Noise produced by manufacturing plants represents a serious environmental problem. Therefore, the obligatory use of effective silencers with "noisy" systems is a legitimate demand of time. Especially strict noise regulations are imposed on the objects located within or in close proximity to residential areas, such as, for instance, boiler plants.
The units having a particularly strong impact on the environment include:
- gas turbines,
- forced-draft machinery (fans, smoke exhausters)
- heating and steam boilers,
- gas-distribution stations and their pipelines.
Our high-performance silencers are specifically designed for suppression of noise produced by such units. Having a number of advantages over their conventional counterparts, these silencers don’t have the limitations inherent in common silencers.
Limitations of conventional silencers :
In most cases industrial noise has a broadband spectrum. During the propagation of such noise in the flue ducts, its suppression by sound-absorbing fiber lining materials is ineffective, since the cooling of flue gases creates condensate that saturates the fiber lining and reduces its effectiveness. It is particularly apparent during cold seasons and in cases when the unit operates intermittently.
Conventional resonance silencers are tuned to a certain limited range of frequencies, and therefore are ill-suited to broadband noise suppression. In addition, they have to be sizable to effectively suppress low frequencies, much larger than the diameter of the corresponding flue duct. It is also important to consider that the silencers create an additional hydraulic resistance, overcoming of which requires significant expenditure of energy.
Noise produced by forced-draft machinery has tonal components having a harmful impact on man. Therefore, in many countries noise regulations include amendment restricting tonal noise. An increased noise from forced-draft machinery in the area of large thermal power plants can be heard within a radius of several kilometers. Therefore, the silencers for such noise must effectively suppress not only the broadband but also tonal noise.
Thus, given the limitations of conventional solutions and modern noise regulations, we can specify the requirements for the “ideal” silencer:
- high efficiency in the wide range of low-frequency spectrum,
- effective suppression of tonal components at given frequencies
- low hydraulic resistance
- small size
- the use of sound-absorbing fiber materials is unwanted, and the construction of silencers should prevent accumulation of condensate.
Our silencers meet all of those requirements:
These silencers fall under a fundamentally new type of acoustic devices - the broadband acoustic reflectors (“acoustic mirror”), ensuring an unimpeded flow of flue gases and reflecting sound waves. They are highly efficient in a wide range of low-frequency spectrum, have low resistance and small size. The design does not apply sound-absorbing fiber materials, is heat resistant and prevents condensate accumulation.
The silencers are specifically designed for use in metallurgical production. They are installed in flue duct gaps or as terminal devices for discharging flue gases into the atmosphere.
The silencers can be used in boiler plants and allow installing boiler plants in proximity to residential areas without violating noise regulations.
Principles of design and operation of the proposed silencer:
The basis of the proposed solution is a distributed system of low-sized high-Q acoustic resonators. Each resonator, excited by an acoustic field, creates an oscillatory flow and can either amplify the exciting field or suppress it, depending on the phase relations.
When using this effect, the objective of the development of the silencer is to correctly select the resonant frequencies, Q-values and the quantity of low-sized resonators in accordance with the required efficiency of suppression and the noise spectrum of a particular source. Whereas, the engineering solution should provide for such an arrangement of the resonators relative to the flue duct, which would ensure the optimal match of their acoustic characteristics and minimum hydraulic resistance. Controlling the broadband spectrum field requires a large number of resonators to cover the entire spectrum and ensure the required efficiency.
The procedure for setting up a silencer for a particular application starts with measurements of noise spectra along the borders of sanitary protection zones or other problem areas. After that, the required suppressing efficiency is determined depending on the sound frequency. Based on these data and using a specially developed technique the number of low-sized resonators, their resonant frequencies, Q-values and dimensions are determined. Thus, the silencer shown in Fig. 3 includes about 600 low-sized resonators.
This approach permits flexible adjustment of a standard silencer to the specifics of a particular noise source. The resonant acoustic mirror principle allows to flexibility modify a set of resonators, ensuring the required suppression efficiency at required frequencies, including the low frequencies for which the sizes and weight of standard types of silencers are unacceptable.
An example of implementation:
In 2008, in one of the metallurgical enterprises of the Urals the Jet Technologies Concern launched two units of scrap metal drying before loading into the electric arc furnace. The units include 75 kW fume exhaust fans; the exhaust gas flow is 20 000 nm3/h. The diameter of each unit’s exhaust pipe is 1.4 m. The noise level on the border of the sanitary-protective zone significantly exceeded the permissible level (Fig. 1, curve 1). To solve this problem the resonant acoustic mirror type silencers have been developed and installed on the exhaust pipes (Fig. 2). Measurements made after the installation of the silencers by both factory services and independent experts have shown that noise levels do not exceed the limitations set by sanitary standards (Fig. 1, curve 2).
Fig. 1. Scrap drying unit’s noise spectra on the border of the sanitary-protective zone (distance - 500 m from the edge of the exhaust pipe): 1 - initial, 2 - after installing the silencer, the maximum permissible noise levels: 3 – day; 4 - night.
Fig. 2. The picture of the resonant acoustic mirror type silencer installed as the terminal device on the exhaust pipe with a diameter of 1.4 m.
The design of the silencer does not include sound-absorbing fiber materials, prevents moisture accumulation and therefore, works effectively in both summer and winter seasons, requiring no maintenance. If compared with standard resonance silencers, it is much smaller and lighter. The silencer has low hydraulic resistance, almost equal to that of the corresponding straight section of the exhaust pipe.
Successful implementation of the resonant acoustic mirror type silencer creates real opportunities for solving the problem of excessive noise in many processes that use powerful forced-draft equipment.
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