Bubble point vs dew point (temperature glide)
Learn bubble point, dew point, and temperature glide for refrigerant blends (zeotropic mixtures), and how to interpret saturation readings safely.
For many refrigerant blends (zeotropic mixtures), "saturation" is not a single temperature at a given pressure. Instead, phase change can occur over a temperature range. Two terms describe the ends of that range: bubble point and dew point.
Definitions (at a fixed pressure)
- Bubble point: the temperature where the first bubble of vapor appears when heating a saturated liquid mixture. This is sometimes called the "bubble temperature".
- Dew point: the temperature where the first drop of liquid appears when cooling a saturated vapor mixture. This is sometimes called the "dew temperature".
What "temperature glide" means
Temperature glide is the gap between dew and bubble temperatures at the same pressure. When the glide is non-trivial, evaporating/condensing in a heat exchanger can happen across a range of temperatures, not at a single flat "saturation temperature".
Why it matters (practical implications)
- Interpreting PT readings: pressure + temperature can map to different mixture states depending on whether you mean bubble, dew, or an in-between quality.
- Superheat/subcooling targets: for blends, the reference "Tsat" depends on whether you're using dew or bubble conventions and where you measure.
- Heat exchanger intuition: glide changes how you reason about approach temperatures and where phase change starts/ends along a coil.
Using FluidTool (safe way to explore)
In many property libraries, you can represent the two "ends" of saturation using quality:
- Q = 0 (saturated liquid) corresponds to the bubble point end.
- Q = 1 (saturated vapor) corresponds to the dew point end.
In FluidTool, select a refrigerant and use a Two-phase input pair (P + Q or T + Q) to probe the saturated region. Compare results at Q=0 and Q=1 at the same pressure to see whether a fluid behaves like a "single Tsat" pure fluid or shows meaningful glide.
Common pitfalls
- Assuming a single Tsat: for blends with glide, "Tsat(P)" depends on which saturation endpoint you mean.
- Mixing conventions: different charts, tools, and procedures may report dew/bubble differently. Always confirm the convention before comparing numbers.
- This is not a service manual: follow OEM/manufacturer procedures and local codes for any field work.
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