Document jmbRMEbwmRmz3zr2wr1MENB72

634P CHAPTER 32 1949 Guide (45\s /400V 36/ ^ \m) ** 1 = (45V /4OOV 36/ X VioO/ X4 = 122hp FAN EFFICIENCY The efficiency of a fan may be defined as the ratio of the horsepower out put to the horsepower input. The horsepower output is expressed by the formula: .. ,, , cfm X total pressure in inches of water Air Horsepower1 =------------;--------------- ---------------------------- 6356 a) When the static pressure is used in the computation in place of total pressure it is assumed that this represents the useful pressure and that the velocity pressure is lost in the piping system and in the air which leaves the sys tem. Since in most installations a higher velocity exists at the fan outlet than at the point of delivery into the atmosphere, some of the velocity pres sure at the fan outlet may be utilized by conversion to static pressure within the system, but, owing to the uncertainty of friction losses which Occur at the places where changes in velocity take place, the amount of velocity pressure which is actually utilized is seldom known, and the static pressure alone may best represent the useful pressure. In the standards for published capacity tables as adopted by the National Association of Fan Manufac turers, the term static pressure refers to the true resistance to air flow. Such tables charge both the inlet and outlet velocity of the fan to the fan per formance, and may be used directly where the static pressure of the system as calculated represents only the actual resistance to flow of the air. The efficiency based upon static pressure is known as the static efficiency and may be expressed as follows: a. .. , cfm X static pressure in inches of water Static Efficiency1 =--------------------- ------------------------------------------ 6356 X Horsepower input ,. (2) Different fans may develop the same capacity against the same static pressure and with the same power input, and therefore operate at the same static efficiency, while maintaining different outlet velocities,. Where a high outlet velocity is desirable or can be utilized effectively,' the static efficiency fails to be a satisfactory measurement of the performance. In many applications of propeller fans, air is circulated without encountering resistance and no static pressure is developed. The. static efficiency is zero and its calculation is meaningless. Because of such situations where the static efficiency fails to indicate the true performance, many engineers prefer to base the calculation of efficiency upon the total pressure. This efficiency is variously known as the total, or mechanical efficiency, and may. be expressed as follows: ... . . _ . , . , cfm X total pressure in inches of water Mechanical or Total Efficiency1 = ------------------------------------------------------------ (3) 6356 X Horsepower input CHARACTERISTIC CURVES In the operation of a fan at a fixed speed the static and. total efficiencies vary with any change in the resistance which is imposed. With different Fans . 635. designs the peak of efficiency occurs when the fans deliver different per centages of their wide-open capacity. . Variations in efficiency accompany variations in pressures and power consumption which are characteristic of the individual designs and which are influenced particularly by the shape and angularity of the blades. Such variations in pressure, power, and efficiency are shown by characteristic curves. Characteristic curves of fans based upon tests performed in accordance with the Standard Test Code for Centrifugal and Axial Fans1 prepared