Under what condition is the pressure/temperature relationship for ammonia valid as per a PT chart?

The pressure/temperature relationship for ammonia as represented on a PT (pressure-temperature) chart is valid specifically when ammonia is in a saturated condition. Saturated conditions mean that the refrigerant exists in a mixture of both liquid and vapor phases at equilibrium. At this point, the pressure and temperature are directly related; for any given pressure, there is a corresponding saturation temperature, and vice versa.

When analyzing the PT chart, the key areas to note include the saturated region, where the refrigerant can transition between liquid and vapor states, and areas beyond this region, such as superheated vapor or subcooled liquid conditions. In the saturated state, the refrigerant is at its boiling point for a given pressure, which is critical for processes like refrigeration where phase change absorbs or releases heat.

While ammonia may also exist in superheated states or in high pressure, those conditions fall outside the reliable saturation parameters defined in a PT chart, making the saturated condition the correct and applicable context for understanding the pressure/temperature relationship.