What is the most likely cause of increased superheat temperature for refrigerating fluid entering a reciprocating compressor?

Increased superheat temperature for refrigerating fluid entering a reciprocating compressor is most likely caused by scale particles lodged in the thermal expansion valve (TEV). When scale particles obstruct the TEV, they hinder the flow of refrigerant through the valve. This restriction can cause an insufficient amount of refrigerant to enter the evaporator, leading to a higher superheat condition. Superheat represents the amount of heat added to the refrigerant vapor after it has fully evaporated. If the flow is diminished due to the obstruction, the refrigerant will not absorb enough heat in the evaporator, causing the vapor that enters the compressor to be hotter than normal because it has spent more time absorbing heat than it typically would.

The other factors such as refrigerant overcharge or low ambient temperature do not directly cause an increase in superheat. Overcharging may actually lead to lower superheat temperatures due to excessive refrigerant being present, while low ambient temperatures can influence system performance in different ways, but would not normally lead to an increase in superheat at the compressor inlet. Similarly, dirty compressor filters may affect the overall system efficiency and pressure drops, but are less directly related to the superheat at the compressor inlet compared to the blockage in the TEV.