Difference Between Evaporator Superheat and Total Superheat

 

1. Evaporator Superheat:

   • Definition: Evaporator superheat is the temperature increase of the refrigerant above its saturation temperature as it exits the evaporator coil. This measurement is taken immediately after the refrigerant has completely boiled off from liquid to vapor within the evaporator.

   • Measurement:

     • Suction Pressure: Measure the suction pressure at the outlet of the evaporator (usually at the evaporator's exit or the low-side service port).
     • Saturation Temperature: Convert this pressure to a temperature using a PT (Pressure-Temperature) chart for the specific refrigerant.
     • Actual Refrigerant Temperature: Measure the actual temperature of the refrigerant vapor at the same location.
     • Calculation: Subtract the saturation temperature from the actual refrigerant temperature.
     $$\text{Evaporator Superheat} = \text{Actual Refrigerant Temperature} - \text{Saturation Temperature}$$

   • Purpose: It is used to ensure that the refrigerant has fully vaporized before it exits the evaporator, preventing liquid refrigerant from reaching the compressor, which could cause damage.

2. Total Superheat:

   • Definition: Total superheat is the temperature increase of the refrigerant from the point it exits the evaporator until it reaches the compressor. This includes both the evaporator superheat and any additional superheat gained as the refrigerant travels through the suction line toward the compressor.

   • Measurement:

     • Suction Pressure: Measure the suction pressure at the compressor's inlet or the low-side service port.
     • Saturation Temperature: Convert this pressure to a temperature using a PT chart.
     • Actual Refrigerant Temperature: Measure the actual temperature of the refrigerant vapor at the compressor inlet or the service port.
     • Calculation: Subtract the saturation temperature from the actual refrigerant temperature.
     $$\text{Total Superheat} = \text{Actual Refrigerant Temperature at Compressor Inlet} - \text{Saturation Temperature at Compressor Inlet}$$

   • Purpose: Total superheat is often used for system diagnostics and charging. It provides a broader view of the refrigerant’s condition as it approaches the compressor, ensuring that the system is properly charged and that the refrigerant is in a fully vaporized state before compression.

When They Are Used:

• Evaporator Superheat:

  • Usage: It is commonly used when adjusting or checking the thermostatic expansion valve (TXV) or other metering devices to ensure proper refrigerant flow and complete vaporization in the evaporator.

• Total Superheat:

  • Usage: It is typically used during system diagnostics, troubleshooting, and when charging the system. Total superheat helps determine if the refrigerant charge is correct and if the system is operating efficiently.

Summary:

• Evaporator Superheat focuses on the temperature rise of the refrigerant as it exits the evaporator coil and ensures no liquid refrigerant enters the compressor.
• Total Superheat encompasses the entire journey of the refrigerant vapor from the evaporator outlet to the compressor inlet and is a broader indicator used for system diagnostics and charging.