Industrial Noise & Vibration Centre

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Fume and scrubber extract fan noise attenuation

Fume and scrubber extract fan noise control techniques

Fume and scrubber extractor fans are common causes of noise complaints. Unfortunately, the passage of fumes and other chemicals (even in small quantities) makes noise attenuation very difficult using conventional silencers that will inevitably clog or corrode. Consequently, traditional mitigation methods tend to be expensive and can reduce system efficiency.

We have developed innovative and low-cost alternative noise reduction technology for these applications that avoids the issues with material choice and clogging in aggressive environments. We've also developed a simple procedure (by email) that allows us to evaluate the optimum noise attenuation method. There are only 2 types of noise from these units, each with its own silencing solution.

  • broadband air noise from the stack, filter or outlet: specially designed silencers and materials can be effective, but can also reduce efficiency
  • fan tones: low-frequency tones are best reduced using aerodynamic fan noise control technology that actually increases fan efficiency. Silencers and barriers are ineffective

dB(A): broadband mid-high frequency air noise

Generated by airflow through the extract fan, this component usually dominates the overall dB(A) noise level and will travel unattenuated if the fan is directly connected to a stack. There will be some noise attenuation if the outlet air passes through filtration (particularly dust extract bag filters). This type of noise is directional, opening up the possibility of simple redirection.

Free Remote Diagnosis

Email us a video from your smartphone for an evaluation of best practice

clogged absorptive fan splitter silencer

The usual option is a bespoke silencer, but great care has to be taken with materials selection and geometry on scrubber fans passing potentially aggressive chemicals to minimise corrosion. Moreover, conventional mufflers will almost inevitably clog and require regular cleaning - which can be an expensive process requiring extensive downtime. Opening silencers to find that they are empty other than a few corroded remains can be a salutatory experience...

Care must also be taken to avoid reducing the efficiency of the fan system as attenuators can generate backpressure that either cuts airflow or requires a higher fan speed, increasing noise and power consumption.

In many cases, we can design retrofit silencing that renders part of the existing system an effective attenuator. This can be much lower cost than conventional monolithic silencers.

Tonal noise: low-frequency fan drone or hum

stack and fan noise control

The "A" weighting filter (dB(A)) progressively filters out low frequencies as they are less damaging to hearing. However, low frequencies (typically below 300Hz) travel long distances and diffract round barriers and buildings. Consequently, whilst typical low-frequency fan tones do not contribute significantly to the overall dB(A) value (they have been filtered out), they are a very common cause of noise complaints.

The best practice solution is to cut the tonal content at source using aerodynamic fan noise control technology. This is a fraction of the cost of silencers, does not reduce fan efficiency, and lasts the lifetime of the fan without maintenance.

Weld fume extract filter fans

extract fan noise attenuation

The aerodynamic noise reduction technology cannot clog and the aero-insert materials can be selected so that they are unaffected by the composition of the chemicals in the air.

This dust extract system generated extensive complaints due to very low-frequency tones 35Hz and 70Hz). We diagnosed the cause as a duct resonance, designing a retrofit duct termination to kill the tones (30dB attenuation) plus a flow control insert (simple steel fabrication) to reduce air turbulence at a duct intersection. This not only reduced broadband noise by 23dB, but it also substantially improved the extract fan system efficiency. The modifications cannot clog and will last the lifetime of the system without maintenance.

paint shop fume extract fan noise control

Paint fume extract fan silencing

A key benefit for these types of fan is that the aero-inserts are unaffected by chemically aggressive atmospheres.

This pair of paint shop spray booth fume extract fans had caused complaints about a low frequency hum both from offsite and from personnel in the workshops. They had previously been fitted with silencers that not only did not work well but also clogged quickly and had to be regularly replaced.

Fitting aerodynamic modifications to both fans permanently cut the 148Hz tone by 95% (13dB), eliminating the complaints.

The modifications had no effect on performance and were installed with no downtime outside normal maintenance and were a fraction of the cost of conventional silencing.

Wet scrubber fan and stack noise control

Exactly the same approach can be used on any size of extract fan system. In this case, the previously used silencer on this wet scrubber fan had had to be removed as it had caused problems. Consequently, the fan noise had to be reduced by 6dB(A) to meet the off-site noise specification. The fan and stack generated a combination of broadband noise plus a tone at 240Hz.

The recommended modifications included:-

  • an aerodynamic insert to attenuate the tonal noise at source
  • a set of retrofit components to convert part of the ductwork into an effective broadband silencer.

wet scrubber fan silencing

As it was a large unit and fan performance was critical, we used Computational Fluid Dynamics (CFD) modeling to determine the optimum geometries for both the aero-insert and the broadband noise control cartridges to maintain system efficiency.

In many cases, removing the fan hum also eliminates the environmental tonal noise penalty that is applied to this type of subjectively annoying "character". In the UK this penalty can be up to 6dB(A) i.e. a measured 50dB(A) with a tone is assessed as equivalent to noise at 56dB(A) with no tone. Hence removing a low-frequency tone that does not affect the measured dB(A) can provide an effective 6dB(A) reduction in the assessed level against a noise specification.

Fume extract and scrubber fan noise reduction: the process

Following this simple procedure ensures the optimum noise control package is used on any of this type of system.

  1. Problem analysis: use a free smartphone frequency analysis app to measure any tones - or email short smartphone video recordings to us for analysis (free service)
  2. Source diagnosis: match tones to fan characteristics as detailed above
  3. Source control: select the optimum noise control measures for the problem source(s)
  4. Check performance: analyse noise levels and frequency content again

Unless this process is followed, then the selection of the noise control measures for these extract systems has been based on guesswork - which can be extremely costly.

We can provide the whole of this process as a turnkey service, usually without a site visit via remote control of noise. Our initial analysis and diagnosis are usually free, providing 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.

scrubber fan noise complaints

Scrubber fan diagnostic noise analysis example

Extensive complaints had been received due to the environmental noise from these 4 ostensibly identical scrubber fans that are run at night. The project noise consultant took photographs and emailed us smartphone recordings at locations close to the fans and off-site (item 1 above).

Analysis of these recordings allowed us to determine the precise cause of the complaints (item 2 above). The main problem was found to be a pair of tones at 214Hz (from scrubber fans 1, 2 and4) and at 226Hz (scrubber 3). As these tones are at the fan blade pass frequency, scrubber 3 was running faster than the others.

Aerodynamic fan noise control offered a simple solution where the components could be manufactured from the same material as the impellers to avoid corrosion.

Scrubber fan noise attenuation analysis

Moreover, the frequency analysis also indicated that scrubber fan 3 had severely damaged bearings and that those on scrubber 4 should also be checked. These conclusions (from smartphone recordings) demonstrate the usefulness of detailed narrow-band frequency analysis.