Centrifugal fans play a very important role in many industries, but when the centrifugal fans of many companies are used, the air volume is often large and small, the partial pressure is high and low, the motor current fluctuates greatly, and there are cycles. Sexual vibration has a great impact on the production of enterprises. In fact, this phenomenon is the surge of the centrifugal fan.
1. What is the surge of a centrifugal fan?
Surge of centrifugal fan is actually caused by the reduction of wind pressure at the outlet of the fan caused by the stalled airflow. At this time, the wind pressure in the air duct is greater than the wind pressure at the outlet of the fan, causing the air flow to flow back. When the wind pressure at the outlet of the fan is greater than the air duct pressure At the same time, the fan will supply air to the air duct again, so that the air flow will flow back and forth, the fan and the pipeline will generate strong vibration, the noise will increase significantly, and there may also be large fluctuations in flow, total pressure and current.
2. The hazard of surge in centrifugal fans
When the centrifugal fan surges, due to the strong pulsation and periodic oscillation of the airflow, the air supply parameters (pressure, flow, etc.) will fluctuate greatly, destroying the stability of the process system. The greater the exhaust pressure and the density of the gas, the more severe the surge and the greater the harm:
1. The blade will vibrate strongly, the impeller stress will be greatly increased, and the noise will be aggravated.
2. The friction and collision of the dynamic and static parts are caused, and the shaft of the fan is bent and deformed. In severe cases, axial movement occurs and the impeller is damaged.
3. Aggravate the wear of bearings and journals, destroy the stability of the lubricating oil film, cause fatigue cracks in the bearing alloy, and even burn out.
4. Damage the interstage seal and shaft seal of the centrifugal fan, reduce the efficiency of the fan, and even cause explosions, fires and other accidents.
5. It affects the normal operation of other equipment connected to the centrifugal fan, interferes with the normal work of the operator, and reduces the accuracy of some measuring instruments or even fails.
3. Reasons for the surge of centrifugal fans
The root cause of centrifugal fan surge is that the intake air volume decreases and reaches the allowable lower limit of the fan. Theory and practice have proved that various factors that can make the operating point of the centrifugal fan fall into the surge area are the reasons for the surge:
1. The intake air temperature rises and the air density decreases, and surges are more likely to occur in summer than in winter.
2. The inlet pressure drops, such as the inlet filter is blocked or the suction negative pressure value is high.
3. The pressure of the pipe network of the outlet system increases, that is, the poor exhaust causes the outlet to block and surge.
4. The working pressure value of the centrifugal fan outlet is set at the edge of the surge area.
5. Surge is prone to occur when the rotational speed of the centrifugal fan is reduced.
4. Measures to prevent the centrifugal fan from surging
1. Avoid arbitrarily increasing the coefficient when selecting a model
The selection of centrifugal fans should be in the high-efficiency area, and do not arbitrarily increase the selection coefficient. If the actual fan flow is much higher than the designed fan flow during operation, it is easy to adjust the fan to Surge area work. For this reason, in the design and selection of centrifugal fans, it is necessary to avoid the working condition range approaching the surge area.
2. Change the performance curve
Once the centrifugal fan surges during the operation of the system, the operating range of the working condition can be adjusted by changing the speed of the fan, and the installation angle of the rotor blade can also be changed. These methods can move the performance curve of the centrifugal fan to the small flow area. At this time, the surge critical line also moves to the small flow area accordingly, which can also expand the stable working condition range of the fan.
3. Add air release valve
Since some workplaces do not have the above adjustment conditions, the shutdown cannot affect the production, but there is a simple and quick adjustment method to eliminate the surge, that is to add a vent valve. Make qv=qv put +qv surge > qvmin, the surge can be eliminated immediately. It must be noted that the gas put into the atmosphere should be harmless gas. The disadvantage of this method is that part of the kinetic energy obtained by the impeller is released in vain, which reduces the overall efficiency of the fan. But because of its simple method and remarkable effect, it is widely used in fan regulation.
4. Add anti-surge ring
At present, in the centrifugal fans used in subways or tunnels, it has become a common trend to add anti-surge rings to the main fan. Instead, the air mass is reversed along the guide vane and flows back to the blade. The disadvantage is that in this case, part of the airflow does useless work, but it can expand the stable working range of the centrifugal fan and reduce the surge area.