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704 CHAPTER 32 i951 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 of 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 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 expensive, and frequently requires more space. While requiring less space than the centrifugal, the Table 1. Good Operating Velocities and Tip Speeds for Ventilating Fans Static Pressure Inches ot Water Forward Curved Blade Fans Outlet Velocity Up Speed Feet per Minute Feet per Minute Backward Tipped and Double Curved Blade Fans Tubeaxial and Vaneaxial Fans Outlet Velocity Up Speed Wheel Velocity Feet per Minute Feet per Minute . Feet per Minute i 1000-1100 1520-1700 800-1100 2600-3100 1100-1500 i 1000-1100 1760,1900 800-1150 3000-3500 1250-1700 i 1000-1200 1970-2150 900-1300 3400-4000 1400-1900 1 1200-1400 2225-2450 1000-1500 3800-4500 1500-2100 I 1300-1500 2480-2700 1100-1650 4200-5000 1650-2350 t 1400-1700 2660-2910 1200-1750 4500-5300 1800-2500 l 1500-1800 2820-3120 .1200-1900 4800-5750 1900-2700 U 1600-1900 3162-3450 1300-2100 5300-6350 2150-3000 H 1800-2100 3480-3810 1400-2300 5750-6950 2350-3300 U 1900-2200 3760-4205 1500-2500 6200-7550 2500-3550 2 2000-2400 4000-4500 1600-2700 6650-8050 2700-3800 21 2200-2600 4250-4740 1700-2800 7050-8550 21 2300-2600 4475-4970 1800-2950 7450-9000 3 2500-2800 4900-5365 2000-3200 8200-9850 Wheel Telocity is the axial 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 of 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 chance 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 class* 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 705 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 class* 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, i 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 lighter 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