Thanks Charlie.... I have no real solid reason for trying to avoid using one, just a concern about over taxing my compressor, maybe?
In real-world application, how much strain does it put on your compressor? What sort of compressor are you using? I've got a makita mac700 and assumed it would just run non-stop when used to generate vacuum...
There are two ways to use a Venturi: 1) continuously (which places a constant load on the compressor, but does not necessarily run the compressor 100% of the time) or 2) with a vacuum reservoir (chamber) and a pressure switch (plus a check valve and solenoid valve) in which case the Venturi only runs when there is insufficient vacuum pressure (placing an intermittent load in your compressor). In the second case the duty cycle will depend entirely upon how much air is leaking past your workpiece and seals as that will determine how many CFM of vacuum must be generated to maintain the desired vacuum pressure.
The second advantage of option 2 is that you may, in theory, get some warning if impending failure as increased air leakage will increase the duty cycle of the Venturi, which might tip you off to failure. However, if I were using one for turning I would probably add a pressure switch that opens its contacts if the vacuum pressure drops more than a couple inches below the target level with those contacts wired to a contactor wired to interrupt power to my lathe (if I had a lathe) so as to bring things to an emergency stop as quickly as possible if an air leak develops. Coupled with a large vacuum reservoir you might just get enough warning, especially if your lathe has an E-stop brake that can be activated (or one could apply DC power momentarily to an induction motor to provide supplemental braking). If it seems a little paranoid it is probably because the thought if a large hunk if wood spinning at several hundred RPM suddenly detaching makes me a little paranoid (but, then, I'm not a turner)!
Whether the Venturi is a strain on your air compressor depends heavily on your air compressor's specifications and whether it is rated continuous duty (many larger units are continuous duty if using synthetic oil). You do not want to use a large Venturi with a small portable air compressor but if you have a large stationary compressor then it can be a good candidate for a Venturi application -- I have a 4HP 80gal compressor (14 CFM at 90 PSI). Every Venturi has a recommended operating pressure (do not exceed this pressure or efficiency will drop) and a stated CFM draw on your compressor at that recommended pressure. You can use this information to calculate your compressor's approximate duty cycle beforehand if you like and take that into consideration in your designs.
When looking at vacuum pumps for your application focus more on the CFM curve for your desired level of vacuum, not on the maximum vacuum attainable . Going off memory here, my recollection is that you will want to maintain at least 20-24" of vacuum to maintain a firm grip on your workpiece so a vacuum pump with at least 26-28" will suffice (be aware that you will lose an inch, or so, to a poppet style check valve, if used) -- but too much vacuum can be destructive to your project. The more CFM the pump can move at the desired vacuum level the better it will perform in this application as that is the critical zone.
It is also a good idea to ensure there is a suitable filter before your vacuum pump to catch any debris (such as sawdust) or dirty water/sap (if using green wood) prior to the pump as mechanical pumps are especially sensitive to such waste entering them. It is still a good idea to similarly protect a Venturi, though they are far less sensitive to such waste unless it is sufficiently "sticky" as to build up on the Venturi.
