Reverse flow theory

Discussion in 'Reverse Flow' started by ribwizzard, Jul 1, 2013.

  1. Thats probably what I will work on, ...just not sure if I want an adjustable dampner or just play around with some plates with different size openings that I can play around with until I get it right, then weld in after I get it the way I want.

    I fab'ed everything up using foam board and have been cutting out all the pieces this week, should go together fairly quickly over the weekend and I promise to get some pics, right now its just a bunch of individual parts.
  2. I've thought a lot about adding fins and channels under the reverse flow plate as suggested, but I really dont want to experiment to far with this one...

    I've done six reverse flows over the past year and changed a few things on each and I'm trying to take the good from each, and trying to determine why one was better than the other. #19 just cooks so good and is so efficient for that size cooker, I believe its due to the over sized firebox and the over sized opening to the cooking chamber. It was burning everything on that end before I redesigned the reverse flow pan, but once I did that it has been awsome. #22 was a perfect little smoker as well, just too small and was a quick build, gave that one to my Mom. But it was similar, A was bigger than B,  All the others I built with A being smaller than B, and even though thy cooked OK, they are not outstanding like those other two.take longer to heat up as well.
  3. Ok, made a cart and set it on there so I could get some pics. Its a little tall, but so am I and once I get racks finished and everything dressed out , it shouldnt look so gangly

  4. Here is the reverse flow plate just set inside, it will be installed about 1/2 inch higher, and that will decrease the slope as well, the white square is where the heat will enter the chamber, black sharpy lines are cooking grate placement,

  5. Here is the reverse flow "dome" I will be working it with a ball peen some to give it more "shape" but now you can see the ideal.

  6. with the reverse flow plate 1/2 inch higher, that will put the bottom about an inch higher, I'm going to "hawg" out this opening come more after I get the deflector plate welded in.

  7. So, I'm going to cap the firebox lid with 3/16" plate,

    question...I'm torn about the ideal of putting insulation under the cap,  ... I like the ideal of not loosing so much radiant heat,  but I also like the ideal of being able to heat up a pot of beans on there as well.

    Any input?

  8. jarjarchef

    jarjarchef Master of the Pit OTBS Member

    Very interesting build. I am curious to see how this turns out for you. You always have great builds and information.
  9. Well, this one is to be my weekend warrior, the one I'll be doing 90% of my smoking on, so I hope it will cook like I think it will.  I have two more to finish for Christmas presents, and that's it for a while.
  10. sqwib

    sqwib Smoking Guru OTBS Member

    Rib... you're on a roll.

    Looking good
  11. smokejumper

    smokejumper Smoking Fanatic

    I think that insulating that section of the top will not buy you much, unless you were to insulate the entire firebox.

    When you are not warming beans, you could be using that area to pre-heat splits as well.
  12. Thanks Smokejumper,

    I decided to just cap it, the air gap should cut down on some heat loss and I wont have to worry about the paint burning off either.
  13. So...trying to decide on pipe dia. for exhaust stack. I want to go big , to reduce pressure.    Point "b" is 3"x 9". I'd like stack to be about 32" long.

    I have both 3.25" and 4" I.d. pipe.  The 3.25 looks good sitting on it, the 4" does look huge. I'm not sure if the 3.25 will give me the flow I want.


  14. And I still cant get my head around what they are using to determine length required. How does 18 inches of 3.25 id pipe = 12 inches of 4.0 id pipe
  15. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    3.25 x 3.25 = 10.56 x .7854 = 8.3 sq. in.

    4 x 4 = 16 x .7854 = 12.6 sq. in.

    12.6 / 8.3 = 1.52

    1.52 x 12" = 18.2"      It's all about the area of the pipe.... 
  16. Both work out to about 150 Cubic inches. Keep the updates coming, lots of interesting ideas in this thread.
  17. But your still pushing air through either a 3.25 in or a 4.0" orifice,  so the beginning of the pipe, the orifice is what will dictate how much volume can enter the pipe, or exit the cooking chamber, right?

    Or does the hot air rising in the pipe actually create that much draft to overcome the orifice size?

    Do you guys think that either18 inches of 3.25 id pipe or 12 inches of 4.0 id pipe will perform the same when it comes to and exhaust stack?
  18. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    From Feldon's calculator......  

    Chimney Size
    Enter the diameter of your chimney pipe to find out how long it should be.
      • A chimney that is too short may produce insufficient draft (drawing of air). A chimney that is too long may cause the air to cool before it exits, reducing effective draft and worse, dripping of exhaust materials onto food!
      • Many horizontal smokers have an exhaust between 30-40 inches in length, but there is no hard and fast formula.
      • If you are building a horizontal smoker with a vertical cooking cabinet, realize that the cabinet partly acts as an exhaust, thus you may greatly shorten the chimney.
  19. sqwib

    sqwib Smoking Guru OTBS Member

    No I don't. but that is only an opinion

    I would opt for shorter length and larger diameter.

    There has to be some restriction the narrower the pipe... adding more length to a narrow pipe will not draft or draw as good as a shorter pipe with a larger diameter.

    Maybe adding a little length to a 4" pipe in place of a shorter 5" pipe would be ok, but there's got to be limitations... lets say a 2" pipe would have to be 8" long, I don't think that would work as well as a shorter larger diameter.

    I may be wrong but wouldn't the pressure increase in a smaller diameter, taller pipe??
  20. I have no idea about the science behind the flow of a smoker, but displacing 150 Cubic inches is still displacing 150 ci same force necessary I think.

    Again I have no knowledge of the details involved, but i did stay at a Holiday Inn Express before.

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