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Keeping the noise down

The Schiller biomass -fired power plant successfully addressed its 'noise issue' by implementing INVC's QFT<br>Credit: PSNH/Northeast Utilities
The Schiller biomass -fired power plant successfully addressed its ‘noise issue’ by implementing INVC’s QFT
Credit: PSNH/Northeast Utilities

Originally developed for the military, so-called ‘quiet fan technology’ is now gaining traction in noise abatement in the power generation sector. Andrew Whittaker explains the potential of this technology.

Anyone who has set foot inside a power station will be aware of what noisy places they are. Boilers, gas turbines, pumps, stationary engines – each element contributes to the general din.

Fans, notably the large centrifugal induced draft (ID) fans in thermal power stations, which pull air through the boiler before discharging the combustion-product gases up the stack, can be a particular problem, creating high and low-frequency noise that can vary with the speed of operation.

The traditional approach to reducing centrifugal fan noise (indeed any fan noise) has been to fit large and costly attenuators and to then encase the offending item and attendant ducts in barriers, enclosures, lagging and other ‘silencing’ apparatus. The noise remains but, in theory, is trapped. Such measures, still widely used as standard, can be expensive to install and maintain, and will often dramatically reduce the efficiency of the fan thereby increasing energy consumption.

However, in the last decade a new approach to tackling the problem of fan noise in thermal power stations has emerged. Quiet fan technology (QFT), developed by the Industrial Noise and Vibration Centre (INVC) in the UK, turns the traditional approach on its head. Instead of installing equipment to dampen the noise, QFT works by reducing the source of the noise itself.

The technology has felt its way in the biomass sector, but is eminently transferable to more conventional thermal power stations. Indeed, by reducing running costs and increasing the efficiency of ID fans, QFT could do much to contribute towards the push for greener credentials in fossil fuel-fired facilities.

Highs and lows of fan noise

To understand QFT we need to consider the noise it sets out to reduce. Typically, the problematic sound issuing from large fans in power stations is low frequency. All fans produce low-frequency noise (and high-frequency noise too), but the problem is worse with larger fans because of their reduced running speed – large fans have to be run at a slower pace than smaller units to avoid imbalances and to keep the blade tip speed down.

In contrast to high-frequency noise, the low-frequency noise is often tonal and capable of travelling much greater distances, and of passing effortlessly through the windows and roofs of nearby buildings. So, when communities living close by to industrial sites complain about the drone, hum or beating noise caused by large fans, the subject of their complaint is this low-frequency noise.

Primarily, the tonal noise comes from the turbulence and pressure pulses produced by the fan blades. The ‘note’ that is created varies with the number of blades and the fan’s running speed. The casings, motors and vibrations in the connected ductwork can also produce noise, all of which, in the case of the ID fan, is pushed out into the environment through the power plant’s stacks. Where more than one fan is in use, the variations in speed between the fans can create that strange ‘beating’ noise, sometimes audible for miles.

As mentioned, the traditional method of treating this noise has been to try and ‘hide’ the source. Attenuators (silencers) are placed in the ductwork leading to and from the fan, their ‘splitters’ working to absorb the noise as it moves away from the blades. Additionally, acoustic enclosures are fitted or lagging is wrapped around the fan housing and ductwork, again in an effort to keep the noise contained, whilst, often, the building in which the fan is housed is itself converted into an acoustic enclosure, helping to buffer the noise from the local environment.

The cost of installing QFT is up to 30 per cent of that for traditional attenuators and other standard silencing methods<br>Credit: INVC
The cost of installing QFT is up to 30 per cent of that for traditional attenuators and other standard silencing methods
Credit: INVC

All of these measures are expensive to install and maintain, and some, silencers in particular, can disrupt the airflow to and from the fan, seriously comprising the fan’s efficiency and increasing power consumption.

Peter Wilson, technical director at the INVC has been studying the different types of fan noise and their impact for nearly three decades. In considering the traditional noise abatement methods he became convinced that a new approach was possible, utilising technology developed by the INVC for much smaller fans.

“Traditional silencing techniques are rather like palliatives in the medical world,” he explains. “They’re very expensive sticking plasters that don’t tackle the fundamental problem – the source of the noise. We set about developing a means to prevent the noise being generated inside the fan in the first place.”

From small beginnings….

Two projects related to Ministry of Defence (MOD) requirements in the UK helped the INVC establish a working model for this new approach.

The first involved developing a practicable air supply for nuclear, biological, chemical (NBC) suits, the protective garments used by military personnel in potentially hazardous environments. The MOD found that the small (7-8 cm diameter) battery-powered air supply fans fitted to the suits they had were emitting a continuous whistle.

Shortly afterwards, the second project found the INVC looking at the new ejector seat fans for the Eurofighter jet. Again, the air supply fans were causing problems; the high-pitched whistle interfering with the pilots’ communication systems.

Adding conventional silencing techniques in either circumstance would introduce materials such as foam or fibreglass into the system, which was unacceptable for these applications.

“We went back to first principles and studied the way that noise is generated within the fans themselves, with the objective of developing ways to reduce the noise at source,” says Wilson. “Noise is simply turbulence and pressure fluctuations in the air that are associated with wasted energy in the fan – fan efficiency curves are the mirror-images of their noise curves: minimum noise occurs at maximum fan efficiency.”

The INVC took their cue from Formula 1 development programmes, in which engineers design aerodynamic features to control the flow of air around the vehicle. They developed sets of aerodynamic aids that could be installed inside the fan casings to reduce the pressure fluctuations created by high-speed air flowing off the impellor interacting with the static fan volute.

By minimising these pressure fluctuations, they found that they could reduce the noise at the point of creation by up to 99.7 per cent; simply by introducing retrofit aerodynamic aids into the fan volute.

Having started out small with tiny air supply fans for the MOD, INVC successfully transferred QFT across to much larger centrifugal fans, all the way up to the huge ID fans used in thermal power stations.

….come big things

In 2011, the new technology was installed at the Schiller wood-burning power station in New Hampshire. At that time, Schiller’s biomass facility was the biggest such unit in the US.

“It’s a 50 MW plant,” says Jim Granger, senior engineer at Schiller. “We had a problem when the unit dropped load and got down to about 15 MW. It didn’t happen that often, but when it did, the 3-metre-wide ID combustion fan created a far-field noise that could be heard across the river.”

Residents of Eliot, the small town situated across the Picatagua river from the plant, asked if something could be done about the noise. A consultant put Granger in touch with INVC and the decision was made to use QFT on the ID fan.

Installing QFT resulted in a 10 dB noise drop at Schiller<br>Credit: PSNH/Northeast Utilities
Installing QFT resulted in a 10 dB noise drop at Schiller
Credit: PSNH/Northeast Utilities

The existing fan was not replaced but adapted, a process that took only 12 hours to complete. As backup, Schiller also installed conventional silencers in the plant’s stack (an operation which took several days).

“After installation, in the near-field of the fan we recorded a 10 dB drop in noise, which is huge,” says Granger. “Because it was near-field we know that the bulk of the sound reduction came from the QFT rather than the silencers in the stack.” The residents of Eliot were also pleased with the results. Granger also recorded a reduction of 20 amps in the power used by the ID fan after the QFT had been installed.

This energy-saving facet has been key to the technology’s growing presence in the power generation sector, as Wilson comments: “Large fans on power stations use a lot of energy. On older stations where they have been in use for a long time, old silencers may lose efficiency due to clogging and corrosion, and require costly maintenance or replacement. In turn, the fan has to work harder.

Typically, the cost of installing QFT is 10 to 30 per cent of that for traditional attenuators and other standard silencing methods. And it’s fit and forget technology; it lasts the lifetime of the fan.”

In the UK, as in the US, the responsibility for the regulation of noise pollution from industry lies with local authorities. Each case is considered in the context of its particular area. The Environment Agency’s Integrated Pollution Prevention and Control (IPPC) regulatory system dispenses operation permits to power stations based on each plant’s environmental conditions, setting the noise levels within which it must remain. Naturally, any plant near a residential area will have relatively strict noise levels with which to comply.

At the Enviropower biomass plant in Lancing, Sussex, the Environment Agency recently found that tonal noise from four ID fans was exceeding the plant’s permit stipulation, impacting on the lives of residents on a nearby housing estate.

The Lancing facility, which produces 5 MW of electricity from non-recyclable waste, supply 8000 local homes, installed QFT on the four fans, successfully reining noise levels back within the level set by the company’s environmental permit.

As at Schiller, the QFT was used here in combination with traditional attenuators; sometimes the QFT functions alone, sometimes as part of this integrated refurbishment. The environmental conditions specific to each individual fan will determine the course of action.

With the UK and other countries increasingly looking to smaller biomass units for future energy production, the case studies at Schiller and Lancing illustrate an encouraging truth – that the financial deterrents of reducing ID fan noise to a level that allows for relatively close cohabitation between power plant and residential development are not insurmountable.

In turn, the possibilities for freeing up brownfield land on which to build housing in relatively close proximity to power plants (both biomass or conventional) are again, encouraging – according to the charity Campaign to Protect Rural England, in 2012 the UK had enough redundant brownfield land to accommodate approximately 1.5 million new homes).

Watching from a distance

Perhaps the power generation sector will take its lead from other industry fields, where QFT has, thus far, found a hihgly receptive audience, keen to find an alternative to those traditional silencing ‘palliatives’.

Steel manufacturer Corus installed QFT on three large centrifugal fans at its Scunthorpe plant in the UK, and found the savings on capital costs, compared to conventional silencers, were in the region of à‚£800,000 ($1.3 million); the savings on power is around à‚£200,000 per annum.

Similarly, QFT was installed on five 1.4 MW fans at BP’s oil refinery in Chicago, in the US. The technology replaced traditional silencing equipment that was being used to dampen the drone from the fans, but which was reducing efficiency and increasing running costs.

Remarkably, given that each refinery fan is the size of a two-storey house, INVC carried out the Chicago work remotely. Site engineers in the US were directed from INVC’s UK base.

Indeed, because QFT takes its lead from the science of acoustics, the team at INVC can often make a diagnosis based on simple audio or video evidence – usually supplied these days via smartphone. Once compatibility with QFT has been established, they can then design the relevant aerodynamic aids based on the particular fan’s specifications.

It all seems too simple. And that, perhaps, has been the reason that QFT has not taken off in the power generation sector at quite the speed it might – the price of silence has always been assumed to be big, expensive and heavily engineered – even while QFT has become established elsewhere as best practice for this type of noise problem.

However, given the sector’s push towards a greener future – and all the associated implications for cost, energy use, land use and so on – it seems logical that QFT should play an important role in how the industry manages its noise in the years ahead.

For more information on the QFT work conducted by INVC, visit www.invc.co.uk.

Andrew Whittaker is a UK-based freelance writer.

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