The definitions
ASHRAE's terminology and Copeland's changeover guidelines give the working definitions:
- Bubble point: the liquid saturation temperature of a refrigerant at a given pressure, where liquid refrigerant first begins to boil.
- Dew point: the saturated vapor temperature at that pressure. Copeland labels dew-point tables as saturated vapor tables, and ASHRAE notes that a zeotropic blend's bubble point is lower than its dew point.
- Glide: the difference between the starting and ending temperatures of boiling or condensing at constant pressure, not counting any superheat or subcooling.
- Zeotropic blend: a blend whose liquid and vapor have different compositions at equilibrium, so its temperature changes as it boils or condenses at constant pressure.
- Azeotropic blend: a blend whose liquid and vapor compositions are the same at a given pressure, so its temperature stays constant as it boils or condenses at that pressure, like a single refrigerant.
Single-component refrigerants such as R-22 and R-32, both described that way by Arkema, have one saturation temperature per pressure. Their bubble and dew points are the same.
How big the glide gets
Glide depends on the blend and changes slightly with pressure. The table gives each blend's dew pressure at 40°F, then the glide at a pressure with a dew point near 40°F and at one with a bubble point near 100°F, as example evaporating and condensing conditions.
| Refrigerant | Dew pressure at 40°F | Glide, evaporating | Glide, condensing |
|---|---|---|---|
| R-407C | 63.2 psig | 11.1°F | 9.0°F |
| R-427A | 61.2 psig | 10.5°F | 8.3°F |
| R-448A | 71.4 psig | 10.4°F | 8.8°F |
| R-449A | 71.6 psig | 9.6°F | 8.1°F |
| R-407A | 69.4 psig | 10.0°F | 7.9°F |
| R-422B | 59.4 psig | 7.6°F | 5.3°F |
| R-454B | 107.0 psig | 2.4°F | 2.4°F |
| R-452B | 108.8 psig | 2.1°F | 2.1°F |
| R-404A | 85.4 psig | 0.9°F | 0.6°F |
| R-410A | 118.4 psig | 0.2°F | 0.2°F |
| R-507A | 89.2 psig | 0.0°F | 0.1°F |
The blends near the top of the table have the largest glides. R-454B and R-452B have a small glide of a few degrees. Arkema describes R-410A and R-404A as near-azeotropic, and their glide is small enough that one column is close for field work. Arkema describes R-507A as an azeotropic blend.
Which column to use
ASHRAE defines superheat as a vapor temperature minus the dew-point temperature at the same pressure, and subcooling as the bubble-point temperature minus a liquid temperature. That settles it:
- Superheat (suction line): dew point. The vapor leaving the evaporator has finished boiling, and the last liquid boils off at the dew point. The superheat calculator uses the dew column for blends.
- Subcooling (liquid line): bubble point. The liquid leaving the condenser finished condensing at the bubble point. The subcooling calculator uses the bubble column for blends.
The cost of getting it wrong is the full glide. On R-407C at 63 psig, the dew point is 39.9°F and the bubble point is 28.8°F. Using the bubble column for superheat overstates it by 11.1°F, enough to make a correctly charged system look badly undercharged. Using the dew column for subcooling on the liquid side makes the same size error in the other direction.
What glide does inside the coil
ASHRAE defines a zeotrope as a blend whose composition and saturation temperature change as it boils or condenses at constant pressure. Copeland's changeover guidelines say that, because of glide, refrigerant entering the evaporator at a given suction pressure is about 10°F colder than the vapor leaving it, not counting superheat, for blends such as R-407C, R-448A and R-449A. That is why Copeland says low-pressure controls set for R-22 may need new cut-in and cut-out points. In the condenser the reverse happens: condensing starts at the dew point and finishes at the bubble point.
Because of this, some literature quotes a single average, and Copeland sets pressure controls for the average evaporator temperature. ASHRAE's definition of saturated condensing temperature for zeotropic refrigerants is the arithmetic average of the dew and bubble temperatures at the condenser inlet pressure. When a spec sheet, control or compressor rating gives one saturation temperature for a blend, check whether it means dew, bubble or average before comparing it with your readings.
Charging and leaks with zeotropic blends
Because the liquid and vapor of a zeotropic blend have different compositions, vapor drawn off the top of a cylinder is not the blend on the label. Chemours' R-407C retrofit guidelines say to remove liquid refrigerant from the cylinder when charging, Arkema says R-427A must be charged in the liquid phase, and AHRI's A2L best practices say a 400-series refrigerant such as R-454B must leave the cylinder as liquid. Single-component refrigerants such as R-32 can be charged as vapor or liquid.
The same composition difference matters after a leak, and manufacturers differ on what to do. Chemours says R-454B's very low glide lets it be topped off after leaks, and that its R-422B product (Freon NU-22B) can be topped off during service without removing the whole charge. Daikin says topping off blends is generally not recommended and calls for evacuating and recharging instead. Follow the refrigerant and equipment manufacturer's guidance for the system in front of you.