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Basic knowledge of reciprocating compressors 1. What is the working process of a compressor? A reciprocating compressor consists of cylinders, pistons, and valves. The working process of a compressed gas can be divided into four stages: expansion, intake, compression, and discharge. Figure 1-1 shows the cylinder of a single-suction compressor. This type of compressor has intake and exhaust valves only at one end of the cylinder; each time the piston moves back and forth, it only draws in air once and expels air once. Figure 1-1 Schematic diagram of a single-stage compressor cylinder: 1 – Cylinder; 2 – Piston; 3 – Intake valve; 4 – Exhaust valve. (1) Expansion: As the piston 2 moves to the left, the volume of the cylinder on the right side increases, causing the pressure to drop, and the remaining gas in the cylinder continues to expand. (2) Suction: When the pressure drops to slightly below the pressure of the gas in the discharge pipe, the gas in the inlet pipe pushes open the suction valve 3 and enters the discharge cylinder. As the piston moves gradually to the left, gas continues to flow into the cylinder until the piston reaches its leftmost position (also known as the left dead center). (3) Compression: When the piston moves to the right in the opposite direction, the volume of the workpiece gradually decreases, thus initiating the process of compressing the gas. Due to the check function of the intake valve, the gas inside the cylinder cannot flow back into the inlet pipe; moreover, since the pressure of the gas in the outlet pipe is higher than that inside the cylinder, the gas inside the cylinder cannot escape outside through the exhaust valve 4. The gas in the outlet pipe cannot flow back into the cylinder either, as the exhaust valve acts as a check valve. Therefore, the mass of gas in the cylinder remains constant; it is only as the piston continues to move to the right that the volume of space containing gas in the cylinder decreases, which causes the pressure of the gas to keep rising. (4) Exhaust: As the piston moves to the right, the pressure of the compressed gas rises to a level slightly higher than the gas pressure in the outlet pipe; at this point, the gas inside the cylinder pushes open the exhaust valve and enters the outlet pipe, where it continues to be exhausted until the piston reaches its rightmost position (also known as the right dead center). Then, the piston begins to move left again, repeating the above action. The piston moves back and forth continuously inside the cylinder, causing the cylinder to cyclically draw in and discharge gas. Each back and forth movement of the piston is called a working cycle, and the distance covered by the piston on each such movement is known as the stroke. Figure 1-2 shows the cylinder of a double-suction compressor. Both ends of this cylinder are equipped with an intake valve and an exhaust valve. Its compression process is the same as that of a single-suction cylinder; the difference lies in the fact that, at any given time, regardless of the direction in which the piston moves, compression occurs in the direction of its movement, while suction takes place behind the piston. In other words, gas can be drawn in and expelled simultaneously regardless of whether the piston moves to the left or to the right. 2· What are the three thermal processes of compressed gases? The energy changes of a gas during compression are related to the gas conditions (i.e., temperature, pressure, volume, etc.). A large amount of heat is generated when compressing gas, causing the temperature of the compressed gas to rise. The greater the degree of compression of a gas, the more it is heated, and thus its temperature rises higher. The heat generated during the compression of gas remains in the gas, raising its temperature; part of this heat is transferred to the cylinder, raising its temperature as well, while a small amount of heat is lost to the air through the cylinder walls. The compression work required to compress a gas is determined by the change in the gas state. Put simply, the amount of power consumed by the compressor is directly related to the heat generated by compressing the gas. Generally, there are three types of processes for compressing gases: (1) Isothermal compression process: During compression, all the heat equivalent to the work done in compression is removed, keeping the temperature of the gas inside the cylinder constant; this type of compression is known as isothermal compression. The compression work consumed during isothermal compression is minimal. But this process is an ideal one; it is very difficult to achieve in actual production. (2) Adiabatic compression process: During this compression process, there is no heat exchange with the outside environment, which results in an increase in the temperature of the gas inside the cylinder. This type of compression, which neither dissipates heat to the outside nor absorbs heat from it, is called adiabatic compression. This compression process requires the most work and is also an ideal process. In actual production, it is very difficult to completely avoid heat loss under any circumstances. (3) Polytropic compression process: A compression process that is neither completely isothermal nor completely adiabatic is called a polytropic compression process. This process lies between an isothermal process and an adiabatic process. In actual production, the compression of gases is always a polytropic compression process. Figures 1-3 show the compression curves of the gas under the above three conditions. Among them, the outermost curve BC represents an adiabatic process and is called an adiabatic curve; the curve BCl in the middle represents the gas compression process under actual conditions, and is called a polytropic curve; the innermost curve BCz represents the gas compression process at constant temperature, and is called an isothermal curve. Gas volume v. Figures 1–3 show the gas compression curves: Bc represents the adiabatic curve; Bcl represents the polytropic curve; Bc2 represents the isothermal curve. It can be seen from Figures 1–3 that the area enclosed by PnCzD during isothermal compression is smaller than the area enclosed by Pnd during adiabatic compression. The size of the area can also indicate the level of power consumption; therefore, the work required for isothermal compression is much less than that required for adiabatic compression. It can also be seen from the graph that the polytropic curve lies between the isothermal and adiabatic curves. Its area, PnClD, is larger than the area PnCzD for isothermal compression, and smaller than the area PnD for adiabatic compression; therefore, the work required during polytropic compression is greater than that for isothermal compression, but less than that for adiabatic compression. The closer a polytropic curve is to an isothermal curve, the less work it requires; conversely, the closer a polytropic curve is to an adiabatic curve, the more work it requires. Therefore, in practical applications, in order to save compression work – that is, the energy consumed in compressing the gas – it is necessary to make the polytropic process as close as possible to an isothermal process. In other words, conditions similar to an isothermal process must be created for gas compression. To make a polytropic process approach an isothermal process, it is necessary to remove the heat generated during the compression of the gas. In actual production, to achieve the above objectives, air and cooling water are commonly used to cool the compressor cylinders and the compressed gas. 1 – Normal power diagram during compression (dashed lines represent the theoretical intake, compression, and exhaust curves); 2 – The clearance volume is higher than normal; 3 – Leakage in the exhaust valve; 4 – Stuck exhaust valve leaf; 5 – High resistance in the intake and exhaust pipes; 6 – The spring of the exhaust valve is too strong; 7 – Leakage in the intake valve or piston rings during compression; 8 – Stuck intake valve; 9 – Leakage in the piston rings; 10 – Inappropriate selection of the valve spring. The better the cooling effect during this process, the more heat can be removed, and the polytropic curve will be closer to an isothermal curve. As a result, more energy can be saved, making the process more economical. Figures 1-3, also known as indicator diagrams, can be plotted on paper using specialized instruments (indicators). Based on these indicator diagrams, it is possible to determine the indicated power, volume coefficient, the polytropic indices for compression and expansion processes, the pressure losses during intake and exhaust, as well as the indicated power lost due to harmful resistances. Furthermore, the operating conditions of all valves, their springs, piston rings, and packing glands are reflected in the indicator diagram. Figures 1-4 show the P-V diagrams for the normal and abnormal operation of the compressor. Based on the characteristics of the indicated work diagram distortion, it is possible to identify the faults that occur in the compressor during operation and their nature. 3·What is multi-stage compression? Multi-stage compression refers to the division of the compressor’s cylinders into several stages, depending on the desired pressure; 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. 4· Why is multi-stage compression needed? When using a single-stage compressor to compress gas to very high pressures, the compression ratio inevitably increases, 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 (around 240°C), causing the lubricant to burn 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 of a reciprocating compressor, it is necessary to 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 space in the passages connecting the valves to the cylinder – has expanded to a pressure slightly lower than the intake pressure, before suction can begin. After expansion, the high-pressure gas occupies a portion 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 raises the difficulties involved in manufacturing 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 **increases the cost of the equipment. Under normal circumstances, the compression ratio per stage of the compressor should not exceed 3-5. 5·What is the production capacity (discharge volume) of a reciprocating compressor? The amount of gas discharged by the compressor per unit of time, converted to the volume of gas in its original intake state, is referred to as the compressor’s production capacity, or also known as the compressor’s discharge volume. Its unit is m3/h or m3/min. 6. What are the main factors that affect the production capacity of reciprocating compressors? (1) Clearance: When the clearance is large, the high-pressure gas in the clearance expands during suction, occupying part of the volume and thereby reducing the amount of air drawn in, which lowers the compressor’s production capacity. Of course, too small a clearance is also undesirable, as this can cause the piston in the cylinder to collide with the cylinder head and damage the machine. Therefore, the gas clearance of the compressor must be adjusted properly. (2) Leakage loss: The production capacity of a compressor is highly dependent on the airtightness of the piston rings, the intake and exhaust valves, as well as the cylinder gasket. The piston ring is fitted over the piston; its function is to seal the gap between the piston and the cylinder, preventing the gas being compressed from leaking to the other side of the piston. Therefore, when installing piston rings, they should be able to expand and contract freely, so as to achieve a good seal without causing excessive friction between the piston and the cylinder. If the piston rings are not installed properly or wear out due to friction with the cylinder, preventing a complete seal, some of the compressed high-pressure gas does not exit through the exhaust valve, but leaks to the other side of the piston at the area where the piston rings are not airtight. As a result, the amount of air expelled decreases, and thus the compressor’s production capacity also drops. In actual production, a decrease in output due to air leakage caused by piston ring wear occurs frequently. If the exhaust valve is not tight enough, during the intake process, some of the high-pressure gas in the outlet pipe will leak back into the cylinder through the loose valve. If the intake valve is not tight enough, some of the compressed gas will also leak back from the cylinder into the inlet pipe during compression. Both of these situations will reduce the compressor’s production capacity. In practice, since the valve plates of gas valves are often eroded by the gas or damaged due to poor quality, air leakage that leads to reduced production also occurs frequently. During the operation of the compressor, air leakage can occur either due to wear resulting from frequent friction between the cylinder gasket and the piston rod, or as a result of poor installation quality. Therefore, air leakage from cylinder packing is also a common issue in actual production. (3) Resistance of the suction valve: The suction valve of the compressor should have a certain capacity to resist gas pressure, and it should open only when the pressure inside the cylinder is slightly lower than the gas pressure in the inlet pipe. If the resistance of the intake valve is greater than normal, the opening speed will be slower, the amount of air entering the cylinder will decrease, and as a result, the compressor’s production capacity will also drop. (4) Temperature of the intake gas: Although the volume of the compressor cylinder remains constant, if the temperature of the intake gas is high, the density of the gas inside the cylinder decreases. The decrease in the mass of gas drawn in per unit time results in a reduction in the compressor’s capacity to compress gas. This is why the compressor’s production capacity is always lower in summer than in winter. Furthermore, although the temperature of the gas in the intake pipe is not high, if the cylinder is not cooled properly and as a result the temperature of the gas in the intake valve chamber becomes too high, the volume of the gas will expand and its density will decrease, which in turn reduces the compression capacity of the compressor. 7. Why must there be a clearance in the cylinders of reciprocating compressors? (1) When compressing gas, some of the vapor in the gas may condense. We know that liquids are incompressible; if there is no clearance in the cylinder, the compressor will inevitably be damaged. Therefore, a clearance must be left in the compressor cylinder. (2) The presence of clearance and the gas remaining in the clearance volume can act as a cushion, preventing the piston from colliding with the cylinder head and causing damage. At the same time, for the purposes of assembly and adjustment, a certain clearance must also be left between the cylinder head and the piston at the dead center position. (3) The compressor is equipped with air valves; there is residual gas between the air valves and the cylinders, as well as in the air passages of the valve seats, and this residual gas cannot be completely expelled by the pistons. This residual gas helps to reduce the impact of gases on the inlet and outlet valves, and it also reduces the impact of the valve plates on the valve seats and the lift limiters (valve covers). (4) Due to the thermal expansion of metals, the piston rod and connecting rod expand and lengthen as the temperature rises during operation. Having a clearance in the cylinder brings many benefits to the compression machine’s assembly, operation, and safe use. However, if the clearance is too large, it not only fails to provide any advantages but also has an adverse effect on the machine’s performance. Therefore, under normal conditions, the clearance volume left in the compressor cylinder is approximately 3-8% of the working volume of the cylinder; whereas for compressors with higher pressure and smaller diameters, the clearance volume is usually 5-12%. 8. Why is intercooling necessary between the different stages of a reciprocating compressor? After compression at each stage, the rising temperature reduces the viscosity of the lubricating oil in the cylinders; simultaneously, carbonaceous substances are formed, which accumulate in important parts such as the valve plates and interfere with their proper functioning. If the temperature is higher than the flash point of the lubricant, there is a potential risk of explosion. Sometimes the compressed gas is a hydrocarbon gas (such as propane), and its physical properties change at high temperatures, such as through polymerization. Generally, the exhaust temperature of a compressor should be well below the flash point of the lubricating oil. When compressing air, the exhaust temperature should be kept below 150°C; for gases such as propane, ethylene, and acetylene, it should be kept below 100°C. Therefore, an intercooler is necessary. In multi-stage compressors, the pressure at each stage is relatively low, and there are inter-stage coolers that cool the gas discharged from each stage to a temperature close to that before it enters the first stage (something that cannot be achieved solely through cooling within the cylinder liners). As a result, the temperature of the gas at the end of compression in each stage does not become too high. Figures 1-6 show the flow diagram of a three-stage compressor. In the flow diagram, the function of the oil-water separator is to separate lubricating oil and water from the gas as it cools, so that these oils and waters are not carried by the gas to the lower cylinders. 9. What is the role of lubrication in reciprocating compressors, what are the types of lubricants used, and what are the lubrication methods? The role of lubrication in compressors is primarily to reduce wear on frictional components and minimize the energy lost due to friction. Additionally, it helps to cool the friction surfaces of moving parts, as well as the pistons and stuffing boxes, thereby improving the reliability of their operation. Therefore, lubrication of the compressor is of great importance. The lubrication of compressors can basically be divided into the cylinder lubrication system and the moving mechanism lubrication system. The lubricating oil used for lubricating cylinders should have a high viscosity, so as to provide effective lubrication and sealing between the piston rings and the cylinder. Secondly, it is also required to have a high flash point and high stability, so that the oil does not evaporate easily or oxidize easily; otherwise, carbon deposits may form (carbon compounds resulting from the oxidation of lubricating oil), and when these deposits burn, explosions can occur. Additionally, carbon deposits accelerate the wear of the cylinder valves, so their formation in the cylinders is extremely detrimental to the operation of the compressor. Therefore, the cylinder lubricant is specialized compressor oil used for lubrication. The oil consumption in the cylinders of air compressors is subject to strict limits. Excessive oil usage is not only uneconomical but also causes contamination of the conduits and associated components, leading to carbon buildup. For low-pressure and medium-pressure compressors, horizontal compressors have an average lubricant consumption of 1 g/min per 400 m2 of lubricated surface, while vertical compressors have an average lubricant consumption of 1 g/min per 500 m2 of lubricated surface. Due to the presence of a cooler and an oil separator after the compressor, high-pressure compressors have higher lubricant consumption; the lubricant consumption per liter of the lubricated surface in each 200# cylinder is 1 g/min, while it is 3 g/min per 100 m2 of the lubricated surface on the piston rod in the stuffing box. During the trial operation of the new compressor (running-in period), the amount of oil added is twice the specified amount. The lubricant flow rate (circulation volume) of the motion mechanism varies depending on whether a lubrication cooler is present; with a cooler, the lubricant flow rate is 0.075 kg/min·kW, while without a cooler it is 0.15 kg/min·kW. The amount of lubricant consumed should be determined based on actual conditions; the figures mentioned above are for reference only. There are generally two methods for lubricating compressor cylinders: (1) The splash method: A rotating mechanism (such as a crankshaft) is used to throw oil from the crankcase onto the cylinder walls in order to supply lubrication to the cylinders. This method is only suitable for reciprocating compressors without crossheads, but the amount of oil supplied cannot be adjusted. Especially when the wiper ring and piston rings do not fit well together, excess lubricant can be carried away by the gas. 2) Forced lubrication method (pressure lubrication): Lubricant is forced into the space between the metals inside the cylinder and between the piston rod and the packing using a lubricator under pressure. The commonly used oil injector is of the single-plunger vacuum drip-type. Compared with the valve-controlled multi-plunger pumps and slide-valve controlled multi-plunger pumps that were used in the past, this type of injector has a simpler structure and more advanced technology, enabling faults to be addressed without shutting down the machine. This type of oil injector is equipped with small oil pumps, with each pump responsible for lubricating one specific point. There are generally two methods for lubricating the moving parts of a compressor: (1) Splashing method: A rotating mechanism is used to throw the lubricating oil in the crankcase into a mist; when some of these oil droplets fall into the oil holes on the bearing surfaces, they can flow to the friction surfaces. (2) Pressure lubrication method: Circulating lubrication is achieved using a gear oil pump, as shown in Figure 1-7, the lubrication system diagram for the moving mechanism. In this method, the lubricating oil passes through the following components in sequence: the oil tank, the oil pump, the filter, the cooler, and the various lubrication points of the moving mechanism, before returning to the oil tank. The circulation system is also equipped with a bypass valve and a pressure gauge to regulate the lubricating oil pressure. I. Introduction to Piston Compressors Piston compressors are the most widely used type of positive-displacement compressor. In petroleum and chemical manufacturing, the main uses of piston compressors are as follows: first, to compress gases for use as power; for example, compressed air can be used as power to drive various pneumatic machinery and tools, as well as control instruments and automation devices ； The second is refrigeration and gas separation; for example, gases are liquefied through compression, cooling, and expansion, which is used for artificial refrigeration (commonly referred to as refrigerators or ice makers). If the liquefied gas is a mixture, its various components can be separated using separation devices – for instance, petroleum cracking gases are first compressed and then their individual components are separated at different temperatures ； Third, it is used for synthesis and polymerization, such as the synthesis of ammonia from nitrogen and hydrogen under high pressure, the synthesis of methanol from hydrogen and carbon monoxide under high pressure, the synthesis of urea from carbon dioxide and ammonia under high pressure, as well as the production of polyethylene under high pressure ； Fourth, it is used for gas transportation or bottling; for example, after being pressurized by compressors, gases such as coal gas and natural gas are transported over long distances through pipelines, as well as various gases used as raw materials in production. It is also used for bottling gases such as nitrogen, oxygen, hydrogen, chlorine, argon, and carbon dioxide. The classification principles for piston compressors are as follows: 1. Based on the arrangement of the cylinders, they can be divided into: (1) Vertical compressors, in which the cylinders are all arranged vertically ； (2) Horizontal compressors, with cylinders arranged horizontally ； (3) Angular compressors, with cylinders arranged at various angles such as V-shaped, W-shaped, L-shaped, or star-shaped ； (4) Symmetric balanced compressors, in which the cylinders are arranged horizontally on either side of the crankshaft; the crank angles of the opposite pairs of cylinders differ by 180°, resulting in essentially balanced inertial forces. 2. Based on exhaust pressure, they can be classified as: (1) Low-pressure compressors, with an exhaust pressure of 0.3~1 MPa (gauge pressure) ； (2) Medium-pressure compressor, with an exhaust pressure of 1~10 MPa (gauge pressure) ； (3) High-pressure compressor, with an exhaust pressure of 10~100 MPa (gauge pressure) ； (4) Ultra-high pressure compressors, with exhaust pressure > 100 MPa (gauge pressure). 3. Based on displacement, they can be classified as: (1) Micro compressors, with a displacement of <0.017 m³/s ； (2) Small compressors, with a displacement of 0.017~0.17 m³/s ； (3) Medium-sized compressors, with a discharge volume of 0.17~1.00 m³/s ； (4) Large compressors, with a discharge volume > 1.00 m³/s. 4. Based on the number of stages required for the cylinder to reach the final pressure, they can be classified as: (1) Single-stage compressors, in which the gas reaches the final pressure after just one compression ； (2) Two-stage compressor: the gas is compressed in two stages to reach the final pressure ； (3) Multi-stage compressor: The gas is compressed through three or more stages to reach the final pressure. 5. Based on the role of the piston within the cylinder, they can be classified as: (1) Single-acting compressors, in which the compression cycle occurs only at one end of the cylinder ； (2) Double-acting compressor, with the same compression cycle taking place at both ends of the cylinder ； (3) Differential compressor, in which two or more compression cycles of different stages take place at one or both ends of the cylinder. 6. Based on the number of columns, they can be classified as: (1) Single-column compressors, with the cylinders arranged along a central line on one side of the machine body ； (2) Twin-screw compressor, with cylinders arranged on two center lines on one or both sides of the machine body ； (3) Multi-row compressors, with cylinders arranged on one or both sides of the fuselage along two or more centerlines. Although there are many types of piston compressors and their structures are complex, their basic design remains roughly the same. For piston compressors without a crosshead, the main components include the cylinder block, crankshaft, connecting rods, pistons, cylinders, and intake and exhaust valves ； For piston compressors with crossheads, in addition to the components mentioned above, there are also crossheads and slideways, piston rods, and stuffing boxes. A piston compressor converts the rotational motion of the drive mechanism into the reciprocating motion of the piston through a crank-slider mechanism; the cylinder and the piston together form the compression volume ； The piston moves back and forth within the cylinder, enabling the gas to undergo processes such as intake, compression, and exhaust within the cylinder; the intake and exhaust valves control the entry and exit of gas into and out of the cylinder ； A packing seal is installed at the cylinder end on the crankshaft side to prevent gas leakage. The piston rings on the piston prevent gas from leaking between the cylinder volumes on either side of the piston. II. Quality requirements for the main components of piston compressors 1. Cylinder: The cylinder is subjected to the pressure of gases, so it must have sufficient strength. Cylinders with a working pressure of less than 5 MPa (gauge pressure) are usually made of HT200 or high-quality cast iron ； When the pressure is below 20 MPa (gauge pressure), it can be manufactured from cast steel ； For cylinders under higher pressure, they can be forged from carbon steel or alloy steel. However, due to the varying complexity of the cylinder structure and cylinder diameter, cylinders under the same working pressure may not necessarily be made of the same material, as it is necessary to consider both sufficient strength and the ease of manufacturing as well as cost. Therefore, when servicing cylinders, it is necessary to make a determination based on the \"product manual\" or sample analysis; the material of the cylinder must not be changed arbitrarily. The piston moves back and forth inside the cylinder, causing friction on the inner wall of the cylinder; therefore, the cylinder wall needs to have good wear resistance as well as optimal lubrication conditions. High-pressure cast or forged steel cylinders have poor friction resistance and are prone to piston seizure; therefore, most steel cylinders are fitted with cast iron cylinder liners that offer better friction resistance. Cylinder liners should preferably be made of high-quality pearlitic cast iron; HT200 is used at low pressures, while HT250, HT300, or HT350 are used at medium and high pressures. The bolts on the cylinders are subjected to alternating loads; therefore, they must be fatigue-resistant, and high-quality steel grade 40 or 40Cr steel is commonly used. 2. Piston assembly: The piston assembly includes the piston body, piston rings, and piston rod. The piston body is a compressive component and must possess sufficient strength and stiffness. The back-and-forth movement of the piston generates inertial forces; therefore, it is preferable to use a piston with low mass. Symmetrically balanced compressors require that these inertial forces be balanced symmetrically. For this reason, the piston body can be made of cast iron, aluminum and aluminum alloys, cast steel, forged steel, or by welding steel plates, depending on the actual requirements. The piston rings rub back and forth in the cylinder along with the piston, so they need to be wear-resistant. It is generally required that the hardness of the piston rings be 10%~15% higher than that of the cylinder wall ； Piston rings that fit with cast iron cylinder surfaces or liners are made of cast iron HT200 or HT250 ； Alloy cast iron is used in conjunction with steel cylinder liners or tungsten carbide cylinder liners. For compressors operating at high speeds and under high pressures, cast-iron rings can be fitted with PTFE inserts or bronze and copper inserts; in some cases, the surface of the piston rings is chrome-plated to reduce wear on the rings and prevent cylinder scoring. The piston rings of oil-free lubrication compressors are made of materials filled with tetrafluoroethylene, graphite, nylon, and other self-lubricating materials. The pistons of various compressors (except vertical compressors) are mostly supported on the working surface of the cylinder. To reduce wear on this cylinder surface, the bearing surfaces of pistons with large diameters are made from wear-resistant materials. The material of the pressure-bearing surface, along with the oil-lubricated pistons, often involves the use of fluoroplastic-filled materials, nylon, and other self-lubricating materials to create support rings in various forms. The piston rod is subjected to alternating compressive and tensile forces from the piston, which requires it to be ductile. Under the action of sealing pressure, the surface of the rod continuously rubs against the packing back and forth; to prevent scuffing, it is necessary for the friction surface of the rod to be hard, generally requiring a hardness of above HRC50. Common materials include high-quality carbon steels such as grade 40 and 45, as well as grades like 33CrMoAlA and 38CrMoAlA. Chrom plating on the surfaces of steel grades 40 and 45 can increase surface hardness and wear resistance; however, if the plating layer is too thick, it tends to peel off. Generally, the thickness of the plating layer after grinding should be maintained at 0.05~0.25 mm ； In addition, high-frequency quenching or carburizing can also be used, while 35CrMoAlA and 38 CrMoAlA alloy steels are often treated by nitriding. 3. Valve: The valve operates under impact loads, therefore it must possess sufficient strength and stiffness. The low-pressure valve seat and lift limiter can be manufactured from HT200 cast iron, and the castings should undergo aging treatment ； High-pressure valves can be made from high-quality carbon steel grades 35, 40, or 45, or from 40Cr steel; the sealing surface of the valve seat must be hardened through high-frequency quenching ； For the valve seats and lift limiters of oxygen compressors exposed to corrosive media, brass (HPb 59—1) and stainless steel (1Cr18Ni9Ti, Cr13) are commonly used. The valve disc is subjected to repeated impact loads and alternating bending loads; therefore, the material used for the valve disc must possess high strength, good toughness, wear resistance, and corrosion resistance. For compressors handling non-corrosive media such as air, nitrogen, hydrogen, propane, etc., the valve plates are usually made of 30CrMnSiA ； For compressors handling corrosive media (such as carbon dioxide compressors and oxygen compressors), the valve plates are typically made from materials such as 1Cr13, 2Cr13, 3Cr13, or 1Cr18Ni9Ti. After quenching and tempering, the hardness of the valve disc should be HRC 40~56. Valve discs are also being made from engineering plastics, which not only helps to save alloy steel but also allows for a larger valve disc travel and a lower spring force, thereby reducing resistance. Engineering plastics are corrosion-resistant and have a wide range of compatibility with different gas types; however, due to limitations in temperature resistance and strength, they are currently mainly used in low-pressure suction valves. 4. Crankshaft: The crankshaft is subject to large, periodically varying inertial forces from the gas, in terms of direction and magnitude, as well as resulting alternating bending and torsional stresses, which in turn cause fatigue and vibration ； At the same time, the crank journal is also subject to severe frictional wear; therefore, the crank material is required to possess properties such as fatigue resistance, wear resistance, and vibration resistance. Crankshafts are commonly forged from high-quality carbon steel grades 40 and 45. With the improvement and development of casting technology in our country, the cranks of small and medium-sized compressors are increasingly made from rare earth-magnesium ductile iron, in order to address the issues of replacing steel with iron and casting materials in place of steel. 5. Connecting rod: The connecting rod consists of a rod body, a large end bearing, and a small end bearing, among other components. The connecting rod body is subjected to alternating tensile and compressive stresses; therefore, it is usually forged from high-quality carbon steels such as grade 35, 40, and 45. In recent years, ductile iron connecting rods have been widely used in small and medium-sized compressors. The big-end bearing is connected to the crankshaft and subjected to rotational friction; therefore, its material is usually a steel or brass casing lined with babbitt alloy ； The small-head tile is connected to the crosshead pin to perform reciprocating motion, and its material is usually tin bronze or phosphor bronze. The cylinder head cover and the big end of the open-type connecting rod are connected by connecting rod bolts; therefore, these bolts are subjected to large alternating loads as well as a preloading force several times greater than the piston force. Connecting rod bolts are typically made of alloy steels with high strength and good plasticity, such as 40Cr, 30CrNi, 35CrMoA, and 40CrMoA ； Nuts are often made of steels such as 35, 35Mn, and 20Cr. 6. Crosshead: The crosshead consists of a crosshead body, slide plate, and crosshead pin, among other components. The crosshead body is connected to the piston rod and bears the lateral forces exerted by the piston. The crosshead body of small compressors is usually cast from HT 21–40 cast iron ； The crosshead bodies of large and medium-sized compressors are commonly made from ZG25, ZG35 cast steels or forged from 40 Number steel. The crosshead pin is connected to the rod end, transmitting all of the piston force ； The material used should possess good toughness, wear resistance, and fatigue resistance; it is commonly made of grade 20 steel and subjected to surface carburizing and quenching to achieve a hard exterior with a tough interior, thus providing both wear resistance and fatigue resistance ； Its surface hardness is required to be HRC=55~62. Skates move back and forth on the track; they need to be wear-resistant, so cast iron or cast steel is often used for the skates, with babbitt alloy poured onto the friction surface ； Removable skates are also available, made from copper alloys and aluminum alloys.