What is the difference between an oil-free dry screw and a water-lubricated screw?

In recent decades, the research of oil-free screw compressor has become increasingly active, and many countries and enterprises have begun to explore the road of oil-free screw compressor on the basis of the original model of oil-injected screw compressor. In the fields that require high gas quality, such as textile, metallurgy, food, chemical industry, medicine, oil and air separation, etc. Oil-free screw compressor can provide high-quality compressed gas to meet various needs and has broad application prospects. Application field In the food manufacturing and packaging industries, when the oil injection screw compressor is used to prepare compressed gas, the lubricating oil in the compressor has undergone many high-temperature oxidation and condensation emulsification in the production process, and its performance has been greatly reduced, showing acidity, which not only can not lubricate the subsequent equipment, but will destroy the normal lubrication. The use of oil-free screw compressor can avoid the negative impact of invalid lubricating oil on the equipment. In pharmaceutical and bioengineering, the pollution of bacteria and phage contained in compressed gas can not be ignored. The pure compressed gas provided by the oil-free screw compressor can prevent bacteria and bacteriophages from breeding in the gas. In electroplating industry, there are some problems such as frosting, scorching, pinholes and cracks on the surface of the coating during production. People use stirring to solve these problems, and stirring needs the support of compressed air. The quality of compressed air has a great influence on the deposition rate and electroplating efficiency of bright nickel electroplating. The application of oil-free screw compressor can effectively avoid frosting, scorching, pinholes and cracks on the coating surface. Therefore, the oil-free screw compressor has been used to prepare pure gas for electroplating [1]. In the automobile spraying industry, impure gases often cause inferior coatings. If compressed air contains oil, there will be some scattered or concentrated small protrusions on the surface of the coating. These bubbles are generally produced in the coating under the topcoat, which is caused by water vapor or dirt under the coating. In addition, the compressed air containing oil may also lead to small pits distributed in dots on the surface of wet coating, forming crater-shaped silicon pits, and sometimes the substrate can be seen at the bottom of the pits, commonly known as fisheye. Nowadays, the oil-free screw compressor has been used in automobile spraying industry to prepare pure gas for spraying, which has improved the spraying level of domestic automobiles. In the textile industry, air-jet looms need dry and oil-free compressed air. In the production process, the fine nozzle will blow compressed air onto the wire harness to form a vortex, thus giving the yarn shape, elasticity and elasticity. The pure compressed air provided by the oil-free screw compressor can ensure the quality of the finished fabric. Classification of oil-free screw compressors and main manufacturers Oil-free screw compressors include dry screw compressors and water-jet screw compressors. Dry screw compressors are mainly twin-screw compressors, and water-jet screw compressors include twin-screw compressors and single-screw compressors. The basic structure of dry screw compressor is shown in Figure 1. At present, the domestic market mainly includes Atlas (AC), Ingersoll Rand (IR), Shouli, Borg and Shengang, Wuxi Compressor Factory and Nanjing Compressor Factory. Wuxi compressor factory, some of which use Kobe steel as the main engine, also produces the main engine, for example, the main engine with 80 cubic meters per minute displacement, and the rotor is sprayed with PTFE anti-corrosion coating. However, the performance of large displacement compressor is not ideal. For example, a compressor with a displacement greater than 100 cubic meter per minute is expected to be improved in terms of large displacement. Nanjing compressor factory introduced the patented technology of Swedish SRM company and jointly developed the oil-free screw compressor. In Borg, Germany, the rotor coating is made of molybdenum and graphite, and the bearing can be replaced by itself in the rotor design, and the cooler design is reasonable. Shouli provides high-quality air source for various new textile electromechanical equipment, and the sealant can be replaced for a long time. Ingersoll rand mainly adopts molybdenum disulfide rotor coating, which is treated by infiltration process, and the input end of its transmission shaft is equipped with improved lip seal. Atlas, the rotor adopts S-AP line and is sprayed with Teflon coating. The gas path system (high and low pressure rotor), oil path system (gearbox, oil pan, oil pump and oil filter) and cooling system (intercooling, aftercooling and oil cooling) are integrated; The gearbox is designed with double bearings, the ball bearing bears radial force and the roller bearing bears axial force. Water-jet screw compressor The process of water-jet twin-screw compressor is shown in Figure 2, and there was GrassAir in the Netherlands earlier. Today it's mainly Atlas and CompAir. In addition, Wuxi compressor factory is also actively engaged in research and development in this area. Atlas and CompAir use ceramic rotors, water-lubricated bearings and no synchronous gears. The rotors mesh with each other to form a water film seal. There are Fu Sheng and Sanshui blowout water single-screw compressors, and small-displacement water-lubricated single-screw compressors are developed. Fu Sheng adopts three wells with single screw head, with displacement 12m3/min and maximum power 120kW. Rotor materials are stainless steel, copper and ceramic bearings. Due to the limitation of materials and technology, it is relatively difficult to manufacture large-scale water-jet single-screw compressor at present, but small-scale water-jet single-screw compressor has high efficiency and relatively low maintenance cost. Because the rotor material and profile rotor material in dry screw compressor are not lubricated by lubricating oil and take away tiny particles, the gas will corrode the rotor, the inner wall of the shell and the air passage. Therefore, on the basis of traditional oil injection screw rotor materials, anti-corrosion coatings, such as PTFE, MoS2, Teflon, etc., are usually sprayed to ensure that the rotor is not deformed and corroded. In the water-jet screw compressor, although injecting water into the compression cavity can play the role of cooling, sealing and maintaining gas purity, improve the efficiency of the compressor and control the exhaust temperature well, water will inevitably corrode the rotor and other components, so it is necessary to choose rotor materials carefully. The selection of general rotor materials is as follows: (1)NSB steel rotor In the LG-30/0.8 water jet torch gas screw compressor manufactured by Shanghai Screw Compressor Development Center, the rotor is made of NSB steel (nickel-phosphorus alloy, which can resist the corrosion of torch gas, H2, H2S and SO2). (2) Polymer ceramic rotors Atlas and GVM in France use polymer ceramic rotors. Its rotor can be accurately machined to the accuracy of several millimeters. The long-term test results show that the material has good wear resistance, reliability and applicability. (3) The application of engineering plastic rotor opens up a new economic way for the material and processing means of screw compressor rotor, and provides good conditions for the development of water-jet screw compressor. The precision injection-molded engineering plastic rotor of the invention is rustless and low in cost, and at the same time, the noise and vibration of the screw compressor are reduced. This creates a good condition for using water spray sealing cooling technology in compression chamber. The profile design of screw rotor is very important for screw compressor, and it is necessary to comprehensively consider the meshing requirements of rotor profile, continuous contact line, machining performance and other factors. In order to complete the national "Tenth Five-Year Plan" 863 project "Fuel Cell Engine", the Compressor Research Institute of Xi Jiaotong University has successfully developed LG300 oil-free water jet screw air compressor which can be used for fuel cells. According to the specific design requirements, the research group specially developed a new rotor line as shown in Figure 4. The profile has the characteristics of bilateral asymmetry, and the tooth curves of all parts are arc or arc envelope, which can reduce the lateral leakage through the contact line and improve the compressor efficiency. In addition, the machinability of the rotor is improved, which is convenient for processing by various methods. Technical difficulties Technical difficulties of dry screw compressor (1) Rotor deformation The performance of dry screw compressor largely depends on the sealing gap when the rotor is engaged. There are many leakage channels between rotors and between rotors and casing, and the leakage of fluid through gaps directly affects volumetric efficiency and total efficiency. In the actual operation process, under the action of thermal and mechanical boundary conditions, the rotor undergoes thermal and mechanical elastic deformation, which makes the actual operation gap differ greatly from the design gap. Excessive clearance will increase leakage and reduce volumetric efficiency; If the gap is too small, it is easy to burn and bite [4]. Although the temperature difference of the whole rotor is not big, the radial deformation of the rotor along the length direction is still different. This is mainly because the shape of the whole rotor is complex, and the cogging is spiral rather than strictly axisymmetric. The radial deformation analysis of the male and female rotors along the contact line shows that although there are differences in the deformation along the axial length, the total deformation of the two rotors is basically unchanged, and the total deformation changes linearly with temperature. The exhaust temperature of dry compressors is usually very high. In order to prevent the rotor from biting or even seriously damaging the rotor teeth, we should consider increasing the design clearance or taking appropriate cooling measures [5]. (2) The rotor coating with poor coating failure will fall off after running for a period of time, and the rotor is exposed to the air, which is affected by impurities in the air and temperature changes. Eventually, it will lead to the decline of running performance and even damage to the machine. Spraying anticorrosive coating is one of the difficulties. At present, molybdenum disulfide, polytetrafluoroethylene and molybdenum stone grinding coatings are widely used. Technical difficulties of water-jet screw compressor (1) Ceramic rotor is not suitable for machine tool processing because of the wear resistance of ceramic materials, so precision casting is adopted, and casting technology is one of the difficulties. The defects such as porosity and cracks in general ceramic products need to be detected by focused X-ray instruments, and the consistency of ceramic body density needs to be monitored by ultrasonic instruments. (2) Water pollution cleaning will inevitably have a certain corrosive effect on the main engine and cooling water system of the screw compressor due to the existence of water, and even form a certain scale in the pipeline. After the scale is formed in the cooling system, it will reduce the cross-sectional area of the water channel, increase the resistance of water circulation and hinder the normal heat exchange. With the continuous thickening of the scale layer, it will cause serious poor cooling of the equipment and bring a series of adverse consequences, such as increasing the power consumption of the compressor and reducing the intake and exhaust volume. Therefore, the treatment method of scale is also very important. The methods of scale treatment generally include chemical reagent method, physical method and manual method. (3) Water lubricated bearings Water lubricated bearings need to change the material properties and lubrication structure of traditional metal bearing systems, organically combine bearings with dynamic sealing devices and other components, and use water as the lubricating medium. At present, there are relatively few manufacturers that can provide water-lubricated bearings, and SKF Sweden is actively engaged in research and development in this field. Research on the development trend of noise reduction As a prominent problem of screw compressor, noise has been put forward many times. Especially in oil-free screw compressors. Therefore, noise reduction research will become a hot spot. With the noise research of other models, the noise research technology will continue to develop and progress, providing sufficient theoretical basis and empirical guidance for the noise research of oil-free screws. Research on water lubrication technology With the increasing exposure of the application limitations of oil-injected screw compressor in many fields and a series of problems existing in dry screw compressor itself, water-lubricated oil-free screw compressor came into being and has a good development prospect. With the development of coating technology and bearing manufacturing industry, some companies, such as Atlas, have used water to lubricate bearings and abandoned the limitation that synchronous gears need oil lubrication. Furthermore, under the general trend of continuous improvement of rotor profile, it is no longer unrealistic to realize the meshing between rotors and form a good water film seal and water lubricate the oil-free screw compressor. Improving compressor performance and improving the profile of oil-free lubricated screw compressor rotor is the key to improve compressor performance. The continuous improvement of screw compressor performance and the continuous expansion of market share are inseparable from the development of rotor profile. Optimizing the blade profile design method will become a research hotspot. As a general machine, compressor has a good application prospect in many fields. Oil-free screw compressor has unique advantages. With the deepening of its research and the continuous improvement of design technology, the performance of oil-free screw compressor will be further improved, and its application field will be more and more extensive. In some medical and other fields, such as dentistry, oil-free pistons and other models are mostly used at present, and there may be a place for oil-free screw compressors in the future. In addition, chemical industry, aerospace, aviation and many other aspects will also be more widely and deeply applied.