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1Combustion noise source composition and characteristics 1.1 Air compressor noise source composition Air compressor is a multi-source sound generator, the noise is mainly the intake noise noise exhaust noise mechanical noise electromagnetic noise.
1.1.1 Intake noise The intake port intermittently draws in air, generating pressure pulsation and transmitting it into the air to form aerodynamic noise. With the intermittent opening of the air intake valve of the air compressor cylinder, when the airflow is intermittently sucked into the cylinder, pressure fluctuations are generated in the vicinity of the air inlet, and are radiated from the air inlet in the form of sound waves, thereby generating intake noise.
1.1.2 Exhaust Noise When the gas is intermittently discharged from the cylinder valve, the airflow generates noise caused by the disturbance. The high-pressure gas generated by the air compressor enters the gas storage tank through the pipeline, and the pressure pulsation occurs as the displacement of the exhaust gas changes, causing the pipeline to vibrate, and the gas storage tank generates a loud sound and forms noise; when the compressed gas passes through the small hole of the valve , spray at the speed of sound, impact the valve outlet or the valve outlet. Form valve noise. When the air compressor is vented, due to the sudden change of the compressed air pressure, the volume expands rapidly and enters the atmosphere at a high flow rate, thereby generating strong eddy current noise at the exit of the pipeline. Although this noise occurs intermittently, due to its frequency, Received date 20080225 and sound level are relatively high, and the exhaust port is often outside, so it has a great impact on the surrounding environment.
1.1.3 Mechanical noise Mechanical noise is mainly caused by friction and wear and uneven force transmission between mechanisms. When the air compressor is running, many components rotate and reciprocate rapidly, causing frictional shocks and causing noise generated by vibration of the machine. There are mainly piston reciprocating motion and cylinder wall friction, so that the cylinder wall vibrates strongly at the natural frequency; the crank rod connecting rod crosshead and other components also have frictional impact during the movement; the unbalanced rotor of the rotor and its assembly meshes with the impact caused by the rotor speed fluctuation Noise; vibration and noise caused by misalignment of the motor and coupling of the open screw compressor; vibration and noise of the evaporator cooling system of the oil separator of the base of the casing including the casing support structure; sliding stagnation in the sliding bearing Acting, it will excite other parts of the compressor to produce high frequency vibrations.
1.1.4 Electromagnetic noise The noise caused by the magnetic field pulsation of the drive motor. The air compressor drive machine is a synchronous motor. When the motor is running, the fundamental magnetic flux and the higher harmonic flux between the stator and the rotor enter the air gap in the radial direction, generating radial force on the stator and the rotor, thereby causing Radial vibration and noise.
1.2 Air compressor noise characteristics The 5408 type maximum speed is 600, and the air compressor of 1 is also taken as an example for analysis.
1.2.1 Air intake noise The fundamental frequency of the air intake noise of the air compressor; 260 bait is the number of revolutions per minute of the air compressor. In addition to the fundamental frequency, there are also Shen 3 harmonics, but the sound level of the high frequency harmonics is lower than the fundamental frequency level. 1 is the measured air compressor noise and spectrum of the air compressor, its fundamental frequency; 260=20 1 noise is between 100120.
As can be seen from 1, the air inlet noise is a wide-band continuum spectrum with low frequency characteristics. The sound pressure level is gradually reduced from the low frequency to the high frequency, that is, the low frequency strong frequency bandwidth, and the total sound pressure level is high. Intake noise is controlled by the intake noise of the negative intake noise or several measures simultaneously. Noise management based on noise source frequency can be implemented in several ways.
2.1 Air inlet noise control The air intake noise of the air compressor can be controlled by installing a muffler. Because the noise of the air inlet of the air compressor is low frequency, the peak appears in the 31.56312523 frequency band. Below 2001, the acoustic energy accounts for 98, 9,200 or more, and only 12,630 or more. The medium and high frequency noise only accounts for about 0.2 of the total sound energy. . According to the frequency analysis measured by the inlet noise, a resistant muffler should be used. The resistant muffler is a muffler that reflects part of the sound wave back to the direction of the sound source through a sudden change in the acoustic characteristics of the pipe to achieve the purpose of muffling. Its muffling performance is a wide frequency band of low frequency and low intermediate frequency. Mainly used to reduce the noise in the low frequency and low frequency bands. The form includes an expansion type resonant expansion type micro-perforated plate type interference type and the like. Fixed air compressors are usually installed on the indoor floor 5, but some of the air intakes of the air compressor are indoors. When the air inlet of the air compressor is indoors, the noise of the air inlet should be reduced to slightly lower than the noise of the air intake. The noise reduction of the air intake muffler should be about 15 or so; when the air inlet of the air compressor is outside, it should be based on The environmental conditions around the equipment room reduce the noise of the air compressor inlet to the environmental noise standard, and the muffler's noise reduction should be 20dBAWl.
In order to ensure the muffler effect of the muffler, the intake muffler should adopt a resistant muffler with loose materials such as fiber-free and non-foaming plastic, a composite micro-perforated composite muffler or a micro-perforated muffler, etc., and should not be loosened with fiber or foam. The material is used as a sound absorbing material to connect a closed cavity having a diameter of 34 times. The hollow air column and the cavity in the small hole form a common vibration system. At the resonant frequency, the air column vibrates at a high speed, overcomes the friction energy and consumes a large amount of sound energy, forms a resistance resonance, and has a good noise reduction effect.
Some air compressors are equipped with an air filter at the air inlet. The air filter has a certain attenuation of the airflow noise of the air inlet, but it cannot meet the noise reduction requirements. Therefore, it is still necessary to add a noise reduction device. Generally, the filter steel frame can be used to set the muffling hundred pages for muffling, and the muffling hundred pages are made of aluminum alloy plate, mainly for rust prevention; the hundred pages are vertical, which is good for dust prevention; The material uses centrifugal glass wool, which has good sound absorption performance and is hydrophobic material, and is suitable for outdoor use.
2.2 Exhaust port noise control Exhaust pressure The air compressor with high flow rate and high flow rate has high exhaust noise. In the exhaust system, a special muffler is required for control. The exhaust muffler requires a large amount of muffling, a wide muffling frequency band, a decompression expansion, and a reduction in the pressure drop of the exhaust vent to reduce the exhaust noise. For air compressors with flow less than 20m3min, the increase of noise increases, which is also related to factors such as the size governing mechanism of the intake valve and the structure of the valve passage. The noise of the air inlet of the air compressor is 740 higher than that of other components. It is the main noise source of the air compressor.
1.2.2 Exhaust noise The exhaust noise is also a wide-band continuum spectrum. It can be seen from 1 that the noise spectrum is obviously medium-high frequency. 5 The noise frequency is more complicated, and the noise is between 80 and 18. The peak value of exhaust noise is more prominent in 1 , and on the left side of the peak, the sound pressure level of the low frequency noise is weak, and the sound pressure level of the high frequency noise on the right side of the peak is strong. The size of the exhaust noise is related to the flow rate of the compressed gas. The higher the flow rate, the higher the pressure, and the higher the speed, the greater the noise. 1.2.3 Mechanical noise The main cause of mechanical noise is the vibration generated by the imbalance of the rotor system. Therefore, the noise frequency is an integral multiple of the rotational frequency of the rotor. The fundamental frequency is the same as the fundamental frequency of the intake noise, and the frequency spectrum is also low frequency. characteristic. For example, 1 is the measured mechanical noise and spectrum, and noise is used in the frequency 2 air compressor noise control method. The air compressor noise control uses three methods and measures of sound absorption and sound insulation. Sound absorption means that when sound waves are incident on the surface of an object, part of the sound energy is absorbed by the object and converted into other forms of energy to reduce noise; the sound insulation is to close the noise source, and the noise is fixed in a small fixed manner in the inner wall of the air flow channel. In order to achieve the purpose of weakening aerodynamic noise. At present, the structure of air compressor muffler at home and abroad has six kinds of resistant muffler impedance composite muffler micro-perforated plate muffler resistant micro-perforated plate composite muffler venturi muffler and group muffler. In actual work, the sound can be used according to the type of noise source and is mainly high frequency, and a resistive muffler can be used. The advantage of the anechoic muffler is that it can silence the sound in a wide range of medium and high frequencies, especially for high frequency sound waves. Resistive muffler utilizes porous sound absorbing materials of different structures in the airflow duct. The so-called resistive material absorbs sound energy and reduces noise. It is the most widely used muffler in various types of muffler, and has wide muffler. In the frequency range, the muffling performance is particularly significant in the mid-high frequency range. Resistive muffler has tube muffler elbow muffler chamber muffler disc muffler, but for air compressors with high exhaust pressure and high flow rate, due to the large noise level, there are both medium and low frequency noise and high frequency. Noise, relying solely on the resistance of the muffler is not the ideal form of impedance composite, resistive and resonant composite, resistant and micro-perforated plate composite. Impedance composite muffler combines the previous advantages. Although the resistive muffler has excellent mid-high frequency noise cancellation performance, the low frequency noise cancellation performance is poor and difficult to improve, while the expansion and resonance muffler are opposite, and the low intermediate frequency has Good noise reduction performance, high frequency noise reduction effect is poor. If the two muffler principles of resistance and resistance are combined into a muffler, a satisfactory muffling effect can be obtained in a wide frequency range. The muffler has an acoustic filter component such as a cavity expansion chamber perforated screen and a hole sound absorbing material. The noise of the body noise of the air compressor is controlled by different body 2.3 noise control, and the sound isolation and sound absorption control technology is considered. The measure is to install an acoustic enclosure or a soundproofing chamber to prevent the noise from spreading, because the noise and electromechanical noise cannot be controlled by the silencer alone. The noise at the air inlet of the air compressor drops by 2, and the radiated noise of the 5 casing will become the main sound source. For small mobile air compressors, soundproof enclosures are often used to make soundproof rooms.
The elimination of the noise of the gas tank is usually done by hanging the sound absorber.
When the compressed air enters the tank, the sound absorbing body in the tank absorbs the sound energy, thereby achieving the noise reduction effect. Reduce the noise of the exhaust pipe, use the method of pipe wrapping or bury the pipe underground; the outer layer is made of 3 thick plated steel plate, the middle layer is 50mm thick and the weight is 150kg, n3 2.4 control the vibration of the air compressor to control the noise of the air compressor The vibration is controlled, and the vibration of the air compressor is mainly transmitted outward through the foundation and the pipeline system. The vibration control is mainly controlled by vibration isolation. The main measures are as follows using a vibration isolator. The vibration isolator forms an elastic connection between the air compressor and the foundation, reducing the amplitude to achieve vibration isolation, which is the most critical part of vibration isolation.
The meter is suspended with a vibration isolation joint. The vibration isolation joint suspends the foundation to cut off the vibration of the air compressor to the soil. The vibration isolation joint is 15,200 wide and filled with dry sand. The sand and industrial blanket are laid under the foundation. The thickness of the blanket is 2, and the vibration isolation groove is used. In some cases, ground trenching may be used to cut off the vibration of the surface wave along the ground.
3 Air compressor noise management example analysis Take a grain processing plant 5408 air compressor of a grain processing plant as an example. The original suction system uses an orifice plate to eliminate the airflow pressure pulsation and the small hole intake noise elimination scheme; the exhaust venting system uses an air filter and a small hole exhaust muffler to reduce noise. The maximum noise of the system exceeds 10048 纟.
The noise control scheme of the improved air compressor is as follows: 1. The air inlet adopts a composite composite muffler 2, and the outer layer of the muffler is made of a 2-thick steel plate, which is lined with a thin layer of polyurethane foam material, and an inner pipe is inserted in the expansion chamber. Using a perforated tube with a perforation rate of 25, the two inner cannula tubes are connected to form two expansion chambers, and the dynamic characteristics are good, and the sound volume is large; 2 an orifice plate capable of reducing the pressure fluctuation of the gas flow is installed at the inlet of the gas storage tank, and the orifice plate is used. The airflow pulsation is eliminated, the pipeline is not changed, and the cost is low; the exhaust venting port adopts a resistive composite muffler 3, and the impedance composite muffler is formed by inserting an empty pipe in the end of the cylinder resistant muffler, and the resistance is mainly Resistive as the main supplement. The resistive section has the function of reducing pressure and reducing the pressure drop of the exhaust air venting, thereby effectively reducing the low-frequency noise-resistant muffler filled with thick glass wool felt between the inner and outer tubes, and has a good sound-absorbing effect on high-frequency noise. After the above comprehensive method, the noise reduction effect is remarkable, the maximum noise of the air compressor intake is less than 70, and the effective noise reduction of the 3146 exhaust venting noise is less than 6, and the effective noise reduction is 2818.
3 Conclusion Test is an important auxiliary means for Shantou quality analysis and evaluation. 1 The test index has a significant correlation with the tasting test index, especially the elastic hardness and adhesion are effective for the taro quality score.
Palatability is an important indicator of the quality of steamed bread. It is based on sensory evaluation or objective measurement of instruments, but it is subject to the influence of the quality of tasters and their hobbies. Looking for the objective and standardized renaturation of the species science has become an urgent question to be solved in the study of Shantou quality.
0,3,0阎俊. Asian food requirements for wheat quality. Mai is a king, Zheng Tiesong. The quality requirements of noodles and slow-head special powders and the wheat variety Zhu Keqing. Industrial production technology of traditional staple food Shantou Beijing China Light Industry Press, 2001.
Liu Aihua. He Zhonghu, Wang Guangrui, and so on. Qi Bingjian, the relationship between wheat quality and steamed bread quality. Su Dongmin. Wheat flour quality and special powder production network. Beijing China Business Press, 2004.
18 Ding 320493, slow-headed wheat flour 3 Chen Dongsheng. Zhang Yan, He Zhonghu, and so on. A research tool for the influence of different water additions on the processing quality of northern steamed bread. Crop Journal. 2005, 6730735.
Chen Haifeng. Yang Qilin, He Weiping. The production and evaluation of the laboratory of Guangxu Shantou is on page 58 4 Conclusions Air compressor noise seriously pollutes the production work environment and must be controlled and controlled. In the practical application of the project, three types of air compressor noise control are adopted. Pathway, control of noise sources; noise reduction on noise propagation path 3 Take protective measures at the sound field acceptance point.
At present, the control of air compressor noise is widely used to install sound muffler with sound insulation and vibration isolation and vibration damping, thus effectively reducing the noise pollution of the compressor. The causes of air compressor noise in food processing enterprises are multifaceted and complex, each with its own characteristics. Governance must combine spectrum characteristics and adopt different measures. Adopting the method of comprehensive noise management, although the investment is large, the noise reduction effect is good and it is easy to meet the requirements.
Zhao Suli. Air compressor noise control several Taiyuan technology, 2005.766263 Sun Wei, Yan Huiping. Classification of industrial mufflers. Mechanical research and application, Li Weimin. Talking about the Method of Noise Control of Air Compressor å® Western Exploration Engineering, Zhang Guangxun. The control blade of coal mine air compressor noise. Coal technology. 2006, 161 Ma Wu. The dynamic optimization of the structure of the stomach 51.0 air compressor muffler. Mechanical Peng Shengqun. Railway air compressor station noise control railway labor safety and health and ring ffi, 2004, 314197
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