Sub-Cooling Subcooling Method

SUBCOOLING METHOD IS USED IN SYSTEMS THAT HAVE A TXV METERING DEVICE CONDENSER Temperatures Pressures States CONDENSER INLET

• High Pressure High Temperature Superheated Vapor

• Saturation Point • (vapor changing to a liquid as heat is removed)

• High Pressure Sub-cooled Liquid

CONDENSER OUTLET Saturated Liquid Pressure and Temperature

Pressure

Saturated Condensing Temperature Condenser Side

Pressure

Sat. Cond. Temp. 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

Checking Sub-cooling

• 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. • 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.

Determine Operating Sub-cooling

111°F

Liquid Line

Suction Line (Vapor)

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

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

• 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

High 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

Summary

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