Charging Considerations and Mistakes – Class 7/6/2021
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The goal of the refrigeration circuit is to move heat around; it absorbs heat from a space and rejects it somewhere else via the refrigerant. We maximize the amount of heat absorbed by allowing the refrigerant to undergo phase changes (liquid and vapor). Many technicians add or remove refrigerant whenever the system does not work as it should, but adjusting the charge is not always the answer.
If a system has too little refrigerant, the refrigerant could boil off too quickly and at a lower temperature; the insufficient amount of molecules don’t exert enough force to keep the pressure high enough to keep the boiling temperature up. So, the evaporator coil could freeze up. Your compressor could also heat up because some of the refrigerant cools the compressor, and you may have an insufficient amount. Systems with low charge also have low capacity because they feed less liquid into the evaporator.
If a system has too much refrigerant, it will run high head pressure. Too much liquid refrigerant in the condenser decreases the amount of space where condensing may occur. As such, the refrigerant will still condense, but it will occur at a higher temperature and pressure. The compression ratio is affected and negatively impacts system performance (poor efficiency, low mass flow rate of refrigerant). Systems with receivers can help control the charge by holding excess refrigerant charge, so long as the receiver doesn’t exceed 80% capacity.
Before adjusting the charge, check the evaporator and condenser airflow. Look at air filters, blower wheels, and coil cleanliness. See what type of metering device you have; a dynamic metering device (TXV, EEV) will generally be charged by subcool, and a fixed orifice (piston, cap tube, header crimp) typically gets charged by superheat. (Check BOTH subcooling and superheat regardless of the metering device!) Assess the blower technology; make sure you are at full airflow and system capacity. Also, check if the variable-speed motor has been set up properly. Know your return and indoor air temperature, and check the outdoor temperature in the shade. Finally, you’ll want to weigh in on the line length and look at the manufacturer’s charging guidelines before adding or removing any charge.
We charge TXV systems by subcooling because they maintain a constant superheat. We charge fixed orifice systems based on superheat because we don’t want to raise the head pressure and overfeed the evaporator by charging via subcooling. Superheat indicates evaporator feeding, and subcooling indicates condenser stacking. In residential HVAC, the superheat can be as low as 5 degrees (F) at the evaporator outlet, and it may approach 25 degrees near the compressor. On the other hand, a solid subcooling range is 5-20 degrees (F).
To charge via superheat, take the indoor wet-bulb temperature in the return and outdoor dry-bulb in the shade. Then, use the charging chart to determine your target superheat. Add refrigerant to decrease the superheat; recover refrigerant to increase the superheat.
When adding refrigerant, you must consider if the system is running or not running. If a system is not running, you can add the charge to the liquid line so that you don’t flood the compressor upon startup. If a system is running, SLOWLY add refrigerant to the suction line.
Common charging errors include mixing refrigerants, loss on the disconnect, and using the wrong charging method. Some techs also fail to purge their hoses, depress Schrader cores, or weigh the charge with a scale. Techs may also make incorrect measurements by taking the temperatures at incorrect locations, failing to zero out their devices, using analog probes instead of digital ones, and taking temperatures readings in full sunlight. Some techs may also charge too quickly, which can flood the compressor.
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