How the noise of Roots blower is generated:
When roots blower is running, it will produce strong noise. According to the mechanism of noise generation, fan noise includes pneumatic noise, mechanical noise and electromagnetic noise; according to the parts of noise generation, there are intake noise, exhaust noise, casing noise, gear noise (using booster or transmission) and motor or turbine noise, etc.
There are two kinds of aerodynamic noise in Roots blower: rotating noise and vortex noise.
The rotating noise of Roots blower is the noise caused by the interaction between the circumferential uneven air flow and the volute when the impeller rotates. The main sound sources are volute, blade and wind tongue. When the impeller rotates, the velocity and pressure of the air flow are not uniform at the outlet of the blade along the circumferential direction. This kind of uneven air flow rushed to the volute before it was flattened, thus forming the pressure pulsation on the volute at any time and becoming a noise source. The stronger the nonuniformity, the greater the noise. The non-uniform exit along the circumference could have been a kind of steady motion, but the existence of the wind tongue has the pressure fluctuation with time, which in turn affects the flow of the impeller, so the air flow on the blade also has the pulsation property with time, so the force on the blade fluctuates with time, making the blade become the second noise source. It, together with the noise source on the wind tongue, is caused by the rotation of the impeller, so it is called rotating noise, also known as discrete frequency noise.
How to control the noise of Roots blower:
Generally, air volume, air pressure, efficiency, power, speed and other indicators are regarded as the main technical parameters of the fan. Strictly speaking, noise should also be listed as the main technical parameters of the fan, which can not be ignored. Therefore, it is very important to control the fan noise.
There are many ways to control the noise of Roots blower:
1、 Reduce the noise of sound source
The most fundamental way to control the fan noise is to reduce the noise of the sound source.
(1) Reduce the excitation force of the sound source.
1. Reduce the impact and eddy noise caused by aerodynamic force. In the design of fans, the first consideration should be given to reducing the noise of fans. Therefore, in terms of aerodynamic performance, it is necessary to minimize the occurrence of air impact noise and eddy current noise. When designing the fan, avoid sharp protruding and sharp turning of the flow passage. Because the fan noise is proportional to the 6th power of the impeller speed, the fan speed should be reasonably selected, and the gap between the impeller and the volute tongue outlet should be controlled. The gap has a great influence on the fan noise. If the gap is too small, the noise will increase rapidly.
2. Improve the dynamic balance accuracy of impeller and shaft of Roots blower to reduce the noise caused by unbalanced weight. If the fan is driven by a gearbox, the dynamic balance accuracy of its gear and coupling shall also be strictly corrected.
3. Maintain good lubrication conditions to reduce friction between components.
4. Pay attention to the good rigidity of the casing to improve its natural frequency.
5. Pay attention to the vibration isolation of the sound source, which can effectively reduce the excitation force of the sound source.
(2) Reduce the response of noise radiation components to excitation force in the system
1. Change the natural frequency of resonance components. The fan designer should note that when the frequency of the excitation force is equal to or close to the natural frequency of a component, the strong resonance of the component and the resulting intense noise will be excited.
2. Adopt excellent damping materials to reduce noise radiation.
3. Use muffler. As the noise level of the air inlet or exhaust port of Roots blower is the largest, the muffler can be designed and installed at the air inlet and exhaust port of the blower, so as to minimize the sound radiation noise at the air inlet and exhaust port of the blower and effectively reduce the noise.
For Roots blower, because of its wide noise frequency band and small allowable pressure loss, it is seldom to use expansion type and resonance type muffler, but to use absorption type muffler to reduce the noise of blower.
(3) Install and use the equipment correctly, use and maintain it reasonably to prevent the fan from making abnormal sound during operation.
2. Noise control of transmission path
The following methods can be used to control the noise in the conveying path of the air inlet and exhaust of the fan.
(1) Sound insulation of sound source
The roots blower uses a properly designed sound insulation cover around the noise source to make the sound source airtight and prevent or reduce the outward propagation of the noise source. In order to prevent or reduce the spread of noise, the fan can also be installed in the sound insulation room with sound absorption performance, and at the same time, a muffler is set at the air inlet to reduce the noise.
(2) Absorption of sound
By using sound absorption principle and sound absorption material, the noise is attenuated continuously in the transmission process. The sound absorption plate can also be set at the outlet of the fan pipe to effectively reduce the noise level. The sound absorption plate can also be installed on the air inlet side of the fan, and the sound absorption effect is good.
(3) Reflection of sound
Based on the reflection principle of sound, the discontinuous structure is adopted to make the sound energy reflect to the sound source, namely the so-called impedance mismatch, so as to block the propagation of noise.