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Fans And Blowers Noise Level
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Fan blade pass frequency is a pure tone produced when the blades of the fan wheel (impeller) rotate past the housing cut-off sheet in centrifugal fans, or the turning vanes, in axial fans. The blade pass frequency is calculated by multiplying the number of blades times the rotating speed in revolutions per minute. If this frequency matches the natural frequency of the ductwork, it can excite the ductwork, which can cause it to resonate, thereby increasing the noise level. Because of this possible increase in sound, and because certain pure tones are irritating to people, the sound output of the blade pass frequency should be investigated when sound reduction is desired. The next factor to consider is the fan design. Generally a fan operating at peak mechanical efficiency will produce less noise, because high efficiencies result from minimal air turbulence within the fan.
The sound generated by some fans can be a potential hazard to personnel in close proximity to the fan, and the sound can be transmitted, via the ductwork connected to the fan, to all areas serviced by the fan. Because of these concerns, fan manufacturers publish sound ratings for their products to serve as a guide for selecting fans to meet sound specifications, and to assist acoustical consultants in predicting the total noise levels in specific environments. This article provides basic information to help understand fan sound ratings and how to apply them. Like any mechanical device, fans generate sound, which emanates naturally from the turbulence of moving air, the mechanics of moving parts of the fan, and from vibration.
Air turbulence within the fan increases the sound coming from the air movement. The noise resulting from air turbulence is a major factor in the sound levels of a fan in a specific application. Further, duct work can transmit this turbulent noise to all areas serviced by the fan. Factors contributing to air turbulence include the resistance to flow, flow separation along fan surfaces, and shock related to abrupt changes in the direction of airflow, pressure, or velocity. A lower noise level can be achieved by reducing air turbulence. This can be done by considering several factors related to air movement when selecting fans.
There are four basic centrifugal fan wheel designs - forward curved, backwardly inclined, radial, and radial tip - and a variety of axial flow wheel designs. Each wheel design has unique sound characteristics due to the way they handle air, and the efficiencies they can achieve. Fan speed does not always determine which fan will be quieter. For example, centrifugal fans have higher amplitudes at lower frequencies, while axial fans exhibit higher amplitudes at the higher frequencies. The amplitude of the blade pass frequency on an axial fan is higher and more pronounced than on backwardly-inclined fans, and commonly will have amplitude peaks at multiples of this frequency.
Of the four centrifugal designs, the backwardly inclined fans are the most efficient, and therefore, the quietest. Those with airfoil-shaped blades offer the highest efficiencies, for clean air environments, while those with single-thickness blades can be used in applications where light dust or moisture is present, although the efficiencies are somewhat lower.
Certain types of axial fans offer the next highest efficiencies. An excellent example is the Vaneaxial fan that uses airfoil shaped blades in an in-line flow design. This fan is used to handle high volumes of clean air at low pressures, which is a typical ventilation application.
Radial fans are typically low efficiency, open designs for special purpose applications, such as bulk material handling, or exhausting / supplying lower volumes of air at higher pressures. A radial fan will be much louder than a backwardly-inclined fan operating under the same volume and pressure conditions.
Radial Tip fans, commonly used to handle larger volumes of air that contains particles or material, exhibit sound characteristics similar to the radial fans. The sound spectra of radial and radial tip fans contain amplitude spikes at various frequencies, and a noticeable spike at the blade pass frequency.
The forward-curved fan design operates at speeds that are much slower than the other fan types, which results in lower noise levels from mechanical operation and vibration. However, because of its modest efficiencies, a forward curved fan may be noisier than a backwardly-inclined fan when operating at comparable volume and pressure. The sound spectrum of the forward-curved fan shows a slower rate of reduction in amplitudes than the other centrifugal types, and because of the large number of blades, the blade pass frequency occurs much later in the spectrum and is not predominant.
The moving components of the fan - the motor, bearings, and drive - produce sound. This too can be transmitted through the system via the fan structure or shaft, or when these components are in the airstream. Motor sound will vary with speed, enclosure, electrical characteristics, and even the manufacturer. Antifriction bearings can be used to reduce bearing noise, and proper drive selection will reduce the likelihood of belt hop, or slap. Of course, proper maintenance must be employed to keep the moving parts running smoothly, and quietly.
Excessive vibration can significantly add to the overall noise level of an installation. This will occur if the fan or any of its components are not adequately balanced, if the fan is installed on an insufficient foundation, or if the fan is not properly isolated from other system components. For example, it is not uncommon for the fan’s support structure or ductwork to have a natural frequency at the fan’s operating speed or blade pass frequency, either of which can cause the system to resonate at that frequency, increasing the sound levels, and the possibility of damaging the installation. These risks can be eliminated by changing the speed of the fan, installing appropriate isolation, and / or detuning of the fan or affected system components.
dBA is a useful measurement for evaluating the overall noise level at a particular location, but this measurement takes into account all of the sound sources affecting that particular location, which include the sounds from all equipment in the area, natural sounds of the environment, and from other environmental factors. Some of these factors are the current physical properties of the air such as temperature, humidity, and pressure, whether the location is outside or inside, the size and material of the room. All of these affect the sound pressure experienced by the listener, and recorded by the sound level meter.
Published fan sound power ratings and corrections only reflect noise created by air turbulence within the fan. Because of the infinite variables, mechanical noise and vibration noise are impossible to accurately predict, and are not included in the rating.
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