To determine a system's subcooling value, from which temperature should the liquid line temperature be subtracted?

The correct choice for determining a system's subcooling value is the temperature from the condenser. To calculate subcooling, you need to compare the temperature of the refrigerant in the liquid line after it has exited the condenser to the saturation temperature of the refrigerant at the corresponding pressure in the condenser.

In more detail, subcooling refers to the process of cooling the liquid refrigerant below its saturation temperature at a given pressure. The saturation temperature is defined based on the pressure within the condenser, which means that the relevant temperature for comparison is the condenser temperature. By subtracting the liquid line temperature from this condenser saturation temperature, you can determine how much additional cooling has occurred beyond the phase change from vapor to liquid.

This measurement is essential as it ensures that the refrigerant entering the expansion valve is completely in a liquid state, preventing possible compressor damage and improving system efficiency. Other temperature sources, such as those from evaporators or suction lines, do not serve this specific purpose in determining the subcooling level effectively.