2.6.1 Design / Operating principle
Roots vacuum pumps belong to the category of technically dry-running rotary displacement vacuum pumps. They are also termed Roots pumps or Roots blowers.
In a Roots pump, two synchronously counter-rotating rotors (4) rotate contactlessly in a single housing (Figure 2.12). 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 rates Ql of less than 10-5 mbar · l / s.
Pump properties, heat-up
Since Roots pumps do not have internal compression or an outlet valve, when the suction chamber is opened its gas volume surges back into the suction chamber and must then be re-discharged against the outlet pressure. As a result of this effect, particularly in the presence of a high pressure differential between inlet and outlet, a high level of energy dissipation is generated, which results in significant heat-up of the pump at low gas flows, which in and of itself transports low quantities of heat.
The rotating Roots pistons can only be provided with relatively weak cooling by comparison with the housing, as there are no contacting surfaces other than the front side. Consequently, they expand more than the housing. To prevent contact or seizing, the maximum possible pressure differential, and thus dissipated energy, is limited by an overflow valve (7). It is connected to the inlet side and the pressure side of the pump-through channels. A weightloaded valve plate opens when the maximum pressure differential is exceeded and allows a greater or lesser portion of the intake gas to flow back from the pressure side to the inlet side, depending upon the volume of gas encountered. Due to the limited pressure differential, simple Roots pumps cannot discharge against atmosphere and require a backing pump. However Roots vacuum pumps with overflow valves can be switched on together with the backing pump, even at atmospheric pressure, thus increasing their pumping speed right from the beginning. This shortens evacuation times.
Rotary vane pumps, rotary piston pumps or screw pumps can be used as backing pumps: These kinds of pump combinations can be employed for all applications in the low and medium vacuum ranges involving high pumping speeds. Liquid ring pumps can also be used as backing pumps.
Gas-cooled Roots pumps
To allow Roots vacuum pumps to work against atmospheric pressure, some models do not have overflow valves with gas cooling (Figure 2.13). In this case, the gas that flows from the outlet flange (6) is re-admitted into the middle of the suction chamber (4) through a cooler (7). This artificially generated gas flow cools the pump, enabling it to compress against atmospheric pressure. Gas entry is controlled by the Roots pistons, thus eliminating the need for any additional valves. There is no possibility of thermal overload, even when operating at ultimate pressure.
Figure 2.13 shows a cross section of a Roots vacuum pump. The direction of gas flow is vertical from top to bottom, enabling the liquid or solid particles entrained in the inlet flow to flow off downward. In phase I, the chamber (3) is opened by the rotation of the pistons (1) and (2). Gas flows into the chamber through the inlet flange at pressure p1. In phase II, the chamber (3) is sealed off against both the inlet flange and the pressure flange. The inlet opening (4) for the cooling gas is opened by the rotation of the pistons. In Phase III, the chamber (3) is filled at the outlet pressure p2, and the gas is advanced toward the pressure flange. Initially, the suction volume does not change with the rotary movement of the Roots pistons. The gas is compressed by the inflowing cooling gas. The Roots piston now continues to rotate (phase IV), and this movement pushes the now compressed gas over the cooler (7) to the discharge side (Phase V) at pressure p2.
Gas-cooled Roots pumps can be used in the inlet pressure range of 130 to 1,013 mbar. Because there is no lubricant in the suction chamber, they do not discharge any mist or contaminate the medium that is being pumped. Connecting two of these pumps in series enables the ultimate pressure to be reduced to 20 to 30 mbar. In combination with additional Roots vacuum pumps, the ultimate pressure can be reduced to the medium vacuum range.
Pumping speed and compression ratio
The characteristic performance data of Roots pumps are: The pumping speed Sth = S0, which is the volume flow rate the pump displaces without counter-pressure, and the (no-load) compression ratio Km = K0 without gas displacement, which is a function of the exhaust pressure p2. Pumping speeds range from 200 m3/ h to several thousand m3/ h. Typical K0 values are between 10 and 75.
The compression ratio is negatively impacted by two effects:
- By the backflow into the gaps between piston and housing
- By the gas that is deposited on the surfaces of the piston on the outlet side and is re-desorbed after rotating toward the suction side
In the case of outlet pressures of 10-2 to 1 mbar, molecular flow prevails in the seal gaps, which results in less backflow due to their lower conductivities. However the volume of gas that is pumped back through adsorption, which is relatively high by comparison with the pumped gas volume, reduces the compression ratio
K0 is highest in the 1 to 10 mbar range, since molecular flow still prevails due to the low inlet pressure in the pump's sealing gaps, and backflow is therefore low. Because gas transport through adsorption is a function of pressure, it is less important than the pressure-proportional gas flow that is transported by the volume flow.
At pressures in excess of 10 mbar, laminar flow occurs in the gaps and the conductivities of the gaps increase significantly, which results in declining compression ratios. This effect is particularly noticeable in gas-cooled Roots pumps that achieve a compression ratio of only approximately K0 = 10.
The gap widths naturally have a major influence on the compression ratio. However to avoid piston scraping they should not be smaller than certain minimum values due to the thermal expansion of the pistons and the housing.
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