If the PT chart indicates bubble point and dew point values, then the refrigerant is ________.

If the PT chart indicates bubble point and dew point values, then the refrigerant is ________.

• A. A pure refrigerant
• B. A zeotropic blend with temperature glide
• C. An azeotropic blend
• D. A nonflammable gas

Answer: (B)

The key idea is that seeing separate bubble point and dew point values on a PT chart means the refrigerant behaves as a mixture whose composition changes during phase change, i.e., a zeotropic blend with temperature glide. In a zeotropic blend, different components boil at different temperatures, so as the mixture boils the vapor and liquid have different compositions. This creates a temperature difference between the bubble point (where the liquid starts to boil) and the dew point (where vapor starts to condense) at the same pressure. That separation of the two curves on the chart is the telltale sign of a zeotropic blend. An azeotropic blend would exhibit no glide; bubble point and dew point coincide. A pure refrigerant would boil at a single temperature at a given pressure, so you wouldn’t have two distinct values either. A nonflammable gas doesn’t directly relate to the presence of bubble vs dew points on the chart.

The key idea is that seeing separate bubble point and dew point values on a PT chart means the refrigerant behaves as a mixture whose composition changes during phase change, i.e., a zeotropic blend with temperature glide. In a zeotropic blend, different components boil at different temperatures, so as the mixture boils the vapor and liquid have different compositions. This creates a temperature difference between the bubble point (where the liquid starts to boil) and the dew point (where vapor starts to condense) at the same pressure. That separation of the two curves on the chart is the telltale sign of a zeotropic blend.

An azeotropic blend would exhibit no glide; bubble point and dew point coincide. A pure refrigerant would boil at a single temperature at a given pressure, so you wouldn’t have two distinct values either. A nonflammable gas doesn’t directly relate to the presence of bubble vs dew points on the chart.