In these several cooling cycle modes, a vacuum pump system is used to maintain the pressure of the vacuum chamber of the superconducting chamber. The normal temperature and normal pressure helium gas at a temperature of 313 K is compressed by the screw compressor system to 1.3 M Pa, and then through the heat exchange cooling of the heat exchangers in the refrigerator cold box and the precooling of the two-stage turboexpander Two-stage turbulence or supercritical turbulence required for 2 K is generated by two-stage throttling, and two-phase turbulent flow is carried out in a gas-liquid separator; H e is obtained mainly by using the above saturated liquid helium or supercritical Turbulent flow, through the interaction of the 2 K part of the throttle valve and the vacuum pump system, maintains the superconducting chamber vacuum adiabatic cell pressure of 3 131 Pa, resulting in a 2K saturated superfluid helium cooling the superconducting cavity. Depending on the cooling cycle, 2 K of saturated helium is returned to the compressor inlet via a low temperature heat exchanger, or low pressure heat transfer, or a cold compressor and heater into the vacuum pump system.
In the schemes A, B, and D, the 2 K refrigeration system uses a low-temperature heat exchanger and a low-pressure heat exchanger. Schemes A and D have a throttle valve and a two-stage cold compressor, respectively, compared with B; It is a combination of a low temperature heat exchanger and a heater, wherein the four cooling schemes of AD recover the low temperature of the low temperature helium gas to the normal temperature in the 2 K system by means of the low temperature heat exchanger precooling and the low pressure heat exchanger. . Scheme D uses a portion of the high-temperature and high-pressure helium gas compressed by the compressor to exchange heat with the low-temperature low-pressure helium gas from the 2 K portion in the low-pressure heat exchanger, and then returns to the cold box of the refrigerator to mix with the high-pressure helium gas; In Scheme E, the low temperature helium at 2 K is rewarmed to normal temperature and is not recovered, that is, consumed by the heater.
2 numerical models, calculation results and analysis
The numerical model of the PK US CA F cryogenic system was based on several cooling cycles as shown. I. The isoentropic compression efficiencies of the compressor and the room temperature pump are taken as 0. 59 and 0, respectively, in the heat calculation process, the pressure loss of the fluid in each of the heat exchangers is ignored, and the efficiency of each heat exchanger is taken as 0.95. 55.
2. 1 cooling cycle process analysis
The calculation results show that, because the A scheme is to use the supercritical turbulent flow to further reduce the temperature to obtain the superfluid helium, the low temperature helium gas returned to the cold box in the circulation mode is cooled by the low pressure heat exchanger and then throttling in the gas liquid separator to generate pressure. 0. 12 M Pa of saturated helium and a part of saturated liquid due to the endothermic evaporation of saturated helium, the flow rate is limited by the liquefaction rate and heat load after the throttling, in order to avoid the negative temperature difference of the heat exchangers in the refrigerator Therefore, it is necessary to ensure that the cold end of the refrigerator has sufficient cold helium gas flow rate; and at the same time, it is necessary to ensure that the cold end outlet temperature of the low pressure heat exchanger cannot exceed the normal temperature, and the outlet temperature and flow rate of the hot end of the low pressure heat exchanger are also limited. Therefore, under the condition that the above two requirements are met, in order to balance the entire cooling cycle system, a throttle valve may be added at the hot end outlet of the low-pressure heat exchanger to meet the demand for the cooling capacity in the high-pressure helium gas in the cold box. The other four cooling cycle schemes use saturated liquid helium to further cool down to obtain superfluid helium.
2. 2 2 K system thermal efficiency analysis
It is known that in the above-mentioned several cooling cycles, the C COP and F FOM of the scheme B are the highest, and the scheme E is the worst, and the C COP values ​​of the schemes B and E are nearly 1.2 times. The results also show that the cooling efficiency of the cooling cycle using the recovery of 2 K partial cooling is higher than that of the cooling cycle of 2 K partial cooling, indicating that it is necessary to improve efficiency in cryogenic cooling. The cold amount of the low temperature gas is recovered as much as possible.
3 % The difference between the compression work of the unit mass flow rate (including the room temperature pump and the main compressor) is 0.3% in the cooling cycle. Change within.
It can be seen that the percentage of helium mass flow rate flowing through each device is different depending on the cooling cycle mode. Moreover, in the above several cooling cycle modes, the mass flow rate of the helium gas in the turboexpander exceeds 50% of the total helium gas mass flow rate, which indicates that more than half of the helium cooling cycle with the turboexpander precooling The helium gas is used to pre-cool the high pressure to flow the helium gas after being expanded and cooled by the turbo expander.
It can be concluded that during the cooling cycle using the low-pressure heat exchanger, the mass flow rate of the helium gas flowing through the vacuum pump system and the low-temperature heat exchanger is basically the same, but the mass flow rate of the vacuum pump system in the cooling cycle modes A, B and C is It is larger than the helium flow rate flowing through the low-pressure heat exchanger, but in the circulation mode D, the opposite is true. This is mainly because the high-pressure helium gas flow flowing through the low-pressure heat exchanger is mainly limited by the temperature of the hot-end outlet of the heat exchanger. Because in the above several cooling cycle modes, the mass flow rate and the inlet and outlet temperature of the negative pressure helium gas in the low pressure heat exchanger are close, and the high pressure helium gas of the low pressure heat exchanger in the circulation mode D shown in the figure is finally cold. The 40 K heat exchanger in the tank is mixed with helium, so the helium outlet temperature is 40 K. For the three cooling cycles of A, B and C, the high pressure helium is finally throttled after cooling by the cooler. The gas-liquid two-phase enthalpy of 0.12 M Pa was obtained in the gas-liquid separator, so that the high-pressure helium gas temperature in the low-pressure heat exchanger was reduced to about 7 K in this mode, thus causing the above difference.
The refrigeration coefficient and quality factor of the system are given in several cases. As the temperature of the helium gas at the hot end of the low-pressure heat exchanger is continuously reduced, the flow rate of the hot fluid flowing through the low-pressure heat exchanger is reduced, but the refrigeration of the entire system is improved. Efficiency is very beneficial, especially when the temperature is lower, the more obvious this effect.
By comparing the thermal calculation and analysis of the above five cooling cycles, the cooling cycle mode B has the advantages of the highest cooling efficiency of the 2 K system, and the total power consumption of the system is also minimal, and the two modes of cooling the PK SCAF are cooled by the cycle mode. The shell superconducting acceleration chamber is better than other cooling cycles. The dotted line in the figure is the fluid that is diverted from the compressor outlet to the low pressure heat exchanger.
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