英语翻译These are machines that move and compress gases．Fans dis

英语翻译
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．

《风扇,鼓风机和压缩机》
这些是转动并压缩气体的机器.风扇在空地或大型的通风管道中放出大量的气体（通常是空气）.它们是低速旋转而产生低压的机器,压力通常为0.04 atm.鼓风机是高速旋转的设备（或利用刀片的移位或利用离心力）来产生最大的压力,大约为2 atm.压缩机也是利用刀片换位或离心运动的机器,产生从2 atm到几千个大气压.既然泵通常是用于移动液体的设备,那么空气泵和真空泵是设计用于压缩空气的.风扇中的流动密度没有明显的变化或基本不变.而鼓风机和压缩机里面的密度发生很大的改变这是可以肯定的,这在论述这些可压缩流动的设备时是必须提到的.
风扇
大多的风扇通常是离心运动的,工作的原理和离心泵相同,而它们的叶轮片可能是弯曲运送的,这可能导致泵中的不稳定,而风扇却不会.典型的风扇叶轮如图18.1所示；它们装有轻的薄片——金属包壳.间隙挺大而且输送压头低在5-60之内.（130毫米至1500毫米）.有时,在通风扇中,几乎所有的额外能量来自于周转率的能量而无关于压位差.在任何情况下,速率的获得吸收相当可观的额外能量并必须估算效率和功率.总效率,有70%认为功率输出是有压力和速度落差的.
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