EP4004378A1

EP4004378A1 - Molecular vacuum pump

Info

Publication number: EP4004378A1
Application number: EP20745185.7A
Authority: EP
Inventors: Ian David Stones
Original assignee: Edwards Ltd
Current assignee: Edwards Ltd
Priority date: 2019-07-29
Filing date: 2020-07-23
Publication date: 2022-06-01
Also published as: GB2586019A; WO2021018729A1; GB201910771D0

Abstract

A molecular vacuum pump comprising: a stator and a rotor and a thermoelectric device arranged to cool the stator such that gas molecules impacting said stator are slowed and a capture rate of gas molecules by the pump is increased.

Description

MOLECULAR VACUUM PUMP

FIELD OF THE INVENTION

The field of the invention relates to the field of molecular vacuum pumps.

BACKGROUND

Molecular or kinetic vacuum pumps such as turbomolecular and drag pumps provide blades or surfaces which drive gas molecules at low pressure from the inlet side towards the outlet or exhaust side of the pump. The effective pumping speed of such a pump depends on both the arrival rate of the molecules to the inlet of the pump and subsequently the probability of the molecules then being captured by the pumping mechanism and transferred from the inlet to the outlet without returning back to the inlet.

Lighter gases typically move faster and have higher arrival rates but lower capture probability in the pumping mechanism than heavier gases. It is this capture property of molecular vacuum pump mechanisms which make them better at pumping heavier molecule gases than they are at pumping lighter, faster moving gas molecules such as Flelium and Flydrogen.

In order to improve pumping speeds, various techniques can be used such as improving clearance tolerances, increasing tip speeds, improving the pumping mechanism design and increasing the size of the pump or inlet. All of these may provide some improvement but each have cost implications.

It would be desirable to be able to improve the pumping speed of a molecular pump.

SUMMARY

A first aspect provides a molecular vacuum pump comprising: a stator and a rotor; said molecular vacuum pump further comprising a thermoelectric device arranged to cool said stator such that gas molecules impacting said stator are slowed and a capture rate of gas molecules by the pump is increased.

The inventor of the present invention recognised that the cooling of the surfaces inside a molecular pump to a temperature below ambient could considerably improve its ability to pump lighter gases such as Hydrogen or Helium by slowing the molecules such that they have an increased probability of being captured and pumped from the inlet to the outlet. In effect a molecule contacting a cooled stator surface will be cooled and reemitted at a lower speed, the lower speed being governed by its lower temperature. This lower speed molecule will have a higher probability of contacting the rotor of the pump which contact will move the molecule towards the pump outlet. By cooling only those surfaces inside the vacuum pump, the arrival rate of molecules to the inlet is maintained while the capture rate is improved yielding an overall improved pumping speed. Further, to simplify construction and cost it is possible to achieve a significant improvement by cooling the stator or static components only.

Although the idea of cooling a stator within a vacuum pump has been considered before, cooling fluids such as liquid Nitrogen have been used to achieve this. Using cooling fluids to cool components of a pump has the disadvantages of requiring not only a supply of cooling fluid and the means to exchange heat in the fluid but also cooling fluid flow paths within, adjacent or close to the stator which paths can be bulky, create vibration and significantly increase the volume, cost and total energy consumption of the device.

The present invention uses a thermoelectric device to provide cooling. Such a cooling device requires a power source, however a molecular pump also requires a power source to drive the rotor and as such the provision of power to such a cooling device does not require undue additional hardware. Furthermore, appropriately arranged thermoelectric devices provide very effective, compact, cost efficient and controllable cooling and have no moving parts, thus avoiding unwanted noise, vibration or wear. In some embodiments, the thermoelectric device comprises a multistage Peltier device.

A multistage Peltier device can cool to low temperatures, consumes relatively low power and can be compact in this kind of application.

In some embodiments, said thermoelectric device is located adjacent to said stator within a vacuum environment during operation of said pump.

It may be advantageous if the thermoelectric device is located within a vacuum environment. More effective cooling may be provided if the Peltier device is itself within the vacuum environment. Thermal losses and transfer within a vacuum environment are low and thus, providing and maintaining a lower temperature within such an environment is easier and consumes less energy where the thermally insulating properties of the vacuum are used to isolate the Peltier device from external heat sources. As the pump being cooled is a vacuum pump, arranging the cooling device within a vacuum environment is straightforward and requires few additional components.

In some embodiments, said stator comprises an annular sleeve within or adjacent to a vacuum enclosure, said thermoelectric device being located adjacent to an outer surface of said sleeve within said vacuum enclosure.

One way that the thermoelectric device can be located within the vacuum environment and provide effective heat transfer between the device and the stator is to provide the stator as an annular sleeve within or adjacent to a vacuum enclosure and locate the thermoelectric device within the vacuum enclosure and adjacent to the stator and in thermal contact with it. In this way the rotor rotates with respect to this annular sleeve and the thermoelectric device cools the sleeve while being insulated by the vacuum from external heat sources. In this way, effective cooling is provided along with effective pumping. In some embodiments, said vacuum pump comprises a pump controller, said pump controller being configured to control operation of said pump and said thermoelectric device.

One advantage of using a thermoelectric device is that it is straightforward to control by simply changing the current/voltage supplied to it. This means that control circuitry such as that used to control the pump can also be used to control the cooling device. This allows control of both the operation and cooling of the pump to be combined.

In some embodiments, said vacuum pump further comprises a DC power source, said DC power source being configured to supply power to said pump controller and said thermoelectric device.

Many vacuum pumps are powered by a DC power source, such a power source is also appropriate for powering a thermoelectric device and as such, a single power source or a power source sharing many common components can be used to power both the pump and the thermoelectric device providing a cost efficient solution.

In some embodiments, said vacuum pump further comprises a power source, said power source being configured to supply power to said pump controller, said pump controller being configured to supply DC power to said thermoelectric device.

Where the pump is powered by an AC power supply the pump controller can be used to rectify this power and provide a DC power source for the thermoelectric device. ln some embodiments, said pump controller is configured to control heating and regeneration of said thermoelectric device by reversing a polarity of voltage supplied to said thermoelectric device.

Depending on the temperatures that the pump is cooled to and the gas or vapour being pumped there may be some requirements to regenerate the surfaces cooled by the thermoelectric device from time to time. Where for example, water vapour is being pumped, there may be some condensation or freezing of this on the surfaces cooled by the thermoelectric device. Thermoelectric cooling devices have a further advantage of being easy to regenerate in that reversing the polarity of the voltage and current flowing through them, allows the temperature of the connected surfaces to be raised rather than lowered and in this way any species that have frozen to the surfaces or condensed on the surfaces can be evaporated in an efficient manner.

In some embodiments, said pump controller is configured to provide control of a pumping process, said pump controller controlling both a rotational frequency of said rotor and a temperature of said thermoelectric device in combination.

The pumping performance of the pump is governed by several factors, including the rotational frequency of the rotor and the temperature of the gas being pumped. In some embodiments, the controller provides control of both rotational speed and gas temperature (via the thermoelectric device) in combination such that each factor can be tuned to provide an overall improved performance for the particular conditions.

In some embodiments, said molecular vacuum pump is configured to control a rotational speed of said rotor and a power supplied to said thermoelectric device in dependence upon a stage in said pumping process.

In some embodiments, said pump controller is configured to control said rotor to commence rotation during an initial pumpdown sequence, and after one of a predetermined time or a predetermined vacuum being reached to change said power supplied to said thermoelectric device.

In some embodiments, said pump controller is configured to supply power to said thermoelectric device to cool said stator after said one of a predetermined time or predetermined vacuum being reached.

In some embodiments, said pump controller is configured to control said thermoelectric device to heat said stator during at least a portion of said initial pump down sequence

In some embodiments, said pump controller is configured to control said thermoelectric device to cool or heat said stator prior to said initial pumpdown sequence.

Different portions of the pumping cycle may pump different mixtures of gases at different pressures. Thus, it may be more advantageous at certain points for the gases to be cooled, whereas at others cooling may not be so advantageous or may even be disadvantageous where condensable gases are being pumped. Thus, control circuitry able to control the thermoelectric device in conjunction with the pump motor may provide a pump whose performance can be tailored to the particular circumstances.

Such control circuitry may for example, start rotation of the rotor of the pump without starting the cooling such that initial evacuation is done without cooling. When a certain vacuum is reached or time elapsed or other measure of suitable operation is achieved, the cooling is started. This can, for example, be used to reduce the total energy consumption by applying power to the cooling device only when there is a vacuum present. ln the same control sequence example, it is also possible to remove the majority of gas and vapours before turning on the cooling device which will reduce the likelihood and volume of condensation on the cooled pump surfaces.

In a different control sequence, the control circuitry can be used with reverse polarity on the cooling device to add heat when starting rotation of the pump to help outgas and evaporate any condensation which already exists on the surfaces of the pump. The polarity can subsequently be reverted to cool the surfaces and increase pumping speed. Such a heating process could be initiated before or during the initial rotation.

Another example of the control circuitry may adapt the rotation frequency and cooling voltage in combination to provide a desired level of performance. For example, by cooling the surfaces and at the same time reducing the rotation frequency, the pumping performance may be maintained whilst offering a lower level of motor power, vibration, noise and stress in the pumping mechanism. Further, if the gas species being pumped is known, the control circuit can be used to regulate cooling such that it is only applied when pumping lighter gases which benefit the most, thus saving power to the cooling device when pumping heavier gases.

In a still further example the controller may turn the thermoelectric device on to cool the pump prior to starting rotation of the pump. The controller may then start the pump and it will operate in a pre-cooled mode which will have an immediate effect on the pumping performance. In some embodiments, the cooling may be stopped when the pump starts accelerating the rotor and pumping gases at higher pressures as the pump motor is using significant amounts of power during this stage. When the pump reaches a more steady state and is consuming less power the cooling may be turned on again. In this way the peak power consumption of the pump is reduced. The ready controllability of the thermoelectric device allows the temperature of the gas molecules to be controlled in a flexible manner and can be used to provide improved pumping speed and compression or, when used in combination with the pump controller can offer other benefits as described above.

In some embodiments, said molecular vacuum pump comprises a drag pump.

In some embodiments, said drag pump comprises a helical rotor drag pump.

In other embodiments, said molecular vacuum pump comprises a turbomolecular pump.

Both turbomolecular and drag pumps can be effectively cooled using

thermoelectric devices and both benefit from this cooling to more effectively pump gases, particularly lighter gases. Furthermore, some types of drag pump, such as a helical rotor pump are not very effective at pumping lighter gasses but have high levels of performance when pumping heavier gases. Thus, using such pumps in conjunction with cooling can overcome their main disadvantage and render them an efficient and effective pump.

In some embodiments, the thermoelectric device is configured to cool said stator to temperatures between 100 and 200 Kelvin

Temperatures between 100 and 200 Kelvin may provide particular improvements in pumping speed particularly to lighter gas molecules and are readily achievable using a multi-stage Peltier device.

Further particular and preferred aspects are set out in the accompanying independent and dependent claims. Features of the dependent claims may be combined with features of the independent claims as appropriate, and in combinations other than those explicitly set out in the claims. Where an apparatus feature is described as being operable to provide a function, it will be appreciated that this includes an apparatus feature which provides that function or which is adapted or configured to provide that function.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described further, with reference to the accompanying drawings, in which:

Figure 1 shows a helical rotor pump and a cooling device according to an embodiment;

Figure 2 schematically shows a turbomolecular pump, cooling device and control circuitry according to an embodiment; and

Figure 3 shows a graph illustrating how a fall in temperature can improve the pumping speed of lighter gases in a molecular pump.

DESCRIPTION OF THE EMBODIMENTS

Before discussing the embodiments in any more detail, first an overview will be provided.

Light gases have high thermal velocities which contribute to a high conductance at the inlet (molecule arrival rate).

The high thermal velocities also lead to poor capture rate, resulting in low pumping speed and low compression.

By cooling the static components, the relative thermal velocities inside the pump are reduced without adversely affecting the inlet conductance. This leads to a higher capture and transfer rate and hence higher overall effective pumping speed.

In embodiments, a multistage Peltier device may be used. Where this is within the vacuum environment, thermal losses will be kept low. Peltier devices require a DC voltage which could be configured to be supplied by a DC pump controller. The power consumption of such a device is low and the device is small and self contained. The device power supply polarity can also be reversed to‘regenerate’ the cooled surfaces, should cryo-pump action or condensation take place.

The stator component(s) are in embodiments cooled to temperatures in the region of 100-200 Kelvin in order to reduce the thermal velocity of the gas molecules exiting the stator element(s). The effect of this speed reduction is to raise the capture rate of molecules and hence raise the pumping speed and compression.

In the case of a helical rotor, the stator is the static cylindrical sleeve around the outside of the rotor.

In the case of a turbomolecular pump, the stators are the static turbo blades and the spacing rings located around the outside of the rotor blades.

Figure 1 shows an example of a vacuum pump 5 which in this case is a helical rotor pump. The pump has a cylindrical sleeve forming a stator 20 with a helical rotor 30 mounted to rotate within stator sleeve 20. There is a multistage Peltier device 10 arranged in contact with the stator 20. Although not shown the Peltier device 10 is located within a housing which is connected via a gas channel to the vacuum environment of the pump and thus, the Peltier device is within a vacuum environment when the pump is operating. As can be seen power leads take DC power to the Peltier device. These are attached to a DC power source in this embodiment which also powers a motor (not shown) for driving the rotor 30.

A controller controls operation of the pump and the Peltier device. The controller controls the Peltier device to cool the stator 20 such that the gas molecules being pumped and contacting the stator 20 are themselves cooled. This reduces the velocity with which the molecules are emitted from the stator and consequently increases the capture rate and thus, the pumping speed and compression of this pump. This is particularly effective at improving the pumping speed and compression of the faster lighter molecules.

Where the gas(es) being pumped contain species liable to freeze or condense at the temperatures attained by the device, then the controller can control the power supply to reverse polarity of the power supplied to the Peltier device when this becomes a problem and a high vacuum is not currently required. This results in the Peltier device warming the stator 20 rather than cooling it and any frozen or condensed substance will evaporate and be pumped away by the rotor. The polarity can then be reversed again and cooling recommenced. The heating process can be done in combination with a gas purge to reduce partial pressure in the pump and accelerate the cleaning process.

Figure 2 schematically shows an alternative embodiment where vacuum pump 5 is a turbomolecular pump which may include drag stages. Vacuum pump 5 has a Peltier device 10 in contact with the stator. In this embodiment the stator comprises a cylindrical sleeve with stator blades extending from it into the pumping chamber of the pump in the turbomolecular stage and may, for example, also have a Siegbahn disc extending into the pumping chamber in the Siegbahn stage. The stator is formed of a metallic material, generally aluminium and effective thermal transfer from the Peltier device to the inner surfaces of the stator is provided by conduction. The molecules travelling through the pump will contact these cooled inner surfaces and cooling of the molecules will be effected. As for the helical rotor pump this will decrease the speed of the molecules being emitted from the stator surfaces and improve the capture of the molecules by the rotating assembly thereby increasing pumping speed and compression.

In this embodiment there is an AC or DC power source 60 which supplies power to the pump motor 40 and the Peltier device 10 via a pump controller 50. Where the power supply is an AC power supply the pump controller rectifies the received power to provide a DC power source. The pump controller 50 converts power from the DC source to provide a waveform for driving the motor and routes DC power to the Peltier device. The controller 50 may be physically attached to the vacuum pump.

In one embodiment, the controller 50 sends power to motor 40 to start rotation of the rotor. Gas is pumped and as there is no cooling at this time, the heavier molecule gases will be preferentially pumped. Once a certain pressure (or other determining measure eg time) has been reached the controller 50 routes power to the Peltier device 10 and the gases being pumped are cooled. This improves the pumping performance particularly for the lighter molecule gases and an improved pumping speed and compression is obtained.

In another embodiment, the controller 50 may reverse the polarity of the power supply connected to the Peltier device 10 during the initial pump down to heat the stator and provide improved outgassing of the vacuum chamber during the initial pumping sequence. Once a certain time has elapsed or vacuum has been reached, the heating of the Peltier device may stop and after a predetermined time the polarity may be reversed and cooling commenced.

In some embodiments, during pumping the Peltier device may be controlled in conjunction with the rotational frequency of the rotor to maintain a certain pumping performance. Thus, were it desirable to reduce vibration levels and stress on the pump, the rotational frequency could be decreased at the same time that cooling is increased so that pumping performance is maintained at a lower rotational frequency.

Figure 3 shows a graph illustrating pumping speed vs stator temperature of a model modelling the performance of a helical rotor drag pump at different stator temperatures. As can be seen the pumping speed for Hydrogen and Helium rises as the temperature falls, and below 200K, the pumping speed of Helium is similar to that of Nitrogen at 300K. Temperatures below 200 K are readily achievable with a 4 stage Peltier device. Thus, from the graph it would seem that light gas pumping performance should be raised by more than 50% in a helical rotor pump according to the models. It should be noted that the pumping speed for all gases is increased by decreasing temperature, however, the effect is far more pronounced for lighter gases.

Cooling also provides a risk of condensation for some gases and thus, it may be desirable to carefully control the cooling of the pump such that cooling is applied where lighter non-condensable gases are predominantly present within the pump Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiment and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents.

REFERENCE SIGNS

5 vacuum pump

10 Peltier device

20 stator

30 rotor

40 motor

50 controller

60 power source

Claims

1. A molecular vacuum pump comprising:

a stator;

a rotor; and

a thermoelectric device configured to cool said stator such that gas molecules impacting said stator are slowed and a capture rate of gas molecules by the pump is increased.

2. A molecular vacuum pump according to claim 1 , wherein said

thermoelectric device comprises a multistage Peltier device.

3. A molecular vacuum pump according to any preceding claim, wherein said thermoelectric device is located adjacent to said stator within a vacuum environment during operation of said pump.

4. A molecular vacuum pump according to claim 3, wherein said stator comprises an annular sleeve within or adjacent to a vacuum enclosure, said thermoelectric device being located adjacent to an outer surface of said sleeve within said vacuum enclosure.

5. A molecular vacuum pump according to any preceding claim, said molecular vacuum pump further comprising a pump controller.

6. A molecular vacuum pump according to claim 5, said molecular vacuum pump further comprising a DC power source, said DC power source being configured to supply power to said pump controller and said thermoelectric device.

7. A molecular vacuum pump according to any one of claims 5 or 6, said molecular vacuum pump further comprising a power source configured to supply power to said pump controller, said pump controller being configured to supply DC power to said thermoelectric device.

8. A molecular vacuum pump according to any one of claims 5 to 7, said pump controller being configured to control cooling by said thermoelectric device by controlling power supplied to said thermoelectric device and to control heating and regeneration of said thermoelectric device by reversing a polarity of power supplied to said thermoelectric device.

9. A molecular vacuum pump according to any one of claims 5 to 8, wherein said pump controller is configured to provide control of a pumping process, said pump controller controlling both a rotational frequency of said rotor and a power supplied to said thermoelectric device in combination.

10. A molecular vacuum pump according to claim 9, said molecular vacuum pump being configured to control a rotational speed of said rotor and a power supplied to said thermoelectric device in dependence upon a stage in said pumping process.

11. A molecular vacuum pump according to claim 9 or 10, said pump controller being configured to control said rotor to commence rotation during an initial pumpdown sequence, and after one of a predetermined time or a predetermined vacuum being reached to change said power supplied to said thermoelectric device.

12. A molecular vacuum pump according to claim 11 , said pump controller being configured to supply power to said thermoelectric device to commence cooling of said stator after said one of a predetermined time or predetermined vacuum being reached.

13. A molecular vacuum pump according to claim 11 or 12, said pump controller being configured to control said thermoelectric device to cool or heat said stator prior to said initial pumpdown sequence.

14. A molecular vacuum pump according to claim 11 or 12, said pump controller being configured to control said thermoelectric device to heat said stator during at least a portion of said initial pump down sequence

15. A molecular vacuum pump according to any preceding claim, wherein said molecular vacuum pump comprises a drag pump.

16. A molecular vacuum pump according to claim 15, wherein said drag pump comprises a helical rotor pump.

17. A molecular vacuum pump according to any preceding claim, wherein said molecular vacuum pump comprises a turbomolecular pump.

18. A molecular vacuum pump according to any preceding claim, wherein said thermoelectric device is configured to cool said stator to temperatures between 100 and 200 Kelvin.