Cooling tower noise reduction
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.
What are the sources of cooling tower noise and how can they be reduced?
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.
- fans: broadband noise plus low-frequency hum (tones)
- water: falling water into the catch tank (mid to high-frequency sound)
- pumps: if there are pumps, these often generate tonal noise over a wide frequency range
- fan drives: motor and (sometimes) a gearbox that generate tonal noise
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.
Cooling tower fan noise silencing
Sound file: typical low-frequency cooling tower fan noise
The 2 cooling tower fan noise sources are:-
- broadband noise from air movement (at mid to high frequencies) that usually dominates the overall dB(A) noise level
- fan hum or tonal noise, almost invariably at low frequencies
Broadband cooling tower fan noise attenuation
- conventional silencers: effective at frequencies above about 300Hz - as long as they are designed to minimise the increase in backpressure. Alternatively, you can make use of directional effects and bespoke silencer elements. We have used these to reduce cooling tower noise by 10dB or more by modifying existing elements to make them acoustically effective.
Cooling tower noise control elements
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- noise barriers: these can also be effective above 300Hz (the higher the frequency, the greater the noise attenuation) and may only be necessary to attenuate the sound in one direction. The main problem with cooling tower noise barriers is that to be effective they must be close to the source. This cuts airflow and free-cooling which reduces the system efficiency, adding to running costs.
Important note: conventional mufflers and acoustic barriers do not attenuate low-frequency sound e.g. the classic cooling tower hum is left virtually untouched.
Cooling tower fan hum tonal noise reduction
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...
Cooling tower falling water noise attenuation
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.
Cooling tower pump noise reduction
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.
- vibration isolation: reduce the transmission of vibration into radiating surfaces (AV mounts or pads - but be aware of alignment)
- vibration damping: fit high performance constrained layer damping to the radiating surfaces (to the motor mounting plate in this picture). This avoids potential alignment issues. Thin sheet coupling guards can also be damped
- motor cowls: these can be very efficient noise radiators. The solution is to fit silencers to the cowls as shown here. This cut the noise by 10dB - 15dB without compromising cooling
Where pump noise is relatively high frequency, then acoustic screens can be effective, as long as pipe runs are not significant noise radiating surfaces.
Cooling tower gearbox noise control
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.
Cooling tower fan silencing by conventional means is often both very difficult and very costly as the problem is usually low frequency in nature so conventional silencers and noise barriers don’t work. Despite this fact, the cooling tower world is awash with placebo silencers and acoustic barriers that are just a waste of money. Alternatively…
… quite often there are low-cost source control alternatives. For example, the dominant environmental noise problem from a food processing plant in the middle of a village was caused by cooling tower noise from a pair of units – despite the large silencers already fitted to the axial fan exhausts. We devised engineering modifications that produced a substantial improvement in the efficiency of the cooling towers that, coupled with measures to reduce the noise from falling water, not only cut the overall noise level from the units by 15dB(A), but also substantially reduced running costs.
The noise from a pair of new cooling towers had ignited complaints from residents across the road from the site. A typical acoustic consultant was drafted-in who recommended the industry default silencers and a noise barrier without carrying out any diagnosis. Not good practice, let alone best practice.
The predicted effect on the plant noise of fitting the typical conventional acoustic package of silencers and a barrier along the edge of the site was to reduce the noise level down to around 56dB(A) at the neighbours - still far too high for the area. Despite a total cooling tower noise attenuation package costing well over £40k, not only would the problem not have been solved but in addition, as the acoustic barrier would have caused serious transport access problems, the company had to look for alternatives.
Our solution was based on a very accurate diagnosis and ranking of the cooling tower fan, drive and pump noise sources to develop an engineering source control package. The final noise reduction measures for both cooling towers comprised:-
- intake and drive motor modifications
- exhaust and pump silencers
- local acoustic screening
This package achieved a noise level of <50dB(A) at a cost of only £6k and without affecting normal site operation or cooling tower efficiency. Around 15% of the cost and more than 4 times the attenuation of the conventional noise reduction measures.
How to select the optimum cooling tower noise control measures >
Cooling tower noise had triggered complaints as the noise was highly tonal. This led to a £31k quote from the supplier to replace the whole fan unit on the recommendation of their noise consultant. The local EHO suggested they contact us for a 2nd opinion.
We asked for a video clip by email, analysis of which allowed us to determine that the problem tone was at the tooth meshing frequency of the pinion gear. As a result, the gearbox was replaced at a cost of £3k. The whole process from 1st phone call to the provision of a detailed proposed solution took 6 hours…
This cooling tower noise reduction project is yet another example of the very poor diagnostic and engineering knowledge of many noise consultants. Far too often, decisions about noise control measures are made based on assumptions about noise sources rather than on accurate and precise diagnosis. There is a simple diagnostic process that must be followed when evaluating cooling tower noise control projects. This case study illustrates the power of simple, accurate diagnosis to remove all guesswork in solving noise problems quickly and at very low cost.
Detailed guide to selecting cooling tower noise attenuation techniques >
