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I. Lubrication characteristics of reciprocating compressors: External lubrication and cooling are applied to the parts that come into direct contact with the compressed gas, while internal lubrication refers mainly to the lubrication, sealing, and rust prevention within the cylinders. After completing its task of internal lubrication, it is discharged along with the compressed gas. The lubricating oil passes through the exhaust valve; part of it is discharged after separation, while the portion that remains unseparated enters the storage cylinder and the pipelines. Therefore, the internal lubrication is of the full-loss type. There are roughly three methods of internal lubrication: (1) pressure oil injection lubrication. This method involves using an oil injector to feed lubricant into the lubrication points of the cylinder at a certain rate. Small-capacity compressor oil injectors are driven by the crankshaft, whereas those in large-capacity compressors are usually driven by a separate motor. When adjusting the pressurized oil injection, the oil supply amount must be increased by 75% based on the designed value in order to overcome the back pressure in the cylinder. The greatest advantage of pressure oil injection is that it enables optimal lubrication of all friction surfaces using the minimum amount of oil; it is widely used for external lubrication in reciprocating compressors and other large-capacity, high-pressure compressors. (2) Splashing lubrication. It is mostly used in small, general-purpose compressors without crossheads. (3) Oil absorption lubrication. This is a lubrication method that involves introducing a small amount of lubricating oil into the compressor intake, and it is commonly used in non-piston-type compressors where splash lubrication cannot be applied. The moving mechanism of reciprocating compressors relies primarily on pressure lubrication; it is characterized by an adequate supply of oil, thorough lubrication, and the ability to effectively remove friction debris. It provides forced lubrication for the main bearings as well as the bearings at both ends of the connecting rod, and is widely used in various compressors. 1. Appropriate viscosity: To ensure good lubrication and sealing of compressed gas, the lubricating oil must have a certain viscosity, and its viscosity increases under high pressure. For example, oil with a viscosity of 20 mm2/s at 1 MPa will see its viscosity increase to 40 mm2/s at 10 MPa. Excessively high viscosity not only results in power waste but also slows down the recovery of the oil film on the surface, leading to increased wear as well as the formation of sludge and carbon deposits. Its low viscosity makes it difficult to establish a reliable oil film between the cylinder pistons, leading to air leakage; therefore, factors such as exhaust pressure, final pressure temperature, and the number of compressor stages need to be taken into consideration when making a choice. The oil used in the external lubrication system of reciprocating compressors is generally turbine lubricating oil with a viscosity range of 32–100. 2. Good thermal oxidation stability ensures minimal sludge formation during operation, as well as resistance to carbon deposition at high temperatures (above 220°C). The oxidation rate is fast at high temperatures; for every increase of 8–10°C in temperature, the oxidation rate doubles or triples. Good thermal oxidation stability is an essential property to prevent oils from oxidizing and forming sludge. European compressor manufacturers specify that for VG32-68, the POT value should be at most 2.0 or 2.5; the maximum value is 3.0 for ranges of 100–150 (POT antioxidant performance test: the percentage increase in Conrad residue value after exposing the oil to air at 200°C under the catalysis of iron oxide, with air flow at a rate of 15 L/hm, for 24 hours). 3. Anti-coking property: The anti-coking property is an important indicator that affects the amount of carbon buildup that occurs in compressor lubricants during use; it is essential to use lubricants that minimize carbon buildup. Excessive carbon buildup not only causes gumming and blockages at the exhaust valves and exhaust pipes, but it also leads to leaks at those valves as a result of the carbon accumulation, allowing the exhaust gases to flow back into the cylinder. This results in repeated compression, which raises the temperature rapidly; and high temperatures further accelerate oxidation. When the temperature rises to the oil’s auto-ignition point, the lubricating oil accumulated in the carbon deposits begins to burn, producing hydrocarbons and free carbon. When this mixture reaches the explosive limits due to high temperature and pressure along with air, an explosion occurs. The explosion limits of general hydrocarbons in air are as follows: CH4 5–15, C2H4 3–12, C2H6 2.8–28.6, C3H8 2.1–9.35, C3H6 2–11.1, C4H10 1.8–8.4, C4H8 1.6–9.9, C5H12 1.4–8, C2H2 5–80. According to statistics, half of the air compressors with an exhaust temperature exceeding 170°C have exploded; therefore, various countries stipulate that the exhaust temperature must not exceed 150°C. Europe sets the standard at 130–140°C, highlighting the importance of controlling temperature. Additionally, a 10% reduction in air intake results in a 20°C rise in temperature; therefore, air intake must be strictly controlled so that the temperature of the incoming air does not exceed 40°C. The air intake opening should not be located indoors to prevent clogging of the air filter. The temperature difference in the cooling tank’s outlet water should not exceed 10°C, and even in summer, the water temperature at the outlet of the cooling tank must not go above 50°C. Regularly remove carbon deposits: generally, clean the exhaust valve after 600 hours, and replace it after 4,000 hours. Try to use naphthenic oil or synthetic oil to reduce carbon deposition; add antioxidants, anti-corrosion agents, and anti-friction additives, but antidegradants or viscosity index improvers are not allowed (to prevent coking and carbon deposition due to thermal oxidation). Additionally, the oil supply to the cylinders should not be excessive; according to various sources, generally, 1 g of oil is required per 400 m2 of friction surface for horizontal compressors, while 1 g of oil is needed per 500 m2 of friction surface for vertical compressors. It is also stated that the maximum oil supply amount should not exceed 50 g/m3. Moreover, the following formula can be used for calculation: Q=120πKDLN, where Q represents the amount of lubricating oil in g/h ； D=cylinder diameter (m) ； L = cylinder stroke (m) ； N=crankshaft speed (r/min) ； K = oil consumption per unit lubricated surface area (g/m2); for horizontal cylinders, K can be taken as 0.025 g/m2, while for vertical cylinders, it can be taken as 0.02 g/m2. 4. The flash point should be high – the flash point is an indicator of a compressor’s safety. Since the temperature of compressed air can exceed the flash point of oils, the flash point of the lubricating oil used should be 30–40°C higher than the exhaust temperature. The flash point is different from the ignition point, and the flash point increases under high-pressure air. For example, an oil with a flash point of 216°C at atmospheric pressure has a flash point of 243°C at 0.8 MPa (8 atmospheres). Moreover, the exhaust temperature of most compressors does not reach 243°C; using oils with a high flash point does not completely solve the problem of explosions. Explosions occur when the oil decomposes at the overheated ignition points caused by carbon deposits, resulting in the formation of gummy hydrocarbons. Oils with a very high flash point and high viscosity are more likely to cause carbon deposits, as well as increasing frictional resistance and wasting energy. Practice has shown that the flash point increases as pressure rises, which means that lubricants under pressure have an increased safety factor. 5. The ignition point is also a safety indicator for compressed lubricating oil. The main cause of fires and explosions in compressors is the thermal decomposition of carbon deposits, which reaches their auto-ignition point. The normal-pressure ignition point of Turbine Oil No. 32 is 360°C; when the air pressure increases to 1 MPa, the ignition point drops to 250°C, and it further drops to 200°C when the pressure rises to 10 MPa. Hydrogen-nitrogen compressors, especially oil-free ones, are primarily used for ammonia synthesis in the chemical industry; they constitute the key equipment in ammonia synthesis plants. It is important to note that the gases being compressed contain carbon monoxide, which is highly toxic to humans, and that a mixture of hydrogen and air at certain concentrations can be explosive. For safety reasons, the exhaust temperature of oil-free nitrogen-hydrogen compressors is generally limited to below 160°C. The lubricating oil used in hydrogen-nitrogen compressors does not require any special oxidation stability properties, but it should have a low sulfur content. II. Structural features of reciprocating compressors: These compressors rely on the reciprocating motion within the cylinders to change the working volume, thereby compressing the gas and increasing its pressure; such compressors are known as reciprocating compressors. The common types of reciprocating compressors are piston-type and diaphragm-type. They draw in, compress, and discharge gas by means of the reciprocating motion of pistons or diaphragms within the cylinder, with the intake and exhaust valves opening and closing automatically. Their advantage is their ability to handle a wide range of pressures; in industrial applications, the maximum pressure can reach 350 MPa (3500 kgf/cm2). The gas pressure is pulsating, and there is vibration during operation; among them, the piston compressor is the most widely used type. Reciprocating compressors are classified into single-stage and multi-stage based on the compression process ； Based on the relative arrangement of cylinders, they are further classified into vertical, horizontal, angular, and opposed-balanced types. Diaphragm compressors use a crank-slider mechanism to drive the diaphragm via a liquid; due to the excellent airtightness of the diaphragm chamber, the purity of the compression medium is very high. Therefore, it is commonly used for compressing precious and rare gases, as well as for transporting media that must not leak, are flammable, highly corrosive, radioactive, or highly toxic. Diaphragm compressors have a very low gas delivery volume of 520 m3 per hour, and are mainly used for research purposes and in special applications in production. The cylinder of a diaphragm compressor does not require lubrication; only the crank rod and hydraulic cylinder need to be lubricated.