Pressure. Temperature. Boiling points. And how the refrigerator goes full throttle.
The idea behind the refrigerator is a simple one: transfer the heat from inside and put it outside. What makes this possible are the phase transitions of the refrigerant in the refrigeration cycle. The refrigerant absorbs heat from the different refrigerator compartments, boils and evaporates, and then loses the heat to the outside when it condenses.
To drive these phase transitions, the refrigerator uses pressure at two important parts of the refrigeration cycle: expansion valve and compressor.
At the expansion valve (also called a metering or throttling device), high pressure liquid refrigerant is forced into much thinner coils. This “throttles” the incoming liquid, impeding its flow significantly and thus causing the pressure to drop rapidly.
When pressure drops, the refrigerant molecules find it easier to escape from the liquid, and hence the boiling point drops too. The refrigerant thus evaporates more readily. Because there is no external energy source, the heat needed for this evaporation comes from the refrigerant itself. As a result, the temperature of the remaining liquid refrigerant drops.
Learn Engineering has a great video that explains this clearly (watch for a minute till the 1:40 minute mark):
It is this cold liquid refrigerant that enters the evaporator coils. It draws heat from the refrigerator compartments, keeping them cold as it boils and evaporates into a hot gaseous refrigerant.
When the hot gaseous refrigerant leaves the evaporator coils, the compressor helps to push it to the condenser coils so that it can condense and lose the heat to the surroundings.
The compressor also plays another important role. It pressurises the refrigerant. When the gaesous refrigerant condenses into a liquid, it stays pressurized, making it suitable to be fed into the expansion valve again (which only works if there is a high pressure liquid to start with).
In the same video from Learn Engineering they describe this clearly too (watch for 1 minute till 2:35 minute mark)
This effectively re-uses the refrigerant in a continuous cycle. The benefits of this ingenuous re-use of the refrigerant are obvious. It also outweighs the trade-off of raising the temperature of the refrigerant when it is pressurized (the reverse of the earlier bit about how a lower pressure lowers the temperature too). That’s pretty cool, yes?
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photos: in order – depositphotos/chiociolla; mrhighsky; belchonock