The difference between the saturated liquid temperature corresponds to the condensing pressure of the condenser and the actual temperature of the liquid at the condenser outlet.
In engineering, the exhaust pressure is generally approximated as the condensing pressure.
And the difference between the saturated liquid temperature corresponding to the exhaust pressure and the temperature of the condenser outlet liquid is used as the degree of subcooling.
The reason for this approximation is that the pressure drop in the condenser is relatively small compared to the evaporator.
The difference between the exhaust pressure and the true condensing pressure is small, and the error caused by using such an approximation can be ignored.
Why a Refrigeration System Must Need Subcooling?
To understand why a refrigeration system needs subcooling, you need to know what happens to the refrigeration system if it is not subcooled.
The answer is when the refrigerant or other medium drops in pressure before it reaches the evaporator.
The saturated liquid will inevitably evaporate due to the reduction of pressure.
Liquid evaporation will absorb the surrounding heat, and the remaining liquid will cool down and reach saturation temperature under the corresponding pressure.
This cycle will result in the refrigerant advancing in the refrigeration system while vaporizing until it reaches the evaporator inlet.
The dryness of the two-phase refrigerant that eventually reaches the evaporator will then be much greater than the designed dryness.
The liquid phase component is reduced, it will not be able to meet the evaporation volume of the evaporator, and the refrigeration effect will of course be reduced.
Therefore the refrigeration effect is reduced.
This is why need subcooling
But, the design parameters of the major condenser plants around the world are different.
Therefore, it is not possible to standardize the subcooling of refrigeration systems.
A subcooling degree of 3-5°C is appropriate for conventional air-cooled condensers on the market.