Glossary
Overview
Vacuum is a relative condition, normally referring to a lower pressure than atmospheric in any given confined volume. A vacuum pump is defined as a pump that removes a gas from a sealed volume in order to leave behind a partial vacuum. A vacuum pump can be visualised as a compressor running with the inlet attached to the vacuum system and the outlet attached to the exhaust.
Pumps can be broadly grouped into four main categories
- Positive displacement pumps
- Non-positive displacement pumps
- Molecular pumps
- Entrapment pumps
Alongside these pump options, we can also consider venturi vacuum pumps, which are not actually pumps at all but rather create vacuum using compressed air and are extensively used for part holding in automated processes.
Positive displacement pumps
Positive displacement pump types include rotary vane pumps, diaphragm pumps, liquid ring pumps, piston pumps, scroll pumps, screw pumps, and a number of other variations on a theme. All operate on the same underlying principle: inside the pump, a mechanism expands a small sealed cavity to create vacuum; because of the pressure differential, air from the chamber is pushed into this cavity; the pump's cavity is then sealed from the chamber, opened to the atmosphere and exhausted.
A positive displacement vacuum pump moves the same volume of gas with each cycle, so its pumping speed is constant. But as the chamber pressure drops, the volume being pumped increasingly contains less and less mass of gas, and the characteristic graph of throughput drops off exponentially, with different technologies capable of achieving a different base pressure.
Non-positive displacement pumps
High-speed, multi-staged centrifugal exhausters and regenerative exhausters are the major types of non-positive displacement pumps. Ideal for central vacuum systems, exhausters maintain a constant vacuum as the volume of air changes with the number of operators. The end result is not a particularly high vacuum, but flow capacity is very high.
Molecular pumps
Molecular pump types include diffusion pumps and turbomolecular pumps. These operate by 'knocking' gas molecules from the vacuum size of the pump to the exhaust side. Diffusion pumps do this by blowing out molecules with jets of oil, while turbomolecular pumps use high speed fans. Both types will stall if exhausted directly to atmosphere, so they must be exhausted to a lower grade vacuum created by a mechanical pump.
Molecular pumps are capable of much higher pumping speeds than mechanical pumps. Molecular pumping is only possible below pressures of around 10mbar, but it is in the high vacuum region where molecular pumping becomes more effective than positive displacement pumping. As with displacement pumps, the characteristic graph for throughput and mass flow rate drops off exponentially.
Entrapment pumps
Entrapment pumps include ion pumps, cryopumps, sorption pumps and non-evaporative getter pumps. These all operate by capturing gases in a solid or absorbed state.
