We can use the vacuum we create to prime a centrifugal pump , evacuate a condenser, lower the pressure in a chemical vessel or do any thing else we can think of where a vacuum might be needed. The pressure side could be used to create a head or pressure in a nozzle application or a hundred other applications you might consider. The capacity needed, along with how much vacuum, or how much pressure, will dictate the type of pump you will need. Atmospheric pressure at seal level is 760 mm. or 29.9 inches. Any less than this amount is considered to be a vacuum.
Hard vacuum starts at one millimeter of mercury.
As we learned from our discussions about conventional centrifugal pumps, these centrifugal do not work very well pumping gas or vapors, so we will be looking at a positive displacement type to do this job and there are a variety of designs available to us. A hydraulically balanced, O-ring design would be desirable. The casing is full of water or some compatible liquid. The rotor throws liquid outward to the stationary casing where the solid ring of liquid rotates at the same speed as the rotor. The elliptical shaped casing causes the rotating liquid to recede from and enter buckets in the rotor.