If the two lines connected to the expansion valve are essentially the same temperature, what does this indicate in a vapor-cycle cooling system?

In a vapor-cycle cooling system, if the two lines connected to the expansion valve are essentially the same temperature, it indicates that the expansion valve is not metering refrigerant properly. This situation suggests that the expansion valve is not allowing the proper amount of refrigerant to flow into the evaporator. Ideally, one line (the inlet from the liquid line) should be cooler than the other (the outlet into the evaporator), reflecting the pressure drop and the refrigerant's phase change as it absorbs heat.

When both lines are at the same temperature, this lack of temperature differential implies that refrigerant is not evaporating effectively, which could be due to the expansion valve being stuck in an open position or receiving insufficient feedback from the system to control the flow adequately. This condition prevents the system from achieving the desired cooling performance, pointing directly to an issue with the expansion valve's metering function.

This situation does not typically reflect normal function, nor does it necessarily indicate a blockage or complete evaporation of the refrigerant. A blockage would manifest as temperature differences due to restricted flow, and the complete evaporation of refrigerant generally wouldn't keep both lines at the same temperature either.