Document DGn3jOd02464rwqj9ZqgGBO1N

536 Chapter 29 1945 Guide 400 rprivand requires 4 hp. If the air temperature is increased to'200 F (density 0.0602 lb) and the speed of the fan remains the same, what will be the static pressure and power? . 0 060? Static pressure = 1 X U` .u/o = 0.80 in. " Power = 4 X ^ = 3.20 hp ,. . . Example S. If the speed of the fan of Example S is increased so as to produce a static pressure of 1 in. of water at the 200 F temperature, what will be the speed, capacity, Vand power? Speed = 400 X 0.075 = 446 rpm 0.0602 Capacity = 12,000 X = 13,392 cfm (measured at 200 F) 0.0602 I .Power = 4 X }/ 0.075 0.0602 = 4.46 hp. Example 4- It the speed of the fan of the previous examples is increased so as to deliver the same weight of air at 200 F as at 70 F, what will be the speed, capacity, static pressure, and power? Speed = 400 X =* 498 rpm Capacity = 12,000 X = 14,945 cfm (measured at 200 F) ,,. . 0.075 . Static pressure = 1 X q qqq2: = . m. Power 0.075 \ 6.20 hp. 0602/ Laws of Homologous Fans The laws applying to different sizes of homologous fans are as follows: Capacity varies as the ratio of size cubed, times the ratio of the rpm. Pressure varies as the ratio of size squared, times the ratio of the rpm squared. Horsepower varies as the ratio of the size to fifth power, times the ratio of the rpm cubed. Example 5. Assuming that a fan with a. 36 in. diameter blast wheel will deliver 12,000 cfm at 70 F at 1 in. static pressure, requiring 4.0 brake hp when'operating at 400 rpm, what is the capacity, pressure and horsepower of a homologous fan having a. 45 in. wheel at the same speed? Capacity = (j^f X X 12,000 = 23,400 cfm Static Pressure = Horsepower = X X X 1 = 1.56 in. X 4 = 12.2 hp FAN EFFICIENCY The efficiency of a fan may be defined as the ratio of the horsepower . output to the horsepower input. The horsepower output is expressed by the formula: . Fans 537 Air Horsepower1 = cfm X total pressure in inches of water 6356 (1) 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 system. 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 pressure 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 Manufacturers, 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 performance, 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: Static Efficiency1 cfm X static pressure in inches of water ' 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 mechanicarefficiency, and may be expressed as follows: Mechanica,l or nT,ota,l Effici.ency,1 = c--f-m----X--6^t3o5,,t6a-l-X-p--r-eH--s-o-s-ru-s-re-e-p-i-on--w-i-ne--cr-hi;-ne--ps--uo--tf--w--a--t-e--r (3) 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 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 Fans* prepared jointly by the. American Society of Heating and Ventilating Engi neers and the National Association of Fan Manufacturers are generally .. Standard Test Code for Centrifugal and Axial Fans. 3rd Edition, 1938, Bulletin No. 103. National Association of Fan Manufacturers.