Industrial Noise & Vibration Centre

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Chiller Noise Reduction

Chiller noise control techniques

Chillers, dry coolers and air condensers are a very common source of noise problems. Unfortunately, installing ineffective noise control measures that are costly and can reduce system efficiency is also common. There is a simple process to determine the best noise attenuation method. Chiller noise control must be based on understanding the 3 typical noise sources, each with their own characteristics and each with a different optimum silencing solution.

  • mid-high frequency air noise: silencers and barriers can work, but can also reduce efficiency
  • compressor hum or drone: low-frequency tones plus harmonics - use engineering source control first
  • chiller fan hum: very low-frequency tones: use aerodynamic noise control technology: silencers and barrier are ineffective

Not using the following process cost one company over £100k. They replaced an air conditioning chiller unit unnecessarily based on the overall dB(A) whereas the complaints were due to a different unit generating a low-frequency fan hum...

Sound file: dry chiller and AHU noise control at source: 12dB(A) attenuation

Broadband mid-high frequency air noise

Caused by air movement and usually dominates the overall dB(A) noise level. By their nature, air-cooled condensing units, air handling units, heat exchangers and air conditioning plant pass high volumes of air through heat exchanger radiators. This airflow velocity through less than ideal paths generates turbulence and therefore sound. This type of noise is reasonably directional and can be attenuated by conventional silencers and acoustic barriers. However, great care must be taken to avoid reducing the efficiency of the chiller:-

  • inlet and exhaust silencers: can generate high backpressures that either cut airflow (reducing cooling) or that require higher fan speeds, increasing noise and power consumption. Bespoke silencer design can be an option to minimise these effects
  • noise barriers: these have to be placed as close to the chiller as possible for maximum attenuation. This reduces airflow and free-cooling, compromising efficiency. Careful location and geometry design can minimise these losses, as can taking advantage of sound directional effects

Chiller compressor noise control (hum)

chiller compressor classic noise signature

Compressor hum is caused by pulsation vibration transmitted into the structure that is then radiated as sound or via direct vibration from badly mounted units. You typically get tonal noise at the rotation speed (usually c 49Hz) and many harmonics over a wide frequency range. Particular high-level tones occur at the pumping frequency from pipework pulsations as shown here. Anyone can easily diagnose this issue using one of the many free spectrum analyser apps on a smartphone. The most effective chiller compressor noise reduction options involve the following:-

  • vibration isolation
  • vibration damping

Once these have been addressed, then enclosure and/or barriers can be added if necessary. Note that enclosing without considering isolation and damping is often completely ineffective.

chiller compressor noise control

Vibration isolation: you cannot use sound as the diagnostic tool due to complex reflections of tones. You must use vibration measurements to determine the transmission paths. However, it is often possible to evaluate common issues by inspection. Is the compressor vibration isolated effectively (not hard mounted, are there any vibration breaks in the pipework, no short-circuits, not mounted off thin panels)? How is the high vibration pipework attached to the frame?

Vibration damping: compressor and pipework vibration are fed into the chiller frame and thin sheet metal panels that then behave as efficient loudspeakers. The simplest (and low cost) solution is to introduce high-efficiency damping to the radiating surfaces to dissipate vibration energy to reduce the noise. This can be applied either by replacing thin panels with laminated versions or by retro-fitting high loss damping in situ e.g. https://sounddampedsteel.com/laminated-metal-how-does-it-work/.

If the compressor is the dominant source, these engineering source control techniques can reduce chiller noise by 5dB - 15dB.

Chiller fan noise attenuation (hum)

chiller fan hum tone

The hum often associated with chiller condenser fans (a very common cause of complaints) is at the blade pass frequency of the fans (speed x number of blades) and is caused by poor aerodynamic flow through the impellers. One of the most common and costly mistakes involves wasting resources on silencers and acoustic barriers in an attempt to deal with the hum. Neither is effective at these low frequencies, typically in the range 40Hz - 200Hz as shown here. Also note that these low-frequency tones do not contribute to the overall "A" weighted noise level (the "A" in dB(A)).

Chiller condenser fan

Once again, this source is easily diagnosed using one of the many free smartphone spectrum analyser apps - no sophisticated instrumentation is needed to avoid the costly mistake of assuming silencers or barriers will work.

The only effective solution to reduce chiller fan noise is either to buy new low noise units or to fit aerodynamic fan modifications to reduce the tones. The latter are retro-fit inserts and geometric flow control devices that eliminate the pressure fluctuations causing the hum at source by up to 99% (20dB) or more. This is a very low-cost solution that can also improve fan efficiency - by as much as 23% on one data centre relief fan application. This can often make these noise control projects self-financing.

Chiller noise control: the process

We follow this simple procedure to ensure that the optimum noise control package is applied. You should do the same. This approach applies to all chiller, AHU, dry cooler, air condenser, HVAC and refrigeration system noise problems.

  1. Analyse the problem: use a free smartphone frequency analysis app to assess tonal content - or email smartphone noise or video recordings to us for a free analysis
  2. Diagnose the source(s): link tones to chiller components as detailed above
  3. Source control: select the appropriate noise control measures (as above) for the problem source(s)
  4. Check noise attenuation performance: reanalyse frequency content and noise levels

If this process has not been followed, then the selection of, and expenditure on noise control measures has been based on guesswork - which can be very costly.

We provide the whole process as a turnkey service, usually without a site visit via remote control of noise. The initial analysis and diagnosis are completely free - we provide you with the results and a detailed evaluation of the costed noise control options using current best practice. This can be followed by detailed recommendations for implementation by local contractors.

Chiller fan noise reduction

Attempts are often made to solve classic low-frequency chiller fan noise using conventional silencers and noise barriers. Despite the fact that there is zero chance that these techniques can work at low frequencies, a fortune is still wasted every year on these useless palliatives. Previous failed attempts on this air cooled chiller (48.5Hz and harmonics) were replaced with aerodynamic techniques that reduced the tones by 95% and improved the fan efficiency – all by email via forwarded video clips and photos.

The company had already tried using a barrier to mitigate the noise complaints to no avail. This is because barriers are almost completely ineffective at frequencies below around 200Hz (drones, hums etc). Chiller and AHU fans regularly generate low-frequency tones as the blade pass hum often falls in this range. Whilst acoustic barriers and attenuators can be used to reduce the overall dB(A) noise level, they do not touch the low-frequency tones that are very commonly the actual cause of complaints. Our analysis of emailed video clips ca immediately confirm whether this is the case, avoiding potentially costly mistakes. In contrast, our aerodynamic axial fan noise reduction technology addresses the low-frequency noise problem by reducing it at source with increased fan efficiency as a common additional benefit.

Click this link for more detailed information on the best approach to noise attenuation for chiller, air condenser, HVAC and refrigeration fans.

data center server farm fan noise attenuation

Data centres require a lot of cooling - which involves noisy chiller fans, air handling units and condensers. Moreover, in addition to the general fan air noise, there are often low-frequency tonal elements (hums) that cannot be silenced using conventional silencers and acoustic barriers. Fortunately, there are low-cost alternatives...

An Abatement Notice was issued re the noise from a large array of chillers and air handling units at this data centre. The company had already spent nearly £100k on noise control with very little success and were considering claiming Best Practicable means (BPM) in court when the council suggested they might like to get a second opinion from us…

Our accurate diagnosis and control technology meant that we could guarantee BPM would be implemented by recommending a range of innovative noise reduction techniques for the AHUs and chiller condenser fans. These reduced the plant noise by 15dB at less than half the previously expected cost. If this approach had been implemented in the first place, the company would have saved over £100k, the council would have saved resources and the complainants would not have had to suffer unnecessary disturbance over a 2 year period.

More on data center, chiller, condenser and heat exchanger fan noise reduction.

Rooftop chiller fan noise attenuation

The cost of reducing the noise from air conditioning system chillers on the roof of a large multi-storey data centre facility to meet the requirements of the Local Authority had been estimated at over £300,000. Fitting the proposed acoustic screens round the condensers would also have required considerable planning (closing streets and bringing-in cranes) and the proposed noise control measures and silencers would have reduced the cooling capacity of the HVAC system. We had a better idea…

The communications company involved approached us in the hope that we could provide a better alternative. A very careful analysis of the noise sources and layout of the air conditioning plant plus some lateral thinking resulted in an elegant solution involving a combination of engineering control at source, novel local acoustic treatments and the clever use of geometry. The result was a set of modifications that were easy to implement in convenient stages and provided a reduction in the condenser axial fan noise of c 15dB(A) at a cost of c £40,000 – and with no effect on the normal operation or the overall efficiency of the system unlike conventional noise barriers and silencers.

More details of the best approach to reducing the noise from data centres, chillers and air conditioning plant >

Axial chiller condenser fan noise reduction

Chiller noise control costs were cut by 80% using new technology. The company running a server farm had spent £100,000 on noise control measures (acoustic barriers and silencers) for their HVAC and AHU plant to no avail – the complaints continued as the noise levels were virtually unchanged. We were invited by the Local Authority to provide both parties with a definitive statement as to what constituted “best practicable means” and to sort the problem.

Careful diagnosis allowed us to generate a best practice noise control package that would eliminate the problem at a cost of c £30k.

These measures included purpose designed silencing, geometry changes, improved acoustic screening and internal modifications to the main Air Handling Units. On the air conditioning condenser units, we by-passed the conventional solution of noise barriers and silencers as these are not only high cost control measures, but they can also reduce the efficiency of the chillers themselves. Our alternative was a set of low cost engineering noise control modifications that not only reduced the overall noise from the HVAC plant at source by 8dB(A), but also eliminated the hum (worth an additional 5dB(A) noise reduction with respect to environmental noise limits) – and with no effect at all on the chiller condenser performance or access.

The result was an overall noise reduction of 12dB(A) and the elimination of the tonal content of the noise from the condensers. If best practice had been used at the start, the company would have reduced the cost of the chiller plant noise control project by 70% and avoided several months of management hassle and a sacking…

More information on the optimum way to reduce noise from server farm, chiller, AHU and data center fans >

Garage chiller noise reduction

Highly tonal noise from these chillers had been the cause of complaint for some considerable time. The company had already attempted acoustic enclosure with the result that the chillers had over-heated.

The EHO sent us sound recordings, photographs and noise levels. Once analysed, we were able to say that the noise problem could be resolved for a few hundred pounds without running the risk of chiller problems. Once this information was passed on to the garage, they contacted us and we supplied them with detailed drawings that they used to get the silencers made locally.

Problem solved…

Chiller noise control case studies

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