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1022 CHAPTER 40 1958 Guide The following text will present data and discussion of methods whereby solutions of the noise problem can be obtained when the allowable room noise level, and the path through which the noise reaches the room, are known..... NOISE GENERATED BY FANS Aerodynamic noise from all types of fans may be broadly divided into a rotational component and a vortex component. The rotational component is associated with the impulse given to the air each time a blade passes a given point and is hence a series of discrete tones at the fundamental blade passing frequency and harmonics thereof. The vortex component of noise is largely due to the shedding of vortices from the fan blades. It is random in character and has a continuous spectrum over a wide range of frequencies determined by the fan. geometry and operation. It may also have many non-harmonic,single frequency components determined by blade geometry and local air velocity. Since the laws of generation of these two types of noise are different, they will vary in importance for different types of fans and operating conditions.6,7 In addition to aerodynamic noise, there are usually several non-aerodynamic sources of noise in equipment involving fans. Such sources in clude noise resulting from unbalance, bearing noise, brush noise, magnetic noise, and belt noise. The number of blades of a centrifugal fan generally is governed by opti mum air flow design. . The noise generation decreases but slightly for more than the optimum number of blades. A shroud around a propeller fan may serve to reduce noise considerably if it! is working properly. Such reduction is generally most effective at the higher harmonics. However, if the flow breaks down over part of the shroud, the noise may become considerably worse than for an unshrouded case. As the operating pressure across axial fans is increased the maximum sound intensity is shifted from the fundamental to higher harmonics. This effect is not observed for centrifugal fans. Effect of Fan Parameters on Noise vs Performance Fan Laws. The principal laws governing the mechanical performance of a fan, whether centrifugal or axial, are fairly well established and are given in Chapter 32 of this Guide. Fixed. Point of Rating. A fixed point of rating of a given size fan at a given speed is one for which the capacity (volume flow) is chosen as a given fraction of the free delivery capacity. Having chosen this capacity ratio, the ratio of pressure to the static no-delivery pressure is determined. This fixed point of rating will have a fixed efficiency within the limitation of certain manufacturing details. The fan laws serve to relate the perform ance of two members of a symmetrical series of fans operating at the same fixed point of rating whether at the same or different speeds. Correlation of Fan Noise with Size, Static Pressure, Speed and Capacity. The empirical relations between the noise generated by a fan and its size, static pressure, speed and capacity are as follows: Change in overall sound power level varies as: 70 logiofsizej/sizei) + 50 logio(speedt/speedi) d2) 20 logio(sizej/sizei) + 25 logio(pressure,/presaurei) 10 logiofcapaeityi/capacityi) + 20 logio(pressurei/pressurei) (13) (14) Sound Control 1023 These relations also apply only-for a fixed point'of rating. A double width fan is essentially two fans of the same size, speed and sound pressure level and therefore its:sound power level will be 10 logio 2 or 3 db greater than for the single one. ,. Example 1: Consider a 36)^-in. diameter fan operating at 700 rpm. If it produces a noise level of 70 dbc what will be the level if a 49-in. diameter fan operating at 1000 rpm is substituted for it? ,. , , . Solution: From Equation 12, _; ,' ,, ' 'V. 49 . : 1000 Change in level = 70 log10' + 50 log!0 r~;: = 16.8 db 36.5 1 700 Noise level of second fan = 70 + 16.8 = 86.8 dbc Fan noise does not follow equations exactly. The radiated sound power of a centrifugal fan, for example, is not al! concentrated on the blade fre quency. While the overall sound power level seems to follow these equa tions fairly closely., the various.components in the different frequency bands may show greater deviation. In the case of axial flow fans there is a wider range in characteristics of the vortex noise, so it would be expected that a slightly greater deviation from these equations exists than for centrif ugal fans. This is particularly true as speed is increased. The funda mental or blade frequency exists over a wider range of pressure in the case of the centrifugal fan. In the axial fan, the harmonics are more easily excited and often the second and third harmonics exceed the level of the fundamental. Specific Plots Curves which describe the characteristics of an entire series of similar fans can be plotted by using dimensionless coordinates.7 To describe the pressure characteristics the pressure coefficient can be plotted against the flow coefficient <j> as shown in Fig. 5, where i't = p )(Z})(R1'M) J 4005 total pressure wheel peripheral velocity pressure (15) P. = M(D)(RPMn 4005 J static, pressure wheel peripheral velocity pressure (16) (CFM)/(Outlet Area) <t> = Or) (D) (RPM) outlet velocity wheel peripheral velocity (17) where D -- wheel diameter, feet. P = static pressure. Pi = total pressure. - Such curves apply to all fans in a similar series, i.e., a series of fans differ ing m size only while preserving complete similarity in shape by haying all inear dimensions changed in the same proportions. An overall specific sound power level PWL,, in dbe as shown in Fig. 5 nb* , defined that will be applicable to all fans in a family.7 It is tamed by means of Equation 18 which follows: