Book - School of Science and Technology

Chapter 17Fans and air treatment equipmentThe fan is the one item of equipment which every mechanical ventilation and airconditioningsystem has in common. A fan is simply a device for impelling air throughthe ducts or channels and other resistances forming part of the distribution system. Ittakes the form of a series of blades attached to a shaft rotated by a motor or other sourceof power. The blades are either in the plane of a disc (propeller fan) or in the form of adrum (centrifugal fan), however a recent development, the mixed flow fan, is a suitablealternative for certain specific applications.There is as yet no other practicable commercial method of moving air for ventilationpurposes, but fans in general suffer from various disadvantages such as low efficiency andnoise. It is the latter which is probably the most troublesome to designers, and to whichmuch careful attention must be given if silent running is to be achieved in the system as awhole: factors involved are air speeds, fan speeds, duct design, materials of construction,acoustical treatment of ducts and provision for absorption of vibration.Fan types and performanceFan characteristicsA comparison of the operation of fans of various types is best understood by studyingtheir characteristic curves. For this purpose consider a fan connected to a duct with anadjustable orifice at the end, as in Figure 17.1. Pressures are measured by water gaugesconnected to a standard Pitot tube, see Figure 16.32. The perforated portion gives thestatic pressure, and the facing tube the total pressure.If the fan is running with the orifice shut, no air will be delivered. Static pressure will beat a maximum, and velocity pressure nil. As the orifice is opened the static pressure willfall and the velocity pressure increase until, with the orifice fully open, the static pressurewill be negligible and velocity pressure at a maximum. Over this range the power requiredto drive the fan will have increased from minimum to maximum, and perhaps will fallaway as the total pressure falls off. The power required to drive the fan, if 100% efficient,would be:H t ˆ V P