A review of your textbooks should give much information. A review of past CR4 discussions of water ring vacuum pumps should also be informative. Click on the provided link for a number of discussions we have had about them.
Vacuum pump, in which water is used as a working media is called water ring type vacuum pump. It is a positive displacement rotary pump. Working principle is very simple. It has an impeller with radial blades which rotates inside an oval shaped casing. So clearances between impeller and casing side-wise are smaller compared to at top and bottom. Casing at top and bottom are connected to suction port which lead to system to be evacuated. Casing where clearances between impeller and casing are smaller are connected to discharge port. Water level is generally kept at 50% hight of impeller (or as per OEM) with a provision to make-up the losses. When impeller rotates water is churned due to centripetal force and pick larger volume of air at suction port which is released at discharge port with higher pressure due to lower clearances between impeller and casing. So, system gets evacuated.
Interestingly, when suction and discharge ports are reversed, it acts like a compressor and called water ring compressor. Liquid other than water also can be used as media. So in general it is Liquid ring vacuum pump/ compressor.
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