A distinction is made between ejector pumps such as water jet pumps (17 mbar < p < 1013 mbar), vapor ejector vacuum pumps (10-3 mbar < p < 10-1 mbar) and diffusion pumps (p < 10-3 mbar). Ejector vacuum pumps are used mainly for the production of medium vacuum. Diffusion pumps produce high and ultrahigh vacuum. Both types operate with a fast-moving stream of pump fluid in vapor or liquid form (water jet as well as water vapor, oil or mercury vapor). The pumping mechanism of all fluid entrainment pumps is basically the same. The pumped gas molecules are removed from the vessel and enter into the pump fluid stream which expands after passing through a nozzle. The molecules of the pump fluid stream transfer by way of impact impulses to the gas molecules in the direction of the flow. Thus, the gas which is to be pumped is moved to a space having a higher pressure.
In fluid entrainment pumps, corresponding vapor pressures arise during operation depending on the type of pump fluid and the temperature as well as the design of the nozzle. In the case of oil diffusion pumps this may amount to 1 mbar in the boiling chamber. The backing pressure in the pump must be low enough to allow the vapor to flow out. To ensure this, such pumps require corresponding backing pumps, mostly of the mechanical type. The vapor jet cannot enter the vessel since it condenses at the cooled outer walls of the pump after having been ejected through the nozzle.
Wolfgang Gaede was the first to realize that gases at comparatively low pressure can be pumped with the aid of a pump fluid stream of essentially higher pressure and that, therefore, the gas molecules from a region of low total pressure move into a region of high total pressure. This apparently paradoxical state of affairs develops as the vapor stream is initially entirely free of gas, so that the gases from a region of higher partial gas pressure (the vessel) can diffuse into a region of lower partial gas pressure (the vapor stream). This basic Gaede concept was used by Langmuir (1915) in the construction of the first modern diffusion pump. The first diffusion pumps were mercury diffusion pumps made of glass, later of metal. In the Sixties, mercury as the medium was almost completely replaced by oil. To obtain as high a vapor stream velocity as possible, he allowed the vapor stream to emanate from a nozzle with supersonic speed. The pump fluid vapor, which constitutes the vapor jet, is condensed at the cooled wall of the pump housing, whereas the transported gas is further compressed, usually in one or more succeeding stages, before it is removed by the backing pump. The compression ratios, which can be obtained with fluid entrainment pumps, are very high: if there is a pressure of 10-9 mbar at the inlet port of the fluid entrainment pump and a backing pressure of 10-2 mbar, the pumped gas is compressed by a factor of 107!
The ultimate pressure of fluid entrainment pumps is restricted by the value for the partial pressure of the fluid used at the operating temperature of the pump. In practice, one tries to improve this by introducing baffles or cold traps. These are “condensers” between fluid entrainment pump and vacuum chamber, so that the ultimate pressure which can be attained in the vacuum chamber is now only limited by the partial pressure of the fluid at the temperature of the baffle.
The various types of fluid entrainment pumps are essentially distinguished by the density of the pump fluid at the exit of the top nozzle facing the high vacuum side of the pump:
The pumping action of a vapor ejector stage is explained with the aid of Fig. 2.46. The pump fluid enters under high pressure p1 the nozzle (1), constructed as a Laval nozzle. There it is expanded to the inlet pressure p2. On this expansion, the sudden change of energy is accompanied by an increase of the velocity. The consequently accelerated pump fluid vapor jet streams through the mixer region (3), which is connected to the vessel (4) being evacuated. Here the gas molecules emerging from the vessel are dragged along with the vapor jet. The mixture, pump fluid vapor - gas, now enters the diffuser nozzle constructed as a Venturi nozzle (2). Here the vapor and gas mixture is compressed to the backing pressure p3 with simultaneous diminution of the velocity. The pump fluid vapor is then condensed at the pump walls, whereas the entrained gas is removed by the backing pump.
Oil vapor ejector pumps are ideally suited for the pumping of larger quantities of gas or vapor in the pressure region between 1 and 10-3 mbar. The higher density of the vapor stream in the nozzles ensures that the diffusion of the pumped gas in the vapor stream takes place much more slowly than in diffusion pumps, so that only the outer layers of the vapor stream are permeated by gas. Moreover, the surface through which the diffusion occurs is much smaller because of the special construction of the nozzles. The specific pumping speed of the vapor ejector pumps is, therefore, smaller than that of the diffusion pumps. As the pumped gas in the neighborhood of the jet under the essentially higher inlet pressure decisively influences the course of the flow lines, optimum conditions are obtained only at certain inlet pressures. Therefore, the pumping speed does not remain constant toward low inlet pressures. As a consequence of the high vapor stream velocity and density, oil vapor ejector pumps can transport gases against a relatively high backing pressure. Their critical backing pressure lies at a few millibars. The oil vapor ejector pumps used in present-day vacuum technology have, in general, one or more diffusion stages and several subsequent ejector stages. The nozzle system of the booster is constructed from two diffusion stages and two ejector stages in cascade (see Fig. 2.47). The diffusion stages provide the high pumping speed between 10-4 and 10-3 mbar (see Fig. 2.48), the ejector stages, the high gas throughput at high pressures (see Fig. 2.49) and the high critical backing pressure. Insensitivity to dust and vapors dissolved in the pump fluid is obtained by a spacious boiler and a large pump fluid reservoir. Large quantities of impurities can be contained in the boiler without deterioration of the pumping characteristics.
Included in the class of fluid entrainment pumps are not only pumps that use a fast-streaming vapor as the pump fluid, but also liquid jet pumps. The simplest and cheapest vacuum pumps are water jet pumps. As in a vapor pump (see Fig. 2.46 or 2.51), the liquid stream is first released from a nozzle and then, because of turbulence, mixes with the pumped gas in the mixing chamber. Finally, the movement of the water and gas mixture is slowed down in a Venturi tube. The ultimate total pressure in a container that is pumped by a water jet pump is determined by the vapor pressure of the water and, for example, at a water temperature of 59°F (15°C) amounts to about 17 mbar.
Essentially higher pumping speeds and lower ultimate pressures are produced by steam ejector pumps. The section through one stage is shown in Fig. 2.51. The markings correspond with those shown in Fig. 2.46. In practice, several pumping stages are usually mounted in cascade. For laboratory work, two-stage pump combinations are suitable and consist of a steam ejector stage and a water jet (backing) stage, both made of glass. The water jet backing stage enables operation without other backing pumps. With the help of a vapor stream at overpressure, the vacuum chamber can be evacuated to an ultimate pressure of about 3 mbar. The condensate from the steam is led off through the drain attachment. The water jet stage of this pump is cooled with water to increase its efficiency. Steam ejector pumps are especially suitable for work in laboratories, particularly if very aggressive vapors are to be pumped. Steam ejector pumps, which will operate at a pressure of a few millibars, are especially recommended for pumping laboratory distillation apparatus and similar plants when the pressure from a simple water jet pump is insufficient. In this instance, the use of rotary pumps would not be economical.
In spite of their low investment costs, water jet pumps and steam ejectors are being replaced in the laboratories more and more by diaphragm pumps because of the environmental problems of using water as the pump fluid. Solvent entering the water can only be removed again through complex cleaning methods (distillation).
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