Document jggo26VrEX2qMn3dBD44Vp3YQ

American Society of Heating and Ventilating Engineers Guide, 19J7 ' Justification for liberal power provision exists also in the possibility of varying demand due to changes in ventilation requirements, intensity of occupation, and weather conditions. ^ The motive power of fans should be determined in accordance with the Standard Test Code for Disc and Propeller Fans, Centrifugal Fans and Blowers, as adopted by the American Society of Heating and Ventilating Engineers and the National Association of Fan Manufacturers Fans may be driven by electric motors, steam engines (either horizontal or vertical), gasoline or oil engines, and turbines, but as previously stated the drive commonly used is the electric motor. REFERENCES Heating, Ventilating and Air Conditioning, by Harding and Willard, Revised Edition 1932. * Fan Engineering, Buffalo Forge Company. Fans, by Theodore Baumeister, Jr. Theories and Practices of Centrifugal Ventilating Machines, by D. Murgue, trans lated by A. L. Stevenson. Mechanical Engineers' Handbook, by Kent. ' Mechanical Engineers' Handbook, by Lionel S. Marks. Constructive Mechanism and the Centrifugal Fan, by George D. Beals. Coal Miners Pocket Book. The Fan, by Charles H. Innes. " Mine Ventilation, by J. J. Walsh (A.S.H.V.E. Transactions, Vol. 23, 1917). Fan Blower Design, by H. F. Hagen (A.S.H.V.E. Transactions, Vol. 28, 1922). The Centrifugal Fan, by Frank L. Busey. Section X, A.S.H.V.E. Code of Minimum Requirements for the Heating and Venti lation of Buildings (Edition of 1929). PROBLEMS IN PRACTICE 1 What information must be supplied to the manufacturer when ordering a centrifugal fan? a. Size of fan (catalog number). b. Type of fan. c. Width of fan (single or double). d. Number of inlets (single or double). e. Fan performance and kind of application. ' /. Direction of rotation (clockwise or counter-clockwise). g. Direction of discharge (top horizontal, down blast, etc.). h. Drive arrangement (see Fig; 6). Style of housing (full, three-quarters, etc.). 2 In selecting fans for quiet operation in public buildings: a. Should the outlet velocity of the fan be limited? b. Should the tip speed of the fan be limited? . o. Because all commercial fans operating at pressures suitable for this class of work would be considered noisy if the fan were to discharge directly into, the .room; and 330 Chapter 17--Fans use the duct system on the fan discharge is depended upon to absorb a reasonable beca" t of fan noise, it is desirable to have a moderate run of duct work with some bends an?elbows included as sound deadeners. Where this duct is of necessity very short, the t! t velocity must be kept down to the lower limits recommended in this chapter or 1 an efficient sound absorber must be used. The experience of the engineer must be y^guide in determining the allowable outlet velocity in each individual case. Tin speed should not ordinarily be limited, because different types of fan blades have firelv different allowable tip speeds tor quiet operation. A fan having a backward H de at the tip can run at much higher tip speed than can a forward curved or a straight blade fan, with the same degree of quietness. 3 Is a direct connected or a belted fan preferable in public building work? Where space is at a premium, direct connection is best. Next in space economy is the hort V-belt drive. The flat belt drive fan requires the greatest floor space. In this Jass of work, pressures are usually so low that even with the high speed fans the motor cost is greater for direct connected units than for belt drive fans. 4 9 a. What type fans are used in industrial work? b. What outlet velocity is suitable? a All of the centrifugal types are suitable; the disc and propeller types are suitable for low pressure work, or they are often used as exhausters. b The outlet velocities on fans for industrial work can be much higher than can those in public building work, where quietness is essential. Fans should be selected with outlet velocities as recommended in this chapter, using the upper limit of velocities. 5 9 Are direct connected or belted fans preferred in industrial work? In industrial applications, fans are often advantageously direct connected to motors. The pressures are usually high enough to use standard motor speeds. The high speed types of fans have limiting horsepower characteristics so that little margin in power must be provided in the driving motor. Belted fans may be used, but where high power is required a special arrangement is often necessary for shaft and bearings on account of the weight of the sheave and the belt pull. 6 A forward curved multiblade fan which requires 5.4 bhp is delivering 22,800 cfm at 70 F against a resistance pressure of 1 in. of water at an outlet velocity ofl440fpm: a. What is the static efficiency?. b. What is the total efficiency? a. 66.3 per cent (see Equation 2). b. 74.5 per cent (see Equation 3). N' 7 9 If the above fan has a 54-in. diameter wheel and operates at 193' rpm, will it be suitable for a ventilating installation where a minimum of noise is desirable? Yes. The tip speed will be 2720 fpm and this, together with the 1440 fpm outlet velocity, falls within the limits given in Table 1 for 1-in. resistance pressure. 8 9 What objectionable feature is inherent in the ordinary propeller fan when it is operating at high resistance pressures? It must operate at a high speed with consequent noise. 9.9 At what point should a fan be selected for operation, and why? At its point of maximum efficiency because the cost of operation and the noise produced will be least. 331