-268.5℃ Helium Compressor

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This post was last edited by Zhang Xinlin from Binglun Environment on 2019-11-5 at 14:39. The helium compressor project for Binglun Environment’s 4.5K (–268.5°C) 2500W large-scale cryogenic refrigerators has been successfully completed and accepted! The cooling temperature is -268.5°C, close to absolute zero! Ultra-low temperature refrigeration systems are of great significance for **, national defense, and other fields, and are widely used in important areas such as satellites, aerospace, and superconductivity. Helium has a low critical temperature and is the hardest gas to liquefy in nature; therefore, it serves as the primary working fluid in ultra-low temperature refrigeration systems. Similar to conventional refrigeration processes, ultra-low temperature refrigeration systems using helium as the working fluid also involve four main thermodynamic processes: compression, condensation, throttling, and evaporation. The compressor is a key component of cryogenic systems; its main function is to supply the ultra-cryogenic refrigeration system with high-pressure, pure helium as the refrigerant, with a purity of 99.995%. The efficiency of the compressor determines the basic performance of the entire cryogenic system. The injection screw helium compressor was first introduced for use in 1979 by the Fermi National Accelerator Laboratory in the cryogenic system of the \"tera-electron volt accelerator\". Compared with other types of compressors such as piston machines, oil-injected screw helium compressors feature large capacity, low vibration, and high reliability, offering broader application prospects in the field of cryogenic refrigeration. Injection screw helium compressors are typically developed through special modifications based on air compressors. Currently, Linde and Air Liquide’s helium liquefiers make use of the series of helium compression products manufactured by German company Caesar. Other companies that possess helium compression technology include Aixen in Germany, Praxair in the United States, Maekawa in Japan, and Houghton in the United Kingdom. Among them, some non-standard models developed by Caesar Company are not exported to the Chinese market due to an exclusive agreement signed with Linde. As for the models that are allowed to be exported to China, in addition to their high prices, there are strict restrictions on their areas of use; they are prohibited from being used in fields such as nuclear technology, aerospace, and national defense, which severely limits the development and application of cryogenic refrigeration technology in our country. In response to the strategic goals of \"Made in China 2025,\" my organization has developed and designed oil-injected screw helium compressors for the ultra-low temperature sector, in order to explore and analyze key technologies. 1. Oil-injected screw helium compressors: The main reason for using oil-injected screw compressors for helium compression is as follows: 1) Helium leaks very easily, and the injection of oil helps to provide sealing; the smaller the sealing gap in the compressor, the higher its efficiency ； 2) Helium has a high adiabatic index, and with intake/exhaust pressures of 1/16 bar (A), the temperature inside the compressor becomes very high during adiabatic compression. Injecting fuel can reduce the temperature rise during the compression process, making it more akin to isothermal compression and thus helping to improve compressor efficiency ； 3) Injecting oil can reduce the adiabatic index of real gases, thereby lowering compressor power consumption ； 4) Screw compressors with oil injection have the advantages of low noise, low vibration, and few vulnerable components ； 5) The twin-screw structure belongs to the positive-displacement compressor category, and it possesses strong compression capability under varying operating conditions. The oil-injected screw helium compressor is mainly composed of a compressor, an oil separator, an oil filter, a cooler, etc., and is supplemented by valves, a control system, etc. to form an independent system. This helium oil-injected screw compressor is designed for an intake pressure of 1.05 bar (A) and an exhaust pressure of 17 bar (A). The operating process of the oil-injected screw helium compressor is shown in Figure 1. Helium and lubricating oil enter the compressor through the suction port and the oil injection port respectively; after mixing, they are compressed into a high-temperature, high-pressure gas-oil mixture. The oil-gas mixture enters the oil separator, where the lubricating oil and helium are separated. The separated lubricating oil is cooled by a cooler and filtered by a filter before returning to the compressor, thus completing the oil circulation cycle. The helium from which the lubricating oil has been separated is cooled by a cooler before being sent to the end where it is used. Figure 1: Schematic diagram of the oil-injected screw helium compressor process. 2. Compressor: The compressor is the key equipment in helium compression. For oil-injected screw compressors, the main factors affecting their performance include the profile, clearance, sealing, and oil injection. 2.1 The profile has been re-optimized to take into account the tendency of helium molecules to leak due to their low molecular weight; the new high-efficiency bilateral asymmetric full-circular arc envelope profile can reduce the area of the leakage triangle, thereby improving the volumetric efficiency of helium injection screw compressors. Figure 2: Profile of the helium injection screw compressor type. Helium is a monatomic molecule with low dynamic viscosity and high permeability, which poses the problem of easy leakage, especially leakage within the rotor gaps. Therefore, it is particularly important to develop specific profiles taking into account the properties of helium. During this development process, a rotor profile specifically designed for helium as the working medium was created. This profile features a bilateral asymmetric fully circular arc envelope, offering advantages such as good sealing performance, short contact lines, smooth meshing, ease of machining using grinding methods, and high efficiency. 2.2 The main factors affecting compressor efficiency include the meshing clearance and the one-sided clearance, among others. The meshing clearance refers to the gap between the teeth during the meshing of the male and female rotors, while the one-sided clearance denotes the one-sided radial gap between the tip of a rotor tooth and the rotor hole. The rotor clearance plays a decisive role in the performance of the compressor; an excessive clearance reduces the volumetric efficiency of the compressor, while too small a clearance can lead to friction between the male and female rotors as well as with the compressor housing, resulting in damage to the compressor. During adiabatic compression of helium, the significant temperature rise results in substantial thermal deformation of the rotor. To avoid friction and ensure stable operation of the compressor, a larger clearance is required; however, an increased clearance reduces the efficiency of the compressor. Using an oil-injected screw can effectively solve the aforementioned problems: injecting an appropriate amount of lubricating oil at the right location in the compressor serves to provide sealing and cooling effects, which has a significant positive impact on improving the compressor’s efficiency. During the operation of the compressor, the deformation of the rotor includes thermal deformation and deformation due to applied forces. The gaps between various components must be larger than the amount of deformation caused by the rotor, so that frictional engagement does not occur under stress, thereby ensuring stable and reliable operation. Using simulation software, a simulated analysis was conducted on the tooth deformation of the rotating rotor caused by forces, as shown in Figures 3 to 5. Figure 3: Force-induced deformation diagram of the female rotor. Figure 4: Force model of the rotor. Figure 5: Force-induced deformation of the rotor. Through simulation analysis, the tooth shape was optimized, reducing the deformation at the tooth tips; this effectively prevented problems such as friction and scoring caused by force-induced deformation of the teeth. It also ensured the accuracy of the design clearance, thereby guaranteeing the safe and stable operation of the compressor. The clearance was redesigned and adjusted for helium compression; this compressor features high-precision rolling bearings to ensure the meshing clearance, discharge-end clearance, one-sided clearance, and other related parameters. 2.3 Sealing: Ordinary process compressors use single-face mechanical seals. Considering the properties of helium, this project has involved special design in collaboration with the shaft seal manufacturers in accordance with API 682 standards, employing a double-face seal structure. A double-end balanced shaft seal is used, with two-stage sealing, resulting in a leakage rate of 0.1 mL/h. This seal has the following features: 1) The static ring adopts a multi-spring design to ensure the flatness of the sealing surfaces. 2) A secure sealed end face can reduce twisting caused by pressure. 3) The FEA finite element analysis method and hydrodynamic algorithms are employed to optimize the design of the sealing end face. 4) It uses a hook drive to provide better torque transmission. 5) The fully assembled sealed end face remains flat despite temperature fluctuations, allowing it to compensate for the manufacturing tolerances of the equipment itself as well as the axial displacement caused by thermal expansion. The double-end face shaft seal (Figure 6) is supplied with oil by a separate auxiliary system; the oil supply pressure is 3 bar higher than the compressor exhaust pressure, ensuring zero leakage at the shaft seal. Figure 6 Schematic diagram of double-end face shaft seal 2.4 Fuel injection: A comprehensive analysis is conducted of the impact of the fuel injection volume on the compression process, and the position and size of the fuel injection ports are optimized considering the compression characteristics of helium gas. Due to the high adiabatic index of helium, the temperature rises rapidly during compression; by injecting lubricating oil right at the point where intake of air into the compressor ends, it is possible to effectively reduce this temperature rise. Meanwhile, methods such as multi-point oil injection and oil atomization are used to keep the amount of oil injected within an optimal range, ensuring the stable and efficient operation of the compressor. 2.5 Compressor energy regulation There are various methods for regulating compressors, including slide valve regulation, variable frequency regulation, and suction throttling regulation. Among them, slide valve regulation and frequency conversion regulation are the commonly used control methods. For the slide valve control method, at load levels below 40%, the air flow rate drops rapidly yet the power consumption does not decrease in proportion; this approach is suitable for applications with high compressor loads. On the other hand, variable frequency control achieves variable flow by changing the speed, allowing for rapid adjustments to energy consumption, but it comes at a higher cost. Given that helium compressors are used in cryogenic applications where they need to work in conjunction with expanders, it is necessary for these compressors to operate smoothly and respond quickly. Taking into account their suitability for such applications as well as their performance under varying conditions, variable-frequency control is employed for these compressors, enabling a linear variation in power and gas flow rate while also allowing for rapid response to changes in load. 3 Design of key components for helium compressors 3.1 Oil separator The outlet of screw compressors dispenses a mixture of the working fluid and lubricating oil; therefore, reliable equipment is needed to separate oil from gas in order to reduce the oil content in the exhaust gas. If lubricating oil enters the subsequent expansion equipment, it will solidify at low temperatures, causing blockages that affect the operational stability of the equipment. This project employs an efficient oil separator that combines multiple separation methods: impact, cyclone, gravity, and interception. The separator can achieve very high separation precision after separation. 3.2 The cooler used is an air-cooled cooler to cool the lubricating oil and high-pressure helium; it has the advantage of being simple to use and requiring no cooling water. The cooler fan is equipped with variable frequency control to regulate the exhaust air temperature and the oil supply temperature; by adjusting and controlling the oil supply temperature, the unit operates under more efficient conditions. 3.3 Pipeline Accessories: Considering the tendency of helium to leak, the pipeline accessories of the unit have been specially designed. Welding should be preferred for all pipelines and valves, in order to minimize the risk of leakage associated with flange connections ； In areas where flange connections are used, a special design is employed to minimize leaks as much as possible. When the unit was filled with helium to 2.0 MPa (G), a helium mass spectrometer leak detector was used to inspect various leak points and welds; the helium leakage rate was below 10-8 Pa·m3/s in all cases. 4 Testing and experimental data: After the development of the injection screw helium compressor was completed, a testing bench was set up to measure the performance of the unit. The test bench process is simplified as shown in Figure 7. The main functions of the test bench are to adjust the operating conditions of the helium compressor with an injection screw, as well as to measure parameters such as the flow rate of the medium and the power of the unit. The testing is carried out in accordance with Methods X and Y specified in GB/T 5773—2016 \"Test methods for performance of positive-displacement refrigerant compressors\"; Method X is the vortex flowmeter testing method, while Method Y is the calorimeter method. A test result is considered acceptable if the testing error is within ±4%. The operating conditions are adjusted via pressure sensors and pneumatic control valves: when the pressure at the unit’s intake is higher than the set value, the opening degree of pneumatic control valve 3 decreases; when the pressure at the unit’s intake is lower than the set value, the opening degree of pneumatic control valve 3 increases ； Pneumatic control valve 1 opens when the pressure at the unit’s exhaust outlet is above the set value and pneumatic control valve 3 has reached its minimum allowable opening; pneumatic control valve 2 opens when the pressure at the unit’s exhaust outlet is below the set value and pneumatic control valve 3 has reached its maximum opening. The flow rate of the medium in the unit is measured using a flow meter in the pipeline. The selection of the main instruments for the test bench is based on GB/T 5773—2016, and the parameters of the main testing components are shown in Table 1. Through experimental testing on the injection screw helium compressor, it was found that under suction/exhaust pressure conditions of 1/16 bar (A), the compressor’s adiabatic efficiency can reach 81%. Performing an uncertainty analysis on the test results helps to assess their accuracy. The uncertainty analysis for these tests included the uncertainty associated with multiple tests as well as the uncertainty of the instruments used; the combined uncertainty was taken as the standard value. Calculations showed that the relative uncertainty was 2.57%. 5 Conclusion The key technologies of oil-injected screw helium compressors lie in the design of the profile, the optimization of clearances, and the oil injection cooling design. Through the development and optimized design of the new type of wire, a suitable contact wire length and leakage triangle were obtained. The development of this profile initially solved the critical problem of internal leakage in helium screw compressors, improving both volumetric efficiency and adiabatic efficiency. The successful development of this oil-injected screw helium compressor can promote the rapid advancement of the cryogenic refrigeration sector in our country. There is room for improvement in both the shape optimization design of the products and the layout of the unit configurations. With the upgrading of these products, more stable and efficient oil-injected screw helium compressors will be able to serve China’s field of deep cryogenic helium refrigeration better.