Noise control of diesel generator set
Diesel unit noise is a source of environmental pollution. It consists of multiple sound sources, mainly aerodynamic noise and surface radiated noise. In actual work, it is feasible to control the noise leakage of the oil engine room, and the selected scheme is comprehensive treatment. If combined with the adjustment of the oil engine room structure, the management will be more simplified.
Keywords: diesel generator set; noise; comprehensive treatment
Article ID (ArTIcle ID): 1009-9336 (2001) 05-0052-07
CLC number: Tk421.6
Document identification code (Document code): B
Due to the characteristics of their own industries, telecommunications companies in modern cities must be able to operate their communication equipment without fail. In order to ensure the uninterrupted operation of communication equipment such as switches, diesel generators are generally used as their own backup power sources. Once the mains power fails, the diesel generators are started immediately and switched by the mains / oil engine conversion screen (ATS) The diesel generator set provides power to the communication equipment to ensure the normal operation of the telecommunications business.
Since most telephone exchanges are located in densely populated areas, these areas have high environmental requirements, and the noise of diesel generator sets often becomes the main source of pollution in the surrounding environment. At present, society has higher and higher requirements for environmental protection. How to effectively control its noise pollution is a difficult task and has great promotion value. This is also a major task of our communication construction and should be given more attention. . In order to do this work, we must first understand and analyze the noise composition of the diesel generator set.
1 Sound source analysis
Diesel engine noise is a complex sound source composed of multiple sound sources. According to the noise radiation mode, diesel engine noise can be divided into aerodynamic noise and surface radiation noise. According to the generated mechanism, the surface noise of diesel engines can be divided into combustion noise and mechanical noise. Among them, aerodynamic noise is the main noise source.
1.1 Aerodynamic noise
Aerodynamic noise is caused by the unstable process of gas, that is, the disturbance of gas and the interaction between gas and objects. The aerodynamic noise radiated directly to the atmosphere includes: intake noise, exhaust noise, and cooling fan noise.
1.1.1 Intake noise
Intake noise is one of the main aerodynamic noises of diesel engines. It is formed by the pressure fluctuations caused by the periodic opening and closing of the intake valve. When the intake valve opens, a pressure pulse is generated in the intake pipe, and as the piston continues to move, it is damped; when the intake valve is closed, a pressure pulse of a certain duration is also generated. Thus, periodic intake noise is generated. The noise frequency component is mainly concentrated in the low frequency range below 200 Hz. At the same time, when the airflow flows through the intake valve flow section at a high speed, turbulent dissociation occurs, resulting in high-frequency noise. Since the intake valve flow section is constantly changing, the turbulent noise has a certain frequency range Mainly concentrated in the high frequency range above 1 000 Hz. When the natural frequency of the air column of the intake pipe is consistent with the main frequency of the periodic intake noise, the resonance noise of the air column will be more prominent in the intake noise.
For turbocharged engines, the turbocharger speed is generally higher, so the intake noise is significantly higher than non-turbocharged engines. The noise of the turbocharger is formed by the rotating noise generated by the blades periodically cutting the air and the turbulent noise formed by the high-speed airflow. It is a continuous high-frequency noise, mainly distributed in the frequency range of 500 ~ 10 000 Hz . At present, most of our company uses turbocharged engines.
The intake noise is related to the design factors such as the engine's intake mode, intake valve structure, cylinder bore, and cam profile. For the same engine, it is most affected by the speed, and doubling the speed can cause the intake noise to increase by 10 ~ 15dB (A).
1.1.2 Exhaust noise
Exhaust noise is the main source of engine noise, and its noise is generally 10 ~ 15dB (A) higher than the engine noise. Engine exhaust is a high-temperature (800 ~ 1000 â„ƒ), high-pressure (3 ~ 4 atmospheres) gas. The exhaust process is generally divided into two stages, namely the free exhaust stage and the forced exhaust stage. Engine exhaust gas rushes out of the exhaust valve at high speed, enters the muffler along the exhaust manifold, and is finally discharged into the atmosphere from the tail pipe. In this process, a wide band of exhaust noise is generated.
Exhaust noise contains complex noise components: exhaust noise based on the number of exhausts per unit time as the fundamental frequency, resonance noise of the air column in the pipeline, air blowing noise at the exhaust manifold, exhaust gas injection and impact noise, Helmholtz resonance noise of the cylinder, Carmen vortex noise and turbulence noise in the exhaust system, etc.
The main factors affecting engine exhaust noise are: cylinder pressure, exhaust valve diameter, engine displacement and exhaust valve opening characteristics. For the same engine, engine speed and load are the most important factors affecting its exhaust noise.
1.1.3 Cooling fan noise
Fan noise consists of rotating noise and turbulent noise. Rotating noise is caused by the blades of the fan cutting the air periodically, causing the pressure pulsation of the air, which takes the blade passing frequency as the fundamental frequency and is accompanied by higher harmonics. Turbulent noise is caused by the turbulent dissociation of the surrounding air due to the movement of the fan, which causes the air to be disturbed and forms the compression and sparse process of the gas. It is a broadband noise.
The cooling fan noise is most affected by the rotation speed, and doubling the rotation speed can cause its sound level to increase by 10 ~ 15dB (A). At low speeds, the fan noise is much lower than the engine noise, and at high speeds, it often becomes the main noise source. At present, the speed of the diesel engine used by our company is mostly 1500 rpm, which belongs to the high-speed oil engine.
1.2 Surface radiation noise
Combustion noise and mechanical noise are difficult to distinguish strictly. Generally, the noise radiated by the pressure vibration due to combustion in the cylinder through the cylinder head, piston-connecting rod-crankshaft-body is called combustion noise. The noise generated by the impact of the piston on the cylinder liner and the mechanical impact vibration between the timing gear, the gas distribution mechanism, the fuel injection system and other moving parts is called mechanical noise. Generally, the combustion noise of direct injection diesel engines is higher than that of mechanical noises, while the mechanical noise of non-direct injection diesel engines is higher than that of combustion noises, but the combustion noise is higher than that of mechanical noises at low speeds.
2 Noise control measures
2.1 Aerodynamic noise control
2.1.1 Intake noise control
Generally, the engine is equipped with an air filter, and the intake noise can be greatly attenuated, becoming a secondary sound source. When other sound sources are further controlled, the intake noise may become the main sound source. At this time, it is necessary to consider the use of a good performance intake muffler. Usually, the intake muffler is combined with the air filter for an integrated design. It can meet the requirements of intake and filtration, and can effectively control the intake noise.
2.1.2 Exhaust noise control
The most effective way to control exhaust noise is to install an exhaust muffler. In reality, the noise reduction effect is often not ideal. The main reason for the analysis is that the structure design of the muffler is not reasonable and there are problems in the processing technology. The latter problem can be improved by improving the process level; the former problem involves the design idea of â€‹â€‹the muffler. Usually muffler design is mainly based on experience. Some design calculation procedures are carried out under some ideal assumptions. Among these assumptions, the actual impact is to ignore the existence of airflow and the existence of high-pressure, high-temperature, high-speed pulsating airflow. The airflow in this state will affect the sound field distribution, sound speed, and sound propagation law inside the muffler, especially the airflow speed has a greater influence. The main reason that the airflow affects the performance of the muffler is the high-speed pulsating airflow regeneration noise of the engine exhaust, and the second is that this airflow will impact the acoustic components such as the muffler's pipes, housing, partition, etc., and then excite the vibration radiation noise. When the structural parameters of the muffler are selected improperly, or the structure is unreasonable, or the processing technology has problems, it will cause the muffler's noise reduction performance to be reduced. At the same time, the high airflow speed will also increase the pressure loss of the muffler and cause the noise reduction performance to decline.
2.1.3 Control of engine surface radiated noise
The control of engine surface radiated noise (combustion noise and mechanical noise) is subject to various limitations in terms of engine performance, which is very difficult from a technical point of view, and the amount of noise reduction is limited. Practice has shown that taking structural measures can reduce the surface radiation noise of the engine to a certain extent, thereby reducing the overall noise. The basic control measures are to increase the structural rigidity and damping, so that under the same excitation force, reduce the surface response of the structure. At the same time, reducing the surface area of â€‹â€‹radiated noise is also an effective measure to control radiated noise.
3 Integrated control ideas
Usually a 500 kW imported unit, the noise in the machine room can reach 105 ~ 108 dB (A). Without treatment, the ambient noise outside the machine room is 70-80 dB (A) or higher, and the noise of domestic units with the same power parameters is greater. At present, China uses the "City Regional Environmental Noise Standard" or "Industrial Enterprise Noise Standard for Noise" when assessing whether the environmental noise is up to the standard. In the standard, there are different noise limits for different regions. Generally in urban areas, it is mostly a first-class area, and the standard limit is 55 dB (A) during the day, and 45 dB (A) at night; in the suburbs, it is mostly a second-class area, and the corresponding standard limit is 60 dB (A) , 50 dB (A) at night. It can be seen from the comparison data that the required noise reduction is very large, and the corresponding control technology is also very difficult.
In actual work, because our company chooses to use the configured engine complete machine, it is very difficult and unrealistic for the unit itself to take control measures. Considering that the noise of the oil engine is generally mainly polluted by the surrounding environment, how to effectively control the external radiation of the oil noise in the engine room is a very realistic and must be resolved. The selected scheme should be able to effectively reduce the environmental noise, and organize the air flow in the machine room to meet the air flow required by the generator set operation to ensure the normal operation of the unit. Simply reducing the leakage of noise and sacrificing the air flow in the oil machine room will cause uneven cooling of the surface of the oil machine, reduce the power generation capacity of the oil machine, and affect the normal use. After years of cooperation with the environmental protection department, the oil machine room has been dealt with noise reduction and accumulated some experience in governance. The main control plan must be determined according to the specific machine room project, which should consider the environmental standards of the area where the machine room is located. Enclosure structure, oil machine model, power, cooling air volume and other factors. The core of the integrated control is the concept of equal sound insulation, that is, a closed enclosure structure is used to isolate the unit from the outside world, reducing the external sound radiation of the sound source. The so-called equal sound insulation concept is that the sound insulation of each part of the entire envelope (such as civil construction parts and doors, windows, etc.) should be equivalent. The channels reserved for the communication between the machine room and the outside world (such as cooling fan outlets, engine exhaust outlets, machine room ventilation vents, etc.) must be designed as noise reduction channels, and their insertion loss should also be equivalent to the sound insulation of the enclosure, only Only in this way can the environmental noise outside the equipment room meet the standard.
We still take a 500 kW imported unit as an example. The interior wall of the oil engine room is designed with a sound-absorbing board, and the sound-absorbing board is used for ceiling. After such sound absorption treatment, the sound insulation of the enclosure structure is increased, and the oil can be reduced The reverberation sound in the computer room can generally have an effect of 3 ~ 5dB (A). For the high-frequency noise in the engine noise, because of its short wavelength, it can be achieved by blocking. Because the low-frequency component of engine noise is more abundant, simple blocking cannot achieve satisfactory results, so the impedance composite structure should be selected for the noise reduction channel, and the propagation of low-frequency noise should be controlled by the noise reduction characteristics of the resistant structure. The noise in the equipment room after effective control can meet the requirements of the corresponding environmental protection standards under the normal operation of the unit, reaching 55 dB (A) during the day and 45 dB (A) at night. This has been confirmed in our previous work.
At present, because there is no uniform standard for the oil engine room of the telephone bureau, the civil engineering design unit does not have a thorough understanding of the performance of the communication power equipment during the civil construction stage, resulting in an unreasonable layout of the oil engine room and the arrangement of the intake and exhaust ports. Difficult to increase noise and increase investment in noise reduction. For example, in an oil engine room that we have treated before, the air inlet of the oil engine room is a fireproof passage and cannot be occupied. The air inlet silencer can only be installed in the oil engine room, and the internal space design of the oil engine room is too small, resulting in the distance of the air inlet silencer. The generator set is too close, which is very inconvenient for maintenance personnel to operate. In order to reduce the occurrence of the above problems and save investment in noise reduction treatment, by summing up previous work experience, it is recommended that the following plan should be adopted in the future construction of the oil engine room: try to reduce the number of oil engine room doors and windows to avoid leakage of oil engine noise ; Try to increase the distance between the air inlet of the oil machine room and the base of the oil machine, and extend the silencing distance. It is best to build a small air inlet room; add an expansion room outside the air outlet of the oil machine and extend the exhaust distance of the expansion room of the oil machine room as much as possible.
If the above scheme can be adopted, the layout of the oil engine room can be more standardized and more reasonable, and the noise reduction control in the later period is more convenient, the construction mode is facilitated, the management work is easier, and the environmental protection requirements are met with less investment.
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