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Multi-stage compression system and method of operating the sameMulti-stage compression system and method of operating the same description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070189905, Multi-stage compression system and method of operating the same. Brief Patent Description - Full Patent Description - Patent Application Claims
RELATED APPLICATION DATA [0001]This application claims benefit under 35 U.S.C. Section 119(e) of co-pending U.S. Provisional Application No. 60/772,715, filed Feb. 13, 2006, which is fully incorporated herein by reference. BACKGROUND [0002]The present invention relates to a system and method to control a centrifugal compression system. In particular, the invention relates to a control system and method that varies the speed of multiple compression stages while maintaining constant speed ratios. [0003]Compressors, and more particularly centrifugal compressors, operate across a wide range of operating parameters. Variation of some of these parameters may produce undesirable efficiency and capacity variations. [0004]Compression of a gas in centrifugal compressors, also known as dynamic compressors, is based on the transfer of energy from a set of rotating blades to a gas such as air. The rotating blades impart energy by changing the momentum and pressure of the gas. The gas momentum, which is related to kinetic energy, is then converted into pressure energy by decreasing the velocity of the gas in stationary diffusers and downstream collecting systems. [0005]The performance of a multi-stage centrifugal compression system is impacted by conditions of the gas at the inlet, such as pressure, temperature, and relative humidity, and by the operating speed of the compressor stages. Specifically, for a given rotational speed of a stage, variations in pressure, temperature, and relative humidity of the gas at the inlet of that compression stage will alter the compressor discharge head and capacity. Additionally, a change in the operating rotational speed of a stage of compression also results in a change in the performance parameters of the overall compressor in terms of discharge head, capacity, and thermodynamic efficiency. It is relevant to note that in dynamic compressors there is a dependent relationship between capacity and compression ratio, defined as the discharge pressure divided by the inlet pressure (in consistent units). Accordingly, a change in gas capacity is always accompanied by a change in the compression ratio. As the operating point of the compressor changes, the efficiency of the compression thermodynamic process will also change. SUMMARY [0006]In one construction, the invention provides a multi-stage compression system including a plurality of centrifugal compression stages. Each stage includes an impeller and a variable speed motor coupled to the impeller. Each variable speed motor is operable at a speed between a first speed and a second speed. The multi-stage compression system also includes a control system that is connected to each of the variable speed motors and is operable to vary the speed of each motor. The speed of each motor is varied simultaneously such that a ratio of the speed of any two variable speed motors remains constant. [0007]In another construction, the invention provides a multi-stage compression system including a first centrifugal compressor stage having a first impeller and a first variable speed motor. The first variable speed motor is operable at a first speed between a low speed and a high speed. The multi-stage compression system also includes a second centrifugal compressor stage having a second impeller and a second variable speed motor. The second variable speed motor is operable at a second speed between a low speed and a high speed. The first speed and the second speed define a first ratio. The multi-stage compression system further includes a third centrifugal compressor stage having a third impeller and a third variable speed motor. The third variable speed motor is operable at a third speed between a low speed and a high speed. The first speed and the third speed define a second ratio and the second speed and the third speed define a third ratio. The multi-stage compression system also includes a control system operable to synchronously vary the first speed, the second speed, and the third speed such that the first ratio, the second ratio, and the third ratio remain constant. [0008]In yet another construction, the invention provides a method of controlling a multi-stage compression system to deliver a gas to a gas utilization system that uses the gas at a variable rate. The method includes operating a first centrifugal compressor stage at a first speed to produce a fluid flow, directing the fluid flow to a second centrifugal compressor stage, and operating the second centrifugal compressor stage at a second speed. The method also includes varying the first speed and the second speed in unison such that a ratio between the first speed and the second speed remains constant. BRIEF DESCRIPTION OF DRAWINGS [0009]FIG. 1 is a cross-sectional view of a centrifugal compression stage; [0010]FIG. 2 is a schematic of a multi-stage compression system including three centrifugal compression stages of FIG. 1; and [0011]FIG. 3 is a performance map of the centrifugal variable speed compression stage of FIG. 1. DETAILED DESCRIPTION [0012]Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein are for the purpose of description and should not be regarded as limiting. The use of "including," "comprising," or "having" and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms "mounted," "connected," "supported," and "coupled" and variations thereof are used broadly and encompass direct and indirect mountings, connections, supports, and couplings. Further, "connected" and "coupled" are not restricted to physical or mechanical connections or couplings. [0013]FIG. 1 illustrates a fluid compression module 10 (sometimes referred to as a compression stage or a compression unit) that includes a prime mover coupled to a compressor 20 and that is operable to produce a compressed fluid. In the illustrated construction, an electric motor 15 is employed as the prime mover. However, other constructions may employ other prime movers such as but not limited to internal combustion engines, diesel engines, combustion turbines, etc. [0014]The electric motor 15 includes a rotor 25 and a stator 30 that defines a stator bore 35. The rotor 25 is supported for rotation on a shaft 40 and is positioned substantially within the stator bore 35. The illustrated rotor 25 includes permanent magnets 45 that interact with a magnetic field produced by the stator 30 to produce rotation of the rotor 25 and the shaft 40. In one construction, the rotor 25 is operable between about 0 RPM and 50,000 RPM, with faster speeds also being possible. Before proceeding, it should be noted that the word "between" as used herein in conjunction with discussions of operating speeds of the motors or compression stages means that the motor or compression stage is operable at any speed between the defined end points (e.g., 0 RPM, 50,000 RPM) and including the end points. Thus, a two-speed motor (i.e., one operable at 0 RPM and 50,000 RPM) is not operable between 0 RPM and 50,000 RPM. Rather, it is operable at 0 RPM and it is operable at 50,000 RPM. A motor operable between two speeds is operable at those two speeds as well as any intermediate speed between the end points. The magnetic field of the stator 30 can be varied to vary the speed of rotation of the shaft 40. Of course, other constructions may employ other types of electric motors (e.g., synchronous, induction, brushed DC motors, etc.) if desired. [0015]The motor 15 is positioned within a housing 50 which provides both support and protection for the motor 15. A bearing 55 is positioned on either end of the housing 50 and is directly or indirectly supported by the housing 50. The bearings 55 in turn support the shaft 40 for rotation. In the illustrated construction, magnetic bearings 55 are employed with other bearings (e.g., roller, ball, needle, etc.) also suitable for use. In the construction illustrated in FIG. 1, secondary bearings 60 are employed to provide shaft support in the event one or both of the magnetic bearings 55 fail. [0016]In some constructions, an outer jacket 65 surrounds a portion of the housing 50 and defines cooling paths 70 therebetween. A liquid (e.g., glycol, refrigerant, etc.) or gas (e.g., air, carbon dioxide, etc.) coolant flows through the cooling paths 70 to cool the motor 15 during operation. [0017]An electrical cabinet 75 may be positioned at one end of the housing 50 to enclose various items such as a motor controller, breakers, switches, and the like. The motor shaft 40 extends beyond the opposite end of the housing 50 to allow the shaft to be coupled to the compressor 20. [0018]The compressor 20 includes an intake housing 80 or intake ring, an impeller 85, a diffuser 90, and a volute 95. The volute 95 includes a first portion 100 and a second portion 105. The first portion 100 attaches to the housing 50 to couple the stationary portion of the compressor 20 to the stationary portion of the motor 15. The second portion 105 attaches to the first portion 100 to define an inlet channel 110 and a collecting channel 115. The second portion 105 also defines a discharge portion 120 that includes a discharge channel 125 that is in fluid communication with the collecting channel 115 to discharge the compressed fluid from the compressor 20. [0019]In the illustrated construction, the first portion 100 of the volute 95 includes a leg 130 that provides support for the compressor 20 and the motor 15. In other constructions, other components are used to support the compressor 20 and the motor 15 in the horizontal position. In still other constructions, one or more legs, or other means are employed to support the motor 15 and compressor 20 in a vertical orientation or any other desired orientation. 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