In a DX evaporator, the refrigerant should leave in which condition?

In a DX (direct expansion) evaporator, the refrigerant is designed to leave in a superheated state. This is crucial for ensuring that the refrigerant vapor is fully vaporized before it enters the compressor. If the refrigerant were to leave in a saturated state, there would be a mix of liquid and vapor, which could potentially lead to liquid floodback to the compressor, causing damage or inefficiencies.

Leaving the evaporator as superheated ensures that the refrigerant is fully gaseous, which not only protects the compressor but also optimizes the thermodynamic efficiency of the cycle. Additionally, operating in the superheated condition allows for improved heat absorption from the cooling load, making the overall system more efficient.

In contrast, sub-cooled refrigerant, which is liquid below its saturation temperature, would indicate that the refrigerant has not absorbed the necessary heat, resulting in a lower efficiency of heat transfer. The concept of trans-critical refers to conditions where refrigerants operate above their critical pressure and temperature, which is not typical for standard DX evaporators and is more relevant to specialized applications using certain refrigerants. Hence, superheated is the ideal condition for the refrigerant to leave a DX evaporator.