Pumping Principles

Pumping is a crucial process in cryogenics and vacuum technology, used for decreasing pressure in a system by removing gas molecules.
The two main types of pumps used in these fields are mechanical pumps and high-vacuum pumps.
Mechanical pumps, also known as roughing pumps, are used to remove bulk quantities of gas down to low vacuum level.

Mechanical Pumps

Rotary-vane pumps are a common type of mechanical pump, using a rotating mechanism to create vacuum and expel gas.
Another common type is the diaphragm pump, which uses a flexible membrane to create alternating volumes for the intake and expulsion of gas.
Mechanical pumps are often used in the initial stages of creating a vacuum, before switching to a high-vacuum pump.

For creating extremely low pressures, high-vacuum pumps are used.
Unlike mechanical pumps, high-vacuum pumps don’t use mechanical methods to remove gases, but rely on principles such as entrapment or momentum transfer.
Turbo molecular pumps are a common type of high-vacuum pump, utilising high-speed rotating blades to transfer momentum to gas molecules and expel them.
Other high-vacuum pumps include cryopumps, which use extremely cold temperatures to trap gas molecules, and ion pumps, which use an electric field to remove ions.

The process of reducing pressure in a vacuum chamber is referred to as pump down.
A quick pump down time is ideal to minimize the exposure of a system’s internal components to the atmosphere, reducing oxidation and contamination.
Pump down is influenced by factors such as the size of the chamber, the volume and type of gas present, and the quality of the vacuum pump.

Pumping speed, measured in volume per unit time, refers to the volume of gas a pump can remove from a chamber in a given period.
Whereas, throughput is the quantity of gas moved by the pump, usually expressed in the same units as pressure times volume.
An effective pump should have high pumping speed and low throughput, indicating it can rapidly reduce pressure while minimizing gas movement that could cause contamination.