Multistage centrifugal compressor

Abstract: By changing a vane height to produce different vanes from master vanes, different impellers are produced, and therefore there can be obtained an inexpensive and high-reliability multistage centrifugal compressor and an impellers therefor. An impeller (2) for multistage centrifugal compressor, which is to be mounted on a rotary shaft (1), comprises a plurality of vanes (2c) disposed between a disk (2a) and a shroud (2b) and separated from one another equiangularly. By changing the vanes (2c) of the impeller (2) only in height without changing the configuration, vanes (3c) can be obtained, and then a different impeller (3) comprising the disk (2a), a shroud (3b) and vanes (3c) is also obtained. Since a whole of the vane (3c) has a configuration identical to a part of the vane (2c), these vane (2c, 3c) can be produced by a single pair of the pressing dies.

Description: The present invention relates to a multistage centrifugal compressor according to the pre-characterizing portion of claim 1 and to a multistage centrifugal compressor system according to the pre-characterizing portion of claim 4. The impeller used in a multistage centrifugal compressor comprises two rotary discs (a disk and a shroud), and a plurality of vanes disposed between the disk and the shroud and substantially equidistantly in a circumferential direction to define passages by means of the disk and the shroud and the vanes. The disk, the shroud and the vanes are so designed and manufactured as to provide a proper velocity distribution of a working gas for every stage. Further, the suction temperature and the suction pressure of the impeller vary for each stage because the fluid to be employed is a gas with compressibility. As a result, the density of the gas varies in the stages, and the desired width of the gas passage of the impeller, that is, the vane height, is made narrower as going downstream(a suction port, a first stage, a second stage,...), and therefore the impellers of the respective stages need to be different in configuration from each other. Consequently, the multistage centrifugal compressor is heretofore produced in such a manner that a suitable configuration of the impeller would be decided for every stage and the disk and the shroud would be manufactured separately by machining in accordance with the decided configuration of the disk and the shroud of the impeller of each stage. The vanes would be shaped into the required configuration by press working and integrated into the disk and the shroud by means of welding or the like. FR-A-2 419415 discloses a multistage centrifugal compressor comprising a plurality of impellers mounted on a rotary shaft, each of said impellers including a disk, a shroud and a plurality of vanes disposed between said disk and said shroud and separated from one another in a circumferential direction, and a casing for housing said plurality of impellers, said casing being formed with a suction port and a discharge port, through said suction port a gas is drawn into said compressor, and the gas drawn is compressed in sequence by rotation of said impellers of each stage and discharged from said discharge port: The impellers of the multistage centrifugal compressor according to FR-A-2 419 415 are divided into three groups, each group comprising two identical impellers. However, the shape of the impellers of different groups is very different compared with one another. In particular the different impellers have very different characteristic angles. FR-A-2 419 416 discloses a multistage centrifugal compressor system in which the impellers of the single centrifugal compressors have different shapes like the impellers as disclosed in FR-A-2 419 415. Meanwhile, there has been employed another method in which a simple shape of a vane, such as a two-dimensional vane, is substituted for the optimum shape of the vane for every stage, and then such simple shaped vane is made by casting or the like. Moreover, there has been known still another method in which a multispindle NC machine tool is used to make the vane of a complicated shape for a half-shrouded impeller with no shroud. US patent no. 4,775,270 discloses an impeller of a centrifugal fluid-type rotary machine including impeller blades each having a concave and convex surface of which the shape is formed of parallel generatrices inclined with respect to a rotary shaft by an angle defined for each impeller blade. A method of manufacturing the impeller includes inclining a main plate material of the impeller with respect to a plane of a table of a machine tool by a desired angle, fixedly mounting the plate on the table after placing the plate on a rotation indexable jig, controlling three axial positions of the plate in the right and left, front and rear and upper and lower direction by a three-axis controller and cutting out the main plate or side plate by a cutting tool to integrally form the impeller blades on the plate. US patent no. 4,322,200 discloses a heavy duty centrifugal pump impeller wherein the relatively heavy gauge shrouds are formed by plastic deformation such as by metal spinning, stretch forming or hydroforming. The hub is similarly formed and a plurality of regularly circumferentially spaced spiral vanes extend axially therebetween so that when the hub, blades and shroud are joined by welding or brazing, there is formed a heavy duty impeller having an axially extending inlet and a radially extending annular outlet. Following the fabrication, the impeller is finished and metallized. In the various methods described above, it is necessary that the impellers of the respective stages are so designed and manufactured as to be different in configuration from each other. These methods for producing an impeller for a centrifugal compressor are disclosed in Japanese Patent Unexamined Publication Nos. 2-161200 and 3-151597. In the former, a shroud is made axially movable for the purpose of suppressing occurrence of surging to obtain a high-efficiency impeller. However, there is given no consideration for the reduction of the number of manufacturing steps in producing the impeller, such as employment of a process of the impeller common to a plurality of stages. On the other hand, in the latter, the width of the passage at an outlet of the impeller is adjustable for the purpose of regulating the flow rate to enhance the efficiency. However, there is also given no consideration for the reduction of the number of manufacturing steps, such as employment of a process and a design common to a plurality of impellers. According to both of these conventional technologies, the impeller has been designed and manufactured for every stage, and no consideration has been given for the reduction of the number of required processing steps in order to manufacture an impeller for centrifugal compressor at low cost. It is an object of the invention to provide an inexpensive multistage centrifugal compressor and an inexpensive multistage centrifugal compressor system without the above-described problems of the prior art. This object is achieved by a multistage centrifugal compressor according to claim 1 and a multistage centrifugal compressor system according to claim 4. According to the present invention, the vanes of the impellers for some stages of the multistage centrifugal compressor can be made from a common master vane blank, and therefore the production cost, the number of processing steps and the number of rejects of the product can be reduced, thereby improving the reliability. Further, the use of a common NC program becomes possible, and therefore an inexpensive and high-reliability multistage centrifugal compressor can be obtained. Moreover, it is possible to obtain a multistage centrifugal compressor in which the reduction in performance or efficiency can be neglected even if the vanes are made from common master vane blank. In addition, the production process is simplified, and therefore it is possible to obtain a high-reliability multistage centrifugal compressor which is less liable to cause manufacturing error as well as defective manufacturing. The vanes constituting the different impellers of the multistage centrifugal compressor according to the present invention can be formed by partially cutting off and press forming the vane blanks. Namely, only a single kind of vane blank (master vane blank) is needed for the different vanes. More specifically, the vanes comprise three-dimensional complicatedly-undulating surfaces. The vane which is formed by press forming a whole master vane blank is used in an impeller for a first stage. The vane which is formed by press forming a master vane blank partially cut off (or almost whole of the master vane blank) is used in an impeller for a second stage. The vane which is formed by press forming a master vane blank more partially cut off (or a substantial part of the master vane blank) is used in an impeller for a third stage. Namely, in an impeller for a later stage, a less part of the master vane blank is used to form a vane. Accordingly, only by putting between two press dies the vane blank which is defined by partially cutting the master vane blank off and has an area required for the vane of the impeller of the stage, the vanes of every stage can be easily produced. Meanwhile, in case of production by an NC machine, it will do to change only the coordinates of the shroud wall surface and the disk wall surface. The coordinates between the shroud and the disk are common to every stage, and therefore the program can be used in common. Further, it is not always necessary that the master vane blank is used for all the stages. The master vane blank may be changed every two stages, for example a first master vane blank for the first and the second stages and a second master vane blank for the third and the fourth stages. Also the master vane blank may be used merely for the first and the second stages and the different vane blanks may be used for the respective stages other than the first and the second stages. Preferred embodiments of the multistage centrifugal compressor according to claim 1 are subject matter of claims 2 and 3. Preferred embodiments of the multistage centrifugal compressor system according to claim 4 are subject matter of claims 5 and 6. Embodiments of the present invention will now be described with respect to the accompanying drawings in which: Fig. 1 is afragmentary sectional view of an impeller for a multistage centrifugal compressor according to an embodiment of the present invention; Fig. 2 is a perspective view of Fig. 1 with a shroud omitted; Fig. 3 is a graph for explaining the flow condition when the vane height is changed; Fig. 4 is a fragmentary sectional view of an impeller for multistage centrifugal compressor according to another embodiment of the invention; Fig. 5 is a longitudinal sectional view of a multistage centrifugal compressor according to an embodiment of the invention; Fig. 6 is a perspective view of vanes which are to be used in the impeller for multistage centrifugal compressor of the present invention; Fig. 7 is a perspective view of vanes to be used in another impeller for multistage centrifugal compressor of the invention; and Fig. 8 is a sectional view of a multistage centrifugal compressor according to another embodiment of the invention. As shown in Fig. 1, an impeller 2 fixedly mounted onto a rotary shaft 1 has a disk 2a and a shroud 2b. Between the disk and the shroud a plurality of vanes 2c are arranged at substantially equal pitches in the circumferential direction, each of the vanes having a three-dimensional configuration as shown in Fig. 2. In the centrifugal compressor, a working gas is drawn in through a suction port 30, and compressed with the rotation of the rotary shaft 1 while passing through the passage defined by the disk 2a, the shroud 2b and the vanes 2c, and then discharged from a discharge port 31 at the upper part of Fig. 1. In manufacturing the impeller having the above construction, if the vane is made by means of press working, a pair of pressing dies, which are different in configuration from the desired vane by an amount corresponding to the plastic deformation thereof, are made by casting or machining. A disk plate, which is preshaped into a meridional surface shape, is put into and pressed by such pressing dies to form a desired meridional surface shape. Since the flow rate of gas varies for every stages due to its compressibility, it is impossible to simply use the similarity principle in designing each stage unlike a hydraulic machine. It has therefore been necessary for each stage to design a detailed configuration of the impeller after obtaining the principal specification data of the impeller based on the gas suction and discharge conditions of the impeller, rotational speed and so on. For this reason, many pairs of pressing dies corresponding to the number of stages is needed to be prepared for producing the multistage centrifugal compressor. In the present invention, the vanes 2c and 3c of the impellers of different two stages of the multistage centrifugal compressor are different in the shape of the meridional surface from each other, but they are partially identical in the configuration to each other as shown in Fig. 1. For example, if the first-stage impeller of the multistage centrifugal compressor has a longitudinal sectional form as shown by solid line in Fig. 1, the configuration of the second-stage impeller, disposed downstream of the first-stage impeller, has a longitudinal sectional form as shown by chain line in Fig. 1. It is noted that a vane 3c of the second-stage impeller is partially identical with the vane 2c of the first-stage impeller, namely the vane 3c is perfectly identical with a part of the vane 2c, while a shroud 3b of the second-stage impeller is different from the shroud 2b of the first-stage impeller. Incidentally, a disk 3a of the second-stage impeller is identical with the disk 2a of the first-stage impeller. In consequence, the vane 2c and the vane 3c can be made of the vane blanks each of which is to be finished in the meridional surface of the corresponding vane by means of a single pair of pressing dies. As apparent from the above, the common pair of pressing dies can be used, and therefore the production cost and the number of manufacturing steps of the vane can be reduced. In this embodiment, the common pair of pressing dies is used for the vanes for the first-stage and second-stage impellers, but this invention is not exclusively for this combination. A common pair of pressing dies may be used for all of vanes for every stages, or may be used for vanes of every two adjacent stages, without departing from the scope or spirit of the invention. The flow states at the inlet ports of the impellers, the vane of each of which impellers is different from each other as shown in Fig. 1, will be described hereinafter by referring to Fig. 3. Fig. 3 shows the relationship between the suction flow rate (volume flow rate) of the impeller and the local relative velocity at the impeller inlet port. The local relative velocity means the difference in vector between the absolute velocity of the gas flowing into the impeller and the rotational speed. It is assumed that the inlet local relative velocity of the impeller 2, comprising the disk 2a, the shroud 2b and the vanes 2c, is Wh when the suction flow rate of the impeller 2 is Q s . Meanwhile, concerning the impeller 3 made by modifying the impeller 2 only in the vane height, namely comprising the disk 3a, the shroud 3b and the vanes 3c, the flow rate is changed from Q s to Q so and the inlet local relative velocity is reduced from W h to W ho . In the impeller with the vanes of reduced height, as compared with the original impeller, the inlet local relative velocity is decreased and as a result the flow loss is also decreased, which is proportional to the n-th (n > 1 ) power of the gas flow velocity. This means that if the impeller 2 comprising the disk 2a, the shroud 2b and the vanes 2c is manufactured in an optimum design, and an impeller of downstream stage, in which the flow rate is smaller than that of the impeller 2, is designed and produced so as to become the impeller in which the vane height is reduced by an amount correspondingly to a decrement in the flow rate, there can be obtained the downstream impeller free from a drastic reduction in efficiency. In other words, the difference of the vane of downstream stage from the optimum vane can be minimized. As is clear from the above, it is more advisable that the vane, whose meridional surface is largest among the vanes made by the common pair of pressing dies, is optimally designed, as compared with that the vane, whose meridional surface is smaller, is optimally designed. Another embodiment of the invention is shown in Fig. 4. This embodiment differs from the embodiment of Fig. 1 in the point that the height of the vane of the impellers, made by the common pair of pressing dies, is changed on the disk side. More specifically, the shroud 2b of the impeller 2 is identical with the shroud 3b of the impeller 3. However, though the vane 2c of the impeller 2 is different at the disk side from the vane 3c of the impeller 3, the vane 2c is identical at the shroud side with the vane 3c. In other words, a whole vane 2c is identical with a part of the vane 3c. Consequently the disk 2a of the impeller 2 is different from the disk 3a of the impeller 3. With such construction, in case of mounting a plurality of impellers 22a - 22g onto a one rotary shaft 1 as a single spindle multistage centrifugal compressor shown in Fig. 5, the diameter of the rotary shaft can be made larger and then the rigidity of the rotating system can be enhanced. In Fig. 5, a static passage is defined by a suction end cover 6a, a discharge end cover 6b, a casing 4, an inner casing 5, and a diffuser/diaphragm 7. In the single spindle multistage compressor a working gas is drawn in through a suction port formed in a portion of the casing located upstream of the suction end cover 6a. The pressure of the gas is increased in sequence as the gas passes through the passages formed between the impellers 22a - 22g of every stage and the diffuser/diaphragm 7, so that the flow rate of the gas at the inlet of the impeller of every stage is reduced in sequence. Accordingly, by using the above-described impeller for the impeller of every stage, there can be obtained a high-reliability multistage centrifugal compressor at low cost. In still another embodiment of the invention, as shown in Fig. 6, the vane 2c has a three-dimensional shape made up of linear segments 21a, 21b, ..., 21n connecting end points 18a, 18b, ..., 18n on the shroud side to end points 19a, 19b, ..., 19n on the disk side, respectively. This kind of vane can be easily manufactured by controlling an end milling machine so as to move an axis thereof along each linear element. If the end milling machine is once programmed to manufacture the vane 2c made up of the linear segments 21a, 21b, ..., 21 under the above-mentioned control based on a stored program, such program can be also applied for manufacturing the vane 3c whose height is changed or reduced and which has a three-dimensional shape made up of linear segments connecting end points 20a, 20b, ..., 20n on the shroud side to end points 19a, 19b, ..., 19n on the disk side, respectively. Therefore, the manufacture of the different vanes can be facilitated and the number of processing steps can be reduced. In the above embodiment, the vane is represented by a plurality of the linear segments. In the embodiment shown in Fig. 7, the vane is represented by a group of points. When a vane A is represented by a group P 1 of points {P 1 (i,j) : i= 1, ..., ℓ ; j = 1, ..., m}, and a vane a whose height is smaller than that of the vane A is represented by a group P 2 of points {P 2 (i, j) : i=1, ..., ℓ ; j= 1, ..., k (k<m) }, the common portion or overlapped portion {P (i, j) : i=1, ..., ℓ ; j= 1, ..., k} of the two vanes A and a may be obtained by the same process (program). In connection with the vane A , only remainder portion {P (i, j) : i=1, ..., ℓ ; j= k, ..., m} of the vane A is obtained by another process (program). Therefore, it is possible to reduce the number of processing steps as well as the cost due to the reduced steps of the total program. For the above programmed process, a multiple spindle NC milling machine is most convenient, but the present invention is not limited to such machine but various kinds of numerically-controllable machine tools are also applicable. Further, the above description has made reference only to the manufacture of the vane, and however it goes without saying that the same technical skill can be used in the case where the vanes and the shroud, or the vanes and the disk are machined as one body. In such case, by welding the disk to a machined product in which the vanes and the shroud are integrated with each other, or the shroud to a machined productin which the vanes and the disk are integrated with each other, a desired impeller can be produced at a low price. In a further embodiment shown in Fig. 8, impellers 8 and 9 mounted on opposite end portions of a rotary shaft 1 are housed within casings 4a and 4b, respectively. The casing 4a is formed with a suction port 12a and a discharge port 13a, while the casing 4b is formed with a suction port 12b and a discharge port 13b. The discharge port 13a is connected to the suction port 12b through an intercooler 14. Even in a multistage centrifugal compressor of the type that single-stage centrifugal compressors are connected together by means of the piping as shown in Fig. 8, it is also possible to reduce the number of processing steps but increase the reliability by employing the above-described various kinds of impellers. It is noted that the use of the intercooler enables the multistage centrifugal compressor to be further enhanced in efficiency. Although the number of stages is two in this embodiment shown in Fig. 8, the invention is not limited to this but can be applied to three stages, four stages, five stages or more. In these cases, the intercooler does not need to be used between every pair of adjacent stages but may be equipped as the occasion demands. In any of the above-described embodiments, all impellers have the same outer diameter, but it is of course possible that the outer diameter of the impeller of downstream stage is made smaller.

Claims: A multistage centrifugal compressor comprising: a plurality of impellers (2, 3, 22a to 22g) mounted on a rotary shaft (1), each of said impellers (2, 3, 22a to 22g) including a disk (2a, 3a), a shroud (2b, 3b) and a plurality of vanes (2c, 3c) having a three-dimensional shape and disposed between said disk (2a, 3a) and said shroud (2b, 3b) and separated from one another in a circumferential direction, and a casing (4, 5) for housing said plurality of impellers (2, 3, 22a to 22g), said casing (4, 5) being formed with a suction port (30) and a discharge port (31), through said suction port (30) a gas is drawn into said compressor, and the gas drawn is compressed in sequence by rotation of said impellers (2, 3, 22a to 22g) of each stage and discharged from said discharge port (31), characterized in that the three-dimensional configuration of each vane (3c) of one (3) of said impellers (2, 3, 22a to 22g) is geometrically identical to a part of the three-dimensional configuration of any vane (2c) of another one (2) of said impellers (2, 3, 22a to 22g). A multistage centrifugal compressor according to Claim 1, characterized in that said one impeller (3) is disposed more remote from said suction port (30) than said other impeller (2) is. A multistage centrifugal compressor according to Claim 1, characterized in that said one impeller (3) is disposed downstream of said other impeller (2) with respect to a gas passage formed within said multistage centrifugal compressor. A multistage centrifugal compressor system including at least two centrifugal compressors connected together through piping, each of said centrifugal compressors comprising: a rotary shaft (1), an impeller (8, 9) including a disk, a shroud and a plurality of vanes having a three-dimensional shape and disposed between said disk and said shroud and separated from one another in a circumferential direction, said disk and said shroud being mounted on said rotary shaft (1), and a casing (4a, 4b) serving to house said impeller (8, 9) and having a suction port (12a, 12b) through which a gas is drawn in and a discharge port (13a, 13b) from which the gas is discharged, characterized in that the three-dimensional configuration of each vane of one (9) of said impellers belonging to one of said at least two centrifugal compressors is geometrically identical to a part of the three-dimensional configuration of any vane of another one (8) of said impellers belonging to the other of said at least two centrifugal compressors. A multistage centrifugal compressor system according to Claim 4, characterized in that an intercooler (14) is disposed between two adjacent centrifugal compressors. A multistage centrifugal compressor system according to Claim 4 or 5, characterized in that said one centrifugal compressor is disposed downstream of said other centrifugal compressor with respect to a gas passage formed within said multistage centrifugal compressor system.

Inventors: EINO TAKASHI, EINO, TAKASHI
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1996
Main classification: F04D29
IPC codes: F04D17, F04D29