Chapter 5 Creation of Vacuum (Vacuum Pumps)

5.1 Rotary Vane Pump (RVP)

Construction
RVPs consist of a cylindrical housing (pump housing) with an eccentrically mounted cylindrical rotor inside. This rotor has a single or multiple slits and rotates in the direction of the arrow (Fig.1). In each of the slits there is a slide-valve or vane, which is pressed tightly again the inner wall, either just by the centrifugal force or, more often, by strong springs. During the rotation they push the air, which has penetrated from the vacuum chamber through the entrance opening, towards the exhaust valve where it is compressed and eventually expelled from the pump. All parts are covered by oil, also the exhaust valve. This serves not only for lubrication but also for the sealing and for the filling of harmful spaces and for the removal of the heat of the compression and of friction, thus for cooling.
Fig.1) Structure of a typical oil-sealed rotary vane pump
Fig.2) Pumping sequence
Working principle
The pumping action is illustrated in Fig.2 and may be briefly described as “induction, isolation, compression, and exhaust.” Referring to Fig.2(a), the half turn of the rotor that concludes with vane V being in the position shown induces air into the pumping chamber.The total volume available to the gas increased and so it expanded to occupy the availablevolume. In Fig.2(b), the movement of vane V past the inlet  isolates the induced air in a crescent-shaped volume. Further rotation, as shown in Fig.2(c), results in the reduction of this volume and compression of the air with a rise of pressure and temperature to a point shown in Fig.2(d) where the pressure is sufficient to open the exhaust valve so that the air is expelled.

This sequence is completed twice per revolution. Energy is expended not only to do work against inertial and frictional forces but also to compress and move the pumped gas, and so pumps run hot, typically at about 75°C. In some versions of the pump the stator exterior is not immersed in oil but is finned and air-cooled, and the oil necessary for the pump’s operation is drawn from a reservoir located on top of the pump and circulated appropriately.
Gas-ballasting procedure
When air that contains water vapor is being pumped, the possibility arises that compression of the air–vapor mixture will cause the condensation of the vapor to liquid droplets. This will happen at a stage of compression when the partial pressure of water vapor in the diminishing isolated volume reaches the saturation vapor pressure at the prevailing temperature. Some of the condensate will mix with and degrade the lubricating oil with undesirable effects. Lubrication will be impaired and temperatures rise. To overcome this problem, which arises frequently in many applications, Gaede suggested the use of the so-called gas-ballasting procedure. In this arrangement, whose implementation is illustrated in Fig.3, atmospheric air is admitted into the post-isolation compression volume in a controlled way so that the pressure necessary to open the exhaust valve is reached and the gaseous mixture expelled before the condition for condensation is reached. The facility may be switched on and off as necessary.
Fig.4 Gas ballasting procedure
Two-stage RVP
To improve the ultimate pressure of the single-stage pump just described, two-stage pumps utilize two similar stages connected in series but with no intermediate exhaust valve, as shown in Fig.(5). Both are driven by the same shaft. The ultimate pressures better than 10−3 mbar may be achieved.
Another reason for the better performance of a two-stage RVP is the fact that with one-stage pumps the lubricating oil gets in contact with the outer air absorbing a part of it, which is released when the oil is circulated between the vacuum side and the atmospheric side, thereby limiting the ultimate pressure. The ultimate pressure of two-stage pumps, especially when only degassed oil is used on the vacuum stage, lies at the limit between medium vacuum andhigh vacuum.
Fig.5) Two stage rotary vane pump
Pumping speed
Pumping speed curves for pumps of typical laboratory size are shown in Fig.6 and include the curves for operation with gas ballast, which naturally differ, reflecting the changed conditions when this facility is used. While speeds of the order of 10m3 per hour (approximately 3ls−1) are typical for a laboratory-sized unit, these pumps are available in a wide range of sizes, with speeds up to a few hundred m3h−1.
Fig.6) Typical pumping speed curves for single- and two-stage rotary pumps.

5.2 Root Pump
Roots pumps are widely used in vacuum technique. They are usually used in combination with roughing pumps (usually rotary pumps), and expand the range of operation far into the medium vacuum range. When two-stage roots pumps are used, even the high-vacuum range can be attained. Their operation principle makes it possible to build units with a very high pump speed (up to 100000 m3/h), which are more economic than other pumps. Because of the high pump speed RPs are especially useful for all applications with high flow rates, like e.g. many technological applications or drift tube experiments etc.
Construction A RP is a rotary pump with two symmetrical rotors, which rotate around each other without touching each other. The rotors have approximately the forms of eights, and they are synchronised by cogwheels (toothed wheels) (Fig.1). The width of the slit between the rotors and the inner wall of the RP and between the two rotors is only a few tenth ofmillimetres. Therefore RP can run with very high rotation speeds without mechanical wear and tear.
Fig1. Schematic: Root pump Principle of operation How gas is displaced can be seen in the sequence of diagrams of Fig2.
Fig2. Principle of operation of Root Pump
Part (c) shows the rotors in a horizontal “T” configuration that identifies the volume isolated at the pressure of the inlet. For each revolution of the two synchronized rotors, a volume equal to four times this is swept (ideally) from inlet to outlet. The ideal, zero leakage, pumping speed is therefore the number of revolutions per unit time multiplied by this volume. It is the free air displacement of the pump when there is no pressure differential Δp between inlet and outlet, and therefore no reverse flow, and is denoted SD. These pumps, also called “lobe pumps” or “Roots blowers,” were originally designed to move gas at high volumetric rates at atmospheric pressure.
In vacuum applications, gas is moved from the inlet at low pressure to the outlet at higher pressure, and therefore there is compression. Both this and viscous friction cause heating of the gas. The rotors and the stator housing become hot in cooling the gas, but heat is taken away externally from the stator housing rather more effectively than it is from the internal rotors and so there is differential thermal expansion and the real danger of mechanical seizure if the rotors expand excessively. To combat this, the rotors may be cooled by an internal oil flow or, with less efficiency, by cold surfaces placed in the flowing gas near the rotors on the outlet side. The effect is sufficiently large that pumps cannot run at full speed and at high pressures with high compression ratios for any reasonable length of time. The most common way to run the pumps and to alleviate these problems is to back them with a rotary pump whose speed is typically one tenth or more of the Roots pump speed.
Leakage
In contrast to other rotary pumps RP are not running in an oil bath. Therefore there is a principal inner leakage, which causes a reduction of the compression ratio to 10 or 100. The inner leakage of a RP and of other dry pumps with a high number of revolutions stems in the first line from the fact that certain surfaces of the pump are exposed once to the intake port of the pump and once to the compression side. During the compression phase, these surfaces (pistons and inner wall) are charged with gas so that a layer of adsorbed gas is formed. During the intake phase this gas is desorbed again towards the low pressure side. The thickness of the transferred gas layers depends on the thickness of the slits between the pistons and the wall.