How do the temperature lines run in the Sub-Cooled and Superheated regions of the enthalpy diagram?

In an enthalpy diagram, the temperature lines represent the relationship between temperature and enthalpy across different phases of a refrigerant. In the sub-cooled region, the temperature lines run vertically. This is because at any given pressure below the saturation point (where the refrigerant is fully liquid), increasing the enthalpy means that the temperature must increase as well, leading to a vertical trajectory on the diagram.

Conversely, in the superheated region, the temperature lines run horizontally. Here, within the confines of the saturated vapor state, an increase in enthalpy does not lead to an increase in pressure; the vapor can absorb heat without a change of state until it reaches the saturation point again, resulting in horizontal progression.

The correct answer highlights the distinct behavior of these regions in the diagram: vertical for sub-cooled and horizontal for superheated. This representation is crucial for understanding how refrigerants behave under different thermal conditions, which plays a vital role in designing and operating refrigeration and air conditioning systems.