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HEATINC VENTILATINC AIR CONDITIONING GUIDE 1944 should have been 0.625 in., this would correspond with a system curve as shown at C and on this curve the fan would deliver 16,400 cfm to the system instead of the design volume of 13,000 cfm. In this example extreme errors have been selected to emphasize the effect the square function of the system Characteristic has in maintaining the fan performance within comparatively narrow limits. In the first example a system estimated at half what it should have been, resulted in a drop of 23 per cent in volume; and in the second example, a system estimated at twice what it should have been resulted in an increase of 26 per cent in volume. * In some instances fans may be applied to variable flow systems. In such cases the limiting systems may be plotted and the effect on fan performance examined. For instance, a system might vary between system A, shown in Fig. 6 as one limit; and system B as the other limit. The fan performance will then fall between points X and Y on the fan curve at a point determined by the system characteristics at that par ticular time. If curves A and B are the limiting systems, the fan per formance will never be outside the points X or Y. SELECTION OF FANS The following information is required to select the proper type of fan; 1. Cubic feet of air per minute to be moved. 2. Static pressure required to move the air through the system. . 3. Type of motive power available. 4. Whether fans are to operate singly or in parallel on any one duct. 5. What degree of noise is permissible. 6. Nature of the load, such as variable air quantities or pressures. In order to' facilitate the choice of apparatus, the various fan manu facturers supply fan tables or curves which usually show the following factors for each size of fan operating against a wide, range of static pressures: (1) volume of air in cubic feet per minute (68 F, 50 per cent relative humidity, 0.075 lb per cubic foot), (2) outlet velocity, (3) revo lutions per minute, (4) brake horsepower, (5) tip or peripheral speed, and (6) static pressure. The most efficient operating point of the fan is usually shown by either bold-face or italicized figures in the capacity tables. Other important factors to be considered in selecting fans are: (1) efficiency, (2) space occupied, (3) sound emission, (4) first cost, and (5) speed (both peripheral and revolutions per minute). These factors are not necessarily shown in the -order of importance. In some instal-. lations space occupied may be of first importance. In others lowest power consumption is desirable. In many cases quietness of operation of the entire system is essential. Practically all fans operate at their lowest sound level when selected at or near the peak of the static efficiency so that in selecting a fan for highest static efficiency the quietest operating range of the fan will also be obtained. Tables 1 and 2 show desirable outlet velocities and tip speeds, or peripheral velocities, for various static pressures. Fans selected accordingly will operate at or near the peak of the static efficiency with resulting low power consumption. and noise 574 ' CHAPTER 30. FANS m_____ 1 r_nAn OuT7t? ATTTJfi VELOCITIES . nn PrtDVTjn Static Pbessubb Inches of Wates x X X 1ix% m m 2 ix 2X 3 Outlet Velocity Fkst pkb Minute 1000-1100 1000-1100 1000-1200 1200-1400 1300-1500 1400-1700 1500-1800 1600-1900 1800-2100 1900-2200 2000-2400 2200-2600 2300-2600 2500-2800 Tip Spesd Fist pee Minot* 1520-1700 1760-1900 1970r2150 2225-2450 2480-2700 2660-2910 2820-3120 3162-3450 3480-3810 3760-4205 4000-4500 4250-4740 4475-4970 4900-5365 levels. Smaller fans with higher outlet velocities may be used if the installation requirements are such as to warrant the additional power and increased sound level. When space for duct expansion from a fan outlet is not available there may be advantages in selecting a larger fan for reducing duct noises, although lower outlet velocities generally result in lower fan efficiencies which cannot always be justified on the basis of increased cost and space requirements. Having selected a fan for its quietest operating point consistent with the requirements of the installation, it must be recognized that ventilating fans, even so selected, emit noise and precautions must be taken in the installation of the fans to prevent this noise from being transmitted to' occupied portions of the building. Fans operating against high static pressures produce more noise than fans operating against low static pressures. Consequently, from a noise standpoint, the system should be designed to operate against the lowest static pressure possible. In many modem air conditioning systems it is necessary to introduce devices into the air stream for conditioning the air in various ways, the result of which Conn Operating Velocities and Tip Speeds for Multiblade Ventilating Static Prebsubb Inches or Water imx IX 2 Outlht Velocity Feet per Minute 800-1100 800-1150 900-1300 1000-1500 1100-1650 1200-1750 1200-1900 1300-2100 1400-2300 1500-2500 1600-2700 1700-2800 1800-2950 2000-3200 575 Tip Spsed F*et pzr Minot* 2600-3100 3000-3500 3400-4000 3800-4500 4200-5000 4500-5300 4800-5750 5300-6350 5750-6950 6200-7550 6650-8050 7050-8550 7450-9000 8200-9850