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This is often accompanied by unwanted fan or air velocity noise making the whole system less than desirable. This can apply to both industrial systems with large fans but also thermal enclosures where smaller fans are present.
The annual UK industrial fan power consumption is estimated at well over 30TW-hours. If best practice was applied within the UK, CO2 emissions alone could be reduced by around 2 million tonnes per annum and electrical costs would also be reduced by a similarly shocking amount.
Efficiency and noise
Often ultimate efficiency and noise are linked in an intimate balance, particularly in an air movement system.
Put simply electrical energy is used by a fan to convert Potential energy such as electricity, into air movement creating kinetic energy. The kinetic energy in an efficient system is safeguarded and used in the most efficient way. If this is not the case and the system is not efficient, the kinetic energy will turn into noise which is in essence, wasted energy within an air movement system.
A simple example of this is a fan discharging directly into a duct work 90° bend or square to round section. The bend in the ductwork or transition piece will absorb energy from the airstream and create noise when the air is disturbed, causing turbulence and reducing the air stream’s kinetic stored energy and efficiency of the system.
To counteract the inefficiency of the system and to compensate for the poor use of energy, fans are often oversized. This leads to an increase in the energy used, carbon footprint and noise!
Generally the main contributor to the noise of an air movement system is the fan itself. There are many reasons fans could be louder than desired and understanding the reasons for potential noise generation can be essential in designing an air movement system.
The fan inlet can be a key place to wage war against noise, as a rule of thumb a minimum of 1.5 times the fan inlet diameter should be maintained around the radius of the fan inlet, this allows for a smooth uninterrupted airflow into the fan inlet and encourages the fan to operate at peak efficiency. The use of a straightener to give a laminar airflow is also good practise to achieve maximum benefit for noise and efficiency. If these conditions are not adhered to, fans can suffer recycling of air at the inlet causing excessive turbulence, poor efficiency and extra production of unwanted noise.
Motor noise can also be a problem in a system that has been designed to be quiet. Often in applications such as office air conditioners, fan coil units or small local air handling units, fans are massively oversized to reduce noise levels and boost efficiencies. As the air noise within fan systems reduces then other noises within a system become apparent.
Motors in applications operating in these environments are often not running at full speed but are subject to large amounts of speed control and different types of motors react differently to speed control.
Traditional AC motors can be controlled with voltage via a triac type controller or frequency via an inverter drive system. Triac control systems can be strident as harmonics are produced and cause electrical noise which can be detected as a motor hum and this is usually produced at frequencies particularly disturbing to the human audible range.
EC/DC motors are the best selection of motor for efficient speed control, the integrated drive and permanent magnet arrangement allow for the motor to be slowed efficiently with minimal electrical noise or loss of efficiency.
It is also important to size ductwork correctly to achieve the correct velocities of air within the system; small cross sectional area ductwork will cause high velocities of air creating unwanted noise and high resistances reducing the system efficiency as a side effect. In general H&V applications should aim to have ductwork velocities between 2-10m/s but the lower the velocity the lower the noise.
Selecting a fan within a system can often be an issue in itself. Manufacturers are able to pick and choose how data is displayed in order to promote their product in the best way.
In general fan assemblies are tested under three test conditions; A5, A6 and A8:
• Test condition A5 is conducted with the discharge of the fan unit connected to insulated ductwork and the measurement is taken of the “free inlet”, this is often the position for the lower noise figure and is the most commonly publicised figure.
• Test condition A6 is a measurement taken at the free outlet or discharge of the fan and is rarely publicised. This measurement can be considerably higher than the A5 test figure.
• Test condition A8 is the general average of the total sound of the unit and is the most useful when comparing two products in order to truly compare the noise generated.
Sound power/Sound pressure
Understanding the difference between sound power and sound pressure is essential when dealing with fan, ductwork and sound attenuators. If sound pressure and power are not clearly identified, incorrect selections and comparisons can rapidly follow.
Sound power is a method of comparing the actual amount of sound energy produced as a figure rather than an audible volume of noise produced. The actual value of this is measured in Watts and then converted into the decibel to keep numbers within a useable range for comparison. This must not be confused with the sound pressure dB unit or the dBA of the weighting scale. Sound power does not have an association with distance from the source noise, for example, noise produced for a fan may be 86 decibels sound power. This will be the same figure at three metres or three miles from the noise source. It is also important to note that sound power figures will always be higher than sound pressure figures due to that the fact the sound power figure is derived from actual energy produced.
Sound pressure is also displayed in dBA but along with the figure there is also a distance involved. For example a fan may produce 71dBA sound pressure at 1m distance, but as the distance increases from the fan the noise level will drop; the fan may produce 65dBA sound pressure at 3m distance.
Understanding all the points above can assist in the correct identification of facts and figures enabling the effective design and commissioning of efficient systems ultimately reducing costs and carbon footprint as an added benefit.
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