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

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

  1. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    Mateo, evening...... 150 CFM...... are you sure...... like positive..... have you tested the air flow with pitot tube and inclined manometer...

    I'm gonna take a guess here.... Slow the exhaust air flow down to about 1 CC volume air change every 5 minutes... 12 CC volumes per hour..... I have no idea what that will do but.... 150 CFM is SCREAMING..... you have a convection oven...
  2. dcarch

    dcarch Smoking Fanatic

    If the garage is part of your house, I hope you have a few good CO detectors.

    Just saying.

    Very interesting project!

  3. lendecatural

    lendecatural Smoking Fanatic

    Pork butt, AKA Boston Butt cuts are quite user friendly to smoke. 

    Like Dave said, you are moving air too quickly through the cook chamber which greatly reduces the smoke of sufficient density available to get into the meat. Here is a link for a forced air system used with a probe in the CC that only moves enough air into the firebox to maintain the correct temperature. I don't know what kind of access you have to components and someone down there with electronics capability but it could be put together much less expensively than it sells for here...
  4. mateo oglesby

    mateo oglesby Fire Starter

    thanks, now I am learning about the forced air system.. I am trying to understand the total air flow path because I have to get the smoke 40 feet in the air before dispersal. Im really confused but I am willing to keep trying...

    If I use forced air I still have to move whatever smoke is exiting vertical 40 feet...maybe if I tie the extractor power to the blower power so that both function simultaneously but then I need a valve to shut off air flow to the chimney being affected by the extractor...

  5. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    How about something like this....

  6. lendecatural

    lendecatural Smoking Fanatic

    Very clever Dave!

    Mateo, have you tried to get it started using the exhaust and then run without the fan once the smoke turns Ninja when everything gets up to temperature?
  7. mateo oglesby

    mateo oglesby Fire Starter

    I got the info yesterday that told me I was pulling air too fast...I inquired in my circles about how to slow the cfm down on the on-off only 153 CFM extractor...once I was up to temp for an hour or so I unplugged the extractor and of course the smoke was perfect... I got excited and started showing people how well it was working and neglected to monitor temp. As temp came down, you guessed it, smoke started seeping from unsealed places on the chimmney and backing up in the kitchen itself.

    At this point my strategy is to try to create the optimum CFM from the top and to effectively seal the 40 ft vertical run.

    any advice on how to lower CFM on existing 1 speed motor?
  8. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    Put a "Tee" in the stack and suck in cold air between the fan and the smoker... use a slide damper to regulate the "make up" air....
  9. smokejumper

    smokejumper Smoking Fanatic

    You have a stack sized for about 10 times the BTUs needed for a cooker.

    An 'extractor" fan has no place on a smoker IMHO.

    If the stack will not draft without an exhaust fan, then you need to insulate the stack.

    That is the only way I know to keep the exhaust from cooling on such a long rise at the typical smoker flow rates.

    The stack diameter can also be reduced to at most 6". 4" would be even better.

    As in any smoker you should control the heat with the firebox intake damper.

    Also, it appears your "RF plate" is made of brick. That defeats the typical function of the RF plate which is to absorb heat and radiate it up into the cook chamber.

    Bottom line is: get rid of all the fans, insulate the stack, and build a small hot fire that you control with the FB air intake.

  10. lemans

    lemans Master of the Pit OTBS Member SMF Premier Member

    I hope the building inspector is a friend of yours.. Unless you have a permit , this build In your garage could void your homeowners insurance.. I would check that out before you light that baby up
  11. lemans

    lemans Master of the Pit OTBS Member SMF Premier Member

    Sorry. Your r not in the states so my post is off base. But I would still be concerned
    Be safe ..
  12. dcarch

    dcarch Smoking Fanatic

    Every time you restrict air flow to a fire, you are guaranteed to generate carbon monoxide.

    That is why now there are building codes requiring balanced and carefully designed fresh air intake for a house.

    For instance, if you have a fireplace in your living room going, the air going out the chimney is much stronger than the smoker’s stack, (or your gas heating boiler) and all the CO will be drawn inside the house.

    For instance, if you have a powerful kitchen exhaust fan turned on, same thing will happen.

    And for instance, if there is a breeze blowing on the opposite side of the house, aerodynamically this will create a suction in the house that will draw the CO into the living space.

    Please be very careful.

  13. radioguy

    radioguy Smoking Fanatic


    Since you are located in Paraguay, I don't know if you have ready access to a variety of materials or have to work with stuff you scrounge up.  10" diameter stack is very large and requires a large fire to draw properly.  For a small fire smoker your diameter should be in the 2-3" range. 

    You can control the speed of your fan with a solid state controller, not sure of your access to items like this.  The controller type would depend on the motor type / size /  you have on hand.  Speed controllers can be had via internet.     This is one site with controller selection tool, they sell on E-Bay too.  Many other sites out there. 

    I do like Dave's' idea on allowing outside air in, this would allow the smoker to "breath" more naturally.  Please be careful of carbon monoxide. 

  14. mateo oglesby

    mateo oglesby Fire Starter

    thanks for all the responses...

    to clarify, my smoker is 2 meters square and my fb is roughly one third of 2 meters square. My stackis 64cm high and 25cm in diameter THEN the stack takes a right and left turn exiting the wall and goes horizontal for 1 meter and then vertical for 12 meters...I am also on ground floor of 4 story building and I must move the smoke to the top of the building.approx 12 meters.

    I have taken advice and installed solid state controller on the extractor at roof level. That extractor will pull at 10% of 150cfm...probably still too much pull but I am working on lowering to almost nothing.

    I need to control the airflow exiting the smoker... there is nothing pre-fab at all with this smoker. All chimney parts were hand made from sheet metal. The chimney itself was sheet metal formed into cylynders with a bent lip that locks in place and gets pounded flat and then the individual lengths connected together by rivets. I am attaching a pic of the stack coming off smoker, the opening to the smoker from fb and the air intake opening on the fb and looking for a simple... really simple design (paraguay) to control airflow from initial entry point to smoker to chimney. 

    Once I have airflow controls, I am sure that the gentle pull of the extractor dialed 95% back will pull the smoke to the roof and solve my problem.
  15. mateo oglesby

    mateo oglesby Fire Starter

    the opening TO smoker FROM firebox is roughly double the air intake size. I dont have flashlight to show inside openings.
  16. mateo oglesby

    mateo oglesby Fire Starter

  17. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    What did you think of my drawing on post #25...

    Pull in room air and remove smoke from the smoker....

    A variable speed fan would work on the "Through the Roof" exhaust stack and still remove smoke from the smoker....
  18. mateo oglesby

    mateo oglesby Fire Starter

    i will show this design to the municipality and comment later
  19. mateo oglesby

    mateo oglesby Fire Starter

    main problem now is controlling air flow
  20. dcarch

    dcarch Smoking Fanatic

    Also consider having all chimney/stack parts removable, including the fan.

    At some point they all need to be cleaned. There will always be creosole buildup.


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