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Unit 3 — Refrigeration System Fundamentals & Maintenance
Section 3 — Pressure and Temperature Relationship

3.3 — Blended Refrigerants & Temperature Glide

Blended refrigerants don’t play by the same rules as pure refrigerants. Instead of a single boiling point at a given pressure, they transition through a range of temperatures — which is why their P–T charts show two values, and why you need to choose the right one for each calculation.

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3.3.1 — Pure Refrigerants vs. Blended Refrigerants

A pure refrigerant (like R‑22 or R‑134a) consists of a single chemical compound. At any given pressure, it has exactly one saturation temperature — making its P–T chart a single column of numbers.

A blended refrigerant is a mixture of two or more refrigerants, each with its own molecular weight and boiling characteristics. Because the lighter (more volatile) component evaporates faster than the heavier one, the blend does not change state at a single temperature. Instead, it transitions gradually through a range, which is called its temperature glide.

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Zeotropic Blend

A refrigerant mixture whose components do not share an identical boiling point and therefore exhibit temperature glide. Examples: R‑407C (R‑32 / R‑125 / R‑134a), R‑407A, R‑404A (near-zeotropic).

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Near-Azeotropic Blend

A mixture whose components have very similar boiling points, resulting in negligible temperature glide (<2°F). Treated like a pure refrigerant for P–T purposes. Example: R‑410A (R‑32 / R‑125).

3.3.2 — Temperature Glide — What It Is and Why It Matters

Temperature glide is the difference in temperature between the point where phase change begins and the point where it ends, at a constant pressure. In an evaporator coil, this means the refrigerant enters the coil at one temperature and finishes boiling at a slightly higher temperature.

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R‑407C Glide

At low-side pressures typical of air conditioning, R‑407C has a temperature glide of approximately 9–11°F (5–6°C). The refrigerant enters the evaporator slightly below the dew point and finishes boiling at the bubble point.

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Glide in the Condenser

In the condenser, the refrigerant enters as vapour and condenses as it gives up heat. The heavier component condenses first, and the process finishes at a slightly lower temperature — glide runs in the opposite direction compared to the evaporator.

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Why glide is generally small enough to manage

While temperature glide sounds complicated, in practice you simply need to know which saturation temperature (bubble or dew) to use for each calculation. Once you learn that rule, blended refrigerants are no harder to diagnose than pure refrigerants.

3.3.3 — Bubble Point & Dew Point — The Two Chart Temperatures

Because a blended refrigerant has a range of saturation temperatures, its P–T chart shows two temperatures for each pressure. Choosing the correct one is essential for accurate superheat and subcooling calculations.

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Bubble Point

The temperature at which the first bubble of vapour forms in a liquid — i.e., where boiling begins. At or below the bubble point, the refrigerant is entirely liquid.

Use for: SUBCOOLING (liquid line)

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Dew Point

The temperature at which the last drop of liquid evaporates — i.e., where boiling is complete. At or above the dew point, the refrigerant is entirely vapour.

Use for: SUPERHEAT (suction line)

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Analogy — The Pot of Mixed Soup

Imagine heating a pot of soup made of two ingredients that boil at different temperatures. The first tiny bubbles appear at a relatively low heat (the bubble point) as the more volatile ingredient begins to vaporise. Only after more heat is added does the entire pot reach full boil and turn to steam (the dew point).

A blended refrigerant behaves exactly the same way inside an evaporator coil, just under controlled pressure rather than open-air heating.

Bubble Point → Liquid Side
  • Found at high-side pressure
  • Used for subcooling calculation
  • Subcooling = Tbubble − Tliquid line
  • Confirms all liquid leaving condenser
Dew Point → Vapour Side
  • Found at low-side pressure
  • Used for superheat calculation
  • Superheat = Tsuction − Tdew
  • Confirms fully evaporated vapour at compressor

3.3.4 — R‑407C — P–T Values & Worked Example

R‑407C (a blend of R‑32 / R‑125 / R‑134a) was widely used as a drop-in replacement for R‑22 in air conditioning systems. Its temperature glide is approximately 10°F (5.5°C), so both bubble and dew point values appear on its P–T chart.

Pressure (psig) Dew Point (°F) Bubble Point (°F) Glide (°F)
6820128
9830228
13840328
19055478
21062548
24070628
29085778
34096888
360101938
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Worked Example — R‑407C Subcooling

Scenario: High-side pressure = 210 psig on an R‑407C system.

Step 1 — Look up the chart: At 210 psig the chart shows Dew = 62°F, Bubble = 54°F.

Step 2 — Choose the correct temperature: For subcooling on the liquid line, use the bubble point → 54°F.

Step 3 — Measure liquid line temperature: Contact thermometer reads 47°F.

Step 4 — Calculate subcooling:

Subcooling = 54°F − 47°F = 7°F subcooling

Interpretation: A subcooling value of 10–15°F is typical for most systems. At 7°F the system is slightly low — worth checking charge and liquid line for flash gas.

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Worked Example — R‑407C Superheat

Scenario: Low-side pressure = 138 psig on the same system.

Step 1 — Look up the chart: At 138 psig the chart shows Dew = 40°F, Bubble = 32°F.

Step 2 — Choose the correct temperature: For superheat on the suction line, use the dew point → 40°F.

Step 3 — Measure suction line temperature: Insulated probe reads 52°F.

Step 4 — Calculate superheat:

Superheat = 52°F − 40°F = 12°F superheat

Interpretation: 12°F is well within the normal 10–15°F target for this type of system. The evaporator is fully utilised and the compressor is protected.

3.3.5 — Fractionation — The Risk of Topping Up a Blend

Because the components of a zeotropic blend have different boiling points, they can separate under certain conditions. If a blended refrigerant leaks as vapour from a system, the lighter, more volatile component escapes faster than the heavier one — leaving behind a mixture with a different composition than the original. This is called fractionation.

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Never Top Up a Fractioned Blend

If a zeotropic blended refrigerant system has suffered a vapour leak, the remaining refrigerant is no longer the correct blend ratio. Adding more of the original blend will not restore the correct composition.

Correct procedure: Recover all remaining refrigerant, repair the leak, evacuate the system to 500 microns or below, and recharge with a full, new charge of the correct blended refrigerant.

Exception — liquid leaks: If the leak was a liquid leak (e.g., a fitting that dripped liquid refrigerant), fractionation is minimal because both components leave at the same ratio as the blend. In that case, topping up may be acceptable — but verify with the equipment manufacturer.

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Charging a blend — always charge as liquid

Blended refrigerants should be charged from the cylinder as a liquid to ensure the correct mixture ratio enters the system. Invert liquid-port cylinders or use the liquid valve. Adding vapour risks fractionation in the cylinder itself.

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