SCROLL-TYPE FLUID DISPLACEMENT DEVICE FOR VACUUM PUMP APPLICATION

Abstract: A scroll-type vacuum pump (10) wherein an expander (40, 50) and a compressor (60, 70) are arranged in series, in two stages, in the same housing (20) and driven by the same shaft (22). The first stage is a scroll-type expander. It is in series with a scroll-type compressor, which is the second stage. The volume of the suction pockets of the second stage, the compressor, is not significantly smaller than the volume of the discharge pockets of the first stage device, the expander. Thus, the amount of heat associated with the re-expansion and compression process is reduced. The two stage pump also includes a double shaft seal mechanism (11, 92) which seals off the suction chamber of the expander from both the ambient and the discharge chamber of the expander. The two stage pump of the invention further includes a labyrinth structure (301-304) at the tip surfaces of the scroll elements to tightly control the axial ( delta ) gap between the tips and bases of the mating scroll elements.

Description: SCROLL-TYPE FLUID DISPLACEMENT DEVICE FOR VACUUM PUMP APPLICATION FIELD OF THE INVENTION This invention relates in general to a fluid displacement device. More particularly, it relates to a scroll-type fluid displacement device for vacuum pump application. BACKGROUND OF THE INVENTION Scroll-type fluid displacement devices are well known. For example, U.S. Pat. no. 801,182 to Leon Creux, discloses a scroll device including two scroll members, each having a circular end plate and a spiroidal or involute scroll element. The scroll elements have identical, spiral geometry and are interfit with an angular and radial offset to create a plurality of line contacts between their spiral curved surfaces. Thus, the interfit scroll elements define and seal off at least one pair of fluid pockets. By orbiting one scroll element relative to the other, the line contacts are shifted along the spiral-curved surfaces, thereby changing the volume of the fluid pockets. This volume increases or decreases depending upon the direction of the scroll elements' relative orbital motion. Thus, the device may be used either to compress or expand fluids. Known scroll-type fluid displacement devices, whether operating as expanders or compressors, can be used as vacuum pumps. However, both face a substantial potential for overheating. Where an expander is used as a vacuum pump, ambient air will re- expand to the discharge pockets because the air pressure in the discharge pockets is much lower than the ambient air pressure. Re-expansion of ambient air in this fashion consumes energy and frequently causes overheating. A discharge valve can be employed to reduce re-expansion of the ambient air to some extent, but, it cannot eliminate re-expansion and such valves frequently malfunction. When a compressor is used as a vacuum pump and the iniet air of the compressor is at atmospheric pressure during the start-up period, or due to leakage to ambient, the heat associated with the re-expansion and compression process is damaging to the compressor because there usually is no lubrication or internal cooling allowed. The re-expansion and compression heat causes excessive thermal growth of the scroll elements, resulting in galling between tips and bases of the scroll elements. U.S. Patent No. 3,994,636 discloses a tip seal mechanism for radial sealing between the compression pockets in a scroll-type fluid displacement device. In this device, as shown in the drawings as PRIOR ART, tip seals 101 and 201 are placed in spiral grooves 102 and 202 formed in the middle of the tips of a scroll vanes 103 and 203, respectively. These tip seals 101 and 201 run continuously along spiral grooves 102 and 202, from the central region to the periphery of the scroll members 103 and 203, respectively. The seals 101 and 201 are urged by either a mechanical device, such as elastic material, or by pneumatic force to contact the bases 204 and 104 of the other scroll member 203 and 103, respectively. This arrangement provides radial sealing. However, the width of the tip seal is smaller than the width of the scroll vane. There are tangential leakage passages A-A and B-B in scroll element 103, for example, at the both sides of the tip seal 101. These leakage passages lower the volumetric and energy efficiency of the scroll device. SUMMARY OF THE INVENTION It is a primary object of the present invention to overcome the above- mentioned shortcomings of a scroll-type fluid displacement device in a vacuum pump application. It is also an object of the invention to provide a scroll-type vacuum pump wherein excessive heat normally associated with the re-expansion and compression process in such a device is eliminated. It is another object of the invention to provide a scroll-type vacuum pump which achieves these ends by, among other things, utilizing and expander and compressor in the same pump. It is still another object of the present invention is to provide a shaft seal mechanism which seals off the suction chamber of the expander from both the ambient and the discharge chamber of the expander. Yet another object of the present invention is to provide a seal arrangement at the tip of a scroll element which effectively provides radial and tangential sealing without tip-base galling. The foregoing and other objects are realized in accord with the present invention by providing an expander-compressor, two stage vacuum pump, built in the same body and sharing the same drive shaft. The first stage is a scroll-type expander. It is in series with a scroll-type compressor, which is the second stage. The volume of the suction pockets of the second stage, the compressor, is not significantly smaller than the volume of the discharge pockets of the first stage device, the expander. Thus, the amount of heat associated with the re-expansion and compression process is reduced. The two stage pump also includes a double shaft seal mechanism which seals off the suction chamber of the expander from both the ambient and the discharge chamber of the expander. The two stage pump of the invention further includes a labyrinth structure at the tip surfaces of the scroll elements to tightly control the axial gap between the tips and bases of the mating scroll elements. The labyrinth structure comprises an arrangement of small lips, with thin and low walls, forming a maze on each tip of each of the scroll elements. When thermal growth of the scroll elements causes the labyrinth lips to press against the base of a mating scroll element, the labyrinth lips are sufficiently weak that the contact pressure between the lips and base deforms the lips on the scroll by removing interferencing material without causing tip or base galling. Thus, the labyrinth lips can produce an extremely close axial clearance between the scroll tips and bases. Radial and tangential leakage flow between compression pockets is significantly reduced because good radial and tangential sealing is achieved. BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS The invention, including its construction and operation, is illustrated more or less diagrammatically in the drawings, in which: FIGURE 1 is a cross-sectional view along the axis of a two stage, scroll-type vacuum pump constructed in accord with the present invention; FIGURE 2 is a cross-sectional view taken transversely through the pump of FIGURE 1 along line 2-2 of FIGURE 1 ; FIGURE 3 is a cross-sectional view taken transversely through the pump of FIGURE 1 along line 3-3 of FIGURE 1 ; FIGURE 4a-4c illustrate the work principle of the first stage of the pump, in accord with the present invention; FIGURES 5a-5c illustrate the work principle of the second stage of the pump, in accord with the present invention; FIGURES 6a-6f illustrate various embodiments of labyrinth lips formed on the tips of scroll elements, in accord to the present invention. DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS Referring now to FIGURES 1-3, a scroll-type vacuum pump constructed in accordance with the present invention is shown generally at 10. The vacuum pump 10 includes a main housing 20 which contains a main shaft 22 supported by a bearing 30. A first scroll member 40 and a fourth scroll member 70 are bolted to the front and rear ends of the main housing 20, respectively. A front bearing housing 90 is bolted to the first scroll member 40. The front bearing housing 90 holds a front shaft seal 92 and a front shaft bearing 94. The main shaft 22 is rotatably supported by the bearing 30 and the bearing 94, and rotates along its axis S1-S1 when driven by an electric motor (not shown) through a pulley 96. The shaft seal 92 seals the shaft 22 to prevent outside air and dirt from entering the pump 10. The main shaft 22 includes a front crank pin 24 and a rear crank pin 26. The central axis S2-S2 of the front crank pin 24 is offset from the main shaft axis S1-S1 by a distance equal to the orbiting radius R or ι of a second scroll member 50. The central axis S3-S3 of the rear crank pin 26 is offset from the main shaft axis S1-S1 by a distance equal to the orbiting radius R or 2 of a third scroll member 60. The orbiting radii R or ι and R or 2 are the radii of the orbiting circles which are traversed by the second scroll member 50 and the third scroll member 60 as they orbit relative to the first scroll member 40 and fourth scroll member 70, respectively. The first and the second scroll members 40 and 50, together, form the first stage of the vacuum pump 10, the expander. The first scroll member 40, also called the expander fixed scroll, includes a circular end plate 41 from which a first scroll element 42 extends. In addition to the circular end plate 41 and the first scroll element 42, the first scroll member 40 includes an axialiy protruding front end 43 to which the front bearing housing 90 is attached. The second scroll member 50, also called the expander orbiting scroll, includes a circular end plate 51 , a second scroll element 52 and an orbiting bearing boss 53. The scroll element 52 is affixed to, and extends from, the front surface of the end plate 51., The orbiting bearing boss 53 is affixed to, and extends from, the front surface of the end plate 51. It could also extend from the rear surface of the end plate 51 in a more traditional design. Scroll elements 52 and 62 are interfit at a 180 degree angular offset and at a radial offset equal to the orbiting radius R or ι. At least one pair of sealed off fluid pockets is thereby defined between the scroll elements 52 and 62, and the end plates 51 and 61. The second scroll member 50 is connected to a driving pin 24 through a front driving pin bearing 27 and front driving slider 28. A front oldham ring 29 prevents rotation of the second scroll member 50. Therefore, when the second scroll member 50 is driven in an orbital motion at the orbiting radius Ron, it is effective to expand fluid in the pockets when the drive shaft 22 is rotated. The third and the fourth scroll members 60 and 70, together, form the second stage of the vacuum pump 10, the compressor. The third scroll member 60, also called the compressor orbiting scroll, has a circular end plate 61 from which a third scroll element 62 extends. An orbiting bearing boss 63 is affixed to, and extends from, the front surface of the end plate 61. The fourth scroll member 70, also called the compressor fixed scroll, includes a circular end plate 71 , a fourth scroll element 72, a discharge hub 73 and reinforcing ribs 74. Scroll elements 62 and 72 are interfit at a 180 degree angular offset, and at a radial offset equal to the orbiting radius R or2 - At least one pair of sealed off fluid pockets is thereby defined between scroll elements 62 and 72 and end plates 61 and 71. The third scroll member 60, is connected to driving pin 26 through a rear driving pin bearing 31 and rear driving slider 32. A rear oldham ring 33 prevents rotation of the third scroll member 60, whereby it is driven in an orbital motion to thereby compress fluid at the orbiting radius R or2 when the drive shaft 22 is rotated. In operation of the compressor 10, air enters the inlet chamber 81 from the intake port 80. From the inlet chamber 81 , the air travels to the suction pockets 82 formed by the first and second scroll members 40 and 50. This air then is expanded by the operation of these two scroll members. The expanded air is discharged through chamber 84, chamber 85 and passage 86 to the suction chamber 87 of the second stage of the vacuum pump, the compressor. The air in the suction chamber 87 then enters the suction pockets formed by the third and fourth scroll members 60 and 70, where it is compressed by the operation of these two scroll members. The compressed air opens the discharge valve 88 and escapes to ambient from the discharge hole 89 and the discharge port 98. FIGURES 4a-4c schematically illustrate the relative movement of interfitting, spiral-shaped scroll elements 42 and 52 of the first and the second scroll members 40 and 50, respectively. In FIGURE 4a, the suction pockets of the expander are shown at 2A. The suction pockets 2A are the innermost pockets formed by the two scroll elements 42 and 52 when the tips of one scroll element are in contact with the tips of the other scroll element. The total volume of the suction pockets is called suction volume. Referring now to FIGURES 4b and 4c, 2B indicates the pockets during the expansion process and 2C indicates the discharge pockets of the expander. The discharge pockets 2C are the outermost pockets formed by the two scroll elements 42 and 52 just before the sealed pockets open to discharge. The volume of the discharge pockets is called discharge volume. FIGURES 5a-5c schematically illustrate the relative movement of scroll elements 62 and 72 of the third and the fourth scroll members 60 and 70, respectively. The suction pockets 3A, formed by the third and the fourth scroll members 60 and 70, are the pair of outermost pockets of the compressor. The pocket undergoing the compression process is shown at 3B in FIGURE 5b. Referring to FIGURE 5c, the discharge volume, i.e., the volume of the innermost pockets of the compressor, is seen at 3C. The relationships of the suction and discharge pockets in the compressor stage of the vacuum pump 10 are opposite to that in the expander stage. According to the present, the volume 3A in the compressor stage must not be significantly smaller than the volume 2C in the expander stage. Preferably, that volume 3A is equal to or greater than 2C. The relationship between the discharge volume of the expander and the suction volume of the compressor is thus important to the performance of the vacuum pump. Air which discharges from the discharge pockets of the first stage, the expander, is sucked in by the suction pockets of the second stage, the compressor. At steady state, the law of mass conservation gives the following relationship: D 2c * V 2c =D 3a 3a ( 1 ), where D 2c and D 3a are the densities of the air in the discharge pockets of the expander stage and in the suction pockets of the compressor stage, respectively, and V 2c is the discharge volume of the expander stage while V 3a is the suction volume of the compressor stage. If the suction volume of the second stage, V 3a , is less than the discharge volume of the first stage, V 2c , i.e., if V 3a < V 2c (2), then D 3a > D 2c (3), and, assuming constant temperature of the air in both volumes, the state equation for an ideal gas leads to the following: P 2c /D 2c = P 3a /D 3a (4). Therefore, Psa > P 2c (5). Since the air pressure in the chambers 84, 85 and 86 is P3a, the air in the discharge pockets of the expander is over-expanded. The air in chambers 84, 85 and 86 will re-expand to the discharge pockets as soon as the discharge pockets of the expander open to the chamber 84. Repetitive re-expansion can overheat both the expander and the compressor. If V3a is not significantly smaller than V2c, the heat generated by the re-expansion of the air may be dissipated to the ambient through the housing and other parts, and overheating might not happen. However, if V 3a > V 2c (6), overheating will never happen. Thus, the invention contemplates a vacuum pump 10 in which operation always produces a suction volume of the second stage which is greater than the discharge volume of the first stage. That is achieved by using the expander-compressor construction hereinbefore described. In another aspect of the invention, optimum shaft sealing is achieved. Referring to FIGURE 1 , the shaft seal 11 is illustrated. The shaft seal 11 comprises a spring seat 12, a spring 13, a rotating ring 14, an "O" ring 15, an orbiting ring 16 and an orbiting "O" ring 17. The orbiting ring 16 seals off the air passage between the front driving pin bearing 27 and the orbiting bearing boss 53. The "O" ring 15 seals off the air passage along the surface of shaft 22. The rotating ring 14 is pushed by spring 13 against orbiting ring 16 to form an air tight contact surface 18. This contact surface 18 seals off any possible air passage along the shaft between inlet chamber 81 and chamber 85. The uniqueness of shaft seal 11 resides in the fact that the relative motion between the rotating ring 14 and orbiting ring 16 is a combination of shaft rotation and the orbiting motion of the orbiting ring 16. A conventional shaft seal 92 is used to seal off chamber 81 from the possible air leakage through the front bearing housing 90 to ambient. Seals 11 and 92, in combination, form the seal mechanism in accord with the present invention. Another aspect of the invention is found in the scroll element tip sealing area. Referring to FIGURES 6a-6f, labyrinth lips 301 , 302, 303, 304 on a tip 300 (only a portion of which is shown) of a scroll element are illustrated. The labyrinth lips are very thin, shallow walls formed on the tips of the scroll elements. They are designed to block the air flow in radial and tangential directions. However, when the labyrinth lips are urged against the base of the other scroll element due to thermal growth of the scroll elements, the labyrinth lips easily bend, or deform or are removed by contact with the base. This avoids tip-base galling. FIGURE 6a shows one form of the labyrinth lips 301. The lips have three longitudinal walls A, B and C, located at both sides and in the middle of the tip 300 of the spiral scroll element. They are connected by diagonal walls D. The lips have a triangular cross section, and the width w and the height h of each (see FIGURE 6b) is small, e.g., 0.5 mm. Other geometric configurations or cross sections of the labyrinth lips are possible, as long as they have weak peaks. Their peaks are easily bent, deformed or removed without galling the base of the mating scroll. A very small axial gap δ, even zero gap, between the tips and bases of the scroll elements is maintained. Thus, excellent radial and tangential sealing is provided. FIGURES 6c and 6d show comb-shaped and square-shaped labyrinth lips 302, 303, respectively. FIGURES 6e and 6f show a combination of the labyrinth lips 304 with a conventional tip seal mechanism. While the above-described embodiments of the invention are preferred, those skilled in this art will recognize modifications of structure, arrangement, composition and the like which do not part from the true scope of the invention. The invention is defined by the appended claims, and all devices and/or methods that come within the meaning of the claims, either literally or by equivalents, are intended to be embraced therein.

Claims: I CLAIM: 1. A scroll-type displacement apparatus, comprising: a) a first scroll member having a first end plate with a base surface from which a first scroll element projects; b) a second scroll member having a second end plate with a base surface from which a second scroll element projects; c) a pair of suction chambers and expansion chambers and a pair of discharge chambers defined between said first and second scroll members when said first and second scroll members are combined in an axially eccentric position shifted in phase by 180°; d) a third scroll member having a third end plate with a base surface from which a third scroll element projects; e) a fourth scroll member having a fourth end plate with a base surface from which a fourth scroll element projects; f) a pair of suction chambers and compression chambers and a pair of discharge chambers defined between said third and fourth scroll members when said third and fourth scroll members are combined in an axially eccentric position shifted in phase by 180°; and g) a shaft for transmitting a drive force to the second and third scroll member, said shaft being supported by at least one bearing; h) wherein the volume of said suction chambers formed by said third and fourth scroll members is not significantly smaller than the volume of said discharge chambers formed by said first and second scroll members. 2. The apparatus of Claim 1 wherein: a) said shaft drives the second and third scroll members such that the first and second scroll members function as a scroll expander and the pair of the third and the fourth scroll members function as a compressor. 3. A scroll-type displacement apparatus, comprising: a) a first scroll member having a first end plate with a base surface from which a first scroll element projects; b) a second scroll member having a second end plate with a base surface from which a second scroll element projects, said second end plate having a bearing hub extending therefrom; c) a shaft for transmitting a drive force to the second scroll member, said shaft being supported by at least one bearing; and d) a seal assembly on said shaft; e) said seal assembly including a seal mechanism having an orbiting piece and a rotating piece, said orbiting piece being stationary in relation to said second scroll member. 4. The apparatus of Claim 3 wherein: a) said orbiting piece includes an O-ring which forms a seal in a radial gap between said second scroll element bearing hub and said first orbiting piece; b) said rotating piece being stationary in relation to said shaft. 5. The apparatus of Claim 4 wherein: a) said rotating piece includes an O-ring which forms a seal in a radial gap between said shaft and said rotating piece; b) said orbiting piece having a surface to surface contact with said rotating piece when said shaft rotates and said second scroll member orbits in relation to said first scroll member; c) said surface to surface contact forming a seal against radial passage of fluid between said contact surfaces. 6. A scroll-type displacement apparatus, comprising: a) a first scroll member having a first end plate with a base surface from which first scroll elements project; and b) a second scroll member having a second end plate with a base surface from which second scroll elements project; c) wherein at the tips of each of said first and second scroll elements are formed labyrinth structures, said labyrinth structures comprising thin, axially extending walls standing up on the said tip of each said scroll element. 7. The apparatus of Claim 6 wherein: a) said labyrinth structures reduce the radial gap between the tips of each scroll element and the corresponding bases of the other scroll element and, when a labyrinth structure contacts the base of a corresponding scroll element during apparatus operation the thin walls of the labyrinth structures deform whereby galling is avoided. 8. The apparatus of Claim 6 wherein: a) at the middle of said tip of each of said first and second scroll elements a groove is formed; and b) a tip seal element made of resilient material of low friction coefficient is closely fit into the groove and yet is free move in said groove. 9. The apparatus of Claim 8 wherein: a) said tip seal element comprises 30% carbon fiber and 70% Teflon.

Inventors: NI SHIMAO, NI, SHIMAO
Applicants: MIND TECH CORP, MIND TECH CORPORATION
Application year: 1999
Main classification: F04C18
IPC codes: F02B53, F04C18, F04C23, F04C25, F04C27, F04C29