Document 29wdgjNv0zXqVY1ONRMp5Ne6

352 CHAPTER 25 1960 Guide . p* static pressure L JI"(r)(D.)(rpm)T 4005 /wheel peripheralX V velocity pressureJ 9/A. (*)(D)(rpm) outlet velocity wheel peripheral velocity (16) (17) TIm data arm for a partrarfer fen. H Quantifies i>,, 4>, and Lws ore dhcvssed b> (to text. Such curres apply fo off fan* a a similar teriex. Rg.5 ....Typical Specific Fan Characteristic Curves, Includ ing Specific Sound Power Level Example 1: Consider ft 36^6-in. diameter fan operating at 700 rpm. If it produces a noise level of 70 db what will be the level if a 49-in. diameter fan operating at 1000 rpm is substi tuted for it? Solution: From Equation 12, Oiange in lovel - 70 log,, ^ + 50 log* ^ - 16.8 db Ww 700 Noise level of second fan = 70 + 16.8 = 86.8 db Fan noise does not follow equations exactly. The radiated sound power of a centrifugal fan, for example, is not all con centrated on the blade frequency. While the overall sound power level seems to follow these equations fairly closely, the various components in the different frequency hanriq 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 equa tions exists than for centrifugal fans. This is particularly true as speed is increased. The fundamental or blade frequency exists over a wider range of pressure in the case of the cen trifugal 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 coor dinates.' To describe the pressure characteristics the pressure coefficient \f/ can be plotted against the flow coefficient 4 as shown in fig. 5, where Jr ()(,) (rpm) |_ 4005 total pressure (wheel peripheral\ velocity pressureJ (16) wheel diameter, feet. p, = static pressure, inches of water. pr = total pressure, inches of water. 5 = quantity of air discharged, cubic feet per minute. A, => outlet area, square feet. Such curves apply to all fans in a arnilm- series, i.e., a series of fans differing in size only while preserving complete simi larity in shape by having all linear dimensions changed in the same proportions. The overall specific sound power level Lws in decibels, as shown in fig. 5, is defined as follows:* where Lwa = Ljr -- 10 logialg-p,1} (18) 9 = quantity of air discharged, cubic feet per minute. pt * total pressure, inches of water. Lw = sound power level actually measured at equivalent point of rating, decibels re 10"** watt. Since static pressure p, is more frequently measured for fans than total pressure p,, p, may be used under most normal operating conditions. When the difference between p, and pi is less than 20 percent, the error will then be less than i db. Near free discharge, p, is near zero and p, must be used. In other words, Lwa is the sound power level that a similar hypo thetical fan would produce when operating at 10,000 cfm and a total pressure of 1 in. of water at an equivalent point of rating. . Although few data are available to confirm the conclusion, it is indicated that such a plot also bolds throughout a series Tb overall power level if first determined from Eqoation 19 and it added oipebraicedb to the ordinate m Fig. 6. From Celeronci 22, Used by per mission. Fig. 6 .... Chart for Determining the Sound Power Band Levels for Ventilating Fans of Two Types. Sound Control 353 of feu? for all eight of the frequency bands provided rotor or blade passage frequencies are not predominant. Approximate Overall Sound Power level In practice, data of the type shown in Fig. 5 are usually not available. Hence, in design an estimate has to be made of the overall sound power level and of the shape of the noise spec trum. Tests on centrifugal fans ranging in rise from 0.01 hp to 100 hp show that the sound power level in a duct either upstream or downstream from the fan can be found within 4 db from the simple relation:10 where Lw =* 100 + 10 !ogteEk, + 10 log,,p, (19) Ese ** rated horsepower of the fan motor. (Note: Static pressure p, may be substituted for total pres sure pt if the difference between p and p, is less than 20 per cent.) The assumption is made that the fan is being operated near the knee on its operating characteristics, le., near maximum efficiency or near the minimum in the curve of specific sound power (upper curve on Fig. 5). It is interesting to note that if the formula for fan mechan ical, or total, efficiency is substituted in Equation 19, it may be brought into the same form as Equation 18. Assuming actual horsepower -- Ei, and 50 percent mechanical effi ciency, Lwa is 105. The example illustrated in Fig. 5 demon strates the fact that all fans of the same power input are not equally noisy, since the minimum value of Lwa fr this par ticular fan is 109 db, 4 db higher than the value calculated in the above manner from Equation 19. It is on the edge o! the stated limit of approximation of 4 db. It should be emphasized that Equation 19 is intended to be used only when actual noise data are not available. _ -- Approximate Octave Band Spectrum Within about 4 db approximation, and assuming no tur bulence in the duct or the coupling between it and toe fan, the octave hand spectrum for a centrifugal fan, relative to the overall sound power level, is given in Fig. 6. limited data on vane-axial fans,u where sound pressure level measurements were made in the discharge duct, indicate 0--VIN 0OCT. //' FREE FI LO -sol----------1----------!---------1^----l---------- 1---------- 1--------- 20 75 ISO 300 OOO 1200 2400 4000 75 150 MO OOO 1200 2400 4000 10000 FREQUENCY &AND IN CYCLES PER SECOND fig'. 7 .... Sound Power Spectrum for Single-stage Axial Fan* The noise h eepressod as the sound power level in db in each of ffte fferee speech interference bonds Utrai). fig. 8 .... Grille Noise for a Grille Area of One Square Foot that the overall sound power level is about the same'as that for centrifugal fans. However, the spectrum shape of the vaneaxial fan noise is similar to that shown in Fig. 6. More recent free field measurements on axial fans indicate that perhaps the drop in level at the high and low ends of toe spectrum are more pronounced* The solid line in Fig. 7 is typical of the sound power spectrum for the axial fans re ported, as taken directly from the free field measurements. Application of a correction for the end reflection losses from the discharge opening (see Fig. 15) gives the dashed curve in Fig. 7, which corresponds to the relative sound power level in the duct. This spectrum shape for the axial fan appears to be based on more extensive data than those of Fig. 6. NOISE GENERATED BY GRILLES AND DIFFUSERS Grille Noise The subject of grille noise has not been covered thoroughly in the literature. The information given in this chapter has been taken from the advertising literature of several grille manufacturers and from recent experiments at the Massachu setts Institute of Technology- In general, grille noise at the lower frequencies is dependent primarily on the size and shape of the grille opening, on toe mass Sow per minute, and on the temperature of the air. At the higher frequencies, the noise is determined primarily by the area of the grille, the velocity of air through the free (open) areas (not the core or nominal area), and the pressure drop across the grille. For two simple types of grille, stamped and deflector vane, power levels are given in Fig. 8. These power levels are given in terms of the speech interference level, Lwai Conversion to the speech interference level in db in the room at a listener's position is described in the section on Determination of Room Levels. In the absence of more complete information, it may be assumed that the power levels of the noise produced by the grilles themselves at low frequencies are inconsequential. Fig. 8 is for a grille core area of 1 sq ft. Correction to other areas of grille is made by the relation: / Decibel additioo = 10 logiA where A = core area, square feet.