Principles of naval engineering - Historic Naval Ships Association

Chapter 17. -COMPRESSED AIR PLANTSVALVE PLATEVALVE PLATE^GUIDESTOP PLATEFigure 17-5.— Diagram of a thin plate air compressor valve.47.154Xa filter, to an oil cooler. From the cooler, theoil is distributed to the top of each main bearing,to spray nozzles for reduction gears, and to outboardbearings. The crankshaft is drilled so thatoil fed to the main bearings is picked up at themain bearing journals and carried to the crankpin journals. The connecting rods contain passageswhich conduct lubricating oil from thecrank pin bearings up to the wrist pin bushings.As oil leaks out from the various bearings, itdrips back to the reservoir in the base ofthe compressor and is recirculated. Oil from theoutboard bearings is carried back to the base bythe drain lines.The discharge pressure of lubricating oilpumps varies, depending upon the pump design. Arelief valve fitted to each pump functions when thedischarge pressure exceeds the pressure forthe valve is set. When the relief valve opens, excessoil is returned to the reservoir.COOLING SYSTEM.— Most compressors arecooled by sea water supplied from the ship's fireand flushing system. The cooling water is usuallyavailable to each unit through at least twosources. Compressors located outside the largermachinery spaces are generally equipped with anattached circulating water pump as a standbysource of cooling water. Some small low pressurecompressors are air cooled by a fanmounted on or driven by a compressor shaft.The path of water in the cooling water systemfor a four- stage compressor is shown in figure17-9. The flow paths are not identical in allcooling water systems, but in all systems it isimportant that the coldest water be available forcirculation through the oil cooler. Valves areusually provided so that the flow of water to thecooler can becontroUedindependentlyof the restof the system. Thus the oil temperature can becontrolled without affecting other parts of thecompressor. Cooling water is then supplied to theintercoolers and the aftercooler and then to thecylinder jackets and heads. A high pressure aircompressor may require from 6 to 25 gallons ofcooling water per minute, while a medium443