Optimum dust and cyclone separator extract fan noise control techniques
Dust collector filter extractor and cyclone separator fan noise can cause complaints. There are several simple, low-cost solutions. Conventional silencing can be difficult as dust in the air will clog the attenuator, reducing the performance and requiring regular cleaning and maintenance. Installing silencers can also reduce the efficiency of baghouse filter systems. In some cases, structural vibration or acoustic resonances can also cause noise problems.
Given the problems with conventional dust extraction fan silencers, how can you reduce the noise from dust collectors and cyclone separators without the maintenance and efficiency compromises associated with attenuators?
Fortunately, there are solutions in the form of elegant, low-cost alternative environmental noise mitigation methods for these traditionally difficult applications.
We've also developed the technology to diagnose and solve these noise problems remotely by email - a much faster and lower-cost process than commissioning noise reports.
What are the causes of cyclone separator and dust extraction system noise?
There is typically only a single major noise source: one or more fans. There may also be subsidiary noise sources such as scrap rattle in ducts, reverse jet filters and acoustic resonances. The mitigation options are:-
- Reduce the fan noise at source using efficient, low-cost aerodynamic technology. This simultaneously cuts the noise at the intake, at the exhaust, through the cyclone and ducts.
- Either damp or acoustically lag cyclones and ductwork to reduce material impact rattle.
- Eliminate acoustic resonances via minor geometry changes
- Cut reverse jet filter noise at source
The following is a detailed explanation of the best practice dust collection, cyclone separator, and bag filter fan noise reduction technique options with examples.
Dust extract noise control
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Low-frequency tonal fan noise
Filter and cyclone fans often generate low-frequency (LF) tones (hum or drone) that can vary in level substantially depending on the backpressure in the filter bags (how often they are cleaned). These tones travel long distances, causing noise complaints. These LF tones at the blade passing frequency of the fan (fan speed x number of blades) are very poorly attenuated by conventional mufflers as absorptive silencers are only effective at mid-to-high frequencies (above about 300Hz).
The best solution is to use aerodynamic fan noise reduction techniques:-
- surgically removes the tones without affecting the system efficiency
- cuts noise at source - intake, exhaust, cyclone and breakout noise are all simultaneously reduced
- very little downtime required to fit
- cannot clog and requires zero maintenance
- often possible to remove or replace existing attenuators to improve efficiency and cut running costs
Scrap and dust extract chopper fan and cyclone sound sample
In the example shown here, the 147Hz tonal noise from this tyre remould scrap and dust extract chopper fan and cyclone was reduced by 32dB and the overall noise by 19dB(A) by fitting an aerodynamic insert at a cost of c £2k instead of the proposed enclosure at a cost >£30k - and that would not have attenuated the tone...
Overall cyclone noise level - dB(A)
The mid to high-frequency noise can be generated by airflow through the extract fan or cyclone separator, by material and scrap impacts on ducts and cyclone surfaces, by system vibration radiated as noise or by reverse jet filters - or by all four. The noise reduction options for these sources are detailed below.
Reducing airflow noise from fans
Residual airflow noise can be reduced by:-
- using conventional silencers (these can be smaller with less impact of efficiency as the low-frequency tones have been removed)
- the addition of simple, low-cost bespoke silencer elements
- modifying existing system elements to convert them into attenuators
Material impact noise and vibration
This takes the form of mid-high frequency "rattling" sounds from the impacts of, for example, wood or metal fragments in duct walls. As the walls are thin material, they behave as very efficient loudspeakers that convert the impact vibration into sound. The same applies to motor, fan or other vibration source radiated by panels as noise. There are 2 fundamental noise control techniques, the choice of which depends on the application.
- duct and panel damping: using dead metal or sound-damped steel to add high-efficiency damping to the duct walls to reduce the response to impacts. Low cost and effective
- acoustic lagging: this is the most commonly used method. It is effective, but will always trap moisture against the duct wall that will eventually cause corrosion
Reverse jet filter noise control
The intermittent high-frequency pulsations from reverse jet air filters are another potential cause of complaints. There are 2 effective noise control techniques:-
- silencers on external air exhausts. We have designed very low-cost units with the benefits of zero backpressure and zero clogging
- it is possible to design internal modifications that reduce the pneumatic noise at source - and which can also reduce compressed air consumption
Cyclone separator noise reduction
The noise from cyclone separators is usually created by the filter system or baghouse fans. Where that is the case, the noise reduction techniques described above provide the best solution. However, in some instances, there can be acoustic resonances in the cyclone or structural vibration that is radiated as noise (particularly if a vibrator is used to move the contents).
Cyclone acoustic resonance
The noise from several cyclone separators that were used to transport and filter PVC granules from the waste plastic included a shrill, penetrating whistle at 4975Hz that dominated the entire production area. As well as being extremely annoying, the overall 110 dB(A) noise level posed a very high risk of hearing loss.
The noise control measures under consideration were high-cost acoustic enclosures.
We diagnosed the cause to be a flow-induced resonance that was eliminated by inserting a small shaped steel sheet, tuned to provide the maximum attenuation. As shown here, this reduced the tone by 50dB and the overall noise level by 25dB(A) at a cost of <£1. Potentially a world record for cost-effective noise control...
How to reduce dust and cyclone filter system noise - the easy way...
All you need is a smartphone and 30 minutes. Send us a few photos and recordings, we will then analyse the data and send you the optimum (and costed) noise attenuation options. The details of the information we need are provided on the Remote control of noise page. This process typically reduces noise control project costs by 50% - 90%.
