PRINCIPLES OF NAVAL ENGINEERING 



equals the pressure on the opposite side of the 

 air inlet valve, the inlet valve closes. Air is in- 

 creasingly compressed as the piston moves 

 toward TDC; the pressure in the cylinder finally 

 becomes great enough to force the discharge 

 valve open against the discharge line pressure 

 and the pressure of the valve springs. During the 

 balance of the compression stroke, the air which 

 has been compressed in the cylinder is dis- 

 charged, at almost constant pressure, through 

 the open discharge valve. 



The basic operating cycle just described is 

 repeated a number of times in double acting 

 compressors and in other stages of multistage 

 compressors. In a double-acting compressor, 

 each stroke of the piston is a suction stroke in 

 relation to one end of the cylinder and a com- 

 pression stroke in relation to the other end of 

 the cylinder. In a double acting compressor, 

 therefore, two basic compression cycles are 

 always in process when the compressor is oper- 

 ating; but each cycle, considered separately, is 

 simply one suction stroke and one compression 

 stroke. 



In multistage compressors, the basic com- 

 pression cycle must occur at least once for each 

 stage of compression. If the compressor is de- 

 signed with two compressing elements for the 

 first (low pressure) stage, two compression 

 cycles will be in process in the first stage at the 

 same time. If the compressor is designed so that 

 two stages of compression occur at the same time 

 in one compressing element, the two basic cycles 

 (one for each stage) will occur at the same time. 



Compressor Components 



A reciprocating air compressor consists of 

 a compressor element, a lubrication system, a 

 cooling system, a control system, and an unload- 

 ing system. In addition to these basic com- 

 ponents, the compressor has a system of con- 

 necting rods, crankshaft, and flywheel for 

 transmitting the power developed by the driving 

 unit to the air cylinder pistons. 



COMPRESSING ELEMENT. -The compres- 

 sing element of a reciprocating air compressor 

 consists of the air valves, the cylinder, and the 

 piston. 



The valves of modern compressorsareof the 

 automatic type. The opening and closing of these 

 valves is caused solely by the difference between 

 the pressure of the air in the cylinder and the 

 pressure of the external air on the intake valve 



or the pressure of the discharged air on the dis- 

 charge valve. On most compressors, a thin plate, 

 low lift type of valve is used. A valve of this type 

 is shown in figure 17-5. 



The design of pistons and cylinders depends 

 primarily upon thenumberof stages of compres- 

 sion which take place within a cylinder. Common 

 arrangements of pistons and cylinders are shown 

 in a previous illustration (fig. 17-2). 



Two types of pistons are in common use. 

 Trunk pistons (fig. 17-6) are driven directly by 

 the connecting rods. Since the upper end of a con- 

 necting rod is fitted directly to the piston wrist 

 pin, there is a tendency for a piston to develop a 

 side pressure against the cylinder walls. To dis- 

 tribute the side pressure over a wide area of the 

 cylinder walls or liners, trunk pistons with long 

 skirts are used. This type of piston tends to elim- 

 inate cylinder wall wear. Differential pistons 

 (fig. 17-7) are modified trunk pistons having two 

 or more different diameters. These pistons are 

 fitted into special cylinders which are arranged 

 so that more than one stage of compression is 

 served by one piston. The compression for one 

 stage takes place over the piston crown; com- 

 pression for theother stage or stages takes place 

 in the annular space between the large and small 

 diameters of the piston. 



LUBRICATION SYSTEM.- Lubrication of air 

 compressor cylinders is generally accomplished 

 by means of a mechanical force-feed lubricator 

 which is driven from a reciprocating or a rotary 

 part of the compressor. Oil is fed from the lu- 

 bricator through a separate feed line to each cyl- 

 inder. A check valve is installed at the end 

 of each feed line to keep the compressed air from 

 forcing the lube oil back to the lubricator. Each 

 feed line is equipped with a sight glass. Lubri- 

 cation begins automatically as the compressor 

 starts up. The amount of oil that must be fed to 

 the cylinders depends upon the cylinder diame- 

 ter, the cylinder wall temperature, and the vis- 

 cosity of the oil. 



On small low pressure and medium pressure 

 compressors, the cylinders may be lubricated by 

 the splash method, from dippers on the ends of 

 the connecting rods, instead of by a mechanical 

 force-feed lubricator. 



Lubrication of the running gear of most com- 

 pressors is accomplished by a lube oil pump 

 (usually of the gear type) which is attached to the 

 compressor and driven from the compressor 

 shaft. This pump draws oil from the reservoir, 

 as shown in figure 17-8, and delivers it, through 



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