Exhaust pipe damper

Discussion in 'Reverse Flow' started by amosis16, May 18, 2010.

  1. I've seen a few builds on here where guys put dampers on the exhaust stack. Is this necessary in a reverse flow?

    I've always smoked on my ECB and UDS and the rule there was to never close the top damper or the smoke will get stale.

    I think I recall reading somewhere about dampering the exhaust pipe as a last ditch effort to get the temp down after you've hut down all your fire box dampers.

    What is the opinion around here? Do I need to put one on my minismoker?
     
  2. woodsman

    woodsman Newbie

    see my thread from just today titled "dry ribs". This past weekend, I actually needed to close off my damper because it got too hot. My fault though. Would have to say leave it wide open all the time and control from the firebox. IMHO
     
  3. adiochiro3

    adiochiro3 Master of the Pit OTBS Member

    I keep the exhaust vent open on every smoker & grill I've used and contorl temp through the intake side. Exhaust vents are for keeping rain and critters out IMHO.
     
  4. bbally

    bbally Master of the Pit OTBS Member

    It is necessary for a proper configured reverse flow. As your skills with the unit increase you will want it. Keeping it open is the best way to start, but when you learn to balance draft and flue you will add a whole new dimension to your smoking options.
     
  5. diesel

    diesel Smoking Fanatic

    bbally,
    My curiosity has been peaked. I am finishing up a reverse flow. Should have a curing fire in it this weekend. I didn't install the damper in the stack based on the "stale smoke thing". Can you tell me a little more about the controlling the draft and flue and how it changes the cooking? Maybe some extra reading. I will search the forum and do some research also. But any advice you have would be appreciated. Thanks!
     
  6. Yes, oh wise one - please explain more! I'm always looking for tricks of the trade to add to my own wisdom gained through experience.
     
  7. got14u

    got14u Master of the Pit OTBS Member SMF Premier Member

    I know you are talking about reverse flows here but smokers in general need a damper. Check out Jambo pits they are only controlled by the damper on the exhaust heck they don't even have intake dampers. And then you have verticals(cabinet) smokers that are also controlled by dampers and intake dampers in conjunction. It ALL depends on how your air flow is configured that is the size diameter of your exhaust the length of it as well. Then also the intakes the size, ect.......So i would say put one on for sure
     
  8. petesque

    petesque Smoke Blower SMF Premier Member

    I have heard of this never close the damper also. But I use it for better control on mine. I thing the correct rule of thumb is never cloce the exhaust damper completly. The other rule is that their are no hard fast rules in smoking, or very few.

    Another discussion is the stack like on a off set. Grill level or out the top? I do mine at grill level.

    Pete
     
  9. Just finished my 250 gallon reverse flow and I also brought my chimney down to the grate level. I have not installed a damper yet but, I have been using a piece of steel to act as a temporary damper. I would have thought that closing the damper some would create more heat in the cooking chamber. But, as I opened the damper further and further, the temps. would rise. The only problem that I am having is getting consistent temps. at the grate level. I have to adjust something (firebox dampers, or the chimney damper about every 20 minutes or so and I am still getting a lot of fluxuation of my temps. Does anyone have any hints ?????
     
  10. bbally

    bbally Master of the Pit OTBS Member

    Stale smoke thing is from the reverse plate gap being to big. This increases the dead volume of the chamber. When the gap is to big (and it is almost also to big) the exchange of new to old is not completed in one volume exchange.

    The second thing for stale smoke to form is to cold a fire in the firebox. Cooler fires make for incomplete combustion. This makes more of the building blocks for creosote available. Without a damper you are not controlling the fire, but cooling the fire. With a damper you control the amount of the fuel fully combusting. This allows you to have completed combustion on a smaller area of your fuel load. Without the damper your flow rate is not dampened enough to create the complete combustion temperature.

    Humidity is the biggest control you add with a damper. Primarily the damper is used when curing sausage. Especially fermented sausages.

    What most people call "Stale smoke" is really suspended creosote which in large concentration has a very bitter taste. However when it is just forming, but is not at heavy enough concentrations to precipitate it tends to create an acid smell to the smoke.

    This forming is caused by, flow rate, temperature gradient and humidity.

    When beginners start using a smoker then generally find it easiest to control the flow rate by using on the firebox draft to control flow. It is the easiest way to control the flow rate, control the air coming into the system and you control the air going out. But without a damper the more you close the draft while leaving the flue open, the larger the temperature gradient through the system.

    This leaves the system in a condition to produce creosote. To combat that most people keep the smoke very light, a thin blue line is what they end up calling it. But it is really a condition of operation called a light smoke. The reason the thin blue line is really ideal for this operating condition of open damper and controlling flue is because it limits the materials that can build creosote. But it also lacks control of humidity. You will see lots of spritzing... lots of water pans, lots of apple juice and lots of foiling to combat humidity loss.

    Now add the damper to the mix. You can control temperature gradient by closing the damper to match the input draft. You can control humidity loss by closing the damper. The damper is a tool, when systems are understood you can use this tool to finely tune your smoker to produce many of the more advanced techniques of smoking. Put it on, if you can not master it you can always just leave it open.

    My combustion knowledge comes from being a process boiler engineer trained a long time ago. Woodruff was the author of the earlier Steam Boiler combustion books we used. Don't even know if his books are still available. Steam Plant Operation or something like that. I end up converting from coal to wood in the formulas. But there is a large body of work published on that now thanks to biomass generators. I will check my engineering associations to see what they have for books you could review.
     
    jweller, steveaggie and fatbastard09 like this.
  11. coyote-1

    coyote-1 Smoking Fanatic

    To paraphrase Pirates of the Carribean, it's more like a guideline than a rule.
    That exhaust damper has many uses. Indeed, closing it and the intake damper will help you shut down a fire. But it also does these things:

    - For smokers that stay outdoors without covering, it keeps rainwater out. This prevents internal rust.

    - In cold or windy weather, it can help retain heat. On windy days, a wide-open stack can often result in backdrafts that instantly drop your smoker temps. Partly covering the stack can, if oriented properly relative to the wind, prevent that problem.
     
    fatbastard09 likes this.
  12. diesel

    diesel Smoking Fanatic

    BBally,
    Thanks for you time and information! I am going to put one in. One thing I have learned from metal fabrication is that if you don't like it you can cut it off.. or add to it.
    So with that I am going to put the damper on. I also need to look into the flow plate opening. I may need to close it up some. I think the calculator program has that in it. Thanks again and I will be sure to provide some qview soon.
     
  13. bbally

    bbally Master of the Pit OTBS Member

    Another thing you might find useful. You will hear of Stack on the grates, and stack at the top. I use to different size pipes on my stacks. I have the smaller stack pipe and then a slight larger stack pipe. On the inside I bring the smaller stack pipe through the chamber half way to the grates. Then cut a larger pipe to slip up over the smaller pipe, cut a hole in the larger pipe, weld a nut on and add a bolt. This allows you to slide the larger pipe up and down the internal stack and control the depth of smoke. Great for switching between pork butts and fish smoking.

    [​IMG]

    [​IMG]
     
  14. "Stale smoke thing is from the reverse plate gap being to big. This increases the dead volume of the chamber. When the gap is to big (and it is almost also to big) the exchange of new to old is not completed in one volume exchange." BBally



    Is the reverse plate gap your talking about between the top of the reverse flow plates and the top of the smoker (area where the cooking grates are)? If so, is there a method to determine the ideal reverse flow plate location (how far below cooking grates)?

    Thanks foe any info
     
  15. duck killer 1

    duck killer 1 Smoking Fanatic SMF Premier Member

    Ok, so at the expense of looking like i don't know what i'm talking about (cuz i'm still trying to figure this out LOL)

    and as i tell the tech's that work for me "the only stupid question is a question that isn't asked" LOL.

    the gap bbally is refering to is the gap at the end of the plate. i too still do not completely understand this, i wish i could wrap my brain around it. i have a strong mechanical background from working on food equipment and refrigeration equipment, but the engineering side eludes me,LOL. the "dead volume thing" as far as i can understand (and i am getting this from context so i could be wrong) is referring to the volume of the cooking chamber compared to the volume of air being exchanged (i think[​IMG]) so if the gap is too big, it allows the cook chamber to fill with new products of combustion faster than it can exhaust (? is this right?) so the products of combustion will then cool off before they are exhausted out the flue and thus soot up.

    i am assuming that "temperature gradient" is refering to heat loss, or temp drop of the products of combustion?

    so i am still confused a little as to the proper gap.

    i have heard and also reasoned out with myself that the proper gap is equal to either the flue volume or the area of the inside diameter, i'm unsure which.

    or,

    it is equal to the area of the space under the baffle plate.

    i feel it is the first statement but since it is easier for me to add to the length of my baffle plate in my smoker i'm building i have decided to go with the second idea. all i will need to do is add a piece of flat steel above and horizontal to the upright that blocks the end of the plate and it will make the gap closer.

    now as far as knowing how to balance the intake and flue dampers, i don't know what to watch for that will tell me which one to open or close. any help or additional info is extremely appreciated! once i figure all this out for myself it will be easier for me to help others with these questions and give something back to this wonderful community!
     
  16. bbally

    bbally Master of the Pit OTBS Member

    The gap I am referring to is the gap from the end wall (opposite the firebox) and the heat plate.  This should be no more than 10 percent over the ID of the exhaust flue.

    The volume under the cooking chamber is also important.  I will post my calculations this weekend for you.
     
    Last edited: Jun 19, 2010
  17. duck killer 1

    duck killer 1 Smoking Fanatic SMF Premier Member

    cool, thanx bbally! i am looking forward to seeing them![​IMG]
     
  18. fourthwind

    fourthwind Master of the Pit OTBS Member SMF Premier Member

    10% over the ID?  or 10% more in area ?   I am looking forward too see your calculations as well.  I want my build to be as functional as possible.

     
     
  19. duck killer 1

    duck killer 1 Smoking Fanatic SMF Premier Member

    [​IMG]   i have been thinking about this and with the information posted so far, i have come up with 2 possibilities:

    IS IT THIS WAY? [​IMG]

    [​IMG]

    OR IS IT THIS WAY?  [​IMG]

    [​IMG]

    can somebody please tell me which one is correct or am i still not getting it?

    THANKS!

    duck[​IMG]
     
  20. fourthwind

    fourthwind Master of the Pit OTBS Member SMF Premier Member

    What makes sense to me is the volume.  If air flow is what we want to control, then we dont want more air and smoke to travel through the gap of the plate than can exit the stack.  I would think that it would cause back pressure in the grill area that could lead to poor draft..  Hopefully Bob will come in and let us know..
     

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