Introduction to Silencers

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This post was last edited by Black gold on 2011-11-8 at 19:53. Muffler 　　A muffler is a device that prevents sound from spreading while allowing air to pass through; it is an important means of reducing aerodynamic noise. A silencer is a device installed in the airflow passage of aerodynamic equipment (such as blowers, air compressors, boiler exhausts, generators, water pumps, and other devices that generate significant noise at their exhaust outlets) or within the intake and exhaust systems, used to reduce noise levels. Types of silencers: There are many types of silencers, but based on their noise reduction mechanisms, they can be divided into six main categories: resistive silencers, reactive silencers, impedance-combined silencers, microperforated plate silencers, orifice silencers, and active silencers. 　　 Resistive silencers primarily use porous sound-absorbing materials to reduce noise. Fixing sound-absorbing materials on the inner walls of the airflow channels or arranging them in the pipes in a certain manner constitutes a resistive silencer. When sound waves enter a resistive silencer, part of the acoustic energy is converted into heat through friction within the pores of the porous material, thereby dissipating it and reducing the intensity of the sound waves passing through the silencer. A resistive silencer is like a pure resistive circuit in electricity, with the sound-absorbing material functioning similarly to a resistor. Therefore, people call this type of silencer a resistive silencer. Resistive silencers are effective at reducing noise in the medium and high frequency ranges, but less effective at reducing noise in low frequency ranges. 　　 A reactive silencer is composed of tubes and chambers with modified interfaces, functioning like an acoustic filter. Similar to electrical filters, each small chamber containing a tube represents a element of the filter; the mass of air in the tube corresponds to inductance and resistance in electronics, and is referred to as acoustic mass and acoustic impedance. The volume of air in the chamber is equivalent to the electrical capacitance, and is referred to as acoustic compliance. Similar to electrical filters, each chamber with a tube has its own natural frequency. When sound waves containing various frequency components enter the first short tube, only those sound waves with frequencies near the natural frequency of the first mesh can pass through the mesh and reach the opening of the second short tube; sound waves with other frequencies cannot pass through the mesh and can only reflect back and forth within the chamber. Therefore, we refer to this structure that has a filtering function for sound waves as an acoustic filter. By selecting appropriate tubes and chambers and combining them, noise with certain frequency components can be filtered out, thereby achieving noise reduction. Resistant silencers are suitable for eliminating mid- and low-frequency noise. 　　By combining resistive and reactive structures in a certain way, an impedance composite silencer is formed. 　　 Micro-perforated plate silencers are generally made from pure metal sheets with a thickness of less than 1 mm; holes are drilled in these sheets using drills with a diameter of less than 1 mm, with a perforation rate of 1% to 3%. By selecting different perforation rates and cavity depths with varying plate thicknesses, it is possible to control the spectral performance of the silencer, enabling it to achieve good noise reduction within the desired frequency range. 　　 The structure of a perforated silencer is a straight tube with a closed end, on the wall of which many small holes are drilled. The principle of the orifice silencer is based on the spectrum of jet noise. If the total area of the nozzle is kept constant but replaced with many small nozzles, as the airflow passes through these small openings, the spectrum of the jet noise shifts toward higher frequencies or ultra-high frequencies, thereby significantly reducing the audible components in the spectrum and thus minimizing disturbance and harm to humans. 　　 The basic principle of an active noise suppressor is to use electronic devices to generate another sound wave in the existing sound field, one with a magnitude equal to that of the original sound pressure but with an opposite phase, so that it can cancel out the original sound field within a certain range. This type of silencer is a set of instrumental devices, mainly consisting of a microphone, amplifier, phase-shifting device, power amplifier, and speaker. Metrics for silencers To determine the quality of a silencer, the following three aspects are primarily considered: 1. The noise reduction performance of the silencer ； (Silencing level and spectral characteristics) 2. Aerodynamic performance of the silencer ； (Pressure loss, etc.) 3. Structural performance of the silencer. (Size, price, lifespan, etc.) Selection and characteristics of silencers The selection of silencers should take into account various factors such as fire protection, moisture resistance, corrosion resistance, cleanliness requirements, the location where they will be installed, the spectral characteristics of the noise source, the natural sound attenuation of the system, the noise generated by air flow in the system, the allowable noise level in the room, the permissible pressure loss, and the cost of the equipment; priority should be given to certain factors depending on the specific circumstances. Generally, the greater the noise reduction capability of a silencer, the higher the pressure loss and the price ； When the noise reduction level is the same, the smaller the pressure loss, the more space the silencer requires. 　　The new type of high-efficiency, anti-backflow series of silencing devices are widely used in industries such as power generation, chemicals, metallurgy, and textiles, for the exhaust gases from various types of boilers and steam turbines ； fan ； Noise reduction for equipment such as safety doors. This series of silencers is developed based on the combined noise reduction principle of absorption, reflection, and obstruction. It offers numerous advantages such as high noise reduction efficiency, small size, light weight, easy installation, and no need for maintenance. 　　• The noise reduction frequency characteristics of the silencer. Anti-backflow mufflers offer excellent performance for noise at various frequencies. 　　• The appropriate wind speed for silencers is generally 6–8 m/s, with a maximum not exceeding 12 m/s; attention should also be paid to the pressure loss of the silencer. 　　• Pay attention to the net cross-sectional area of the silencer; when connecting the air duct to the silencer, it may be necessary to enlarge it if required (when there are restrictions on wind speed). 　　• For the installation of noise-reducing devices such as silencers, independent load-bearing suspension rods or bases are required ； The sound source device must be connected via a flexible connector. 　　• When two sound-dampening elbows are used in series, the distance between the connections of the two elbows should be greater than 2.5 times the length of the diagonal of the elbow cross-section. 　　• For environmental systems with high temperatures, high humidity, oil mist, and water vapor, micro-porous structure noise suppression devices are generally used; in environments with strict cleanliness requirements such as operating rooms, recording studios, and clean production facilities, micro-porous structure noise suppression devices should also be employed. 　　• When the piping of adjacent rooms is connected, be mindful of the mutual influence of indoor noise through the piping; if necessary, install noise reduction measures on the air inlets. Application areas of the silencer: 1. It is suitable for TA-type steam safety valve silencers used in various types of steam safety valves. It is applicable to the safety valves of power plant boilers such as PCV/ERV safety valves (pressure relief valves), drum safety valves, superheater safety valves, reheater safety valves, and other pressure-relief devices. Its steam venting must not only avoid excessive noise but also ensure that the steam flow can escape smoothly without affecting the discharge capacity, operating threshold, or reseating action of the safety valve. For this reason, common noise-reduction methods such as multi-stage throttling and orifice injection should not be used. Such limitations have always been a challenge in the design of safety valve silencers, and they are also the reason for the poor noise reduction performance of safety door silencers. The new TA-type steam exhaust silencer for safety valves utilizes a combined spray-impedance noise reduction principle; in this design, larger holes are used to increase the flow area, replacing the smaller holes used for flow control. This approach enhances the noise reduction mechanisms based on resistance and impedance, resulting in a higher level of noise suppression. 　　2. TB-type ignition and exhaust silencer suitable for steam equipment such as boilers. The TB-type boiler ignition and exhaust silencer is designed by incorporating the most effective silencing principles of the PB-type exhaust silencer. High-pressure steam enters the pressure-reduction chamber after being regulated in flow within the silencer; after further expansion and pressure reduction in a large volume, it emerges as low-pressure steam. During this process, part of the internal energy of the steam stream is converted into sound energy of a certain frequency. Although the noise level is significantly reduced, exhaust noise often remains above the standard values due to various reasons such as the exhaust stream deviating from the designed parameters. To this end, a sound-insulating and sound-absorbing cover with a composite multi-material structure was designed externally for the noise-reducing unit. This composite sound-insulating and sound-absorbing cover is designed based on the spectral characteristics of the residual noise emitted by the noise-reducing unit, in order to effectively absorb that residual noise. When installed as required, the total noise reduction can reach 36–42 decibels. 　　3. TD type low-pressure steam silencers suitable for various low-temperature and low-pressure thermal systems. A large number of low-pressure boilers with a pressure of 0.13 Mpa or less are in use in various industrial and mining enterprises. There are also many low-pressure thermal devices in power plant thermal systems, such as backpressure turbines, deaerators, auxiliary extractors, turbine oil pumps, continuous boiler blowdown systems, and periodic blowdown systems. In accordance with the requirements of the exhaust pipe, the aforementioned equipment is equipped with silencers that are lightweight and easier to install. For different noise reduction applications, specialized exhaust silencers for TDb-type back-pressure steam turbines, silencers for TDy-type deaerators, and other series of noise reduction products have been designed to meet the requirements of users. 　　4. A specialized silencer of the TC type, suitable for use in boiler commissioning and purging processes as well as after the installation and maintenance of industrial pipelines. The TC-type pipeline purging silencer is a specialized exhaust silencing device designed to reduce the noise generated by the discharge of debris from the outlet of the purging pipes during the commissioning of power station boilers, as well as by waste gas emissions. It can also be used for flushing, noise reduction, and removing debris in other industrial pipelines as well as civil pipelines after installation or maintenance. 　　5. PB series orifice injection silencers for pipeline gas discharge. The PB series orifice injection silencers are a new generation of noise reduction products developed in the 1990s. Based on the principle of noise reduction through pressure reduction and orifice injection, their optimized design results in noise reduction performance, safety characteristics, and service life that far exceed those of similar products available on the market today. They are well-suited to meet the noise reduction requirements associated with the exhaust gas discharge from boilers and pressure vessels in various industries such as power, metallurgy, petroleum, chemicals, textiles, papermaking, and food processing, where pressures range from low to ultra-high and sub-critical levels. 　　6. CF-type fan silencers suitable for use in ventilation ducts. Fans are a type of general-purpose mechanical equipment with a wide range of applications; production in various industries such as electricity, mining, machinery, metallurgy, and chemicals relies on fans. The noise generated by fans during operation often becomes a source of harm to workers’ health and a disturbance to the environment. In particular, for fans located near residential areas, the aerodynamic noise emitted from their air inlets and outlets is a major factor contributing to environmental pollution; it constitutes a form of public nuisance and has been one of the main targets for industrial sectors in China in their efforts to control noise pollution in recent years. 　　7. CP-type diesel generator exhaust silencers suitable for use in reducing noise from diesel generator exhausts. Diesel generators are widely used in industrial and mining enterprises due to their good economic efficiency and performance. The exhaust gases emitted during their operation contain a certain amount of dust (i.e., the black smoke that is commonly seen), and this phenomenon occurs as a result of incomplete combustion of fuel. Therefore, conventional ventilation and exhaust silencers, such as perforated plate silencers and those equipped with sound-absorbing fillers, are absolutely not suitable for use in such environments, as they can only provide temporary noise reduction. 　　8. XB-type noise reduction and sound absorption panels for ventilation ducts. The XB-type panels are of the resistive type; when sound waves pass through ducts lined with sound-absorbing material, this material absorbs the energy of the sound waves, thereby achieving noise reduction ； Through optimized design and the use of new type of suction lining materials, this series of silencers achieves improved noise reduction performance. 　　9. Industrial sound-absorbing and sound-insulating enclosures for equipment noise reduction. Sound-absorbing and sound-insulating enclosures are widely used; they are employed in the noise control of various sound-generating devices such as fans and water pumps. Its main design principle is to effectively control noise sources along the propagation path of sound waves. Since the external dimensions and ventilation/heat dissipation requirements vary for each sound source device, it is often necessary to design, manufacture, and produce them based on the specific conditions of the sound source in use. 　　10. Silencers for use in the **defense industry – Gun silencers are devices used to reduce the noise generated by gunfire. Ordinary silencers can only reduce noise, striving to make it as quiet as possible. 　　The working principle of a gun silencer is to allow the **gases that propel the bullet to do work to release their heat energy in a much larger enclosed space relative to the barrel, thereby reducing pressure and flow velocity and preventing excessive shock to the air near the muzzle (which is what causes muzzle noise – the main source of noise when a gun is fired). The principle behind the silencers commonly used in the world today is to reduce noise by lowering the **speed and flow rate of the gas exiting the barrel, thereby reducing the peak pressure levels. The specific measures involve installing several sound-dampening partitions or wires with small holes inside the silencer, or attaching a rubber silencer with a smaller diameter at the muzzle, in order to reduce the maximum peak value at the muzzle (that is, to dissipate in advance the work that the gases need to do). At the same time, subsonic bullets are used to reduce the whooshing sound produced as the bullets travel through the air. No matter what method you use to reduce the noise of a gun, you always have to sacrifice some of its performance, and this also causes the mechanism to function improperly. So the silencer has no effect in reducing recoil, nor does it lessen the force of the backstroke. Modern firearms generally have cage-shaped or barrel-shaped suppressors installed at the muzzle; by adjusting the cutting angle of the exhaust holes, a certain degree of recoil reduction can be achieved. However, once a silencer is installed, the recoil-reducing effect of the muzzle device is lost. In addition to its usual functions of noise reduction and flame suppression, the silencer has an additional advantage: it includes a weight at the muzzle, which helps to prevent the muzzle from jumping upward, reduces dispersion, and improves accuracy.