What is multi-stage compression? Why multiple stages of compression?

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What is multi-stage compression? Why multiple stages of compression? Immerse yourself in reading with a novel reader. I. What is multi-stage compression? Multi-stage compression refers to the division of the compressor’s cylinders into several stages, depending on the pressure required; the pressure is increased stage by stage, and an intercooler is installed after each compression stage to cool the high-temperature gas resulting from that compression. In this way, the exhaust temperature at each stage can be reduced. As shown in the figure below, it is the P-V diagram of a multi-stage compressor. BC is the adiabatic curve, and BK is the isothermal curve. When the gas enters the first-stage cylinder at pressure P1 and is compressed to that same pressure within the cylinder, if it is an adiabatic process, the gas state is represented by point a on line BC1. During the compression process, if it is cooled by the coolant in the cylinder water jacket, the gas state will be at point b in the diagram. As can be seen from the figure, this can save work associated with the Bab area. The gas in state b passes through the intercooler after the first-stage cylinder, where its temperature drops and its volume changes from point b to point c (with pressure P2 remaining constant). Similarly, during the second stage of compression, the work saved by avoiding the use of Cade area can be represented by the area fdgh; the work saved in the third stage can also be expressed in this manner ； The work saved in the fourth stage can be expressed by the area igCj. The more levels there are, the closer the lines connecting points B, b, c, e, f, h, i, and j will be to the isotherm curve, resulting in greater savings in work. II. Why multi-stage compression? Using a single-stage compressor to press the gas to very high pressures necessarily results in an increased compression ratio, and the temperature of the compressed gas also rises significantly. The greater the increase in gas pressure, the higher the gas temperature rises. When the pressure ratio exceeds a certain value, the final temperature of the compressed gas rises above the flash point of conventional compressor lubricants (200–240°C), causing the lubricant to burn down into carbon residue and resulting in lubrication difficulties. The work consumed by a multi-stage compressor is significantly reduced compared to that of a single-stage compressor; the more stages there are, the greater the energy savings. At the same time, the more stages there are, the lower the temperature of the gas after compression, and the greater the volume of gas that can be drawn into the cylinder. During the suction process, a reciprocating compressor must wait until the high-pressure gas remaining in the clearance volume of the cylinder – where the clearance volume refers to the space between the piston and the cylinder at the end of exhaust, when the piston is at its dead center position, as well as the spaces in the passages connecting the valves to the cylinder – has expanded to a pressure slightly lower than the intake pressure, before it can begin to draw in air. After expansion, the high-pressure gas occupies part of the cylinder volume, reducing the volume available for the cylinder to draw in air. Obviously, the higher the pressure ratio, the higher the gas pressure remaining in the clearance, and the greater the volume occupied by the residual gas after expansion, which significantly reduces the compressor’s capacity. At the same time, the length, thickness, and diameter of the compressor components must all be increased accordingly; otherwise, they will not be able to handle the loads applied to them. As a result, not only does this increase the cost of the compressor, but it also adds difficulties to the manufacturing of these components. Therefore, to achieve a higher final pressure, multi-stage compressors must be used. However, the number of stages in the compressor should not be too high either, because with each additional stage, more components such as cylinders, valves, piston rods, and connecting rods are required, which complicates the compressor’s structure and significantly increases the cost of the equipment. Under normal circumstances, the compression ratio per stage of the compressor does not exceed 3 to 5.