slide damper from BBQ Smoker Supply..BUT what is the proper way to determine what size damper I need? They offer a few different sizes and I don't want to weld one in that is either too small or too large.

    (example: http://www.bbqsmokersupply.com/index.php?main_page=product_info&cPath=3_22&products_id=77)

    is there a science to the smokestack pipe size and length, if so how do you determine that?

    Thanks in advance for any help, and remember this will be my first smoker build so if you see anything that looks off please don't hesitate to speak up. I am open to any advise/opinions. The pipe is getting cut right now and I should have it home in the next few days.
    Last edited: Aug 24, 2015
  2. jckdanls 07

    jckdanls 07 Master of the Pit Group Lead OTBS Member

  3. iridelow

    iridelow Fire Starter

    Holy cow that's some kind of formula....I've been messing with that for some time and I must be doing something wrong. I'm not coming up with anything remotely close to correct. Ill keep plugin away! thanks for the link.

    Does it matter that I am not doing a reverse flow?
    Last edited: Aug 25, 2015
  4. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    I'm not coming up with anything remotely close to correct.

    How do you know ????
  5. iridelow

    iridelow Fire Starter

    I figured it out. I was using the formula incorrectly to figure my football shape cutout.
  6. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    Volume of the Cook Chamber.... Use the Inside Diameter of the tank...

    Diameter X Diameter X 0.7854 X Length = Volume in cubic inches ..35.25x35.25x0.7854x72=70,265 cu. in.

    Volume in cubic inches X 0.004 = FB/CC opening in square inches=281 sq. in.

    Volume in cubic inches X 0.004 = Area under the RF plate in square inches = "

    Volume in cubic inches X 0.004 = Area required at the end of the RF plate in square inches = "

    Volume in cubic inches X 0.33 = minimum volume of the Fire Box > 23,187 cu. in.

    Volume in cubic inches X 0.022 0.017 = Exhaust Stack Volume in cubic inches, above the CC.... (ESV)
    (The increase in volume ~30% shows a dramatic improvement in equalized temperatures across the cooking surface... edit 6/19/15 )

    Exhaust Calculation..

    ESV in cubic inches______1546_______________________ ... = Stack Length in inches (36" +/-) = 31" stack.. 8" inside diameter
    0.7854 X Stack Diameter X Stack Diameter

    Adjust the diameter of the stack, until the proper length is achieved... be sure to measure the actual internal diameter of the pipe used..
    this is for round stacks only..
  7. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    Radius = 17.625
    ED.......= 7.25"
    Segment area = 144 sq. in. which is half of what you need...

    Soooooo, A football that is 14.5" wide/fat at the middle is what you need....

    Chord AB will be 34.5" that is the width of the RF plate for a RF smoker....

    Firebox air inlets......

    Volume in cubic inches X 0.001 = FB air inlets in square inches...

    Recommended upper and lower air inlets... Upper air inlet directly across from the FB/CC opening to facilitate moving heat from the FB to the CC, and insuring good air flow through the CC.. The lower air inlet should be situated at or below the fuel grate.... The two air inlets can share the designated square inches of opening....

    70,265 x 0.001 = 70 sq. in. 56 sq. in. lower and 14 sq. in. upper...

    If you are really going to build a "TRUE" SFB smoker with tuning plates and all that stuff.... Several of the dimensions can be reduced by 25% 33%.... If that's the case, use Feldon's calculator but.... when cranking out numbers, be sure the FB is at 100% of recommended because Feldon's uses the FB size to crunch all the other numbers, then make the FB larger for actual use.....
    Last edited: Aug 25, 2015
  8. iridelow

    iridelow Fire Starter

    That is exactly what I'm coming up with now thanks for your help Dave.

    I do plan to do a true side fire smoker with tuning plates, etc. My plan is to get the main structure assembled, once that is done I will add the odd and end features once the pit is standing on its own. I do however want to keep my firebox larger because I do grill over a flame quite often and would like the extra space. Will having a firebox larger than I necessarily need hurt me in any way?
  9. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    Nope... FB can be as big as you need.... The FB air inlets need to be below the fire grate and directly across from the FB/CC opening..
  10. iridelow

    iridelow Fire Starter

    That's great. Thanks Dave
  11. iridelow

    iridelow Fire Starter

    Alright well I got my material back home so in the next few weeks I should start having some progress. (ill post a few pictures tonight or tomorrow)

    One area that has me concerned is once I get ready to cut the door/doors (not sure if I am going to do 2 small doors or 1 big door yet). my plan is to lay the doors out on the pipe, cut the section where the hinges will land, weld the hinges in place, and then cut the remainder of the door out. Will this work without the door springing or warping on me? If I need to weld other braces in place I can do that I just wasn't sure how the material will react. It is pretty thick (.375") and Ill be cutting by hand not a torch. 
  12. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    The hinge pivot point, needs to be behind the door seal, so the door opens properly... If you use spacers, make them thicker than the door seal... That way, you can slide the door seal under the hinge .....

    Last edited: Aug 27, 2015
  13. iridelow

    iridelow Fire Starter

    I'm glad you specified that Dave. I thought the pivot point had to be directly over the cut between the door and tank. My hinges are elevated a bit so I don't think I will need the spacer.

    Heres my hinges I will be using. I have 4 of them total which should be plenty for my door

  14. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    If you want to run a continuous door seal, you will need a spacer.... at least on the door....
  15. iridelow

    iridelow Fire Starter

    hmm..... ill have to think on that one. Not exactly sure what I want to do on that one.
  16. jckdanls 07

    jckdanls 07 Master of the Pit Group Lead OTBS Member

    you could always put your door seals on the inside of the tank.. it will just make your opening just a tad bit smaller ... but it will look better with the seal on the door itself... this way it conceals the cut ...
  17. iridelow

    iridelow Fire Starter

    Well here is how it sits now. We are going to pick up some more metal today probably and I will post pictures as we make progress,

  18. iridelow

    iridelow Fire Starter

    I'm still gathering a couple other items before we dive into this project

    Hers my hinges, I still need 2 more for my firebox door

  19. iridelow

    iridelow Fire Starter

    well things have been rather quiet at the house. We have been travelling a lot, wife is about to have a baby, and I just took a promotion at work. I haven't forgot about this thing im still gathering a few parts, and kicking some ideas around in my head. Just last week we have started moving on this thing again.

    I did pick up some threadolets from work for my gauges.

    We started laying out the doors (didn't get a picture of the finished layout....Ill get one before we cut the doors out)

    I got a torch and a plasma back to the house so we should start making progress pretty quickly

    Eventually I plan on putting this pit on a trailer but for now I am just going to make some legs so I can get it stable, off jack stands, and remove the hoist for safety and make this pit useable. The trailer build will come at a later time. I have been messing around in AutoCAD with the legs and hopefully this week ill have something productive that will work for the pit.

    Last edited: Mar 3, 2016
  20. iridelow

    iridelow Fire Starter

    What valves are yall using for grease drains? I'm thinking 1" - 1.1/2" valve just because I know if I have this size pit loaded down with meat it will have a lot of drippings

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