Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Section 5 EXHAUST FANS The exhaust fan on a pneumatic dust-collection system creates the differences in pressure that cause air to move through the system. In practical terms, the fan must meet the system's requirements for airflow and static pressure. These and other criteria for selecting fans are covered in this section. The main classifications of fans are axial flow and centrifugal. Types of fans in each classification are shown in Figures 5-1 and 5-2. The performance curves of axial-flow fans make them generally too inflexible for dust-collection systems in grain elevators. However, they are sometimes used as in-line pressure boosters. The most common exhaust fan in grain elevators is the centrifugal type with straight or radial blades (see Figure 5-2). Selecting a Fan To select the proper exhaust fan for a dust-collection system, the designer must have the following information; 1. Airflow required by the system (cubic feet per minute). 2. Static pressure (pressure drop) across the system. 3. The kind of material that will pass through the fan (abrasive, corrosive, etc.). 4. The flammability or explosivity of the material. 5. Whether direct drive or belt drive is best. 6. Limitations on space. 7. The permissible level of noise. 8. The operating temperature. Points 4, 5, and 8 call for brief discussion. If the material to be handled is explosive or flammableâand grain dust is bothâthe exhaust fan should be nonsparking. Also, a fan motor that is to operate in the air stream should be approved for Class II, Group G. The installation should meet the standards of the National Board of Fire Underwriters, the National Fire Protection Association, and state and local ordinances. Direct-drive exhaust fans make a more compact assembly than belt-driven fans. Direct drive also insures constant speed by avoiding the slippage that can occur when belt drives are not maintained properly. Direct drive limits the speed of the fan to the speed of the motor (excepting direct-current motors), whereas belt drive permits fan speed to be changed quickly. However, belt drives can be sources of ignition if they fail and so should not be used inside elevators. 37
38 DISC FAN II 1 BMP SP VOLUME - CFM PROPELLER FAN II BMP SP VOLUME-CFM VANE-AXIAL FAN ui BMP VOLUME - CFM FIGURE 5-1 Axial flow fans. (from "Industrial Ventilation", 16th Edition, 1980)
39 BACKWARD CURVED BLADES s VOLUME - CPU STRAIGHT OR RADIAL BLADES VOLUME-CFM n il FORWARD CURVED BLADES VOLUME - CFM FIGURE 5-2 Centrifugal fans. (from "Industrial Ventilation", 16th Edition, 1980)
40 The operating temperature of an exhaust fan determines the kind of bearings it should have. Sleeve bearings are satisfactory for fans operating at up to 250Â°F. Ball bearings are required at 250Â°F to 550Â°F, and special cooling devices are required at higher temperatures. The designer generally should follow the manufacturer's recommendations. Fan Size and Speed The size and speed of the exhaust fan for a specific installation should be selected for maximum efficiency. That is, the fan should supply the required airflow and static pressure at minimum horsepower and thus minimum operating cost. The designer usually can determine the optimum size and speed of the fan from a rating table published by the manufacturer. The best form of table is a multirating table. It gives the airflows, or capacities, for a fan of a given size over the entire range of static pressures and fan speeds. The table also gives the horsepower required over the entire range of static pressures and speeds. When a fan is running at a given speed, its capacity varies with the static pressure in a manner characteristic of the fan. A range of capacities and the corresponding static pressures at constant speed can be plotted to give a curve that is characteristic of the fan. A typical characteristic curve is shown in Figure 5-3. The second curve in Figure 5-3 shows how capacityâor airflow through the systemâvaries with the resistance, or static pressure, across the system. The point where the two curves intersect represents the capacity of the fan in a given system at constant speed. In other words, a particular fan running at constant speed in a particular system can have only one capacity, or airflow. That capacity can be changed only by changing the speed of the fan or the resistance of the system. The slope, or degree of steepness, of a fan's characteristic curve is especially important in a grain-dust system. Figure 5-4 shows a relatively flat characteristic curve (left) and a relatively steep one (right). It can be seen in Figure 5-4 that a slight increase in the static pressure of the system causes a smaller loss of capacity for the fan with the steep characteristic curve than for the one with the flatter curve. The point is significant because a dust-collection system may occasionally become overloaded with dust. When ducts become overloaded, the resistance of the system rises and airflow declines correspondingly. If the fan's characteristic curve is too flat, airflow may fall to the point where velocity is below the transport velocity, and the system will clog. Thus when choosing among otherwise adequate fans for a particular job, the best selection is the one with the steepest characteristic curve.
41 I characteristic^ ^ \/ -System resistance (varies approx. as 0 Capacity CFM FIGURE 5-3 Typical point of rating. (from "Industrial Ventilation", 16th Edition, 1980) Volume Volume POOR SELECTION GOOD SELECTION Fan with flat pressure curve gives wide volume Fan with steep pressure curve gives small volume variation with pressure change. variation with pressure change. EFFECT OF FAN CURVE SLOPE FIGURE 5-4 Fan selection. (from "Industrial Ventilation", 16th Edition, 1980)
42 A flat characteristic curve may also cause problems during start-up of a fabric filter. The fabric has very little resistance during start-up because the mat of dust has not yet formed on it. If the fan's curve is too flat, the system will tend to handle much more than the design airflow. As a result, more than the desired amount of dust will be conveyed and will tend to overload the filter and system. Generally speaking, an oversize fan tends to operate in a flat section of its characteristic curve. To avoid the problems described above, therefore, the size of the fan selected should exactly match the requirements of the system. An actual example of fan selection is shown in Figure 5-5. The design capacity of the system is 20,200 cfm, and the design static pressure is 16 inches of water (4 inches across the bags and 12 inches owing to friction pressue drop). The fan was selected to give design volume flow at 4 inches pressure drop across the bags. Note that almost doubling the pressure drop across the bags, to 7.6 inches, reduces volume flow only 10 percent. This relationship reflects a good fan selection. Even the best fan cannot overcome poor maintenance. Note in Figure 5-5 that when pressure across the bags climbs to an excessive 13.6 inches, the fan will deliver only 70 percent of design volume flow. Under these conditions, the ductwork probably would clog. If the design is for a 4" pressure drop across the bag this problem would occur more rapidly. Fans for Nonstandard Conditions Rating tables and performance charts for exhaust fans normally are based on the density of clean, dry air at standard temperature and pressure. Standard temperature is 70Â°F; standard pressure is sea-level pressureâ407.5 inches of water or 29.92 inches of mercury. At these conditions, the density of dry air is 0.075 Ib/cu ft. If air is not at standard conditions, or if it is humid (contains water vapor), its density will not be 0.075 Ib/cu ft. The normal rating tables and performance charts, therefore, will not be accurate guides to fan selection unless corrections are made to account for the difference in density. The density of air increases as temperature and humidity decrease and as pressure increases. Normal fluctuations in temperature, pressure, and humidity change the density of air very little and can be ignored. Suppose, however, that a fan will be operating at an altitude of 2,000 ft, where pressure is lower than at sea level. At standard temperature (70Â°F) and no humidity, the density of the air will be 0.070 Ib/cu ft. This density is enough lower than standard (0.075 Ib/cu ft) to require that corrections be made when using the normal tables and charts to select a fan with proper characteristics.
43 22 20 18 rr 16 LU 5 14 LL O ffi 12 I O ? 10 z LU cc 8 D m R cc !l Q. STORAGE SYSTEM System No. 4 Design Conditions: 20200 SCFM @ 16" S.P. Duct Velocity - 4000 fpm SP - Static Pressure TP â Total Pressure HP â Horsepower SE - Static Efficiency Octane Band TE - Total Efficiency PWL-Outlet 1 2 3 4â¢ 5 6 7 8 109 105 104 107 102 98 94 90 â 80 â 5 60 ^o: 40 â 5 20 cc co S.P. PERFORMANCE CURVES No. 730 Single Type C-25 Outlet Coned to 6.0 ft2 70 F 29.98" bar. 1780RPM 0.075 Ibs/C.F. Sound Power Levels - Fan Discharge indB Re 10~12 Watts â¢Denotes Blade Frequency Friction S.P. S.E. Wheel Diameter - 36V Load Limit Horsepower - 73.0 Design S.P. SP Bags = 4.0" SP Friction = 12.0" Total S.P. = 16.0" 4 8 12 16 20 24 28 32 STANDARD CUBIC FEET PER MINUTE X 1000 FIGURE 5-5 Typical fan performance chart.
44 Fan Motor and Drive The motor and drive for a fan are selected on the basis of the horsepower required to produce the necessary static pressure and capacity. The required horsepower is determined from data supplied by the manufacturer of the fan. However, the final selection of the motor and drive must take into account two additional factors; 1. Volume flow in the system may well be 10 percent more than design volume flow until a mat of dust has built up on the filter fabric. 2. Start-up of a system in cold weather will require more than design horsepower because of the higher density of the air. For these reasons, it is suggested that the motor and drive be designed to provide 10 percent more than the design airflow at the lowest temperature expected for the system. This precaution will prevent the drive from cutting out because of overload. Fan Installation Manufacturers' data on the performance of fans are obtained from tests conducted under ideal conditions. A fan installed in a dust-collection system, however, normally does not operate in ideal conditions. To obtain maximum performance, therefore, certain guidelines should be observed when installing a fan. A fan's inlet and discharge ducts should be designed to minimize nonuniform (turbulent) flow. The reason is that nonuniform flow increases pressure drop and so decreases volume flow. Figure 5-6 shows the losses in volume flow caused by various inlet fittings. The figure also shows the increases in static pressure that the fan must provide to compensate for those losses. The situation is generally the same for fan-discharge fittings (Figure 5-7) . Elbows in discharge ducts, for example, should be avoided to minimize pressure drop.
45 DESCRIPTION % LOSS IN CFM IF NOT CORRECTED X INCREASE NEEDED IN FAN SP TO COMPENSATE 3 piece elbow R/0 - .5 I2 30 I3 Olj^N *:Â° 5 5 II I I **^.// \ *Â» Piece elbow R/D = ' -Â° 6 It I3 9 9 Â«t PC R \ ^\ 8-Â° it 1 I bow / il / \ . | * or more piece R/o - I.0 2.0 8.0 5 it ll 9 9 elbow V } / Mitered elbow I6 U2 Square Oucts with Vanes "" -s, â¢Â»,NÂ»N|, NÂ° A9"*" 8 I7 8 6 US I8 I3 II 9 ABC "> J 5 Rectangular Elbows without /anes* al I cases use of 3 '5 <T <N ng , equal I y spaced tes will reduce loss d needed sp increase I/3 the values for i/aned el bows . *m urn 5 .-- -25, Sr J. .5 W I .0 2.0 7 it I5 9 9 The maximum included angle of any e - ement of the transition should never â exceed 30Â°. If it does, additional tt , I.00, & -- .5 losses wi occur. If angle is less W W I .0 than 30Â°and L is not longer than the 2.0 I2 5 it 30 II 9 transition may be ignored. If it is - - k.OQ, 6- - =â¢ .6 longer, it will be beneficial because w W I .0 elbow will be farther from the fan. 2.0 I5 8 39 I8 9 <t Each 2'/2 diometers of straight duct between fan and elbow or inlet box will reduce the adverse effect about 20%. For example, in the case of the poorest 3 pc. elbow above: No duct . L/D = 2V? 5 7Vi I0 CFM loss - I2% Additional fan 10% SP needed 7% S% 30% 24% I8% I 2% 6% FIGURE 5-6 Probable effects of various inlet connections. (These losses do not include friction losses.) (from "Industrial Ventilation", 16th Edition, 1980)
46 High S.P. Loss Moderate S.P. Loss Low S.P. Loss No loss S.P. Regain S.P. = static pressure FIGURE 5-7 Effects of fan discharge methods upon static pressure. Fan Location Nonuniform flow, as well as other problems, can be avoided or minimized by selecting the optimum location for a fan. If at all possible, the installation should conform to these guidelines; 1. Locate the fan downstream from the filter to minimize erosion and abrasion. 2. Locate the fan so as to avoid elbows and other obstructions in the inlet. 3. Arrange the direction of rotation and the discharge of the fan so that it exhausts in the direction finally desired, thus avoiding unnecessary bends. 4. As noted previously, do not install V-belt drives inside an elevator because they can be a source of ignition in the event of failure. 5. Locate small fans on the filter itself. Locate larger fans so as to minimize vibrationâusually they are best mounted on a concrete pad at ground level. 6. Locate fans for easy inspection and maintenance.
47 Modifying a System The capability of the exhaust fan must be carefully checked when a dust-collection system is to be modified. When replacing a cyclone with a fabric filter, for example, it is extremely important to ensure that the existing fan can perform satisfactorily in the modified system. A fabric filter generally will entail higher pressure loss than a simple cyclone. The fan must produce correspondingly higher static pressure to keep the system operating properly. If it cannot, it should be replaced by a fan of greater capacity. It may be possible to obtain the necessary increase in static pressure by speeding up the existing fan. However, an exhaust fan should be speeded up only after the fan's manufacturer has been consulted. Booster Fans Booster fans may be used in some circumstances. Where additional static pressure is required, as in the case above, it might be obtained by putting a second fan in series with the existing fan. In other instances, small booster fans may be used to provide additional static pressure in particular sections of complex dust-collection systems. In any event, extreme care should be exercised when considering the use of booster fans. Generally they will handle grain dust and so must be spark-proof and otherwise properly selected for a dust location.