Smoker Build question

Discussion in 'Reverse Flow' started by sqwib, Dec 16, 2009.

  1. sqwib

    sqwib Smoking Guru OTBS Member

    I got my tank and have someone working on the firebox.
    I pretty much have all the info I need too start this weekend, but have a few questions.

    1) when i cut the hole for the firebox entering the smoke chamber, do I need to put a plate in the smoke chamber above the firebox inlet SO THAT the heat will go underneath the plate or is that only for reverse flow?

    2) If I don't need the plate how far should the meat rack be above the fire box inlet?

    3) Should I use plates in the smoke chamber below the firebox inlet for thermal mass and if so would 1/8" plates be ok? and what distance from the meat rack should the plates be.
    I'm only going to get 1 shot at this and don't want to mess it up
  2. roksmith

    roksmith Meat Mopper OTBS Member

    Yea, even if you don't go reverse flow, you still need at least a deflector plate to get the fire/heat going down and not directly up under the racks otherwise the rack surface nearest the firebox will be completely useless.
  3. sqwib

    sqwib Smoking Guru OTBS Member

    Ok so I need a plate the length of the chamber but it needs to be above the entry hole of the firebox. correct? can the plate be 1/8" plate steel?
    I have a 20" diameter tank, if I install the meat rack at 1/2 of that it will be 10 " from the bottom and my firebox hole into the smoke chamber needs to be 7” x 8.5”, should I make the hole something like 4.5" high x 12" wide as opposed to the 7" high x 8.5" wide.
    I know the opening must be a certain size but does it matter how it is oriented?
    I am a bit afraid that if i put the deflector plate above the inlet hole of the firebox that the deflector plate will be too close to the meat and act more of a convection than smoker.

    Like I said before I got one shot at this and don't want to mess it up.
  4. roksmith

    roksmith Meat Mopper OTBS Member

    1/8th in steel is OK, 1/4 would be better and less likely to warp from the heat.
    The orientation of the hole between the firebox and cooking chamber doesn't matter, but you do want the opening in the firebox to be as high in the firebox as you can. Once the fire is up an cooking, it won't matter so much, but if your hole to the smoke chamber isn't high, the smoke may tend to back up a bit when you first start it up.

    You'll want to leave as much space as you can between the deflector or baffle and the cooking grates as you can, but a few inches should be ok. Heat will radiate up from the plate, but that's much better than the flames jumping up into the bottom of the meat. Some folks like to put a water pan between the baffle and the cooking grates. This would help keep the direct heat down to a minimum. It also helps catch some of the dripping grease and stops flame ups and general goo build up.
  5. meat hunter

    meat hunter Master of the Pit SMF Premier Member

    Sqwib, do you have any photos of what you have to work with? I take it you do not want this to be a reverse flow?
  6. sqwib

    sqwib Smoking Guru OTBS Member

    I understand the hole to the fire box being at the highest point but when you say "the hole to the smoke chamber isn't high smoke may back up" this is where i am at a loss.
    If the firebox is 18" high and the chamber is 20" high and I mount The firebox midway on the smoke chamber then the highest point would be where the meat rack is, shouldn't the smoke inlet into the chamber be lower, because if I add deflector plate at that level then they will be at the same level as the meat rack.
    I was thinking of having the top of the fire box at around 1/3rd the height of the tank so the inlet will be lower than the meat rack. does this make sense?
  7. sqwib

    sqwib Smoking Guru OTBS Member

    You know I think the more I read the more overwhelmed I get, note the quote in my signature, that is from the heart.
    Not sure on how to do a regular one let alone a reverse flow.

  8. meat hunter

    meat hunter Master of the Pit SMF Premier Member

    Ok, first things first. RELAX[​IMG]. Like you said, you have one chance to do it so lets no rush into anything. It took me nearly 2 months of asking questions before I built mine. And a reverse flow is no harder to build than a regular cross flow one. Nice thing about a reverse flow, you can maintain a constant temp better, but they do use a tad bit more fuel, wood. Let me get some pics of some builds for you to look at. Give you an idea of how your firebox should be placed, spacing and all that other good stuff. This may take me a day to do it as I am smoking some venison pastrami as we speak, or type rather. When I go in to night at work, I will look for some posts on here with some diagrams for you. Very very simple.
  9. sqwib

    sqwib Smoking Guru OTBS Member

    Im sorry I am starting to confuse myself now.

    The hole to the smoke chamber is dictated by the cut out in the fire box, so that means if I cut the hole out in the firebox at the top I will need to mount the top of the firebox at least 4" or so below the cooking grates.
    So if my diameter is 20" and the rack is halfway and I need 4" then I should cut the smoke chamber 6" from bottom.

    Then when I do the deflector plates, they will go directly up to the top of the firebox hole so the heat can travel underneath.

    Will 4" be enough?
    Does this sound like a go?
  10. sqwib

    sqwib Smoking Guru OTBS Member

    Thanks I greatly appreciate it, meanwhile I'll smoke some deer jerky, smoking the good stuff always calms me down[​IMG]
  11. meat hunter

    meat hunter Master of the Pit SMF Premier Member

    What are you using for a firebox? Round or square? Dimensions?
  12. sqwib

    sqwib Smoking Guru OTBS Member

    heres what I got so far

    Smoking chamber
    80 gallon tank = 6,160 Cubic inches smoking chamber

    Stack Design
    Exhaust Chimney Volume
    324 cubic inches needed. Need 4” pipe at 25.8” or 5” pipe at 16.5”. 10” of the pipe will extend down to the level of the meat racks.
    Not sure if I should add the 10" to the required height
    • 4" stack 24" above smoke chamber.
    • cut diagonal and lower to meat rack
    • mount opposites side of firebox
    • install a damper
    Firebox design Firebox ¼” plate steel
    18” High x 18” wide x 20” deep = 6,480 Cubic inches. 6160 is the minimum for the 1/3rd rule of thumb slightly larger than needed but its better to be bigger than smaller.

    Firebox air inlet opening, under coals
    We will need 4, 2.5” circular holes underneath the coals for the inlet
    Sliding plate with 2.5" holes spaced at 2.5" for air intake control

    Firebox to cook chamber opening
    The firebox hole into the smoke chamber needs to be 8.5” x 7”
    • ¼" plate steel
    • Air Intake
    • air inlets are all below the "wood level"
    • Thermal plate 2" from top
    • ash grate 4" from bottom
  13. meat hunter

    meat hunter Master of the Pit SMF Premier Member

    Thats what Im talking about, good info.
    I take it then the firebox is square. Are you limited to the 8.5 x 7 you have stated in your design? Would you be able to go wider and make the height shorter? Reason I ask this is the more room between the meat grate and the opening the better, as its best to have some room from where your tuning plates start to the grate. Unless of course that won't be an issue, then never mind.

    On your stack design, I would go with the 4" pipe, and don't see a need to run it any higher than 24" out the top. Some people will argue that running the stack down to the grate is the way to go, others say the opposite, to have it flush with the top. I am of the later style. I run a reverse flow and its not as common on them as it is on a cross flow. The reason I went this route is that in order for the smoke to escape the main chamber, it has but only one exit, the chimney. Which means that the whole chamber from top to the bottom of the stack will be filled with smoke. And sometimes that smoke can become overpowering and stale and thats not what one wants coming in contact with his meat. The smoke should just gently roll by kiss the meat. But there is a way to have the best of both worlds on this one, and its a modification that many on here do. Run that stack only an inch or 2 in from the top, and if you have to, use dryer vent to do the rest. Adjustable and it won't take up grate space like a solid pipe would. Also, if you ever put in a second rack, the chimney pipe is not in your way. You simply use a large hose clamp and clamp the flexible dryer vent to the stack. Another reason that the stack is run down to grate level is to keep more heat in. In a cross flow design, once the heat enters the main chamber, is starts to go up right away and the opposite end of the chamber can be quite cooler then the end where the firebox is at. Having the stack all the way down allows for more heat to stay in, but since you are going to go with tuning plate, you can adjust to a degree, how much and where you want to focus the heat. It is more or less a perforated reverse flow design.

    Your tuning plates should start at the top of the cutout of your firebox. Look at the designs I included here. The first one is from a site I had bookmarked about a year ago. I am also including the text that came with it.

    To help understand tuning plates, the gaps let heat and smoke rinse as shown below. The wider a gap is, the more heat and smoke rises in that immediate area, the hotter that area becomes. Since the firebox end is much hotter, the gaps at the firebox end are usually very small to reduce the heat at this end and help guide more heat to the far end (in fact the first tuning plate in the Liberator is welded to the firebox and the next adjustable plate is butted up against the first with no gap). Although the first gap has little or no space between plates, sequential gaps are slightly bigger than the one proceeding.

    Typically tuning plates are used to even out the heat across the pit but they can also be used to create "zones". If you were to want one area hotter for cooking chicken and a cooler area for slow smoking brisket at the same time, these zones can be made by opening the gaps or removing a tuning plate while closing the gaps at the opposite end.

    This image was made my me. Nope, I didn't make a career out of graphic design, go figure[​IMG].
    What I wanted to show in this one was the firebox in relation to the main chamber. If you can get your hole dimensions shllow and wider, it would be best as the heat and smoke are entering from a lower point, if that makes sense. The area with the X's in it is the area that would be cut out.
  14. sqwib

    sqwib Smoking Guru OTBS Member

    ok here is what i got


    1)Red Line represents the cooking grate level
    2) the blue line represents the tuning plates 4" below the cooking grate and where the top of the firebox will be welded
    3) the black area represents the opening into the smoke chamber.
    which is 16" x 6-1/2" high

    The illustrations you showed me is for a basic horizontal, correct? can you show me what i would need to do to make it a reverse flow, that way I can form an opinion?
    I am not limited to the 8.5 x 7 only the cubic inches look at my pic and you'll see how much I can safely open up the smoke chamber...its roughly 16" x 6-1/2" opening in the smok chamber when its cut out for the firebox. That well give me a 4" space between the cooking grate and tuning plates
  15. sqwib

    sqwib Smoking Guru OTBS Member

    Ahh so the tuning plates do not go to the end.... there's a deflecting plate there.
    Just spoke to the firebox builder and he said ¼ steel would be hard to do because he wants to bend The steel so he said ⅛ would be easier. So I suggested 2 layers then ¼ steel welded on the back and ¼ for the door. Will this work?
  16. roksmith

    roksmith Meat Mopper OTBS Member

    To make the above diagram a reverse flow.. basically turn the tuning plates into one solid plate running all the way from the firebox to a few inches from the other end and then move the stack to the other end of the cook box.
    You are basically forcing the smoke down the length of the cook chamber then back across the meat on the way to the stack.

    There are some alterations you can make depending on what you are trying to accomplish, but those are the basics.

    Don't worry about messing something up.. it's nothing that can't be fixed with a cutting tourch and welder. I had mine back in for alterations every winter for the first few years.
  17. sqwib

    sqwib Smoking Guru OTBS Member

    Ok then...reverse flow it is.
    How far should the plate be from the opposite end of the firebox 4-6"?

    And if Doing the reverse flow, I would butt the plate up to the firebox and weld it in place, correct?

    Is there a preference to 1 solid piece or multiple peaces, I know it will need to be at least 2 pieces so it will fit in the tank.
  18. rickw

    rickw Master of the Pit OTBS Member

  19. bbq engineer

    bbq engineer Master of the Pit OTBS Member SMF Premier Member

    Hey SQWIB,

    Meat Hunter has put up some good info, and as additional info, I will direct you to this post from Coley Smoking BBQ. He also has some good graphics and description that may also help clear things up. (oops, Rick beat me to it, but here are direct links to a couple of great 3 and 21 in Rick's mentioned link).

    Tuning Plate design:

    Reverse flow:

    Take lots of pics of your build...I think this is going to be good.
  20. meat hunter

    meat hunter Master of the Pit SMF Premier Member

    The diagrams the bbq and dave put up are very informative. I was looking for them last night but I could not find them. Thats why they are OTBS members LOL.

    How far from the end of the firebox does the plate go? Rule of thumb, as many inches from the end and it is off the bottom of the main chamber. So, if that plate is 5 inches off the bottom, then 5 inches from the end.

    You can use either one or two pieces. Depending on how large you make your door, you may get it in in one piece.

    Yes, you but the plate up to the firebox and weld it. Im not one to blow my own horn, but I will include the one I made last year. Keep in mind thou that this smoker was more or less a project to see how cheap one could make a smoker by using material that was either laying around, very very cheap or free. Total invested in it was under 50 bucks and it works like a champ. There are some pics in there that should give you an idea of how things work, espeically the heat sink/V shaped plate.

    Also, like Roksmith said. Don't worry about messing anything up because with a torch and a welder, anything can be fixed. I too cut here, welded there and tweaked it until I got it to where I wanted it. This is going to be a fun build for you and you have some good people here who will help you best they can to make a great unit. Remember, take your time, don't rush, measure twice and cut once....

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