GB2570502A

GB2570502A - Scroll vacuum pump

Info

Publication number: GB2570502A
Application number: GB1801435.7A
Authority: GB
Inventors: Turner Neil
Original assignee: Edwards Ltd
Current assignee: Edwards Ltd
Priority date: 2018-01-29
Filing date: 2018-01-29
Publication date: 2019-07-31
Also published as: WO2019145680A1; GB201801435D0

Abstract

A scroll vacuum pump 10 has two intermeshing scrolls 12, 14, at least one of which rotates, whereby a fluid is compressed between the scrolls and drawn from an inlet 25 to an outlet 27. The rotating scroll has a heat pipe cavity 38, 39 for receiving a coolant, such as ether, alcohol or water. The scroll’s motion causes the coolant to cycle between hotter and colder regions of the heat pipe and cool the scroll. Preferably the cavity comprises a hotter hollow portion 40, 66 in the scroll plate 31, 35 and a colder hollow portion 44, 64 in the scroll shaft 45, 47. The coolant is a phase change fluid which, in use, flows as a liquid into the plate hollow portion to be heated and evaporated, then returns as gaseous coolant to the shaft hollow portion to give off heat, preferably with the help of cooling fins 62, and condense back to liquid. The shaft hollow portion may taper outwardly to facilitate coolant flow into the plate hollow portion by centrifugal action. Both scrolls may rotate, a flexible torque coupling 23 being provided to allow a relative orbiting motion between drive scroll 14 and driven scroll 12.

Description

The present invention relates to scroll vacuum pumps and more a particularly to a way of cooling scroll vacuum pumps.

Scroll pumps are used in numerous fields such as superchargers for automobiles and compressors for refrigerant. The present invention is specific to their use as vacuum pumps. Use in vacuum pumping presents different challenges to their use in positive pressure pumping. Often scroll pumps used in vacuum pumping are required to be clean and may encounter significantly higher compression ratios as compared to positive pressure pumps (e.g. 10,000:1 compared to 10:1).

A typical scroll vacuum pump known hereto comprises one scroll which is stationary and one scroll which orbits relative to the stationary scroll on a support bearing on a cranked drive shaft. A benefit of having a fixed scroll is that it is relatively easy to arrange for it to be cooled, for example by forcing cooler air over its outer surface in order to remove excess heat. As the fixed scroll is fixed it can form part of a pump housing and is therefore open to ambient temperature for cooling.

A disadvantage with the fixed and orbiting scroll arrangement is that the imbalanced mass of the orbiting scroll has to be counter-balanced by a correcting balance mass. This is due to a need for the correction mass to correct a force imbalance that arises from a radial offset between the scroll and its axis of rotation. A further correction mass is required to correct the couple imbalance arising from the axial offset between the scroll and the correcting balance mass.

The requirement for correction masses has been a primary limitation of orbiting mechanisms in scroll pumps and has significantly limited their potential size, scroll centre offset and operating frequency.

Another problem experienced by scroll pumps is distortion and swashing, which limits the accuracy with which a tip of one scroll wall can be spaced closely to a

-2scroll plate of an opposing scroll. In order to compensate for this problem the spacing can be made large to avoid contact in use but inevitably this large spacing is the cause of leakage. More typically compliant seals are located at the tips of the scroll walls which bridge the spacing to provide effective sealing. However, these seals are worn away during use and the particulates or dust created from the seals can be the cause of undesirable contamination. The seals also require periodic replacement requiring disassembly of the pump and this adds to the cost of ownership and increases pump downtime.

It is an object of the present invention to provide an improved scroll vacuum pump.

According to a first aspect of the present invention there is provided a scroll vacuum pump comprising a scroll mechanism having first and second intermeshing scrolls that cause compression of fluid between the scrolls from an inlet to an outlet of the scroll mechanism, at least one scroll (one or both scrolls) comprising a heat pipe formed by a heat pipe cavity in the scroll for receiving a coolant, whereby motion (e.g. rotation) of the scroll causes the coolant to cycle in the hollow pipe cavity between hotter and colder regions of the heat pipe for cooling the scroll.

Ideally the scroll pump includes a scroll plate having a scroll wall for intermeshing with a scroll wall of the other scroll and a drive shaft connected to the scroll plate, wherein the heat pipe cavity is formed by a hollow portion of the drive shaft and a hollow portion of the scroll plate, and the scroll plate forms the hotter region and the drive shaft forms the colder region of the heat pipe.

The scroll pump is arranged so that motion/rotation causes: liquid coolant to flow through the drive shaft hollow portion to the scroll plate hollow portion where heat exchanged with the scroll plate causes a phase change of the coolant to the gas phase; and gaseous coolant to flow from the scroll plate hollow portion to the drive shaft hollow portion where heat exchanged with the drive shaft causes a phase change of the coolant to the liquid phase.

-3Preferably in use the drive shaft hollow portion is outwardly tapered to cause liquid to flow towards the scroll plate hollow portion by centrifugal action generated by motion of the drive shaft.

In some embodiments the drive shafts preferably also include heat exchange elements for cooling the drive shaft.

Preferably the scroll plate includes a hollow portion whose radial dimension is such that motion/rotation of the scroll causes coolant to flow radially outwardly over the plate by centrifugal action.

In some embodiments scrolls are supported by respective support bearings for rotation in the same rotational direction about respective axes one axis offset from the other axis.

Optionally one of the scrolls is a drive scroll coupled to a motor for rotating the drive scroll and the other scroll is a driven scroll coupled to the drive scroll by a coupling for rotating the driven scroll. In this way torque is supplied to the driven scroll from the drive scroll indirectly via a gear box or other form of clutch or torque coupler.

In one particularly preferred arrangement the drive scroll and the driven scroll are coupled by a flexible torque coupling that allows relative orbital motion between the scrolls but resists a rotational motion between the scrolls.

Ideally the scroll mechanism has an inlet at an outer radial portion and an outlet at an inner radial portion and the drive shaft of one of the scrolls comprises an axial exhaust bore in fluid communication with the outlet for conveying pumped fluid (gas or liquid) from the scroll mechanism.

Ideally the heat pipe cavity of the drive shaft of said one of the scrolls surrounds and is generally concentric with the axial exhaust bore.

-4The heat pipe cavity of the drive shaft of said one of the scrolls may surround and be generally concentric with the axial exhaust bore.

In use, the heat pipe cavity(s) may be filled with a coolant selected to phase change at or close to the operating temperature of the scroll plate.

In use, the heat pipe cavity(s) may be filled with a coolant selected from any one of ether, alcohol or water.

In use, the heat pipe cavity(s) may be evacuated or pressurised to a pressure other than atmosphere to change the temperature at which the coolant undergoes phase change.

In embodiments described herein phase-change cooling occurs inside the scroll and its stub shaft. The bore of the cavity in the stub is tapered so as to drive condensed liquid towards the scroll. Inside the scroll, the cavity expands out towards the periphery so the fluid has to flow over the whole of the radial extent of the scroll plate.

When the fluid boils on the surface of the relatively hotter scroll, the vapour returns to the relatively colder end of the cavity in the stub through the central core. There can be features on a stub end to promote heat transfer with the environment (fins) and dissipate the heat of fusion liberated on condensation.

The heat pipe works by transporting the liquid using local, centrifugal gravity to push it downhill. Both scrolls can be cooled in the same way.

At least one of the stubs may be shaped and dimensioned to carry an exhaust gas flow, and in that stub the heat pipe cavity will probably surround the exhaust pipe.

Preferred embodiments of the invention will now by described, by way of example only and with reference to the Figures in which:

-5Figure 1 is cross sectional view of a scroll vacuum pump;

Figure 2 shows a coupling between scrolls of the scroll vacuum pump shown in Figure 1; and

Figures 3 shows a scroll vacuum pump with housing.

Referring to the Figures there is shown in Figure 1 a longitudinal section along axes A and B of a scroll vacuum pump 10. Scroll 12 is a driven scroll supported by support bearing 18 for rotation about axis A and scroll 14 is a drive scroll supported by support bearing 20 for rotation about axis B. The axes are offset by a distance C. The drive scroll is coupled to the driven scroll so that when the drive scroll is rotated by a motor 21 it drives rotation of the driven scroll. The coupling 23 causes one scroll to orbit relative to the other scroll, whilst both scrolls rotate in the same direction about respective axes.

The coupling 23 is an anti-rotation device that restricts relative angular movement between the scrolls and allows relative orbiting motion. Such an anti-rotation device may comprise for example three cranked shafts, an Oldham coupling or a device (sometimes referred to as a ‘frog’) as described in the Applicant’s published International Patent Application WO-A-2011/135324, the contents of which are hereby incorporated by reference. The applicant’s earlier device is used in the present example and described for completeness below with reference to Figure

2.

This drive/driven scroll mechanism is contrasted with typical scroll mechanisms that comprise a fixed/orbiting scroll mechanism. In the former mechanism, a single scroll orbits around its axis and is unbalanced in itself. In the latter mechanism both scrolls rotate about respective axes and are generally balanced. Since it is the imbalance in the fixed/orbiting mechanism that is the cause of distortions and swashing, a drive/driven scroll mechanism allows a much tighter clearance to be

-6set between the scrolls. This may mean that tip seals can be dispensed with in the present design or that if used their wear is significantly reduced. Additionally, since both scrolls are generally balanced pumps can be rotated at higher frequencies and be more compact, or have larger radial offsets, greater capacity, or larger scrolls.

Referring again to Figure 1, similarly to a fixed/orbiting scroll mechanism, in a drive/driven scroll mechanism the scrolls intermesh to cause compression of fluid between the scrolls from an inlet 25 to an outlet 27 of the scroll mechanism during relative orbiting motion. The inlet is located at an outer radial portion of the mechanism and the outlet is located at an inner radial portion.

The driven scroll comprises a scroll plate 31 having a scroll wall 33. The driven scroll comprises a scroll plate 35 having a scroll wall 37. The scroll walls intermesh and cause pockets of pumped fluid to be trapped between the walls (and the plates) and on orbiting motion the pockets are compressed as the fluid is pumped from the inlet to the outlet. The scroll plates are connected to or formed integrally with respective shafts 45, 47 that are supported for rotation by bearings 18, 20.

The drive shaft of one of the scrolls (in this example shaft 45 comprises an axial exhaust bore in fluid communication with the outlet 27 of the mechanism for conveying pumped gas from the scroll mechanism and from the pump. The axial bore 41 is centred about axis A of the driven scroll for balancing.

As described above, a fixed/orbiting scroll arrangement has an advantage that one scroll is stationary and can be readily cooled, but also disadvantages that the orbiting scroll requires balancing and balancing the scroll causes other disadvantages. In this regard, a radial counter-weight is fixed to a scroll shaft to balance the off-centre mass of the scroll about the axes. Additionally, an axial imbalance about a shaft support bearing is corrected with an additional axial weights. It is not possible with an orbiting scroll to correct the axial imbalance at the cause of imbalance and therefore axial correction gives rise to loading on the

-7shaft. The drive/driven scroll arrangement is more inherently balanced about the scroll axes and what little imbalance exists can be corrected at the cause of imbalance by removal or addition of mass (e.g. by drillings or counter-weights). However since both scrolls are moving neither scroll can be readily cooled. There follows in more detail below a description of how one or both scrolls may be cooled.

At least one of the drive or driven scrolls comprises a heat pipe. As shown in Figure 1, both scrolls comprise a heat pipe. In the driven scroll 12, the heat pipe is formed by a heat pipe cavity 38 and in the drive scroll 14 the heat pipe is formed by a heat pipe cavity 39. The heat pipes also comprise a coolant and the heat pipe cavities are arranged for receiving the coolant. The coolant may be introduced to the cavities at any point during assembly and possibly on site prior to use, or may be introduced during manufacture. When the coolant has been introduced, rotation of the scrolls causes the coolant to cycle in the heat pipe cavity between hotter and colder regions of the heat pipe for cooling the scrolls.

Referring first to the driven scroll 12, the heat pipe cavity is formed by a hollow portion 44 of the shaft and a hollow portion 40 of the scroll plate. The scroll plate forms the hotter region and the shaft forms the colder region of the heat pipe. The scroll plate is hotter because it receives heat from the pumped gas when the gas is compressed between the scrolls. The shaft is colder because it is spaced away from the pumped gas.

In use, liquid coolant is caused to flow through the shaft hollow portion 44 to the scroll plate hollow portion 40 by rotation of the scroll where heat is exchanged with the scroll plate. The hotter scroll plate heats the coolant and causes evaporation or a phase change of the coolant to the gas phase. Gaseous coolant is caused to flow from the scroll plate hollow portion to the drive shaft hollow portion where heat is exchanged with the shaft. The gaseous coolant loses heat to the shaft causing a phase change of the coolant to the liquid phase. This condensation causes a reduction in pressure in the shaft hollow portion, whilst the evaporation causes an increase in pressure in the scroll plate hollow portion thereby producing a pressure

-8differential that causes the gaseous coolant to flow from the scroll plate hollow portion to the shaft hollow portion.

In this example, the shaft hollow portion is outwardly tapered to cause liquid to flow towards the scroll plate hollow portion by centrifugal action generated by motion of the shaft. The radially outer surface of the shaft hollow portion is tapered uniformly in this example like a cone, whilst the inner radial surface is not tapered like a cylinder. Liquid coolant is caused to flow along the outer surface towards the scroll plate leaving a space adjacent the inner surface to receive coolant vapour. The shaft hollow portion may have other configurations whereby rotation of the scroll causes flow of liquid coolant towards the scroll plate although the illustrated configuration functions well and is readily manufactured. The shaft hollow portion may be formed by drilling or casting.

The heat pipe cavity 38 of the shaft 45 is concentric with respect to the axis A in order to balance the shaft about the axis. The cavity also surrounds and is generally concentric with the axial exhaust bore 41 so that both the exhaust bore and the cavity are balanced about the axis.

The scroll plate hollow portion has a radial dimension such that rotation of the scroll causes coolant to flow radially outwardly over the plate by centrifugal action. The radial dimension may, as illustrated, be greater than the radial dimension of the outer scroll wall in order to allow coolant to contact as much of the hotter part of the scroll plate as possible. However, the scroll plate becomes hottest at the radial central portion where compression is the highest and therefore the plate hollow portion may have a reduced radial extent.

As illustrated the plate hollow portion 40 is formed by an annular space or void between opposing axial plate portions 42a and 42b. The plate hollow portion may be formed by drilling or by casting, or by additive manufacturing methods such as 3D printing. Alternatively one plate portion may be fixed by fasteners to the other plate portion after casting. In an alternative configuration, the plate hollow portion

-9may be formed by a plurality of shaped channels that fan out in the plate from the centre towards the periphery.

The drive scroll 14 in Figure 1 also comprises a heat pipe and heat pipe cavity 39 for receiving a coolant. The arrangement of the drive scroll heat pipe will be described briefly because it is similar to the driven scroll.

Heat pipe cavity 39 comprises a drive shaft hollow portion 64 and a plate hollow portion 66. The drive shaft hollow portion is shaped to cause the flow of liquid coolant towards the plate hollow portion on rotation of the drive scroll. The drive shaft hollow portion 64 is concentric about axis B, but is different from the shaft hollow portion 44 since the drive shaft does not accommodate an exhaust bore in this example. In other examples the drive shaft may accommodate an exhaust bore instead of the driven shaft or both shafts may accommodate respective exhaust bores. In the illustrated example, the drive shaft hollow portion is generally conical without a central cylindrical surface. Similarly, the plate hollow portion 66 is an annular or disc-shaped void between axial plate portions 43a, 43 but does not comprise an interruption to accommodate an exhaust bore.

In order to promote cooling of the shaft 45 and drive shaft 47, the shafts are provided with heat exchange elements, or cooling fins, 62 to increase the surface area for heat dissipation away from the shafts. The cooling fins may be formed integrally with the shafts or may be fixed to the shafts subsequent to shaft manufacture.

The coolant is selected so that liquid to gas phase change (boiling or evaporation) occurs at around the desired operating temperature of the scroll. Since the cooling effect is produced by phase change in the embodiments, only once the scrolls are at or around a preferred operating temperature does significant cooling occur to resist temperature rise above the operating temperature. The operating temperature of the scrolls may in examples be between around 50°C and 100°C, or between around 60°C and 80°C or around 60°C to 70°C. Therefore in the latest

-10case a coolant may be selected that changes phase at 60°C to 70°C. An ether or an alcohol may be selected with an appropriate phase change temperature.

The temperature at which the coolant changes phase is dependent also on the pressure in a heat pipe cavity. In one example the heat pipe cavity is evacuated to a pressure less than atmosphere prior to use in order to reduce the phase change temperature. Although not illustrated an evacuation duct may be provided for evacuating gas from a cavity. A reduction in cavity pressure is useful for example if the coolant is water and the desired phase change temperature is less than 100°C, particularly since water is an inexpensive coolant.

The selection of coolant may be optimised dependent on the angle of taper of the shaft hollow portions and the speed of rotation of a scroll to achieve the desired flow of liquid coolant towards the plate hollow portions. A coolant is selected that has a viscosity that the taper and the speed produce the desired circulation.

Optionally a wick 70 may be included in the heat pipe cavity in order to improve flow of liquid coolant at shallow tapering angles of the cavity, although a wick may not be a preferred solution since it may be detrimental to unimpeded circulation of coolant in the cavity.

The anti-rotation device 23 is shown in more detail in Figure 2. The device comprises a central body portion 52 having a plurality of arms 54, 56 extending from the body. Each of the arms has a connecting portion 58 at an end thereof. The arms are arranged in two perpendicular pairs. One of the pairs (either pair 54, 54 or pair 56, 56) is connected to one of the scrolls 12,14 and the other of the pairs is connected to the other of the scrolls 12, 14. In Figure 1, the first pair 54 is connected by fasteners 58 to the drive scroll 14 and the second pair 56 is connected by fasteners 60 to the driven scroll 12. The arms 54 flex to allow movement of the orbiting scroll in the 'y' direction and the arms 56 flex to allow movement in the 'x' direction.

-11 The anti-rotation device 23 is lubricant free and therefore can be located in a high vacuum region without contaminating the flow path through the scroll arrangement or causing the migration of lubricant upstream of the pump to a processing tool. The anti-rotation device may be made of a light-weight material, such as plastics (e.g. PTFE, acetal or nylon), so that even though it is not in balance in use its out of balance mass is low. In contrast, in a fixed/orbiting scroll arrangement an orbiting scroll is made of a metallic material (such as steel) and consequently its out of balance mass is high.

Figure 3 shows a scroll vacuum pump with a pump housing 70. The pump housing houses the scroll mechanism and forms a vacuum space 72 inside the housing sealed from a low vacuum space 74 external to the housing. The vacuum space is annular and is in fluid communication with the scroll inlet 25 through 360 degrees of rotation of the scrolls about their axes. The vacuum space is in fluid communication with a housing inlet 76 for pumping gas at vacuum pressure.

The vacuum space 72 is sealed from the low vacuum or atmospheric space 74 by seals 78, which in the this example are annular shaft seals located between the scroll shafts and the housing. The shaft hollow portions 44, 64 extend externally from the housing 70 for cooling in the low vacuum space away from the hotter vacuum space 72. The low vacuum space may also be enclosed by a housing part (not shown). The scroll outlet or exhaust 27 is in fluid communication with the exhaust bore 41 that extends within the shaft 12 and radially inward of the shafts seals 78.

The invention has been described by way of examples only and it will be appreciated that variation may be made to the embodiments without departing from the scope of protection as defined in the claims.

Claims

1. A scroll vacuum pump comprising a scroll mechanism having first and second intermeshing scrolls that cause compression of fluid between the scrolls from an inlet to an outlet of the scroll mechanism, one or both scrolls comprising a heat pipe formed by a heat pipe cavity in the scroll for receiving a coolant, whereby motion of the scroll causes the coolant to cycle in the heat pipe cavity between hotter and colder regions of the heat pipe for cooling the scroll.

2. A scroll pump as claimed in claim 1, wherein said at least one scroll comprises: a scroll plate having a scroll wall for intermeshing with a scroll wall of the other scroll and a shaft connected to the scroll plate, wherein the heat pipe cavity is formed by a hollow portion of the drive shaft and a hollow portion of the scroll plate, and the scroll plate forms the hotter region and the drive shaft forms the colder region of the heat pipe.

3. A scroll pump as claimed in claim 2, wherein motion/rotation causes:

liquid coolant to flow through the drive shaft hollow portion to the scroll plate hollow portion where heat exchanged with the scroll plate causes a phase change of the coolant to the gas phase; and gaseous coolant to flow from the scroll plate hollow portion to the drive shaft hollow portion where heat exchanged with the drive shaft causes a phase change of the coolant to the liquid phase.

4. A scroll pump as claimed in claim 2 or 3, wherein the drive shaft hollow portion is outwardly tapered to cause liquid to flow towards the scroll plate hollow portion by centrifugal action generated by motion of the drive shaft.

5. A scroll pump as claimed in any one of the preceding claims, wherein the drive shafts incorporates heat exchange elements for cooling the drive shaft.

6. A scroll pump as claimed in any one of the preceding claims, wherein the scroll plate hollow portion has a radial dimension such that motion/rotation of the scroll causes coolant to flow radially outwardly over the plate by centrifugal action.

7. A scroll pump as claimed in any one of the preceding claims, wherein the scrolls are supported by respective support bearings for rotation in the same rotational direction about respective axes one axis offset from the other axis.

8. A scroll pump as claimed in claim 7, wherein one of the scrolls is a drive scroll coupled to a motor for rotating the drive scroll and the other scroll is a driven coupled to the drive scroll by a coupling for rotating the driven scroll.

9. A scroll pump as claimed in claim 8, wherein the drive scroll and the driven scroll are coupled by a flexible torque coupling that allows a relative orbiting motion between the scrolls but resists a rotational motion between the scrolls.

10. A scroll pump as claimed in any one of the preceding claims, wherein the scroll mechanism has an inlet at an outer radial portion and an outlet at an inner radial portion and the drive shaft of one of the scrolls comprises an axial exhaust bore in fluid communication with the outlet for conveying pumped gas from the scroll mechanism.

11. A scroll pump as claimed in claim 10, wherein the heat pipe cavity of the drive shaft of said one of the scrolls surrounds and is generally concentric with the axial exhaust bore.

12. A scroll pump as claimed in any one of the preceding claims, wherein, in use, the heat pipe cavity(s) is filled with a coolant selected to phase change at or close to the operating temperature of the scroll plate.

13. A scroll pump as claimed in any one of the preceding claims, wherein, in use, the heat pipe cavity(s) is filled with a coolant selected from any one of ether, alcohol or water.

5

14. A scroll pump as claimed in any one of the preceding claims, wherein, in use, the heat pipe cavity(s) is evacuated or pressurised to a pressure other than atmosphere to change the temperature at which the coolant undergoes phase change.