The Professor: Checking on Refrigerant Overcharge
This column will discuss refrigerant overcharge. The check sheet shown depicts a refrigeration system with an overcharge of refrigerant. The system is a low-temperature HFC-134a unit with a TXV/receiver. (See the chart below.)
• High discharge temperature
• High condenser subcooling
• High condensing pressures
• Higher condenser splits
• Normal to high evaporator pressures
• Normal superheats
• High compression ratio
High Discharge Temperatures:
With an overcharged system, the high compressor (superheated vapor) discharge temperature of 240° is caused from the high compression ratio. A discharge temperature of 225-250° is considered the maximum discharge temperature in order to prevent system breakdown from excessive heat. Liquid backed up in the condenser from the overcharge of refrigerant will flood some of the condenser’s internal volume at its bottom 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’s 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 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 in the condenser. A forced-air condenser used in refrigeration should have at least 6-8° of liquid subcooling in the condenser. 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 causes a reduced condenser internal volume and raise condensing pressures. Now that the condensing pressures are raised, there is more of a temperature difference between the surrounding ambient and condensing temperature, causing greater heat flow. This compensates for the reduced condenser’s 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, thus higher condensing temperatures, there will be a greater temperature difference (split) between the ambient and condensing temperature. A dirty condenser will also give a system high condenser splits, but the condenser subcooling will not be as high as 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 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 also have a tendency to overfeed refrigerant to the evaporator 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.
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.
Again, it is the system check sheet that will tell the service technician whether a system is overcharged or not. Service technicians must install pressure gauges and thermistors or some other sort of temperature sensing devices in order to systematically troubleshoot a refrigeration system correctly.
Publication date: 09/06/2010
SYMPTOMS
Symptoms can include:• High discharge temperature
• High condenser subcooling
• High condensing pressures
• Higher condenser splits
• Normal to high evaporator pressures
• Normal superheats
• High compression ratio
Refrigerant overcharge. (Click on the chart for an enlarged view.)
With an overcharged system, the high compressor (superheated vapor) discharge temperature of 240° is caused from the high compression ratio. A discharge temperature of 225-250° is considered the maximum discharge temperature in order to prevent system breakdown from excessive heat. Liquid backed up in the condenser from the overcharge of refrigerant will flood some of the condenser’s internal volume at its bottom 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’s 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 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 in the condenser. A forced-air condenser used in refrigeration should have at least 6-8° of liquid subcooling in the condenser. 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 causes a reduced condenser internal volume and raise condensing pressures. Now that the condensing pressures are raised, there is more of a temperature difference between the surrounding ambient and condensing temperature, causing greater heat flow. This compensates for the reduced condenser’s 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, thus higher condensing temperatures, there will be a greater temperature difference (split) between the ambient and condensing temperature. A dirty condenser will also give a system high condenser splits, but the condenser subcooling will not be as high as 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 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 also have a tendency to overfeed refrigerant to the evaporator 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 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.
Again, it is the system check sheet that will tell the service technician whether a system is overcharged or not. Service technicians must install pressure gauges and thermistors or some other sort of temperature sensing devices in order to systematically troubleshoot a refrigeration system correctly.
Publication date: 09/06/2010
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