Conventional Refrigeration
A domestic refrigerator relies on this cycle of evaporation and condensation using a low boiling point refrigerant to produce a cooling effect. Inside the compartment, the liquid refrigerant is made to expand rapidly before evaporating, undergoing the liquid-gas phase transition. As it does this, the refrigerant absorbs energy from its surroundings, cooling the inside of the compartment. The efficiency of the refrigerator cycle is improved by coiling several meters of tubing inside the compartment, ensuring the refrigerant has ample time to evaporate and remove as much heat as possible. This coil of tubing, known as a heat exchanger, is hidden inside one or more walls of the compartment and is usually surrounded by metal fins to improve heat transfer.
The gaseous refrigerant is returned to the outside of the refrigerator compartment and is forced to condense, releasing the energy it has absorbed as heat. Physically this is achieved by the use of a mechanical compressor, driven by an electrical motor, and by passing the refrigerant through another heat exchanger, mounted on the rear or underside of the refrigerator.
Our cryostats use the same evaporation/condensation process as a conventional domestic refrigerator. However, in our cryostats we use liquid helium as the refrigerant.
Cooling from room temperature to 4 K is easily achieved by keeping the cryostat immersed in a bath of liquid helium. Further cooling to below 1 K is achieved by pumping on a container of liquid helium. As the helium refrigerant undergoes the liquid-gas phase transition, it absorbs energy from its surroundings, cooling the cryostat. Pumping on the container removes the energetic gaseous helium allowing the liquid-gas phase transition to continue.
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