Vacuum Generation: Multi-Stage Roots Pumps - Know-How

4.5 Multi-stage Roots pumps – Vacuum generation

4.5.1 Design / Operating principle

Single-stage and multi-stage Roots pumps belong to the category of dry-running rotary displacement vacuum pumps. They are also termed Roots pumps or Roots blowers. The multi-stage Roots pumps described in this chapter belong to the ACP range (adixen clean pump).
In a pump, two synchronously counter-rotating rotors (1) rotate contactlessly in a single housing (Figure 4.9). The rotors have a figure-eight configuration and are separated from one another by a narrow gap. Four to six pairs of rotors are located on the rotor shafts. Each rotor cavity is separated from the others by stator disks with a gas orifice. The gas conveyed is pumped from the inlet port (3) to the outlet port (4).
One shaft is driven by a motor (5). The other shaft is synchronized by a timing gear (6) in the gear box. Lubrication is limited to the gear box and encapsulated low pressure and high pressure bearings (9), which are sealed off from the suction chamber (7) by shaft seals (8).
Because there is no friction in the suction chamber, a Roots vacuum pump can be operated at high rotation speeds of up to 6,000 rpm. The symmetrical distribution of the rotor mass around the shaft axis also results in perfect dynamic balancing, which means that the pump operates extremely quietly in spite of its high speeds.

Figure 4.9: Operating principle of an air cooled multi-stage Roots pump

4.5.2 Application

Dry, air cooled multi-stage Roots pumps in the ACP range can often be used as a direct replacement for oil- lubricated rotary vane pumps in the available pumping speed range thanks to their design and performance parameters. Advantages of multi-stage Roots pumps include:

No lubricant in the gas displacement area
No contamination of the media pumped with oil or particles
No operating fluid disposal problems
Constant vacuum parameters with long-term stability
High reliability due to minimal number of wearing parts
With one-phase or three-phase frequency converter constant operating parameters at every power supply and mains frequency, universal mains power connection
Air cooling, no installation expenses and operating costs for water cooling
Can be continuously operated against a high inlet pressure
Long service intervals resulting in low operating costs
Remote control option with frequency converter
Ideal adaptation to the application due to adjustable rpm, reduced noise level, energy use and power input
Minimal leak rate
Ideal backing pump for turbopumps and Roots pumps
Certified to UL/CSA and SEMI S2

Vapors

Without additional measures, there is a danger of condensation inside the pump with oil-free pumps due to their comparatively low operating temperature. Similarly to rotary vane pumps, multi-stage Roots pumps are fitted with a gas ballast valve to increase the water vapor tolerance and the water vapor capacity of the pump. Condensate formation inside the pump increases the ultimate pressure, results in corrosion and at worst to total failure of the pump.
ACP pumps can convey media with a moisture content of up to 5 % without gas ballast. With the standard pump gas ballast, the water vapor tolerance is 10 hPa and the water vapor capacity 100 g · h^-1. With an increased gas ballast throughput in special pump versions (CV, Condensable Vapors) water vapor tolerances of about 100 hPa and water vapor capacities of 700 to 1,000 g · h^-1 can be attained, depending on the ambient temperature.
As for rotary vane pumps: before evacuating the vapors, the pump must have warmed up for at least half an hour with gas ballast. The higher temperature of the pump block reduces condensation. Additional measures to reduce condensation include obtaining the lowest possible outlet pressure and separate removal of condensates. Backpressure at the outlet must be prevented with a vertical exhaust gas line. If an extraction system is available, the outlet should be connected to it.

Dust and particles

The multi-stage Roots pumps in the ACP series can, at worst, become blocked and fail completely as a result of uncontrolled immissions of large particles. For this reason filters are fitted on the intake side in dust-laden processes or, for instance, in glass-making to protect from breakage.

Corrosive gases

Pumps in the ACP series are not suitable for pumping down large quantities of corrosive gases. Multi-stage Roots pumps are available for this purpose for corrosive gas processes in the semiconductor, solar and coating industry (see Chapter 4.6). However, ACP series pumps can be used to handle at least traces of corrosive gases. In this case, versions with inert gas purge are used in which the bearings are protected with an inert gas curtain and process gases are diluted by introducing inert gas in one of the pump stages.

Leak rate

Their high tightness makes multi-stage Roots pumps the perfect solution for long-term use and gas circuits with expensive gases (⁴He, ³He, isotope-marked gases).

Light gases and high vacuum

Multi-stage Roots pumps have an outlet valve to prevent humidity and oxygen from diffusing back to the suction chamber which in corrosive gas applications would cause chemical reactions to occur in the pump. This valve must be opened periodically (e. g. through gas ballast) in applications where a vacuum requires to be generated in order to prevent light gases from collecting and diffusing back.
Even a slight vacuum at the exhaust or the use of gas ballast is enough to displace even a continuous gas throughput of helium, such as in an evaporator cryostat, with almost the same performance data as nitrogen or air.
In applications where the use of carrier gas is prohibited (such as in closed loop applications), there must be a vacuum at the pump outlet.

4.5.3 ACP portfolio overview

The ACP is an air cooled, dry multi-stage Roots pump for applications in the low and medium vacuum range. The pumping speeds of this product range from 14 to 37 m³ · h^-1. Ultimate pressures of below 0.1 hPa are attained. Their pumping speed and the attainable final pressure depend on the rotation speed selected by the frequency converter.
Regardless of the model in question, ACP pumps can be continuously operated in their particular operating range at any pressure.
ACP pumps can be configured differently and optimized for a wide range of applications by adding gas ballast or sealing gas at different places in the pump.

4.5.3.1 Air cooled multi-stage Roots pumps

The versions shown in Table 4.14 can be supplied with either a three-phase or a single-phase mains power connection. Irrespective of the mains voltage and frequency, all pumps are equipped with a frequency converter.

Air cooled multi-stage Roots pump configurations
Model	Standard version			”G“ version for corrosive gases	”CV“ version for condensable media
	Without gas ballast	Manual gas ballast valve	Standard applications with a humidity > 5% or Permanent gas ballast with inlet filter		Manual gas ballast valve	Permanent gas ballast with inlet filter
ACP 15
ACP 28
ACP 40

Table 4.14: Air cooled multi-stage Roots pump performance data

Air cooled multi-stage Roots pumps
Model	Pumping speed	*Ultimate pressure with/without gas ballast**	Applications
ACP 15	14 m³ · h^-1	3 · 10^-2 hPa	Noncorrosive gases, argon, backing pumps for turbopumps
ACP 28	27 m³ · h^-1	3 · 10^-2 hPa
ACP 40	37 m³ · h^-1	3 · 10^-2 hPa
ACP 15	14 m³ · h^-1	1 · 10^-1 hPa*	Standard applications with a humidity > 5% or desorption from chamber walls, regeneration of cryo pumps
ACP 28	27 m³ · h^-1	1 · 10^-1 hPa*
ACP 40	37 m³ · h^-1	1 · 10^-1 hPa*
ACP 15G	14 m³ · h^-1	1 · 10^-1 hPa*	Load locks and transfer chambers, backing pumps for turbopumps with high incidence of light gases, process gases with low corrosive gas concentration, particle-laden process gases, oxygen and oxidating media
ACP 28G	27 m³ · h^-1	1 · 10^-1 hPa*
ACP 40G	37 m³ · h^-1	1 · 10^-1 hPa*
ACP 28CV	27 m³ · h^-1	2 · 10^-1 hPa*	Drying, distillation, applications with vaporizing fluids
ACP 40CV	37 m³ · h^-1	2 · 10^-1 hPa*	Drying, distillation, applications with vaporizing fluids

Table 4.15: Air cooled multi-stage Roots pump performance data

4.5.3.2 Accessories

Particle filter (IPF)

If process-related dusts and particles are present, the pump must be fitted with an upstream particle filter to prevent the pump from becoming blocked. There are various types available to fit the connection flange on the pump. The effective filter mesh is 25 µm. Other filter types and mesh sizes are available on request.

Silencer (ES)

The silencer ES 25S on the outlet side reduces the noise level of the pump particularly at high working pressures near to atmospheric pressure. It does not have any influence on the noise level of the pump near the ultimate pressure.

Sound protection hoods (NRC and SEK)

Sound protection hoods reduce the sound pressure level of the ACP pumps even at operating pressures close to the ultimate pressure. Type NRC hoods reduce the sound pressure by about 5 to 6 dB(A) and limit the maximum ambient temperature to 35 °C. Hoods of type SEK reduce the sound pressure by about 10 dB(A) and limit the maximum ambient temperature to 30 °C.