| Mixed-refrigerant low-temperature cryogenic system is using refrigerants with different boiling point so that the lowest temperature can reach to-193?.These cryogenic system are widely used in natural gas liquefaction,electronics industry,biomedical engineering and other fields because of advantages of wide cooling temperature zone,simple structure,strong adaptability and so on.The refrigerated space tends to be more wider in most small and medium-sized mixedrefrigerant low-temperature cryogenic system,other units space is reduced.Because of the requirements for process manufacturing,disk casing heat exchanger with solder is widely used,but due to the cooling process,there is a large temperature change between cold(hot)import and export,the effects of heat conduction between the coil cannot be ignored.The are optimized and analyzed firstly.Through the establishment of mathematical model which concerned the influence of heat transfer performance of space structure for casing heat exchanger,the impact of the radial thermal conduction on heat transfer performance mechanism had been analyzed.Compared with the heat transfer performance of the casing heat exchanger in different thermal contact resistance,the simulation results showed that the larger the thermal resistance was,the better the heat transfer performance would be.Then a casing heat exchanger test bench was built for heat transfer research.The experimental results showed that there existed radial heat conduction on casing heat exchanger which was soldering.It also showed the inexistence of radial heat conduction in heat exchanging process without soder and the heat transfer performance was more superior.When imported temperature difference between cold and hot flows was 45~85 ?,the heat transfer capability of casing heat exchanger without solder increased by 2%~12%.In view of the problems of low energy efficiency,cooling speed,high discharge pressure and temperature of compressor in mixed-refrigerant low-temperature cryogenic system,theoretical and experimental studies were working and two low-temperature refrigeration system in different cooling temperature zone were designed and set up.The process modeling and parameter optimization helped to analyse these problems.Aiming at the cooling zone before-90 ?,a single-stage compression auto-cascade refrigeration(ACR)system was designed.R134 a,R32 and R14 were using as refrigerant components.Genetic algorithm and Aspen Plus were adopted to optimize the ACR system in this paper.The maximum thermodynamical perfectness ? was as the goal and the concentration demand laws of optimal operating components were got at last.The simulation results showed that with the reduction of evaporation temperature,the high boiling components R134 a reduced gradually and the low boiling components R14 increased gradually.The range of change had been greatly reduced.Accordingly,an microflow control strategy of component concentration was proposed.The related experiment was demonstrated by connecting an expansion gas tank and a micro-flow control valve at the outlet of the evaporator.The results showed that this control solution can speed up the cooling rate of the system and reduce the compressor's total work efficiency by 8.9%.The high pressure of the system is well controlled.Moreover,the safety and reliability of the whole system were improved.The study also designed a flexible ultra-low temperature refrigeration system for mixed refrigerant in the cooling zone of-150?.R600 a,R170,R290,R50 and R728 were the five main refrigerants.Advantages of the microflow control strategy of component are highlighted in the ultra-low temperature zone.The power consumption was reducing by 10.39% while increasing the cooling rate of the system.Meanwhile,the system adjusted the operating concentration of the mixed refrigerant adaptively. |
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