Balance piston and seal arrangement

Abstract: A labyrinth seal (62) is provided to isolate the transmission chamber (30) from a balance piston (59) of a centrifugal com­ pressor, the labyrinth seal (62) being pressurized with re­ frigerant vapor from the motor chamber, which vapor is at a pressure slightly above that in the transmission, to thereby minimize the efficiency losses that would otherwise occur from leakage of the vapor into the transmission and to the compressor suction (60).

Description: This invention relates to a centrifugal compressor and to a method of pressurizing a seal. This invention relates more particularly to a method and apparatus for providing a seal between an oil-fed transmission chamber and the relatively low pressure area in a balance piston adjacent the impeller. In order to counteract the aerodynamic thrust that is developed by the impeller of a centrifugal compressor, it is well known to employ a balance piston consisting of a low pressure cavity behind the impeller wheel. Because of the tendency for lubricating oil to leak from the transmission into this low pressure area, it is also common practice to install a seal device between the balance piston and the transmission. A mechanical seal, such as a carbon face seal, is typically used for this purpose. However, besides being very intricate, delicate and expensive, these mechanical seals introduce substantial mechanical losses due to viscous drag from relative motion between mating surfaces. An alternative is a labyrinth seal which is simple, rugged, inexpensive and, since it is noncontacting, there is virtually no mechanical losses due to rubbing. The disadvantage, however, is that in order to be entirely effective, it is necessary to pressurize the labyrinth seal. One known way to do so in a centrifugal compressor is to fluidly connect a source of high pressure gas from the discharge line to the center of the labyrinth. In this way, oil leakage from the transmission is substantially eliminated. A disadvantage of such a pressurized labyrinth seal as recognized by the Applicants is that the high pressure gas at the labyrinth will tend to flow into the balance piston and the transmission chamber, and if the flow becomes excessive, the overall efficiency of the compressor will suffer. Further, the flow into the balance piston will tend to degrade its performance. In particular, with regard to efficiency losses in higher pressure systems, such as a centrifugal compressor designed for an operation with a high density refrigerant such as R-22, the pressure differential between the compressor discharge line and the transmission, and even more so, the pressure differential between the discharge line and the balance piston, can be sufficiently high that there will be a substantial flow of refrigerant gas to the balance piston and into the transmission. The transmission is vented by means of a pipe back to compressor suction. Also, the balance piston is ported to compressor suction. Thus, any high pressure gas leaking in either direction ends up being re-compressed and is therefore cause for a loss in efficiency. In US-A-3 650 634 there is described a seal arrangement for a centrifugal compressor according to the preamble of claim 1. Specifically, US-A-3 650 634 discloses a seal arrangement of the type having a balance piston cavity to counteract the thrust load on the impeller, a labyrinth seal interposed between the cavity and a transmission chamber, a source of pressurized fluid to pressurize the cavity, and a source of pressurized gas which is maintained at a pressure that is slightly above the pressure in the transmission chamber. A method of pressurizing a labyrinth seal according to the preamble of claim 5 is also known from US-A-3 650 634. It is an object of the present invention to provide an improved labyrinth seal arrangement for maintaining an effective and efficient seal between the transmission and a low pressure cavity of a balance piston structure in a centrifugal compressor. To achieve this, the seal arrangement of the invention is characterized by the features claimed in the characterizing part of claim 1 and the method of the invention is characterized by the features of claim 5. Specifically, according to the invention, a conduit fluidly interconnects the source of pressurized gas to the intermediate portion of the labyrinth seal. The gas is delivered to the intermediate portion to pressurize the seal at a pressure greater than the pressure in the balance piston cavity. Advantageous embodiments of the invention are claimed in the subclaims. Among the advantages of the invention may be included that it reduces the leakage of high pressure labyrinth seal gas to the balance piston cavity. Yet another advantage of the present invention is the provision in a centrifugal compressor for a labyrinth seal arrangement which is economical to manufacture and reliable and effective in use. These objects and other features and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings. Briefly, in accordance with one aspect of the invention, the labyrinth seal, between the transmission and balance piston of the centrifugal compressor, is pressurized by a source of pressurized gas which is maintained at a pressure slightly above the pressure in the transmission. This slight pressure differential is sufficient to prevent oil from migrating out of the transmission and yet is not so great as to cause excessive amounts of gas to flow into the balance piston and transmission. By another aspect of the invention, the gas which is supplied to pressurize the labyrinth is taken from a motor chamber which is vented to the cooler. Refrigerant gas, generated in the motor chamber during the motor cooling process, is allowed to flow to the cooler in a manner controlled by a back-pressure valve. This valve acts to maintain a fixed pressure differential between the motor shell and the cooler and to thus provide a source of pressurized gas to the labyrinth seal at a pressure that is slightly above that in the transmission and not significantly higher than that in the balance piston. In the drawings as hereinafter described, a preferred embodiment is depicted; however, various other modifications and alternate constructions can be made thereto without departing from the true spirit and scope of the invention. Figure 1 is a longitudinal cross-sectional view of a centrifugal compressor having the balance piston and seal arrangement of the present invention incorporated therein. Figure 2 is an enlarged view of a portion thereof showing details of the labyrinth seal portion of the invention. Referring now to Figure 1, the invention is shown generally at 10 as embodied in a centrifugal compressor system 11 having an electric motor 12 at its one end and a centrifugal compressor 13 at its. other end, with the two being interconnected by a transmission 14. The motor 12 includes an outer casing 16 with a stator coil 17 disposed around its inner circumference. The rotor 18 is then rotatably disposed within the stator winding 17 by way of a rotor shaft 19 which is overhung from, and supported by, the transmission 14. The transmission 14 includes a transmission case 21 having a radially extending annular flange 22 which is secured between the motor casing 16 and the compressor casing 23 by a plurality of bolts 24, with the transmission case 21 and the compressor casing partially defining a transmission chamber 30. Rotatably mounted within the transmission case 21, by way of a pair of axially spaced bearings 26 and 27 is a transmission shaft 28 which is preferably integrally formed as an extension of the motor shaft 19. The collar 29, which is an integral part of the shaft or attached by shrink fitting, is provided to transmit the thrust forces from the shaft 28 to the thrust bearing portion of the bearing 26. The end of shaft 28 extends beyond the transmission case 21 where a drive gear 31 is attached thereto by way of a retaining plate 32 and a bolt 33. The drive gear 31 engages a driven gear 34 which in turn drives a high speed shaft 36 for directly driving the compressor impeller 37. The high speed shaft 36 is supported by journal bearings 39 and 40. In order to reduce windage losses in the transmission 14 and to prevent oil losses from the transmission chamber 30, the transmission chamber 30 is vented to the lowest pressure in the system (i.e., compressor suction pressure) by way of passage 55, tube 65, and compressor suction pipe 75. As will be explained hereinafter, this flow path can be a cause of lost efficiency unless provision is made in accordance with the present invention. In order to cool the motor 12, liquid refrigerant is introduced from the condenser (not shown) into one end 41 of the motor 12 by way of an injection port 42. Liquid refrigerant, which is represented by the numeral 43, enters the motor chamber 45 and boils to cool the motor 12, with the refrigerant gas then returning to the cooler by way of a conduit 44. A back pressure valve 46 is included in the conduit 44 in order to maintain a predetermined pressure differential (i.e., about 0.34-0.41 bar (about 5-6 psi)) between the motor chamber 45 and the cooler, which typically operates at about 5.5 bar (80 psia). Compressor suction pipe 75, at the point where transmission vent tube 65 is connected, is typically at a pressure 0.07-0.14 bar(1-2psi) less than the cooler. This establishes a transmission pressure of about 5.37-5.44 bar (78-79 psia). Thus, the pressure in the motor chamber is maintained at 5.86-5.93 bar (85-86 psia), which is about 0.41-0.55 bar (6-8 psia) or 7.6-10.3%above that in the transmission chamber 30. Also, fluidly communicating with the motor chamber 45 is an opening 47 in the annular flange 22 of the transmission case 21. A line 48 is attached at its one end to the opening 47 by way of a standard coupling member 49. At the other end of the line 48 is a coupling member 51 which fluidly connects the line 48 to a passage 52 formed in flange member 53 as shown in figure 1 and as can be better seen in figure 2. The bearing 40 functions as both a journal bearing to maintain the radial position of the shaft 36 and as a thrust bearing to maintain the axial position thereof. An oil feed passage 54 is provided as a conduit for oil flowing radially inwardly to the bearing surfaces, and an oil slinger 50 is provided to sling the oil radially outward from the shaft 36. An annular cavity 56 then functions to receive the oil which is slung off from the bearing 40 and to facilitate the drainage of oil through a passage 57 and back to the sump 58. It is this path which, together with the flowpath mentioned above, can be a cause of loss in efficiency unless corrective provisions are made as will be described hereinafter. In order to provide a counteraction to the aerodynamic thrust that is developed by the impeller 37, a "balance piston" is provided by way of a low pressure cavity 59 behind the impeller wheel 37. A passage 61 is provided in the impeller 37 in order to maintain the pressure in the cavity 59 at the same low pressure as the compressor suction indicated generally by the numeral 60. This pressure (downstream of the guide vanes 70) typically varies from around 5.3 bar (77psia) at full load, down to 2.75 bar (40 psia) at 10% load. Since the pressure in the transmission casing is higher (i.e., equal to the compressor suction pressure upstream of the inlet guide vanes 70, or about 5.37-5.44 bar (78-79 psia) than that in the cavity 59, and especially at part load operation, a labyrinth seal 62 with its associated teeth 63 is provided between the bearing 40 and the impeller 37 to seal that area against the flow of oil from the transmission into the balance piston 59. This concept is well known as is the further concept of pressurizing the labyrinth seal by exerting a high pressure gas thereon. If, as is customary, high pressure gas from the discharge line is used to pressurize the labyrinth seal 62, then the substantial pressure differential will cause the high pressure vapor, (i.e. around 13.8 bar(200 psia)) to flow from the labyrinth seal 62 to the low pressure sections of the system to thereby reduce the efficiency thereof. This flow can occur in two directions as indicated by the arrows in figures 1 and 2. It can flow along passage 61 to the compression suction 60 or it can flow along passage 57 to the sump 58, from where it can flow as indicated by the arrows in figure 1, through the vent opening 55, the tube 65, the suction pipe 75 and finally to the compressor suction 60. In order to prevent these losses, the labyrinth seal 62 has instead, been pressurized with the refrigerant vapor in the motor chamber 45, which vapor passes through the line 48, the passage 52, and a passage 66 in the labyrinth seal 62. Thus, the labyrinth seal 62 is pressurized at the motor casing pressure of 5.86-5.93(85-86psia),which is 0.41-0.55bar(6-8psi)above the transmission pressure. With this pressure differential being so minimized, the losses that would result from the labyrinth pressurization gas leaking back into the transmission and eventually into the compressor suction 60 is therefore also minimized. Similarly, with the pressure differential between the labyrinth seal 62 and the compressor suction 60 being minimized, the losses that result from the leakage of labyrinth pressurization gas leaking directly into the compressor suction 60 by way of the passage 61 is also minimized. It will therefore be seen that the present invention not only provides the advantages of using a labyrinth seal for isolating the transmission chamber 30 from a balance piston in a centrifugal compressor, but also provides a novel and practical means of pressurizing the labyrinth seal in a manner which optimizes the efficiency of the system.

Claims: A centrifugal compressor (13) of the type having a balance piston cavity (59) to counteract the thrust load on the impeller (37), a labyrinth seal (62) interposed between the balance piston cavity (59) and a transmission chamber (30), a source of pressurized fluid (60) to pressurize the balance piston cavity (59), and a source of pressurized gas (45) which is maintained at a pressure that it slightly above the pressure in the transmission chamber (30), characterized by a conduit (48) which fluidly interconnects said source of pressurized gas (45) to an intermediate portion (66, ) of the labyrinth seal (62), said gas being delivered to the internal portion (66, ) for pressurizing the labyrinth seal (62) at a pressure that is greater than the pressure in the balance piston cavity (59). A centrifugal compressor as set forth in claim 1, characterized in that said compressor (13) is driven by an electric motor (12) which is cooled by refrigerant injected into its chamber (45) and wherein said source of pressurized gas is the motor chamber (45). A centrifugal compressor as set forth in claim 1, characterized in that said source of pressurized gas (45) is maintained at a pressure in the range of 7.6-10.3% greater than that in the transmission chamber (30). A centrifugal compressor as set forth in claim 1, characterized in that the source of gas is a source of vapor (45) fluidly interconnected to the labyrinth seal (62) for pressurizing said seal (62) and preventing the flow of oil from the transmission chamber (30) to the balance piston cavity (59). A method of pressurizing a labyrinth seal (62) of the type disposed between an oil containing transmission chamber (30) and a balance piston cavity (59) of a centrifugal compressor (13), comprising the steps of: establishing a source of pressurized fluid (60) for pressurizing the balance piston cavity (59), and establishing a source of vapor (45) at a pressure slightly above the pressure in the transmission chamber (30), characterized by the steps of: fluidly interconnecting said source of vapor (45) to an intermediate portion (66) of said labyrinth seal for pressurizing the seal (62) at a pressure greater than that in the balance piston cavity (59) so as to prevent the flow of oil from the transmission chamber (30) to the balance piston cavity (59). The method as set forth in claim 5, characterized in that said source (45) of vapor is at a pressure in the range of 7.6-10.3% greater than that in the transmission chamber (30) The method as set forth in claim 5, characterized by including the step of pressurizing a motor chamber (45) and using said motor chamber as said source (45) of vapor.

Inventors: SISHTLA VISHNU MOHAN, ZINSMEYER THOMAS MICHAEL, SISHTLA, VISHNU MOHAN, ZINSMEYER, THOMAS MICHAEL
Applicants: CARRIER CORP, CARRIER CORPORATION
Application year: 1990
Main classification: F04D29
IPC codes: F04D29, F16J15, F04D15