Document RjVGwBZZNXQ6pwpdjJp8YORRB

586 CHAPTER 32 1965 Guide And Data Book? ; ;i:i it %i !i i S Rg. 4 .... Percentage Performance Curves of an Axial-Row Fan Performance curves of a typical forward-curved blade centrifugal fan are shown in Fig. 2. The pressure rises from, free delivery toward no delivery, with a characteristic drop at low capacities, because a large share of the pressure is hoing generated by conversion of velocity, which is small at low capacity. The maximum efficiency occurs at approxi mately maximum pressure. The horsepower curve reflects, the increase in energy by rising rapidly from no delivery to, free delivery. The sound is a minimum at maximum effi ciency, and rises toward free delivery as the velocities in- Performance curves of a typical backward-curved blade, centrifugal fan are shown in Fig. 3. The pressure is con stantly rising from free delivery nearly to point of nonde livery. The horsepower reflects the energy-velocity relatioifc- ship by rising to a value as the capacity increases, and then decreasing with further increase in capacity to give. a self-limiting horsepower characteristic. The maximum horsepower coincides approximately with the maximum, ef ficiency. The sound is again a minimum near maximum effi-: ciency, but is little or no higher at free delivery than at low capacities. Between the extremes of forward and full-backward- curved blades, there exists a number of intermediate designs which show varying degrees of similarity to the curves in figs. 2 and 3. A common variation is a fan having modified backward, single, or double-curved blades, and fixed inlet vanes. 8uch vanes applied to a partially backward-curved impeller give the steep, constantly rising pressure character istic, and the self-limiting horsepower feature of the full- backward-curve impeller. They also stabilize the flow enter ing the tmpolW 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 pass ing through the impeller, and its conversion into static' pres sure. They are thus inherently high-velocity fans, and are very dependent on blade conformation for good characteris tics. For that reason, an air-foil section, such as developed in wind tunnels for aircraft work1 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 b&de show definite characteristics. To absorb energy, the air must be given a tangential motion in passing the im peller,* and, when operating against higher pressures, must have guide vanes (see v&neaxial fans) to obtain best effi ciencies. While axial-flow fans are inherently a higher capacity type centrifugal fans, they also may be designed with widely varying characteristics. As with acentrifugal-fan, the pres- sure rises generally from free delivery to no delivery, but; tnKonxtal and vftnAftxml' fans may have a drop in pressure when the capacity decreases below a certain volume, a con dition pigTM found in the case of the centrifugal fan having forwarti-eurved blades. The pressure drop is caused by the gpmi condition for both fans, Le., the air flow over the blades is such that a stall condition occurs and the blades cease to function in the normal manner. Tubeaxial and vaneaxial fn.no may nlsn have performance curves somewhat resembling those of a centrifugal fan with backward-curved blades. Fig. 4 shows the. performance curves fora typical design. The horsepower curve may be flat, with a self-limiting characteristic as in a backward-curved blade centrifugal fan, or it may have a generally downward trend from no delivery to free delivery, with the maximum'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 that for a centrifugal fan. Fans' 587 - The sound curve, which may have a minimum value com- ; parable to centrifugal fans, is again lowest'near maximum efficiency, but has a characteristic rise when the fan is operating at Jow 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 horsepower is highest at no delivery, and decreases toward free delivery, in contrast to a centrifugal fan. Maximum total efficiency is obtained at a higher percentage of free delivery than for other types. free delivery. To the left of peak pressure, the air flow across the fan blades is unsteady, producing pressure fluctuations and noise. This condition also exists near free delivery, blit usually to a lesser extent Kg. 6 shows an undesirable fan and system combination which should be avoided by nsirig a different fan or reducing the system resistance. Fans having a steep dip in their pressure curve to the left of peak pressure may intersect the system, curve in several places. -1 -Fans operating in parallel* on the same system may produce' a similar undesirable condition. Parallel fans are here consid ered as fans operated with separate drives. Referring to Kg. SYSTEM CHARACTERISTICS 7, a single fan is shown operating at point B, the fan and sys tem intercept The parallel fan is now brought into 'service Any ventilating system consisting of ductwork, heaters, by opening the outlet dampers. The additional volume pro air washers, filters, etc., has a system characteristic which is duced by the second fan is indicated by the hnri>ntftl dis individual to that system, and is independent of' any-fan which may be applied to the system. This characteristic may tance between the one-fan curve and the system curve. These abscissae are replotted as a second system curve be expressed in cqrve form in exactly-the *m< manner that (labeled limit Curve), starting at A. If this limit curve inter fan characteristics may be shown. Typical system charac teristic curves are. shown as A, B, and C in Kg. 5. These sects the fan curve in the undesirable region, unstable opera tion will result curves are drawn'to follow the simple parabolic law which Satisfactory parallel operation can be obtained by dividing states that the static pressure or resistance to flow of air a portion of the original system into separate resistances in varies as the square of the volume flowing through the sys tem. Heating and ventilating systems follow this law very series with each fan. The series resistance will then be a part of each fan -package and will have the effect of reducing both closely, and no serious error is introduced by its use. the system and the fan curves so as to avoid the undesirable When the characteristic curve of a constant-speed fan of intercept. Kg. 8 shows the resulting fan and system curves. a given size'is superimposed upon a system characteristic System and fan characteristics are affected by air density. curve, the relation between the two is at once apparent. The only point common to the two curves is the point at the in tersection, of the system characteristic curve and the fan The fan laws show that pressure and horsepower vary directly with the air density. System resistance is also affected by density in direct proportion. - characteristic curve, and it is at this point that the combina tion .will operate. In Kg. 5, system characteristic.curves A, B,- and C cross the fan characteristic curve at points X, Y, and Z. The fan whose curve is shown, when applied to sys tems having characteristic curves A, B, and C, will'deliver 10,000, 13,000 or 16,400 .cfm, respectively. ,, ; In-some instances, fans may be applied to,variable-flow systems. In such cases, the limiting systems may be plotted and* the effect on fan performance examined.' For instance, a system might have a characteristic curve .between A, FAN ARRANGEMENTS Centrifugal fan arrangements have been standardized by the Air Moving and Conditioning Association. Kgs. 9, 10, and 11 show the accepted designation for arrangement of drive, rotation, discharge, and motor position, for belt drive. Axialflow fans are either belt driven or direct connected, in'ac cordance with individual manufacturer's arrangements. Usu ally a choice of antifriction or sleeve bearings is available. shown in Kg. 5, as one limit, and B as the other limit The fan performance will then fall between points X and Y on FAN CONTROL the fan curve at a point determined by the system charac teristics at that.particular time. If A and axe the limiting characteristic curves of the systems, the fan performance will never be outside the points X or Y. ' ' In order to avoid unstable operation, the fan and system intercept should always be between peak'fan pressure and In some heating and ventilating systems iff is desirable to vary the volume of air handled by the fan. This may be accomplished by a number of methods. Where the change is made infrequently, the pulley .or sheave on the driving motor or fan may be changed to vary the speed of the fan and alter the air volume. Dampers may be placed in the duct system to vary the volume. Variable-epeed pulleys or transmissions, such as fan belt change boxes, or electric or hydraulic couplings, may be used to vary the fan speed. Variable-speed motors and variable inlet vanes on fans'may Rg. 8 .... Parallel Fans with Series Resistance