Document YGdBK69v4jbmgLVbpp7QQB0mN

542. ' Chapter 29______________________________ . '_________________________ 1945 Guide. system A, shown in Fig. 4 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, 60 per cent relative humidity, 0.075 lb per cubic foot), (2) outlet velocity, (3) revolutions 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. Table 1 shows desirable outlet V, / Table 1. Good Operating Velocities and Tip Speeds for Multiblade Ventilating Fans Static Pressure Inches of Water M yV2s % % v i va m 2 2H 2J4 3 Forward Curved Blade Fans 'Backward Tipped and Double Curved Blade Fans Outlet Velocity Feet per Minute Tip Speed Feet per Minute - 1000-1100 1000-1100 1000-1200 1200-1400 1300-1500 1400-1700 15004800 1600-1900 1800-2100 1900-2200 2000-2400 2200-2600 2300-2600 2500-2800 . 1520-1700 1760-1900 1970-2150 2225-2450 2480-2700 . 2660-2910 2820-3120 3162-3450 3480-3810 3760-4205 4000-4500 4250-4740 4475-4970 4900-5365 Outlet 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 Tip Speed Feet per Minute 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 % .-<y Fans 543 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 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. Fans for schools, churches, residences, and all public buildings should be selected for lower outlet velocities and tip speeds than would be required for other types of in stallations. 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 .modern air conditioning systems it is necessary to introduce devices into the air stream for conditioning the air in various ways, the result ofwhich is to set up a rather high static pressure against which the fan must operate. In such cases the sound level at the fan may be too high to be neglected and special sound treatment of the installation must be con sidered. When a fan is operating against higher pressures it should be located in a room either removed from the occupied areas, or in a room which has been acoustically treated to prevent sound being carried through the walls to adjoining spaces. The fan should be mounted on a resilient base along with its driving motor to absorb any noise or vibration which might be transmitted to the floor and thence to the building structure. AH ducts should be connected to fans with unpainted canvas, or other flexible material, to prevent any vibrations being transmitted to the duct work. Ducts leading into the fan room or from the fan, should be - acoustically treated on the interior and in special cases, should be pro vided with sound traps or filters. Many ventilating systems encounter noises which are connected with the fan in no way. Noises due to high duct velocities,.abrupt turns, grilles, etc., may be present. Treatment of such problems is covered in Chapter 32. If double width, double inlet fans are selected, care must be taken that both inlets have the same free area. If one inlet of a fan is obstructed more than the other, the fan will not operate properly, as oiie half of the wheel will deliver more air than the other half. Proper installation and location of fans is as important as the selection of the size of fan for that particular installation. All fans will work according to their Character istic curves if they are properly selected and properly installed and operated. FAN DESIGNATIONS In order to prevent misunderstandings, which may cause delays and losses, the arrangements of fan drives adopted by the National Association of Fan Manufacturers and indicated in Fig. 5 are suggested.