Why is VM bad at heads separation

[Arphaxad]

The other main drawback of a VM is it is less effective for heads extration when compared to a good LM design. This results in a slightly higher proportion of your total run being heads

I read that on the beginners board a couple weeks back but for the I've of me I can't figure out why this would be.

Can anyone shed some light on this.

[rad14701]

It's not really a matter of standard VM is bad at heads separation... It's more of the side effect of the heads laden vapor coating the large amount of surface in the product liebig causing a long smear into the hearts cut transition... With a LM/VM combo the LM is used to remove the compressed and condensed foreshots and heads as liquid... Then the VM branch can be used to collect the hearts... And if there is enough alcohol left to collect tails they could also be drawn off using the LM section... Using the VM liebig for the hearts alone keeps that section as clean as possible...

[Arphaxad]

Would that mean that if it was a pure vm with a very short Liebig, that would be better than a vm with a long Liebig?

Or more to the point the volume of the Liebig gas chamber needs to be minimized?

[Mud Mechanik]

Yes a shorter condenser would help with this but too short makes more problems with not condensing all of your vapors. My VM does an excellent job at heads seperation and the smear is very miniscule, making conservative cut eliminates the problem.

[Coaster]

@ Arphaxad,

Suggest reviewing the “Some Still Drawings” thread located in the ‘Research and Theory’ sub-forum that has diagram drawings for LM/VM Still builds. These diagram drawings provide component dimensional information.

Regards,
Coaster

[MudDuck]

Would adding a second VM takeoff work as well as adding a LM takeoff? May be easier to build.

[myles]

Would adding a second VM takeoff work as well as adding a LM takeoff? May be easier to build.

There is an easier solution than this. Any of the designs that allow you to remove the heads in a liquid form, and so maintain a clean product vapour path will do well. Previously I have also tried the CM over VM option, to take heads off as vapour from the upper section of the column.

However, I have reverted to the liquid option. I am now choosing to use a heads trap on the reflux condenser, that I can remove the heads from.

If you are still in the design stage, I think it is worth considering the modification that allows you to return your reflux condensate back into the column BELOW the VM splitting chamber, and not through it.