CHAPTER 17 



COMPRESSED AIR PLANTS 



Compressed air serves many purposes 

 aboard ship, and air outlets are installed in var- 

 ious suitable locations throughout the ship. The 

 uses of compressed air include (but are not lim- 

 ited to) the operation of pneumatic tools and 

 equipment, diesel engine starting and control, 

 torpedo charging, aircraft starting and cooling, 

 air deballasting, and the operation of pneumatic 

 control systems. The systems that supply com- 

 pressed air for the various shipboard needs are 

 discussed in chapter 9 of this text; in the present 

 chapter we are concerned with the equipment 

 used to compress the air and supply it to the 

 compressed air systems. 



Compressed air represents a storage of 

 energy. Work is done on the working fluid (air) 

 so that work can later be done by the working 

 fluid. Air compression may be eitlTer an adiabatic 

 or an isothermal process. ^ Adiabatic compres- 

 sion results in a high internal energy level 

 of the air being discharged from the com- 

 pressor. However, much of the extra energy 

 provided by adiabatic compression may be dis- 

 sipated by heat losses, since compressed air is 

 usually held in an uninsulated receiver until it is 

 used. Isothermal compression is, in theory, the 

 most economical method of compressing air in 

 that it requires the least work to be done on the 

 working fluid. However, the isothermal com- 

 pression of air requires a cooling medium to re- 

 move heat fromthe compressor and its contained 

 air during the compression process. The more 

 closely isothermal compression is approached, 

 the greater the cooling effect required; in a com- 

 pressor of finite size, then, we reach a point at 

 which it is no longer practicable to continue to 

 strive for isothermal compression. 



In actual practice, the process of air com- 

 pression is approximately isothermal when con- 



1 Thermodynamic processes are discussed in chapter 

 8 of this text. 



sidered from start to finish, and approximately 

 adiabatic when considered within any one stage 

 of the compression process. In order to achieve 

 some benefits from each type of process (iso- 

 thermal and adiabatic) most air compressors are 

 designed with more than one stage and with 

 a cooling arrangement after each stage. Multi- 

 staging and after- stage cooling have the further 

 advantages of preventing the development of ex- 

 cessively high temperatures in the compressor 

 and in the accumulator, reducing the horsepower 

 requirements, condensing some of the entrained 

 moisture, and increasing volumetric efficiency. 



The general arrangement of a multistage 

 compressed air plant with after-stage cooling is 

 illustrated in figure 17-1. This illustration shows 

 a reciprocating air compressor with two stages; 

 however, the same general arrangement of parts 

 is found in any type of compressed air plant that 

 utilizes multistaging and after- stage cooling. 



The accumulator shown in figure 1 7- 1 is found 

 in all compressed air plants, although the size of 

 the unit varies according to the needs of the sys- 

 tem. The accumulator (also called a receiver) 

 helps to eliminate pulsations in the discharge line 

 of the air compressor, acts as a storage tank 

 during intervals whenthedemandfor air exceeds 

 the capacity of the compressor, and allows the 

 compressor to shut down during periods of light 

 load. Overall, the accumulator functions to re- 

 tard increases and decreases in the pressure 

 of the system, thereby lengthening the start- 

 stop- start cycle of the compressor. 



COMPRESSOR CLASSIFICATIONS 



Air compressors are classified in various 

 ways. A compressor may be single acting or 

 double acting, single stage or multistage, and 

 horizontal, angle, or vertical, as shown in figure 

 17-2. A compressor may be designed so that 

 ONLY one stage of compression takes place 



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