Does a vacuum pump have to run constantly in a pick & place system?

I'm reading this article about which of vacuum pump or venturi vacuum generator is more efficient for a pick and place system: https://fluidpowerjournal.com/pump-or-venturi/

The example application is as follows:

Here’s a typical vacuum-lifting application: a small end-of-arm tool consisting of eight small vacuum cups of Ø40 mm (Ø1.5″). These eight cups are picking up a flat plastic sheet, which is, of course, non porous. Cycle rate is one sheet pick every 15 seconds, or four sheets a minute. The piece is being held for only three seconds during transfer. What’s the most efficient? Pump or venturi?

The conclusion appears to be venturi, but I find the argument a bit odd:

The pump will run continuously with the vacuum being turned on and off via a vacuum solenoid valve. The venturi will run only when required (during the lift part of the cycle), turned on and off using a compressed air solenoid valve. The vacuum pump uses 0.25 hp all day long during a 24-hour shift. The venturi uses 0.9 hp for every three seconds of a 15-second cycle. Therefore, the venturi uses on average 0.18 hp of compressed air over the same cycle period. Consequently, the venturi is “more efficient” in overall running costs.

Does anyone know why the vacuum pump would need to run constantly for 24hrs whilst the pressure pump would not? Could the vacuum pump not be switched off just like the pressure pump? In fact, can the solenoid valve give a good seal in the venturi case?

• In order to keep the chamber pressurized the vacuum pump must be turned on and maintain the seal if I am not mistaken. I suppose if you count for some leakage in the chamber the pump will also periodically have to turn on again to maintain pressure? – Morten Nissov Feb 2 at 18:11
• vacuum pump keeps chamber evacuated ... and has to maintain vacuum – jsotola Feb 2 at 19:41
• @MortenNissov Does the pressure pump not need to be turned on likewise? – John M. Feb 3 at 4:39
• @jsotola But the venturi creates a vacuum too though. Doesn't the pressure pump also need to maintain pressure? – John M. Feb 3 at 4:41
• @JohnM. yes, the compressor does need to maintain the chamber pressurized .... my comment was about the use of terminology – jsotola Feb 3 at 5:01

The problem with vacuum pick and place systems is the amount of time it takes to create the vacuum, so your design will revolve around your requirements.

Vacuum

If you run the vacuum continuously then, as you approach your target, you only have to wait long enough for the channels on your pick-up head to evacuate.

If you switch off the vacuum pump, air rushes into the tubing, equalising at atmospheric pressure. The next time you want to pick, you need to evacuate all of the tubing between head and pump. This could take a long time, even if you have a vacuum 'reservoir' much larger than the volume of your tubing.

In between these would be a vacuum solenoid close to the pick-up head. Here most of the tubing would stay evacuated, and you would only have to evacuate the pick-up head channels and the short tubing to the solenoid. This does require you to run power and control all the way to the head, though.

The greater the volume of tubing on the open air side of the solenoid, the greater the time would be from switching the solenoid to their being sufficient vacuum.

Venturi

With a venturi generator, you can site it next to the head, keeping the volume to evacuate low, while putting the solenoid at the other end of the tube. High pressure air will rush into that tube much more quickly than the air could be evacuated, so you effectively get the best of both worlds. Fast actuation, but no need to add solenoids and run power and control to the head.

Comparison

Price and the difference in pressure differentials are the big advantages of compressed air. Air compressors can easily and cheaply supply air to a venturi generator, and can cope more easily with large changes in demand.

A typical workshop compressor might store 100l of air at 8 bar and be rated at 410 litres/minute (14.5cfm). This could run venturi generators needing 80 litres/minute with the 5:1 duty cycle suggested in your quote. During that 15 second cycle, pressure would fall from 8 bar to around 7.8 bar until the pump kicks in again. This variation should be negligible in terms of pick-up performance.

We could do something similar with a vacuum vessel, have a higher flow vacuum pump, a vacuum 'storage' vessel, and run it with a 3:12s duty cycle, but there are problems with this. We would need a much higher flow rate (and cost) pump. Vacuum vessels can have at most 1 atmosphere of pressure differential, in practice much less, so a 100l tank could never 'store' more than 100l of vacuum, and a 0.3bar (10"Hg) vacuum vessel would need to be around 2500 litres in size, to have a similar pressure variation over the duty cycle as with a 100l compressed air solution. Flow rate is affected significantly by current pressure in the storage vessel:

Summary

In summary, almost all of the vacuum based pick and place machines I've worked on have used venturi generators. Vacuum pumps always have to be sized according to maximum required vacuum, assuming no storage, whereas compressed air systems can usually be sized based on average use.

If you need constant vacuum, pumps may be better, but if demand varies significantly from second to second, as it probably does in a pick and place machine, venturi may be a better fit.