Surge is a characteristic characteristic of single-stage centrifugal refrigeration compressors, i.e. speed refrigeration compressors. It is manifested in the fact that when the single-stage centrifugal refrigeration compressor is running under low load (less than 25% of the rated load), it is prone to "surge", resulting in periodic increase in noise and vibration, and even damage to the compressor in serious cases.
This may be due to the special structure and operation mode of the single-stage centrifugal refrigeration compressor, as it is a speed-based refrigeration compressor rather than a positive displacement refrigeration compressor (such as reciprocating and rotary refrigeration compressors).
The surge of the centrifuge unit is one of the characteristics of the single-stage centrifugal unit, which is caused by the fact that the discharge pressure of the compressor is less than the pressure of the condenser, resulting in the compressor being unable to exhaust, but the compressor continues to suck air, so that the unit has violent vibration and noise. Generally speaking, "surge" occurs when the load of the unit is less than 30% of the total load of the unit.
Mainly due to the unit operating load is too low, generally speaking, one is the whole system load is too low, and the centrifugal unit must be used when it may appear, can take measures, if the centrifugal unit has been used, it can be set in the computer system to ensure that the minimum operating load of the unit is more than 30% (this is the most stupid way), the best solution is the size of the unit used by the system, that is, to ensure that the minimum load of the whole system is greater than the 30% load of the smallest centrifugal unit used. Or a combination of centrifugal and screw units, which is an inherent characteristic of centrifugal compressors, but shifts after the surge point as the speed changes. This is due to the fact that the discharge pressure of the compressor is less than the pressure of the condenser, resulting in the backflow of pressure and the impact on the impeller. High-speed centrifuges are particularly susceptible to surge (e.g., Carrier, York's single-stage centrifuges), which are generally overcome by hot gas bypass, and by limiting the opening of the guide impeller to limit the increase in condensing pressure.
Since the low-speed multistage centrifuge is generally 2900 rpm due to its low speed, and the surge point is far away from the working point, this model can operate at a low load of 10% when operating (such as Trane's multistage centrifuge) Surge is unique to centrifuges, but not just single-stage, multistage centrifuges will still breathe, Trane three-stage centrifuges will not breathe? The reason for this is that the suction volume is low at low loads, so the discharge pressure may be lower than the condensing pressure, so the condenser gas backflow is repeated. It is characterized by loud sound and large current fluctuations.
Generally, the surge is caused by the low load operation of the unit or the cooling water inlet temperature is too high or the suction pressure is too low, and the elimination method is to eliminate the causes of the above three problems, or to do hot gas bypass surge control, it can be achieved by opening the bypass valve of the compressor or directly venting a part of the gas to maintain the minimum flow rate of the compressor. However, since the passage of gas through the bypass or venting means wasting energy, it is often desirable to determine the surge flow as accurately as possible in order to avoid unnecessary waste in practice. However, determining surge flow is not an easy task. Because it is not a fixed value, but is related to other parameters, it is also necessary to take into account the surge system for other parameters that also have an impact. Therefore, through different measurement methods, a variety of control schemes are formed. Choosing a surge control system for a particular purpose depends on a number of factors, including: the type of compressor; changes in load; The simplicity and reliability of the measuring cell and the accuracy required for the surge control system, etc.
Why does surge occur?
1. First of all, we can be sure that this project must use a centrifuge, so we know that the centrifuge is actually a speed-type model. The compressor mainly obtains kinetic energy by the impeller after the refrigerant gas enters the compressor through the air inlet, and the high-speed gas enters the diffuser pipe after going out of the impeller, the refrigerant speed is reduced, and the pressure is increased, so as to complete the process of compressing the refrigerant.
2. Causes of surge. Mainly with the speed of the refrigerant at the outlet of the impeller (can be divided into radial velocity and tangential velocity, which are related to the impeller,) when the unit runs at partial load, the radial velocity decreases accordingly with the decrease of the load, which causes the reduction of tangential velocity and the angle of the impeller, when the angle is small to a certain value, the gas of the compressor can not be compressed out, and vortex is generated in the impeller, at this time the high-pressure gas in the condenser will flow backwards into the impeller, so that the compressed gas increases in a short time, and after the gas is discharged, it flows back again, and so on, At this point, the compressor enters a surge state.
3. How to solve surge.
A. Improve the design of impeller and diffuser tube. (Mainly done by the product development department)
B. The use of frequency conversion technology is expensive.
C. The hot gas bypass method is adopted, and the energy consumption of part load operation is high, which is not economical.
D. Multi-stage centrifugation technology is adopted.
There are several ways to prevent surge:
1. Product design: imported guide vane + adjustable diffuser linkage control (Meitong has this patent), which can prevent the exhaust pressure from being less than or equal to the condensing pressure due to the decrease of refrigerant flow rate at low load of the centrifuge;
2. System scheme: according to the actual operation habits, select the appropriate size of centrifugal unit + screw unit; Fundamentally avoid the centrifugal unit running at low load! Centrifugal compressors are velocity compression types, which do not directly increase the pressure of the inhaled refrigerant gas, but increase the speed of the inhaled refrigerant gas. The high-velocity refrigerant from the compressor exhaust port passes through the diffuser before entering the condenser, converting the kinetic energy of the refrigerant gas into pressure potential energy (velocity decreases, pressure increases). This is the compression principle of centrifugal compressors.
When the load is reduced to less than 30%, the refrigerant flow rate is reduced; If the shape of the compressor outlet diffuser is not adjustable (relatively large), the refrigerant gas can not be raised to a pressure value higher than the condensing pressure, and the refrigerant circulation pressure difference is insufficient, resulting in violent turbulence of the refrigerant at the compressor outlet, aggravated vibration, and the compressor can be damaged in serious cases! The simple solution, like blocking part of the faucet outlet, will increase the water pressure and the water can shoot farther!