Document 2jj9BDGrmqZj5gE7a5kYxQED6

570 CHAPTER 32_______ 1946 Guide' 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 mechanical efficiencyf and may be expressed as follows:^ Mechanical or Total Efficiency * = . ^hes of water 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 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, lyith the Standard Test Code for Centrifugal and Axial Fans1 prepared jointly by the American Society of Heating and Ventilating Engi neers and the National Association of Fan Manufacturers are generally plotted to show total and static pressure, mechanical and static efficiency,. and horsepower in relation to air delivery as a basis. Results may also Fic. 1. Operating Characteristics of Axial Flow Airfoil Type Fan be plotted against per cent of wide open volume or discharge. Examples < of fan performance curves are shown in Figs. 1, 2 and 3. In the selection of all but very small fans, power consumption is usually, a major consideration. It must be borne in mind that the horsepower at , peak efficiency alone may be misleading, as actual operation is apt to occur at some point on the pressure-volume curve varying considerably, from that specified, due to inaccuracies of the estimated system resistance or to fluctuating resistance caused by damper or louver.adjustments. To cope with such variations a fan should be selected having a high efficiency over a wide range, that is, aflat or broad efficiency curve is more desirable than.a sharp or narrow curve which, though reaching a high, peak, falls off rapidly to either side of a narrow range. When the point of operation' varies only within narrow limits and both volume and pressure require ments are' accurately known in. advance, the designer can select a fan operating at maximum efficiency, irrespective of performance over' the entire range. Fig. 2. - Operating Characteristics of a Fan with Blades Curved'Forward