Parameters and classifications

The lowest pressure which can be achieved by a pump at its inlet, is determined either by the leakage in the pump itself, or by the vapour pressure of the fluid utilized in the pump. This pressure determines the low pressure end of the pres- sure range in which the various pumping types are effective (fig. 1).

The pressure range of a single pump is that range in which the pumping speed of that pump can be considered useful (fig. 2). Pumps of the same type but of different sizes or constructions may have adjacent pressure ranges, so that the pressure range of a specific pumping method can be larger (fig. 1) than that of an individual pump (fig. 2).

The pumping speed of the pumps is not constant, but is a function of the pressure. The pumping speed vs. pressure curve of pumps has either a shape of a curve decreasing as the pressure decreases (e.g. rotary pumps), or of a curve increasing first with decreasing pressure, reaching a ma mum and then decreasing as the pressure decreases (e.g. diffusion pumps, Root's pump).

The classification of the pumps, according to the pressure range, h sum rized in fig. 1, while the typical variation of the pumping speed is show fig. 2, expressed as percents of the maximum pumping speed of each type of pump.

The exhaust pressure is the pressure against which the pump may be operated.

From this point of view the vacuum pumps may be broadly divided into three - Pumps which exhaust to atmosphere, usually known as roughing or backing pumps. The removal of the atmospheric air from the system to some accept- able operating pressure is referred to as roughing out the system. The main- tenance of a required low pressure at the outlet of another pump, is referred to as backing. Mechanical rotary pumps, and ejectors are the typical roughing and backing pumps.

- Pumps which exhaust only to sub-atmospheric pressures, require a backing pump (in series) to exhaust to atmosphere. Diffusion, Root's pumps, and mole- cular drag pumps are of this type, which require a backing pump.

- Pumps which immobilize the gases and vapours within the system require no outlet. These are the pumps based on ionization or on sorption.