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1.1 What is vacuum?

A vacuum is defined as a diluted gas, or the corresponding state at which its pressure or density is lower than that of the ambient surrounding atmosphere. Because atmospheric pressure fluctuates locally over the Earth's surface and lessens as altitude above sea level increases, it is not possible to specify a general upper limit for the vacuum range.

2.1 Classification of vacuum pumps

In connection with vacuum pumps, a distinction is made between gas-displacement vacuum pumps and gas-binding vacuum pumps. While gas-displacement vacuum pumps can be used without limitation, gas-binding vacuum pumps have a limited gas absorption capacity and must be regenerated at certain process-dependent intervals.

Gas-displacement pumps, which are also referred to as gas transfer pumps, are classified either as positive displacement pumps or kinetic vacuum pumps. Positive displacement pumps displace gas from sealed areas to the atmosphere or to a downstream pump stage. Kinetic pumps displace gas by accelerating it in the pumping direction, either via a mechanical drive system or through an aligned vapor stream that is condensed at the end of the pumping section. Gas-binding vacuum pumps either bind the gas to an especially active substrate through gettering or condense the gas at a suitable temperature.


2.2 rotary vane pump

Design / Operating principle

 Figure 2.2: Operating principle of a rotary vane pump

A rotary vane vacuum pump is an oil-sealed rotary displacement pump. The pumping system consists of a housing (1), an eccentrically installed rotor (2), vanes that move radially under spring force (3) and the inlet and outlet (4). The outlet valve is oil-sealed. The inlet valve is designed as a vacuum safety valve that is always open during operation. The working chamber (5) is located inside the housing. Rotor and vanes divide the working chamber into two separate spaces having variable volumes. As the rotor turns, gas flows into the enlarging suction chamber until it is sealed off by the second vane. The enclosed gas is compressed until the outlet valve opens against atmospheric pressure. In the case of gas ballast operation, a hole to the outside is opened, which empties into the sealed suction chamber on the front side.

Operating fluid, oil

Pump oil, which is also called as operating fluid, has multiple tasks to perform in a rotary vane pump. It lubricates all moving parts, fills both the harmful space under the outlet valve as well as the narrow gap between inlet and outlet. It compresses the gap between the vanes and the working chamber and additionally ensures an optimal temperature balance through heat transfer.

Multi-stage pumps

Rotary vane vacuum pumps are built in single- and two-stage versions. Two-stage pumps achieve lower ultimate pressures than single-stage pumps. Moreover, the effects of the gas ballast on the ultimate pressure are lower, as the ballast gas is only admitted in the second stage.

Vacuum safety valve

Depending upon the type of pump in question, rotary vane vacuum pumps can be equipped with a vacuum safety valve. The vacuum safety valve disconnects the pump from the vacuum recipient in the event of intentional or unintentional standstill, and uses the displaced gas to vent the pumping system in order to prevent oil from rising into the recipient. After switching on the pump, it opens after a delay once the pressure in the pump has reached the approximate pressure in the recipient.

2.3 Roots vacuum pump

Design / Operating principle

Figure 2.3: Operating principle of a Roots pump

Roots vacuum pumps belong to the category of technically dry-running rotary displacement vacuum pumps. They are also termed Roots pumps or Roots blowers.

Operating principle

In a Roots pump, two synchronously counter-rotating rotors (4) rotate contactlessly in a single housing (Figure 2.3). The rotors have a figure-eight configuration and are separated from one another by a narrow gap. Their operating principle is analogous to that of a gear pump having one two-tooth gear each that pumps the gas from the inlet port (3) to the outlet port (12). One shaft is driven by a motor (1). The other shaft is synchronized by means of a pair of gears (6) in the gear chamber. Lubrication is limited to the two bearing and gear chambers, which are sealed off from the suction chamber (8) by labyrinth seals (5). Because there is no friction in the suction chamber, a Roots vacuum pump can be operated at high rotary speeds (1,500 – 3,000 rpm). The absence of reciprocating masses also affords troublefree dynamic balancing, which means that Roots vacuum pumps operate extremely quietly in spite of their high speeds.


The rotor shaft bearings are arranged in the two side pieces. They are designed as fixed bearings on one side and as sliding internal rings on the other in order to enable unequal thermal expansion between housing and piston. The bearings are lubricated with oil that is displaced to the bearings and gears by splash disks. The driveshaft feedthrough to the outside is sealed with radial shaft seal rings made of FPM that are immersed in sealing oil. To protect the shaft, the sealing rings run on a protective sleeve that can be replaced when worn. If a hermetic seal to the outside is required, the pump can also be driven by means of a permanent-magnet coupling with can. This design affords leakage ratesQlof less than 10-5 mbar · l / s.