Document 65OOwZRx7o1yJqeYYj3vM1rBE

640. CHAPTER 32 1949 Guide range of the fan will also be obtained. Table 1 shows desirable outlet velocities and tip speeds, or peripheral velocities, for various static pres sures. 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 conditions 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. 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 installations. Having selected a fan for its quietest operating point consistent with the requirements of the installation (note sound level curves on Figs. 1, 2 and 3), it must be recognized that ventilating fans, even so selected,emit noise and precautions must be taken in the installation of the fans to pre vent 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 stand point, the system should be designed to operate against the lowest static pressure possible. In many modem air conditioning systems it is neces sary to introduce devices into the air stream for conditioning the air in various ways, the result of which 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 considered. 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 pre vent 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. All ducts should be connected to fans with un- Table 1. Good Operating Velocities and Tip Speeds fob Multiblade Ventilating Fans Static Pressure Inches or Water M Vs y% Vs M' Vs 1 m u$. IK 2 2M 2J4 3 Forward Curved Blade Fans Backward Tipped and Double Curved Blade Fans '' :::_________________ Outlet Velocity Feet per Minute Up Speed Outlet Velocity Feet per Minute ' , Feet per Minute Tip Speed Feet per 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 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 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 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 Fans 641 painted 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 provided with sound traps or filters. Many ventilating sys tems 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 42. 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 one half of the wheel will deliver, more air than the other half. Proper installation and location are as important as the selection of the size of fan for that par ticular installation. All fans will work according to their characteristic curves if properly selected and properly installed and operated. FAN DESIGNATIONS Fig. 5 shows the names and definitions of types of fans published by the National Association of Fan Manufacturers In order to prevent misunderstandings, which may cause delays and losses, the arrangements and designations adopted by the National Asso ciation of Fan Manufacturers* and indicated in Figs. 6 and 7 should be used. Fig. 5. Names and Definitions of Types of Fans Propeller Fan A propeller fan consists of a propeller or disc type wheel within a mounting ring or plate and including driving mech anism supports either for belt drive or direct connection. Tubeaxial Fan A tubeaxial fan consists of a propeller or disc type wheel within a cylinder and including driving mechanism supports either for belt drive or direct connection. Vaneaxlal Fan A vaneaxial fan consists of a disc type wheel within a cyl inder, a set of air guide vanes located either before or after the wheel and including driving mechanism supports either for belt drive or direct connection. Centrifugal Fan A centrifugal fan consists of a fan rotor or wheel within a scroll type of housing and including driving mechanism sup ports either for belt drive or direct connection.