GB1117246A - Improvements in or relating to refrigerators operating at very low temperatures - Google Patents

Abstract

1,117,246. Refrigerating. UNITED KING- DOM ATOMIC ENERGY AUTHORITY. 26 Jan., 1967 [16 March, 1966], No. 11500/66. Heading F4H. In a refrigerat on system a superfluid solution of helium-3 in helium-4 is diluted reversibly by the addition of helium-4 through a superleak, thereby doing external work. As shown, the system comprises a boiler 4 which is connected by a U-tube 3 to a cooling chamber 1 surrounding a container 2 for the article to be cooled, and has a vapour outlet 6 to an external pumping system. A helium-3 condenser 9 is connected by line 12 to a phase-separator 13 which is connected by a line 14, containing a valve 15, and a line 16 to an expansion chamber 17. A line 22 containing a superleak 21 connects line 16 to a load chamber 23 and a line 18 containing a valve 19 connects line 16 to the cooling chamber 1. A heat switch 25 provided between the boiler and load chamber is switched " on " and " off " by a solenoid 29 surrounding a superconducting link 28 which reverts to its normal state when the solenoid is energized; when the switch is " on " the temperature of chamber 23 is reduced to that of the boiler and when the switch is "off" chamber 23 may be heated by a heater 24. In operation the system is initially operated in substantially the same manner as that described in Specification 945,223, i.e. a phase boundary is established in phase-separator 13 between concentrated helium-3 and a dilute solution of helium-3 in helium-4 and the temperature falls to approximately 0À07‹ K. which is the lowest temperature obtainable by dilution. Heat switch 25 is open and heater 24 on so that load chamber 23 is filled with pure helium-4 by the fountain effect acting through superleak 21. Valve 19 is now closed, heater 24 turned off, and heat switch 25 closed thus lowering the temperature of the helium-4 in chamber 23 so that the fountain pressure becomes less than the osmotic pressure of helium-3 in tube 16 and helium-4 begins to flow through superleak 21 and into the expansion chamber 17. Simultaneously helium-3 flows across the phase boundary in separator 13 so that the concentration of helium-3 in the helium-4 in chamber 17 is only slightly less than in the separator. Valve 15 is closed when chamber 17 is about one eighth full, helium-4 continues to flow into chamber 17 so that the concentration of helium-3, and hence its osmotic pressure and temperature, falls. Boiler 4 has meanwhile continued to lower the helium-3 concentration in the cooling chamber 1 since valve 19 is closed. When the helium-3 concentration in chamber 17 is less than that in chamber 1, valve 19 is opened, heater 24 is energized and switch 25 opened. Helium-4 thus flows back into chamber 23 and helium-3 flows from chamber 17 to the boiler. Valve 19 is closed when chamber 17 is empty and chamber 23 full and the cycle is begun again when the fountain pressure is sufficiently high. In a modification (Fig. 2, not shown) the load chamber and heat switch are replaced by a capillary tube connecting the superleak with the boiler.