Last month’s column analyzed a closed-door, medium-temperature refrigeration system that incorporated a liquid high-side receiver and a thermostatic expansion valve (TXV) as the metering device. The system utilized R-134a as the refrigerant, and the analysis from the system check discovered that the refrigeration system was low on refrigerant. As a reminder, Table 1 shows the measured and calculated values for the undercharged system.
Now consider the same system — a medium-temperature refrigeration system incorporating a liquid high-side receiver, a TXV as the metering device, and R-134a refrigerant — but the measured and calculated values are listed in Table 2, along with this detailed system analysis.
|Compressor discharge temperature||195°F|
|Condenser outlet temperature||78°F|
|Evaporator outlet temperature||10°F|
|Compressor inlet temperature||50°F|
|Low side (evaporator) pressure||3.94 in. Hg (minus 20°F)|
|High side (condensing) pressure||86.4 psig (80°F)|
Table 1: The first set of measured and calculated values for a closed-door, medium-temperature refrigeration system that incorporated a liquid high-side receiver and a thermostatic expansion valve (TXV) as the metering device and utilized R-134a as the refrigerant, which was analyzed in the July 2 issue of The NEWS.
|Compressor discharge temperature||240°F|
|Condenser outlet temperature||90°F|
|Evaporator outlet temperature||30°F|
|Compressor inlet temperature||40°F|
|Low side (evaporator) pressure||8.8 psig (20°F)|
|High side (condensing) pressure||172 psig (120°F)|
TABLE 2: The second set of measured and calculated values for a closed-door, medium-temperature refrigeration system that incorporated a liquid high-side receiver and a thermostatic expansion valve (TXV) as the metering device and utilized R-134a as the refrigerant, which is analyzed in this article.
Compressor discharge: With an overcharged system, the high compressor (superheated vapor) discharge temperature of 240°F 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 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 there is too much 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 condenser outlet temperature from the condensing pressure/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. However, subcooling amounts do depend on system piping configurations, 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 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, causing unwanted inefficiencies from the higher compression ratio.
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. Remember, the condenser split is the difference between the condensing temperature and the ambient temperature.
Normal to high evaporator pressures: The TXV will try to maintain its evaporator superheat, and the evaporator pressure will be normal to slightly high, depending on the amount of overcharge. If the refrigerant overcharge is excessive, the evaporator’s higher pressure will be caused by the decreased mass flow rate through the compressor from high compression ratios, causing low volumetric efficiencies.
The evaporator will 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 refrigerant overcharge. As mentioned above, it may overfeed slightly during its opening strokes, but it 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 low volumetric efficiencies, which results in low refrigerant flow rates.
In summary, there are seven symptoms or telltale signs of a system that has too much refrigerant.
- High discharge temp
- High subcooling in the condenser
- High pressures in the condenser
- Higher condenser splits
- Normal-to-high evaporator pressures
- Normal superheats
- High compression ratio
A system check is the best way for service technicians to determine whether or not a system is overcharged. They simply have to install gauges and thermistors on the refrigeration system and take readings to systematically troubleshoot a refrigeration system correctly.
Publication date: 8/6/2018