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

When the two lines connected to the expansion valve in a vapor-cycle cooling system are essentially the same temperature, it suggests that the expansion valve is not metering the refrigerant effectively. In a properly functioning system, a temperature difference between the lines would be expected. The evaporator absorbs heat and causes the refrigerant to evaporate, resulting in a drop in pressure and temperature before it enters the expansion valve. If both lines are at similar temperatures, it indicates that the refrigerant is not experiencing the necessary pressure drop that occurs due to the valve's metering function. This lack of temperature difference typically means that the refrigerant is not being properly regulated through the expansion valve, which can impair the overall cooling efficiency of the system.

In a well-functioning system, the expansion process should cool the refrigerant significantly, leading to a clear temperature difference between the high-pressure (inlet) and low-pressure (outlet) sides of the valve. Therefore, a situation where both lines are nearly the same temperature points directly to issues related to the proper operation of the expansion valve.