The principle of operation of single-stage Roots pumps corresponds to the operating principle of multi-stage pumps as described in Chapter 4.5. In the Roots vacuum pump, two synchronously counter-rotating rotors (4) rotate contactlessly in a housing (Figure 4.16). The rotors have a figure-eight configuration and are separated from one another and from the stator 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) with compression rings. Because there is no friction in the suction chamber, a Roots vacuum pump can be operated at high rotation speeds (1,500 – 3,000 rpm ). The absence of reciprocating masses also affords trouble-free dynamic balancing, which means that Roots vacuum pumps operate extremely quietly in spite of their high speeds.
Design
The rotor shaft bearings are arranged in the two side covers. They are designed as fixed bearings on one side and as movable (loose) bearings on the other to enable unequal thermal expansion between housing and rotor. The bearings are lubricated with oil that is displaced to the bearings and gears by splash disks. The driveshaft feedthrough to the outside on standard versions 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 a can. This design affords leakage rates QI of less than 10-6 Pa m3 s-1.
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 that only transport low quantities of heat. The rotating Roots pistons are relatively difficult to cool compared to the housing, as they are practically vacuum-insulated. Consequently, they expand more than the housing. To prevent contact or seizure, the maximum possible pressure differential, and so also the 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 weight-loaded 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 on the throughput. Due to the limited pressure differential, standard Roots pumps cannot discharge against atmospheric pressure 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 start. This shortens evacuation times.
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Figure 4.16: Operating principle of a Roots pump
Backing pumps
Single-stage or two-stage rotary vane pumps or external vane pumps are used as oil-lubricated backing pumps. Screw pumps or multi-stage Roots pumps can be used as dry backing pumps. Pump combinations such as these can be used for all applications with a high pumping speed in the low and medium vacuum range. Liquid ring pumps can also be used as backing pumps.
Gas-circulation-cooled Roots pumps
To allow Roots vacuum pumps to work against atmospheric pressure, some models are gas-cooled and do not have overflow valves (Figure 4.17). In this case, the gas that flows from the outlet flange (6) through a cooler (7) is re-admitted into the middle of the suction chamber (4). 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.
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Figure 4.17: Operating principle of a gas-cooled Roots pump
Figure 4.17 shows a cross-section of a gas-circulation-cooled Roots vacuum pump. The direction of gas flow is vertical from top to bottom, enabling the liquid or solid particles entrained in the inlet stream 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 (5) 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 to 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 hPa. 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 hPa. 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 and compression ratio. The theoretical pumping speed
is the volume flow rate which the pump displaces without counterpressure. The compression ratio K0 when operated without gas displacement (inlet flange closed) depends on the outlet pressure p2. Pumping speeds range from 200 m3 · h-1 to several thousand m3 · h-1. Typical K0 values are between 10 and 75.
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Figure 4.18: No-load compression ratio for air for Roots pumps
The compression ratio is negatively impacted by two effects:
By the backflow into the gaps between the piston and housing
By the gas that is deposited by adsorption on the surfaces of the piston on the outlet side and re-desorbs after rotating toward the suction side.
In the case of outlet pressures of 10-2 to 1 hPa, molecular flow prevails in the seal gaps,which results in less backflow due to their low 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 hPa range, since molecular flow still prevails due to the low inlet pressure in the pump’s sealing gaps, and backflow is therefore low. Since gas transport through adsorption is not a function of pressure, it is less important than the pressure-proportional gas flow that is transported by the pumping speed.
At pressures in excess of 10 hPa, 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 have a major influence on the compression ratio. Due to the different thermal expansion of the pistons and the housing, they must not, however, fall below certain minimum values in order to avoid rotor-stator-contact.
Due to their low compression ratios, Roots pumps must always be operated as pump combinations for vacuum generation. Their achievable final pressures will be a function of the ultimate pressure of the selected backing pumps. Due to gas transport through adsorption, it is no longer practical to use Roots pumps in the range below 10-4 hPa. The behavior of the pumping speed and ultimate pressure of pumping stations with various backing pumps is shown in Figure 4.19. The curves clearly show that the pumping speed of this kind of pump combination rises by a factor of 8 and its ultimate pressure reduces by a factor of 15 relative to the backing pump.
4.7.2.1 Backing pump selection
Rotary Vane Pumps
If a negative impact on the function is unlikely due to the process, a rotary vane vacuum pump is the most cost-effective backing pump for a Roots vacuum pumping station. Rotary vane vacuum pumps have ultimate pressures of around p < 1 hPa over a broad pressure range at constant pumping speed. A Roots vacuum pumping station achieves ultimate pressures of approximately 10-2 hPa with the gas ballast valve open. Water vapor can be extracted with these kinds of pumping stations, as well as many solvent vapors and other vapors that have sufficiently high vapor pressures and do not chemically decompose the pump oil. Examples of these include alcohols, halogenated hydrocarbons, and light normal paraffin as well as many others besides.
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Figure 4.19: Pumping speed of pumping stations with Okta 2000 and various backing pumps
Liquid ring vacuum pumps
Liquid ring vacuum pumps are a suitable solution for extracting vapors that chemically attack and decompose the backing pump oil or that have such low vapor pressure that condensation in the backing pump cannot be avoided, in spite of gas ballast. However they will only achieve an ultimate pressure that is determined by the vapor pressure of the operating fluid. If 15°C water is used, an ultimate pressure of approximately 20 hPa can be expected at the liquid ring vacuum pump, and it is then already working in the cavitation range. Cavitation occurs near the ultimate pressure of the pump. The operating fluid vaporizes on the intake side and the vapor bubbles suddenly collapse on the pressure side. This destroys the pump in the long term. A liquid ring pump which operates cavitation-free through an air supply attains an ultimate pressure of approximately 25 to 30 hPa and a combination of a Roots pump and a liquid ring pump achieves a pressure of about 1 hPa. A liquid ring vacuum pump should not be used with fresh water when evacuating environmentally harmful substances. In this case, a closed circulation system must be provided to advance a suitable operating fluid over a cooled heat exchanger in order to extract the heat of compression.
Liquid ring vacuum pump with gas jet device
The combination of Roots vacuum pump, gas jet device and liquid ring vacuum pump achieves an ultimate pressure of 0.2 hPa. If lower pressures need to be achieved, an additional Roots vacuum pump must be connected upstream.
Gas-circulation-cooled Roots vacuum pumps
Since Roots vacuum pumps are technically dry pumps, their use is advisable when pumps with liquid-sealed suction chambers cannot be used.
Their applications include:
Extracting and compressing helium on cryostats
Extracting and compressing SF6
Clean recovery of a wide variety of gases and vapors in a wide variety of processes, e.g. distillation
Evacuating molecular sieves, etc.
Pumping down and recirculating toxic substances in closed loop systems
Evacuating extremely large-volume vessels
Roots pumping stations with gas cooled Roots pumps can be configured with a wide variety of inlet characteristics. In extreme cases, it is possible to achieve a virtually constant pumping speed throughout the entire pressure range of 1,000 hPa to 10-3 hPa, and the individual pump stages can be selected in the ratio of 2:1 to 3:1. To do this, however, the Roots vacuum pumps must be equipped with correspondingly powerful motors, and outlet valves to the atmosphere must be provided instead of overflow valves.
Screw Pumps
With the HeptaDry screw pumps, a complete line of technically dry pumps is available that offer pumping speeds of 100 to 600 m3 · h-1. As stand-alone pumps (see also Chapter 4.4), they cover an extensive pressure range in the low and medium vacuum segments. Due to their internal compression, they can work continuously with relatively low drive power throughout the entire inlet pressure range of 0.1 to 1,000 hPa. In combination with OktaLine Roots pumps, it is even possible to achieve ultimate pressures of 5 · 10-3 hPa.
Multi-stage Roots Pumps
Multi-stage Roots pumps in the ACP range make for compact pumping stations with a pumping speed of up to 285 m3 · h-1. Combining an ACP backing pump and a Roots pump makes it possible to achieve final pressures of up to 5 · 10-3 hPa.
Corrosive gas versions of Roots pumping stations are described in Chapter 4.6.
Roots pumps can be supplied in a number of different versions:
Standard pumps with shaft seal rings and a cast iron housing
Hermetically sealed standard pumps with magnetic coupling and a cast iron housing (M series)
Gas-circulation-cooled Roots pumps with shaft seal rings (G series) or magnetic coupling
Roots pumps for potentially explosive environments and for displacement of explosive gases (ATEX series)
4.7.3.1 Standard pumps
The characteristic performance data of the standard pumps are shown in Table 4.19. These performance data also apply to all other series. The maximum differential pressures are a function of the overflow valves. In the ATEX series, these maximum differential pressures are smaller than for the other series in order to satisfy the temperature requirements specified by the ATEX guidelines. The housings for these pumps are manufactured of GG cast iron and are tested at 100 kPa over-pressure. The seal to the atmosphere consists of radial shaft seal rings. The standard pumps are characterized by their robust, compact design as well as by their high compression ratios, which result in high pumping speeds for the pump combination, even with small backing pumps, and thus afford short pump-down times. The vertical direction of flow renders this pump largely insensitive to dusts and liquids.
Roots pumps OktaLine | ||||
Model | Rated pumping speed | Maximum differential pressure | Maximum differential pressure | Applications |
Okta 250 | 290 m³ · h-1 | 75 hPa | 50 | Industrial / chemical applications: E. g. oil regeneration, transformer drying, steel degassing, freeze-drying, leak detection systems, metallurgy, packaging industry, electron beam welding. Large-area coating: e. g. photovoltaics, wear protection, optical coatings Research & development: e. g. accelerators, simulation chambers. |
Okta 500 | 560 m³ · h-1 | 75 hPa | 50 | |
Okta 1000 | 1,180 m³ · h-1 | 45 hPa | 63 | |
Okta 2000 | 2,155 m³ · h-1 | 35 hPa | 70 | |
Okta 4000 | 4,325 m³ · h-1 | 25 hPa | 63 | |
Okta 6000 | 6,485 m³ · h-1 | 20 hPa | 63 | |
Okta 8000 | 8,370 m³ · h-1 | 27 hPa | 70 | |
Okta 18000 | 18,270 m³ · h-1 | 10 hPa | 70 | |
Table 4.19: OktaLine performance data
4.7.3.2 Standard pumps with magnetic coupling
The M series can be used for processes that place the most rigorous requirements on sealing and require the longest service intervals. For the most part, this series is identical to the standard series, however it is additionally characterized by a hermetically sealed magnetic coupling instead of radial shaft seal rings. This means that it is virtually wear-free in operation. The integral leakage rate of the pump is less than 1 · 10-6 Pa · m3 · s-1. This precludes the possibility of oil leaks, nor is there any exchange between the process gas and the environment. M series standard pumps are suitable for all applications shown in Table 4.19. In addition, however, these pumps can also be employed in industrial / chemical applications for pumping toxic gases, as well as for superclean gas applications: e.g. for CVD and PVD processes in the semiconductor industry or for evacuating load locks / transfer chambers and for the production of flat screens. The M series is available in sizes that range from 250 m3 · h-1 to 6,000 m3 · h-1.
4.7.3.3 Explosion-protected pumps
The ATEX series is available for processes in potentially explosive environments, or for evacuating explosive gases.
The ATEX series is available with pumping speeds between 500 m3 · h-1 and 4,000 m3 · h-1. They are PTFE sealed and made of GGG 40.3 nodular graphite cast iron. They meet the explosion protection requirements of directive 94/9/EC, category 3G, group IIB, temperature class T3 X.
Generally speaking, additional measures and / or components are required for safe pump operation, such as: startup and shutdown procedures, special backing pumps, flashback arrestors and pressure sensors. The entire system must be designed and operated in compliance with the appropriate explosion protection regulations.
4.7.3.4 Gas-circulation-cooled Roots pumps
Gas-circulation-cooled Roots pumps can be operated without a backing pump. Large pressure ranges and very high differential pressures are the ideal use for this type of pump. Continuous use is possible at high differential pressures since the gas that is heated up by compression is cooled on the pressure side and partly fed back into the suction chamber. This makes them suitable for operation at atmospheric pressure if they are used in combination with gas coolers. Gas-circulation-cooled Roots pumps are available in sizes from 500 m3 · h-1 (18.5 kW drive power) up to 8,000 m3 · h-1 (200 kW drive power).
Splinter shield inserts are offered as accessories for all OktaLine series Roots pumps.
The following oils for lubricating the gearing and the bearings are available as lubricants (Table 4.11):
Mineral oil P3 (in 0.5 l to 200 l containers)
Perfluoropolyether F5 (in 0.5 l to 50 l containers)
Diester oil D1 (in 0.5 l to 200 l containers)
Caution: Different kinds of oil should not be mixed. The pumps are prepared at delivery for one of these types of oil.
Since many Roots pumps are installed in pump combinations, it is possible to integrate the following accessories on an as-needed basis:
Electrical controllers
Measuring instrumentation for temperature and pressure
Pressure regulation systems
Heat exchangers and condensers
Soundproofing encapsulation for indoor and outdoor installation
Silencers
Dust separators
Flushing devices
Vibration isolation
Liquid separators
Gear chamber evacuation
Sealing gas supply
Measurement connections
In the case of many Roots pumps, it is possible to use the measurement connections on the inlet and pressure sides of the pump. To do this, the existing locking screws can be replaced with small ISO KF flange unions. This enables appropriate temperature sensors and pressure sensors to be connected for monitoring the pump.
Sealing gas connection
When pumping solvents or reactive gases, the risk exists that the lubricant will be significantly diluted as a result of condensation. Reactive gases or vapors can also attack parts of the gear chamber. For the most part, this risk can be avoided by admitting a sealing gas in the area of the shaft feedthrough between working space and gear chamber. Inert gases, mostly nitrogen (N2), are used as the sealing gas.
Gear chamber evacuation
In the case of all processes in which large Roots vacuum pumping stations must reach certain pressures in short cycle times (fast evacuation), it is practical to pump down the gear chambers of a Roots pump via an oil separator, by means of a separate vacuum pump in each chamber. This prevents gas from flowing out of the gear chamber and into the suction chamber, thus enabling the desired working pressure to be reached faster. The desired working pressure will determine whether it is possible to connect the gear chamber toward the backing-vacuum side of the Roots pump.
Flushing devices
A flushing device can be used for processes in which deposits form in the suction chambers. The design of this device will be coordinated individually with the customer on the basis of the specific requirements. The flushing device for standard pumps requires the use of sealing gas to prevent the flushing liquid from reaching the bearings or gear chambers.
Surface protection
If the media to be pumped down are corrosive, components that come into contact with the product can be provided with durable surface protection. The plasma-polymer thin-layer system consists of a bonding agent layer, a corrosion-protection layer and a non-stick coating. The thickness of the layer is less than 1 mm. Upon request, the suction chamber can be phosphated, vented with nitrogen and vacuum sealed in order to provide short-term surface protection, e. g. for warehousing and shipment.
Seals
Roots vacuum pumps come factory-equipped with O-rings made of FKM. For special applications, all pumps can be equipped with the specific O-rings or seals that are required for the respective application.
Pfeiffer Vacuum can supply standard Roots pumps with oil-lubricated single-stage and two-stage rotary vane pumps as well as with a selection of dry pumps. Please refer to Chapter 3.1, for more information.
In addition to standard pumping stations, Pfeiffer Vacuum’s vacuum system group designs and manufactures customized pumping stations (Roots pumping stations and turbo pumping stations).
