Sub-Cooling

Subcooling Method

SUBCOOLING METHOD IS USED IN SYSTEMS THAT HAVE A TXV METERING DEVICE

CONDENSER Temperatures

Pressures States

• High Pressure High Temperature Superheated Vapor

• Saturation Point • (vapor changing to a

liquid as heat is removed)

CONDENSER INLET

CONDENSER OUTLET

• High Pressure Sub-cooled Liquid

Saturated Liquid Pressure and Temperature

Pressure

Saturated

Condensing

Temperature

Condenser Side

Sat. Cond. Temp.

Pressure

Saturated Liquid Pressure

Sub-cooling

Charging Method

METERING DEVICES - TXV

Charging Thermostatic Expansion Valve (TEV) Systems

• The TEV will maintain design superheat under a variety of conditions

• For this reason, these systems must be CHARGED using sub-cooling

• Proper sub-cooling will maintain a positive liquid seal at the TEV entrance

• Determine the required sub-cooling from the IO or the unit datasheet (usually from about 8°F to about 12°F

Temperature Probe Test Point Locations

• LIQUID LINE SERVICE VALVE (Smaller of the two copper lines)

• INSULATE THE PROBE FOR A MORE ACCURATE READING

• Allow the unit to operate for 15 to 20 minutes before checking the sub-cooling.

• This is to insure stable operation. If the temperatures and pressures will not stabilize, look for other problems before attempting to check sub-cooling.

• Attach an accurate thermometer to the liquid line near the inlet to the metering device if possible. A condenser outlet reading may be taken but will be in error by the amount of liquid line temperature/pressure losses.

Checking Sub-cooling

• Record the temperature reading.

• Connect a manifold gauge set to the liquid service valve and record the pressure. Again, discharge pressure may be used but allowances must be made for condenser coil pressure drop.

• Using a Pressure/Temperature Chart, find the saturation temperature for the liquid pressure obtained.

• Subtract the line temperature from the saturation temperature. The difference is the amount of sub-cooling.

Suction Line (Vapor)

Liquid Line

111°F

Determine Operating Sub-cooling

Measure Liquid Line Temperature

118 PSIG 417 PSIG

• Measure Liquid Line Pressure

• Convert Pressure to Saturation Temperature

• Subtract SATURATION Temperature from

ACTUAL Temperature

• The Difference is OPERATING Sub-cooling

Checking Subcooling

EXAMPLE: a. Liquid Line Pressure = 417 b. Corresponding Temp. °F. = 120° c. Thermometer on Liquid line = 111°F. To obtain the amount of sub-cooling subtract

111°F from 120°F. The difference is 9° sub-cooling.

Sub-cooling should be 9 ºF (+ or - 2°F)

Add charge to raise subcooling. Recover charge to lower subcooling.

Sub-cooling = Sat. Liquid Temp. - Liquid Line Temp.

• Low sub-cooling indicates a condenser that is starved for liquid refrigerant

• Long liquid lines

• High vertical separation

• Liquid lines exposed to high ambient temperatures

• Low condenser air flow

• Inadequate condenser size

Low Sub-cooling

• High sub-cooling indicates a condenser flooded with liquid refrigerant

• Refrigerant overcharge

• Low outdoor ambient temperatures

• Liquid line restriction

• Incorrect or faulty metering device

High Sub-Cooling

Proper measurement and interpretation of Superheat & Sub-cooling is the only way to

determine proper operation and performance of HVAC systems

The Service Technician must develop these skills and use them correctly

Summary