How is subcooling measured, and what does it indicate about system performance?

• A. Subcooling is the difference between saturated liquid temperature at high-side pressure and the actual liquid temperature; adequate subcooling indicates proper refrigerant condition and helps metering.
• B. Subcooling is the same as ambient air temperature.
• C. Subcooling measures evaporator efficiency at low side.
• D. Subcooling is the difference between condenser head pressure and suction pressure.

Answer: (A)

Subcooling is the difference between the saturated liquid temperature at the high-side pressure and the actual liquid temperature after the condenser. You determine it by finding the saturated liquid temperature from the condenser pressure (or using a PT chart), then measuring the actual temperature of the liquid leaving the condenser and entering the metering device. Adequate subcooling means the refrigerant is fully condensed and cooled below its condensation temperature before it reaches the expansion device, which helps ensure that only liquid, not vapor, enters the metering device and reduces flash gas, leading to more accurate metering and stable evaporator performance. It also reflects how well the condenser is rejecting heat and whether the refrigerant charge is correct. If subcooling is too low, condenser cooling or charge may be insufficient; if it’s too high, there may be overcharge or condenser airflow/restriction issues.