Categorization of Vacuum Pumps

Authored by: Igor Bello

Vacuum and Ultravacuum

Print publication date:  November  2017
Online publication date:  November  2017

Print ISBN: 9781498782043
eBook ISBN: 9781315155364
Adobe ISBN:


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Vacuum pumps differ in physical principles, operational range, obtained ultimate pressure (or vacuum), pumping speeds, sizes, weights, and many other parameters that are used to select the most suitable pump for a vacuum system at a good price. Some fundamental parameters and terms used at pump characterizations are listed below:

Pumping speed is the volumetric gas flow passing via a considered pumping area or captured on a considered pumping area at a given pressure as a result of the pumping action. Then, in the SI unit systems, the unit of pumping speed is m 3/s. However, manufacturers of vacuum pumps usually give pumping speed in m 3/h for pumps operating in low vacuum ranges and in l/s for pumps operating in high and ultrahigh vacuum ranges. Producers of dry pumps for rough vacuum range also give pumping speed in “cubic feet per minute” (CFM).

Ideal pumping speed is the theoretical pumping speed at the inlet of a pump, with no effect working against the pumping action. This parameter is clarified in the Chapters 11.5 and 14.4, where we illustrate calculations of ideal pumping speeds.

Intrinsic pumping speed is the actual pumping speed as measured at the direct connection of a pump to a large chamber.

Nominal pumping speed is the intrinsic speed at the inlet port of a pump and normally at pressure of a plateau or maximum of a pumping characteristic that is usually given by manufacturers.

Effective pumping speed is the pumping speed that includes effects of gas backstreaming, leaks, desorption, vaporization of materials, permeation, and influence of duct conductance. This is the actual pumping speed that is determined at an arbitrary place of the vacuum system. The effective pumping speed is smaller than the intrinsic and nominal pumping speeds.

Selective pumping speed is the term expressing that different gases are pumped at different pumping speeds. This property is explained in examples in the chapters on individual pumps. For instance, selective pumping is discussed in Chapter 14.3 on diffusion pumps, where pumping speed for different gases differ because of their different molecular masses as well as gas diffusion rates.

Pumping selectivity is the alternative term for selective pumping, which denotes dissimilar pumping speeds of a pump for different gases. This is illustrated mathematically in some cases of pumps later.

Pumping characteristic is the plot of the pumping speed against the actual inlet pressure.

Coefficient of (volumetric) quality is given by the ratio of nominal and theoretical, ideal pumping speeds.

Ultimate pressure is the theoretical equilibrium value of pressure on the inlet of a pump at which pumping speed is zero. It is the lowest achievable pressure by the given pump. It is the value of pressure that does not decrease by further pumping. Apparently, a pump may have lower ultimate pressure than the ultimate vacuum pressure of a vacuum system on which the pump is installed since the ultimate pressure depends on the secondary sources of gases and conductance of a vacuum system. It should be noted that the zero value of pumping speed results from the dynamic conditions of pumping action at which the gas amount removed per unit time (throughput at known temperature) is equal to that returning into the vacuum system from various secondary sources.

Base pressure is a similar term to ultimate pressure for evaluation of the pump or quality of vacuum achieved in vacuum systems. The base pressure is higher than ultimate pressure, and it is achieved under conditions as defined by ISO 21360-1.

Allowed inlet pressure is the highest pressure value at which a pump can operate or start to operate.

Allowed outlet pressure is the highest pressure against which a pump can operate under steady pumping conditions.

Compression ratio is the ratio of the outlet and inlet pressures under a steady pumping regime. The pump may have dissimilar pumping ratios for different gases, which is a characteristic of pumping selectivity.

Operation pressure range is the region of pressures under which a pump can operate and exhibits pumping speed.

Roughing pumps can be used to reduce pressure from atmospheric pressure to a start-up pressure of capture pumps. After starting the capture pumps, the capture pumps do not need any pumping backup. Therefore, the roughing pumps are in standby mode.

Backing pumps is the term used in connection with momentum transfer pumps (molecular pumps, turbomolecular pumps, or diffusion pumps) that cannot operate against atmospheric pressure but operate against lower pressures induced by backing pumps. The backing pumps continuously maintain the pressure below the maximum allowed outlet pressure in the backup lines of kinetic transfer pumps.

Crossover is the transition point in the pumping process when the pumping of a backing pump is overtaken by a high-vacuum pump.

Dry vacuum refers to a vacuum environment that is uncontaminated by substances used as functional fluids or materials in pumps. Dry vacuum is void of these substances, or their fractions are relatively negligible in the environment after pumping processes. Dry vacuum is considered to be absent of hydrocarbons or vapors of substances originating in functional and sealing media. Such a vacuum environment is often denoted as contamination free, though this is hardly achievable. In this context, the devices used for the production of contamination-free vacuum are called “dry pumps.”

Pumps can be categorized into two major groups, namely, gas transfer pumps and capture pumps. Transfer pumps can be classified into subcategories of (i) displacement pumps and (ii) kinetic pumps (momentum transfer pumps). Displacement pumps can operate on the principle of periodically increasing and reducing their working volume. As a result of this action, gases are taken in an operation volume, transported, and expelled from the pump interior. In the category of displacement pumps, we discuss piston oil pumps; single-rotor pumps such as rotary oil pumps (vane, piston, and trochoidal pumps), rotary water ring pumps, dry piston pumps, dry rotary pumps, dry diaphragm pumps and scroll pumps; and then two-rotor pumps including Roots pumps and claw/hook pumps; and screw pumps. Hablanian 384 discusses the major advances in transfer pumps.

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