Principles of naval engineering - Historic Naval Ships Association

Chapter 17. -COMPRESSED AIR PLANTSSOLENOIDFigure 17-10.'d/;////// //!>l/////////>>IIMl///JfJ/Jj'/)EXHAUST47.15(-Magnetic typeunloader.on centrifugal principles; most accurately,perhaps, it might be called a rotary- centrifugal~compressor.The rotary-centrifugal compressor is used tosupply low pressure compressed air. Becausethis compressor is capable of supplying air thatis completely free of oil, it is often used as thecompressor for pneumatic control systems andfor other applications where oil-free air is required.The rotary- centrifugal compressor, shown infigure 17-11, consists of a round, multi-bladedrotor which revolves freely in an elliptical casing.The elliptical casing is partially filled withhigh-purity water. The curved rotor bladesproject radially from the hub. The blades, togetherwith the side shrouds, form a series ofpockets or buckets around the periphery. Therotor, which is keyed to the shaft of an electricmotor, revolves at a speed high enough to throwthe liquid out from the center by centrifugalforce, resulting in a solid ring of liquid revolvingin the casing at the same speed as the rotorbut following the elliptical shape of the casing.This action alternately forces the liquid to enterand recede from the buckets in the rotor at highvelocity.To follow through a complete cycle of oper*-ation, let us start at point A_. The chamber (1) isfull of liquid. The liquid, because of centrifugalforce, follows the casing, withdraws from therotor, and pulls air in through the inlet port. At(2) the liquid has been thrown outward from thechamber in the rotor and has been replaced withatmospheric air. As the rotation continues, theconverging wall (3) of the casing forces the liquidback into the rotor chamber, compressing thetrapped air and forcing it out through thedischarge port. The rotor chamber (4) is now fullINLETDISCHARGEINLET.PORTDISCHARGE— PORT147.119Figure 17-11.— Rotary- centrifugal compressor.447