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

In recent decades, the oil-free screw compressor research is increasingly active, many countries and enterprises in the original oil-injected screw compressor models on the basis of, began to explore the oil-free screw compressor road. In the gas quality requirements of the field, such as textile, metallurgy, food, chemical, pharmaceutical, petroleum and air separation and other needs of pure oil-free compressed air places, oil-free screw compressors can provide high-quality compressed gas to meet the needs of a variety of applications, so the application of the broad prospects. Application areas in the food manufacturing and packaging industry, the use of oil-injected screw compressor preparation of compressed gas, the production process after a number of high-temperature oxidation and condensation emulsification, the compressor lubricant performance has been greatly reduced, and acidic, the subsequent equipment not only can not play a role in the lubrication, but also will destroy the normal lubrication, the use of oil-free screw compressor to avoid the failure of lubricating oil on the negative impact of the equipment. In the pharmaceutical, biological engineering, compressed gas containing bacteria and phage contamination can not be ignored, oil-free screw compressor to provide pure compressed gas, can avoid bacteria and phage breeding in the gas. In the electroplating industry, the production process exists in the plating surface blooming, burning, pinholes, cracks and other problems, people use stirring to solve such problems, stirring needs the support of compressed air. The quality of compressed air has a great influence on the deposition speed of bright nickel plating and the plating efficiency. The application of oil-free screw compressors can effectively avoid the occurrence of problems such as blooming, burning, pinholes, cracks, etc. on the plated surface. Therefore, nowadays, oil-free screw compressors have been applied to prepare pure gas for plating [1]. In the automotive coating industry, impure gases often result in poor quality coatings. If the compressed air contains oil, the surface of the coating appears as a number of small scattered or concentrated bumps, these blisters are usually produced in one of the coats under the topcoat, and the blisters are caused by the presence of water vapor or dirt underneath the coating. In addition, oil-laden compressed air may also cause small pits distributed in spots on the surface of the wet coating, forming crater-like silica pits, with the substrate sometimes visible at the bottom of the pits, commonly known as fisheyes. Today the automotive spraying industry has begun to apply oil-free screw compressors to prepare pure gases for spraying, resulting in improved spraying of domestic automobiles. In the textile industry, air jet looms require dry and oil-free compressed air. During the production process, fine nozzles blow compressed air onto the harness to form eddy currents that give the yarn shape, elasticity and flexibility. The pure compressed air provided by oil-free screw compressors ensures the quality of the finished cloth. Oil-free screw compressor classification and the main manufacturers of oil-free screw compressor including dry screw compressor and water-injected screw compressor two categories. Dry screw compressor is mainly a double screw compressor, water-injected screw compressor includes a double screw compressor and single screw compressor. Dry screw compressor dry screw compressor basic structure as shown in Figure 1, currently in the domestic market mainly Atlas (AC), Ingersoll Rand (IR), Shouli, Berg (BOGE) and Kobe Steel (KOBELCO), Wuxi Compressor Works and Nanjing Compressor Works and other companies' products. Wuxi Compressor Factory, part of the use of Kobe Steel's host, the plant also produces hosts, such as displacement of 80 cubic meters / minute host, its rotor sprayed PTFE anti-corrosion coating. However, in the large displacement compressor performance is not ideal, such as displacement greater than 100 cubic meters / minute compressor, is expected to be upgraded in the large displacement. Nanjing Compressor Factory, the introduction of Sweden's SRM company's patented technology, co-developed the oil-free screw compressor. Germany Berg (BOGE), the rotor coating with molybdenum-graphite material, rotor design can be self-replacing bearings, the cooler design is reasonable. Shouli, for a variety of new textile electromechanical equipment to provide a high-quality air source, the sealant can be a longer period of time without replacement. Ingersoll Rand, which mainly uses molybdenum disulfide rotor coating, which is treated by an infiltration process, is fitted with improved lip seals on the input end of its drive shaft. Atlas, the rotors are S-AP type lines with Teflon coating. Integralized structure of three major systems, including air circuit system (high and low pressure rotors), oil circuit system (gear box, oil sump, oil pump and oil filter) and cooling system (intercooling, after-cooling and oil cooling); gear box adopts double-bearing design, with ball bearings to withstand radial force and roller bearings to withstand axial force. Water-injected screw compressor water-injected twin-screw compressor process shown in Figure 2, earlier there is the Netherlands GrassAir. now mainly Atlas and CompAir (CompAir). In addition Wuxi compressor factory is also actively engaged in research and development work in this field. Atlas and CompAir use ceramic rotors, water-lubricated bearings, no synchronous gear. The rotors mesh with each other and form a water film seal between them. For water-injected single screw compressors, there are Fuso and Mitsui, which develop small-displacement water-lubricated single screw compressors. Fusheng using Mitsui's single screw head, displacement up to 12m3 / min, the maximum power up to 120kW, rotor materials for stainless steel and copper, ceramic bearings. Due to material and technology limitations, it is relatively difficult to manufacture large water-injected single screw compressors at present, but small water-injected single screw compressors have high efficiency and relatively low maintenance costs. Rotor materials and profiles rotor materials in the dry screw compressor, due to the lack of lubricant lubrication and take away the tiny particles, the gas will be corrosive to the rotor, the shell wall, the airway, so on the basis of the traditional oil-injected screw rotor materials, usually coated with anticorrosion layer, such as spraying polytetrafluoroethylene, molybdenum disulfide, Teflon and so on to achieve to ensure that the rotor is not deformed, not corroded role. In the water-injected screw compressor, due to the injection of water into the compression chamber, although the role of cooling, sealing and maintaining the purity of the gas is taken to improve the efficiency of the compressor, but also good control of the exhaust temperature, but the water will certainly have a corrosive effect on the rotor and other components, so it is necessary to carefully select the rotor material. The choice of rotor materials are generally the following: (1) NSB steel rotor in the CSIC Seven Eleven Institute of Shanghai screw compressor development center manufactured by the LG a 30/0.8-type water-jet flare gas screw compressor, the rotor used in the NSB steel (nickel-plated phosphorus alloy, resistant to corrosion in the flare gas, H2, H2S, SO2). (2) polymer polymerization ceramic rotor Atlas and France GVM and other companies use polymer polymerization ceramic rotor. Its rotor can be accurately processed to a few percent millimeter accuracy. The results of long-term tests show that this material is good wear resistance, and has good reliability and applicability. (3) engineering plastics rotor engineering plastics rotor, opened up the screw compressor rotor material and processing means of new economic ways, and for the development of water jet screw compressor provides good conditions. Precision injection molding of engineering plastic rotor, stainless steel, low cost, while reducing the noise and vibration of the screw compressor. For the compression chamber using water jet seal cooling technology to create a good condition. Screw rotor profile for screw compressor, profile design is critical, need to consider the rotor profile engagement requirements, continuous contact line, processing performance and other factors. Xi'an Jiaotong University Compressor Research Institute for the completion of the national "Tenth Five-Year" 863 Program "major special projects for electric vehicles" in the "fuel cell engine" project, developed a successful fuel cell can be applied to the LG300, a fuel cell engine. In order to fulfill the project of "fuel cell engine" in the "major project of electric vehicle" of National Tenth Five-Year "863 Program", the Research Institute of Compressor Technology has successfully developed the LG300 oil-free water-injected screw air compressor which can be applied to fuel cell [3]. To meet the specific design requirements, the group developed a new type of rotor wire as shown in Fig. 4. This type of line is characterized by bilateral asymmetry, and all the constituent tooth curves are circular arcs or circular arc envelopes, which reduces the lateral leakage through the contact line and improves the compressor efficiency. In addition, it improves the machinability of the rotor and facilitates the use of a variety of methods for machining. Technical difficulties of dry screw compressor technical difficulties (1) rotor deformation dry screw compressor performance, depends largely on the rotor engagement sealing gap. In the rotor, rotor and casing between the existence of a number of leakage channels, fluid leakage through the gap directly affects the volumetric efficiency and total efficiency. In the actual operation process, the rotor in the thermal and force boundary conditions under the action of thermal and force elastic deformation, which makes the actual operating gap and design gap difference. The gap is too large to cause increased leakage, volume efficiency decreases; gap is too small and easy to burn bite accident [4]. Although the temperature difference of the entire rotor is not large, but the radial deformation of the rotor along the length direction still exists. This is mainly due to the complexity of the entire rotor shape, teeth, grooves are spiral, rather than a strictly axisymmetric structure. The analysis of radial deformation along the contact line of the negative and positive rotor found that although there are differences in deformation along the axial length direction, but the total deformation of the two rotors is basically unchanged, and the total deformation with the temperature is a linear trend. Dry compressor exhaust temperature is often high, in order to prevent the occurrence of rotor bite or even serious damage to the rotor teeth, should be considered to increase the design gap or take appropriate cooling measures [5]. (2) Coating Failure Poor rotor coatings will peel off after a period of operation, and the rotor will be exposed to the air and be subjected to the effects of airborne impurities and temperature changes. Eventually, this leads to degradation of operational performance and even damage to the machine. The process of spraying anti-corrosion coatings is one of the difficulties. Today, the use of more molybdenum disulfide, PTFE, and molybdenum - stone grinding coating. Water jet screw compressor technology difficulties (1) ceramic rotor manufacturing due to the wear resistance of ceramic materials, not suitable for machine tool processing, so the use of precision casting, and casting process is one of the difficulties. General ceramic products, such as porosity, cracks and other defects, need to be detected by micro focus X-ray instrumentation, as well as to maintain the consistency of the density of ceramic blanks, ultrasonic instrumentation is required to monitor. (2) water pollution cleanup due to the presence of water, is bound to screw compressor host and cooling water system caused by certain corrosive effects, more in the pipeline will form a certain scale. Scale formation in the cooling system, will reduce the cross-sectional area of the water flow path, increase the resistance of the water cycle, hindering the normal heat exchange, with the continuous thickening of the layer of scale, will cause serious cooling equipment and bring a series of bad results, such as the compressor to increase the power consumption, reduce the amount of suction, reduce the amount of exhaust, etc., therefore, the way of scale treatment is also very important. Scale treatment is generally chemical reagent method, physical method, artificial method and so on. (3) water lubricated bearings water lubricated bearings need to change the traditional metal bearing system material performance and lubrication structure, the bearings and sealing devices and other components of the organic air into one, with water as the lubricating medium. At present, can provide water lubrication bearing manufacturers are relatively few, Sweden's SKF is actively engaged in the research and development work in this area. Development trend of noise reduction research noise as a screw compressor a more obvious problem has been raised many times. In the oil-free screw compressor is particularly prominent. Therefore, for the noise reduction research will become a hot spot, along with other models of noise research, noise research technology will continue to develop and progress, to the oil-free screw noise research to provide sufficient theoretical basis and experience to guide. Water lubrication technology research with the oil-injected screw compressor in many areas of application limitations of the increasingly exposed and dry screw compressor itself a series of problems, water lubrication oil-free screw compressor is timely proposed, there is a very good prospect. With the development of the coating process and bearing manufacturing industry, some companies such as Atlas has adopted the water lubricated bearings, while abandoning the limitations of the original synchronous gears need oil lubrication. Furthermore, in the rotor shape line continuously improved under the general trend, can be realized between the rotor mesh and form a good water film seal, water lubrication oil-free screw compressor will no longer be an impractical idea. Improve the performance of the compressor to improve the oil-free screw compressor rotor profile is to improve the performance of the compressor is the key. Screw compressor performance continues to improve and its market share continues to expand, is inseparable from the development of the rotor profile. Optimize the design of the profile, will become a research hot spot. Summarize the compressor as a kind of general-purpose machinery, in many fields have good prospects for application. Oil-free screw compressor has unique advantages, with the deepening of its research and design technology continues to improve, the performance of oil-free screw compressor will be further improved, and its application areas will become more and more extensive. In some medical and other fields, such as dental field, the current use of oil-free piston and other models, there may be a place for oil-free screw compressors in the future. In addition, in the chemical industry, aerospace, aviation and many other aspects will be more widely and y applied.