Document 6BoK9oeo6Yb11YpN5wmoX8Kyd

202 CHAPTER 14 1965 Guide And Data Bode Sound Cot*TM1 203 (a) For o position straight m fmat of outlet. (b) For e position 45 dog to ride of outlet, (e) Dud ero section areo " 1* 'Rg. 8 .... Directivity Factor Q for Use in Rg. 6 '-.i Only at points more t-hpn 45 deg from the center line of an outlet will the beameffect produce a shadow zone.in which the direct sound level is than it would be for uniform radia tion. It is, therefore, not justified to base nalanlatinps on a directivity of less than 2 or 4, depending upon outlet location. . The directivity factor for various source locations and can be estimated from Fig. 8 as a function of frequency. STEP 3--CALCULATION OF REQUIRED DUCT ATTENUATION ' '' Noise is readily transmitted by ducts, unless they are acous tically treated. The attenuation required to reduce the duct noise below the power levels calculated in Step 2 depends on: ' 1. The amount of sound generated by the fan, and how it is divided among the various duct branches.4* Rg. 9 .... Sound Power-Spectra of Two Types of 12| in. Diameter Centrifugal Ventilating Fans Delivering 1120 cfm Against 1A in. Static,r " r Rg. 10.... Sound Power Spectrum of a Vaneaxial Fan '1 with 5 blades at 3550 rpm 2. The natural attenuation provided even by untreated ducts. 3. Sound generated by other sources, such as air flow. - 1 4. Cross transmission between rooms. Since each of these items depends on frequency, separate calculations should be made for various octave hands Sound Generated by Centrifugal Fans Selection of as quiet a fan as practicable will often reduce the required duct attenuation. When centrifugal fans for cen tral systems are selected, the following items should be con sidered: ' '* 1. Sound level, as well as the usual selection factors should be considered in evaluating various fan types. > 2. The fan should be selected to operate near its efficiency peak when handling the required air quantity and static pressure. 3. The system should be designed for smooth air now, And .. resistance should.be as low as economically feasible. 4. Duct connections to the fan inlet should be designed to provide a smooth approach of the air.41 To evaluate the effect of fan design, it is necessary to con sider the entire sound spectrum.4*-44 The sound power spectra4* shown in Fig. 9 illustrate the following'facts about ventilating fans of the centrifugal type: ' 1. When fans of different design are considered for a given application, neither the rpm nor the tip speed indicates which fan will be quieter. Backward-curved fans, if properly selected, are usually somewhat more efficient and a little quieter than forward-curved fans, although their tip speed is higher. On the other hand, forward-curved fans are more compact and lighter! Their lower speed may have other advantages, such as lessbear ing noise and more tolerance in regard to wheel unbalance. ' 2. The sound power spectra of centrifugal ventilating fans fall off from low to high frequency at an average rate of 4 to 6 db per octave. The overall power level, being a logarithmic suni, is pri marily a measure of-the noise at' frequencies below 150 cpa. Experience points to the 150-300 cpa hand as being the most critical one in silencing centrifugal central station fans, with the 75-150 and 300-600 cpa bands being'next. Noise comparisons of different fan types can therefore be based on the sound power level in these bands, but not on overall levels. 3. The sound spectra of centrifugal fans for air-conditioning systems should be relatively smooth, without prominent purs tone components. The spectrum for the backward-curved fan in Fig. 9 shows a peak at the blade passage frequency (number of blades times revolutions per second). Tins is typical for fans hav ing a relatively small number of blades because of the distinct pressure held around each blade. The peak is more pronounced for high flow rates at relatively low static: it may be aggravated by a cut-off located too close to the wheel periphery. Because of the added annoyance of pore tones, the sound output at blade Bg. 11 .... Comparison of Two Similar Centrifugal Fans of Different Size and Speed, Selected to Deliver 6000 cfm Against 1.5 in. Static Pressure frequency should be closely examined when selecting fans for critical applications. . The-sound power spectrum of a vane&xial fan,* as illus trated in Fig. 10, is quite different from that of a centrifugal fan. Relatively little sound is generated at low frequencies. The spectrum usually has a very strong peak at the blade passage frequency and often has secondary peaks at multiples'" of this frequency. - Proper siring of the fan is the most important factor in assuring a' minimum of fan noise for any given type of fan. The noise will be lowest if the fan is sized to operate near the peak of its efficiency curve when delivering the required air volume (cfm) against a required static pressure.41 Noise and efficiency of any fan are not related on an energy basis because of the exceedingly small amount of energy- required to produce noise. However, the same factors which produce noise in a fan also tend to reduce the fan efficiency. As Jig. 11 shows, the noise energy may easily be doubled (increased by 3 db) for a drop in efficiency of only a few percent! \ , An undersized fan will operate.under the right-hand part of the curve. Its efficiency will be lower and the noise higher than that of the optimum .sized fan. The primary cause of both tower efficiency and higher noise appears to be the higher air flow velocities through the fan-4*.To avoid excessive noise, the fan should be large enough so that "the velocity pressure corresponding to the fan outlet, velocity is between 15!and 1 25 percent of the staticpressure for forward-curved fans, and ; between 7 and 13 percent'for backward-curved fans. ` Oversized fans are not only uneconomical, but .are also ' ooisier than optimum sized fans. This is due to separation of air flow over the fan blades!' which occurs when a fan operates to tiie left of the peak of the static efficiency curve shown in Fig. il. Flow separation has leas effect-on efficiency..than it has on noise. The efficiency of aircrversized fanis, therefore, not a good measure of its quietness. ' Since fan design and' application .details have such an im portant effect, fan sound-ratiiigB .from'.-.the!manufacturer would be consulted. It must be. borne in mind, however, that the environment arid'technique used'to obtain threw rat-' Rg. 12 .... Horsepower and Pressure Factor! : " 10 log^W-p), for Use with Eq. U and Table 10 ingg are of the utmost importance. These should' always be clearly specified, and their significance in any selection er com parison dearly understood.1-4* 'If actual test data are not available, the approximate fan sound power level for a centrifugal fan of the type used for central stations can be estimated for each octave band from Equation M.4*- Lwi -- Ln + 10 logit (IF.-pO (14) trAere - - Lfr/ ** approximate sound power level of the fan, decibels re - 107" watts.' ', ; Lwt ** approximate base sound power level for central station, centrifugal fan*, decibels re 10~u watts. . Table 10.... Approximate Base Sound Power levels* for Central Stations.Fans of the Centrifugal Type,b--,i in db re 10-1* watt Static Efficiency* at Operating - Faint, Percent 106 125 Octave Bond Center, cpa 212 425 650 1700 3400' 6900 250 500 1000 2000 4000. .8000 Over 65 . 75- 75- 70- 65- ' 60- ' 551' 5085 80 75 70 65 60 55 50 to 65 - 30 to 50 80- * 80- 75- 70- '65- 60- 5590 85 80 75 70 65- 60 ' 85- 85- 80- 75- 70- 65- 60100 95- 90 85 80 75 70. Under 30 Unpredictable > - - * Vahua of sound power level uncertain far 37.fi to 75 freqaeocy. -,t ' b Base sound pcnrtr levels are baaed on referenoc quantitiea f 1 bp and 1 in. static pressure as defined by Equation 14. * Values are baasd on tbe cress sound power delivered by tbs fan to one duet system (supply or return). An equal amount b abo delivered to the otber - 4 Poor air approach wavtiUnas at the {an inlet will add materially to tbs noisejnaereted. . -< ,t~ The paiametere do not apply to ovarsbed (ana. operatisd at ao outlet velocity below, tbs ranee recommended by tbs {an mnnslactasrer Idf U^arfhaa i than IS add 6 db to level in band in'which bladefraP