An air conditioning unit with a thermostatic expansion valve (TXV) is charged to the subcooling of the liquid line leaving the condenser because the superheat is fixed. The superheat is fixed at 8Â°F to 12Â°F in most residential air conditioning systems.
Subcooling is the amount of liquid held back in the condenser. This allows the liquid to give up more heat below saturated pressure-temperature. For every 1Â° of subcooling at the same condensing pressure, capacity will increase 0.5%. Increasing subcooling with an increase of discharge pressure and compression ratio decreases capacity. Add 5Â° of subcooling for every 30 ft of liquid line lift. To measure subcooling:
1.Obtain refrigerant saturation pressure-temperature. Take a pressure reading of the liquid line leaving the condenser.
Refrigerant saturation temperature is the pressure-temperature when the refrigerant is turning from a high-pressure vapor into a high-pressure liquid (giving up heat). At saturation pressure-temperature, both liquid and vapor are at the same temperature.
2.Convert pressure to temperature with a pressure-temperature chart.
3.Take a temperature reading at the leaving liquid line of the condenser.
4.Compare both the saturated temperature and leaving liquid line temperature. Subtracting one from the other, the difference is the amount the refrigerant has cooled past saturated temperature.
Manufacturers should be able to identify the amounts of subcooling they have designed into a system. A low charge will give a low subcooling. An overcharge will give a high subcooling along with a high compression ratio.
Do not worry about a few bubbles in the sight glass. Sight glasses will not always be clear with a full charge. The zeotropes refrigerant group is known for their fractionation. It is possible to never have a clear sight glass. To determine what the subcooling should be in a system see Table 1.
FIXED-ORIFICE and R-22An air conditioning system with afixed orificemetering device is charged to the superheat of the suction line leaving the evaporator. Superheat is the gas temperature above the saturated temperature.
Superheat can be split into two types of heat: superheat of the evaporators, and total superheat entering the compressor.
The evaporators superheat must be figured at the evaporator outlet not at the compressor inlet. Total superheat is figured at the compressor inlet.
To measure evaporator superheat:
1.Take a pressure reading of the suction line-leaving evaporator to get refrigerant saturation pressure-temperature. Refrigerant saturation temperature is the pressure-temperature when the refrigerant is turning from a low-pressure liquid to a low-pressure vapor (absorbing heat). At saturation pressure-temperature, both liquid and vapor are at the same temperature.
2.Convert pressure to temperature with a pressure-temperature chart. If a reading is obtained at the compressor, not at the evaporator leaving line, you may have to add a few pounds of pressure due to pressure drop in the suction line.
3.Take a temperature reading at the leaving suction line of the evaporator. Compare both the saturated temperature and the leaving suction line temperature.
4.Subtract one from the other; the difference is the amount the refrigerant gas has heated past saturated temperature. This is superheat.
Manufacturers should be able to identify the amounts of superheat they have designed into a system. A low charge will give a high superheat. An overcharge will give a low superheat along with a higher compression ratio. To determine what superheat in a system should be, see Table 2.
TOTAL SUPERHEAT METHODSome residential air conditioning systems with fixed orifice may be charged by the total superheat method. Various equipment manufacturers furnish charts with their units that explain the proper procedures to the installing or servicing technician.
This method, similar to evaporator superheat method, is effective only when the indoor conditions are within 2Â°F of desired indoor comfort conditions and the suction pressure and temperature is stabilized.
To measure total superheat: 1.Read and record the outdoor ambient air-drybulb temperature entering the condenser.
2. Read and record suction line pressure and temperature at the suction service valve or service port at the compressor.
3. From Table 3, the reading at the intersection of vapor pressure and outdoor ambient temperature should coincide with the actual vapor line temperature.
4. If the vapor line temperature is not the same, adjust the refrigerant charge. Adding R-22 will raise suction pressure and lower suction line temperature. Removing R-22 will lower suction pressure and raise suction line temperature.
Caution: If adding R-22 increases both suction pressure and temperature, the unit is overcharged.
This method is useful when performing preventive maintenance or corrective service on residential air conditioning. Remember to always refer to the manufacturer’s recommendations whenever possible.
REFRIGERATIONThe use of a sight glass for charging is common in refrigeration.
It is better to charge a system first by measuring the operating condition (discharge and suction pressures, suction line temperature, compressor amps, super heat, subcooling and coils temperature deferential) before using the liquid line sight glass.
If the sight glass is close to the exit of the condenser or if there is very little subcooling at the sight glass, bubbles may be present even when the system is properly charged. If a system is charged to full sight glass, overcharging may be the result, decreasing efficiency.
Note: Follow the manufacturer’s recommendation for superheat and subcooling.
SUMMARYATXVis designed to maintain a constant superheat. Overcharg-ing a TXV will raise subcooling, increase system pressures, and decrease system efficiency. Under-charging a TXV will decrease subcooling, increase superheat, decrease system capacity, and lower refrigerant velocity, leaving oil in the evaporator.
An automatic expansion valve (AXV) is a constant evaporator pressure valve and not normally used in a/c.
A fixed orifice is the simplest metering device made and the most critical to charge. Overcharging fixed orifices will lower superheat, increase pressures, decrease efficiency, and flood the compressor with liquid refrigerant. Undercharged, the fixed orifices will raise superheat, lower pressure, lower capacity, and lower refrigerant velocity, leaving oil in the evaporator. Always refer to the manufacturer’s recommendations on charging fixed orifices.
The process of charging to superheat and subcooling improves an air conditioning system’s efficiency, capacity, and lessens equipment failures. Always let the system stabilize 10 to 20 min after adjusting the charge. This takes time, but improves efficiency and capacity.
Remember, when changing refrigerants, all superheat and subcooling adjustments have to be checked and recorded. The procedure of recording adjustments is called base-lining. This procedure not only saves time, money, and aggravation, but it is a sign of a professional.
Holder, CM, BSME, is a member of the Refrigeration Service Engineers Society Southern San Joaquin Chapter, Bakersfield, CA. He is a refrigeration and air conditioning specialist at National Technical Transfer Inc., P. O. Box 4558 Englewood, CO 80155; 800-922-2820.
Publication date: 10/16/2000