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Lower Vent Shutter for the PBC

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    Lower Vent Shutter for the PBC


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    There have been a number of posts regarding adjusting the lower vent opening on the PBC. I decided to toss the supplied circular cover and use one with a straight edge, basically a piece of thin aluminum sheet about 4" wide by 5" high. Now, how to place it to get the "right" area? The vent hole is 3" in diameter, so it has an area of 7.07 sq in. So the sea-level setting of "1/4 open" translates to 0.88 sq in, and "1/2 open", to 3.53 sq in. A little searching on the internet will yield formulas for the area of a Circular Segment as a function of its height. The height is the distance from the circle's circumference to the straight line cutting across the circle. (Now, aren't you glad you stayed awake in geometry?) Here's an abbreviated table; the first column is the fraction of the full open area, the second column is the segment height IN MILLIMETERS.

    Open Height
    1/8 5.5 mm
    1/4 9 mm
    3/8 12 mm
    1/2 15 mm
    3/4 24 mm

    At 9mm, opening or closing the shutter 1 mm changes the % of full opening by about 4%, so it's pretty fine control. It's not a linear relationship because, well, a circle is not linear. If you want a full table, email me.

    To make the vent shutter, get a piece of thin Al; Home Depot has 1 foot squares 0.020 thick. Cut a piece with strong scissors to about 4" wide and about 5" tall. Gently roll it to fit the curve of the barrel. I made some guide rails for mine as shown in the picture, but you can just use a magnet (also at HD) to hold it in place. Measure up from the bottom of the vent hole to your desired set point (9mm for 1/4 open), and file a notch in the edge of the vent hole as reference. I marked a mm scale on my guides.

    Now you can reproducibly set and adjust the vent opening. The bonus is, you can easily close it completely after the cook, plug the hanging bar holes with wine corks, and save the charcoal for next time. If you need help emptying 4 wine bottles, let me know.
    Attached Files

    #2
    Thanks for the info!

    Comment


      #3
      Nice post Doc H. How has this worked for temperature control? How does the temp profile differ from a standard PBC? What temps can you cook at with your mod?

      Comment


      • Doc Hazard
        Doc Hazard commented
        Editing a comment
        It shouldn't be any different from the standard PBC, given the same opening area. The comments I've read seem to say that the temperature is not a strong function of the lower vent size. That's generally been my experience as well. It has an effect, but modifying the upper, exhaust vent area is much more effective. I guess that makes sense, since for a fixed and relatively small exhaust, as in the PBC, it doesn't matter much how big the possible input is: no matter the size no more air is going to go in than can get out. Now, if you cracked open the top, or took it off, to get searing temps, then the results would be much more sensitive to the input area. I like the shutter primarily because I know what the setting is and I can reproduce it. And mostly, because I can easily close it completely to snuff out the fire after a short cook.

      #4
      Looks good, been planning something similar myself once the current version wears off

      Comment


        #5
        So you rolled it to conform to the barrel shape. But the original is not rolled correct? Mine is flat and does not hug/conform to the barrel.

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        • Doc Hazard
          Doc Hazard commented
          Editing a comment
          I threw away the original round vent cover. The shutter is made from very thin Al sheet (0.020 inches). I'm sure you could use a couple thicknesses of heavy Al foil. "Roll" is too strong a word: I kind of bent the sheet to match the curvature of the barrel, and of course, my rails force it to match. But you can just use a magnet to hold the sheet of foil. The real info here, such as it is, is the area vs. segment height. ---Doc.

        #6
        IMO the lower vent design on the PBC seems like an afterthought. It's not cut to fit (can't completely close), every few cooks the nut inside seems to come loose during a cook and I end up propping it open with a little wire or nail or whatever is around.

        Comment


          #7
          That is neat. I'd like to have 4 very small vents, whatever would equal the 1/4 gap. See how that would roll with 4-6 briskets hanging in that sucker.

          I'm with Whitey for now. My lower is set for everything. I manipulate upper vents for temp control, if necessary.
          Last edited by Jerod Broussard; February 10, 2015, 04:41 PM. Reason: I does not equal I'd

          Comment


          • Doc Hazard
            Doc Hazard commented
            Editing a comment
            Well, 4 holes each 0.53 inches diameter would give you the 0.88 sq in opening of a 1/4 open vent. Given the mods you've done, this should be easy for you. How about a similar shutter in the top, or near the top of the barrel? ---Doc.

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