After reading this article, completely get rid of the phenomenon of centrifugal refrigeration compressor surge

After reading this article, completely get rid of the phenomenon of centrifugal refrigeration compressor surge

Centrifugal refrigeration compressor is a speed compressor, which is a kind of impeller rotating machinery. It relies on the high-speed rotating impeller to do work on the gas to increase the gas pressure.

　　 Features of centrifugal refrigeration compressor:

　　 (1) Small size, light weight and small footprint.

　　 (2) Good dynamic balance characteristics and low vibration.

　　 (3) Few wear parts and long continuous operation cycle.

　　 (4) High heat transfer performance.

　　(5) It is easy to realize multi-stage compression and throttling, and realize various evaporation temperatures.

　　(6) Stepless adjustment can be carried out economically.

　　(7) If the economical industrial steam turbine is directly driven, the energy saving effect will be better.

　　(8) The speed is high, and the requirements for shaft end sealing are high.

　　 (9) When the condensing pressure is high, surge will occur.

　　 (10) When the cooling capacity is small, the efficiency is low.

　　 1. The mechanism of surge

　　 The basic working principle of centrifugal compressor is to use the high-speed rotating impeller to do work on the gas, add mechanical energy to the gas, so that the gas pressure increases, the speed increases, and the gas obtains pressure energy and velocity energy. A diffuser element with a gradually expanding flow area is installed behind the impeller. After the high-pressure gas flows out of the impeller, it flows through the diffuser for speed reduction and expansion, so that the gas flow rate is reduced, and the pressure continues to rise, that is, part of the gas velocity The energy is transformed into pressure energy, completing the compression process.

The boundary layer separation phenomenon in the diffuser flow channel: the flow of the air flow in the diffuser flow channel is converted into kinetic energy from the work done by the impeller on the air flow. The air flow in the boundary layer mainly depends on the kinetic energy transferred from the main flow. When the air flows, it must overcome the friction of the wall. As the speed decreases along the direction of the flow channel and the pressure increases, the kinetic energy of the mainstream also decreases.

　　 When the kinetic energy transmitted to the boundary layer by the mainstream is not enough to overcome the pressure difference and continue to move forward, the airflow in the boundary layer finally stagnates, and then vortexes and backflows occur, which separates the airflow boundary layer. The flow of gas in the impeller is also a diffusion flow, and this boundary layer separation phenomenon will also occur when the flow rate decreases or the pressure difference increases.

When the gas flow in the flow channel decreases to a certain value, the direction of the inlet air flow of the blade channel is very inconsistent with the blade inlet angle, and the angle of attack α is greatly increased, causing serious separation of the air flow boundary layer in the flow channel on the non-working surface, causing the inlet and outlet of the flow channel A strong air pulsation appears.

When the flow is greatly reduced, due to the unevenness of the air flow and the unevenness of the runner profile, it is assumed that the phenomenon of air separation occurs in the B runner, so that the effective flow area of the B runner is reduced, making the original flow A part of the air flow passing through the B flow channel will flow to the adjacent A flow channel and the C flow channel, which changes the original air flow direction of the A flow channel and the C flow channel, which reduces the attack angle of the C flow channel. The angle of attack of the A channel is increased, so that the air flow in the A channel is separated, and the attack angle of the B channel is reduced to eliminate the separation phenomenon, so the separation phenomenon is transferred from the B channel to the A channel. In this way, the separation zone rotates and moves in the direction opposite to the direction of rotation of the impeller. This phenomenon is called rotational separation.

　　 The diffuser also has a spin-off. During the operation of the compressor, when the flow rate is continuously reduced to the Qmin value, the severe rotation separation as described above occurs in the compressor flow path, and the flow is severely deteriorated, causing the compressor outlet discharge pressure to drop suddenly and greatly, which is lower than the condenser The airflow will flow backward to the compressor until the condensing pressure is lower than the discharge pressure of the compressor outlet. At this time, the reverse flow stops, the displacement of the compressor 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 is gradually reduced, and the gas flows back again. Repeatedly, periodic air flow oscillations are generated in the system. This phenomenon is called surge.

The compressor reaches the minimum displacement point and the severe airflow rotation separation is the internal cause, and the compressor performance curve status and the position of the operating point are the conditions. The internal cause can only occur when the conditions are promoted, the unique phenomenon-surge .

　　 When the centrifugal water chiller is running under partial load, the opening of the compressor guide vane is reduced, and the flow of refrigerant participating in the cycle is reduced. As the compressor displacement decreases, the ability of the impeller to reach the pressure head also decreases. As the cooling water temperature remains unchanged because the cooling tower has not changed, 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 occurs when the displacement is small to a certain limit point. Whether the chiller is operating near the surge point depends mainly on the operating conditions of the chiller. When the surge occurs, the specific surge point can only be measured by testing the machine, that is, continuously reducing its flow.

　　 Due to the decrease of the gas flow in the compressor impeller flow passage, according to the compressor characteristic curve, the operating point of its operation leads to the direction of high compression ratio. At this time, the change of the airflow direction produces a larger positive angle of attack at the impeller inlet, causing serious airflow "detachment" on the non-working surface of the impeller blades, increasing aerodynamic losses, and creating a negative pressure zone at the impeller outlet, causing the condenser The original positive pressure airflow in the upper part or in the volute "backflows" in the direction of pressure drop and returns to the impeller to increase the mixing flow in the impeller channel and the impeller to resume normal operation.

　　At this time, the compressor operating point has not deviated from the surge point (zone), and the above-mentioned airflow "backflow" will occur again. This periodic back and forth pulsation of airflow is the root cause of compressor surge.

　　 2. The danger of surge

　　 Surge is an unstable operating state caused by the operating conditions of the centrifugal compressor in the region of small flow and high pressure ratio. When the compressor surges, there will be periodic oscillations of the airflow. Surge brings serious damage to the compressor, which will cause the following serious consequences:

　　(1) The performance of the compressor is significantly deteriorated, and the gas parameters (pressure, displacement) have a large pulsation.

　　 (2) The noise increases.

　　(3) Greatly intensify the vibration of the entire unit. Surge causes the compressor rotor and stator components to undergo alternating dynamic stress: pressure imbalance causes strong vibration, which causes the center of the unit to shift, bearings wear, and seal gaps increase; even the rotor and stator components collide, etc.: impeller Dynamic stress increases.

　　 (4) The current is pulsating.

　　 (5) The pulsation frequency of the small cooling capacity unit is higher than that of the large unit, but the amplitude is smaller.

Different from the general mechanical vibration, the airflow at the outlet of the compressor is repeatedly inverted, spit out, and hit back and forth, which makes the main motor alternately full and no-load, and the ammeter pointer or the compressor outlet pressure gauge pointer produces a large and irregular strong vibration And beating. The compressor rotor moves back and forth along the axial direction in the machine, accompanied by metal friction and impact sound.

　　 Three, anti-surge measures

　　 1. The principle of hot gas bypass surge protection

　　 Once the surge condition is entered, adjustment measures should be taken immediately to reduce the outlet pressure or increase the inlet flow. From the above mechanism of surge, in centrifugal chillers, pressure ratio and load are the two major factors that affect surge. When the load gets smaller and smaller, when it 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 to keep the unit away from the surge point to achieve 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 Pressure ratio, while increasing the displacement, thus avoiding the occurrence of surge.

　　2, change the compressor speed

The compressor's performance curve will move with the change of compressor speed, which can increase the stable working condition area. It is suitable for steam turbines and gas turbines driven units. It is a relatively economical adjustment method, but the adjusted operating point is not Must be the highest efficiency point. However, for the motor-driven unit, in order to facilitate the speed change, it is necessary to use the DC unit or the frequency conversion method, which will greatly complicate the equipment, and at the same time, the cost is also high.

　　3, multi-stage compression

　　Multi-stage compression to reduce compressor speed. Generally, a surge in any one level of a multi-level machine will affect the normal operation of the entire machine. Using multi-stage compression, under the same pressure ratio operating conditions, the speed of the compressor can be greatly reduced and the stable operating condition area can be enlarged.

　　4、Adopt rotating diffuser to adjust

　　 When the flow rate decreases, the diffuser generally produces severe spin-off first, causing surge. When the flow changes, if the inlet geometric angle of the diffuser channel can be changed accordingly to adapt to the changed working conditions, so that the angle of attack α is not too large, the performance curve can be moved to the small flow area by a large margin, and the expansion and stability The working condition range greatly reduces the surge flow rate and achieves the purpose of anti-surge. The anti-surge control method has been specifically applied in Carrier's products, but hot air bypass is still required when the load is low.

　　5, movable diffuser cavity

　　 As mentioned above, the causes of surge in centrifugal chillers are pressure ratio and load. When the operating pressure ratio of the unit is constant (lifting force), the operating load of the unit will affect whether the unit surges. For a centrifugal unit, when the operating load decreases, the guide vanes of the compressor gradually close, and the suction volume decreases. If the channel area of ​​the diffuser cavity remains unchanged, the gas flow rate decreases: when the gas flow rate cannot overcome the diffuser cavity When the resistance is lost, the airflow will stagnate. Due to the drop of the kinetic energy of the gas, the transformed pressure energy is also reduced: when the gas fluid pressure is less than the pressure of the exhaust pipe network, the airflow will reverse and surge.

　　 IV. Conclusion

　　Hot gas bypass, changing compressor speed, multi-stage compression, rotating diffuser adjustment, and diffuser slider design can effectively avoid "surge", which has good energy-saving effects for centrifugal chillers.