Liquid ring and dry screw vacuum pumps are pumps that use the positive displacement principle, but there are some differences. Liquid ring pumps use liquid to create the desired vacuum, while dry vacuum pumps do not.

The vacuum in a dry screw pump is created by two parallel arranged screw rotors rotating in opposite directions. These rotors capture the gas entering through the inlet and transport it to the gas discharge or pressure side. As the gas is compressed, there is no contact between the rotors. This eliminates the need for any working fluid or lubricant in the compression chamber, which is why the technique is named "dry".

However, the internal gearbox in the dry screw pump does require lubrication. Over time, this lubricant finds its way into the pump chamber during operation, and in a way that renders the claim of "dry" technology bogus.

The liquid ring pump has a cylindrical housing partially filled with a sealing fluid, and the shaft has a multi-blade impeller eccentrically placed in the housing. The centrifugal force generated by the rotating impeller forms a liquid ring. It is this force that forces the fluid ring against the wall inside the pump chamber.

Due to the eccentric position of the impeller in the pump chamber, the blade entry depth increases and decreases continuously. This helps to increase the volume of the impeller unit on the inlet side, thereby creating a vacuum. What happens on the discharge side is that the blade moves further into the liquid ring, causing an increase in pressure that leads to discharge through the discharge port.

The inherent design of liquid ring pumps allows the use of a variety of liquids as sealants, which can be oil, solvent, water, or any other liquid compatible with the process. The liquid remains in contact with the vapor or gas that needs to be compressed, so the compression of the gas is almost isothermal. Depending on conditions, the discharge temperature can range from 45 to 150 degrees Fahrenheit and is generally kept under control in any process application. Since the discharge temperature is controlled in the liquid ring pump, a variety of gases can pass through or be condensed, which provides a great degree of flexibility.

Screw vacuum pumps can be designed with equal pitch, allowing gas to be delivered at a constant discharge while the main part of the compression occurs, or they can be designed with progressive pitch, in which steam or gas is compressed to the unloading port during delivery. In both cases, the compression is adiabatic, no heat leaves or enters the process, and the emission temperature can be as high as 450 degrees Fahrenheit. These high temperatures when using the technology have led to safety concerns.

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