ROTARY COMPRESSORS

Abstract: A rotary compressor comprising a rotary power source, a crankshaft (12) driven by the rotary power source, a working volume (V1,V2) a piston (18) formed with a dynamic seal (21) disposed in the working volume (V1), apparatus for sealing the working volume (V1,V2) from the volume (V3) surrounding the crankshaft (12) and apparatus (40) for substantially eliminating the build up across the apparatus for sealing of a differential pressure caused by alternating strokes of the piston (18).

Description: The present invention relates to compressors generally and more particularly to compressors, having a votary power source and reciprocating piston. One of the major factors limiting the operating lifetime of cryocoolers and other devices employing compressors is contamination of the working gas volume by lubricants and other debris. For example conventional Sterling cycle cryocoolers have a measured reliability of 300 hours mean time before failure (MTBF). Four major failure modes have been identified in these cryocoolers: rotary bearing failure, compressor piston seal failure, contamination of the working gas volume by debris and lubricants and helium leakage. Seals do not effect total sealing of the working volume. As a result of this, contaminant particles are transported into the working volume by gas which escapes from the working volume and which then returns to the working volume. As an additional seal, the use of a bellows between the piston and the compressor housing or crankcase has also been proposed, note e.g. FR-A-703152 which discloses a compressor according to the precharacterizing portion of independent claim 1. In FR-A-703152 the outer side of the bellows is exposed to suction or compressor inlet pressure and the inner side of the bellows in communication with the crankcase volume is filled with oil for damping bellows vibration. The bellows is known to provide a nominally absolute seal with gas leaking therepast at less than 0.0000001 CC Helium/sec but the prior art did not solve the problem of differential pressure across the bellows and therefore, in use, the bellows underwent deformations which caused early fatigue failure. In FR-A-2052125 there is disclosed a gas pump having a piston supported by means of a bellows to a cylinder. There is a suction chamber on one side of the bellows and a pumping chamber on the other side thereof. A reciprocable stem controls an opening in the piston through which the gas can flow from the suction chamber into the pumping chamber. The stem actuates the piston and opens a discharge valve. The stem is connected by a sealing bellows to the cylinder. The object of the invention is to provide an improved compressor avoiding premature fatigue failure of the bellows. To achieve this, there is provided, in accordance with the invention, a compressor comprising a rotary power source, a crankshaft driven by the rotary power source, a housing defining a working volume; a piston formed with a dynamic seal and located in said working volume, means for sealing the working volume surrounding said piston from a crankcase volume surrounding said crankshaft, characterized by means for generally eliminating the build up across said means for sealing of a differential pressure caused by alternating strokes of said piston. The working volume includes first and second volumes and the crankcase volume is a third variable volume. The dynamic seal separates the first and second volumes and the bellows separates the second and third volumes. In accordance with a preferred embodiment of the invention, the second and third volumes are selected such that the pressures therein are generally equal. In order that the pressures therein be maintained equal, the second and third volumes always are in a generally constant ratio. Additionally in accordance a preferred embodiment of the invention, in order to accommodate the effects of gas leakage past the dynamic seal and the effects of temperature gradients, the apparatus for eliminating also comprises apparatus defining a gas flow path extending between the second and third volumes, i.e. the working volume surrounding the piston and the volume surrounding the crankshaft. Further in accordance with a preferred feature of the invention, there is also provided apparatus for preventing transport along the flow path of contaminant particles from the volume surrounding the crankshaft to the working volume surrounding the piston. Further in accordance with an embodiment of the invention, there can also be provided a lubricated piston guide. The compressor will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings, in which: Fig. 1 is a schematic partial cross-section of a portion of a compressor, constructed and operative in accordance with an embodiment of the invention; and Fig. 2 is a schematic partial cross-section of a portion of a compressor, constructed and operative in accordance with an alternative embodiment of the invention. Referring to Fig. 1 there is shown a portion of a compressor constructed and operative in accordance with an embodiment of the invention and comprising a crankcase 10 for an eccentrically mounted crankshaft 12. A connecting rod 14 is bearing mounted at one end onto crankshaft 12 and at another end, the connecting rod is mounted onto a pin 16 which is mounted onto a piston 18. Piston 18 is slidably mounted in a housing in the formula sleeve 20 and includes a clearance seal or other dynamic seal 21 to substantially prevent the flow of gas past piston 18. The housing defines a working volume separated by the dynamic seal 21 into first and second variable volumes, shown as V1 and V2 in the drawing. First volume V1 may communicate by means of a channel 26 with a regenerator and heat exchanger (not shown) of a Sterling cryocooler or any other suitable compressor output device. Piston 18 is mounted for movement along an axis 28 towards and away from a crankcase closure member 30, a bellows-type sealing member, referenced 32, being provided between piston and closure member 30. Defined by crankcase 10 and respective inner surfaces 34, 36 and 38 of closure member 30, bellows 32 and piston 18 is a third variable or crankcase volume V3, while the second volume V2 is defined between volumes V1 and V3 around bellows 32. The second and third volumes are selected such that the pressures therein are generally equal. In order that the pressures therein be maintained equal, the second and third volumes always are in a generally constant ratio. This is achieved in the illustrated embodiment by causing the ratio between the cross sectional area of piston 18 (AP) and the effective cross sectional area of bellows 32 (AB) to satisfy the following relationship with the second and third volumes, assuming equal initial pressures in volumes V2 and V3: V2/V3 + 1 = AP/AB It has been found that during a start-up period, that is, between the time that the compressor is activated and the time that steady state conditions prevail, there exists a pressure differential across bellows 32. Such a pressure differential may also occur due to leakage past the dynamic seal 21. Although the pressure differential may be in the order of only 49 kPa (0.5 atm), it is, nonetheless, desirable to eliminate it. There is provided an assembly, referenced generally 40, for permetting communication between volumes V2 and V3 so as to substantially eliminate the pressure differential across the bellows, thus preventing premature failure thereof. Assembly 40, defining a gas flow path, extends between volumes V2 and V3, the assembly comprising a first conduit 42, a gas filter 44 and a second conduit 46. It will be appreciated that any excess pressure tending to build up in either volume V2 or V3 is dissipated by means of assembly 40. Furthermore, as it has been found that the dynamic seal 21 does not constitute a perfect seal, gas filter 44 is provided to ensure that any contaminant particles that might otherwise have flowed from volume V3 into volume V2 and from there into volume V1, are prevented from doing so. There is provided a valve 48 for opening and closing the flow path defined by assembly 40. According to a preferred embodiment of the invention, valve 48 is opened only during start-up periods. There is also provided, therefore, a pressure responsive control unit 50 for governing the opening and closing of valve 48. Control unit 50 may comprise a conventional pressure responsive valve. It will be appreciated that any other suitable apparatus may be provided as an alternative to control unit 50. To dissipate excess pressures that would otherwise build up within volume V2, a third conduit 52 is provided, and includes one-way valve apparatus 54 so as to prevent undesired flow of gas from volume V3 into volume V2. With reference now to Fig. 2, there is show a partial cross-section of a portion of a compressor which is generally similar to the compressor shown in Fig. 1, common components being indicated by similar reference numerals In the embodiment of the invention shown in Fig. 2, connecting rod 14 is mounted, by means of pin 16, onto a first portion 60 of piston 62, a second portion thereof, referenced 64, including dynamic seal 21. Bellows 32 are mounted between piston 62 and sleeve 20, by means of first and second mounting elements, referenced 66 and 68. There is also provided a lubricated guide 70 for piston 62, typical lubricants being oil, grease or any other conventional lubricating material.

Claims: 1. Compressor comprising: a rotary power source, a crankshaft (12) driven by said rotary power source, a housing (20) defining a working volume; a piston (18; 62) formed with a dynamic seal (21) and located in said working volume, means (32) for sealing the working volume surrounding said piston (18; 62) from a crankcase volume (V₃) surrounding said crankshaft (12), characterized by: means (40) for generally eliminating the build up across said means (32) for sealing of a differential pressure caused by alternating strokes of said piston (18; 62). 2. Compressor according to claim 1, characterized in that said means for sealing comprises bellows (18; 62). 3. Compressor according to claim 2, characterized in that said working volume includes a first and a second variable volume (V₁, V₂), said first volume (V₁) communicating with a compressor output device, and said second volume (V₂) surrounding said bellows (18; 62), and that said crankcase volume (V₃) surrounding said crankshaft (12) is a third variable volume (V₃). 4. Compressor according to claim 3, characterized in that said dynamic seal (21) separates the first and second volumes (V₁, V₂) and said bellows (18; 62) separates the second and third volumes (V₂, V₃). 5. Compressor according to claim 3, characterized in that the pressures in the second and third volumes (V₂, V₃) are maintained generally equal. 6. Compressor according to claim 5, characterized in that said second and third volumes (V₂, V₃) are maintained in a generally constant ratio. 7. Compressor according to claim 6, characterized in that the ratio between the cross sectional area of said piston (AP) and the effective cross sectional area of said bellows (AB) to the ratio of the second and third volumes (V₂, V₃) satisfy the following relationship with the second and third volumes (V₂, V₃), assuming equal initial pressures at V2 and V3: V₂/V₃ + 1 = AP/AB 8. Compressor according to any one of claims 1 to 7, characterized in that said means for generally eliminating comprises means defining a gas flow path extending between the working volume surrounding said piston (18; 62) and the crankcase volume (V₃) surrounding said crankshaft (12). 9. Compressor according to claim 8, characterized by including selectable means for permitting gas flow along said flow path. 10. Compressor according to claim 8, characterized by comprising means for preventing transport along said gas flow path of contaminant particles from the crankcase volume (V₃) surrounding said crankshaft to the working volume surrounding said piston (18; 62). 11. Compressor according to claim 10, characterized in that said means for preventing comprises gas filtering means (44). 12. Compressor according to claim 1, characterized by including means (52, 54) for permitting one-directional gas flow from the working volume surrounding said piston (18) to the crankcase volume (V₃) surrounding said crankshaft (12). 13. Compressor according to claim 1, characterized by including a lubricated piston guide (70).

Inventors: KUSHNIR, MARK, NOVOTNY, SHLOMO, YARON, RAN
Applicants: DIGITAL EQUIPMENT CORPORATION
Application year: 1989
Main classification: F04B39
IPC codes: F04B39, F04B9, F04B53