Firebox Design Review and Critique

Discussion in 'Reverse Flow' started by okiedave, Sep 1, 2012.

  1. okiedave

    okiedave Smoke Blower

    I'm kicking over potential designs for a reverse-flow smoker and I'd like y'all's input.  First, the basics: everything will be made of 3/8" plate, the cook chamber is 55" x 24" OD x 3/8."  Any framing (mostly in the firebox) will be 1" square tubing.  Why these design choices?  Because that's what they had at the scrapyard!  Yes, it'll be sturdy enough to survive a war, and heavy enough to anchor a battleship.

    Definitely going to be reverse-flow.  I think I want an insulated firebox (mostly to limit the temperature of the outside of the box--I have an inquisitive four-year-old).  I want to have the option of adding a BBQ Guru (or homebrew equivalent, more likely) later.

    With the pipe I have, the interior volume is 23,350 cubic inches.  That gives a target firebox volume of 7,784 cubic inches.  A firebox with interior dimensions of 20" x 20" x 20" is 8,000 cubic inches, or within 3% of target without requiring precision measuring.

    I've modeled a firebox design in SketchUp.  Please take a look and let me know what you think of it.  I've not built a smoker before, so I'm running strictly on theory here.

    The door is 20" x 14," and will be framed with the same 1" square tubing.  The firebox-to-cooking-chamber opening measures at 75 square inches (according to SketchUp); the target area, according to the calculator (thank you, Tom!) is 64 square inches, so I think everything is fine there.  It's a circular section running all the way to the corners of the box, with a radius of 12" to match the pipe.  That makes it about 5 3/8" at its widest point.

    The recommended air inlet is 24 square inches; I put two 4" x 3" inlets on each side.  Either side alone should give me sufficient airflow, but having a second set on on the opposing side lets me use whichever one is more convenient in the prevailing wind.

    The attached pictures show the box framed in with interior skins attached (but no exterior skin).  I'll provide the SketchUp file to anyone who'd like it, but I don't have permission on the forum to attach files.

    Thoughts?  Suggestions?
  2. Hi Dave,

    I like your numbers and design. The plan to double skin (insulate) your fire box will serve you well. I think you are well on your way...

    Please keep us posted...
  3. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    Dave, Morning and welcome to the forum.....  I like the plan....  Are you building a fire grate that sits above the air intakes to the firebox ???  That would work well and leave plenty of room for ash to build up during a smoke....  

    When installing the RF plate inside the smoker, think about placing it 2" or so above the firebox opening... that will allow for more space for air flow of heat/smoke etc and figure on about 115 Sq in at the end of the RF plate also....  friction loss in those 2 areas can be a problem in the future if not increased during the build...  I recommend those 2 areas be 1.5 times the firebox opening size....    

    Take lots of pics during the build....  Pics is what keeps us old timers going....   Dave 
    Last edited: Sep 4, 2012
  4. okiedave

    okiedave Smoke Blower

    Yes, I'll set the fire grate about 6" off the lower deck.  I also expect the RF plate to be several inches above the firebox ceiling, and possibly running 8" or so of horizontal baffle from the ceiling into the cook chamber, above the RF plate, to prevent the firebox end from getting too hot; I'll build it without and see if it's necessary on the trial run.
  5. Do yourself a favor and put 2 stacks on it. Open both during the run up to temp to get that air moving. Then close one to cook. Wish I had added this to my custom MeadowCreek 120.
  6. okiedave

    okiedave Smoke Blower

    Interesting idea.  What about using one oversized stack, and damping it partway?
  7. I put two on mine, (check out pics in my profile)  but to do over would probably just use a bigger one on the firebox end. 

    Dont ever use the exhaust dampner to control heat, that should always be done with firebox intake dampner.

    And while we are talking bout that. those vents should be the most important part of the firebox design. All you temp control centers around them. I recomend having a seperate vent below the firegrate (to control heat) and one above it (to control smoke) . I always build a pie shaped vent into the bottom of my firebox directly under where my fire will be. While Im getting the pit fired up, or when I need to make up heat due to rain or opened door , I can open it up and let 'er rip. The rest of the time I keep it closed unless I need to dump some of the ash building up under the grate.
  8. smoke_chef

    smoke_chef Smoking Fanatic SMF Premier Member

    Looks great. I'm jealous of your cad skills. I'll be watching this close. Post as much of the details as possible. I need all the help I can get when it comes time to build mine. (Jan/Feb 2013)
  9. okieleo

    okieleo Fire Starter

    Hows it going? Have you been able to work on this thing anymore?
  10. okiedave

    okiedave Smoke Blower

    I got sidetracked by life, and haven't done anything more.  I was just about to start back up this week, but the weather kept me from it; the past couple days have been nice, but I've been helping with the cleanup following the tornado.

    Bad, bad stuff up there.  I watched it go by from two miles away.

    I have decided on a couple of changes.  First, I'm not going to make it double-walled; that was turning into a significant fabrication challenge as I thought about it, and I really don't need the heat retention here in Oklahoma like I might further north.  I don't lack for wood, so if I burn a few extra sticks, so be it.  It'll also save me a few hundred pounds on the firebox, which pleases me.  I can always go back and do it later if need be (though admittedly at an even greater challenge).

    Second, I'm going to move to the high-low vent system as suggested, instead of the vents on each side.  I'll put them on the right, opposite the opening into the cook chamber.  The door will remain in the front, so I can stoke it from the cooking station.

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