Solving large fan noise problems has conventionally involved many days or weeks of downtime, high capital expenditure and a reduction in system efficiency. There is now an elegant alternative that can be installed in hours at a fraction of the cost - and that can actually improve system efficiency…
Traditional noise control techniques assume that the noise generated by the fan is an immutable fact. As a result, most typical fan noise control projects are based on high cost, antediluvian palliative techniques that haven’t changed since the 19th Century. The only noise control options that are considered revolve around attempts to reduce the noise energy escaping from the system by using silencers, lagging and enclosures. This is very costly in terms of capital expenditure, installation downtime and increased maintenance and running costs, the latter due to the effect that silencers can have on system efficiency.
The Industrial Noise and Vibration Centre (INVC) has developed a game-changing alternative technology based on a deep understanding of the noise generation mechanisms in fans. Coupled with Computational Fluid Dynamic (CFD) modelling, this is then used to design retro-fit aerodynamic aids that change the flow within the system to reduce fan noise at source. This is similar to the way that racing car and aircraft designers develop sophisticated aerodynamic aids that enhance performance by controlling air flow around the vehicle. Moreover, fan efficiency is intimately related to fan noise due to the fact that the minimum noise occurs when the fan is operating at maximum efficiency - noise is caused by energy wasted as turbulence.
Where applicable, this approach can mean that it may be possible to dispense with conventional silencers entirely, increasing the efficiency gains still further, by as much as 20%. Moreover, the INVC can evaluate the options for most fan systems anywhere on the planet from a couple of photographs and a smartphone recording sent by email.
How well does it work in practice?
Process extract fan: The 6MW variable speed fan shown in Figure 2 generated such high levels of low frequency tonal noise that working in the control room was almost unbearable. Considerable resources had already been spent on lagging to try to contain the problem with little success. The next step would have been to cost the installation of inlet and exhaust silencers which would have required moving the fan.
Not surprisingly, the aerodynamic silencing option was chosen instead. A sophisticated computer model of the fan was created as the basis for the design of the aerodynamic inserts. This resulted in a 17dB reduction in the typical 190Hz blade pass tone and 12dB of attenuation at the first harmonic. In principle, the whole project - information capture, design, manufacture and install - could have been completed within a month.
Dryer fan: The noise from the fan / stack system shown in Figure 3 created an environmental noise problem. The quote for conventional silencing including structural steelwork and stack modifications was Euro115,000 plus weeks of costly downtime. The engineers also considered that silencers would cause an unacceptable reduction in fan efficiency, which was not in keeping with the company’s drive to reduce its energy use and
carbon footprint.
After online research for alternatives, the company forwarded the fan details and audio recordings to INVC, which analysed them and assessed the options. The airflow inside the fan was then modelled and an aerodynamic insert designed to be fitted inside the fan casing. This was manufactured and installed by a local mechanical contractor in a matter of hours during a normal shut-down period, meaning that zero additional downtime was incurred.
Post modification, the problem fan noise was eliminated (20dB attenuation, See Figure 4) at around 10% of the cost and without the reduced efficiency of silencers. Instead, there was a 6% reduction in fan energy consumption, which meant that the noise control project paid for itself in 10 months.
Refinery fans: In another example, using aerodynamic technology to silence five 1.4MW fans at a refinery (15dB attenuation) saved over US$1m from the US$1.25m capital cost quoted for conventional silencers. Installation also required negligible downtime and there was no effect on fan performance.
Fan noise analysis by email
Based on INVC’s extensive experience with fan noise control, it has developed technology that allows it to evaluate the silencing options for any fan anywhere via email. All the end user needs is a smartphone. INVC combines information from photographs and drawings of the fan / stack system with mechanical features (fan speed and number of blades) and an analysis of smartphone audio or video to determine the options. Sites anywhere in the world with fan noise problems can get an evaluation of whether there are alternative options to conventional silencing that could cut costs by 80% within a few days.
Acoustic camouflage
Acoustic camouflage is an innovative solution to some difficult environmental noise problems where sound with ‘character’ causes complaints. It can either be used in conjunction with noise control measures or where noise control would be too expensive or impractical. Based on research into the subjective response of people to noise characteristics, Acoustic Camouflage uses bespoke sound signatures from an array of loud-speakers to ‘camouflage’ noise nuisance features in the noise from the perception of complainants. In one project, INVC eliminated noise complaints by increasing the overall noise level by 8dB(A).
Placebo silencers
Some industries are so wedded to the concept that silencers are necessary that, despite solving a serious environmental dryer noise problem for the manufacturer using low cost aerodynamic technology, INVC still had to fit unnecessary ‘placebo silencers’ in order to get paid…
Cement works – Converting existing system elements into silencers
It is often possible to design retro-fit components that convert parts of the existing system into effective silencers. At the cement plant shown in Figure 5, a plant upgrade created a very serious noise problem over a wide area that had to be solved very quickly. Conventional silencing would have required an unacceptable extended shutdown period. The INVC’s alternative was to design a set of tuned stack silencer elements that were prefabricated off site and then installed via existing access doors over a weekend. These reduced the overall noise by 19dB(A) with no effect on system efficiency at a tiny fraction of the cost of conventional silencing.
Industrial Noise and Vibration Centre (INVC)
The INVC is a multi-award winning engineering consultancy specialising in developing elegant engineering solutions to noise problems instead of relying on conventional (and high cost) palliative techniques. Innovative INVC fan noise control technology is increasingly being used to solve fan noise problems across the world as it typically provides capital cost savings of around 80%, needs ~10% of the down-time to fit and it improves system efficiency compared with conventional silencing.