Acoustic barrier not a total solution to reducing cooling towers noise

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1 第 35 卷第 1 期声学技术 Vol.35, No 年 2 月 Technical Acoustics Feb., 2016 Acoustic barrier not a total solution to reducing cooling towers noise IU King Kwong, YEUNG Heung Ho Andrew, FENG Xue Zhen Vita (Supreme Acoustics Research Limited; Supreme Environmental Research (Shenzhen) Ltd) Abstract: Rapid urban development in China has created many environmental noise issues. Six large cooling towers were located at 15 m away from a residential building and the measured noise level at the noise receiver had exceeded the national environmental standard by 25 db(a). Traditional noise control measures by erecting acoustic barrier was not effective while acoustic enclosure with silencers might affect the cooling performance of the cooling towers. The challenge of this project is not only the high demand on noise attenuation but also the minimization of pressure drop and elimination of air ventilation short circuit problem. The authors provided a total solution to the noise and air circulation problems. Acoustic enclosure with 3 m and 1.8 m long silencers for air intake and discharge was designed and built to reduce the noise level at the NSR from 75 db(a) to 49 db(a). The system and product designs to improve the acoustic and aerodynamic performance are worth for discussion. Key words: cooling tower noise; silencer; acoustic enclosure; acoustic barrier 0 INTRODUCTION A landmark commercial development project in Chengdu has 6 sets of cooling towers installed in outdoor cooling plant. Each cooling tower has the dimensions of 3.5 m (W) x 10.5 m (L) x 4.5 m (H) with air flow rate of 120 m 3 /s. Residential buildings were located around the cooling plant, the nearest noise sensitive receiver (NSR) was at 15 m from the cooling tower. The measured noise level at 1 m away from the building façade on 5/F was 75 db(a) which exceeded the National Noise Criteria significantly. The noise emitted from the cooling towers caused extreme annoyance to the neighbours since it exceeded the background noise by over 25 db(a). To reduce the noise level emitting from the cooling plant, noise barrier in height of 4 m was erected at the perimeter of the plant (see Fig.1). However the noise level could only be reduced by 7 db(a) which was well below expectation. The noise barrier had also created hot air shortcircuit problem. Having made a series of investigations into the issue, the authors provided a total solution to the noise and air circulation problems as presented in the present paper. The noise barrier was demolished and a new design consisting of intake silencers, discharge silencers and acoustic enclosure was proposed. The measured noise level at the NSR Received: Oct. 21, 2015; Revised: Dec. 28, 2015 Corresponding author: Iu King Kwong, kkiu@supremeacoustics.com was fully complied with the National Noise Criteria and the hot air short-circuit problem was overcome as well. Fig.1 Noise Barrier Installed at the Plant Perimeter 1 ORIGINAL DESIGN NOISE BARARRIER The cooling tower plant was located at the basement with open roof for air circulations. The six cooling towers were installed at the four sides of the plant. The hot air was discharged vertically from the top of cooling towers with air flow rate of 120 m 3 /s per each cooling tower while the cool air was drawn sideward from the centre of the plant room. Accordingly, the centre of the plant was used as a shared intake opening for all 6 cooling towers and the intake airflow rate was 720 m 3 /s. To reduce the excessive noise, noise barrier in height of 4 m was installed at the perimeter. Accord-

2 64 声学技术 2016 年 ing to ISO , the barrier attenuation should not be taken to be greater than 20 db. Moreover, the noise sources in this project were not point sources, the equation presented in the ISO and other simple prediction models, such as Z. Maekawa model, was not applicable. Estimation of the noise reduction by installation of barrier would be far more difficult in this case. Furthermore, the erected barrier had increased the hot air short-circuit problem as illustrated in Fig.2. Fig.2 Picture showing the Air Circulation Path in Existing Cooling Tower Plant 2 TOTAL SOLUTION After a series of feasibility study, the design of using acoustic enclosure and silencers was identified to be the best solution to solve the noise problem. However the challenge of this design was to fulfill both the hot air short-circuit problem and the ultimate noise reduction requirement, which is minimum 25 db(a). Installation of silencers would induce air flow resistance which is the pressure drop of the silencers. To minimize the effect on the cooling load of the cooling towers, the pressure drop induced by installation of the silencers shall not be over 50 Pa. The main noise sources of a typical cooling tower are fan noise, water sparkling noise and motor noise. Noise would radiate from the intake side, discharge side and breakout through tower casing. The noise contributions from each path and the corresponding frequency spectra shall be measured at the beginning of design stage. The noise level at 1 m in front of the intake opening was 91 db(a) while that at 2 m above the discharge opening was 86 db(a). To reduce the hot air short-circuit problem, the discharge air shall be diverted away from the centre space over the plant room. It was proposed to install an elbow plenum on top of the discharge silencer to divert the hot air to discharge horizontally and away from the cooling tower plant. By doing so, the orientation of discharge openings would be changed to face directly to the residential building around the plant. It was predicted that the noise level at the NSR would be increased from 75 db(a) to 80 db(a). Accordingly, the new design would require to attenuate the noise level by 30 db(a). The formula to predict the noise levels at NSR is: L p2 = L p1 + A + D + IL + 3 L p2 is the sound pressure level at 1 m in front of the building façade of NSR, in decibel, db; L p1 is the sound pressure level in front of the air opening, in decibel, db; A is the distance attenuation of sound, in decibel, db; D is the directivity factor, in decibel, db; IL is the insertion loss of silencer, in decibel, db; The 3 db factor is correction factor of façade reflection. It is remarked that there were 6 cooling towers and the noise would emit from the air intake, air discharge and tower casing. This is a multisource cooling tower plant and predictions shall be conducted for each source and path. The resultant noise levels shall be calculated at each NSR around the plant. It has been mentioned that elbow plenums would be installed on top of the discharge silencers to reduce the hot air short-circuit and inefficient heat dissipation problem, the distance between the discharge outlets and intake air opening would still be too close. To overcome this problem, the height of the intake opening was increased. The picture showing the air circulation paths after installation of the silencers with air plenums is shown in Fig.3. Fig.3 Picture showing the Air Circulation Path in the Cooling Tower Plant according to the New Design Acoustic panels were used to build the air discharge elbow plenums and air intake plenums. The inner face of the acoustic panels was sound absorptive which acted as lined ducts to further reduce the noise

3 第 1 期 IU King Kwong, et al:acoustic barrier not a total solution for reducing cooling towers noise 65 emitting from the silencers. Furthermore, the acoustic panels, having much higher damping property than steel sheet duct work, would eliminate the secondary noise generated by duct vibration induced by air flow fluctuations. As shown in Fig.4, the noises at high frequencies were relatively higher than the low frequency noises. This was due to the combined noises from water sparkling and centrifugal fan running with blade-passing frequency at 3.15 khz. Typically, duct silencers would have good insertion loss at mid-frequencies, while the high frequency noise attenuation drops significantly due to beaming effect. This will be explained in more details in the next section. Fig.4 Noise Levels Measured in front of Intake & Discharge Air Opening Besides the silencers, 100 mm thick acoustic panels were used to cover the remaining openings of the cooling tower plant to reduce the noise emission. Considering both the noise and inefficient heat dissipation problems, a total solution was offered to reduce the noise level at NSR to comply with the nighttime noise criteria of 50 db(a) as defined in GB PRODUCT DESIGN As discussed in previous sections, the silencers used in this project shall have excellent sound attenuation performance at high frequencies while the aerodynamic performance shall be improved to reduce the induced pressure loss. A simple dissipative silencer consists of a straight duct and sound absorptive lining, of circular or rectangular duct cross-section and without any fittings. In accordance with ISO 14163, the insertion loss of a simple dissipative silencer can be described by the following equation: D i = D s + D a l D s is the discontinuity attenuation, in decibels, db; D a is the propagation loss along the silencer, in decibels per metre, db/m; l is the length of the silencer, in metres, m. The propagation loss D a of a silencer can be expressed by Piening s proportionality: D a (U / S) α U is the length, in metres, m, of the duct perimeter lined with sound-absorbent material; S is the cross-sectional area of the duct, in square metres, m 2 ; α is the sound absorption coefficient of the lining. As the air and sound wave pass through the airway of the silencer, the acoustic absorptive lining would absorb some of the sound energy. The amount of energy absorbed can be represented by its airflow resistivity r, it is related to the fibre diameter and bulk density as in the following equation: r (η / a 2 ) (ρ c / ρ u ) 3/2 ρ c is the bulk density, in kilograms per cubic metre, kg/m 3, of the compressed absorber material; ρ u is the bulk density, in kilograms per cubic metre, kg/m 3, of the uncompressed absorber material; η is the viscosity of the gas, in newton seconds per square metre, N s/m 2 ; a is the average diameter of the fibres, in metres, m. The insertion loss of a splitter silencer is determined by the percentage of open area of the airways between splitters in the whole cross-sectional area and the length of silencer. At low frequencies the propagation loss increases with absorber thickness and frequency. In mid-frequency range the duct width is the same as half of the sound wavelength, a maximum value is achieved which is inversely proportional to the airflow resistance of the absorptive material. The total specific airflow resistance perpendicular to the splitter should not significantly exceed 2 kn s/m 3. At higher frequencies the propagation loss will drop significantly the width of airways between splitters is greater than half of the wavelength. This is called beaming effect. To increase the sound attenuation of a silencer in high-frequency range, the width of airway between

4 66 声学技术 2016 年 splitters should be reduced or the splitters should be separated longitudinally and placed along the duct with an off-set, see Fig.5. Fig.5 Insertion Loss D i vs Frequency f and Pressure Loss Coefficient ζ for Different Splitter Configurations From Fig.5, both of the methods would increase the pressure loss, but the off-set method can only provide an additional attenuation of less than 6 db while the pressure loss coefficient ζ was almost doubled. After a series of estimation and consideration, the method of reducing the width of silencer airway was chosen to increase the sound attenuation at high frequencies. The induced pressure loss shall be reduced by designing the splitters into streamline shape. In the final design, a 3 m long intake silencer with 47% airway open area and 1.8 m long discharge silencer with 33% airway open area at each discharge opening were chosen. According to the flow rate of cooling towers, the air face velocities at intake and discharge silencers would be 3.9 m/s and 3.3 m/s, the pressure loss induced by NAP Silentflo silencers at these face velocities were tested to be 17 Pa and 26 Pa respectively. splitter nose and tail will affect the aerodynamic performance as well as the acoustic performance. Furthermore, the quality of sound absorption material and how it is installed in the splitters will lead to deviations from the predicted acoustic performance. Laboratory test is the most accurate method to validate the performance of silencers. To achieve the design requirement of acoustics and aerodynamics of the silencers, Supreme Acoustics has been performing a lot of product designs and laboratory tests in accordance with the testing standard ASTM E477 and ISO Fig.7 shows the laboratory test facilities which are the silencer test rig in length of 50 m connected into the reverberation room for diffuse field sound pressure measurements. The sound absorptive material in the splitters is a major acoustic component of a silencer. Measurement of specific airflow resistance is useful during product development and for quality control to ensure the consistency of acoustic performance of the silencers during manufacture. Supreme Acoustics has been conducting many airflow resistance tests for the sound absorptive materials according to ISO 9053:1991 to ensure the qualities of the silencers and for research purpose. Fig.8 shows the testing instrument. Fig.6 Final Construction after Installation of Silencers and Acoustic Enclosure 4 Laboratory Test Although the silencer performance can be predicted by simple estimation formulae, the design of Fig.7 Silencer Test Rig and Reverberation Room in Supreme Acoustics Testing Centre

5 第 1 期 IU King Kwong, et al:acoustic barrier not a total solution for reducing cooling towers noise 67 to cooling towers and air-cooled chillers. Acoustic barrier, being very popular for reducing traffic noise, is not a total solution to reducing the cooling towers noise, especially for those projects requiring more than 10 db(a) insertion loss. Reference Fig.8 Airflow Resistance Measurement System 5 Conclusions After construction of the complete noise control system, the noise level at the NSR 5/F was measured to be 49 db(a) which complied with the national standard. The noise control system design has also solved the hot air ventilation short circuit problem with pressure loss controlled within the design specification. With more product research work, laboratory testing and experienced system design to suit each individual project, it is confident that acoustic enclosure with silencers is the best noise reduction solution [1] International Organization for Standardization. Acoustics - guidelines for noise control by silencers [J]. 1998, 1(1). Geneva. ISO 14163: [2] International Organization for Standardization. Acoustics - Attenuation of Sound during propagation outdoors Part 2: General method of calculation[j]. 1996, 1(1). Geneva. ISO : [3] American Society for Testing and Materials. Standard Test Method for Laboratory Measurements of Acoustical and Airflow Performance of Duct Liner Materials and Prefabricated Silencers[J]. 2013, 1(1). West Conshohocken, PA. ASTM E477-13, [4] International Organization for Standardization. Acoustics - Laboratory measurement procedures for ducted silencers and airterminal units - Insertion loss, flow noise and total pressure loss[j]. 2003, 1(1). Geneva. ISO 7235: [5] International Organization for Standardization. Acoustics - Materials for acoustical applications - Determination of airflow resistance[j]. 1991, 1(1). Geneva. ISO 9053: [6] Guobiao Standards. Environmental quality standard for noise[j]. 2008, 1(1). Beijing. GB [7] Guobiao Standards. Emission standard for community noise[j]. 2008, 1(1). Beijing. GB 隔声屏障 - 不能治理冷却塔噪声 姚景光, 杨香灏, 冯雪珍 ( 盈达声学科研有限公司 ; 盈达环科声学科研 ( 深圳 ) 有限公司 ) 摘要 : 现时中国大陆城市急速发展, 做成大量环境噪声问题 某室外空调机房配备了六台冷却塔, 与附近最近的居民楼距离只有 15 m, 测得的噪声比国标的规定高出了 25 db(a) 传统的噪声治理是利用隔声屏障去阻挡冷却塔的噪声, 可是效果不显着, 而使用隔声罩与消声器则可能会影响冷却塔运作 此项工程最大的挑战除了要做到高消声效果外, 还要解决消声器的压降与进出风短路问题 作者提供了一个整体的降噪治理和解决进出风短路的方案, 最终把居民楼的噪声由 75 db(a) 降至 49 db(a) 此系统和产品设计非常值得讨论 关键词 : 冷却塔噪声 ; 消声器 ; 隔声罩 ; 隔音屏障中图分类号 :TB533 文献标识码 :A 文章编号 : (2016) DOI 编码 : /j.cnki