Book - School of Science and Technology

Fan types and performance 477120100PressurePowerPressurePowerEfficiency50EfficiencyPercent efficiency:power:static pressure0120100500120100(a) Forward curvePressurePowerEfficiency(c) PropellorPowerPressure(b) Backward curvePowerPressureEfficiency(d) Axial flowPressure50PowerEfficiencyEfficiency0(e) Mixed flow(f) Tangential0 20 40 60 80 100 0 20 40 60 80 100Figure 17.3 Fan characteristicsPercent volume at full openingOrdinary propellerFrom the curves it will be observed how the pressure falls away continuously, and thestatic efficiency reaches but a low figure. Thus, this type is unsuitable where any considerablerun of ducting is used. To generate any appreciable pressure its speed becomesunduly high, and hence the fan is noisy. Its main purpose is for free air discharge where itsvelocity curve would rise towards a maximum at full opening. It should be noted that thefan power is a maximum at closed discharge, and, as the motors supplied with these fansare not usually rated to workat such a condition, the closing or baffling of the dischargeor suction may cause overloading.Axial flowThis type shows a great improvement over the ordinary propeller fan, both as regardsefficiency and pressure. The fan-power curve is self-limiting. Hence these fans may safelybe used in conjunction with a system of ductwork, being often more convenient than acentrifugal, particularly for exhausting. They run at higher speed than a centrifugal fan to