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770 CHAPTER 33 1955 Guide Often the service determines the type of fan. -When operation occurs with little or no resistance, and particularly without a duct system, the propeller fan is indicated for convenience and low cost. When resistance is low the power required is low, and efficiency becomes a secondary im portance. When a duct system is involved the 'Choice is usually made between a centrifugal fan and a tubeaxial or vaneaxial. At times the capacity-pressure-speed relationship (specific speed)9 dictates a choice. Usually, space, efficiency, sound, cost and serviceability must all be con sidered.10 In general, centrifugal and axial fans are comparable in effi ciency and sound, but the latter are lighter and require considerably less space, especially if arranged for straight-through operation. The compari son eannot 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 expensive, and frequently requires more space. While requiring less space than the centrifugal, the Table 1. Good Operating Velocities and Tip Speeds fob Ventilating Fans Static Pressure Inches of Water i : i& s i i U 1* l! 2 21 21 3 Forward Curved Blade Fans Backward Tipped and Double Tubeaxial and Curved Blade Fans Vaneaxial Fans Outlet Velocity Tip Speed Outlet Velocity Top Speed Wheel Velocity* Feet per Minute Feet per Minute Feet per Minute Feet per Minute 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 1100-1500 1250-1700 1400-1900 1500-2100 1650-2350 1800-250) 1900-2700 2150-3000 2350-3300 2500-3550 2700-3800 * Wheel velocity is the rinl mean air velocity through the inside diameter of the housing cylinder at the point of wheel location. axial flow fan is inherently less accessible for service. When high-tempera ture 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 permits less variation in air 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 Fans 771 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 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 41). Minimizing the resistance may be the best insurance against noise. On the other hand, sometimes the lowest overall cost results from selecting the mmimum size of system equipment, and then installing adequate acoustical and vibration treatment. jUc^s sbuld be connected to. fan outlets and inlets by means of un painted canvas or other flexible material. Access should be provided in e 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 ransmission. The lighter building constructions which are common today, ia e it desirable to mount fans and driving motors on resilient bases esigned to prevent transmission of vibrations through floors to the buildntf nniC*'Urf' Conduits, pipes and other rigid members should not be ac ^ns. Noises due to high velocities, abrupt turns, grilles and ,,ilpfr lte?Js nf connected with the fan, may be present. Treatment of Prblems, as well as the design of sound and vibration absorbents, are covered in Chapter 41. FAN APPLICATIONS usedan^ ran aPP^caf'ons and the corresponding types of fan commonly ch are usted in the following paragraphs. Reference is also made to the peers where the applications are discussed. enlral System Supply Fans (Chapter 30) are usually of the centrifugal