Natural gas liquefaction is a low temperature process. The purified raw natural gas enters the heat exchanger for low-temperature cooling, condensation and undercooling, throttles and depressurizes at the temperature of - 160 ~ 165 ℃, and enters the LNG storage tank. The relationship between the heat and temperature of natural gas cooling, condensation and undercooling is as follows: 1) Types of refrigeration cycle In order to liquefy natural gas, the heat in the process of natural gas cooling, condensation and supercooling must be removed, that is, the corresponding cooling capacity is required to exchange with it. The refrigeration system equipped in the liquefaction process is to make the heat exchanger achieve the minimum temperature difference between cold and heat flow, so as to obtain high refrigeration efficiency. The refrigeration system for natural gas liquefaction is very mature, and the commonly used processes include: Step refrigeration cycle Mixed refrigerant refrigeration cycle Mixed refrigerant refrigeration cycle with Precooling Expansion mechanism cold cycle 2) Step refrigeration cycle The step refrigeration cycle was first applied to liquefied natural gas products in 1939. It was installed in Cleveland in the United States, using NH3 and C2H4 as the first and second stage refrigerants. In the first stage, propane or Freon is used as refrigerant. The purified natural gas is cooled to - 35 ~ - 40 ℃ in propane or Freon cooler. After separating the heavy hydrocarbons above pentane, it enters the second stage for cooling. The propane gas evaporated from the propane cooler is pressurized by the compressor, re liquefied after cooling by the water cooler, and recycled to the propane cooler. In the second stage, ethane or ethylene is used as the refrigerant, and the natural gas is cooled to - 90 ~ - 100 ℃ in the second stage and liquefied into the third stage for cooling. After pressurization and water cooling, the gas evaporated from ethane or ethylene cooler is cooled and liquefied in propane cooler and recycled to ethane or ethylene cooler. In the third stage, methane is used as the refrigerant. The liquefied natural gas is subcooled to - 150 ~ - 160 ℃ in the methane cooler, and then depressurized through the throttle valve. After the temperature drops to - 162 ℃, it is pumped to the LNG storage tank. The gas evaporated from the methane cooler is pressurized and water-cooled, cooled in the propane cooler, liquefied in the ethane or ethylene cooler, and recycled to the methane cooler. The classical step refrigeration cycle includes several relatively independent and connected cooling stages. Since the refrigerant is generally compressed by multi-stage compressor, in each cooling stage, the refrigerant can evaporate under several pressures and be divided into several temperature levels to cool the natural gas, and the refrigerant evaporated under each pressure enters the corresponding compressor stage for compression. In each cooling stage, only the refrigerant is different, and the operation process is basically similar. The advantages of step refrigeration cycle are: ① Low energy consumption; ② The refrigerant is pure and has no proportioning problem; ③ Mature technology and stable operation The disadvantages of step refrigeration cycle are: ① There are many units and the process is complex; ② There are many auxiliary equipment, and there should be equipment specialized in producing and storing a variety of refrigerants; ③ The pipeline and control system are complex and inconvenient to maintain