PREDICTION OF FAN SOUND POWER Fan noise is a function of the fan design (K w ), volume flow rate (Q), total pressure (P t ) and efficiency (c). The sound power generation of a given fan performing a given duty is best obtained from the fan manufacturer s actual test data taken under approved test conditions. However, if such data are not readily available, the octave band sound power levels for various fans can be estimated by the following procedure. Fan noise can be rated in terms of the specific sound power level, which is defined as the sound power level generated by a fan operating at a capacity of 1m3/s (or 1 cfm) and a pressure of 1 Pa (or 1 in. of water). By reducing all fan noise data to this common denominator, the specific sound power level serves as a basis for direct comparison of the octave band levels of various fans and as a basis for a conventional method of calculating the noise levels of fans at actual operating conditions. Blade Passage Frequency ( Bf ) Recent study shows that on a specific sound power level basis, small fans are somewhat noisier than large fans. While any such size division is necessarily arbitrary, the size divisions indicated are practical for estimating fan noise. Fans generate a tone at the blade passage frequency, and the strength of this tone depends, in part, on the type of fan. To account for this blade passage frequency, an increase should be made in the octave band into which the blade frequency falls. The number of decibels to be added to this band is called the blade frequency increment (BFI). Blade frequency (Bf ) is : B f = (rpm x no. of blades)/60 The number of blades and the fan rpm can be obtained from the fan selection catalogue. If this catalogue is unavailable, Table 1 may be used for estimation. 87
Table 1. Octave band in which blade frequancy incriment will occur Specific sound power levels and blade frequency increments are listed in Table 2. Table 2. Specific Sound Power Levels (db re 1pW) and Blade Frequency Increments (BFI) for Various Type Fans 88
Point of Operation The specific sound power levels given in Table 2 are for fans operating at or near the peak efficiency point of the fan performance curve. This conforms with the recommended practice of selecting fan size and speed so that operation falls at or near this point; it is advantageous for energy conservation and corresponds to the lowest noise levels for that fan. If, for any reasons, a fan is not or cannot be selected optimally, the noise level produced will increase and a correction factor C as shown in Table 3 shall accounts for this. Table 3. Correction factor for Static peak eff. This correction factor should be applied to all octave bands. Prediction of Fan Sound Power (Lw) Sound power levels at actual operating conditions may be estimated by using the actual fan-volume flow rate and fan pressure, as: where : Lw = estimated sound power level of fan (db re 1pw) Kw = specific sound power level (see table 2) Q = flow rate, m3/s (cfm) Q1 = 0.000472 when flow is in m3/s (1 when cfm) P = pressure drop in pascals (in.h2o) P1 = 249 when pressure in pascals (1 when in.h2o) c = correction factor in db, for point of fan operation. Values of the estimated sound power level are calculated for all eight bands, and the BFI is added to the octave band in which the blade passage frequency falls. 89
Sample Calculation A forward curved fan BOX-T 500 is selected to supply 4.15 m3/s at 750Pa. It has 41 blades and operates at 904rpm with static efficiency of 56%. What is the estimated sound power level? Step 1 : Obtain the specific sound power level (Kw) from Table 2 for forward curved. Step 2 : Calculate the additional sound power levels due to the volume flow rate and pressure. Step 3 : Calculate the Bf to determine the BFI falls at which octave bands. Bf = (rpm x no. of blades)/60 = (904x41)/60 = 617 Hz The BFI falls on 500Hz octave bands. (i.e between lower f, 355Hz to upper f, 710 Hz) Step 4 : Determine correction factor c for off-peak efficiency. From catalogue performance data, this fan shows a peak efficiency of 62%. % of peak static efficiency = (56/62)x100 = 90.3. From Table 3, c = 0. Combine all 4 steps as shown in the Table 4. Lw(Linear) = 98.2 db LwA = 85.8 db(a) 90
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