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Fan and Blowers Noise Level

Date Added: November 03, 2012 04:51:01 PM
Author: Oleg Tchetchel
Category: Education

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. 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. The first factor to consider is the fan’s blade pass frequency, which 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. 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. For additional information please refer to http://www.buffaloblower.com/blowers/index.html. Oleg Tchetchel Buffalo Blower Canada Industrial Process Ventilating Engineers buffaloblower@buffaloblower.com http://www.buffaloblower.com/oem/index.html http://www.buffaloblower.com/inquiry/index.html
 
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