Chapter 17. -COMPRESSED AIR PLANTS 



RECIPROCATING AIR COMPRESSORS 



Operating Cycle 



Reciprocating air compressors are suffi- 

 ciently similar in design and operation so that 

 the following discussion applies in large part to 

 all reciprocating compressors now in naval use. 



Medium pressure air compressors are of the 

 two-stage, vertical, duplex, single-acting type. 

 Many medium pressure compressors have dif- 

 ferential pistons; this type of piston has more 

 than one stage of compression during each stroke 

 of the piston. (See fig, 17-4, A.) 



Modern air compressors are generally 

 motor-driven (direct or geared), liquid- cooled, 

 four-stage, single-acting units with vertical or 

 horizontal cylinders. Cylinder arrangements for 

 high pressure air compressors installed on Navy 

 ships are illustrated in part B of figure 17-4. 

 Small capacity high pressure air systems may 

 have three-stage compressors. Large capacity, 

 high pressure, air systems may be equipped with 

 five- or six- stage compressors. 



Let us consider first the operating cycle that 

 occurs during one stage of compression in 

 a single-acting reciprocating air compressor 

 such as the one shown in figure 17-3. The oper- 

 ating cycle consists of two strokes of the piston: 

 a suction stroke and a compression stroke. 



The suction stroke begins when the piston 

 moves away from top deadcenter (TDC). Theair 

 under pressure in the clearance space (above the 

 piston) expands rapidly until the pressure falls 

 below the pressure on the opposite side of the air 

 inlet valve. At this point the difference in pres- 

 sure causes the inlet valve to open and air is ad- 

 mitted to the cylinder. Air continues to flow into 

 the cylinder until the piston reaches bottom dead 

 center (BDC). 



The compression stroke starts as the piston 

 moves away from BDC and compression of the 

 air begins. When the pressure in the cylinder 



2STAGE, 3 CYL. RADIAL 



^ , 1^ i 2-STAGE WITH 



1 ^ ^ --- 



DIFFERENTIAL 

 PISTON 



2-STAGE, VERTICAL, WITH 

 2 DIFFERENTIAL PISTONS 



A 



r 



^^ 



III 



^^i 







II 



1:1 



4 ^^ 



-^2 1 



•I 2 





4-STAGE ARRANGEMENTS 



3-STAGE ARRANGEMENTS 

 B 



47.153 



Figure 17-4.— Air compressor cylinder arrangements. A. Low and medium pressure 

 cylinders. B. High pressure cylinders. 



441 



