In theory at least, vacuum pumps are ‘merely’ air compressors run backwards — with the inlet attached to a machine you want to apply vacuum to and the outlet open to the air. In fact, for smaller, at-home uses, an air compressor and a vacuum pump are literally the same machine, you just decide which end you want to use and attach whatever your attaching to the appropriate end.

In industrial uses, however — in sizes that affect entire machining plants or other large-scale operations — the machines differ in small ways that enhance the efficiency of one operation over the other. And only very specifically-made machines should be used as both a vacuum generator and a compressor at the same time; the doubled load will run any machine not carefully built to withstand it.

There are three things you need to know about a vacuum pump: the strength of the vacuum it can produce, the rate at which it moves air, and the amount and quality of electricity it takes to use.

Vacuum Strength
Vacuum strength is measured in absolute pressure (mmHg), where the smaller the number, the power powerful the vacuum. Standard atmospheric pressure is 760 mmHg at sea level, so anything less than that is a form of vacuum. Most large pumps are rated once, for continuous-duty use. Small pumps, which can have problems with overheating at high loads, usually have a continuous-duty rating and an intermittent-duty rating showing how much it can produce for short times before it needs a break.

Flow Rate
Vacuum pumps are flow rated according to how quickly they can move air when both sides of the pump are at equal pressure (i.e. open to the air.) Of course, as the vacuum on one side of the pump increases, air flow decreases. Manufacturers can provide the curves that show what the flow rates should be as the vacuum increases.

Power Requirements
Vacuum pumps use relatively little power compared to air compressors. The aforementioned pressure-flow curves should also include the amount of drive power required as the vacuum levels change (and thus allow you to derive efficiency rates by dividing power needed by air moved at each point along the curve.)