A vacuum pump is a device which induces pressure difference between two regions of space. Creating a vacuum in a system requires moving all molecules of gas out of the system. However, molecules will only move if there is a difference in pressure between the low and high pressure regions.
There are two types of vacuum pumps, a transfer and a trapping pump.
The transfer pump, also known as a kinetic pump, transfers gas from the inlet of the pump to the outlet. This is achieved by a rotating part of the pump. The rotation accelerates the movement of molecules of the gas, thus creating a region of low pressure.
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Once the pump reaches its vacuum capacity any exchange of gas between the container and the pump is stopped by a valve.
On the other hand, the trapping pump, otherwise referred to as a capture pump, is usually located in the container being evacuated. The pump works by removing the gas molecules through condensation on the internal surface.
Basically, should the gas molecules come in contact with the refrigerated surface of the pump, the gas is then condensed and removed as a liquid.
Vacuum pumps can be damaged over time. As such, basic maintenance is a requirement in order to keep the equipment running and in good condition.