In what condition would a low refrigerant charge cause high superheat?

A low refrigerant charge can indeed lead to high superheat conditions, particularly when utilizing an automatic expansion valve (AEV). The AEV controls the amount of refrigerant entering the evaporator based on the superheat of the vapor exiting the evaporator. When there is insufficient refrigerant in the system due to a low charge, the evaporator cannot absorb enough heat, causing the refrigerant that is present to heat up more than normal before it exits the evaporator. This results in increased superheat since superheat is the temperature difference between the refrigerant vapor and its saturation temperature.

In this scenario, the AEV is unable to adequately adjust for the low refrigerant charge, often leading to an inadequate flow of refrigerant into the evaporator, which exacerbates the high superheat condition. If the refrigerant charge were normal, the AEV would maintain a proper balance, preventing high superheat levels from occurring.

While the other options may involve different methods of refrigerant flow regulation, they do not specifically explain the direct relationship between low charge and high superheat as effectively as the AEV does. For instance, a capillary tube is a fixed orifice and does not react to superheat changes in the same way as the AEV