Patent Application
20110150628
The invention relates to a fluid energy machine, in particular a compressor or pump, having a housing, a motor, at least one impeller, at least two radial bearings, at least one shaft which extends along a shaft longitudinal axis and supports the at least one impeller and a rotor of the motor, wherein the shaft is borne in the radial bearings, wherein the motor has a stator which at least partially surrounds the rotor in the area of the motor, and a gap which extends in the circumferential direction and along the shaft longitudinal axis and is at least partially filled with a fluid is formed between the rotor and the stator, as well as between the rotor and the radial bearings.
Fluid energy machines such as these are the object of particularly intensive research efforts at the moment, because they offer the capability to be embodied without seals. The motor, which is generally in the form of an electrical drive, and the impeller of the fluid energy machine, for example a compressor impeller, can be arranged jointly in a single housing, sealed in a gas-tight manner from the environment, such that the shaft does not require any bushing to the outside. In this case, without any seals means that no shaft seal has to seal a gap between a moving component and a stationary component from the environment. Nevertheless, some seals are required, for example in the area of the impellers, and are normally in the form of labyrinth seals. The rotor and the stator of the motor are surrounded by the process fluid since, preferably, no shaft seal is also provided between the compressor and the motor in the housing. The process fluid is correspondingly located in the gaps between the rotor and the stationary components—that is to say between the stator of the motor and the rotor, in the bearings and in the back-up bearings. If the rotor or the split cage is excited to oscillate, with the oscillation changing the gap height in one of the circumferential gaps at a circumferential position, and if the fluid has a significant circumferential velocity in the gap between the rotor and the split cage, then the local reduction in the gap height results in acceleration in the resultant Couette flow which, in accordance with Bernoulli's flow law, leads to a local pressure reduction, as a result of which the forces which reduce the gap height are increased in addition to the stimulated reduction of the gap height. These aerodynamic or hydrodynamic forces increase as the fluid density increases and, if sufficiently pronounced, can lead to a contact between rotating and stationary parts, even resulting in damage. It is essential to prevent a reduction such as this in the availability of the fluid energy machine.
A split-cage motor which has at least one motor without any bearings and drives a pump impeller arranged at the side is known from WO 97/08808. The arrangement proposed there is suitable only for operation of small fluid energy machines, since the impeller, which is in each case arranged at a free shaft end, has a restricted size and mass, from the rotor-dynamic point of view. A multistage embodiment is not feasible in the described manner. Hydrodynamic instability in the gap flow is not discussed.
Against the background of the problems described above, the object of the invention is to provide a fluid energy machine of the type mentioned initially, which has particularly high availability, in particular with the aim of improving the operational reliability of a large fluid engine machine.
The invention achieves the object by means of the features additionally stated in the claims. The dependent claims, which refer back thereto, include advantageous developments of the invention.
According to the invention, an impeller should be understood as meaning a rotating component which feeds a process fluid depending on the purpose of the machine, or is driven thereby. By way of example, this could be an impeller of a compressor. Correspondingly, for example, a plurality of centrifugal impellers may be arranged in-line or back-to-back in a centrifugal compressor. The additional application of forces to the rotor of the motor by means of separate magnetic fields which are produced by the closed-loop control system, controlled by the stator, ensures a more secure position of the rotor and an increased level of concentricity of the shaft longitudinal axis with respect to the split cage. Therefore, the rotor-dynamic and flow phenomena of hydrodynamic instability, as described above, do not occur as early and a further operating range can be made use of without any risk.
It is particularly expedient to design the housing to be gas-tight, with at least one inlet and one outlet being provided for the process fluid to be fed by the fluid energy machine, or the driving process fluid. In this sense, a gas-tight housing for the purposes of the invention should be understood as meaning that there is no need to provide a shaft seal in order to pass the shaft out of the housing.
According to the invention, at least one axial bearing is provided for defined bearing of the shaft in an axial position. This axial bearing is preferably in the form of a magnetic bearing, in the same way as the at least two separate radial bearings.
The saving of a complex shaft seal naturally results in the disadvantage that the motor must be insensitive to exposure to the process fluid, which is frequently chemically aggressive.
In this case, for example, the process fluid may be natural gas, which is compressed under water and, in addition to the chemically aggressive nature, also results in the difficulties of a widely fluctuating pressure and coarse impurities.
In this case, it is expedient to protect at least the interior of the stator of the motor against the process fluid, such that a so-called split cage can be provided in the gap between the rotor and the stator, which separates an area in which the process fluid flows around the rotor from an area in which the interior of the stator is arranged.
In this case, the stator is advantageously kept at a suitable operating temperature by a separate cooling system by means of a cooling fluid, with the rest of the components of the machine preferably being cooled by means of the process fluid. In particular, the bearings, which are preferably in the form of magnetic bearings, can be cooled by means of the process fluid.
In this case, the split cage is subject to particular requirements. In order to prevent it from being excessively heated, because of eddy currents being induced in the alternating magnetic fields of the stator, it should be electrically non-conductive. In addition, it must be sufficiently mechanically robust, since high pressure differences can occur between the process fluid and the stator cooling fluid, which is generally separated from the process fluid by means of the split cage. For acceptable efficiency, the wall thickness of the split cage must not be excessively thick. Furthermore, the split cage must be chemically resistant to the process fluid.
In another preferred development of the invention, components of the fluid energy machine are cooled by means of the fluid to be fed, or the process fluid, in particular the bearings, which are in the faun of magnetic bearings. Furthermore, the fluid energy machine is preferably designed such that the process fluid at least partially flows around the rotor.
In order to allow the motor to exert lateral forces in order to stabilize the concentricity of the rotor with respect to the split cage, it is expedient for the motor to have at least two winding systems with different numbers of pole pairs.
It is also expedient for the closed-loop control system to be connected to position and/or oscillation sensors, and to use their signals as input signals to drive the motor. These sensors can likewise be used for closed-loop control of radial magnetic bearings, as a result of which there is no need for additional components. Additionally or alternatively, the closed-loop control system can be linked to measurements of the electrical currents through the motor windings or to measurements of the magnetic fluxes on the motor, and these can be used as an input signal for exerting lateral forces on the rotor, in order to drive the motor.
The advantages of the invention are particularly evident for a multistage compressor or a multistage pump with a number of impellers corresponding to the number of stages.
In a preferably physically short arrangement, the shaft can be borne by means of two separate radial bearings, which are arranged at the shaft ends and enclose the combination of the motor and compressor between them.
With regard to the rotor dynamics, it is particularly expedient for the motor and/or the impellers to be arranged along a shaft longitudinal axis, and for a radial bearing to be provided in each case at both ends of the motor along this shaft longitudinal axis, and for a further radial bearing to be provided on the side of the impeller or of the impellers facing away from the motor. This arrangement, which comprises three separate radial bearings, is ideally suitable for good rotor dynamics for multistage compressors.
In conjunction with the closed-loop control system according to the invention, which drives the motor such that radial forces are also exerted with respect to the shaft longitudinal axis, in addition to torques for driving the fluid energy machine, an arrangement such as this has high availability, even in extreme operating areas.
The invention also relates to a method for operation of a fluid energy machine of the abovementioned type, in which an additional drive for production of radial forces with respect to a shaft longitudinal axis is superimposed by means of a closed-loop control system for driving the motor for controlling a drive torque, and in which at least two further radial bearings are provided adjacent to the motor.
The invention will be explained in the following text using the description of one specific exemplary embodiment and with reference to a figure. Further embodiment options will be evident to a person skilled in the art from the disclosure, which options which can be ascribed to the invention and differ from the described exemplary embodiment. In the figure:
The figure shows a longitudinal section through a fluid energy machine 1, and a closed-loop control system 2, in the form of a block diagram, each illustrated in a simplified form. The fluid energy machine 1 has a compressor 3 and a motor 4, connected by means of a common shaft 5 and arranged along a shaft longitudinal axis 6 in a housing 7 which provides an external gas-tight seal. The gas-tight housing 7 is gas-tight to the extent that no bushing is provided for the shaft 5, which would have to be sealed by means of a shaft seal. To this extent, the fluid energy machine 1 can be described as having no seals, although shaft seals are located between the individual stages of the compressor 3, in order to cope with the pressure difference produced in the stages.
The housing 7 has an inlet 8 and an outlet 9 for process fluid 10, which is compressed by means of the compressor 3. In addition to a main flow 13 through the inlet 8 and the outlet 9 and a plurality of impellers 11 of the compressor 3, a smaller proportion of the process fluid 10 flows from the last impeller 11 along a secondary flow path 12 as far as the first impeller 11.
The motor 4 has a rotor 15 and a stator 16, with the rotor 15 being supported by the shaft 5. The shaft 5 is borne in a first radial bearing 17 and a second radial bearing 18, as well as an axial bearing 19. Dashed lines in the figure show a third radial bearing 20, which can optionally be provided. In this part, the shaft 5 can also be formed by means of a quill shaft 21 (illustrated by dashed lines) in the area between the compressor 3 and the motor 4, such that it bends easily.
The compressor 3 has three stages, and correspondingly has three impellers 11, but may also have fewer or more stages.
The bearings 17, 18, 19 and 20 are in the form of magnetic bearings, and the secondary flow path 12 extends along these bearings, in order to cool them. The process fluid 10 along the secondary flow path 12 cools not only the magnetic bearings 17-20 but also the rotor 15 of the motor 4. The stator 16 is separated from the rotor 15 by a gap 22, with the secondary flow path 12 extending through the gap 22. In order to ensure that the interior of the stator 16 is not subjected to the process fluid 10, it is encapsulated, and is separated toward the gap 22 by a so-called split cage 24.
The stator 16 is cooled by means of separate stator cooling 25. The cooling fluid 26 which circulates in the stator cooling may be at a different pressure than the process fluid 10 which is present in the gap, with the split cage 24 absorbing the pressure difference.
The motor 4 transmits a torque 30 to the compressor 3, in order to drive the compression process. In this case, the closed-loop control system 2 controls the rotation speed of the fluid energy machine 1, with a rotation speed sensor 31 measuring the rotation speed on the shaft 5, and passing on the measured value to an inverter 40 in the closed-loop control system 2. The inverter 40 supplies the stator 16 with the appropriate drive for the nominal rotation speed value, and with a current at the required voltage and frequency. A combined regulator and amplifier 41 in the closed-loop control system 2 passes the appropriate nominal values for the rotation speed to the inverter 40. The combined regulator and amplifier 41 is furthermore connected to two position sensors, a first radial position sensor 50 and a second radial position sensor 51, whose measured values are used by the combined regulator and amplifier 41 to appropriately drive the first radial bearing 17 and the second radial bearing 18 such that the shaft 5 remains in its nominal spatial position. In addition, the combined regulator and amplifier 41 uses the signals from the radial position sensors 50, 51 in order to cause the inverter 40 to produce a drive for the stator 16 of the motor 4, which is superimposed on the drive for driving the compressor, resulting in a discrepancy in the concentric position of the shaft 5 with respect to the split cage 24, and which drive results in additional radial forces 60 on the shaft 5.
In order to allow the motor 4 to produce the additional radial forces 60, a first winding system 71 and a second winding system 72 are provided in the stator 16, which winding systems 71, 72 have different numbers of pole pairs.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2008 038 787.8 | Aug 2008 | DE | national |
This application is the U.S. National Stage of International Application No. PCT/EP2009/060383, filed Aug. 11, 2009 and claims the benefit thereof. The International Application claims the benefits of German application No. 10 2008 038 787.8 DE filed Aug. 13, 2008. All of the applications are incorporated by reference herein in their entirety.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2009/060383 | 8/11/2009 | WO | 00 | 2/11/2011 |
