ACHRNEWS

The Professor: System Overcharge

January 14, 2008
John Tomczyk

Many technicians have a tendency to add refrigerant to a refrigeration system simply because it is not cooling to its potential. Often, this overcharges the system with refrigerant and can make matters worse.

The service technician must be able to recognize an overcharged system and recover some of the refrigerant to bring it down to the correct charge. The system checklist below shows a system with an overcharge of refrigerant. The system shown is a R-134a refrigeration system incorporating a TXV and receiver. 

REFRIGERANT OVERCHARGE

MEASURED VALUES IN °F:
Compressor discharge temperature: 240
Compressor outlet temperature: 90
Compressor inlet temperature: 15
Ambient temperature: 70
Box temperature: 40
Compressor volts: 230
Compressor amps: high
Lowside (evaporator) pressure: 8.8 psig (5°)
Highside (condenser) pressure: 172 psig (120°)

CALCULATED VALUES IN °F:
Condenser split: 50
Condenser subcooling: 30
Evaporator superheat: 10
Compressor superheat: 20

SYMPTOMS

Here are some of the symptoms:

• High discharge temperatures - With an overcharged system, the high discharge temperature of 240° is caused from the high compression ratio. Liquid backed up in the condenser will flood some of the condensing surface area causing 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 because of the backed up liquid.

• High condenser subcooling - Because of the overcharge of refrigerant in the system, the condenser will have too much liquid backed up at its bottom, causing high subcooling. Remember, any liquid in the condenser 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. A forced air condenser should have at least 8° of liquid subcooling. However, subcooling amounts do 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 charge, the higher the subcooling.

• High condensing pressures - Subcooled liquid backed up in the condenser will cause a reduced condensing surface area and raise condensing pressures. Now that the condensing pressures are raised, there is more of a temperature difference between the ambient and condensing temperature, causing greater heat flow to compensate for the reduced condensing surface area. The system will still reject heat but at a higher condensing pressure and temperature.

• High condenser sp1its - Because of the higher condensing pressures, thus higher condensing temperatures, there will be a greater temperature difference (split) between the ambient and condensing temperature.

• Normal to high evaporator pressures - Since these systems have TXV metering devices, 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 from high compression ratios, causing 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 have a tendency to overfeed on its opening stroke due to 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 should catch up to itself if 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 and causes low volumetric efficiencies causing low refrigerant flow rates.

OVERCHARGED SYSTEMS

If we are dealing with a capillary tube-metering device, the same symptoms occur, with ex-ception to the evaporator superheat. Remember, capillary tube systems are critically charged to prevent floodback of refrigerant to the compressor during low evaporator loads. The higher head pressures of an overcharged system will have a tendency to overfeed the evaporator, thus decreasing the superheat.

If the system is overcharged more than 10 percent, liquid can enter the suction line and get to the suction valves or crankcase. This will cause compressor damage and eventually failure.

Publication Date: 01/14/2008