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Reduction of noise characteristics of exhaust ventilation plant

  • Added: 09.07.2014
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Description

Course design-Acoustic calculation of fans. Calculation of UZM ventilyatorarascht decrease in UZM on the way of distribution of noise in vozdukhovodakhrascht BRIDLES in reference point No. 1 Calculation of BRIDLES in reference point No. 2 Calculation of the required soundproofing ability ventilating kameryobshchy information on mufflers a muffler shumaproektirovaniye on a muffler vsasyvaniiproektirovaniye on forcing

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Additional information

Contents

Introduction

1. Source Data

2. Calculation of fan UZM

3. Calculation of UMM reduction along the path of noise propagation in ducts

4. Calculation of SPD at design point No.

5. Calculation of SPD at design point No.

6. Calculation of required soundproofing capacity of ventilation chamber

7. General information on noise silencers

8. Design of suction silencer

9. Design of discharge silencer

Conclusion

List of literature

Appendix A

Appendix B

Introduction

Noise - sounds of all kinds that interfere with the perception of useful sounds or disturb silence, as well as sounds that have a harmful or irritating effect on the human body - is one of the harmful production factors. Since the purpose of ventilation and air conditioning systems is to provide the rooms with a calculated amount of air to create and maintain the specified conditions in the rooms, all sounds accompanying the movement of air are undesirable.

Permissible noise levels at permanent workplaces in industrial premises, residential and public buildings, as well as in the built-up area are normalized by sound pressure levels in eight-octave bands with average geometric frequencies of 63.125, 250, 500, 1000, 2000, 4000, 8000 Hz. In buildings equipped with mechanical ventilation and air conditioning systems, the latter are often the main source of indoor noise.

Noise sources in ventilation and air conditioning systems are fans, pumps, chiller compressors and electric motors. In addition, noise is generated when air flows in air ducts, as well as when it passes through plenum holes and when it enters exhaust ducts. Noise generated during operation of the specified equipment propagates in the building through the air of rooms and air in air ducts, and in the form of vibration is transmitted through construction structures and various communications (along the walls of pipes, air ducts, etc.) and excites vibrations of structures in the audible frequency range. The noise level created by the fan is usually much higher than the noise levels created by other sources. Mechanical, structural and aerodynamic noise occurs during fan operation.

The reason for the formation of mechanical noise is the mechanical vibrations of the structure elements of the ventilation plant, impacts in bearings, power effects of unbalanced rotating masses, etc. The reduction of mechanical noise is achieved by careful fabrication of fans in plants and their high-quality installation on site. Special attention is paid to the dynamic balancing of the fan impeller, the coaxiality of the fan and electric motor shafts, and the condition of the coupling couplings. During operation, it is necessary to lubricate in a timely manner, replace worn parts with new ones, tighten bolt connections, etc.

Fluctuations arising from fan operation can be transmitted to air ducts and the base on which the plant is mounted. These fluctuations cause structural noise propagating through the building. Reduction of structural noise level is achieved by installation of fans on vibration isolators. In order to prevent transmission of fan vibration to air ducts, the latter are connected to fan nozzles by means of soft inserts made of elastic materials (rubber, rubberized fabric, etc.).

1. Source Data

Given:

An exhaust ventilation system is installed in the catering unit of the hospital. Centrifugal fan VTs470 No. 5 is installed on the street. Noise from the fan propagates through the air ducts to the work place through the grille and through the air to the hospital building, which is located next to the food unit at a distance of 42 meters.

Fan capacity Q = 5000 m3/h = 1.39 m3/s.

Pressure pV = 680 Pa.

Rotation speed n = 1300 rpm.

Required:

• calculate the SPD created by the ventilation plant in octave frequency bands for design points No. 1 and No. 2;

• determine the required noise reduction at the design points;

• design noise silencers at suction and discharge, ensuring reduction of SPD to standard values.

7. General information on noise silencer

High noise in workplaces and residential buildings is often created during the operation of ventilation, compressor and gas turbine plants, compressed air discharge systems, test stands for various engines. Reduction of noise of aerodynamic origin is achieved by installation of silencers in channels and air ducts on the way of noise propagation from its source to the place of air and gases suction or discharge. Silencers are divided into absorption (dissipative), reactive (reflex) and combined. Noise in absorption silencers is reduced due to absorption of sound energy by sound absorbing materials and structures used in them, and in reactive ones - as a result of sound reflection back to the source. Combined silencers have the property of both absorbing and reflecting sound. The selection of the type of silencer depends on the design of the installation to be plugged (bench, system, etc.), the spectrum and the required noise reduction.

When propagating noise through pipelines, air ducts, channels, silencers of various structures are widely used to reduce it, the choice of which is determined by the noise spectrum, the necessary silencing and the operating conditions of a particular plant. Reactive silencers are used to reduce noise with pronounced discrete components and in narrow frequency ranges. It is important that the use of silencers of any type does not impair the operation of the plugging machine. The efficiency of noise silencers can reach 30-40 dB or more. If in the working area it is not possible to reduce noise to permissible values ​ ​ by general technical means, then the administration is obliged to provide personal protective equipment operating in this area and mark it with safety signs. Personal anti-noise protection equipment includes inserts (Noise reduction by 5... 20 dB); headphones (efficiency at high frequencies up to 45 dB); helmets used at high noise levels (more than 120 dB).

The most widespread in general industrial ventilation plants were absorption (dissipative) type silencers - tubular, plate, cylindrical, lined air ducts from inside the PMA, since fans have a wide-range noise spectrum. Silencer structures are selected depending on the transverse dimensions of the air duct, the allowable air flow rate, the required reduction in the SPD and the place for installation of the silencer.

Tubular silencers are usually used for the cross section of ducts up to 500 X 500 m or a diameter of up to 500 mm, cylindrical - with a diameter of up to 700 mm, and plate - with large dimensions. Plates are installed in silencers parallel to air flow at certain distance from each other. The thickness of the plates is chosen based on the maximum in the noise spectrum - the higher the frequency of the drowned sound, the thicker the silencer plates should be. Usually, the thickness of the plates is 100-200 mm, less often 400-600 mm.

Conclusion

In this project, the noise situation in two design points was considered, with corresponding calculations, and measures to eliminate noise pollution in the design points were indicated.

Acoustic calculation was carried out including:

identification of the IS and determination of its noise characteristics;

Determination of allowable spares for calculation points;

determining the noise propagation paths from the FI to the design points;

determination of expected SPD in RP prior to implementation of noise suppression measures;

determination of required reduction of SPD in RP;

selection of measures to ensure required reduction of spares in RP;

calculation and design of noise silencing devices.

As a result of noise suppression measures, noise decreased at all octave frequency bands and does not exceed permissible values.

Drawings content

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Лист 1.cdw

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Лист 2.cdw

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Лист 3.cdw

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Лист 4.cdw

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