Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGAIR INLETINTERCOOLERJAFTERCOOLERAIROUTLETCOOLINGf .WATER r"*"OUTLETCOOLING WATERINLETACCUMULATORFigure 17-1.— General arrangement of multistage compressed airplant.147.118within one compressing element, or so that morethan one stage takes place within one compressingelement. In general, compressors are classifiedaccording to the type of compressing element,the source of driving power, the method bywhich the driving unit is connected to the compressor,and the pressure developed.TYPES OF COMPRESSING ELEMENTS .-Air compressor elements may be of the centrifugal,rotary, or reciprocating types. The reciprocatingtype isgenerally selected for capacitiesbelow 1,000 cfm and for pressures of 100 psi orabove, the rotary type for capacities up to 10,000cfm andfor pressures below 100 psi, and the centrifugaltype for 10,000 cfm or greater capacitiesand for up to 100 psi pressures.Most of the compressors used in the Navyhave reciprocating elements (fig. 17-3). In thistype of compressor the air is compressed in oneor more cylinders, very much like the compressionwhich takes place in an internal combustionengine.SOURCES OF POWER. -Compressors aredriven by electric motors, internal combustionengines, steam turbines, or reciprocating steamengines. Most of the air compressors in navalservice are driven by electric motors.DRIVE CONNECTIONS.- The driving unitmay be connected to the compressor by one ofseveral methods. When the compressor and thedriving unit are mounted .on the same shaft, theyare close coupled. Close coupling is often usedfor small capacity compressors that are drivenby electric motors. Flexible couplings are usedto join the driving unit to the compressor wherethe speed of the compressor and the speed of thedriving unit can be the same.V-belt drives are commonly used with small,low pressure, motor-driven compressors, andwith some medium pressure compressors. In afew installations, a rigid coupling is used betweenthe compressor and the electric motor ofa motor-driven compressor. In a steam turbinedrive, compressors are usually driven throughreduction gears.PRESSURE CLASSIFICATION.-In accordancewith General Specifications for Ships of theUnited States Navy, compressors are classifiedas low pressure, medium pressure, or high pressure.Low pressure compressors are thosewhich have a discharge pressure of 150 psi orless. Medium pressure compressors are thosewhich have a discharge pressure of 151 psi to1,000 psi. Compressors which have a dischargepressure above 1,000 psi are classified as highpressure.Most low pressure air compressors are ofthe two- stage type with either a vertical V (seefig. 17-3) or a vertical W arrangement of438