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VM described (was) Re: Reflux Woes still no azetrope etho.

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  • burrows206
    Hi Harry, I posted this a good while back and think it applies now in view of your posting pasted from file snip---- I found this on a distilling forum about a
    Message 1 of 47 , Jan 7, 2009
      Hi Harry,
      I posted this a good while back and think it applies now in view of
      your posting pasted from file
      snip----
      I found this on a distilling forum about a year ago (well it's a
      couple of years now and I was just starting to learn to distill
      properly then) and I can't remember who posted it so if Mike N
      recognises this thank you (I know now, that it was Mike Nixon). The
      question asked was:-
      Quote---

      Can anyone please explain the working principle of a vapour
      management still head?
      What are its advantages/disadvantages over the Nixon-stone type?
      I've seen pictures of this type of head on www.homedistiller.org but
      have yet to find a description of how they work.
      =====================
      Mikes answer------ again quote

      Here we go. I'll try and make it as short as possible.
      If you feed vapour to the middle point of a horizontal tube, then it
      will split into two streams.
      If the cross-sectional area of one arm of the tube is A. and the
      other is B, then the vapour will divide in that ratio, A/B.
      You can control that cross-sectional area by means of gate valves,
      and this is useful for some applications.
      If you feed vapour into the middle point of a vertical tube, density
      now becomes a factor and this can be used to control the behaviour of
      a distilling column depending on the composition of the vapour inside.
      First thing to note is that vapour rises in a distillation column not
      because if convection, as some think, but because it is pushed by
      vapour generated in the boiler. All vapour in a distillation column
      therefore rises at the same rate no matter what its density, and at a
      rate determined by the power pumped into the boiler.
      When vapour meets the middle point of the vertical tube, some will be
      driven further up the column and some will enter the tube. In
      practise, this middle feed point is sealed off by a gate valve in a
      side arm during stabilisation of the column, and the vertical tube
      comprises the path up to the top condenser and the path down to the
      product condenser. It is a 'kinked' vertical tube fed with vapor at
      its middle point. When the gate valve is opened, vapour enters the
      side arm and, if it is less dense than air, will try to rise. If it
      is denser than air then it falls. It cannot rise in the side arm as
      that arm only goes down, but can fall as it points down to the
      product condenser and collecting vessel. The gate valve controls
      amount that can enter the side arm.
      The relative densities of steam at 100C, air at room temperature, and
      ethanol vapor at 78C are 0.6/1.0/1.6
      If the vapour in the column is ethanol, then it will fall down
      through the product arm, and if it is steam then it will continue up
      to the top condenser. The changeover point, when the vapor in the
      column is the same density as air, if when the vapour comprises 45.5%
      ethanol and 54.5% steam. The volume of vapour falling down through
      the side arm therefore tails off and finally stops as the vapour mix
      in the column approaches and then reaches this point. As this
      represents the onset of tails, the side arm being at the top of the
      column, a vapour management still automatically stops delivering
      product before it is contaminated with a significant amount of
      tails. It is also the reason why you cannot test the operation of
      such a still using steam alone. Subsequent collection of tails can
      be done by turning off the top condenser, blocking that path with a
      rag in the top vent, and fully opening the gate valve. A rag is
      recommended for blocking the top vent as a gate valve would present
      the possibility that both gate valves could be closed together,
      sealing the whole still.
      Sorry if that still seems long winded. I would seriously welcome any
      summary that manages to cover everything more concisely.
      Mike N


      --- In Distillers@yahoogroups.com, "Harry" <gnikomson2000@...> wrote:
      >
      >
      > Comments Inline...
      >
      > --- In Distillers@yahoogroups.com
      <mailto:Distillers@yahoogroups.com> ,
      > "rye_junkie1" <rye_junkie@> wrote:
      > > Here is where things are getting blurry for me. On the one hand
      it is
      > > good to go to 2" diameter. It increases Cubic feet as well as can
      > > handle higher wattage's and therefore faster vapor speeds?.
      >
      > .....Higher wattages, yes. Faster vapor speed, no. Faster vapor
      speed
      > interferes with separation. Result, lower proof. Higher wattages
      will
      > produce more VOLUME of vapor. 45 liters of vapor are produced every
      > minute per kilowatt of heat applied (Nixon & McCaw, 2001).
      >
      > This vapor, fed into a larger diameter column will keep the vapor
      speed
      > within specs, but you will have more vapor volume to separate per
      > timeslice.
      >
      >
      > On the
      > > other hand I need to restrict my current columns collar from 1"
      down
      > > to 3/4". This doesnt make sense to me. Is it because of the VM
      set
      > > up and the way that works?
      >
      > ........The collar in your case serves a dual purpose. Yes it
      centers
      > the reflux, but it also applies the necessary restriction to create
      > turbulence to the vapor stream, Turbulence is required to allow the
      > vapor to branch into the available pathways, one going up & one
      going
      > sideways. As the vapor leaves the top of the packing, it is smooth
      > flow, non-turbulent. This is a known phenomenon in Fluid
      Mechanics.
      > Mesh screens are commonly used as 'Flow Straighteners' for fluids
      (gases
      > & liquids).
      >
      > So, if there is no turbulence, the vapor goes into 'Laminar Flow'.
      This
      > is when the layers closest to the outside of the stream are
      travelling
      > slowly, and the inner layers near the center are travelling much
      > quicker. Result...nearly all of the vapor stream goes up into the
      > condenser, bypassing the side takeoff branch.
      >
      > Adding some form of obstruction (your collar; there are other
      simpler
      > ways) after the vapor leaves the packing, creates turbulent vapor
      flow
      > and the stream splits to enter both available pathways in a ratio
      > determined by the diameters of the openings. 2" up & 2" sidearm is
      a
      > 2:2 (50%:50%) ratio. 2" up & 1" sidearm is 2:1 (66%:33%) ratio.
      >
      > From this you should be able to easily figure out that the addition
      of a
      > valve in the sidearm to adjust the diameter of the takeoff branch
      is how
      > the reflux ratio is controlled, and why it is controllable.
      >
      >
      >
      > > Don't think I haven't considered reworking this thing back to an
      > > offset head. When it was in that configuration I was getting 92%
      with
      > > Marbles and propane. Now I switch to Structured packing, VM and
      > > electricity and I gain a %.
      > > Is any of this VM stuff explained in your Book? If so it may be
      time
      > > to pick up a copy. I should have it anyway.
      > >
      > > Mason
      > >
      > .........The above is a pretty good guide. VM is described (not
      > completely) in The Compleat Distiller by Nixon & McCaw. I don't
      know if
      > Riku has it in his book.
      >
      >
      > Slainte!
      > regards Harry
      >
    • duds2u
      I found that there was a very small amount of play in the T piece that allowed me to do it so I didn t have to bend anything. ... pooling ? ... fitting
      Message 47 of 47 , Jan 9, 2009
        I found that there was a very small amount of play in the "T" piece
        that allowed me to do it so I didn't have to bend anything.

        --- In Distillers@yahoogroups.com, "Zapata Vive" <zapatavive@...> wrote:
        >
        > How did you manage to "raise the ends of the cross to prevent
        pooling"?
        >
        > I think that means you bent the two horizontal arms of the "T"
        fitting slightly up? I thought of that, but couldn't think of a good
        way, especially without throwing them out of round. I also didn't
        really feel like messing around with it too much, that fitting cost me
        more than my column!
        >
        > Tips much appreciated on this, I've got a friend thinking of
        building, and if I could convince him his would be even better than
        mine, he'd take the bait for sure!
        >
        >
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