airflow control design advice, please...Reverse flow offset

Discussion in 'Reverse Flow' started by mateo oglesby, May 29, 2014.

  1. mateo oglesby

    mateo oglesby Fire Starter

    I am almost finished with this offset, reverse flow smoker... I am at a critical point where I will begin closing the unit. and sealing it. I need to design an air flow control mechanism for the connection point between fire chamber and smoke chamber....

    last photo shows direct view of air flow. The bottom opening is reverse flow...

    any thoughts or ideas would be greatly appreciated.

  2. lendecatural

    lendecatural Smoking Fanatic

    Most of the guys run RF smokers without using a damper mechanism between the fire box and the cook chamber. Temperature is typically controlled by adjustment of the air intake vents. Have you used a calculator to determine the requirements of the openings? What is the size of your cook chamber and your chimney?

    For reverse flow, it is recommended to multiply the firebox to cook chamber area by 1.5. That resulting area should be used for the area under the RF plate and on the end where the smoke turns to enter the food grate area.
  3. mateo oglesby

    mateo oglesby Fire Starter

    I am at 142% according to calculator, not bad for a shot in the will be intereststing with no goes...
  4. lendecatural

    lendecatural Smoking Fanatic

    Not bad at all! How does the chimney look? If you plug in your numbers and get the results, go to the bottom of the page and copy and paste from the email link box. It will save all your numbers and you can post it here for us to have a look. Once you get that girl fired up she should hold temperatures wonderfully!
  5. lendecatural

    lendecatural Smoking Fanatic,14,0,86.63,1.55&fc=17.15,24.25,231.00

    Your firebox is 142% bigger than it needs to be, that's not a bad thing at all. The calculator figures some parameters based on the firebox so to make the numbers come back out correct, I adjusted it back close to 100% and then applied those numbers to get a firebox to cook chamber opening of 231 sq-in. and multiplying by 1.5, you need an opening and area under the RF plate of 346.5 sq-in and the same at the end of the RF plate where it turns up to the grates. It looks like you only have 1/3 of the required area under the RF plate for good air flow. What are the dimensions of the lower opening going into the cook chamber?
  6. mateo oglesby

    mateo oglesby Fire Starter

    sorry so long to respond...

    demensions for opening...

    17 X 5 (2)
  7. mateo oglesby

    mateo oglesby Fire Starter

    The openings(2) are just slightly larger than called for... however, bottom opening  is RF and the top opening goes straight into cook chamber.

    Can I run it like that?

    Also, where do I position chimney, what diameter and how long does flu need to be?

    The cook chamber is more or less 2 mtrs sq. rectangle...and it is indoor... I am attaching more photos to try and illustrate... the hole at the top right of photo is for chimney...

    where to position atop the chamber and what length off of the chamber and what diameter?

    waiting for photo, will post shortly...
  8. mateo oglesby

    mateo oglesby Fire Starter

  9. mateo oglesby

    mateo oglesby Fire Starter

    that round hole above the rollup garage door is the vent hole...

    If I leave the top firebox opening direct to chamber and position chimmney in the center of smoke chamber vs. the end closest to firebox farthest from vent.... will I get good flow?
  10. So just to get this straight, you have two openings from the FB to cook chamber. One underneath some sort of floor or Rf plate the other flows directly into the cooking area? And both of these openings are individually the correct size for flow or only when added together?

    Personally I can't see running them together, warm air rises and will take the path of least resistance. You may get some flow threw the lower vent but I imagen the upper will command more, at which point you have an offset, and should place the chimney on the far side.

    Unless your plan is to open the upper vent to warm up your box then close it allowing the rf opening to do your cook.

    I wouldn't know where to place a chimney in this scenario. As a RF would place it closest to the FB, but then your whole cook chamber would not warm. At the begining.

    I'm not saying you couldn't run both snd get a balance out of them. Would take some fiddling for sure, also wondering about the long chimney running all the way outside affecting flow.
  11. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    Mateo, morning...... Who designed that smoker..... It's a little different than I am familiar with.... Have you seen that design in operation... If so, how well did you think it performed.....

  12. mateo oglesby

    mateo oglesby Fire Starter



    Hopefully, I will be proud to say I designed it myself. It started out as just an offset(1m2 fb and 2 m2 smoke chamber. After studying where to place chimney, I decided to add an rf plate. Once the build started, I had to leave the area for 10 days or so and while I was gone, the bricklayer goofed and the result was one fb 2 cook chamber openings is above the line instead of both below. I decided over the weekend to change the fb 2 cook chamber opening to reflect original design. That will place both openings below the line and I am confident, based on good calcs that the air will flow.

    I am only left with a few questions...regarding chimney height.

    1. Is this referring to the length of chimney from the top off the cook chamber going vertical?

    2. What about from that point to my exit point on the wall (this is indoor project) According to calcs, at 8 inches diameter, it calls for 40 inches and at 10 it calls for 25 inches...If I go with 10 inches dia. and 25 in length, do I the come off the top at angle and shoot upwards to the hole in the wall? How will the additional upwardly angled 100 inches of chimney affect flow?

  13. mateo oglesby

    mateo oglesby Fire Starter

  14. mateo oglesby

    mateo oglesby Fire Starter

    Im almost ready to put in chimney...any response to my posts?
  15. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    Last edited: Jun 14, 2014
  16. mateo oglesby

    mateo oglesby Fire Starter

    can you give link to thread or comment further please
  17. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    Feldon's is designed for steel smokers.... I don't believe it will work when building a brick smoker...
  18. mateo oglesby

    mateo oglesby Fire Starter

    well, we will test that theory...I am putting in chimney on Monday and tuesday, and will fire it up shortly thereafter.

    I have a hybrid of offset and RF going and I am using his calcs for chimney diameter and height, my opening for air intake and fire chamber exit just happen to line up with his. I am venting the top opening to the chimney side of the smoke chamber with a damper to shut off completely when smoke chamber gets to temp. The bottom opening is set up reverse flow to other side of chamber opposite the chimney. It will be interesting...
  19. mateo oglesby

    mateo oglesby Fire Starter

    Ok, I am 3 smokes into this masonry smoker down here in the capital city of Paraguay, Asuncion. I am ventilating with a 40 ft stack that extracts the smoke at 150 CFM. I am not posting pictures because I got pride. Just not pride in my BBQ, YET. Here is my delimma... I have so sell my food cheap. REAL CHEAP. REAL DAMN CHEAP. Brisket is out of the question and I am looking for a cut of meat that is newbie friendly but can also showcase the smoke flavor profile. I am getting good penetration of the red into the meat I am using but I am not getting a deep penetration at the surface of the meat. Is this because I am using an extractor? 

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