The invention relates to scroll pump tip sealing.
Known scroll compressors, or pumps, comprise a fixed scroll, an orbiting scroll and a drive mechanism for the orbiting scroll. The drive mechanism is configured to cause the orbiting scroll to orbit relative to the fixed scroll to cause pumping of a fluid between a pump inlet and a pump outlet. The fixed and orbiting scrolls each comprise an upstanding scroll wall extending from a generally circular base plate. Each scroll wall has an end, or tip, face disposed remote from and extending generally perpendicular to the respective base plate. The orbiting scroll wall is configured to mesh with the fixed scroll wall during orbiting of the orbiting scroll so that the relative orbital motion of the scrolls causes successive volumes of gas to be enclosed in pockets defined between the scroll walls and pumped from the inlet to the outlet.
A scroll pump may be a dry pump in which the scrolls are not lubricated so the internal working clearances are not sealed with a fluid such as oil. In this case, to prevent back leakage, the tip of each scroll wall is provided with a tip seal to seal against the base plate of the other scroll. The tip seals are located in channels defined in the tips of the scroll walls and are typically made of PTFE. There may be a small gap between the base of each channel and the opposing face of the tip seal so that, in use, fluid occupying the gap forces the tip seal towards and against the base plate of the other scroll. The tip seals close the gap between the scrolls caused by manufacturing and operating tolerances and reduce the leakage to an acceptable level.
Typically, a tip seal is narrower than its channel so that there is a radial clearance between the tip seal and the opposed sidewalls of the channel. During relative orbiting motion of the scrolls, the tip seal is urged against one sidewall for part of its motion and against the other sidewall for another part of its motion. As the tip seal moves back and forth between these positions, leakage is increased because there is a leakage path formed from one side of the seal to the other side of the seal. Known tip seals typically have an aspect ratio of height to radial width which is 1:1. That is, the radial width of the tip seal is equal to the height of the tip seal so that the tip seal has a square cross-section. Accordingly, the tip seal is relatively stiff in the radial, or widthways, direction. When the tip seal moves radially between the sidewalls of the tip seal channel, this relative stiffness slows the movement of the tip seal, thereby increasing leakage.
For some vacuum applications, such as those involving exposure to radioactivity, it is advantageous, or may even be essential, to use an oil free scroll pump. However, where there is to be exposure to radioactivity, it is not possible to use PTFE as the tip seal material.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter. The claimed subject matter is not limited to implementations that solve any or all disadvantages noted in the background.
The invention provides a scroll pump as specified in claim 1.
The invention also includes a scroll pump tip seal as specified in claim 15.
The invention also includes a method of providing a tip seal in a scroll pump as specified in claim 28.
The Summary is provided to introduce a selection of concepts in a simplified form that are further described in the Detail Description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended. to be used as an aid in determining the scope of the claimed subject matter.
In the following disclosure, which is given by way of example only, reference will be made to the drawings, in which:
Referring to
The fixed scroll 22 comprises a spiralling, or involute, scroll wall 28. The scroll wall 28 extends perpendicularly from a major surface 30 of a generally circular base plate 32 and has an end, or tip, face 34 that is spaced from the major surface 30. The tip face 30 may be generally parallel to the major surface 30. The orbiting scroll 20 comprises a spiralling, or involute, scroll wall 36. The scroll wall 36 extends perpendicularly from a major surface 37 of a generally circular base plate 38 and has an end, or tip, face 40 that is spaced from the major surface 37. The tip face 40 may be generally parallel to the major surface 37. The orbiting scroll wall 36 co-operates, or meshes, with the fixed scroll wall 28 during orbiting movement of the orbiting scroll 20. Relative orbital movement of the scrolls 20, 22 causes successive volumes of gas to be trapped in pockets defined between the scrolls and pumped from the inlet 24 to the outlet 26.
The scroll pump 10 may be a dry pump in which the scrolls 20, 22 are not lubricated so that there is no lubricant present to seal the working clearances between the scrolls. In order to prevent, or at least reduce, back leakage, via respective gaps 42, 44 between the tip faces 34, 40 of the scroll walls 28, 36 and the opposed major surfaces 30, 37 of the base plates 32, 38, respective tip sealing arrangements are provided to close the gaps 42, 44. The tip sealing arrangement for the fixed scroll 22 can be seen in
Referring to
Referring to
As best seen in
Providing seal segments that are assembled in overlapping relationship as illustrated by way of example in
Providing a segmented tip seal comprising a plurality of discrete seal segments that are fitted contiguously end to end in a channel, or groove, defined in the tip of a scroll wall may allow the use of relatively inflexible materials that would otherwise not be suitable for forming a tip seal. Furthermore, it may allow the use of materials that may be desirable for particular operating environments, but are not considered suitable for tip seal manufacture because processing them to form a tip seal would be difficult or wasteful of the bulk material. For example, tip seals are commonly made of PTFE, but PTFE is not a suitable material if the scroll pump is going to be exposed to radioactivity. Providing a tip seal as a plurality of seal segments allows the possibility of making the tip seal from polymer materials that have a higher flexural modulus than PTFE and can at least cope better than PTFE with exposure to radioactivity, or even the possibility of making the tip seal from a metal. As examples of suitable polymers, a segmented tip seal may be made of a polymer from the polyimide (PI), polyaryletherketone (PAEK), polysulfone (PSU) or polyamide-imide families. Examples of suitable family members of these high performance polymers include polyesteretherketone (PEEK) from the PAEK family, polyethersulfone (PES) from the PSU family and polyethermide (PEI) from the PI family. These polymers may have a flexural modulus which is at least 1.5 GPa, preferably greater than 2.0 GPa. For example, PEI may have a flexural modulus of 3.4 to 5.4 GPa, PES may have a flexural modulus of 3.4 to 5.6 GPa, VESPELĀ® from the family may have a flexural modulus of 3.7 to 20 GPa and PEEK may have a flexural modulus of 1.32 to 20 GPa. The polymers used may have a density that is lower than that of PTFE. For example, the density of the polymer used may be less than 1.6 g/cm3 and preferably less than 1.5 g/cm3. PEEK may have a density of 1.32 to 1.51 g/cm3, PEI and PES may have a density of 1.27 to 1.51 g/cm3 and VESPELĀ® may have a density of 1.37 to 1.54 g/cm3.
Since the segmented polymer tip seals may be operating in a dry environment, it may be desirable to add a filler such as graphite to the polymer material in order to provide a self-lubricating property.
A metal tip seal may be made of bronze, which has the advantage that bronze is a material approved for nuclear applications. Using bronze as the segmented tip seal material may also be desirable as bronze has self-lubricating, non-galling, properties, which may be advantageous since the tip seal will be in sliding contact with the opposite scroll. Other metals showing good non-galling properties that may be suitable for producing a segmented tip seal, perhaps in an alloy containing the metal, include cobalt, copper, gold, iridium, nickel, palladium, platinum, rhodium and silver.
As previously described, the tip seal may be provided only at the radially innermost end of the scroll walls and the portion of the tip face without a tip seal may form the remainder of the tip sealing arrangement. In other examples, a tip seal may be provided along at least substantially the entire length of the scroll wall. The seal segments may all have substantially the same length. Alternatively, different length seal segments may be provided. In examples in which different length seal segments are used, relatively short seal segments may be used at the radially innermost end of the scroll walls where the curvature of the scroll wall is greatest and relatively longer segments may be used as the curvature of the scroll wall decreases. In some examples, a single seal segment may be used for one or more of the radially outer turns of the scroll wall, while a plurality of seal segments is used for just one of the radially inner turns of the scroll wall. It may be advantageous to use relatively shorter length seal segments in at least some examples as using relatively longer length seal segments may require the provision of a larger number of seal segments with different curvature to take account of the changing curvature of the scroll wall. However, using relatively longer seal segments may be beneficial in reducing assembly times and reducing the number of potential leakage paths through the tip seal.
In some examples the seal segments may have a length in the range 20 to 100 mm, while in other examples the seal segments may have a length in the range 20 to 60 mm. In some examples, at least one of the seal segments may have a curved length in the range of 1 to 5% of the curved length of the tip face between the radially innermost and radially outermost ends 50, 52 of the scroll wall. In other examples, there may be at least one seal segment having a curved length in the range of 1 to 2% of the curved length of the tip face. In still other examples, at least one of the seal segments may have a curved length of about 1.5% of the curved length of the curved length of the tip face.
Each of the seal segments may be made of the same material. However, in some examples, a relatively more flexible polymer (such as a polymer from the polyimide or PEEK families) may be used to make one or more seal segments used at the radially innermost end of the scroll wall, while a metal is used to make one or more seal segments used towards the radially outermost end of the scroll wall. In either case, using a plurality of seal segments disposed contiguously end to end allows the possibility of providing a tip seal made of materials suitable for pumping environments for which a PTFE tip seal would not be suitable, where it would otherwise be difficult or unduly wasteful of material to use the more suitable material.
As previously described, the tip seal may be pressed against an opposed major surface of a scroll base plate by fluid disposed between the base of the channel in which the tip seal is housed and the opposing face of the tip seal. The fluid pressure across the tip seal will vary between a relatively lower pressure adjacent the pump inlet and a relatively higher pressure adjacent the pump outlet. In examples in which one or more of the seal segments is made of a metal, it may be desirable to provide voids within the segment, or segments, to reduce the overall density of the seal segment. Otherwise the fluid pressure may be insufficient to press the tip seal against the opposed scroll base plate where the pressure differential across the tip seal is relatively low. Thus, a segmented tip seal may comprise one or more seal segments having a relatively lower density disposed towards the end of the tip seal disposed closest to the pump inlet and one or more seal segments having a relatively higher density disposed towards the end of the tip seal disposed closest to the pump outlet. The overall density of a metal seal segment may be reduced by making the segment from a foamed metal as illustrated by
The greatest wear to a scroll pump tip seals should occur at the ends of the scroll walls disposed adjacent the pump outlet :26 where the operating pressures should be highest. Providing a segmented tips seal gives rise to the possibility of replacing only those seal segments that are worn sufficiently to require replacement and leaving the remaining seal segments in situ for continued use. This may be both more cost efficient in terms of materials usage and is also more environmentally friendly. Furthermore, having relatively short lengths of new tip seal to wear in following a maintenance operation may be beneficial since the volume of dust produced during wearing in of the tip seal should be reduced.
Although elements have been shown or described as separate embodiments above, portions of each embodiment may be combined with all or part of other embodiments described above.
Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are described as example forms of implementing the claims.
Number | Date | Country | Kind |
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1603333.4 | Feb 2016 | GB | national |
This application is a Section 371 National Stage Application of International Application No. PCT/GB2017/050446, filed Feb. 22, 2017, which is incorporated by reference in its entirety and published as WO 2017/144870 A1, on Aug. 31, 2017 and which claims priority of British Application No. 1603333.4, filed Feb. 26, 2016.
Filing Document | Filing Date | Country | Kind |
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PCT/GB2017/050446 | 2/22/2017 | WO | 00 |