Cooling tower silencing can be achieved at a fraction of the cost of conventional techniques that involve high capital expenditure and increased running costs due to the use of acoustic enclosures, barriers and attenuators. Moreover, the common low-frequency fan hum that remains (as silencers and barriers do not attenuate the hum) can still cause complaints. However, there are low-cost environmental noise reduction alternatives that use aerodynamic and other engineering techniques to provide a win-win of both cooling tower noise attenuation and improved system efficiency.
Details of how to reduce cooling tower noise as efficiently and cost-effectively as possible are provided below.
There are multiple potential causes of cooling tower noise. The following are the 4 common sources, each with its own noise characteristics and each with a specific optimum attenuation option.
The key to success is to rank these sources both in terms of the overall noise and in terms of the sound "character" (tones) that is a major cause of complaints and then select the appropriate noise control technique as described below.
Sound file: typical low-frequency cooling tower fan noise
The 2 cooling tower fan noise sources are:-
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Important note: conventional mufflers and acoustic barriers do not attenuate low-frequency sound e.g. the classic cooling tower hum is left virtually untouched.
A very common cooling tower noise problem is a low-frequency hum or drone at the fan blade pass frequency, typically at frequencies below 200Hz. In this example, the tone is at 64Hz and harmonics. These tones travel large distances, passing through glazing virtually unattenuated and are a very common cause of noise complaints. They are at far too low a frequency to be attenuated by conventional means.
The only effective noise control technique is to alter the aerodynamics of the airflow through the fan. This can be achieved by retro-fitting aerodynamic fan noise reduction aids to eliminate the tones. This is not only low cost and can be fitted with minimal downtime, but it can also increase fan efficiency. This technology has been used to reduce cooling tower hum noise by around 99% (20dB) whilst simultaneously increasing fan efficiency making projects self-financing. In one case the noise reduction modifications for a data center cooling fan noise attenuation project provided 16dB attenuation and increased fan efficiency by up to 23%... A case where noise control technology should be used even if the fans are quiet...
Whilst this can be screened using acoustic barriers, this can also hamper airflow and hence reduce cooling. There are floating mats available to muffle this sound, or we have used floating swimming pool heat retention balls with great success.
In addition, it is sometimes also practical to include waterproof acoustic absorbent within the cooling tower chamber to reduce reverberation and hence the noise radiated.
Sound file: typical tonal cooling tower pump noise
Typical pump noise is tonal at the rotational speed of the pump and at the vane passing frequency (plus harmonics). Depending on the dominant path, there 3 control techniques that can be used to reduce the noise at source.
Where pump noise is relatively high frequency, then acoustic screens can be effective, as long as pipe runs are not significant noise radiating surfaces.
Sound file: typical cooling tower gearbox noise
Some cooling towers have a gearbox drive to step-down the motor speed. This can be a source of tonal noise - typically in the mid frequency range (250Hz - 700Hz).
In most cases, the source of the tone (drone) is easily identified - remotely from emailed audio or video files if the mechanical details of the gearbox are available, or via measurement (ideally vibration) on the gearbox itself. In addition, there are often side-bands (tones) either side of the gearmesh frequency, spaced apart by the rotational speed of one of the gears. We can identify the precise gear pair from this feature.
In this instance, the 541Hz tone in the boundary narrow band noise signature (blue trace) has side-bands at multiples of the motor speed (24.2Hz) which is very obvious close to the gearbox (red trace). This enabled us to recommend simply changing the worn pinion to eliminate this feature of the noise instead of spending a fortune on conventional acoustic barriers.