Introduction to Anti-surge Measures for Centrifugal Refrigeration Compressors

The basic working principle of the centrifugal compressor is to use the high-speed rotating impeller to work on the gas, add mechanical energy to the gas, increase the pressure of the negative gas, increase the speed, and obtain the pressure energy and speed energy of the gas. A diffusing element with a gradually expanding flow area is arranged behind the impeller. After the high pressure gas flows out from the impeller, it flows through the diffuser to reduce the pressure and expands the pressure. The negative gas flow rate decreases, and the pressure continues to rise, that is, a part of the velocity of the gas. Can be converted into pressure energy, complete the compression process. The boundary layer separation phenomenon in the diffuser flow passage: the activity of the airflow in the diffuser flow passage, the kinetic energy converted from the work performed by the impeller on the airflow, and the airflow activity in the boundary layer, mainly relying on the kinetic energy transmitted from the mainstream, within the boundary layer When the airflow is active, the friction of the wall surface is overcome. As the velocity decreases along the flow path, the pressure increases and the kinetic energy of the mainstream decreases. When the kinetic energy transmitted from the main stream to the boundary layer is insufficient to advance against the pressure difference, the airflow in the ultimate boundary layer stagnates, and vortex and backflow occur, and the boundary layer of the negative airflow separates. The activity of the gas in the impeller is also a diffusing activity, which occurs when the flow rate is reduced or the pressure difference is increased.

When the gas flow rate in the flow channel is reduced to a certain value, the direction of the inlet flow of the blade channel is inconsistent with the inlet angle of the blade, and the angle of attack α is greatly increased. The boundary layer of the gas flow in the flow channel is seriously separated on the non-working surface, so that the inlet and outlet of the flow channel are severely separated. A strong airflow pulsation occurs. When the flow rate is greatly reduced, due to the unevenness of the airflow activity and the non-uniformity of the flow line profile, it is assumed that the airflow separation occurs in the B flow path, so that the effective flow area of ​​the B flow path is reduced, so that the original flow is required. A part of the air flow passing through the B flow path flows to the adjacent A flow path and the C flow path, thus changing the direction of the original flow path of the A flow path and the C flow path, which reduces the angle of attack of the C flow path. The angle of attack of the A channel is further increased, thereby separating the airflow in the A channel, which in turn reduces the angle of attack of the B channel and eliminates the separation phenomenon, so that the separation phenomenon is transferred from the B channel to the A channel. Thus, the separation zone is rotationally moved in a direction opposite to the direction in which the impeller rotates. This phenomenon is called rotational detachment.

The diffuser also has a rotational detachment. During the operation of the compressor, when the flow rate is continuously reduced to the Qmin value, severe rotation detachment occurs as described above in the compressor flow path, and the activity is seriously deteriorated, so that the compressor outlet pressure suddenly drops greatly, lower than the pressure of the condenser. The airflow flows back to the compressor until the condensing pressure is lower than the compressor outlet pressure. 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 is reversed again. This is repeated, and a periodic airflow oscillation phenomenon is generated in the system. This phenomenon is called surge.

It is an internal cause that the compressor reaches the minimum displacement point and produces serious airflow rotation and disengagement. The performance curve condition of the compressor and the position of the operating point are conditions. The internal cause can only occur under the condition of the condition. Vibration.

When the centrifugal chiller is running at part load, the compressor guide vane opening is reduced, and the refrigerant flow involved in the cycle is reduced. The displacement of the compressor is reduced and the ability of the impeller to reach the indenter is also reduced. While the cooling water temperature remains unchanged because the cooling tower has not changed, then rotational stall or surge may occur at this time.

Surge is an inherent characteristic of a speed centrifugal compressor. Therefore, for any compressor, this phenomenon occurs when the displacement is small to a certain limit point. Whether the chiller is operating around the surge point depends mainly on the operating conditions of the unit. In what state the surge occurs only through the test of the machine, that is, the flow is continuously reduced, and the specific surge point can be measured.

Due to the reduction of the gas flow rate in the compressor impeller flow passage, according to the characteristic curve of the compressor, the operating condition point of the compressor is directed to the direction of high compression ratio. At this time, the change of the airflow direction produces a large positive angle of attack at the impeller inlet, so that the non-working surface on the impeller blade produces a serious airflow "disengagement phenomenon", the aerodynamic loss increases, and a negative pressure zone is generated at the impeller exit, causing the condenser The original positive pressure airflow in the upper part or the volute is “backflowed” in the direction of pressure drop, and is returned to the impeller to increase the mixing flow in the impeller flow passage, and the impeller returns to normal operation.

When the compressor operating point is still not separated from the surge point (zone), the "backflow" of the above airflow will occur. This periodic round-trip pulsation of airflow is the root cause of compressor surge.

2, surge running state

Surge is an unstable operating state of the operating conditions of a centrifugal compressor in a small flow, high pressure ratio region. When the compressor surges, a periodic oscillation of the airflow will occur. Surge can cause serious damage to the compressor, which can lead to the following serious consequences:

1) The performance of the compressor is significantly deteriorated, and the gas parameters (pressure, displacement) are greatly pulsated. 2) The noise is increased. 3) greatly aggravate the vibration of the entire unit. Surge causes the components of the rotor and stator of the compressor to undergo alternating dynamic stresses: pressure imbalance causes strong vibrations, damage to the seals and bearings, and even collisions of the rotor and stator components, etc.: the impeller dynamic stress increases. 4) The current is pulsating. 5) The pulsation frequency of the small cooling unit is higher than that of the large unit, but the amplitude is small.

Different from the general mechanical vibration, repeated backflow, spitting, and round-trip impact of the airflow at the compressor outlet causes the main motor to alternately appear full load and no load. The ammeter pointer or the compressor outlet pressure gauge pointer produces a large and irregular strong shake. And beating. The compressor rotor oscillates back and forth along the axis in the machine, accompanied by metal friction and impact sounds.

3, anti-surge measures

3.1 Hot gas bypass surge protection principle

Once entering the surge condition, immediate adjustments should be taken to reduce the outlet pressure or increase the inlet flow. 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 is getting smaller and smaller, when it reaches a certain limit point, surge will occur, or when the pressure ratio is too large, surge will occur. The use of 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 can avoid the surge point and achieve the purpose of protection. Connected from the condenser to a connecting pipe of the evaporator. When the operating point reaches the surge protection point and does not reach the surge point, the hot gas bypass solenoid valve is opened by the control system, and the hot gas from the condenser is discharged to the evaporator, which is lowered. The pressure ratio, while increasing the amount of exhaust, thus avoiding the occurrence of surge.

3.2 Changing the compressor speed

When the compressor speed changes, the performance curve of the compressor will move along with it, which can increase the stable working condition area. It is suitable for the steam turbine and gas turbine dragging unit. It is a relatively economical adjustment method, but the adjusted working point is not It must be the highest efficiency point. However, in order to facilitate the shifting of the motor, the DC unit or the frequency conversion method is used, which greatly complicates the equipment and at the same time is expensive.

3.3 Multi-level compression

Multi-stage compression to reduce compressor speed. Surge on any level of a multi-stage machine will affect the normal operation of the entire machine. Multi-stage compression can greatly reduce the speed of the compressor and increase the stable working condition under the same pressure ratio.

3.4 Adjusted with a rotating diffuser

When the flow rate is reduced, a severe rotational detachment is generally caused first in the diffuser to cause surge. When the flow rate changes, if the inlet geometry angle of the diffuser flow passage can be changed correspondingly to adapt to the changed working condition, so that the angle of attack α is not large, the performance curve can be greatly moved to the small flow area, and the stability is expanded. The range of working conditions greatly reduces the surge flow and achieves the purpose of anti-surge. The anti-surge control method has been specifically applied in Carrier's products, but hot gas bypass is still required at low load.

3.5 movable expansion chamber

As mentioned above, the surge occurs in the centrifugal chiller due to the pressure ratio and load. When the pressure ratio of the unit operation is constant (lifting force), the operating load of the unit will affect whether the unit has surge. For the centrifugal unit, when the running load is reduced, the guide vanes of the compressor are gradually closed, and the suction amount is reduced. If the passage area of ​​the expansion chamber is constant, the flow rate of the gas is lowered: when the flow rate of the gas cannot overcome the expansion chamber When the resistance is lost, the airflow will be stagnant. Due to the decrease of the kinetic energy of the gas, the pressure energy of the conversion will also decrease: when the pressure of the gas fluid is less than the pressure of the exhaust pipe network, the airflow will reverse and surge will occur.

4, the conclusion

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

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