My venturi is horizontal, similar layout to GL's drawing, as are several other peoples.
Jim advocates that a venturi should be vertical with one swept bend between it and the processor to create back pressure. Personally I think the requirement is slightly different ... I think the jet from the throat needs to "fill the pipe" for the venturi to function correctly, the two are not quite the same. If a venturi is accurately made (mine are drilled on a lathe), I recon the tendency is for the jet from the throat to travel up the centre of the pipe without touching the sides. It can be encouraged to touch the sides by a bend and gravity can assist, especially if the venturi is vertical.
Given time I'd like to try some experiments with diverging jets which should, by design, fill the pipe.
With the last venturi I made I believe it wasn't "filling the pipe" despite having a swept bend down stream, so I fitted a jet breaker™ after the venturi and it works fine. The jet breaker™ is simply a brass rod inserted through opposing holes in the pipe and soldered in place. This, as the name suggests, breaks up the jet forcing the flow to fill the pipe. It seems to work really well. If I'm right it should be possible to only have a short, straight run into the processor, but don't hold me to that as I haven't tried it!
Personally I prefer the venturi up high, above the fluid level in the tank, that way, if you forget to close valves the vent pipe work can't fill with oil.
Re valve V7 ... yes, you need it.if you omit it, when you open valve V4 to entrain Methoxide you most likely won't. This is because the flow will take the path of least resistance which will be to suck air from the vapour circuit.
