Bubcool and Dewperheat (Bubble and Dew Point explained)

When using the RefTools app, you’ll notice that you can’t switch between bubble and dew point for single-component refrigerants (like R-22); the reason for that is that single-component refrigerants don’t have glide. On the other hand, refrigerant blends have glide, and you can flip the switch between the dew and bubble point on refrigerants like R-410A (although that one has very little glide). Refrigerant blends with no glide are called azeotropes; ones with very little glide (like R-410A) are near-azeotropes. However, some refrigerants have significant glide and are known as zeotropes or zeotropic refrigerants.

The dew point and bubble point have to deal with saturation, which is the point at which fluids are changing state (liquid to vapor or vice versa), and the fluids obey the pressure-temperature relationship. Refrigerant blends with glide have a range of saturation temperatures and pressures rather than a clear-cut temperature-pressure relationship. The bubble point is the lower temperature of the two; the dew point is the highest end of that range.

A blend in the evaporator will begin boiling at the bubble point. On the RefTools app, you can see that R-407C’s bubble point at ~63 PSI is 28.9 degrees (F). The dew point is significantly higher, 40 degrees. Those values indicate that the temperature at the evaporator inlet should be 28.9 degrees for a 40-degree outlet temperature.

We use the bubble point to calculate subcooling (subcool), and you can remember that with “bubcool.” On the other hand, we use the dew point to calculate superheat, so we call that “dewperheat.”