Vapor Charging for Proper Superheat

Superheat is the heat added to refrigerant after all the refrigerant has changed to a vapor. By knowing the amount of superheat in the system suction line, you can tell if the system is properly charged. Also, maintaining superheat is critical because it ensures that no liquid refrigerant returns to the compressor, where it can cause damage and possible failure.

The superheat method can be used to check and adjust the charge in an operating system. It is only used for systems with a fixed-orifice metering device such as a capillary tube or metering piston. This method takes into consideration the operating conditions of the system and establishes a required superheat. The required superheat is then reached by adjusting the refrigerant charge in the system to obtain the correct superheat temperature in the system suction (vapor) line (Figure SP-4-8).

pic1 67 Vapor Charging for Proper Superheat

Required Superheat/Subcooling Calculators can be used when charging HCFC-22 systems (Figure SP-4-9). When used in conjunction with the temperatures and pressures measured in an operating system, this calculator is used to find the values for the system’s required superheat and the required vapor line temperatures. Complete instructions for charging by superheat are printed on the calculator.

pic1 68 Vapor Charging for Proper Superheat

A detailed procedure and illustrated examples for charging by superheat using the Superheat Calculator are provided later in this section. A brief overview of the procedure and the use of the calculator follows.

After the system has run for at least 15 minutes and operation has stabilized, the following measurements are taken:
• Indoor wet bulb temperature of the air entering the evaporator coil.
• Outdoor dry bulb temperature of the air entering the condenser coil.
• System suction (vapor) line pressure.
• System suction (vapor) line temperature.

The measured temperatures for the indoor and outdoor air entering the evaporator and condenser, respectively, are used with the calculator to find the required level of superheat (°F). Following this, the value for the required superheat and the measured system vapor line pressure are used with the calculator to find the required vapor line temperature (°F). Then, the actual vapor line temperature is compared to the required vapor line temperature to determine if an adjustment in the system refrigerant charge is needed. A tolerance of ± 5 ° F is allowed before any adjustment is required (Figure SP-4-10).

pic1 69 Vapor Charging for Proper Superheat

If the measured vapor line temperature is too high, vapor refrigerant must be added to lower the temperature of the vapor line. If the measured vapor line temperature is too low, remove (recover) refrigerant to raise the temperature of the vapor line. This procedure must be repeated each time refrigerant is either added or removed, because the system pressures and temperatures will change, but the required superheat remains the same.

If a calculator is not available, or you are working on a system that uses a refrigerant other than HCFC-22, the charging by superheat method can still be used. Superheat and suction line temperature tables are attached to the unit or given in the manufacturer’s instructions. Use them to find the required vapor line temperature for a correctly charged system.


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