I. Introduction
With the continuous upgrading of screw compressors, their performance has been continuously improved. Improving the vibration and noise of compressors has gradually become a new challenge that screw compressor technology development needs to face. At the same time, it has also become an important "selling point" for manufacturers to enhance their product competitiveness, especially in some special applications where there are extremely stringent requirements for the vibration and noise of screw compressors and their systems.
Furthermore, the vibration and noise issues of screw compressors not only cause noise pollution but also affect machine performance and reliability. Therefore, vibration reduction and noise reduction technology for screw compressors is gradually becoming a core technology, and low vibration and low noise are a major trend in the future development of screw compressors.
II. Vibration Noise Generation Mechanism
Figure 1 shows a typical structure of a twin-screw compressor, which mainly consists of a housing and a pair of parallel helical rotors and intake/exhaust ports contained within the housing. The compressor and electric motor are encapsulated in the same housing, with the electric motor coaxial with the male rotor. Driven by the electric motor, the male and female rotors mesh and rotate like gears, and the working volume enclosed by the rotor tooth tips and the inner wall of the housing periodically expands and contracts, realizing the intake, compression, and exhaust processes.
Based on the working principle of screw compressors, the vibration and noise generation mechanism of screw compressors can be divided into mechanical vibration and noise generated by mechanical contact and fluid vibration and noise induced by airflow pulsation.
2.1 Mechanical vibration and noise
Mechanical noise is generated by the vibration of solids. When mechanical parts are running, under the action of impact, friction, alternating stress, or magnetic stress, the parts collide, rub, and vibrate with each other, thus producing sound. In screw compressors, the mechanical vibration noise source comes from rotating parts, mainly the meshing rotors and supporting bearings. In particular, the vibration noise generated during the meshing of male and female rotors is the main source of mechanical vibration noise in screw compressors.
① Meshing rotor vibration noise
The male and female rotors are the core components of a screw compressor. During operation, they are subjected to radial and axial gas forces, as well as forces from the transmission mechanism and bearing support. These forces change periodically during the compressor's operation, serving as the excitation source for mechanical vibration and noise. In a screw compressor, the male rotor directly drives the female rotor to rotate synchronously through tooth surface contact. Mechanical vibration and radiated noise are unavoidable during this meshing process, making it the primary source of mechanical vibration and noise. In actual operation, because the rotors are metal components and inherently flexible, misalignment or imbalance caused by machining or assembly errors often leads to radial vibration during rotation, generating abnormal noises, which can also become excitation sources for vibration and noise during the movement of the male and female rotors.
② Support bearing vibration noise
The bearings used in screw compressors are mainly divided into sliding bearings and rolling bearings. The vibration of sliding bearings is mainly caused by insufficient lubrication or abnormal friction, which causes the oil film to break and leads to "stick-slip" vibration between metals; the vibration of rolling bearings is mainly caused by the periodic impact of discrete rolling elements on the raceway [4]. In comparison, the vibration noise of rolling bearings is greater than that of sliding bearings, but rolling bearings can provide precise operating accuracy and withstand higher speeds. Therefore, rolling bearings are mainly used in screw compressors to bear axial and radial forces, while sliding bearings are generally only used in some large screw compressors.
With the machining accuracy and assembly error of mechanical parts effectively controlled, the mechanical vibration and noise of screw compressors are effectively controlled. On the contrary, fluid vibration and noise gradually emerge and become the main source of vibration and noise.
2.2 Fluid vibration noise
Fluid dynamic noise refers to the noise generated by fluid vibration caused by fluid flow or solid movement in a fluid. With in-depth research on mechanical vibration noise and the improvement of machining and assembly precision, mechanical vibration noise has been effectively controlled. However, fluid noise induced by airflow pulsation has become the main noise source of screw compressors. According to its location and characteristics, it can be divided into inter-tooth volumetric noise, exhaust noise, and intake noise.
① Interdental volumetric noise
When the screw compressor is in the state after intake and before exhaust, the inter-tooth volume is not connected to the intake or exhaust ports. During this process, the only communication channels between the inter-tooth volume and the outside world are the leakage triangle, tooth tip clearance, meshing clearance, and end face clearance. The gas medium within the inter-tooth volume is continuously compressed as the volume decreases. Simultaneously, a small portion of the medium enters adjacent inter-tooth volumes or the intake-side inter-tooth volume through the aforementioned leakage channels. This process generates not only fluid flow noise but also noise from the gas flow through the gaps under pressure differential. When the inter-tooth volume is connected to the oil, liquid, or air injection ports, the additional gas-liquid flow can lead to even more intense flow noise. The leakage triangle has a relatively large area compared to other leakage channels. Furthermore, the leakage triangle connects two inter-tooth volumes with unequal pressures. These two relatively independent acoustic elements are also subject to external excitation and resonance, resulting in even greater hydrodynamic noise.
②Exhaust noise
In the initial stage after the rotor meshing chamber connects with the exhaust port, the high-pressure gas in the exhaust chamber quickly flows back into the meshing chamber under the influence of the pressure difference, causing a rapid increase in the pressure inside the chamber. Under the action of inertial force, overshoot occurs, making the pressure in the meshing chamber greater than the exhaust pressure, while the pressure in the exhaust chamber is at a low point. As the opening of the exhaust port increases rapidly and the exhaust volume decreases, gas begins to flow into the exhaust chamber. At this time, the changes in the gas velocity flowing into the exhaust chamber and the gas pressure in the exhaust chamber are relatively stable, mainly affected by the rate of change of exhaust volume and the flow area of the orifice.
During the exhaust process, the rotor meshing chamber exhausts gas successively, resulting in periodic changes in volume. Within each cycle, the speed and pressure also change periodically under various forces, forming exhaust gas flow pulsation and inducing aerodynamic noise.
③ Inhalation noise
Intake noise and exhaust noise have certain similarities. During the process of the working volume being connected to the intake and exhaust ports, the working volume increases or decreases periodically, accompanied by periodic changes in the connection area between the working volume and the intake and exhaust ports. This causes drastic changes in fluid flow characteristics, generating large airflow pulsations and inducing aerodynamic noise.
Sangfors et al. conducted extensive research on identifying the main vibration and noise sources of screw compressors, all pointing out that vibration and noise values at the fundamental frequency of airflow pulsation and its integer multiples are relatively large. Airflow pulsation caused by the periodic connection between the working volume and the intake and exhaust ports is the main cause of vibration and noise in screw compressors. Furthermore, because the gas density in the exhaust chamber is much greater than that in the intake chamber, the aerodynamic noise induced by exhaust airflow pulsation is more significant.
III. Vibration Control Technology
During operation, the male and female rotors of a screw compressor mesh with each other, generating mechanical vibrations that are transmitted to the casing and mounting feet via bearings. Therefore, improving rotor machining accuracy, reducing shaft assembly errors, and optimizing bearing clearance can suppress screw compressor vibration at its source. Furthermore, designing and installing vibration damping pads can further isolate vibration transmission along its path, thereby reducing screw compressor vibration.
3.1 Vibration Excitation Source Reduction
① Improve machining accuracy and reduce assembly errors
Improving rotor machining accuracy, reducing rotor surface roughness, and improving assembly processes to reduce shaft assembly errors are all measures that reduce mechanical vibration generated during rotor meshing. By controlling the vibration excitation source of the compressor at its source, mechanical vibration generated during compressor operation can be effectively reduced. Experimental studies by Jin Chunmei et al. have shown that improving rotor machining accuracy, changing from milling to grinding, reduces surface roughness, effectively controls vibration during compressor operation, and also reduces medium- and high-frequency noise to a certain extent.
② Reduce the clearance of the support bearing
Reducing the clearance of the support bearing can improve the rotational accuracy of the rotor, reduce the eccentricity during rotor meshing, and reduce the vibration noise induced by the unbalanced mass of the rotor during high-speed operation. Yin Yufeng et al. [7] found through theoretical and experimental research that the radial clearance of the rolling bearing has the most significant effect on the vibration noise of the bearing. As the radial clearance increases, the vibration noise increases accordingly and shows a good linear relationship.
3.2 Vibration transmission path isolation
① Improve the stiffness of structural components
Increasing casing rigidity reduces casing vibration response. Adding reinforcing ribs in the radial and circumferential directions of the screw compressor casing can increase casing rigidity, reduce casing vibration response, and prevent the transmission of vibration excitation from the compressor rotor and bearings to the casing.
②Design and install vibration damping pads
Design and install vibration damping pads to isolate the vibration transmission of the screw compressor. Based on the rotor profile of the screw compressor, the motor operating speed, its own weight, and actual vibration reduction requirements, design vibration dampers and install them on the screw compressor feet. This can prevent the vibration of the feet from being transmitted to the mounting surface, effectively reducing the vibration on the compressor mounting foundation.
IV. Noise Control Technology
The periodic intake, compression, and exhaust processes of screw compressors inevitably generate airflow pulsations, which in turn induce aerodynamic noise. While mechanical vibration noise has been effectively controlled thanks to in-depth research and improved machining and assembly precision, fluid noise induced by airflow pulsations has become the primary noise source for screw compressors. Therefore, measures such as applying exhaust end-face attenuation devices, Helmholtz airflow pulsation attenuation chambers, and perforated plate pulsation attenuators can attenuate the amplitude of airflow pulsations at their source, reducing the aerodynamic noise induced by airflow pulsations. Designing a casing with a double-wall structure and installing soundproof enclosures can also impede noise transmission along the transmission path, effectively reducing the overall noise of the compressor.
4.1 Source control of aerodynamic noise induced by airflow pulsation
① Exhaust end face airflow pulsation attenuation device
Based on the principle of acoustic interference, a side channel is designed to generate side airflow pulsations with the same amplitude but opposite phase as the airflow pulsations inside the main pipe. The two pulsations are superimposed and cancel each other out, thereby achieving the purpose of attenuating the airflow pulsations. Its structural schematic diagram is shown in Figure 2. When the length of the side channel is an integer multiple of half the wavelength of the fluid medium, the amplitude of the airflow pulsations in the exhaust pipe is minimized, and its attenuation effect is shown in Figure 3.
Based on the principle of half-wavelength tubes, Zhou Minglong et al. designed an airflow pulsation attenuation device on the exhaust end face, taking into account the periodic characteristics of airflow pulsation in screw compressors and the internal space of the compressor structure. This device attenuates the amplitude of airflow pulsation at the exhaust source, reducing the aerodynamic noise induced by airflow pulsation. Figure 4 shows a specific structure applied to the exhaust end face of a screw compressor.
② Helmholtz airflow pulsation attenuation chamber
The Helmholtz resonator is a common noise reduction device in acoustics. It mainly consists of a short tube and a cavity, as shown in Figure 5. Under certain conditions, it can be used to reduce the amplitude of airflow pulsation in the exhaust chamber of a screw compressor.
The natural frequency fr of the Helmholtz resonator can be calculated based on its structural dimensions.
Where c is the velocity of sound in the fluid medium;
S—Cross-sectional area of the short pipe;
L—Effect length of the short tube;
V—Cavity volume.
When the frequency of the incident sound wave pi is close to the natural frequency of the Helmholtz resonator, strong vibrations are generated in the short tube of the Helmholtz resonator, which consumes sound energy by overcoming frictional resistance, thereby reducing the amplitude of the downstream sound wave.
Based on the principle of Helmholtz resonators, Wu Xiaokun et al. designed a Helmholtz airflow pulsation attenuation chamber on the exhaust bearing housing of a screw compressor. The amplitude of exhaust airflow pulsation is attenuated by more than 30%, and the vibration acceleration of the machine foot under the fundamental frequency of airflow pulsation can be reduced by 36.2%-40.9%.
③ Perforated plate pulsation attenuator
The structure of the perforated plate pulsation attenuator is shown in Figure 6. Its pulsation attenuation mechanism is that the system composed of each perforation on the perforated plate and its corresponding cavity is similar to the Helmholtz airflow pulsation attenuation cavity. The perforated plate pulsation attenuator can be regarded as a parallel connection of many Helmholtz airflow pulsation attenuation cavities.
According to Academician Ma Dayou's classic theory, the attenuation frequency fMPA of the perforated plate pulsating attenuator can be expressed as:
Where c is the velocity of sound in the fluid medium;
t—thickness of the perforated plate;
d—perforation diameter;
D—Depth of the perforated plate cavity;
P—Perforation rate (perforation area/total area 100%).
Based on the design principle of perforated plates, Liu Hua et al. [15] designed a broadband perforated plate airflow pulsation attenuator for aerodynamic noise induced by exhaust airflow pulsation in variable frequency screw compressors. After applying the airflow pulsation attenuator, the exhaust noise value of the airflow pulsation fundamental frequency decreased by more than 3.0 dBA in the operating speed range of 3000 to 4500 rpm; in the high-speed operating range of 4500 to 5100 rpm where the exhaust noise is relatively large, the exhaust noise value decreased by 5.0 dBA to 7.5 dBA, realizing noise reduction in the entire frequency range of variable frequency screw compressors.
4.2 Noise transmission path sound insulation
① Double-wall design of the casing
The screw compressor casing employs a double-wall structure, which can impede the transmission of vibration and noise, reducing the overall noise level of the compressor. Companies like Gree and Dalian Refrigeration use double-wall structures for their compressor casings to reduce the outward radiation of noise, effectively isolating it. Furthermore, the use of liquid cooling methods (such as oil cooling or water cooling) not only hinders noise transmission but also eliminates the need for a fan, further contributing to lowering the overall noise level of the screw compressor.
② Soundproof enclosure design
Based on the noise spectrum characteristics of the screw compressor, the design of the soundproof enclosure structure and the optimization of the sound-absorbing material of the soundproof enclosure can effectively reduce the far-field noise of the compressor. Cheng Shuangling et al. [16] reduced the noise of the screw compressor by nearly 10 dBA by optimizing the soundproof enclosure structure and sound-absorbing material.
4.3 Attenuation of airflow pulsation
At present, airflow pulsation attenuation and suppression are mainly aimed at specific operating conditions. When the operating conditions of the compressor change significantly, especially the variable speed operating conditions of the variable frequency screw compressor, the attenuation effect of the airflow pulsation attenuation device weakens or even disappears. In order to meet the airflow pulsation attenuation effect under different operating conditions and broaden the airflow pulsation attenuation frequency range, multiple attenuation devices are often passively connected in parallel or in series. This not only sacrifices the attenuation effect, but also leads to the attenuation device being too large to install or even passively increasing the compressor volume. Therefore, it is urgent to automatically adjust the attenuation frequency of the airflow pulsation attenuation device according to the operating conditions and airflow pulsation characteristics of the compressor and effectively reduce the airflow pulsation attenuation device of the screw compressor. A frequency-adaptive airflow pulsation attenuator designed by Zhou Minglong et al. [17] based on the operating characteristics of the compressor will become a new trend.
4.4 Active vibration reduction
During the operation of a screw compressor, the detected compressor vibration signals are processed in real time. Through a specific control strategy, the actuators are driven to apply external excitation (such as force or torque) to the compressor, ultimately suppressing compressor vibration and reducing mechanical vibration radiation noise. Currently, active vibration reduction technology in China is still in the mechanism research stage and is far from practical application. However, based on the promising development prospects of active vibration reduction technology and in-depth research into the vibration generation mechanism of screw compressors, active vibration reduction technology will gradually be applied to the field of screw compressor vibration reduction and noise reduction.
4.5 Active noise reduction
Active noise reduction utilizes the principle of destructive interference of sound waves. It introduces an electroacoustic device to generate an additional noise source that is superimposed on the unwanted original noise, thereby reducing or suppressing the noise.
Active noise cancellation has excellent low-frequency noise reduction effects and is most suitable for controlling low-frequency harmonic noise. Currently, it is mainly used in headphones and automobiles. With the development of active technology and in-depth research on screw compressor noise, active noise cancellation will gradually be applied to the field of screw compressor noise reduction.
V. Development Trends of Vibration and Noise
5.1 Rotor Material
① Rotor material replacement. With the improvement of the performance of non-metallic materials and the increase in processing precision, their excellent vibration reduction and noise reduction performance has gradually become apparent. Under the condition of meeting the usage requirements, the male rotor of the screw compressor can adopt a structure of injection-molded non-metallic material on a metal steel core, while the female rotor adopts a metal material, thereby reducing the mechanical vibration and noise generated during the meshing of the male and female rotors.
② Rotor surface treatment. A self-lubricating sealing coating is sprayed onto the rotor surface. On the one hand, the sealing properties of the coating can reduce the meshing clearance between rotors and reduce the hydrodynamic noise induced by fluid flow in the leakage channels between the teeth. On the other hand, the self-lubricating properties of the coating can reduce the friction coefficient of rotor meshing and reduce the mechanical vibration noise generated during rotor meshing.
5.2 Rotor Profile
① Increase the number of rotor teeth. Increasing the number of rotor teeth in a screw compressor increases the overlap coefficient during rotor meshing, allowing the meshing load to be evenly distributed across more tooth surfaces, reducing the pressure per unit tooth surface, and lowering the mechanical vibration noise generated during rotor meshing. Furthermore, with fewer rotor teeth, the rotor meshing frequency is low, resulting in longer wavelengths of low-frequency noise with strong diffraction capabilities and longer propagation distances, making low-frequency noise control more difficult. Conversely, with an increased number of rotor teeth, the rotor meshing frequency shifts to higher frequencies, making it easier to absorb and attenuate during propagation, thus making high-frequency noise easier to control and resulting in lower far-field noise levels for the compressor.
② Optimize tooth profile design. Based on theoretical and experimental research, optimize the rotor tooth profile design, such as increasing the torsion angle to increase the overlap coefficient, increasing the meshing line length to reduce the load per unit meshing line, etc., to reduce the tooth surface contact force during rotor rotation, reduce the mechanical vibration and noise generated during rotor meshing, and make the rotor vibration smooth and noise stable during operation.
VI. Conclusion
This paper comprehensively introduces the mechanism of vibration and noise generation in screw compressors and corresponding control measures. Xi'an Jiaotong University has been committed to the research of screw compressors. Based on thermodynamic and dynamic calculations, rotor profile optimization, oil injection optimization, and exhaust gas flow pulsation research, certain achievements have been made in the study of screw compressor vibration and noise. However, numerous factors influence vibration and noise, and these factors interact and restrict each other, increasing the difficulty of vibration and noise reduction in screw compressors. This results in a certain gap between theoretical calculations and experimental results, and a systematic design theory and method for vibration and noise reduction has not yet been formed for engineering applications. Therefore, further efforts are needed to reduce compressor vibration and noise theoretically and apply it to practice.
