Document BRq3V1VqjaQZmVxy95OZ5Qb0j

Fig. 1. Operating Characteristics of Axial Flow Airfoil Type Fan plotted to show total and static pressure, mechanical and static efficiency, and horsepower in relation to air delivery as a basis. Results may also 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 requirements are accurately known in advance, the designer can select a fan. ... ^ > operating'at maximum efficiency, irrespective of performance over the entire range. Generally, fans are selected either at the peak of the static efficiency or to the right of the peak depending on the requirements of the particular installation. Fans selected to the right of the peak will be smaller but will require more power, run at higher speeds and may have a higher sound rating. Where first cost is important and added horsepower and noise are not important, smaller fans may be used. Where efficient and quiet opera tions are most important, fans are selected at or near the peak of the static efficiency curve. Fans are not ordinarily selected to the left of the peak of the static efficiency curve as this results in larger, more costly fans, requiring more power and in some cases producing objectionable noise. The curves in Figs. 1, 2, and 3 show operating characteristics for axial flow and two classes of centrifugal fans, namely, forward-curved multiple blade and backward-inclined blade design, for comparison purposes'. These curves are not applicable for rigid comparison or actual selection, but are shown to indicate variations in operating characteristics. The curves in Fig. 1 of a typical axial flow fan show characteristics of non-overloading horsepower and high efficiency. These results are obtained by producing a more uniform pressure throughout the blade annulus, so that back flow doesnot occur except at high pressures. This avoidance in turbulence has a tendency to reduce noise. Fans of this type are operating against static pressures as high as 4 in. water. The capacity and efficiency of. axial flow fans when operating above the low pressure range can be improved by the use of either inlet or outlet guide vanes or both. The effect of such vanes is to increase the level of the PIJsslJre volume curve, and properly designed vanes on" the discharge S(u t^e ^anhave the advantage of eliminating the rotational component of the air stream, thus restoring uniform axial flow. As high pressures usually require large hubs in proportion to the fan diameter, performance is improved by the use of round-nosed or conical forms mounted co axially with the direct-connected fan (sometimes partly or wholly enclosing the motor) so as to make the changes in velocity to and from the tan blade annulus as uniform as space conditions permit. When axial