a system is equipped with a fixed metering device or capillary tubes,
the total superheat varies with the outdoor air temperature and the heat
load on the evaporator (air temperature and humidity). When properly
charged the total superheat will be within 3ºF of the target superheat
for the current load conditions. Overcharging and undercharging both have a considerable impact on efficiency and humidity removal so
the correct charge is critical to proper operation and creature comfort.
The total superheat method is the most accurate method of charging
fixed orifice or capillary tube systems. These systems are critically
charged and require correct airflow and accurate test instruments to
properly and accurately charge the system. An R410a and R22 charging calculator or the "Non TXV Charging Guide"
are required to determine the target total superheat. Because these
systems do not "control superheat" it is critical that the airflow be as
close to possible to correct prior to charging as the system will be
charged to the airflow. The total heat in the air will be the driving
factor for the evaporator performance so low or high airflow will have a
direct impact on the charge and the system performance.
Video Demonstration of Analog and Digital Method of Charging a System for Correct Subcooling.
Charging by weight is one of the most accurate and fastest methods of
charging a system that has a known refrigerant charge. Typically this
method requires an already evacuated system that is ready for charge, or
removal of the existing charge so the proper charge can be weighed in. Because exact length of the line set is often hard to determine within a few inches, we recommend the initial charge be determined by either the Total Superheat or the Subcooling method. If the exact charge is known, use a high accuracy high resolution digital scale to weigh in the entire charge. If using the Accutools DS220R scale, a CM500 charging module will allow the user to program in the charge and automatically shut off the refrigerant tank when the proper amount of refrigerant has been dispensed. Alternately, the FieldPiece Wireless Digital Scale is a great selection.
If provided by the manufacturer and the system is new and or completely clean, the system can be accurately charged by measuring the suction pressure, measuring outdoor air temperature and then adding charge until the liquid pressure is at the chart requirement. After the charge is complete, verify proper temperature split to assure acceptable system airflow. This is the common method used by Rheem and Rudd and others for initial charging. Because this method is pressure and temperature driven, cleanliness of the coils is critical to your success. Typically the chart supplied with the equipment is specifically for that piece of equipment and cannot be used with other models.
In order to verify charge by the approach method, the evaporator airflow must be properly set, and the evaporator coil and condenser coil completely clean. Find the required approach in the manufacturers installation instructions, which will be specific to the condenser you are servicing. Measure the outdoor air temperature entering the condenser from top to bottom of the coil outside of line of site of the sun and obtain an average condenser entering air temperature. Measure the liquid line temperature with an accurate clamp probe thermometer. Determine the temperature difference between the liquid line temperature and the outdoor air temperature. This temperature difference is the approach temperature. Adding refrigerant will decrease the approach and removing refrigerant will increase approach. Allow 10-15 minutes of operation for the approach to stabilize after refrigerant is added or removed.
After the airflow and the refrigerant charge have been set, verify that the system is operating at its rated capacity by measuring the actual airflow and measuring the change in enthalpy across the evaporator coil. Enthalpy is determined by converting the return and supply wet-bulb measurements using the enthalpy chart provided here or at the beginning of this document. Use the Commissioning Worksheet to determine the capacity of the system.
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