Lubrication characteristics of compressors and refrigerators

I. Lubrication characteristics of compressors 1. Overview A gas compressor is a power device that converts mechanical energy into gas pressure energy; it is commonly used in pneumatic tools to provide gas power. It is also frequently employed in industries such as petrochemicals, drilling, and metallurgy for transporting gases such as oxygen, hydrogen, ammonia, natural gas, coke oven gas, and inert gases. Compressors can be classified into low-pressure compressors – those with an exhaust pressure of less than 1.0 MPa – based on their exhaust pressure ； Medium-pressure compressor—discharge pressure 1.0–10 MPa ； High-pressure compressor—discharge pressure 10-100MPa ； Ultra-high pressure compressors — with an exhaust pressure greater than 100 MPa, etc. Low-pressure compressors are of single-stage type, while medium-pressure, high-pressure, and ultra-high-pressure compressors are multi-stage types, with up to 8 stages being possible. Currently, ultra-high-pressure compressors for polyethylene with pressures reaching 343 MPa have been developed abroad. Depending on the compression medium, compressors can also be referred to as air compressors, oxygen compressors, nitrogen compressors, hydrogen compressors, and so on. The structural working principles of compressors can be roughly classified as follows: In hydrocarbon gases, the main components of petroleum cracking gas and petroleum waste gas are hydrogen, methane, butane, ethylene, propylene, etc ； The main components of coke oven gas and city gas are hydrogen, methane, carbon monoxide, carbon dioxide, and nitrogen, among others ； The main component of natural gas is methane. Hydrocarbon gases can be produced and transported as single-component gases, or as gases with mixed components. In gas compressors, the composition and properties of the compressed medium are closely related to the lubrication method used and the choice of lubricant materials. 2. Lubrication methods and characteristics of gas compressors. In compressors, lubrication not only reduces friction and wear but also serves to provide sealing, cooling, and to reduce operating noise. Good lubrication conditions are an essential guarantee for the long-term reliable operation of compressors. However, for certain types of compressors such as rotary vane compressors, since a certain gap can always be maintained between the rotors and between the rotors and the casing, resulting in no sliding contact, oil-free lubrication is possible ； Small dry screw and vane compressors can also operate without lubrication ； For reciprocating compressors that operate at extremely low temperatures (-20 to -50 degrees or lower) or compress high-purity gases, an oil-free lubrication method can also be employed to prevent the condensation of the lubricating medium or the mixing of lubricant into the compressed gas. In such cases, either a labyrinth-type sealing structure is used for the piston and piston rod, or piston rings and packing materials made of anti-wear materials such as graphite or polytetrafluoroethylene are utilized. This section will focus on the lubrication method for oil-lubricated compressors. The lubrication components of gas compressors can be generally divided into two categories. The first category includes the internal parts that come into direct contact with the compressed gas, such as the cylinders, pistons, piston rings, piston rods, exhaust valves, and sealing gaskets in reciprocating compressors ； The gas chamber, rotor (rotating element), exhaust valve, etc., of a rotary compressor. The second type is external transmission mechanisms in which oil does not come into contact with compressed gas, such as the components in the crankcase of reciprocating compressors, crank pins, crank bearings, rod sliders, slides, and crossheads ； Bearings, speed increase gears, etc., for rotary and speed-type compressors. Oil-free gas compressors require consideration of the impact on the external lubrication system in the event of a gas leak. For medium and high-pressure compressors of large and medium capacity, with multiple stages and crosshead drive, the lubrication of the internal and external components mentioned above is carried out through separate independent systems, allowing the use of different lubricating fluids or oils as required for each system. External component lubrication is a forced-circulation lubrication system that supplies oil under pump pressure. This system not only allows for individual adjustment and distribution of the oil supply to each lubrication point, but also features separate oil pumps, oil tanks, coolers, and filters, which enable the lubricating oil to be thoroughly cooled and filtered. As a result, the oil remains clean and at a relatively constant temperature over an extended period of time. The lubrication of the internal components is achieved by using a multi-nozzle oil injector to force pressurized oil into the packing seals of the cylinders and piston rods. The oil injector is essentially a small piston pump; typically, its suction (or pressure) piston is driven by mechanisms such as the cam on the compressor crankshaft. Therefore, when the compressor stops operating, the oil injector also ceases to supply oil. The oil injector can directly draw oil from the oil reservoir and push it to the lubrication points. Its oil supply is intermittent, and the amount of oil supplied by the injector each time can be easily adjusted by controlling the working stroke of the plunger. The oil injector can be used alone, or several injectors can be combined together for centralized oil supply. When using an oil injector for forced oil supply, attention should be paid to the position of the oil supply fitting on the cylinder; it should be placed between the first and second piston rings near the cylinder head at the dead center position of the piston. This ensures adequate lubrication throughout the entire stroke length, as well as aiding in sealing. The amount of oil required for lubrication inside the cylinder can be approximately calculated using the following formula: Q=120 kDLN. Here, 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 lubrication surface area (grams/m²); for horizontal cylinders, k can be taken as 0.025, while for vertical cylinders, k can be taken as 0.02. The calculated Q value is only an approximate figure for the amount of lubricating oil required; the actual amount can be adjusted accordingly based on the compressor’s operating conditions such as noise level, heat generation, and degree of wear, as well as factors like the type of lubricant used, operating temperature, and the amount of condensate present. It is also advisable to stop the compressor and check the degree of wetting inside the cylinders and valve assemblies to determine the appropriate amount of oil. For multi-stage piston compressors, lubrication is usually required only for the first or second stage cylinders. If each stage of a multi-stage cylinder is equipped with an intercooler and an oil-gas separator to remove the oil contained in the gas, then each cylinder stage requires separate lubrication; however, the amount of lubricating oil needed for the later stages is much less than that required for the earlier stages. 3. Selection of lubricants 1) Choice of lubricant type In oxygen compressors, oxygen causes mineral lubricants to oxidize rapidly, leading to combustion and explosion in the compressor; therefore, oil-based lubrication should be avoided, and instead oil-free lubrication methods or water-based emulsions, or distilled water mixed with 6–8% industrial glycerin, should be used for lubrication ； In chlorine compressors, hydrocarbon-based lubricating oils can react with chlorine to form hydrogen chloride, which is highly corrosive to metals (cast iron and steel). Therefore, oil-free lubrication or solid (graphite) lubrication is typically used. For ethylene compressors and similar devices used to compress high-purity gases, in order to prevent lubricant from mixing into the gas and affecting the quality and performance of the product, mineral oil is generally not used for lubrication; instead, medical-grade white oil or liquid paraffin is often employed. Mineral oil lubrication is widely used in compressors for gases such as ordinary air, inert gases, hydrocarbon gases, nitrogen, and hydrogen. For selecting a lubricant when the compressor compresses different gases, refer to the section on compressor oil selection under \"Lubricant Selection\". 2) Selection of lubricant viscosity: In multi-stage air compressors, the compressed gas output from the preceding cylinder is usually cooled to a temperature slightly above that at the time of intake before being sent to the next cylinder. Since the gas has been compressed, its relative humidity is high; when this humidity exceeds the saturation point, moisture in the gas may condense. This moisture has a cleaning effect, which can cause the lubricant on the surface of the cylinders to be removed ； Secondly, in hydrocarbon gas compressors, it not only dissolves in the lubricating oil to reduce its viscosity, but the condensed liquid hydrocarbons also act as a cleaning agent for the steel walls, just like water. Therefore, for multi-stage, high-pressure hydrocarbon gas compressors with high exhaust temperatures, as well as air compressors operating in environments with high air humidity, oils with higher viscosity are preferred; such oils have better adhesion to metals and are beneficial for sealing purposes. For medium and low-pressure hydrocarbon gases and air compressors, L-DAA100 compressor oil is suitable, while for high-pressure multi-stage compressors, L-DAA150 compressor oil is appropriate. The viscosity requirements for rotary oil-injected compressors are similar; at lower pressures, N32 rotary compressor oil with a dynamic viscosity of 5 mm2/s at 100°C is used, while at higher pressures, N100 rotary compressor oil with a dynamic viscosity of 11–14 mm2/s at 100°C is employed. Secondly, to prevent the cleaning effect of condensed liquid hydrocarbons and moisture in the air on the lubricating oil, a lubricating oil composed of a mixture of animal oil (such as lard or beef tallow) with a mass fraction of 3-5% and mineral oil can be used. Animal oil has a strong adhesion to metals, which helps it resist \"water washing\" and prevents the loss of lubricating oil. 3) Substitutes for lubricating oil: In reciprocating rotary positive-displacement compressors that use oil for lubrication, in addition to using compressor oil of the appropriate grade, antirust and antioxidant turbine oil, aviation lubricants, and cylinder oil can also be used as substitutes. However, the performance of these substitute oils should not be lower than the quality standards of the corresponding compressor oil, or they must meet the requirements for use under specific conditions. When a gas compressor is oil-lubricated, the same grade of lubricating oil can be used for lubricating both the external and internal components, or different grades can be employed. However, regardless of the type of lubricant used for the internal components, mineral-based lubricating oil should be used for lubricating the external driving components. 4. Use and maintenance of the lubrication system in gas compressors. The most common faults in gas compressors include abnormal wear and sticking between the piston and piston rings, as well as in the rotor components and sliding bearings; seizure of the crosshead slider; and abnormal heating. All these failures are directly or indirectly related to improper use and maintenance of the lubrication system and lubrication devices. As operators and maintenance personnel, in addition to learning and mastering the general knowledge regarding the composition principles, performance characteristics, and operational requirements of lubrication systems and their components, they should also continuously accumulate experience through practical work and strengthen daily maintenance efforts to ensure that compressors and their lubrication systems remain in good working condition at all times. The following points should be noted during routine inspections and periodic maintenance: (1) Pay attention to keeping the lubricating oil clean; dirty or deteriorated lubricating oil can lead to a vicious cycle that accelerates component wear. For forced-circulation lubrication systems, it is important to replace and clean the filter elements in filters that are clogged in a timely manner ； The oil in tanks or reservoirs should be kept away from exposure to air, in order to prevent dust and dirt from mixing into the oil. For pressure-free tanks and reservoirs, an air filter can generally be used to allow communication with the atmosphere ； During medium and major overhauls, samples of the oil should be taken for analysis of its composition; if the criteria for oil change are met or exceeded, all or part of the oil should be replaced with fresh oil. Generally, the oil can be replaced every 3 months to half a year (or after 2,000–4,000 hours of use). (2) Regular inspections should be carried out: It is necessary to regularly check whether solid carbon particles and sludge have accumulated in the cylinders, valves, and exhaust pipes. Such accumulations should be removed promptly once detected; otherwise, it may lead to combustion explosions in the cylinders, increased exhaust resistance, and abnormal heating. (3) Attention should be paid to the operating condition of the compressor: It is necessary to monitor its operating condition, regularly check the lubrication and wear status of the cylinder valves, and adjust the amount of lubricant as appropriate to avoid either too much or too little lubricant. In forced circulation systems, an abnormally low or significantly dropping lubrication pressure often results in insufficient lubricating oil. In such cases, the machine should be shut down for maintenance promptly, and replacement parts should be used if necessary. (4) Attention should be paid to maintaining the oil temperature in the lubrication system: the normal temperature of the oil in the tank or reservoir should be between 40–50°C. If the oil temperature is too high, the viscosity of the oil decreases, making it more susceptible to oxidation and degradation; if the temperature is too low, the viscosity increases, resulting in poor fluidity. In either case, this can lead to insufficient lubrication. In addition to being directly measurable through a temperature gauge, oil temperature can also be indirectly indicated by the temperature of the cooling water in the cylinder’s cooling circuit or the oil cooler; both excessively high and low cooling water temperatures can affect the operating temperature of the lubricating oil as well as its viscosity.

Introduction to Refrigeration Oil I. The Function of Refrigeration Oil In refrigeration systems, refrigeration oil is commonly referred to as lubricating oil, and it plays an important role in the operation of refrigeration compressors. There are mainly the following aspects: 1. It serves as a lubricant, reducing friction and wear on the moving parts of machinery and extending their service life. 2. Lowering temperature: The refrigeration oil circulates continuously within the refrigeration compressor, enabling it to remove a large amount of heat generated during the compressor’s operation. This helps keep the machine at a lower temperature, thereby improving the efficiency and reliability of the refrigeration compressor. 3. Sealing function: Refrigerant oil serves to seal the shaft seals as well as the space between the cylinder and the piston, preventing leakage of the refrigerant. 4. Providing power for the unloading mechanism: In refrigeration compressors equipped with an unloading device, the oil pressure of the refrigerant oil is used as the power source for the unloading mechanism. II. Performance indicators of lubricating oils and their selection (I) Performance indicators of lubricating oils 1. Viscosity Viscosity is a key performance indicator for lubricating oils. Different refrigerants have varying requirements regarding viscosity; for example, R12 can dissolve in lubricating oils, which reduces their viscosity. Therefore, lubricating oils with higher viscosity should be chosen. In a compressor, both too high and too low viscosity of the lubricating oil are undesirable. Excessively high viscosity increases the frictional power and heat generation in the compressor, raises the starting torque, and reduces the efficiency of the machine; too low viscosity, on the other hand, accelerates wear in bearings and other components due to the inability to establish a sufficient oil film. Therefore, the viscosity must be moderate. The viscosity of lubricating oil changes significantly with temperature (for example, when the temperature rises from 50°C to 100°C, the viscosity of mineral oil decreases to 1/3–1/6 of its original value). Therefore, lubricating oil with a minimal effect of temperature on viscosity should be selected. 2. Cloud point: The cloud point of lubricating oil is the temperature at which, as the temperature drops to a certain value, paraffin begins to precipitate out of the oil (that is, the oil becomes cloudy). The lubricating oil used in refrigeration compressors should have a cloud point lower than the evaporation temperature of the refrigerant. Especially in fluorine-based systems, a portion of the lubricating oil dissolves in the refrigerant and flows with it throughout the cooling system. If paraffin precipitates out of the oil, it can accumulate at the throttle valve and cause blockages, or it can accumulate on the heat transfer surfaces of the evaporator, reducing the efficiency of heat transfer. 3. Freezing point: The temperature at which the lubricant stops flowing when cooled under test conditions is called the freezing point. For lubricating oil used in refrigeration compressors, the lower the freezing point, the better. The normal freezing point should be below -40°C. When the lubricant and refrigerant dissolve into each other, the freezing point decreases. 4. Flash point: The lowest temperature at which a lubricant (in an open container) heats up to such an extent that its vapor comes into contact with a flame, causing ignition, is called the flash point. The flash point of the lubricating oil used in refrigeration compressors should be 25–35°C higher than the exhaust temperature, to prevent the oil from burning and coking. Generally, the flash point of lubricating oils used for ammonia, R12, and R22 should be above 160–170°C. 5. Chemical stability and antioxidant properties: Lubricating oils should possess good chemical stability and antioxidant properties. Otherwise, under high temperatures or in the presence of metal catalysts, they may react with refrigerants and other substances, resulting in the formation of harmful substances such as coke and acidic compounds. 6. Moisture content and mechanical impurities: Lubricating oil should not contain moisture, as moisture not only reduces the evaporation pressure and raises the evaporation temperature, but it also accelerates the chemical changes in the oil and its corrosive effect on metals. Moisture in Freon compressors can also cause the \"steel plating phenomenon,\" whereby copper components react with Freon, resulting in the release of copper that accumulates on the steel surfaces of components such as bearings and valves. As a result, the thickness of these tables increased, ruining the bearing clearance and causing the machine to malfunction. This phenomenon occurs more frequently in hermetic and semi-hermetic compressors. Generally, new oil contains no moisture or mechanical impurities, as the lubricating oil used in refrigerators undergoes strict dehydration treatment during its production process. However, dehydrated lubricating oil has a strong hygroscopicity, so during storage, transportation, and refilling, it should be kept away from contact with air as much as possible. The substances that remain after dissolving and diluting the lubricating oil with gasoline or benzene and filtering it through filter paper are known as the mechanical impurities of the lubricating oil. Mechanical impurities in lubricating oil accelerate the wear of components, reduce the oil’s insulating properties, and block the lubrication channels; therefore, the fewer impurities there are, the better. Generally, it is specified that their concentration should not exceed 0.01%. 7. Breakdown voltage: The breakdown voltage is an indicator of the insulating properties of lubricating oil. Pure lubricating oil has excellent insulating properties, but these properties decrease when it contains impurities such as water, fibers, and dust. For semi-hermetic and hermetic compressors, it is generally required that the breakdown voltage of the lubricating oil be above 25 kV. Because the lubricant comes into direct contact with the motor windings. (II) Specifications and selection of domestically produced refrigeration oils: Currently, the specifications for refrigeration oils in China are in accordance with the standard “ZBE34003—86” issued by the General Petroleum and Chemical Corporation. Products conforming to this standard are classified into five viscosity grades—N15, N22, N32, N46, and N68—based on their average kinematic viscosity at 40°C. All these grades can be used in refrigerators that utilize ammonia as a refrigerant. Its main performance indicators are shown in Table 2–1. However, the previously established specifications for refrigeration machine oils were classified into four grades—13, 18, 25, and 30—based on their dynamic viscosity at 50°C. When making a selection, refer to the comparison table of viscosity grades for new and used refrigeration oils. In practice, N32 (grade 18) is generally used for R12 compressors, N46 (grade 25) for R22 compressors, while N22 (grade 13) or N46 (grade 25) is used for ammonia compressors.

Use of synthetic lubricants in compressors: Compressors typically use paraffin-based or naphthenic mineral oils as lubricants. In recent years, compressors have seen significant advancements in materials and design, resulting in increasingly stringent requirements for lubricating materials. Due to the loss of viscosity caused by the mutual solubility of mineral oil and hydrocarbon gas when they come into contact ； Chemical raw gas carries mineral oil into the processing process, causing catalyst poisoning ； Mineral oil tends to form carbon deposits in compressors, and it can no longer meet the requirements of compressors operating under more demanding conditions. Synthetic lubricants exhibit significantly superior performance compared to mineral oils, such as a high viscosity index, low pour point, resistance to hydrocarbon dilution, low volatility, good thermal stability, hydrolytic stability, emulsion resistance, chemical stability, and compatibility with rubber and metals. There is a wide range of options for synthetic lubricants; therefore, it is necessary to examine the characteristics of various synthetic oils in order to select the appropriate lubricant for improved compressors. 　 1 Types of compressors and their lubrication requirements. Based on the manner in which they compress gas, compressors are divided into two main categories: positive displacement and dynamic compressors. Positive displacement compressors rely on pistons that move back and forth within cylinders or rotors that rotate to change the volume of gas, thereby compressing the gas and increasing its pressure. A velocity compressor uses a high-speed rotating impeller to impart high velocities to the gas, which are then rapidly reduced in speed within a diffuser, converting the gas’s kinetic energy into potential energy, that is, pressure energy. Based on their structure and the manner in which they compress gas, compressors can be classified into the following types: Compressors – Positive displacement type – Reciprocating type – Piston type, Diaphragm type; Rotary type – Vane type, Screw type, Liquid ring type, Rotor type; Dynamic type – Centrifugal type, Axial flow type. Additionally, depending on the compression medium and application, compressors can also be divided into power compressors and process compressors. The former uses air as the compressing medium, and is primarily used to drive pneumatic machinery, tools, and for material transportation ； The latter uses gases as the compression medium, and is employed for the compression and transportation of gases in industrial processes. The various gas compressors that are widely used today include centrifugal, piston, vane, and screw types. Due to the relatively demanding lubrication conditions, synthetic oils are required; some compressors that use special gases as working media also need synthetic oils with specific properties. 2 Types and Properties of Synthetic Oils Ester oils (diesters and polyol esters), polyolefins (PAO), polyalkylene glycols (PAG), and silicone oils can meet the requirements of most gas compressor applications, and they are currently the most widely used synthetic lubricants in gas compressors. It has been recently discovered that hydrocracked saturated hydrocarbons are acceptable lubricants for compressors. Synthetic oils possess unique physical properties, offering significant advantages over mineral oils in terms of temperature limits, viscosity-temperature characteristics, and volatility. 2.1 Temperature limits Many lubrication problems addressed by synthetic oils are related to temperature. To prevent water and other refrigerants from condensing in the compressor system, the operating temperature of the compressor is high, exceeding the heat resistance of ordinary lubricants. When using mineral oil, due to the gelling effect of wax and poor viscosity-temperature characteristics, it is difficult to start the compressor in the field or in environments without heating; choosing synthetic oil can prevent these problems. 2.2 Viscosity-temperature characteristics The viscosity-temperature characteristics of synthetic oils and mineral oils were compared, showing that synthetic oils can provide effective lubrication at extreme temperatures. Synthetic oils have a high viscosity index, which provides higher viscosity under conditions of high-temperature operation, facilitating easier startup of the compressor at low temperatures. The synthetic oil with high viscosity during operation provides a high volumetric efficiency for screw compressors. 2.3 Volatility: Synthetic oils have low volatility, and this property reduces oil compensation while preventing impurities from entering the next stage. The vapor pressure of synthetic oil is lower than that of mineral oil. 3 Applications of synthetic oils in compressors. The compression media for compressors include air, hydrocarbon gases, chemically processed gases, inert gases, industrial gases, and refrigerant gases. Now, synthetic oils for gas compressors will be introduced separately according to different compression media. 3.1 Application of synthetic oils in air compressors 3.1.1 Centrifugal air compressors – Synthesized and hydrocracked saturated hydrocarbons are used for lubricating the seals, gears, and bearings in centrifugal air compressors. The main purpose of using synthetic oil in such compressors is to save energy. In some industrial gas production plants, high-power (up to 5.88 MW) centrifugal air compressors are used; the total power consumption amounts to $43,800 per month. PAO boasts long hydrolytic stability and excellent water separation properties. By using PAO, the energy savings over one year can offset the additional costs incurred over 9 months due to the use of synthetic oils. Furthermore, it extends the oil change interval, prolongs the service life of oil filters and maintenance intervals, and reduces downtime. **The factory selects synthetic oil based on safety considerations. The specially formulated PAO has good chemical stability, which prevents nitration and ensures the safe use of the compressor. 3.1.2 Polyol esters of the ISO 68 viscosity grade can be used in high-temperature rotary vane air compressors. These types of compressors operate continuously at low pressure and 315.6°C. The operating temperature of the lubricating oil is 148.9°C. 3.1.3 Screw air compressors: For screw air compressors operating under conditions of 0.689 MPa and 82°C, PAO is a better choice than diesters, as it exhibits better long-term hydrolytic stability, is compatible with the rubber and plastic materials used in the compressor, and fits well within air compression systems. They are the results of accelerated life tests for various lubricants used in oil-fed screw compressors. The test utilized a 25 HP screw compressor, operating at 104.4–110°C (typically 71–82°C). The accelerated life of lubricants with a high viscosity index (HVI) is equal to that of diesters (1800 h), **exceeding that of mineral oils refined with these two solvents. The service life of HVI lubricating oil under normal operating conditions (single-stage screw air compressors, 0.689 MPa, 71–82°C, relatively clean air) is much longer. Air compressor lubricants that come into contact with food, and are in compliance with the U.S. Food and Drug Administration (FDA) regulation 21 C FR178.3570, are formulated using food-grade PAO base oils. The original food-grade screw air compressor lubricant had a service life of 40,000 hours. With the addition of FDA-compliant additives, the lubrication life of food-grade PAO in compressors can reach up to 8000 hours. For multi-stage high-pressure screw air compressors that operate at temperatures above 93°C under continuous heavy load, using polyol esters is better than PAO, as it can double the service life. Dimethicone has excellent viscosity index (VI=400), thermal-oxidative stability, and hydrolytic stability; however, its metal-to-metal lubricity is poor. Additives designed to improve its lubricity have been developed. The improved dimethyl silicone oil was introduced to the market in the 1970s, and compressors using it have accumulated over 40,000 hours of operation, with satisfactory results. The biggest problem is the high cost of replacing lubricating oil; the key issue lies in the high costs associated with the design and maintenance of the gas-oil separator. 3.1.4 Reciprocating air compressors One of the main reasons for using synthetic oil in air compressors is cleanliness, namely to reduce sludge, deposits, and carbon buildup. Organic ester-based axle box lubricants improve cleanliness, facilitate heat transfer, reduce friction and the resistance of moving parts, thereby minimizing maintenance and downtime and reducing power consumption by 7.2%. The use of synthetic oil in the crankcase saves 3% in fuel consumption. With a fuel cost of $0.04 per 1500 hours, the savings can offset the additional costs associated with using synthetic oil for at least 6 months. For the one-way cylinder lubrication of large reciprocating air compressors, a common problem is the formation of carbon deposits and sludge in the cylinders and exhaust valves, leading to frequent repairs and shutdowns. Furthermore, residue carbon, rust, excessive lubricant, and high pressure can cause the compression system to catch fire automatically at temperatures below 149°C, even leading to explosions. Using polyol esters can solve these problems. After switching to synthetic oil, the cost savings can offset the additional expenses incurred from this switch over a period of 1.6 years. 3.2 Application of synthetic lubricants in gas compressors 3.2.1 Chemical processing gas compressors Synthetic oils have successfully solved the lubrication problems in chemical processing gas compressors. Chemically processed gases may react with ordinary mineral oils. Due to the tendency of mineral oil to inhibit catalysts, many chemical plants prohibit the use of such lubricants. Typical chemical processing gases include chloromethane, sulfur dioxide, hydrogen chloride, and ammonia; mixed processing gases include CO₂, chlorosiloxanes, chlorohydrocarbons, and gases containing trace amounts of mineral acids. When reactive gases are compressed, the compressor equipment and lubricating oil suffer from chemical corrosion, which intensifies as temperature and pressure increase. Suitable lubricants include PAO, silicone oils, and hydrogenated cracked saturated hydrocarbons. PAO and hydrocracked saturated hydrocarbons have excellent chemical resistance. Choosing the right base oil can resolve lubrication problems in compressors caused by the reaction between acids and other reactive compounds, but pure oxygen compressors can only use perfluorohydrocarbon oils and chlorinated hydrocarbon oils. Hydrocracked saturated hydrocarbons are a relatively new type of lubricant. Crude oil fractions were subjected to hydrogenation at 300°C and 20,785 kPa, during which sulfur, nitrogen, and oxides were completely decomposed, and aromatic compounds were converted into saturated naphthenes. Two more vacuum distillations are carried out to remove volatiles and other impurities. Wax removal and a second hydrogen treatment eliminate residual impurities. The base oil is a mixture of stable isoparaffins and naphthenes, with lubricity similar to that of PAO. Mixing it with other synthetic oils can further improve the lubricating properties of the oil. Silicone oil fluorinated is a partially fluorinated polysiloxane that is completely inert to almost all compounds, except for pure oxygen and certain strong oxidizing agents; it is used in compressors for handling chloromethane, chlorine, and HCl. However, the use of silicone fluids is limited by their high cost (over $800 per gallon). Alkylmethylsilicone oils form tough, thick films when in use, and possess natural lubricity. This oil forms a long-lasting protective film on the surfaces of metal components, providing effective lubrication in applications involving HCl, CO2, and chloromethane containing a small amount of sulfuric acid. 3.2.2 Inert gas and industrial gas compressors: Gas purity is very important in the use of inert gas and industrial gas compressors, and it is necessary to consider the potential effect of lubricants to inhibit catalyst activity. Only base oils of the highest quality can lubricate compressors for inert gases such as N2, He, and CO2, as well as industrial gases like H2. PAO, hydrocracked saturated hydrocarbons, and PAG are immiscible with the aforementioned gases, providing better volumetric efficiency in rotary compressors and offering satisfactory performance for industrial gas compressors. 3.2.3 Hydrocarbon gas compressors In addition to the aforementioned hydrocarbon refrigerants, hydrocarbon gases also include natural gas, well gas, and landfill gas. In light hydrocarbon gas compressors, mineral oil can also be used as a lubricant due to its low dilution level. However, synthetic lubricants are reliable in these applications; their unique viscosity properties and compatibility with hydrocarbons improve the efficiency of the system. Many heavy hydrocarbon gas compressors must use synthetic oil. Hydrocarbons are completely soluble in most lubricants, with the solubility depending on the operating pressure and temperature of the compressor. Dilutability of propane for ISO 220 polypropylene glycol at different temperatures and pressures. At a certain temperature, as pressure increases, the dilution effect of hydrocarbon gas on oil increases ； And under certain pressure, the temperature rises while the dilution decreases. Hydrocarbon gases dissolve in oil, reducing its viscosity. If the dilution is too high, the viscosity of the oil decreases to such an extent that it can no longer provide sufficient lubrication, causing the equipment to fail prematurely. Given these characteristics of hydrocarbon gases, synthetic oils are increasingly being used in hydrocarbon gas compressors, including landfill gas compressors, well gas compressors, various high-boiling-point vapor recovery compressors (such as for gasoline, butene, and hexane vapors), and natural gas pipeline booster compressors. The following provides a brief overview of the synthetic oils suitable for various hydrocarbon gas compressors. For low-boiling hydrocarbon gases, PAO is a better choice as a lubricant, especially for waste gases containing small amounts of strong acids and chlorine impurities. Appropriate anti-wear agents and antioxidants also need to be added to the lubricant for garbage gas compressors. Well gas is a mixture of hydrocarbons with low and high boiling points, water, and hydrogen sulfide; the PAG formulated for these compressors provides excellent water resistance and dilution resistance, as well as protection against corrosion caused by high concentrations of hydrogen sulfide. PAG has a strong ability to prevent hydrocarbon gas dilution, making it suitable for steam recovery compressors. The cost of large screw compressors for recovering hexane and butylene vapors is $5,000 per day. PAG offers excellent performance, with very low oil carryover and replenishment requirements. The lubricant only needs to be replaced once a year to maintain the equipment. Alkylmethylsilicone oil mixtures are used for lubricating the cylinders of booster compressors in large natural gas pipelines due to their excellent oil film retention capability. The cylinder oil supply system needs to be improved. Thanks to the increased efficiency of the lubricating oil used and reduced maintenance, the cost of system improvements is recouped in less than a year; moreover, lubricant consumption is reduced by 20 times. Natural gas and other hydrocarbon gases are also used as fuel for gas turbine engines. The compressor provides gas with the appropriate flow rate and pressure for the continuous operation of the turbine; in this case, even if the gas contains a small amount of mineral oil, carbon deposits can form at the gas inlet of the turbine, hindering flow and causing misfires. Therefore, synthetic oil must also be used. 3.3 Applications of synthetic oils in refrigeration compressors The lubricating oil used in compressor refrigeration systems and heat pumps plays a crucial role in the effectiveness of the system. Since the lubricant and refrigerant can react inside the compressor, it is crucial to choose the right lubricant for the refrigerator. 3.3.1 Compressors using propane as a refrigerant 3.3.2 Compressors using ammonia as a refrigerant PAO and HVI hydrogenated isooalkanes are widely used as lubricants in ammonia refrigeration systems due to their excellent performance advantages. Both PAO and HVI hydrocarbons feature many branches, enabling them to form a \"three-dimensional\" thick oil film that improves lubricity. Using a modified Falex machine, the performance of PAO and naphthenic base oil in the presence of ammonia was compared; it was found that the wear value of PAO is low. Experience from manufacturers of ammonia refrigeration systems shows that both PAO and HVI refrigeration oils possess good anti-wear properties, and their service life is longer than that of mineral oils. Since PAO and HVI lubricants are composed of highly saturated molecules, they possess excellent thermochemical stability, which extends the service life of the lubricant and reduces sludge and carbon deposits. PAO has good viscosity-temperature properties, meeting the requirements for low-temperature ammonia applications set by many refrigeration system manufacturers. HVI oils have better viscosity-temperature properties than mineral oils, but additives are required to lower their pour point and improve their viscosity-temperature characteristics. PAO and HVI lubricants have low solubility, produce little foam, and exhibit good oil-gas separation properties. Experience shows that by using appropriate separators and PAO or HVI oils, the oil carryover can be reduced by 30% to 90%. As a result, the refrigeration system becomes cleaner, improving heat transfer efficiency. The use of HVI oil in reciprocating and rotary screw compressors reduces fuel consumption by 64% to 83%. In addition, other advantages of PAO and HVI oils are their low initial water content and good compatibility with plastic materials. Esters are not compatible with ammonia. Ester-based lubricants produce sludge in ammonia refrigeration systems, and over time this sludge turns into a solid. 3.3.3 Compressors using halogenated hydrocarbons as refrigerants – Sealing tube tests have shown that PAO is extremely stable in an environment with chlorofluorocarbon refrigerants. The high viscosity index of PAO can improve the cold start of refrigeration systems. PAO contains no wax and has no flocculation point, eliminating the possibility of wax blocking the expansion valve and control valves. PAO has a wide range of viscosities, and by selecting the appropriate viscosity grade, it is possible to compensate for the dilution that occurs upon contact with soluble refrigerants, such as in R-12 chlorofluorocarbon refrigeration systems used in oil-injected screw compressors. R-12 is soluble in most oils. Naphthenic mineral oils have a low initial viscosity, which cannot compensate for the oil dilution under the operating conditions of compressors; therefore, PAO is widely used in R-12 screw compressors. PAO has a high critical melting temperature (36.7°C), and it is no longer used in most R-22 chlorofluorocarbon refrigeration systems. However, since PAO’s other properties meet the requirements for compressor lubrication, it can be used in R-22 systems with a special design. The ester-based oil formulated for R-22 systems exhibits excellent thermal stability, solubility down to -37.8°C, and good chemical stability. These esters fall within the ISO 200 viscosity range, but they are currently expensive (35–70 dollars per gallon). The R-114 refrigerant holds special significance in heat pump applications. Experience of use in the United States and Europe shows that PAO is particularly suitable for industrial heat pump operations at high temperatures. PAO has been used in R-114 systems at compression temperatures up to 120°C. PAO was chosen because it offers excellent stability when used in R-114 refrigeration systems, availability of viscosity grades, good low-temperature performance, and a high viscosity index. The usage report indicates that the PAO has been in operation for thousands of hours, with no wear on the compressor and no degradation of the lubricating oil. Low-viscosity PAO delivers excellent performance in low-temperature R-13 and R-503 refrigeration systems, with evaporator temperatures as low as -46°C. 4 Conclusions
a. The use of synthetic oils and hydrocracked saturated hydrocarbons as lubricants can effectively improve the operating performance of compressors. This includes extending the oil change interval, reducing wear, decreasing the frequency of component replacements, ensuring a clean operating process, saving energy, minimizing the risk of fire and explosion, reducing pollution, and cutting down on downtime and production losses. b. Air, chemically processed gases, industrial gases, and inert gases, particularly refrigerant gases and hydrocarbon gases. Synthetic oils are used in compressors, with PAO, organic esters (diesters and polyol esters), silicone oils, and hydrogenated cracked saturated hydrocarbons being able to meet the requirements of most compressors. c. For cylinder lubrication in large reciprocating compressors, the existing pressurized oil injection lubrication system needs further improvement in order to achieve the maximum benefits of using synthetic lubricants.