Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGequals the pressure on the opposite side of theair inlet valve, the inlet valve closes. Air is increasinglycompressed as the piston movestoward TDC; the pressure in the cylinder finallybecomes great enough to force the dischargevalve open against the discharge line pressureand the pressure of the valve springs. During thebalance of the compression stroke, the air whichhas been compressed in the cylinder is discharged,at almost constant pressure, throughthe open discharge valve.The basic operating cycle just described isrepeated a number of times in double actingcompressors and in other stages of multistagecompressors. In a double-acting compressor,each stroke of the piston is a suction stroke inrelation to one end of the cylinder and a compressionstroke in relation to the other end ofthe cylinder. In a double acting compressor,therefore, two basic compression cycles arealways in process when the compressor is operating;but each cycle, considered separately, issimply one suction stroke and one compressionstroke.In multistage compressors, the basic compressioncycle must occur at least once for eachstage of compression. If the compressor is designedwith two compressing elements for thefirst (low pressure) stage, two compressioncycles will be in process in the first stage at thesame time. If the compressor is designed so thattwo stages of compression occur at the same timein one compressing element, the two basic cycles(one for each stage) will occur at the same time.Compressor ComponentsA reciprocating air compressor consists ofa compressor element, a lubrication system, acooling system, a control system, and an unloadingsystem. In addition to these basic components,the compressor has a system of connectingrods, crankshaft, and flywheel fortransmitting the power developed by the drivingunit to the air cylinder pistons.COMPRESSING ELEMENT. -The compressingelement of a reciprocating air compressorconsists of the air valves, the cylinder, and thepiston.The valves of modern compressorsareof theautomatic type. The opening and closing of thesevalves is caused solely by the difference betweenthe pressure of the air in the cylinder and thepressure of the external air on the intake valveor the pressure of the discharged air on the dischargevalve. On most compressors, a thin plate,low lift type of valve is used. A valve of this typeis shown in figure 17-5.The design of pistons and cylinders dependsprimarily upon thenumberof stages of compressionwhich take place within a cylinder. Commonarrangements of pistons and cylinders are shownin a previous illustration (fig. 17-2).Two types of pistons are in common use.Trunk pistons (fig. 17-6) are driven directly bythe connecting rods. Since the upper end of a connectingrod is fitted directly to the piston wristpin, there is a tendency for a piston to develop aside pressure against the cylinder walls. To distributethe side pressure over a wide area of thecylinder walls or liners, trunk pistons with longskirts are used. This type of piston tends to eliminatecylinder wall wear. Differential pistons(fig. 17-7) are modified trunk pistons having twoor more different diameters. These pistons arefitted into special cylinders which are arrangedso that more than one stage of compression isserved by one piston. The compression for onestage takes place over the piston crown; compressionfor theother stage or stages takes placein the annular space between the large and smalldiameters of the piston.LUBRICATION SYSTEM.- Lubrication of aircompressor cylinders is generally accomplishedby means of a mechanical force-feed lubricatorwhich is driven from a reciprocating or a rotarypart of the compressor. Oil is fed from the lubricatorthrough a separate feed line to each cylinder.A check valve is installed at the endof each feed line to keep the compressed air fromforcing the lube oil back to the lubricator. Eachfeed line is equipped with a sight glass. Lubricationbegins automatically as the compressorstarts up. The amount of oil that must be fed tothe cylinders depends upon the cylinder diameter,the cylinder wall temperature, and the viscosityof the oil.On small low pressure and medium pressurecompressors, the cylinders may be lubricated bythe splash method, from dippers on the ends ofthe connecting rods, instead of by a mechanicalforce-feed lubricator.Lubrication of the running gear of most compressorsis accomplished by a lube oil pump(usually of the gear type) which is attached to thecompressor and driven from the compressorshaft. This pump draws oil from the reservoir,as shown in figure 17-8, and delivers it, through442