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308 CHAPTER 22 1959 Guide MOTIVE POWER Heating, ventilating, and air-conditioning fans are usuaiiy driven by electric motors, although other prime movers may be used. The small sizes of fans, and especially those operat ing in the higher speed range, are equipped with directconnected motors. For larger size fans, and those operating at lower speed, V-belt drives are generally used. In selecting the size of motor for operating a fan, it is advisable to select at least the standard size next larger than the fan requirements. Direct-connected motors do not require so great a safety factor as belted units. Justification for liberal power provision exists only in systems where it is possible that larger volumes of air may be required at intervals, and made available by use of bypass dampers, thus greatly reducing the system resistance. If such a sys- tern includes a fan with forward-curved blades, it would be necessary that the motor be sized for the mmmnm volume and duty. If such a system includes fans with backwardcurved blades, the volume peak would not make it necessary to provide additional motor power. In selecting fans for such 1 I 1 f * Ai N> I -aw, SI No2-SW,Sl No 3- SW, SI No3-0W,0l CoonUr-ClockwiaiB | * i Counter-Clockwise Top Angular Down Clockwise Top Angular Doers Clockwise Bottom Angular Up I i ! \ < 3 Ne-SW,SI No 0-SW,Sl Rg. 6.... Arrangement of Fan Drives Air. 1, SW, SL For belt drive or direct connection. Wheel overhung. Two bearing* on beae. Ait. 2, SW, SL For belt drive or direct eooneetioa. Wheel overhung. Bearing* in bracket supported by fan booaag. Air. 3, SW, SL For belt drive or direct connection. One bearing oa each aide and supported by tan tuning Not recommended in owe 27 in. diameter wheel tad smaller. Air. 2, DW, DL For belt drive or-direct connection. One bearing on each ada and supported by fan hcm*g Air. 4, SW, SL For direct drive. Wheel overhung on prime mover shaft. No bearings on fan. Baaa or equivalent for prime mover. Arr. 7, SW, SL For belt drive or direct connection. Arrangement No. 8 phza baaa for prime mover. Not recommended in sues 27 in. diameter and Air; 7, DW, DL For belt drive or direct eocmectioa. Arrangement No. 3 phis base for prime mover. Arr. 8, SW, SL For belt drive or direct connection. Arrangement No. 1 Arr. 9, SW, SL For belt drive. Arrangement No. 1 designed for mount ing prime mover on ride of beee. Counter-Clockwise Top-Angular Up Counter-Clockwise Bottom Angular Up Counter-Clockwise Bottom Angular Down Note: Direction of flotation is determined from the drive ride for either single or double width or tingte or double Met fons. (The drivng tide of a tingle Met fan it considered to be the ride opposite the Met regardless of the oefoat location of the drivej for fan inverted for ceiling suspension, direction of notation end discharge it determined when fan it resting on floor. Rg. 7.... Designation of Direction of Rotation and Discharge i I l i! Fans 309 a system, sound ratings should be given careful considera tion^ WUere a system is constant, and has uu provision for volume change that would materially reduce the resistance, and when the resistance calculations are reasonably precise, there is no necessity for too liberal a motor allowance (even where fans with forward-curved blades are used) if the fan has been property selected. Fig. 5.shows that the system re sistance varies as the square of the volume, and that the fan static pressure varies approximately inversely as the volume, thus greatly offsetting the trend toward both in crease in air delivery and motor load. Reference to Fig. 5 indicates that there is no justification for allowing large spare motor capacity. It is generally more economical to operate motors well loaded. Since the power consumption of fans varies as the cube of the speed, very little starting torque is required of the motor. Refer to Chapter 45 for characteristics of various types of motors. FAN SaECTION The following information is required to select the proper type and size of fan: 1. Capacity in cubic feet per minute. 2. Static pressure or system resistance. 3. Air density if other t-han standard. 4. T^pe of application or service. 5. Arrangement of system. 6. Prevailing sound level or use of space served. 7. Nature of load. 8. Type of motive power available. In order to facilitate the choice of apparatus, the various fan manufacturers 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 percent 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 is usually shown by either boldface or italicized figures in the capacity tables. Often the service determines the type of fan; When opera tion occurs with little or no resistance, and particularly without a duct system, the propeller fan is indicated for con venience and low cost. When resistance is low the power Table 1.... Good Operating Velocities and Tip Speeds for Ventilating Fans Static Forward Curved Blade Backward Tipped and Double Carved Blade lubeoxtaf and Vaneanal Fans Water Outlet Velocity Tip Speed Fpro Outlet Velocity Up Speed Fpm Wheel Velocity* K 800-1100 1300-1500 800-1100 2500-3100 1100-1500 H 80O-125C 1600-1850 80O-125C 3000-3750 1250-1700 H 900-145C 1850-2150 900-1450 3400-4250 1400-1900 % 1000-1600 2050-2350 1000-1600 3800-4760 1500-2100 K 1100-1750 2250-2600 1100-1750 4150-5200 1650-2350 % 1200-1900 2450-2850 1200-1900 4500-5600 1800-2500 l 1250-2000 2600-3000 1250-2000 4800-6000 1900-2700 IK 1400-2300 2900-3350 1400-230G 5350-6700 2150-3000 IK 1550-2500 3200-3700 1550-2500 5900-7400 2350-3300 IK 1700-2700 3400-3950 1700-270C 6350-7950 2500-3550 2 1800-2900 3700-4300 1800-2900 6800-8500 2700-3800 2K 1900-3050 3900-4500 1900-3050 7200-9000 2900-4050 2H 2000-3200 4100-4750 2000-3200 7600-9500 3100-4300 3 2200-3500 4500-5200 2200-3500 8300-10500 3300-4600 3K 2400-3800 4900-5650 2400-3800 9000-11500 4 2550-4050 5200-6000 2550-4050 9600-12000 4K 2700-4300 5500-6350 2700-4300 10000-12500 5 2850-4550 5800-6700 2850-4550 10500-13000 5K 2950-4750 6100-7050 2950-4750 11000-13750 6 3100-5000 6400-7400 3100-5000 12000-15000 7 3350-5400 6900-7950 3350-5400 12700-16000 8 3600-5750 7350-8500 3600-5750 13500-17000 9 3800-6100 7800-9000 3800-6100 14500-18000 10 4000-6400 8200-9450 4000-6400 15000-18500 required is low, and efficiency becomes of secondary im portance. When a duct system is involved the choice is usu- . ally made between a centrifugal fan and a tubeaxial or vaneaxial. At times the capacity-pressure-speed relationship (specific speed)* dictates a choice. Usually, space, efficiency, sound, cost, and serviceability must all be considered.1* In general, centrifugal and axial fans are comparable in effi ciency and sound, but the latter are tighter and require con siderably less space, especially if arranged for straightthrough operation. The comparison cannot be made on the cost of fans only, but the difference in cost of ductwork, mounting, and servicing must be included. A vaneaxial is more efficient and quieter than a tubeaxial, but is more ex pensive, and frequently requires more space. While requir ing less space than the centrifugal, the axial flow fan is inherently less accessible for service. When high-temperature S air or air containing corrosive elements is being conveyed, motors and bearings should be located outside of the air stream. This requirement 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 per mits less variation in. air delivery when the resistance varies. Likewise, a fiat sound curve minimizes the change of moving into a region of increased noise. A fan having a. high effi-