Subcooling

What is Subcooling?

Subcooling is the condition where the liquid refrigerant is colder than the minimum temperature (saturation temperature) required to keep it from boiling and, hence, change from the liquid to a gas phase.
The amount of subcooling, at a given condition, is the difference between its saturation temperature and the actual liquid refrigerant temperature.

It increases the efficiency of the system since the amount of heat being removed per pound of refrigerant circulated is greater. In other words, you pump less refrigerant through the system to maintain the refrigerated temperature you want. This reduces the amount of time that the compressor must run to maintain the temperature. The amount of capacity boost which you get with each degree of subcooling varies with the refrigerant being used.
Subcooling is beneficial because it prevents the liquid refrigerant from changing to a gas before it gets to the evaporator. Pressure drops in the liquid piping and vertical risers can reduce the refrigerant pressure to the point where it will boil or flash in the liquid line. This change of phase causes the refrigerant to absorb heat before it reaches the evaporator. Inadequate subcooling prevents the expansion valve from properly metering liquid refrigerant into the evaporator, resulting in poor system performance.

Remove the cap on the liquid line access fitting. Attach the high pressure (red) gauge hose on the refrigeration gauge manifold. Purge the air from the hose by momentarily opening the high side gauge.
Turn on the A.C. unit and allow to operate for 10 minutes. Record the high side pressure on the red gauge. This measurement is in psig or pounds per square inch gauge pressure.
Place the digital temperature thermometer on the liquid line near the condensing unit. Put tubing insulation around the thermometer to more accurately measure the copper tubing temperature. Record this temperature.
Using a pressure temperature chart, find the saturation temperature for the pressure you recorded in step 1. The difference between the measured temperature and the temperature from the pressure temperature chart is the subcooling. Adequate subcooling is needed to overcome any liquid line pressure drops. A.C. charging charts for TXV systems need subcooling readings to properly charge these units.

Read high-side pressure while system is running. Measure temperature on liquid line close to unit. Cross-reference the temperature to a chart. Subtract the two numbers. Should be 8 to 10 degrees.