Temperature glide calculator

How it works

Temperature glide is “the temperature difference between the saturated vapor temperature and the saturated liquid temperature at a constant pressure”, in John Tomczyk’s definition (ACHR News). The saturated vapor temperature is the dew point, where the last drop of liquid boils off. The saturated liquid temperature is the bubble point, where the first bubble of vapor forms.

The tool takes your gauge pressure and the optional elevation. It converts the reading to the sea-level basis the PT data use, then reads both the bubble and the dew column of the blend’s 1°F PT table, interpolating linearly. It reports:

• Bubble point (liquid): use for the liquid line and subcooling.
• Dew point (vapor): use for suction and superheat.
• Midpoint: the average of the two. Tomczyk puts the average evaporator temperature “at the midpoint of the effective temperature glide.”
• Glide: dew minus bubble.

Only blends are in the picker. Single-component refrigerants such as R-22, R-32, R-134a and R-290 have zero glide: bubble and dew are the same temperature.

Why zeotropic blends glide

A blend is a mix of refrigerants with different boiling points. As it evaporates or condenses, the more volatile component goes to vapor first, so the vapor and the liquid do not have the same makeup. Tomczyk describes this as “a change in their composition by preferential evaporation or condensation of the more or less volatile component in the blend”, which is fractionation.

As boiling goes on, the liquid left behind holds more of the less volatile components, and its boiling temperature rises at the same pressure. So in an evaporator at constant pressure, the refrigerant temperature rises as it boils and reaches the dew point when the last liquid is gone. In a condenser the refrigerant starts condensing at the dew point, its temperature falls as it condenses, and it is fully liquid at the bubble point.

Glide varies widely from blend to blend. Our data show the range:

The full list for every blend is in the table under the tool. Near-azeotropes such as R-410A glide so little that bubble and dew are within a few tenths of a degree. The azeotrope R-507A glides less than a tenth of a degree.

Worked example

The tool opens with R-454B at 108 psig.

1. Dew pressures: 107.49 psig at 40°F, 109.60 at 41°F. Dew point = 40 + (108 − 107.49) ÷ 2.11 = 40.2°F.
2. Bubble pressures: 106.77 psig at 37°F, 108.88 at 38°F. Bubble point = 37 + (108 − 106.77) ÷ 2.11 = 37.6°F.
3. Glide = 40.2 − 37.6 = 2.7°F (from the unrounded values 40.24 − 37.58). Midpoint = 38.9°F.

A blend with large glide: R-448A at 40 psig gives a dew point of 16.5°F and a bubble point of 5.4°F, so the glide is 11.2°F. That is Honeywell’s suction example, where their chart gives “~16°F” dew.

Which value to use

• Superheat: dew point. Tomczyk: “When calculating superheat values, HVAC service technicians must always use the dew point values from the chart.” The superheat calculator does this.
• Subcooling: bubble point. “When calculating subcooling values, HVAC service technicians must use the bubble point values from the chart.” The subcooling calculator does this.
• Evaporator or condenser temperature for coil performance (approach, TD): the midpoint, per Tomczyk.

The Chemours and Honeywell charts are laid out the same way: saturated vapor for superheat, saturated liquid for subcooling. Mix them up on R-448A and the superheat in the example above reads 11°F too high.

Why a single glide number is wrong

Glide changes with pressure. R-448A glides 11.2°F at 40 psig and 9.6°F at 196 psig (dew 94.8°F, bubble 85.2°F). R-407C glides 11.1°F at a 40°F evaporator and 9.0°F at a 100°F condenser.

Glide also depends on the reference point. “Glide at 100°F” can mean the glide at the pressure where the bubble point is 100°F, or at the pressure where the dew point is 100°F. For R-407C those come out at 9.0°F and 9.3°F. A spec sheet’s single glide figure is valid only at its stated condition. That is why this tool computes glide at your pressure and the table states both conditions.

Some older printed charts give only one column. The Chemours guide lists saturated vapor at 50°F and below and saturated liquid at 52°F and above. Read such a chart at the “wrong” end and the error equals the glide.

Liquid charging and fractionation

Because vapor and liquid differ in composition, a zeotropic blend should leave the cylinder as liquid. Pulling vapor off the top draws the more volatile components first. The system gets a different mix from the one on the label, and the cylinder’s remaining contents shift as well. Charge liquid into the system the way the refrigerant and equipment manufacturer’s instructions describe. The subcooling calculator reminds you to add blends as liquid when it calls for refrigerant.

Leaks follow the same physics. Wherever vapor escapes from a space holding both liquid and vapor, it can carry away more of the volatile components. How far the composition moves depends on the blend’s glide and on the leak, and a field gauge cannot measure it. After a significant leak on a high-glide blend, bubble and dew readings may not match the chart exactly. Follow the refrigerant manufacturer’s guidance on topping off versus recovering and recharging. Near-azeotropes with a few tenths of a degree of glide are much less exposed.

Reading the result

• A glide near zero means the blend acts like a single fluid. Either column works.
• With a glide of several degrees, always pick the column for the job: dew for suction, bubble for liquid.
• A pressure outside the chart returns a message instead of a value. Check the refrigerant selection and units.

Common mistakes

• Using one “glide” figure from a brochure at every pressure.
• Using the midpoint for superheat or subcooling. CEC defines superheat against evaporator saturation temperature (RA3.2), and the refrigerant makers’ charts put that at the dew point.
• Vapor-charging a zeotropic blend from the top of the cylinder.
• Forgetting elevation at altitude. Enter it so the gauge reading is converted to the chart’s sea-level basis. More in how to read a PT chart.