Brief Introduction of Common Lubrication Methods of Air Compressor

Air compressor type variety, wide range of uses, and different types of air compressor, the lubrication requirements vary greatly. In the air compressor parts, the lubricating oil plays a role of preventing wear, sealing, cooling, minimizing the influence of air on the viscosity of the lubricating oil and the reaction of additives with air, and preventing corrosion.

At present, there are many types of air compressors, but no matter what type of air compressor, its lubrication system structure and function are similar. This paper mainly discusses the industry commonly used reciprocating air compressor, screw air compressor, centrifugal air compressor lubrication system, and other types of air compressor lubrication system in the structure and principle with these three types of air compressor lubrication system similar or close.

When the air compressor is used in the industrial field, the compressed air is different according to the requirements of use. One is in contact with the lubricating oil, and the other is not in contact with the lubricating oil when the air is compressed. Mainly, centrifugal air compressors include oil-free screw compressors and oil-free reciprocating air compressors. These two structures are different from oil-containing compressed air compressors. When considering air compressor lubrication, it must be recognized that compressed air will cause temperature rise. The more compressed air, the higher the final temperature. When the required exhaust pressure is high, the compressed air will be divided into two or more stages for compression. The air will be cooled between the adjacent two stages to limit the temperature. At an appropriate level, this method also improves the efficiency of the air compressor and reduces the power loss corresponding to the reached temperature range, which is reflected in the structure of various air compressors.

In actual work, the basic task of air compressor lubrication is to use the liquid layer formed between the opposite friction surfaces to reduce the friction of the moving parts of the air compressor, reduce the power consumption of the friction surface, and also play a role in sealing the working volume of compressed air. effect. Air compressors of different structural types have different requirements for the quality and performance of lubricating oil due to working conditions and lubrication characteristics.

Lubrication System of 1. Reciprocating Air Compressor

the lubrication system of the reciprocating air compressor can be divided into two types: internal lubrication of the part in direct contact with compressed air and external lubrication system of the part not in contact with compressed air. The internal lubrication system mainly refers to the lubrication, sealing, rust prevention and corrosion prevention inside the cylinder; the external lubrication system is the lubrication and cooling of moving parts. Usually in large-capacity compressors, high-pressure compressors and compressors with crossheads, the internal lubrication system and the external lubrication system are independent, using suitable internal oil and external oil respectively. In the small non-crosshead compressor, the lubrication system of the moving parts is also used to lubricate the inside of the cylinder, and the internal and external lubrication is common.

1. Lubrication of internal parts of cylinder

in a reciprocating compressor, except where the cylinder is open to the crankcase, oil is typically applied to the cylinder wall at one or more points using a mechanical forced lubricator. In some cases, the cylinder is supplemented with oil by adding additional oil to the suction valve chamber. For some small bore, high pressure multi-stage compressor cylinder, only to the suction valve cavity oil. For cylinders that are not open to the crankcase, the oil added to the cylinder is essentially carried out by the compressed air and collects in the exhaust piping, piping and other system components such as the air reservoir. The cylinder, which is open to the crankcase, carries the oil from the oil pool through the connecting rod and the concave-convex part on the crank and throws it for lubrication. When using this splash lubrication method, the piston has an oil control ring similar to that in a car engine to prevent the cylinder from supplying too much lubricating oil.

The compressor needs little lubrication, or the amount of lubricating oil required is very small, usually by the adjacent cylinder wall diffusion to the valve, or by the air flow to the valve in the form of mist. However, when the air compressor inlet is extremely humid, it is sometimes necessary to provide additional lubrication by means of a forced lubricator connected to the suction valve chamber. A valve controller is used in some types of pressure regulating systems to keep the valve open or closed, such as in an unloader valve. When metal piston rod seals are used in double-acting machines, the seal groove is lubricated by force. If non-metallic seals are used, the seal groove is properly lubricated with oil from the compressor cylinder, and in some cases, a mechanical forced lubricator is used.

2. External lubrication (I. e. lubrication in motion)

in fact, all reciprocating compressors use oil to lubricate the running parts in an oil sump at the bottom of the compressor. Oil from the bearings, crossheads, and cylinders with open crankcase will flow back to the oil pool due to gravity. However, in order to deliver the oil in the oil pool to the respective lubricating parts, various methods or a combination of these methods are used.

The distribution of oil to the lubricated parts may all be done by splashing. If so, one or more of the cups or part of the connecting rod will be immersed in the oil, carrying the oil up and flinging it to the internals to be lubricated. Many horizontal compressors are an overflow lubrication system that lubricates the bearings and crossheads. The oil is taken up from the oil pool by the disc on the curved cup and then scraped by the oil scraper. The oil is introduced into the bearing through the oil passage or It flows to the bearing surface in a step flow manner. The pump pumps the oil from the oil pool, delivers the main bearing and connecting rod bearing at a certain pressure, and then enters the toggle pin bearing (sleeve) and crosshead through the drilled oil passage to lubricate them.

3. Factors affecting the lubrication of reciprocating compressors

first, let's look at the factors of lubrication of cylinder components. The operating temperature of the compressor cylinder is a very important factor, which will affect the viscosity, oxidation and deposit formation of the oil. Because the viscosity of the lubricating oil will decrease at high temperatures, when the operating temperature is high, it is necessary to use a higher viscosity oil to maintain a suitable lubricating film. The thin oil film on the exhaust valve, valve cavity and pipe is heated by the hot metal surfaces in contact with it, and is constantly purged by the compressed gas discharged from the cylinder. This is a very severe oxidation condition, so all compressor oils will have some degree of oxidation, and the degree of oxidation depends on the conditions they experience and their ability to resist this chemical reaction.

The oxidation of lubricating oil is gradual, and the oxidation products formed at the beginning are soluble in oil, but as the oxidation proceeds, they become insoluble in oil and mainly precipitate in the exhaust valve and exhaust pipe. These are the hottest parts. After further baking, these deposits will be transformed into materials with high carbon content. These deposits will interfere with the valve seat seal, so that the hot high-pressure gas leakage back into the cylinder, these high-temperature gas will also heat the suction of the air suction, so that it at the beginning of compression and discharge temperature rise, this recompression process will lead to reduced efficiency and flow reduction. Although there is a tendency for the temperature to reach equilibrium at a higher value for a given leakage rate, the effect is cumulative because this effect occurs at each stroke. The deposit will also affect the smooth exhaust passage and cause the cylinder exhaust pressure to increase, and the temperature will also increase with the increase in exhaust pressure. The abnormally high exhaust gas temperature caused by this effect will cause the oil to oxidize faster, thereby promoting further accumulation of deposits and further temperature rise. This cycle can also eventually lead to a fire or explosion if appropriate measures are not taken.

Compressed air often has a variety of pollutants, hard particles will form abrasive wear on the cylinder surface, and bring interference with the piston ring and valve seat trouble. Some pollutants have a catalytic effect on oil oxidation, and some entrained chemicals can react directly with the oil to form deposits. The solid deposits adhere to the oil lubricated surfaces and contribute to deposit buildup in the exhaust valve and the exhaust passage. In many cases, the deposits found in the compressor are mainly pollutants plus a small amount of carbonaceous material formed by the oxidation of oil. If these types of pollutants are found, the gas filter at the suction end of the compressor should be improved. The lubricating oil added to the compressor cylinder will be oxidized. Most of the oil supplied to the cylinder will eventually leave the cylinder through the exhaust valve, and the temperature at the exhaust valve is the highest. Therefore, maintaining the lubricating oil supplied to the cylinder at a minimum level will help minimize the formation of deposits in these parts and reduce the hidden danger of bringing too much lubricating oil to downstream equipment.

Secondly, the influence of water molecules on the lubrication system. The water in the air will enter the cylinder of the compressor at a very high speed with the compressed air. Especially when the cylinder is cooled below the air dew point during the no-load period, air condensation will occur in the cylinder, and the water formed It will replace the oil film on the metal surface and make it rust. Although the amount of rust formed during each idling period may be small and may be worn away the next time the compressor is started, over time this process can lead to excessive wear. In addition, rust will also promote oil oxidation, rust particles will accelerate the formation of deposits. If such a possibility exists, consideration should be given to the use of oils having good rust-preventing ability and oils reinforced by the use of additives that can adhere to metal surfaces. This type of oil will help reduce the possibility of moisture and other liquids coming into contact with metal surfaces when the compressor is unloaded.

Third, the factors that affect the lubrication of running parts. The factors that generally affect the lubrication of compressor bearings are load, speed, temperature, and the presence of water and pollutants. The main requirement for adequate lubrication during operation is that the lubricating oil used has a suitable viscosity at the operating temperature. Much of the oil circulating in the compressor crankcase is splashed into fine beads or oil mist, or is thrown by rotating parts. Therefore, the speed and degree of oil oxidation depends on its operating conditions and the ability of the oil to resist this chemical change. Relative to the compressor cylinder, the exhaust valve and the exhaust pipe, the conditions for oil oxidation in the crankcase are mild. However, the cylinder oil needs to be constantly replenished, and the oil in the crankcase may work for several dry hours.

2. screw air compressor lubrication system

screw air compressor has a single rotor and double rotor two kinds of design, there are lubricating oil and compressed air contact and non contact two kinds of structure.

Twin-rotor screw air compressor has two structural forms. In the oil flood lubrication type, the lubricating oil is injected into the cylinder to absorb the heat generated when the air or gas is compressed. The lubricating oil can also play a sealing role between the rotors. Because the cylinder is lubricated by the oil of the rotor, so the machine does not have a synchronous gear, they need an external circulation system to control the oil temperature, the need for oil removal system mixed in the discharge of compressed air or gas oil. In this air compressor, in addition to the gears and bearings need oil lubrication, lubricating oil also lubricates the contact surface of the rotor, it is necessary to minimize gas leakage, the use of lubricating seals.

In the dry screw air compressor, no oil is given in the air cavity, and the compressor oil does not contact the compressed air, only the bearings, synchronous gears and transmission mechanisms are lubricated. Because the rotors are non-lubricated, synchronous gears are required to maintain mutual contact between the rotors. These compressors are used to provide oil-free air and gas. However, because there is no oil seal between the rotors, the operating speed must be relatively high in order to minimize gas leakage, usually requiring water cooling of the cylinders and oil cooling of the rotor bearings. Since the dry screw air compressor needs to lubricate the main parts of the gear and bearing, the journal bearing can be ordinary radial sliding bearing or rolling bearing, and the thrust bearing can be tilting pad bearing or angular contact rolling bearing, the gear is precision cutting straight tooth or helical tooth gear.

The single-screw air compressor is a conical screw with a toothed wheel. The gear teeth sweep the threaded cavity of the screw, causing the air to be compressed through an upward decreasing space. The toothed star wheel is mainly used to take up the stress acting on the wheel and contact the side of the screw cavity. The lubricating oil is injected into the casing to play the role of lubrication, cooling and sealing, and its structure and function are basically the same as the twin-screw air compressor.

When the oil-containing screw air compressor selects lubricating oil, its working conditions are relatively harsh. Because the lubricating oil is repeatedly heated and cooled at a higher circulation speed in the compressor cylinder, the contact area with oxidation is greatly increased., The thermal intensity is high, and it is continuously catalyzed by copper, iron and other metals in the cooler, and the oil is easy to oxidize and deteriorate. The mixed condensate will emulsify the lubricating oil, and the particulate impurities, suspended dust and corrosive gases absorbed will accelerate the aging of the oil. Then, the good oxidation stability, suitable viscosity, and anti-emulsification of the oil are to ensure effective cooling, sealing, anti-corrosion and lubrication. At the same time, in order to make the compressor oil get a good separation and recovery from the compressed air, it is necessary to choose Non-volatile oil. In addition, when the lubricating oil enters the oil separator, the oil foam produced causes the oil separating element to be seriously immersed in the oil, resulting in increased resistance, causing internal overload of the compressor, and increasing fuel consumption. Therefore, the compressor oil should also have good anti-foam. Sex.

3. centrifugal air compressor lubrication

in the centrifugal air compressor, the multi-blade rotor rotates at a high speed in the casing, and the air or gas enclosed between the impeller blades is accelerated and thrown outward and forward. When the air leaves the end of the blade, the pressure increases and the speed is high, and then it enters the diffuser ring. In the diffuser ring, the increasing area in the direction of the gas flow causes a decrease in velocity and an increase in pressure. After that, the air enters the spiral shell, and the area in the air flow direction increases again, so that the speed is further reduced and the pressure is increased. As a result of the outward flow of air through the vanes, the inlet pressure drops and air is drawn into the compressor. As the pressure of the compressed air increases, the flow becomes larger, and the speed of the impeller increases. The centrifugal air compressor needs multiple impellers to pressurize and increase the speed, reaching three, four or even higher levels to meet the requirements of the working conditions. Therefore, the centrifugal air compressor requires a special lubricating oil station to lubricate and cool the moving parts of the compressor.

The bearings and gears of the centrifugal air compressor need to be lubricated, but the impeller and casing in contact with the air do not need to be lubricated. Therefore, the compressed air produced is oil-free. Oil film seals, contact seals and non-contact seals are used in compressors, and seals in contact with lubricating oil must be supplied with oil and cooled by lubricators to enhance the sealing effect.

When the compressor is working, the lubricating oil station and cooling system are started first. The plunger pump injects the lubricating oil from the oil station into the moving parts in the compressor for lubrication and cooling, and then through the heat exchange device, the high-temperature lubricating oil after operation is cooled., And finally return to the lubricating oil station to form a cycle.

When the impeller of the centrifugal air compressor moves at high speed, it will produce axial thrust and radial thrust. For axial thrust, thrust bearings are generally used to support all or part of the load and to accurately position the impeller within the casing. Thrust bearings are usually angular contact ball bearings, annular thrust bearings, fixed and tilting pad bearings. The main shaft bearing of the compressor is generally a radial sliding bearing, but sometimes a rolling bearing is also used. Radial sliding bearings include thrust rings, fixed shoes and tilting-pad bearings, all of which are lubricated with oil. For a compressor driven by a speed increasing gear set, it is possible to lubricate the drive gear and one or more bearings with the same system, and to provide pressure circulating oil from the bearing lubrication system via the oil pump of the main drive. Due to the high speed of centrifugal air compressors, synthetic oils containing anti-rust and anti-oxidation additives are often used. The pitch line speed of some gear drives is very high, especially in the gear split shaft type high pressure compression, in order to reduce the inlet shear heat of the gear mesh, it is necessary to limit the viscosity of the lubricating oil.

Management and Maintenance of Lubrication System of 4. Air Compressor

at present, there are many types of air compressors used in industry. Although the structures of various types of air compressors are different, the selection of lubricating oil for lubrication system must first ensure the safe and normal operation of compression, and the reasonable selection of lubricating oil can save energy and protect the environment.

1. Reasonable choice of compressor oil

this point is beneficial to prolong the service life of the equipment, improve the reliability of the equipment operation, and prevent the occurrence of accidents, so it must be very cautious.

The quality selection of compressor oil is mainly the viscosity parameter. The choice of viscosity is related to the type, power, oil feeding method and working conditions (outlet temperature and pressure) of the compressor, as well as the temperature change in winter and summer. The viscosity of the oil is required to form an oil film on the lubrication part, and at the same time play the role of lubrication, friction reduction, sealing, cooling, and corrosion protection. In general, the choice of compressor oil is to meet the performance and working conditions of the compressor.

2. Correct control of oil supply and oil change indicators

the amount of lubricating oil supplied to the compressor should be minimized under the premise of ensuring lubrication and cooling. Excessive oil supply will increase the carbon deposit in the cylinder, make the valve not close tightly, reduce the compression efficiency, and even cause combustion, explosion, and waste of lubricating oil; too little oil supply will cause the lubrication and cooling effect to be unsatisfactory, causing the compressor to overheat, Increase mechanical wear. Therefore, it is necessary to correctly control the oil supply according to the pressure, displacement and speed of the compressor, as well as the lubrication method and the viscosity of the oil. Generally, it is required to cover the cylinder comprehensively, without lumpy oil film, and not to flow out from the bottom of the cylinder to achieve reasonable Oil supply conditions.

The oil change period of the compressor needs to be formed with the structure of the compressor, and the flow rate, operating conditions, lubrication methods and lubricating oil quality are different. It can be determined according to the changes in the quality and performance of the oil during use.