What is the effect of undercooling/subcooling

Under cooling of the liquid refrigerant in the condenser is a normal practice, which ensures that the refrigerant will enter the expansion valve only in a liquid state.

In actual practice, the high pressure and high-temperature vapor refrigerant from the compressor are cooled and liquefied when it passes through the condenser.

The liquid refrigerant also continues to dissipate heat to the cooling medium of the condenser and its temperament gets reduced. This phenomenon is called sub-cooling or under-cooling. The pressure of the liquid refrigerant remains the same but temperament is reduced during undercooling.

The effect of sub-cooling can be seen from the and the effects are:

  • The temperature of liquid refrigerant entering the expansion valve will increase from t3-t3′.
  • Flashing of the liquid refrigerant in the expansion valve is avoided.
  • Increase in refrigerating effect from (h1-h4) to (h1-h4).
  • No extra work is required for under cooling.
  • The net effect of under-cooling is to increase the COP of the system.


The liquid refrigerant, which is not sub-cooling before entering the expansion valve, will evaporate to some extent during the passage of the narrow opening of the expansion valve. This is called flashing.

The non-occurrence of flashing in the expansion valve will increase the refrigerating effect. The increase in refrigerating effect will lead to correct superheating of vapor, which will require only optimum compression work, thereby minimum power per ton is achieved.

Advantages and disadvantages of under-cooling


  1. Flashing of the liquid refrigerant in the expansion valve is avoided.
  2. Increase in refrigerating effect will increase the refrigerating capacity.
  3. Only optimum compression work is required.


  1. Installation of perfect sub-cooling is not possible.
  2. Sub-cooler will lead to higher condensing temperature and pressure.
  3. Over sub-cooling is undesirable.

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