There are several names for this class of pumps: Roots pumps, (Roots) blowers or mechanical boosters. Roots pumps extend the range of applications for so-called backing pumps to lower pressures, especially in applications with high gas loads. Their main working range is between 1 mbar and 1 × 10-3 mbar. By connecting several of such pumps in series even lower working pressures down to 1 × 10-4 mbar can be achieved.
Roots pumps are positive displacement pumps without internal compression. This generally means, that they are not able to compress to atmosphere directly. Running in combination with a corresponding backing pump will always work. When selecting a combination of backing pump and roots pump, the ratio of the pumping speeds of the two pumps is decisive. This is referred to as gradation. Typical gradations are between 1:2 and 1:8. This means that Roots pumps typically have a pumping speed 2 to 8 times higher than that of the attached backing pump. The gradation determines the pressure difference between the inlet and outlet of the roots pump at the respective operating point. In principle, the required motor power is directly dependent on and proportional to this pressure difference. To avoid overloading the motor and overheating the pump, the pressure difference across the roots pump must not exceed a certain value. Leybold offers different types of Roots pumps, which are protected against excessive loads in different ways. While classic Roots pumps are protected by a pressure switch, Roots pumps which are operated by frequency converters and roots pumps with internal bypass lines can run permanently.
- Increasing the available pumping speed up to 250 bis 7000 m³/h
- Clean, hydrocarbon-free vacuum
- Relatively insensitive to dust and dry dirt
- Small installation space
- Different mounting positions applicable
- Low energy consumption
Classic Roots pumps
Classic Roots pumps are equipped with simple flange-mounted or integrated motors that are directly supplied with mains voltage and run at a constant speed. Since the motor load increases with rising suction pressure, these Roots pumps may be cut in below a certain pressure only. Therefore, this type is only suitable for very long pumping cycles or for stationary operation at low pressure. For fast, cyclical pumping, switching the Roots stage on and off constantly is not practicable. In order to protect the Roots pump reliably a pressure gauge monitoring the pump’s cut-in point is always required.
There are several principles available to seal the drive shaft under vacuum against atmospheric pressure. With the three different series WA, WS and WH Leybold offers competent solutions for different requirements here.
RUVAC WA series
The abbreviation WA stands for Roots pumps with flange-mounted motor. In WA pumps the drive shaft is fed through shaft seals to the outside. This allows simple and cost-effective standard motors to be flange-mounted. Due to this design, the drive train of the WA pump is slightly longer. However, the main disadvantage is that these pumps are not completely hermetically sealed. If the application requires the pumped medium not be discharged into the environment under any circumstances, a WS or WH series pump has to be used. Moreover, sealing the shaft by sealing rings and oil reservoir represents an additional maintenance expense.
RUVAC WS series
The abbreviation WS stands for Roots pumps with canned motor. In WS pumps, the motor rotor and stator are separated by a thin-walled cartridge. Thus, an absolutely hermetic sealing is guaranteed and pumping of aggressive or toxic media is possible. Motors of the WS series are available both water-cooled and air-cooled. Compared to the WA series the shaft sealing rings are omitted, making the WS pump easier to maintain.
RUVAC WH series
The abbreviation WH stands for Roots pumps with hermetically sealed motor. Like the pumps of the WS series, WH pumps are air tight and suitable for pumping toxic gases, which must not be discharged into the environment. Unlike canned motor pumps, WH pumps feature a plastic encapsulation of the entire motor stator, separating it from the motor rotor running in the vacuum. By eliminating the metallic can and applying smaller gaps this technology achieves higher efficiencies. The pumps of the WH series feature integrated water cooling and are particularly efficient.
Roots pumps with frequency converter
An elegant and very efficient solution to protect Roots pumps from overload is the use of frequency converters. These converters adapt the pump's motor speed to the load and enable safe operation in all working ranges. At high pressure, the pump rotates more slowly and accelerates to lower pressures until the maximum speed is reached. Moreover, all Leybold Roots pumps using frequency controllers can also be operated at higher speeds than the corresponding Direct-Online version. This allows to increase the achievable pumping speed in the lower pressure region significantly. Leybold Roots pumps are available with integrated frequency converters and can also be operated with external converters.
Roots pumps with bypass line
Another way to keep the pressure difference across the Roots pump below the critical value Δpmax is to return some of the gas pumped back to the roots pump's suction area. This is done by means of an internal, so-called bypass line controlled by a pressure differential valve. The resulting back-flow reduces the effectively delivered gas flow and consequently the pressure difference across the pump. Therefore, the roots pump can already be switched on at atmospheric pressure and monitoring the inlet pressure additionally is not necessary. The advantage of the bypass line over frequency converters are the very quick reaction of the valves and the ability to maintain the pump’s rotating speed. At extremely short pumping cycles the pump with bypass line can thus achieve a decisive speed advantage. Anyway, under certain circumstances the combination of frequency converter and bypass line may also make sense to get an optimum pump down performance. There are pump variants with integrated bypass line for each of the WA, WS and WH series available. These pumps are identified by the abbreviations WAU, WSU or WHU.