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SilverSwede

Return line Above or Below? that is the question

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I posted this in the equipment forum and got no replies so i am trying again here.

I have seen two designs on the return line from the column(s) (whisky column, vodka column or gin basket, or quite commonly these drain into a common return line) back to the pot. In one of the designs the return enters the pot above the liquid line in the vapor zone. I have heard by the people that promoted this design that it has to be that way or the still becomes vapor locked and floods if the return is under the liquid line and to never fill the still above the return line entrance point. 

On the other end I have heard that if the return line is above the liquid line vapor will enter the return line and "fight" the returning liquid.

Interested in  operating and engineering opinions, pros and cons with both designs.

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Posted (edited)

It is very important that vapor be prevented from going up the return line.  As you said, the vapor will "fight" with the liquid because they are going in different directions.  The way to stop the vapor going back up the return line is to install a liquid seal.  This seal can either be inside the pot (by having the return line extend to below the liquid surface) or external to the pot by installing a U or P, but not an S, trap.

I have seen vapor locking occur with an internal seal, ie one that enters the pot above the liquid level but extends down below the surface.  This is because it is quite tricky to get the sizing of the line just right to be able to flush any incondensible gases down the seal leg.  However, this is easily fixed by drilling a small hole (approx 1/8") in the elbow inside the pot above the liquid level. This is big enough to allow the small quantity of trapped gases to escape into the pot, but small enough to limit vapors from the pot entering the return line and causing problems.

Edited by meerkat
clarity

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You will want that liquid return line from you column to your pot to extend below the surface of the liquid level in the pot. Ive seen stills where the return was above the liquid level in the pot and the column would not function correctly. I modified the return to extend below the liquid level and this resolved the column problems the owner was having

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The problem is not that the return line discharges above the level in the pot.  The problem is that there is nothing to stop the vapors flowing up the return line.  If the discharge is above the liquid level then there must be an external liquid seal in the return line to block the vapors.  There are pro's and con's for internal and external liquid seals, but both can work perfectly well.

A similar situation exists with the downcomer on a bubble cap tray.  If the lower end of the downcomer does not have a liquid seal - either with an inverted cap or by extending the downcomer to below the liquid level on the tray below, then the vapor will flow up the downcomer rather than through the bubble caps.

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Will the return under the level not contribute to slow cycling of the flow? How about having a smal (1/4") went line to the vapor zone. Wouldn't the flow be better (like having a vent line on a sink drain?). (Tip your head to the right)

return line vent.jpg

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I haven't done it this way, but that is not to say you can't make it work.  It seems unnecessarily complicated and I always prefer the KISS approach.  Hopefully you will get a comment from someone who has tried it.  

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Our return line is internal below the liquid level. We never experience any backup into the bottom of our column due to slow flow.

 

 

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Ours is below as well. It originally had a piece of pipe that returned it to mere inches from the bottom the kettle. With thick rye mashes we would get some backing up, so we shortened the pipe by about a foot, so it still goes well down into the liquid, but we haven't had the problem since.

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