Teach You to Completely Get Rid of the Surge Phenomenon of the Centrifugal Refrigeration Compressor
2025-03-25
The centrifugal refrigeration compressor belongs to the speed-type compressor and is a kind of mechanical equipment with a rotating impeller. It does work on the gas by the high-speed rotating impeller to increase the pressure of the gas. So, what should we do when the centrifugal refrigeration compressor experiences the surge phenomenon? After reading this article, I believe it can help everyone completely get rid of the surge phenomenon of the centrifugal refrigeration compressor.
Characteristics of the Centrifugal Refrigeration Compressor:
(1) Small in external dimensions, light in weight, and occupying a small floor area.
(2) Good dynamic balance characteristics and small vibration.
(3) Few wearing parts and a long continuous operation cycle.
(4) High heat transfer performance.
(5) Easy to achieve multi-stage compression and throttling, and realize multiple evaporation temperatures.
(6) Capable of economically carrying out stepless adjustment.
(7) If it is directly driven by an industrial steam turbine with high economy, the energy-saving effect will be better.
(8) High rotational speed and high requirements for shaft end sealing.
(9) Surge will occur when the condensing pressure is high.
(10) When the refrigerating capacity is small, the efficiency is low.
I. The Mechanism of Surge Generation
The basic working principle of a centrifugal compressor is to use a high-speed rotating impeller to do work on the gas, add mechanical energy to the gas, increase the gas pressure and speed, so that the gas obtains pressure energy and kinetic energy. There is a diffuser element with a gradually expanding flow passage area behind the impeller. After the high-pressure gas flows out from the impeller, it then passes through the diffuser for speed reduction and pressure increase, reducing the gas flow rate and continuously increasing the pressure, that is, converting part of the gas kinetic energy into pressure energy to complete the compression process.
The boundary layer separation phenomenon in the diffuser flow passage: The flow of the gas in the diffuser flow passage comes from the kinetic energy converted from the work done by the impeller on the gas flow. The gas flow in the boundary layer mainly relies on the kinetic energy transmitted from the mainstream. When the gas in the boundary layer flows, it needs to overcome the friction force of the wall surface. Since the speed decreases and the pressure increases along the flow passage direction, the kinetic energy of the mainstream also decreases continuously.
When the kinetic energy transmitted from the mainstream to the boundary layer is not enough to make it overcome the pressure difference and move forward, the gas flow in the boundary layer will eventually stop, and then vortices and backflow will occur, causing the separation of the gas flow boundary layer. The flow of gas in the impeller is also a kind of diffusing flow. When the flow rate decreases or the pressure difference increases, this boundary layer separation phenomenon will also occur.
When the gas flow rate in the flow passage decreases to a certain value, the direction of the gas flow at the impeller passage inlet is very inconsistent with the blade inlet angle, and the angle of attack α increases greatly, causing serious separation of the gas flow boundary layer in the flow passage on the non-working surface, resulting in strong gas flow pulsation at the inlet and outlet of the flow passage.
When the flow rate decreases significantly, due to the unevenness of the gas flow and the unevenness of the flow passage profile, it is assumed that the gas flow separation occurs in the B flow passage. In this way, the effective flow passage area of the B flow passage decreases, so that part of the gas flow that originally passed through the B flow passage will flow to the adjacent A flow passage and C flow passage. This changes the original gas flow direction of the A flow passage and C flow passage. It makes the angle of attack of the C flow passage decrease somewhat, and the angle of attack of the A flow passage increase more, thus causing the gas flow separation in the A flow passage. In turn, it makes the angle of attack of the B flow passage decrease and eliminates the separation phenomenon, so the separation phenomenon is transferred from the B flow passage to the A flow passage. In this way, the separation zone rotates and moves in the direction opposite to the rotation direction of the impeller, and this phenomenon is called rotating stall.
The diffuser also has rotating stall. During the operation of the compressor, when the flow rate continuously decreases to the Qmin value, the serious rotating stall described above occurs in the compressor flow passage, and the flow deteriorates seriously, causing the compressor outlet exhaust pressure to suddenly drop greatly, lower than the pressure of the condenser. Then the gas will flow back to the compressor until the condensing pressure is lower than the compressor outlet exhaust pressure. At this time, the backflow stops, the compressor displacement increases, and the compressor resumes normal operation.
In fact, the total load of the compressor is very small, which limits the displacement of the compressor. The displacement of the compressor gradually decreases again, and the gas flows back again. Repeating this process, a periodic gas flow oscillation phenomenon occurs in the system, and this phenomenon is called surge.
The compressor reaching the minimum displacement point and generating serious gas flow rotating stall is the internal cause, and the condition of the compressor performance curve and the position of the operating point are the conditions. Only when the internal cause is promoted by the conditions can the unique phenomenon - surge occur.
When the centrifugal chiller is operating at part load, the opening of the compressor guide vanes decreases, and the refrigerant flow rate participating in the cycle decreases. The compressor displacement decreases, and the ability of the impeller to reach the head also decreases. While the cooling water temperature remains unchanged because the cooling tower has not been changed, then rotating stall or surge may occur at this time.
Surge is an inherent characteristic of the speed-type centrifugal compressor. Therefore, for any centrifugal compressor, this phenomenon will occur when the displacement is reduced to a certain limit point. Whether the chiller is operating near the surge point mainly depends on the operating conditions of the unit. At what state the surge occurs can only be determined by testing the machine, that is, by continuously reducing its flow rate, the specific surge point can be measured.
Due to the decrease in the gas flow rate in the compressor impeller flow passage, according to the compressor characteristic curve, its operating condition point moves towards the direction of high compression ratio. At this time, the change of the gas flow direction generates a large positive angle of attack at the impeller inlet, causing a serious "separation phenomenon" of the gas flow on the non-working surface of the impeller blades, increasing the aerodynamic loss, and generating a negative pressure area at the impeller outlet, causing the original positive pressure gas flow in the upper part of the condenser or the volute to "flow back" along the pressure drop direction and return to the impeller, increasing the mixed flow rate in the impeller flow passage and making the impeller resume normal operation.
If the compressor operating condition point still has not escaped from the surge point (area) at this time, the above-mentioned "flow back" of the gas flow will occur again. This periodic back-and-forth pulsation of the gas flow is exactly the fundamental cause of the compressor surge.
II. The Harmfulness of Surge
Surge is an unstable operating state that occurs in the small flow rate and high pressure ratio area of the operating conditions of the centrifugal compressor.
(1) Significantly deteriorate the performance of the compressor, and cause large fluctuations in gas parameters (pressure, displacement).
(2) Increase the noise.
(3) Greatly intensify the vibration of the entire unit. Surge makes the components of the compressor rotor and stator withstand alternating dynamic stresses: the pressure imbalance causes strong vibration, making the unit center shift, the bearing wear, the sealing clearance increase; and even the collision between the rotor and stator components occurs: the dynamic stress of the impeller increases.
(4) The current pulsates.
(5) The pulsation frequency of small refrigerating capacity units is higher than that of large units, but the amplitude is small.
Different from general mechanical vibration, the repeated backflow, discharge, and back-and-forth impact of the gas flow occur at the compressor outlet, causing the main motor to alternately appear full load and no load, and the pointer of the ammeter or the pressure gauge at the compressor outlet to produce large, irregular and strong shaking and jumping. The compressor rotor moves back and forth axially in the machine, accompanied by metallic friction and impact sounds.
III. Anti-surge Measures
1. The Principle of Hot Gas Bypass Surge Protection
Once entering the surge operating condition, adjustment measures should be taken immediately to reduce the outlet pressure or increase the inlet flow rate. From the above mechanism of surge generation, in the centrifugal chiller, the pressure ratio and load are the two major factors affecting the surge. When the load becomes smaller and smaller, and reaches a certain limit point, surge will occur, or when the pressure ratio reaches a certain limit point, surge will occur.
Using hot gas bypass for surge protection is to control the opening or closing of the hot gas bypass through the surge protection line, so that the unit is far away from the surge point and achieves the purpose of protection. A connecting pipe is connected from the condenser to the evaporator. When the operating point reaches the surge protection point but not the surge point, the hot gas bypass solenoid valve is opened through the control system, and the hot gas from the condenser is discharged to the evaporator, reducing the pressure ratio and at the same time increasing the exhaust volume, thus avoiding the occurrence of surge.
2. Changing the Compressor Rotational Speed
When the compressor rotational speed changes, the performance curve of the compressor will move accordingly, which can increase the stable operating condition area. It is suitable for units driven by steam turbines and gas turbines, and is a relatively economical adjustment method, but the working point after adjustment is not necessarily the highest efficiency point. However, for units driven by electric motors, in order to facilitate speed change, a DC unit or a frequency conversion method needs to be used, which will greatly complicate the equipment and also increase the cost.
3. Multi-stage Compression
Multi-stage compression is used to reduce the compressor rotational speed. Generally, the occurrence of surge in any stage of a multi-stage machine will affect the normal operation of the entire machine. By adopting multi-stage compression, under the same pressure ratio operating conditions, the compressor rotational speed can be greatly reduced, and the stable operating condition area can be increased.
4. Using a Rotating Diffuser to Adjust the Flow Rate
When the flow rate decreases, serious rotating stall usually occurs first in the diffuser, leading to surge. When the flow rate changes, if the inlet geometric angle of the diffuser flow passage can be correspondingly changed to adapt to the changed operating conditions, so that the angle of attack α is not too large, the performance curve can be greatly shifted towards the small flow rate area, the stable operating condition range can be expanded, the surge flow rate can be greatly reduced, and the purpose of anti-surge can be achieved. This anti-surge control method has been specifically applied in Carrier's products, but hot gas bypass still needs to be used at low load.
5. Movable Diffusion Cavity
As mentioned above, the reasons for the occurrence of surge in the centrifugal chiller are the pressure ratio and load. When the pressure ratio (lifting force) of the unit operation is constant, the operating load of the unit will affect whether the unit surges. For the centrifugal unit, when the operating load decreases, the guide vanes of the compressor gradually close, and the suction volume decreases. If the passage area of the diffusion cavity remains unchanged, the gas flow rate decreases: when the gas flow rate cannot overcome the resistance loss of the diffusion cavity, the gas flow will stagnate. Due to the decrease in the gas kinetic energy, the converted pressure energy also decreases: when the gas fluid pressure is lower than the pressure of the exhaust pipe network, the gas flow flows back and surge occurs.
IV. Conclusion
Hot gas bypass, changing the compressor rotational speed, multi-stage compression, adjusting with a rotating diffuser, and the design of the diffuser slider can all effectively avoid "surge" and have a good energy-saving effect on the centrifugal chiller.
Characteristics of the Centrifugal Refrigeration Compressor:
(1) Small in external dimensions, light in weight, and occupying a small floor area.
(2) Good dynamic balance characteristics and small vibration.
(3) Few wearing parts and a long continuous operation cycle.
(4) High heat transfer performance.
(5) Easy to achieve multi-stage compression and throttling, and realize multiple evaporation temperatures.
(6) Capable of economically carrying out stepless adjustment.
(7) If it is directly driven by an industrial steam turbine with high economy, the energy-saving effect will be better.
(8) High rotational speed and high requirements for shaft end sealing.
(9) Surge will occur when the condensing pressure is high.
(10) When the refrigerating capacity is small, the efficiency is low.
I. The Mechanism of Surge Generation
The basic working principle of a centrifugal compressor is to use a high-speed rotating impeller to do work on the gas, add mechanical energy to the gas, increase the gas pressure and speed, so that the gas obtains pressure energy and kinetic energy. There is a diffuser element with a gradually expanding flow passage area behind the impeller. After the high-pressure gas flows out from the impeller, it then passes through the diffuser for speed reduction and pressure increase, reducing the gas flow rate and continuously increasing the pressure, that is, converting part of the gas kinetic energy into pressure energy to complete the compression process.
The boundary layer separation phenomenon in the diffuser flow passage: The flow of the gas in the diffuser flow passage comes from the kinetic energy converted from the work done by the impeller on the gas flow. The gas flow in the boundary layer mainly relies on the kinetic energy transmitted from the mainstream. When the gas in the boundary layer flows, it needs to overcome the friction force of the wall surface. Since the speed decreases and the pressure increases along the flow passage direction, the kinetic energy of the mainstream also decreases continuously.
When the kinetic energy transmitted from the mainstream to the boundary layer is not enough to make it overcome the pressure difference and move forward, the gas flow in the boundary layer will eventually stop, and then vortices and backflow will occur, causing the separation of the gas flow boundary layer. The flow of gas in the impeller is also a kind of diffusing flow. When the flow rate decreases or the pressure difference increases, this boundary layer separation phenomenon will also occur.
When the gas flow rate in the flow passage decreases to a certain value, the direction of the gas flow at the impeller passage inlet is very inconsistent with the blade inlet angle, and the angle of attack α increases greatly, causing serious separation of the gas flow boundary layer in the flow passage on the non-working surface, resulting in strong gas flow pulsation at the inlet and outlet of the flow passage.
When the flow rate decreases significantly, due to the unevenness of the gas flow and the unevenness of the flow passage profile, it is assumed that the gas flow separation occurs in the B flow passage. In this way, the effective flow passage area of the B flow passage decreases, so that part of the gas flow that originally passed through the B flow passage will flow to the adjacent A flow passage and C flow passage. This changes the original gas flow direction of the A flow passage and C flow passage. It makes the angle of attack of the C flow passage decrease somewhat, and the angle of attack of the A flow passage increase more, thus causing the gas flow separation in the A flow passage. In turn, it makes the angle of attack of the B flow passage decrease and eliminates the separation phenomenon, so the separation phenomenon is transferred from the B flow passage to the A flow passage. In this way, the separation zone rotates and moves in the direction opposite to the rotation direction of the impeller, and this phenomenon is called rotating stall.
The diffuser also has rotating stall. During the operation of the compressor, when the flow rate continuously decreases to the Qmin value, the serious rotating stall described above occurs in the compressor flow passage, and the flow deteriorates seriously, causing the compressor outlet exhaust pressure to suddenly drop greatly, lower than the pressure of the condenser. Then the gas will flow back to the compressor until the condensing pressure is lower than the compressor outlet exhaust pressure. At this time, the backflow stops, the compressor displacement increases, and the compressor resumes normal operation.
In fact, the total load of the compressor is very small, which limits the displacement of the compressor. The displacement of the compressor gradually decreases again, and the gas flows back again. Repeating this process, a periodic gas flow oscillation phenomenon occurs in the system, and this phenomenon is called surge.
The compressor reaching the minimum displacement point and generating serious gas flow rotating stall is the internal cause, and the condition of the compressor performance curve and the position of the operating point are the conditions. Only when the internal cause is promoted by the conditions can the unique phenomenon - surge occur.
When the centrifugal chiller is operating at part load, the opening of the compressor guide vanes decreases, and the refrigerant flow rate participating in the cycle decreases. The compressor displacement decreases, and the ability of the impeller to reach the head also decreases. While the cooling water temperature remains unchanged because the cooling tower has not been changed, then rotating stall or surge may occur at this time.
Surge is an inherent characteristic of the speed-type centrifugal compressor. Therefore, for any centrifugal compressor, this phenomenon will occur when the displacement is reduced to a certain limit point. Whether the chiller is operating near the surge point mainly depends on the operating conditions of the unit. At what state the surge occurs can only be determined by testing the machine, that is, by continuously reducing its flow rate, the specific surge point can be measured.
Due to the decrease in the gas flow rate in the compressor impeller flow passage, according to the compressor characteristic curve, its operating condition point moves towards the direction of high compression ratio. At this time, the change of the gas flow direction generates a large positive angle of attack at the impeller inlet, causing a serious "separation phenomenon" of the gas flow on the non-working surface of the impeller blades, increasing the aerodynamic loss, and generating a negative pressure area at the impeller outlet, causing the original positive pressure gas flow in the upper part of the condenser or the volute to "flow back" along the pressure drop direction and return to the impeller, increasing the mixed flow rate in the impeller flow passage and making the impeller resume normal operation.
If the compressor operating condition point still has not escaped from the surge point (area) at this time, the above-mentioned "flow back" of the gas flow will occur again. This periodic back-and-forth pulsation of the gas flow is exactly the fundamental cause of the compressor surge.
II. The Harmfulness of Surge
Surge is an unstable operating state that occurs in the small flow rate and high pressure ratio area of the operating conditions of the centrifugal compressor.
(1) Significantly deteriorate the performance of the compressor, and cause large fluctuations in gas parameters (pressure, displacement).
(2) Increase the noise.
(3) Greatly intensify the vibration of the entire unit. Surge makes the components of the compressor rotor and stator withstand alternating dynamic stresses: the pressure imbalance causes strong vibration, making the unit center shift, the bearing wear, the sealing clearance increase; and even the collision between the rotor and stator components occurs: the dynamic stress of the impeller increases.
(4) The current pulsates.
(5) The pulsation frequency of small refrigerating capacity units is higher than that of large units, but the amplitude is small.
Different from general mechanical vibration, the repeated backflow, discharge, and back-and-forth impact of the gas flow occur at the compressor outlet, causing the main motor to alternately appear full load and no load, and the pointer of the ammeter or the pressure gauge at the compressor outlet to produce large, irregular and strong shaking and jumping. The compressor rotor moves back and forth axially in the machine, accompanied by metallic friction and impact sounds.
III. Anti-surge Measures
1. The Principle of Hot Gas Bypass Surge Protection
Once entering the surge operating condition, adjustment measures should be taken immediately to reduce the outlet pressure or increase the inlet flow rate. From the above mechanism of surge generation, in the centrifugal chiller, the pressure ratio and load are the two major factors affecting the surge. When the load becomes smaller and smaller, and reaches a certain limit point, surge will occur, or when the pressure ratio reaches a certain limit point, surge will occur.
Using hot gas bypass for surge protection is to control the opening or closing of the hot gas bypass through the surge protection line, so that the unit is far away from the surge point and achieves the purpose of protection. A connecting pipe is connected from the condenser to the evaporator. When the operating point reaches the surge protection point but not the surge point, the hot gas bypass solenoid valve is opened through the control system, and the hot gas from the condenser is discharged to the evaporator, reducing the pressure ratio and at the same time increasing the exhaust volume, thus avoiding the occurrence of surge.
2. Changing the Compressor Rotational Speed
When the compressor rotational speed changes, the performance curve of the compressor will move accordingly, which can increase the stable operating condition area. It is suitable for units driven by steam turbines and gas turbines, and is a relatively economical adjustment method, but the working point after adjustment is not necessarily the highest efficiency point. However, for units driven by electric motors, in order to facilitate speed change, a DC unit or a frequency conversion method needs to be used, which will greatly complicate the equipment and also increase the cost.
3. Multi-stage Compression
Multi-stage compression is used to reduce the compressor rotational speed. Generally, the occurrence of surge in any stage of a multi-stage machine will affect the normal operation of the entire machine. By adopting multi-stage compression, under the same pressure ratio operating conditions, the compressor rotational speed can be greatly reduced, and the stable operating condition area can be increased.
4. Using a Rotating Diffuser to Adjust the Flow Rate
When the flow rate decreases, serious rotating stall usually occurs first in the diffuser, leading to surge. When the flow rate changes, if the inlet geometric angle of the diffuser flow passage can be correspondingly changed to adapt to the changed operating conditions, so that the angle of attack α is not too large, the performance curve can be greatly shifted towards the small flow rate area, the stable operating condition range can be expanded, the surge flow rate can be greatly reduced, and the purpose of anti-surge can be achieved. This anti-surge control method has been specifically applied in Carrier's products, but hot gas bypass still needs to be used at low load.
5. Movable Diffusion Cavity
As mentioned above, the reasons for the occurrence of surge in the centrifugal chiller are the pressure ratio and load. When the pressure ratio (lifting force) of the unit operation is constant, the operating load of the unit will affect whether the unit surges. For the centrifugal unit, when the operating load decreases, the guide vanes of the compressor gradually close, and the suction volume decreases. If the passage area of the diffusion cavity remains unchanged, the gas flow rate decreases: when the gas flow rate cannot overcome the resistance loss of the diffusion cavity, the gas flow will stagnate. Due to the decrease in the gas kinetic energy, the converted pressure energy also decreases: when the gas fluid pressure is lower than the pressure of the exhaust pipe network, the gas flow flows back and surge occurs.
IV. Conclusion
Hot gas bypass, changing the compressor rotational speed, multi-stage compression, adjusting with a rotating diffuser, and the design of the diffuser slider can all effectively avoid "surge" and have a good energy-saving effect on the centrifugal chiller.
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