PRINCIPLES OF NAVAL ENGINEERING 



AIR INLET 



INTERCOOLER 



J 



AFTERCOOLER 



COOLINGf . 



WATER r"*" 

 OUTLET 



AIR 

 OUTLET 



COOLING WATER 

 INLET 



ACCUMULATOR 



147.118 



Figure 17-1.— General arrangement of multistage compressed airplant. 



within one compressing element, or so that more 

 than one stage takes place within one compres- 

 sing element. In general, compressors are clas- 

 sified according to the type of compressing ele- 

 ment, the source of driving power, the method by 

 which the driving unit is connected to the com- 

 pressor, and the pressure developed. 



TYPES OF COMPRESSING ELEMENTS .- 

 Air compressor elements may be of the centrif- 

 ugal, rotary, or reciprocating types. The recip- 

 rocating type isgenerally selected for capacities 

 below 1,000 cfm and for pressures of 100 psi or 

 above, the rotary type for capacities up to 10,000 

 cfm andfor pressures below 100 psi, and the cen- 

 trifugal type for 10,000 cfm or greater capacities 

 and for up to 100 psi pressures. 



Most of the compressors used in the Navy 

 have reciprocating elements (fig. 17-3). In this 

 type of compressor the air is compressed in one 

 or more cylinders, very much like the compres- 

 sion which takes place in an internal combustion 

 engine. 



SOURCES OF POWER. -Compressors are 

 driven by electric motors, internal combustion 

 engines, steam turbines, or reciprocating steam 

 engines. Most of the air compressors in naval 

 service are driven by electric motors. 



DRIVE CONNECTIONS.- The driving unit 

 may be connected to the compressor by one of 



several methods. When the compressor and the 

 driving unit are mounted .on the same shaft, they 

 are close coupled. Close coupling is often used 

 for small capacity compressors that are driven 

 by electric motors. Flexible couplings are used 

 to join the driving unit to the compressor where 

 the speed of the compressor and the speed of the 

 driving unit can be the same. 



V-belt drives are commonly used with small, 

 low pressure, motor-driven compressors, and 

 with some medium pressure compressors. In a 

 few installations, a rigid coupling is used be- 

 tween the compressor and the electric motor of 

 a motor-driven compressor. In a steam turbine 

 drive, compressors are usually driven through 

 reduction gears. 



PRESSURE CLASSIFICATION.-In accord- 

 ance with General Specifications for Ships of the 

 United States Navy, compressors are classified 

 as low pressure, medium pressure, or high pres- 

 sure. Low pressure compressors are those 

 which have a discharge pressure of 150 psi or 

 less. Medium pressure compressors are those 

 which have a discharge pressure of 151 psi to 

 1,000 psi. Compressors which have a discharge 

 pressure above 1,000 psi are classified as high 

 pressure. 



Most low pressure air compressors are of 

 the two- stage type with either a vertical V (see 

 fig. 17-3) or a vertical W arrangement of 



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