Two significant applications of the Joule-Thomson effect involve the Hampson-Linde cycle and the vapour-compression cycle.
The Hampson-Linde cycle, also known as the ‘Linde refrigerator’ (see diagram below), employs the Joule-Thomson effect to liquefy gases.
The cycle is as follows:
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- The gas is compressed to allow it to circulate throughout the system. This raises the temperature of the gas because the kinetic energy of the gas molecules increases as they collide with the moving wall.
- The gas cools as it passes through the heat exchanger and cools further when it expands through the Joule-Thomson orifice. The system is designed with specific pressures before and after the orifice to ensure the gas cools upon expansion, as described by the Joule-Thomson effect.
- The low pressure gas is routed to the heat exchanger, cooling the warmer incoming gas, before returning to the compression chamber.
- Warm replenishment gas for replacing any liquefied gas is mixed with the returning gas. The net temperature of the gas after mixing is lower than its temperature in step1. Steps 1 to 4 is repeated until the air is cool enough to condense into the compartment below the orifice.
The vapour-compression cycle is used in household refrigerators and air conditioning units.
The cycle is as follows:
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- Compression: The refrigerant gas is compressed by the compressor, allowing it to circulate throughout the system. This is not an isenthalpic process because work is done on the gas, which increases both the pressure and temperature of the refrigerant.
- Condensation: The hot, high-pressure refrigerant gas then passes through the condenser coils, usually located on the back or bottom of the appliance. As it cools, it condenses into a high-pressure liquid.
- Expansion: The flow of the high-pressure liquid refrigerant through an expansion valve, known as the Joule-Thomson orifice, can be modelled using the Joule-Thomson experiment. This causes the refrigerant’s pressure to drop, resulting in a mixture of liquid and vapour. The refrigerant is carefully selected based on its inversion temperature to ensure it cools upon expansion, as described by the Joule-Thomson effect.
- Evaporation: The low-pressure refrigerant then enters the evaporator coils (inside the refrigerator or air handler of an air conditioning unit). Here, it absorbs heat from the surrounding environment, which allows it to fully evaporate into a gas. This process cools down the air inside the refrigerator or the room.
- Cycle repeats: The refrigerant gas is then drawn back into the compressor to repeat the cycle. The desired temperature of the room or the interior of the refrigerator is monitored by a thermostat, which then signals the appropriate compression pressure to apply for subsequent cycles.
The desired temperature of the room or the interior of the refrigerator is monitored by a thermostat, which then signals the appropriate compression pressure to apply for subsequent cycles.
