Principles of naval engineering - Historic Naval Ships Association

Chapter 17. -COMPRESSED AIR PLANTSRECIPROCATING AIR COMPRESSORSReciprocating air compressors are sufficientlysimilar in design and operation so thatthe following discussion applies in large part toall reciprocating compressors now in naval use.Medium pressure air compressors are of thetwo-stage, vertical, duplex, single-acting type.Many medium pressure compressors have differentialpistons; this type of piston has morethan one stage of compression during each strokeof the piston. (See fig,17-4, A.)Modern air compressors are generallymotor-driven (direct or geared), liquid- cooled,four-stage, single-acting units with vertical orhorizontal cylinders. Cylinder arrangements forhigh pressure air compressors installed on Navyships are illustrated in part B of figure 17-4.Small capacity high pressure air systems mayhave three-stage compressors. Large capacity,high pressure, air systems may be equipped withfive- or six- stage compressors.Operating CycleLet us consider first the operating cycle thatoccurs during one stage of compression ina single-acting reciprocating air compressorsuch as the one shown in figure 17-3. The operatingcycle consists of two strokes of the piston:a suction stroke and a compression stroke.The suction stroke begins when the pistonmoves away from top deadcenter (TDC). Theairunder pressure in the clearance space (above thepiston) expands rapidly until the pressure fallsbelow the pressure on the opposite side of the airinlet valve. At this point the difference in pressurecauses the inlet valve to open and air is admittedto the cylinder. Air continues to flow intothe cylinder until the piston reaches bottom deadcenter (BDC).The compression stroke starts as the pistonmoves away from BDC and compression of theair begins. When the pressure in the cylinder2STAGE, 3 CYL. RADIALr^^III^^i II 1:11-^24 ^^•I 2^1^, i 2-STAGE WITH1 ^ ^ ---DIFFERENTIALPISTON4-STAGEARRANGEMENTS2-STAGE, VERTICAL, WITH2 DIFFERENTIAL PISTONSA3-STAGEARRANGEMENTSB47.153Figure 17-4.—Air compressor cylinder arrangements. A. Low and medium pressurecylinders. B. High pressure cylinders.441