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:
- The gas is compressed to allow it to circulate throughout the system. This raises the temperature of the gas.
- The gas cools as it passes through the heat exchanger and cools further when it expands through the Joule-Thomson orifice.
- 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:
- The refrigerant in its vapour phase is compressed to allow it to circulate throughout the system. This raises the temperature of the refrigerant.
- The superheated vapour cools as it passes through the first heat exchanger and condenses into a liquid.
- The liquid cools further when it expands through the Joule-Thomson orifice, turning into a mixture of liquid and vapour.
- The cold mixture is routed to the second heat exchanger, where warm air blown across it is cooled by the mixture.
- The refrigerant exits the evaporator with a higher temperature and reverts to its vapour form. It returns to the compressor and the cycle repeats.
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.
