Document reDrGKkMEZ616gp05o5Enw4nr

826 CHAPTER 32 1957 Guide Table 1. Good Operating Velocities and Tip Speeds for Ventilating Fans Static Pbessube Inches op Wateb X % X % X % 1 IX IX m 2 2X 2X 3 3X 4 *X 5 5X 6 7 8 9 10 Fobwabd Curved Blade Fans Backward Tipped and Double Curved Blade Fans Tubeaxial and Vaneaxial Fans Outlet Velocity Tip Speed Outlet Velocity" " Tip Speed Wheel Velocity* Feet per Minute Feet per Minute Feet per Minute Feet per M inute Feet per Minute 800-1100 800-1250 900-1450 1000-1600 1100-1750 1200-1900 1300-1500 1600-1850 1850-2150 2050-2350 2250-2600 2450-2850 800-1100 800-1250 900-1450 1000-1600 1100-1750 1200-1900 2500-3100 3000-3750 3400-4250 3800-4750 4150-5200 4500-5600 1100-1500 1250-1700 1400-1900 1500-2100 1650-2350 1800-2500 1250-2000 1400-2300 1550-2500 1700-2700 1800-2900 1900-3050 2600-3000 2900-3350 3200-3700 3400-3950 3700-4300 3900-4500 1250-2000 1400-2300 1550-2500 1700-2700 1800-2900 1900-3050 4800-6000 5350-6700 5900-7400 6350-7950 6800-8500 7200-9000 1900-2700 2150-3000 2350-3300 2500-3550 2700-3800 2900-4050 2000-3200 2200-3500 2400-3800 2550-4050 2700-4300 2850-4550 2950-4750 3100-5000 3350-5400 3600-5750 3800-6100 4000-6400 4100 4750 4500-5200 4900-5650 5200-6000 5500-6350 5800-6700 6100-7050 6400-7400 6900-7950 7350-8500 7800-9000 8200-9450 2000-3200 2200-3500 2400-3800 2550-4050 2700-4300 2850-4550 29504750 3100-5000 3350-5400 3600-5750 3800-6100 4000-6400 7600-9500 8300-10500 9000-11500 9600-12000 10000-12500 10500-13000 11000-13750 12000-15000 12700-16000 13500-17000 14500-18000 15000-18500 3100-4300 3300-4600 Wheel velocity is the point of wheel location. mean air velocity through the inside diameter of the housing cylinder at the may determine the type of fan to be used. Where the system resistance is indefinite or variable, the pressure, horsepower and noise characteristics of centrifugal fans usually indicate their selection. Under such conditions, a steep and constantly rising pressure curve permits less variation in an delivery when the resistance varies. Likewise, a flat sound curve mini mizes the change of moving into a region of increased noise. A fan having a high efficiency over a wide range is more desirable than one which reaches an even higher maximum efficiency, but decreases more rapidly on either side of a narrow range. A self-limiting horsepower curve may permit more accurate selection of motor size. The selection of size of fan usually involves balancing cost and space against sound and efficiency. Unless the pressure involved is so high that a smaller fan running at greater speed requires a higher class2 of construc tion, the smallest fan is the cheapest in first cost of the fan only. However, as .the cost of the driving equipment is also involved in the total installation cost, fan efficiency must be considered for that reason as well as its bearing on the cost of operation. In some cases where large fans operate long hours per year, selection at absolute maximum efficiency is indicated. Generally! however, the power saving by selecting for optimum efficiency does not justify the extra cost, and a slightly smaller fan gives the best balance of cost and efficiency. Reference to Figs. 2 to 4 shows that all types tend to have a minimum sound near maximum efficiency so, when noise is a con sideration, selection approaching maximum efficiency is indicated. Too large a fan may not only mean an unnecessary investment and an increased Fans 827 power consumption and sound, but may also give faulty performance if of a type having an unstable pressure characteristic at low capacity. Table 1 shows the outlet velocities and impeller tip speeds recognized as good practice for various static pressures for centrifugal, tubeaxial and vaneaxial fans applied to average heating, ventilating and air conditioning applications. Fans for churches, schools, residences and other buildings having a low prevailing noise level should be selected for lower than average outlet velocities. FAN INSTALLATION In designing heating, ventilating and air conditioning systems, the characteristics of the fans available for use therewith should not be ignored. If double inlet fans or multiple fans in parallel are used, care must be taken that both inlets have the same free area and general approach conditions. The dimensions of the ductwork and the size of the various devices whose individual resistances determine the static pressure, dictate the fan selec tion. Often a minor modification of the system may permit use of a smaller motor, and even a lower class2 of fan, with considerable saving of cost. Invariably, the sound generated is affected, as fans operating at high pressure produce more noise than at lower pressure (see Chapter 40). Minimizing the resistance may be the best insurance against noise. On the other hand, sometimes the lowest overall cost results from selecting the minimum size of system equipment, and then installing adequate acoustical and vibration treatment. All ducts should be connected to fan outlets and inlets by means of un painted canvas or other flexible material. Access should be provided in the connections for periodic removal of any accumulations tending to unbalance the rotor. When operating against high resistance, or when ambient noise levels are low, it is preferable to locate the fan in a room removed from occupied areas or acoustically treated to prevent sound transmission. The fighter building constructions which are common today, make it desirable to mount fans and driving motors on resilient bases designed to prevent transmission of vibrations through floors to the build ing structure. Conduits, pipes and other rigid members should not be attached to fans. Noises due to high velocities, abrupt turns, grilles and other items not connected with the fan, may be present. Treatment of such problems, as well as the design of sound and vibration absorbents, are covered in Chapter 40. FAN APPLICATIONS Many fan applications and the corresponding types of fan commonly used are listed in the following paragraphs. Reference is also made to the chapters where the applications are discussed. Central System Supply Fans (Chapter 29) are usually of the centrifugal type, since this application requires a wide range of satisfactory and quiet operation against relatively high pressures. They can readily be connected to apparatus of large cross-section on the inlet side, and to relatively small ducts on the outlet side. Comparative sizes have been standardized among manufacturers,2 and tuost rating tables cover a range of 700 to 500,000 cfm, and static pressures from \ to 15 in. of water. Central system exhaust fans are predominately centrifugal, but the space