Rapid Expansion of Liquid Refrigerants!

Expansion phase of liquid refrigerant in air conditioners, happens at the critical separation point between high pressure, and low pressure side of air conditioner systems.

It is simply the reverse of compression.

As explained in how air conditioners work, this stage follows compression and condensation stage.

Compression and condensation of the refrigerant has already turned it from vapour, to liquid. It will be in that state, irrespective of temperature change within the contained space (i.e. the tubing), as long as the space can withstand the pressure rise or fall.

Compress it, condense it, hold the pressure. Get these three in harmony, and you’ll have liquid refrigerant forever.

The critical factor:

At this moment, the only factor that is holding the state of the refrigerant is the high pressure within the air conditioner. We can’t rely on ambient temperature alone (if you’re living on earth) to keep it liquefied. Condensation page explains it detail.

But we want liquid refrigerant to start vaporising. Thus, we deliberately put a restriction after the condenser.

This restriction has two functions:

  • allowing compression to happen

  • accelerate and allow liquid refrigerant to expand into low pressure side

And this restriction is called an expansion valve.


Expansion can happen in two ways, similar to compression. Expand,

  • slowly with temperature rise, or

  • rapidly with pressure decrease

However, construction of the valve that holds high pressure on one side, and facing low pressure on the other, determines the way refrigerant liquid expands. Rapidly due to pressure drop. In other words, the refrigerant goes through a throttling process.

Pressure taken off immediately, molecules become active almost immediately, volume (V) increases rapidly.

Throttled liquid will expand rapidly

If adiabatic compression meant increase in refrigerants temperature, the reverse will mean drop in surrounding temperature. This happens as the refrigerant liquid starts to vaporise.

Throttling as "dessert":

Throttling act helps expansion and evaporation of liquid through atomising or breaking a large amount of liquid, into mist.

It is widely used in atomising fuel for vehicles, and air conditioner refrigerants.

Real life and safe experiment? First,

  • borrow a mist spray, then,

  • fill the bottle with water, next,

  • spray into your kitchen sink, finally,

  • adjust the nozzle until you get nice mist

You can feel the “compressor” (your hand) have to put in a considerable energy for compression, and at the same time, watch expansion happen.

Cooling has not even started. It is still “warming” up! Evaporation is still around the corner.

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