Compressors are machines for the compression of gases and vapors (to pressure of 2000 atm. and higher). Piston compressors are used for producing the higher pressures, whereas centrifugal compressors are used for low to medium pressures. The crankshaft is driven by suitable prime mover. A connecting rod between the crankshaft and the piston transforms the rotary motion of the former into a to and fro motion of the piston in the cylinder. The valves are spring loaded and react to variations in pressure produced by the piston movement . In performing the suction stroke the piston causes a lowering of pressure in the cylinder, so that the inlet valve opens against the restraining pressure of its spring and allows the gas to flow into the cylinder. Then, when the piston begins to form its return stroke, the inlet closes because the increase in pressure within the cylinder. When the piston has completed this stroke i.e. has arrived at top dead center, the pressure of the gas compressed in the cylinder is so high that the delivery valve opens against the restraining pressure of its spring, allow the gas to flow into the delivery pipe . The spring of the delivery valve need not be as powerful as might at first to be supposed. Actually there is only pressure acting in the delivery pipe , at the rear of the valve, and this pressure is only a little below that which is produced by the compression stroke of the piston. Also, it is this pressure in the delivery pipe that keeps the delivery valve closed during the suction stroke. Usually , the compressor feeds the compressed gas into an intermediate vessel called a receiver in the case of an air compressor form which it is supplied to consumer equipment.
The operating cycle of a compressor is characterized by the so-called indicator diagram . It shows the relation between pressure and volume during one revolution of the crankshaft, i.e. during on to and from movement of the piston line 1 represents the suction stroke: the gas volume in the cylinder increases while the low pressure remains constant . Line 2 represents the compression: the pressure rises while the volume decreases. Then comes the discharge of the compressed gas represented by line 3 : the volume of the cylinder decreases at constant high pressure . Line 4 shows that when the piston performs the suction stroke, the inlet valve cannot immediately open, as the residual gas in the cylinder at the end of the compression stroke must expand until the pressure in the cylinder has fallen below the pressure in the inlet pipe. Not till then does the inlet valve opens some dead space and therefore an undesirable gas residue , so that only a portion the suction stroke is really utilized for drawing gas into the cylinder. If high pressure is required, compression will have to be achieved in several stages, i.e. the gas passed from one cylinder to next. As gases become heated when they are compressed, it is necessary to provide cooling between the successive stages. Theses multi-stage compressors are usually of the double acting type i.e. when the piston travels in one direction, the gas is compressed on one side and suction is produced on the other side of piston: the opposite occurs during the return stroke.
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