Checking Subcooling

CHECKING SUBCOOLING

Expansion valve systems are normally charged by using the subcooling method. Liquid sub-cooling is required so that you have only liquid entering the expansion valve with no bubbles present. Vapor bubbles present in the refrigerant will cause low refrigerant flow. Low refrigerant flow will cause a loss of capacity and efficiency in a cooling system.

Liquid sub-cooling is normally measured at the liquid line service valve. The manufacturer will usually specify the required sub-cooling on the outdoor unit service panel. It’s usually between 5F and 15F, but always go by the manufacturer’s requirements.

HOW TO MEASURE SUBCOOLING

Take a pressure reading at the liquid line service port.. Use a temperature-pressure chart or the gauge temperature reading to convert the pressure to the saturated condensing temperature of the refrigerant.

Next, attach an accurate digital thermometer to the liquid line. (It’s usually best to insulate the thermometer probe so that it’s not affected by the ambient air.) The temperature that you read with the thermometer should be lower than the saturated condensing temperature.

The difference between the measured liquid line temperature and the saturated condensing temperature is the liquid subcooling.

(Saturation Temperature) - (Measured Liquid Line Temperature) = Subcooling

EXAMPLE:

• R22 Liquid Line Pressure = 250 psig
• Saturation Temp at 250 psig = 117 deg
• Measure Liquid Line Temp = 102 deg

117 deg - 102 deg = 15 deg SUBCOOLING

ADJUSTING CHARGE

• Add refrigerant to increase subcooling.
• Recover refrigerant to reduce subcooling.