Turbo compressor and method of controlling the same

Abstract: A turbo compressor (21) in which a rotor (22) having impellers (31) is supported by a magnetic bearing (27) is constructed such that a control valve (30) is provided in a thrust balancing line (29) connecting a suction line (24) of the compressor to a balancing chamber (33) formed outside a balancing drum (32b) provided at one end side of a rotor (22). The control valve (30) serves to regulate a rate at which a gas flows through the thrust balancing line (29), and is adapted to be controlled in relation to a load imposed upon the magnetic thrust bearing.

Description: BACKGROUND OF THE INVENTION This invention relates to a turbo compressor having a rotor supported by a magnetic bearing and a method of controlling the turbo compressor. More particularly, the present invention can be suitably applied to a high-pressure type of single-shaft multi-stage centrifugal compressor. A conventional turbo compressor will be described below with reference to Fig. 3. Fig. 3 is a longitudinal sectional view of a single-shaft multi-stage centrifugal compressor. A rotor 2 to which a plurality of impellers 10 are attached is incorporated in a casing 1 and is supported by oil bearings 7. The rotor 2 is rotated by a motor to draw a gas being compressed through a nozzle 4. Driving energy is thereby applied to the gas by the impeller 10, and the gas thereby increased in pressure is discharged through a nozzle 5. The pressure of the gas is higher at a compression stage closer to the rear end, and the inter-stage labyrinth diameter is smaller than the mouth labyrinth diameter. Therefore the static pressure acts on one impeller as an axial force in the direction from the discharge side to the suction side. To reduce this thrust force to a magnitude within the range of an allowable load capacity of the thrust bearing corresponding to the axial-end diameter of the rotor, a balancing drum 11 is provided at an axial end of the final-stage impeller, and a chamber 12 communicating with the suction side is formed outside the balancing drum 11. In this type of turbo compressors using oil bearings, the friction loss of the bearings is generally several ten kilowatts. If the above-mentioned oil bearings are replaced with non-contact type magnetic bearings, the running cost of the compressor can be reduced and the lubrication system can be maintenance-free. A turbo compressor using a balancing drum, is disclosed in JP-A-62-258195. The load capacity of magnetic bearings, however, is on the order of about 1/10 of that of oil bearings (ordinarily, several kg/cm 2 or less in terms of surface pressure), and a problem of an increase in the size of a thrust bearing portion is therefore encountered. That is, if a magnetic bearing is used, the diameter of a journal bearing portion is increased in comparison with oil bearings and the inside diameter of the thrust bearing is also increased, because the load capacity of the magnetic bearing is small. Also there is a need to increase the area of a thrust collar, i.e., the diameter of the thrust collar in comparison with oil bearings. If the thrust bearing is disposed outside of the journal bearing and if the thrust collar is large, a problem of a natural oscillation frequency of the compressor rotor in a high-order mode is reduced close to the number of revolutions in steady operation arises. If the thrust bearing is disposed inside the journal bearing, a problem of a further increase in the diameter of the shaft sealing device, a problem of the allowable peripheral speed of the shaft sealing device being exceeded, and a problem of difficulty in constructing the compressor are encountered. A thrust force acting on the compressor shaft is calculated from a static pressure on an impeller portion, fluid force caused by a gas flow, leaks through gaps, and so on. However, the thrust force is changed owing to the difference between design conditions and operating conditions or by a change in these conditions or a change with time (such as wear of labyrinths). Therefore the reliability of the bearing system cannot be satisfactorily high unless a thrust bearing having a sufficiently large load capacity is selected. For this reason, a large thrust bearing having such a large capacity is required in the conventional compressors. Also known by JP-A-1 187 395 is a compressor in which the magnetic thrust bearing is controlled on the basis of the signal provided by a rotor position detection sensor configured as a thrust displacement probe. This does not permit to obtain a correct stabilization of the rotor during operation of the turbo compressor. SUMMARY OF THE INVENTION An object of the present invention is to provide a turbo compressor having a magnetic thrust bearing reduced in size by being arranged so that a thrust force acting on the magnetic thrust bearing is constantly small and stable. Another object of the present invention is to provide a turbo compressor in which only a small, generally constant thrust force can act on a magnetic thrust bearing even if the thrust force acting on the rotor is changed or varied to a large extent owing to the difference between design conditions and operating conditions of the compressor or by a change in these conditions or a change with time (such as wear of labyrinths). As claimed, the turbo compressor of the invention comprises a magnetic thrust bearing having a pair of thrust bearing portions disposed on the opposite sides of a thrust collar provided on a rotor having at least one impeller, a balancing drum provided at one end side of the rotor and a balancing chamber formed at an outer position in the compressor relative to said balancing drum. A thrust balancing line is provided connecting said balancing chamber to a suction line of the compressor and a control valve is provided in said thrust balancing line and operable to regulate a rate at which a gas flows through said thrust balancing line to control a pressure in said balancing chamber so that a thrust force acting on said balancing drum is regulated. A rotor position detection sensor is provided for detecting an axial displacement of said rotor and means for controlling said magnetic thrust bearing on the basis of a signal from said rotor position detection sensor. According to the invention, the magnetic thrust bearing is controlled so that the control current for the inner thrust bearing portion is greater than the control current for the outer thrust bearing portion. Futhermore, comparison means are provided for comparing a control signal for the inner magnetic thrust bearing portion with a reference signal, the opening of said control valve being controlled on the basis of a deviation signal from said comparison means. The arrangement of a turbo compressor having these features may be such that the outside diameter of the balancing drum is greater than the inside diameter of a mouth seal provided on the suction side of the impeller. As also claimed, the method of the invention for controlling a turbo compressor comprising a magnetic thrust bearing having a pair of thrust bearing portions for supporting a rotor having at least one impeller against a thrust force acting on the rotor, a balancing drum provided at one end side of the rotor, a balancing chamber formed at an outer position in the compressor relative to the balancing drum, a thrust balancing line connecting the balancing chamber to a suction line of the compressor, comprises, according to the invention : controlling the rate at which a gas flows through the thurst balancing line in relation to a load imposed upon the magnetic thrust bearing and to a comparison between a reference signal and a control signal for the inner magnetic thrust bearing portion, so that the thrust force acting on the rotor is maintained in a predetermined range. In accordance with the present invention, to limit a thrust force acting on the magnetic thrust bearing to a small magnitude with stability, the chamber on the outside of the balancing drum is connected to a suction side of the compressor by a thrust balancing line, a control valve for regulating the pressure in the chamber is provided in the thrust balancing line, and the opening of this control valve is controlled so that a load upon the magnetic bearing is reduced. For example, in a case where a thrust bearing is provided at an axial end on the suction side, a thrust force acts in the direction from the discharge side to the suction side in order to prevent a thrust collar from coming off (, which direction is hereinafter referred to as a positive thrust direction). For this effect, the compressor is arranged so that, when the valve is slightly throttled from the fully opened state, that is, when the pressure in the chamber on the outside of the balancing drum is slightly higher than the suction pressure, an appropriate positive thrust force acts on the magnetic bearing. In the case of a magnetic bearing, to set this positive thrust force to a value on the order of about 1/10 of that in the case of oil bearings, the force acting actually on the magnetic thrust bearing is detected, and the opening of the valve is adjusted in accordance with the detected value. That is, the opening of the valve is controlled so that the difference between electric currents for energization of a load-side (positive-thrust-side) bearing portion and an anti-load-side (negative-thrust side) bearing portion of the magnetic thrust bearing is equal to a predetermined value. This thrust force regulating apparatus can function not only during steady operation of the compressor but also in a situation where operating conditions are changed or during transient operation. It is thereby possible to obtain a suitable small positive thrust force with stability. The thrust force in the compressor of the present invention can therefore be reduced in comparison with the conventional compressors, so that the size of the magnetic thrust bearing can be reduced, and the load capacity can be sufficiently increased if the size of the magnetic thrust bearing is not changed. If the diameter of the balancing drum is greater than the impeller mouth labyrinth diameter, a thrust force in the negative thrust direction can be increased to an extent corresponding to the difference between these diameters. The degree of freedom of controlling the thrust force in the positive thrust direction can be thereby increased, so that this thrust force can be easily adjusted to a small magnitude. These and other objects, features and advantages of the present invention will become apparent from the following description of an embodiment of the present invention taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram of a turbo compressor in accordance with the present invention; Fig. 2 is a cross-sectional view of an essential portion of the compressor shown in Fig. 1, showing the function of a balancing drum; and Fig. 3 is a partially fragmentary front view of a conventional turbo compressor. DESCRIPTION An embodiment of the present invention will be described below with reference to the accompanying drawings. Fig. 1 shows a longitudinal sectional view of a turbo compressor to which the present invention is applied. A rotor 22 to which a plurality of impellers 31 are attached is incorporated in a casing 21 and is supported by journal magnetic bearings 27. A gas to be compressed enters through a suction nozzle 24, is compressed successively by the impellers 31, and flows out of a discharge nozzle 25. Because the pressure of the compressed gas is increased from the suction nozzle to the discharge nozzle, an axial thrust force is applied to the rotor 22 in the direction from the discharge side to the suction side. A magnetic thrust bearing 28 is therefore incorporated in this compressor. Further, a balance labyrinth 32a and a balancing drum 32b are provided outside a final-stage impeller, and a chamber (balancing chamber) 33 communicating with the suction nozzle through a control valve 30 is formed outside the balancing drum 32b. It is necessary for the magnetic thrust bearing 28 to have a reaction force corresponding to the above-mentioned thrust force. This reaction force is controlled as described below. The position of the rotor in the axial direction is detected by a rotor axial position detection sensor 34, a signal 50 from this sensor and a reference position signal 41 are compared by a comparator 42, and a deviation of the former from the reference signal 41 is converted into a control signal 44 (44a, 44b) by a signal processing circuit 43. The control signal 44 is amplified by power amplifiers 45a and 45b to produce control currents 46a and 46b. These control currents flow through the magnetic thrust bearing 28 to generate a magnetic force acting as a thrust force. In the example shown in Fig. 1, since each impeller, except for the final-stage impeller, receives a thrust force applied in the direction from the discharge side to the suction side (positive thrust force), the rotor as a whole ordinarily has a thrust force in the direction from the discharge side to the suction side. Accordingly, a magnetic force generated for an inner thrust bearing portion 28a (i.e., suction force) is greater than a magnetic force generated for an outer thrust bearing portion 28b. That is, the thrust bearing portion control currents 46a and 46b are controlled so that the control current 46a is greater than the control current 46b. The control system is therefore arranged in such a manner that the thrust bearing portion control signal 44a is extracted as a thrust bearing portion control signal 47 and is compared with a reference signal 51 by a comparator 53, and a control signal is produced in accordance with a deviation from the reference signal and is converted into a control value 49 by a converter 48 to be supplied to a control valve 30 such as an air control valve. The opening of the control valve 30 is thereby controlled according to the change in the thrust force. An insensible band 52 is provided in the control line to the control valve 30 to correct the opening of the control valve only when the deviation signal becomes greater than a certain level, thereby preventing hunting. In this embodiment, the output from the rotor axial position detection sensor 34 is set to 0 only when the rotor is at a normal position. If the rotor is displaced in the direction from the discharge side to the suction side (plus direction), the output from the sensor 34 is set to a plus value in accordance with the extent of displacement. If the rotor is displaced in the direction from the suction side to the discharge side (minus direction), the output from the sensor 34 is set to a minus value in accordance with the extent of displacement. The values set as reference signals 41 and 51 are therefore 0. In this embodiment, as shown in fig. 2, the outside diameter D1 of the balancing drum 32b located outside the final-stage impeller is greater than the inside diameter D2 of the mouth labyrinth 55 on the final-stage impeller. Also, the balancing chamber 33 outside the balancing drum communicates with a compressor suction portion. It is therefore possible to apply a large thrust force to the balancing drum 32b in the minus direction (in the direction from the suction side to the discharge side). That is, as indicated by a number of small arrows in Fig. 2, a thrust force acts on the rotor (the final-stage impeller 31 and the balancing drum 32b). Accordingly, as is apparent from Fig. 2, a thrust force in the minus direction corresponding to a difference ℓ by which the outside diameter of the balancing drum is greater than the inside diameter of the mouth labyrinth is produced between the final-stage impeller and the balancing drum. This thrust force in the minus direction acts oppositely to the thrust force acting on the impeller at each stage in the plus direction (the direction to the suction side), and these forces can cancel out with each other. In this embodiment, the outside diameter D1 of the balancing drum 32b is selected so that when the control valve 30 is fully open, the thrust force applied to the entire rotor is a very small force in the plus direction or a force in the minus direction. As the control valve is throttled from this state, the pressure in the balancing chamber is increased, so that the force in the minus direction is reduced while the thrust force in the plus direction is increased. Preferably, the diameter D1 of the balancing drum 32b should be determined so that the thrust bearing position control current 46a is greater than the thrust bearing portion control current 46b and is stably maintained when the control valve 30 is partially open. The control can be thereby performed so that a certain thrust force in the direction to the suction side (plus direction) can be applied to the rotor during normal operation. It is thereby possible to stabilize the rotor and to prevent a thrust collar 23, a nut 54 and other members from coming off. During steady operation of the thus-constructed turbo compressor, if the rotor thrust force is changed by surging or a change in an operating condition such as suction pressure or discharge pressure, the turbo compressor in accordance with this embodiment operates as described below. A situation where the thrust force in the direction to the suction side (plus direction) is increased by a change in the operating conditions for some reason will be discussed below. If the axial position of the rotor is shifted in the direction to the suction side, a detected signal 50 and the reference signal 41 is compared with each other, and the inner thrust bearing portion control signal 44a is increased in accordance with the deviation from the reference signal. The inner thrust bearing portion control current 46a is thereby increased and the magnetic force (attraction force) of the inner thrust bearing is increased, so that the rotor is returned to the original position. On the other hand, when the inner thrust bearing portion control signal 44a is increased, it is compared with the reference signal 51, and the control valve 30 is operated in accordance with the deviation from the reference signal 51 (in the opening direction in this case). When the control valve 30 is opened, the pressure in the balancing chamber 33 is reduced and the force acting on the balancing drum 32b in the direction of the discharge side (minus direction) is increased, so that the thrust force acting on the entire rotor in the direction of the suction side is reduced. In accordance with this embodiment, this control operation is consecutively repeated to prevent occurrence of a large thrust force acting on the thrust bearing even if the operating conditions are changed. Thus, the thrust force produced to act on the rotor can be controlled through the steady and transient operations so as to be prevented from being excessively increased, so that the size of magnetic thrust bearing 28 can be reduced. If the size of the magnetic thrust bearing can be reduced, a reduction in the rotating mass outside the journal magnetic bearing 27 as well as a reduction in the axial length of the shaft can be achieved, thereby facilitating supporting a turbo compressor by means of a magnetic thrust bearing, which turbo compressor is conventionally supported by oil bearings. A complete magnetic bearing system can be thereby achieved to realize an oil-free design and, hence, an economical turbo compressor reduced in running cost, maintenance cost or the like. According to the present invention, in a turbo compressor using a magnetic thrust bearing, the thrust force acting on the thrust magnetic bearing can be constantly made small with stability, and the size of the magnetic thrust bearing can be reduced. As a result, the present invention makes it easy to obtain an economical turbo compressor using a magnetic thrust bearing support. The present invention also ensures that only a small, generally constant thrust force can act on the magnetic thrust bearing even if the rotor thrust force is changed to a large extent owing to the difference between design conditions and operating conditions of the compressor or by a change in these conditions or a change with time (such as wear of labyrinths). It is therefore possible to obtain a turbo compressor having smaller bearing portions having improved reliability.

Claims: Turbo compressor comprising: a magnetic thrust bearing having a pair of thrust bearing portions (28a, 28b) disposed on the opposite sides of a thrust collar (23) provided on a rotor (22) having at least one impeller (31); a balancing drum (32b) provided at one end side of the rotor (22); a balancing chamber (33) formed at an outer position in the compressor relative to said balancing drum; a thrust balancing line (29) connecting said balancing chamber (33) to a suction line (24) of the compressor; a control valve (30) provided in said thrust balancing line (29) and operable to regulate a rate at which a gas flows through said thrust balancing line to control a pressure in said balancing chamber so that a thrust force acting on said balancing drum is regulated; a rotor position detection sensor (34) for detecting an axial displacement of said rotor; means for controlling said magnetic thrust bearing on the basis of a signal from said rotor position detection sensor (34); characterized in that said magnetic thrust bearing is controlled so that the control current for the inner thrust bearing portion (28a) is greater than the control current for the outer thrust bearing portion (28b) and in that comparison means (53) are provided for comparing a control signal (46a) for the inner magnetic thrust bearing portion (28a) with a reference signal (51), the opening of said control valve (30) being controlled on the basis of a deviation signal (49) from said comparison means. Turbo compressor according to claim 1, characterized in that means (52) are provided for transmitting a control valve opening control signal only when the deviation signal becomes greater than a predetermined value. Turbo compressor according to any preceding claim characterized in that the diameter (D1) of the balancing drum (32b) is chosen so that, when said control valve (30) is partially open, the control current (46a) for the inner thrust bearing portion (28a) is greater than the control current (46b) for the outer thrust bearing portion (28b) and is stably maintained. Turbo compressor according to claim 3 characterized in that the outside diameter (D1) of the balancing drum (32b) is greater than the inside diameter (D2) of a mouth seal (55) provided on the suction side of the final stage impeller (31). Method of controlling a turbo compressor comprising a magnetic thrust bearing (28) having a pair of thrust bearing portions for supporting a rotor (22) having at least one impeller (31) against a thrust force acting on the rotor, a balancing drum (32b) provided at one end side of the rotor, a balancing chamber (33) formed at an outer position in the compressor relative to the balancing drum, a thrust balancing line (29) connecting the balancing chamber to a suction line of the compressor, characterized by controlling rate at which a gas flows through the thrust balancing line (29) in relation to a load imposed upon the magnetic thrust bearing (28) and to a comparison between a reference signal (51) and a control signal for the inner magnetic thrust bearing portion (28a), so that the thrust force acting on the rotor is maintained in a predetermined range.

Inventors: MIURA HARUO, TAKEDA KAZUO, MIURA, HARUO, TAKEDA, KAZUO
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1992
Main classification: F04D29
IPC codes: F04D29, F04D27, F16C32