Industrial Noise & Vibration Centre

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Chiller AHU Fan Noise Control – classic low frequency problem solution

Chiller fan noise reduction

Attempts are often made to solve classic low-frequency chiller AHU 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. As air handling units (AHUs) on industrial sites, offices, supermarkets and other retail premises are such a common noise problem that you would expect better solutions based on a better understanding of the issues.

AHU noise control failings...

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 (heard as 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 can 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.

What are the air handling unit noise sources?

Typically, there are only 3 sources/types of noise generated by AHUs. They are:-

  • Broadband air noise ("rushing sound") from the fans across a wide range of frequencies. This dominates the headline dB(A) noise level and can be attenuated by conventional silencers, barriers and enclosures. However, these can significantly reduce the system efficiency, adding to the cost.
  • Low-frequency tonal noise ("hum or drone") at the blade pass frequency of the fans. These do not usually contribute anything at all to the overall dB(A) figure - and yet are a very common cause of complaints. They cannot be attenuated significantly using conventional means, the only effective solution is to use low-cost aerodynamic technology, technology that will often increase fan efficiency, reducing the cost still further (self-financing).
  • Mechanical noise due to vibration transmitted into the support structure and then radiated as sound. Tonal at the motor speed and higher harmonics. The best solution is usually very low-cost retrofit high-performance damping.

When considering the options to reduce air handling unit noise, it is vital to diagnose the precise nature of the noise problem and which source(s) are responsible to avoid wasting money on the wrong solution. For example, the solution recommended by consultants to cut complaints re noise from roof-mounted AHUs in London was to replace one of the units with a quieter model. This cut the noise on the roof by 7dB(A). However, the complaints continued as they were caused by low-frequency fan tones from a different unit.

The lack of a correct diagnosis cost that client over £100k...

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