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The Venturi Effect - does the length of chimney matter?

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    The Venturi Effect - does the length of chimney matter?

    Expert opinions wanted. Thoughts/experiences Chimney length, diameter etc




    After building a UDS and having temperature issues, bricettes burning out etc.....Initially I thought it was lack of air intake. 4x 1/2” boosted it to 2x2” + 2x1.5”....it looks like it was just a case of the chimney damper chocking out the bricettes. Removal of chimney damper ensured the temp rocketed and no burn out.

    #2
    Depends on how long is long?

    Comment


      #3
      I believe I've heard that discussed here. Hopefully someone more knowledgeable will come along. Ahumadora

      Comment


        #4
        Can you post photos?

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          #5
          What size is your chimney? You don't say here and I can't find your original post. If it is currently 4" take it to 6". A 6" diameter is better than a 4" diameter, but don't think you need to go that big for your small barrel. Your on the edge of being right.
          Last edited by mountainsmoker; August 5, 2019, 10:23 AM.

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            #6
            My wife is kind enough to say chimney size doesn't matter.

            Comment


              #7
              Since you are talking about a UDS, I assume the chimney is threaded pipe nipple into the 2" threaded bung hole on the lid?

              I would think diameter on a UDS would matter more than length of the exhaust. You need the total area of your exhaust to probably be enough to when wide open, it matches the total intake area at the bottom. So if you are using two 2" and 2 1.5" intakes, a single 2" exhaust is not going to match that. If you remember your geometry, the area of a circle is PI x radius squared. So for a 2" diameter pipe, the total area is exactly 3.14 square inches, since the radius is 1", and 1x1=1.

              All that said, a Weber Kettle, a Weber Smokey Mountain, and most UDS smokers get by with one of those daisy wheel style exhausts on top, where MOST output you will see is the four exhaust holes wide open. I've not measured, but at MOST those four holes are probably 1" in diameter, and 4 1" holes is also 3.14 square inches of total area....

              So it makes sense to me you need that bung hole and any chimney on it wide open to get maximum air flow through your UDS. I don't think "draft" is going to matter much, since all of the heat is going up. I think draft matters more on a big offset, where it is sealed well, and you need to draw the heat across the cooker to the exhaust.

              Comment


                #8
                I did quite a bit of research when i built my UDS. The key thing I figured out, is you need to be able exhaust just as much as you take in, if not more. If you are generating a lot of smoke but not exhausting it at a good rate, you can snuff out your fire. I did 8 1” holes in a weber style for my exhaust and i am very successful. I installed two of these for intake https://udsparts.com/dampers/air-inl...lavalockr.html

                but really i use the equivalent of one, two is nice so i can catch the wind from both sides if needed
                Attached Files

                Comment


                  #9
                  Initially had a small piece of wood covering the 2” bung (as aftermarket chimneys have the tear drop damper)....as soon as I removed the wood. Boom!

                  Yes bleeding obvious in hindsight!!!....and may not have needed to increase my previous 4x 0.5” inlets. Needless to say it will now get much and enable me to cook roasts etc in the UDS.

                  My curiosity remains....would the chimney length affect the air velocity?

                  Regards
                  Attached Files

                  Comment


                  • jfmorris
                    jfmorris commented
                    Editing a comment
                    I don't think more length in this type cooker would help - in fact, it might make things worse if you go beyond a certain length, as all pipe and tubing has a resistance to flow, whether you are talking hot gasses or liquids. The longer the pipe, the greater the cumulative resistance to flow.

                  • IowaGirl
                    IowaGirl commented
                    Editing a comment
                    I agree with jfmorris. A little chimney is okay. A long one is not unless you have a large temperature difference to drive the gas flow. And you don't have that.

                    You didn't explain in your first thread about the piece of wood over the outlet.
                    That was an pretty important detail to not include.

                  #10
                  So what’s the purpose of the damper?

                  Comment

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