Principles of naval engineering - Historic Naval Ships Association

PRWCTPLES OF NAVAL ENGINEERINGINLETVALVE> DISCHARGESZZZ^^ZZ^^ VALVE^lpressure air compressor may require from 10to 20 gallons per minute.As previously noted, cooling of the air is requiredfor most economical compression.Another reason for cooling the air between stagesand after the last stage is to condense any moisturethat may be present. The resulting condensateis then drained off. If the moisture is notremoved from the air, it will be carried into theaccumulator or into the air lines, where it cancause serious trouble.The intercoolers used between stages and theaftercooler used after the last stage are of thesame general construction except that the aftercooleris designed to withstand a higher workingpressure than the intercoolers. Water-cooledcoolers may be of the straight shell-and-tubetype or of the coil type. In coolers designed forair pressures below 250 psig, the air flows eitherthrough the tubes or over and around them; incoolers designed for air pressures above 250psig, the air flows through the tubes. In straightf]f\iaFigure 17-6. — Trunk Piston.47.155.147.155.2Figure 17-7.— Differential Piston.tube coolers, baffles are provided to guide the airand water. In coil type coolers, the air passesthrough the coil and the water flows around theoutside. Air-cooled coolers may beof the radiatortype or may consist of a bank of finned coppertubes located in the path of a blast of air suppliedby the compressor fan.Both intercoolers and aftercoolers are fittedwith moisture separators on the discharge sideto remove moisture (and also any oil that may bepresent) from the air stream. Various designsof moisture separators are in use; the removalof liquid may be accomplished by centrifugalforce, impact, or sudden changes in the velocityof the air stream.CONTROL SYSTEM. -The control system ofa reciprocating air compressor may include oneor more devices such as automatic temperatureshutdown devices, start-stop controls, constantspeedcontrols, and speed-pressure governors.444