Julian - my processor was built by jrl so just what he does as standard. There's probably a few inches from the T-piece for the vapour return and the bottom of the methanol pipe which is in the collection drum. I would descibe it as a continuous trickle. What is a plenum?
The last venturi I saw from Jim was a three port unit, so I think you can be reasonably sure it's ok.
A plenum is a void space. It is beneficial to have the central 15mm pipe or pipes of the condenser discharge into quite a large volume. That way the air flow slows right down giving the liquid Methanol a chance to drop out. If you run the air flow through the Methanol collection vessel, this will have the same effect.
The diagrams on the wiki just show straight pipe work, so it might be worth modifying them to show a plenum or at least adding a note about it. Or perhaps adding to the plumbers delight page.
At the end of the day, your demeth times seem reasonable compared to mine, I wouldn't spend too much time changing things. Probably the quickest most effective alteration you could make would be to increase the temperature, you don't say what temperature you currently use.
These days I run the heater set to 65°C throughout processing, except during the introduction of Methoxide. That way the Bio is well up to temperature at the start of demeth.
