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These are machines that move and compress gases.Fans discharge large volumes of gas(usually air)into open spaces or large ducts.They are low-speed machines that generate very low pressures,on the order of 0.04 atm.Blowers are high-speed rotary devices(using either positive displacement or centrifugal force)that develop a maximum pressure of about 2 atm.Compressors,which are also positive-displacement or centrifugal machines,discharge at pressures from 2 atm to several thousand atmospheres.Note that while pump generally refers to a device for moving a liquid,the terms air pump and vacuum pump designate machines for compressing a gas.
In fans the density of the fluid does not change appreciably and may be assumed constant.In blowers and compressors,however,the density change is too great to justify this assumption,and in discussing these devices compressible flow theory is required.
Fans
Large fans are usually centrifugal,operating on exactly the same principle as centrifugal pumps.Their impeller blades,however,may be curved forward;this would lead to instability in a pump,but not in a fan.Typical fan impellers are shown in Fig.18.1 ;they are mounted inside light sheet—metal casings.Clearances are large and discharge heads low from 5 to 60 in.(130 to 1,500 mm) .Sometimes,as in ventilating fans,nearly all the added energy is converted to velocity energy and almost none to pressure head.①In any case,the gain in velocity absorbs an appreciable fraction of the added energy and must be included in estimating the efficiency and power.The total efficiency,where the power output is credited with both pressure and velocity heads,is about 70 percent.
These are machines that move and compress gases.Fans discharge large volumes of gas(usually air)into open spaces or large ducts.They are low-speed machines that generate very low pressures,on the order of 0.04 atm.Blowers are high-speed rotary devices(using either positive displacement or centrifugal force)that develop a maximum pressure of about 2 atm.Compressors,which are also positive-displacement or centrifugal machines,discharge at pressures from 2 atm to several thousand atmospheres.Note that while pump generally refers to a device for moving a liquid,the terms air pump and vacuum pump designate machines for compressing a gas.
In fans the density of the fluid does not change appreciably and may be assumed constant.In blowers and compressors,however,the density change is too great to justify this assumption,and in discussing these devices compressible flow theory is required.
Fans
Large fans are usually centrifugal,operating on exactly the same principle as centrifugal pumps.Their impeller blades,however,may be curved forward;this would lead to instability in a pump,but not in a fan.Typical fan impellers are shown in Fig.18.1 ;they are mounted inside light sheet—metal casings.Clearances are large and discharge heads low from 5 to 60 in.(130 to 1,500 mm) .Sometimes,as in ventilating fans,nearly all the added energy is converted to velocity energy and almost none to pressure head.①In any case,the gain in velocity absorbs an appreciable fraction of the added energy and must be included in estimating the efficiency and power.The total efficiency,where the power output is credited with both pressure and velocity heads,is about 70 percent.
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