PRINCIPLES OF NAVAL ENGINEERING 



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THIRD STAGE COOLER 



WATER OUTLET- 



e 



Qgr — 



FIRST ST ACE COOLER 



WATER INLET- 



H' WATER OUTLET 



OIL OUTLET 



OIL COOLER 



Figure 17-9.— Cooling water system for multistage air compressor. 



47,157 



Combined speed and pressure governors are 

 usually furnished for compressors which are 

 driven by reciprocating steam engines. Neither 

 the start-stop nor the constant- speed control is 

 entirely satisfactory for this type of compressor. 



UNLOADING SYSTEM. -Air compressor un- 

 loading systems are installed for the removal of 

 all but the friction loads on the compressors. An 

 unloading system automatically removes the 

 compression load from the compressor while the 

 unit is starting and automatically applies the load 

 after the unit is up to operating speed. For units 

 that have the start- stop type of control, the un- 

 loading system is separate from the control sys- 

 tem. For compressors equipped with the 

 constant- speed type of control, the unloading 

 system is an integral part of the control system. 



A number of different unloading methods are 

 used, including closing or throttling the com- 

 pressor intake, holding intake valves off their 

 seats, relieving intercoolers to atmosphere, 

 opening a bypass from the discharge to the intake, 

 opening up cylinder clearance pockets, using 



miscellaneous constant- speed unloading de- 

 vices, and using various combinations of these 

 methods. 



As an example of a typical compressor un- 

 loading device, consider the MAGNETIC TYPE 

 UNLOADER. Figure 17-10 illustrates the un- 

 loader valve arrangement. This typeof unloader 

 consists of a solenoid-operated valve connected 

 with the motor starter. When the compressor is 

 at rest, thesolenoid valve is deenergized, admit- 

 ting air from the receiver to the unloading mech- 

 anism. When the compressor approximates nor- 

 mal speed, the solenoid valve is energized, 

 releasing the pressure from the unloading mech- 

 anism and loading the compressor again. 



ROTARY-CENTRIFUGAL AIR COMPRESSORS 



The one non- reciprocating type of air com- 

 pressor that is found aboard ship is variously 

 referred to as a rotary compressor, a centrifugal 

 compressor, or a "liquid piston" compressor. 

 Actually, the unit is something of a mixture, 

 operating partly on rotary principles and partly 



446 



