The Professor: The Perils of Overcharging
Too much refrigerant can cause a number of system issues
There’s an old saying that too much of anything — even a good thing — isn’t necessarily a good thing. That certainly holds true for refrigerants.
The system check below shows a refrigeration system with an overcharge of refrigerant. The system is a low-temperature thermostatic expansion valve (TXV)/receiver system incorporating R-134a as the refrigerant.
Compressor Discharge Temperature: 240°F;
Condenser Outlet Temperature: 90°;
Evaporator Outlet Temperature: 15°;
Compressor In Temperature: 25°;
Ambient Temperature: 70°;
Box Temperature: 10°;
Low Side (Evaporator) Pressure: 8.8 psig (5°); and
High Side (Condensing) Pressure: 172 psig (120°).
Condenser Split: 50°;
Condenser Subcooling: 30°;
Evaporator Superheat: 10°; and
Compressor Superheat: 20°.
The symptoms of an overcharged system are:
High discharge temperature;
High condenser subcooling;
High condensing pressures;
Higher condenser splits;
Normal to high evaporator pressures;
Normal superheats; and
High compression ratio.
Let’s take a closer look at these symptoms.
High discharge temperature — With an overcharged system, the high-compressor (superheated vapor) discharge temperature (in our example above, 240°) is caused by the high compression ratio. A discharge temperature of 225° to 250° is considered the maximum discharge temperature in order to prevent system breakdown from excessive heat. Liquid backed up in the condenser from an overcharge of refrigerant will flood some of the condenser’s internal volume at its bottom and cause high head pressures. All of the heat being absorbed in the evaporator and the suction line, along with motor heat and high heat of compression from the high compression ratio, has to be rejected into a smaller condenser internal volume because of the backed-up (overcharged) liquid refrigerant.
High condenser subcooling — Because of the overcharge of refrigerant in the system, the condenser will have too much liquid backed up at its bottom. This will cause high subcooling. Remember, any liquid in the condenser at a temperature lower than the condensing temperature is considered subcooling. You can measure this at the condenser outlet with a thermometer or thermocouple. Subtract the condensing out temperature from the condensing temperature to get the amount of liquid subcooling in the condenser. A forced-air condenser used in refrigeration should have at least 6° to 8° of liquid subcooling in the condenser. However, subcooling amounts depend on system piping configurations and liquid line static and friction pressure drops.
Condenser subcooling is an excellent indicator of the system’s refrigerant charge — the lower the refrigerant charge, the lower the subcooling; the higher the refrigerant charge, the higher the subcooling.
High condensing pressures — Subcooled liquid backed up in the condenser will cause a reduced condenser internal volume and raise condensing pressures. When the condensing pressures are raised, there is a greater temperature difference between the surrounding ambient and condensing temperature, which causes greater heat flow. This compensates for the condenser’s reduced internal volume. The system will still reject heat, but at a higher condensing pressure and temperature.
High condenser splits — Because of the higher condensing pressures and temperatures, there will be a greater temperature difference (split) between the ambient temperature and condensing temperature. A dirty condenser will also give a system high condenser splits, but the condenser subcooling will not be as high with an overcharged system.
Normal to high evaporator pressures — Since this system has a TXV metering device, the TXV will still try to maintain its evaporator superheat, and the evaporator pressure will be normal to slightly high, depending on the amount of overcharge. If the overcharge is excessive, the evaporator’s higher pressure would be caused by the decreased mass flow rate through the compressor as high compression ratios cause low volumetric efficiencies. The evaporator would have a harder time keeping up with the higher heat loads from the warmer entering-air temperature. The TXV will also have a tendency to overfeed refrigerant to the evaporator on its opening stroke because of the high head pressures.
Normal evaporator superheats — The TXV will try to maintain superheat even at an excessive overcharge. As mentioned above, the TXV may overfeed slightly during its opening strokes, but then it should catch up to itself if it’s still in its operating pressure ranges.
High compression ratios — The condenser flooded with liquid during the overcharge will run high condensing pressures. This causes high compression ratios, low volumetric efficiencies, and low refrigerant flow rates.
CAPILLARY TUBE SYSTEMS
If we are dealing with a capillary-tube metering device, the same symptoms occur with exception to the evaporator superheat. Remember, capillary tube systems are critically charged to prevent liquid floodback of refrigerant to the compressor during low evaporator loads. The higher head pressures of an overcharged system incorporating a capillary tube as a metering device will have a tendency to overfeed the evaporator, thus decreasing the superheat.
If the capillary tube system is severely overcharged, liquid can enter the suction line and get to the suction valves or crankcase. This will cause compressor damage and eventually failure.
Remember, it is the system check that will tell the service technician whether a system is overcharged or not. Service technicians must always install pressure gauges and thermistors (or some other sort of temperature-sensing devices) in order to systematically troubleshoot a refrigeration system correctly.
Publication date: 12/5/2016