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.