Document qmyb7qXdQ1vB1754Dxbn8YYBq

682' CHAPTER 32 1950 Gulden stabilize the flow entering the impeller when adverse flow conditions exist in the approach to the inlet. ' Axial flow fans develop none of their static pressure by. centrifugal force, but all from the change in velocity in passing through the impeller, and its conversion into static pressure. They are thus inherently high velocity fans, and are very dependent on blade conformation for good characteristics. For that reason, ;an air foil section, such as developed in wind .tunnels for aircraft work8 is frequently used: Since any shape of blade can-only be correct for a narrow range of capacity at constant speed, the performance curves for any blade sh6w definite characteristics: To absorb energy, the air must be given a tangential motion in passing the impeller,7 and whenoperating against'higher pressures, must have guide vanes (see vaneaxial fans)To obtain best efficiencies...- Fans... 683 downward trend from no delivery to free.delivery with the maximu.rp.at no delivery, contrary to that of a centrifugal fan. The;type of guide vanes in a vaneaxial fan has a distinct bearing on the shape of the horsepower curve. The maximum efficiency tends to occur at a percentage of free delivery capacity higher than for a centrifugal fan. The sound curve, which may have a minimum value comparable to centrifugal fans, is again lowest near maximum efficiency, but has a charac teristic rise when the fan is operating at. low capacities and the stall point of the blade section is reached. Since propeller fans are designed for operation near free delivery, less attention is given the-regaining of velocity to static pressure, and-the pressure curve rises constantly from free delivery to no delivery. The >While dxfdZ _^ow fans are inherently a higher capacity'type than centrif ugal fans,-they,too, may be designed with widely varying characteristics. As-with a centrifugal fail; the pressure rises generally from free delivery to no delivery, but tubeaxial and vaneaxial fans may have a drop in pressure, when the capacity decreases below a certain volume, a'condition also found in' the case of the centrifugal fan having forward-curved blades. The pressure drop is caused by the same condition for both fans, t.e., the staticpressure is largely dependent on conversion, of velocity pressure, and velocity pressure is small at low capacity. , Tubeaxial and vaneaxial fans may also have performance curves resembling somewhat those of a centrif ugal;.fan. with backward-curved blades. Fig. 4 shows the performance curves for a typical, design. , i-: The:horsepower curve may be flat-.with a. sehdimiting-characteristic as in;a backward-curved blade centrifugal fan,,or it may-have a.generally horsepower' is highest at no delivery,-, an'd: decreases toward: free, delivery/ m contrast to-a centrifugal fan. Maximum total efficiency-is-'obtained at a higher percentage of free delivery than for other types. - -'-ui :.- SYSTEM CHARACTERISTICS '0 . . Any ventilating .system, consisting; of .duct work, heaters, Air .waSbiera,' filters, etc., has a system characteristic which is individuaVto that system, and is independent of any fan which may be applied to the system.' This characteristic may be expressed in curve form-in exactly the same manner that fan characteristics may be shown. Typical .'system "characteristic curves are shown as A, B andC in- Fig. 5. These curves are drawn,to follow the simple parabolic law in which the static'pressure or resistance to flow of air varies as the square of the volume flowing'through the.systein. - Heating and ventilating systems follow this law very, closely and no serious error is'introduced by its use. \ When a constant speed fan curve for a {pven size fan is super-imposeji