In various production enterprises, centrifugal fans will be used for ventilation or induced draft fans. Generally, when designing and selecting models, the main reference is the wind pressure and efficiency of the fans, but the actual situation is that the output of some fans cannot meet the production requirements, so The number of blades, the length of the blades, and the width of the blades will be increased to meet the fan processing requirements.
1) Reasons for using blade lengthening
Lengthening the blades and making the flow channel smooth and reasonable can effectively improve the static pressure and air volume of the fan. This has been proved by good practice in power plants and some fan users, but it will increase the power consumption of the fan. Therefore, it is necessary to check the fan equipped with the motor. capacity; at the same time, the efficiency of the fan will also be reduced and cause greater noise. According to the test, although the efficiency of the fan has decreased after the transformation, the reduction range is controlled at 10%, which is usually within the acceptable range. Therefore, it is feasible to increase the fan output by lengthening the fan blades.
2) the centrifugal fan blade lengthening method
1. The method of lengthening the leaves
There are two ways to lengthen the centrifugal fan blades, namely lengthening along the inside of the fan inlet and lengthening along the impeller outlet.
(1) Lengthening along the interior of the inlet: The lengthened blade can be extended along the interior of the inlet to change the inlet diameter of the impeller; when it extends inwardly according to the original flow channel, it also changes the inlet installation angle of the blade.
(2) Lengthening along the outlet of the impeller: Extending along the outlet of the impeller, at this time, the outlet installation angle of the impeller is changed, and the outer diameter of the impeller is increased. At this time, another steel plate must be welded to complement the rear disc.
2. The choice of blade lengthening method
Due to the complexity of the process, the method of lengthening along the fan inlet is usually adopted. The blade is lengthened along the inside of the fan inlet to make it a chamfered shape, that is, the diameter of the top of the blade near the front disk is larger than the diameter of the bottom of the blade near the rear disk. At this time, the airflow can better adapt to the unevenness of the flow passage. Because the blades are elongated and chamfered along the inside of the inlet to reduce the deflection of the inlet airflow, the airflow at all points on the edge of the impeller is less impinged and enters the impeller. The advantages of this method are:
(1) When the fan is running under the design conditions, this method can reduce the impact loss of the fan inlet, reduce the wear and blockage of the inlet at the front end of the flow channel.
(2) The airflow adapts to the uniform change from the tip to the bottom of the blade during the turning process, so that the airflow velocity gradient on the inlet flow section tends to be more uniform, reducing the separation loss caused by the internal curvature of the flow channel, and further reducing the The occurrence of vortex and secondary flow at the top of the impeller.
In short, this method makes the air flow into the blade channel relatively well, improves the flow quality of the internal flow field of the fan, and improves the performance of the fan.
3) The influence of the centrifugal fan blade lengthening
1. When the fan impeller blades are lengthened by 5% and 10%, the flow rate at the operating point increases by 4.9% and 10.5% on average, the total pressure increases by 10% and 22.1% on average, and the shaft power increases by 15.7% and 30.2% on average.
2. When the fan impeller blades are lengthened by 5% and 10%, the A sound level is increased by 1.6 and 2.3dB on average. After spectrum analysis, lengthening the impeller blades reduces the distance between the impeller and the volute tongue and increases the rotational noise; at the same time, the flow in the volute deteriorates and the eddy current noise increases. The increase in noise can be improved by corresponding noise reduction measures.
4)Precautions for blade lengthening
1. The blade lengthening technology is mature, the process is simple, the construction time is short, and the site is easy to implement. It is an effective measure to rapidly increase the fan output. The length of the blade should be within 10%, and the method of increasing the outlet angle of the blade can be used to improve the performance.
2. After the blades are lengthened, in addition to checking the power margin of the motor, the strength of the fan should also be analyzed to ensure long-term safe operation.
3. When lengthening the blades of the centrifugal fan, the welding quality should be controlled.
The above is the centrifugal fan manufacturer’s explanation of the centrifugal fan blade lengthening method and its impact on the fan. I hope it can be helpful to everyone. It is not difficult to see that the lengthening of the blades has a negative impact on the efficiency and noise of the fan, but we have to admit that a small sacrifice of efficiency and noise can greatly improve the performance of the fan, so many companies are using the method of lengthening the blades.
