7.2 Temperature Glide, Fractionation & Blends

7.2 Temperature Glide, Fractionation & Blends

Many modern refrigerants are multi-component blends formulated to replace phased-out CFCs and HCFCs. To properly charge, commission, and troubleshoot these systems, an HVAC contractor must understand the thermodynamic distinctions between pure compounds, azeotropic mixtures, and zeotropic blends, as well as the diagnostic implications of temperature glide and fractionation.

---

1. Pure Compounds, Azeotropes, and Zeotropes

Refrigerants are grouped into three primary thermodynamic categories based on how their molecular components behave during phase transitions:

A. Pure Compounds

• Thermodynamic Behavior: Composed of a single chemical molecule (e.g., R-22, R-134a, R-32, R-12). At a given pressure, a pure compound boils and condenses at a single, unchanging saturation temperature.
• Temperature Glide: Exactly $0.0^\circ\text{F}$.
• Charging Protocol: Can be charged into a system from a supply cylinder as either a vapor or a liquid without altering the chemical composition.

B. Azeotropic Blends (500-Series)

• Thermodynamic Behavior: A mixture of two or more refrigerants that forms a constant-boiling solution. At equilibrium, the liquid phase and vapor phase possess the exact same chemical formulation.
• Representative Blends: R-500 (73.8% R-12 / 26.2% R-152a), R-502 (48.8% R-22 / 51.2% R-115), and R-507A (50% R-125 / 50% R-143a).
• Temperature Glide: Negligible to zero ($0.0^\circ\text{F}$). Azeotropes condense and boil at a fixed temperature just like a pure compound.
• Charging Protocol: Can be transferred as either liquid or vapor without risk of composition shift.

C. Zeotropic Blends (400-Series)

• Thermodynamic Behavior: Blends composed of two or more refrigerants with different boiling points that maintain their individual phase-change behaviors. When heated at constant pressure, the constituent with the lowest boiling point (highest vapor pressure) vaporizes first, followed progressively by the higher-boiling constituents.
• Near-Azeotropes: Zeotropic blends exhibiting very small temperature glide. For example, R-410A has a glide of less than $0.3^\circ\text{F}$ ($0.15^\circ\text{C}$), and R-454B has a glide of approximately $1.5^\circ\text{F}$. In the field, these behave nearly like azeotropes in heat exchangers, but are chemically 400-series blends.
• High-Glide Zeotropes: Blends exhibiting substantial temperature glide. For example, R-407C (23% R-32 / 25% R-125 / 52% R-134a) has a glide of $9^\circ\text{F}$ to $11^\circ\text{F}$ ($5^\circ\text{C}$ to $6^\circ\text{C}$) across operating pressure ranges.

---

2. The Physics of Temperature Glide

Temperature glide is the observable temperature change that occurs during constant-pressure phase changes in zeotropic refrigerants:

Temperature Glide Definition: The numerical difference between the starting boiling temperature (Bubble Point) and the ending vaporization temperature (Dew Point) of a zeotropic blend at a constant pressure across an evaporator or condenser coil.

$\text{Temperature Glide} = T_{\text{dew (vapor)}} - T_{\text{bubble (liquid)}}$

Phase Progression Through System Heat Exchangers

1. In the Evaporator Coil: Saturated liquid enters the evaporator at the Bubble Point. As the refrigerant absorbs heat, the lower-boiling component (the "light end," e.g., R-32 in R-407C) evaporates first. As it travels through the coil tubing, the remaining liquid becomes increasingly rich in the higher-boiling component (the "heavy end," e.g., R-134a). Consequently, the boiling saturation temperature progressively climbs along the length of the coil from inlet to outlet, even if pressure remains constant.
2. In the Condenser Coil: Superheated discharge vapor enters the condenser and cools to the Dew Point, where the first liquid droplet condenses (rich in the higher-boiling component). As heat is rejected to outdoor air, the condensing saturation temperature progressively drops until the final vapor bubble condenses at the Bubble Point.

---

3. Bubble Point vs. Dew Point on P-T Cards

Standard Pressure-Temperature (P-T) charts for 400-series zeotropic blends list two distinct saturation temperature columns for every given pressure reading: the Bubble Point and the Dew Point.

Field Diagnostic Protocol & Worked Example

A technician evaluates a commercial rooftop air conditioning system charged with R-407C:

• Suction Line Measurements: Suction pressure = $65.0\text{ psig}$. Suction line pipe temperature = $52.0^\circ\text{F}$.
• Liquid Line Measurements: High-side liquid pressure = $240.0\text{ psig}$. Liquid line pipe temperature = $98.0^\circ\text{F}$.
• P-T Chart Reference for R-407C:
  • At $65.0\text{ psig}$: $\text{Bubble Point} = 29.5^\circ\text{F}$; $\text{Dew Point} = 40.0^\circ\text{F}$.
  • At $240.0\text{ psig}$: $\text{Bubble Point} = 110.0^\circ\text{F}$; $\text{Dew Point} = 121.0^\circ\text{F}$.

Calculations:

1. Evaporator Superheat: Always use the Dew Point ($40.0^\circ\text{F}$): $\text{Superheat} = T_{\text{suction line}} - T_{\text{dew}} = 52.0^\circ\text{F} - 40.0^\circ\text{F} = 12.0^\circ\text{F}$ (If the technician had mistakenly used the Bubble Point of $29.5^\circ\text{F}$, calculated superheat would appear to be an erroneous $22.5^\circ\text{F}$, leading them to misdiagnose a starving evaporator and overcharge the system).
2. Condenser Subcooling: Always use the Bubble Point ($110.0^\circ\text{F}$): $\text{Subcooling} = T_{\text{bubble}} - T_{\text{liquid line}} = 110.0^\circ\text{F} - 98.0^\circ\text{F} = 12.0^\circ\text{F}$ (If the technician had mistakenly used the Dew Point of $121.0^\circ\text{F}$, calculated subcooling would appear to be an erroneous $23.0^\circ\text{F}$, causing them to mistakenly remove refrigerant).

---

4. Fractionation Mechanics, System Leaks & Servicing Protocols

Fractionation is the physical separation of a zeotropic blend into its individual component parts due to differences in vapor pressure:

The Standing Vapor Leak Scenario

When an idle air conditioning system containing a high-glide zeotropic blend (such as R-407C) develops a slow pinhole leak in a vapor line or coil, the component with the highest vapor pressure (the lighter constituent, such as R-32) boils off and escapes at a faster rate than the less volatile constituent (such as R-134a).

• Thermodynamic Consequences: The refrigerant remaining inside the system becomes "fractionated"—disproportionately rich in heavy, lower-pressure components. This changes the chemical formulation, alters compression ratios, elevates compressor discharge temperatures, reduces cooling capacity by 15% to 20%, and renders the manufacturer's published P-T charts invalid.

Servicing Protocols: Low-Glide vs. High-Glide Blends

• Near-Azeotropes / Low-Glide Blends (R-410A, R-454B, R-404A): Because temperature glide is minimal ($< 0.3^\circ\text{F}$ to $1.5^\circ\text{F}$), the composition change during a leak is negligible (less than 1% to 2%). Industry standards and EPA guidelines permit repairing the leak and topping off the remaining charge with fresh liquid refrigerant.
• High-Glide Zeotropes (R-407C, R-401A, R-402A): If a system loses more than 20% to 30% of its charge from a vapor leak while idle, the remaining charge has suffered severe fractionation. The contractor must recover the entire remaining altered charge, repair the leak, evacuate the system to 500 microns, and recharge with fresh virgin refrigerant weighed in as a liquid to the manufacturer's nameplate specification.

---

5. The Liquid Charging Mandate for 400-Series Blends

Inside any refrigerant supply cylinder containing a 400-series zeotropic blend, the vapor space above the liquid is fractionated: it contains a higher concentration of the low-boiling components than the liquid sitting at the bottom.

          ZEOTROPIC REFRIGERANT CYLINDER PHENOMENON
     ┌──────────────────────────────────────────────────┐
     │  VAPOR HEADSPACE: Fractionated!                  │
     │  (Enriched in high-volatility 'light ends')       │
     ├~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~┤
     │  LIQUID PHASE: Correct Chemical Blend Ratio!     │
     │  (Must be charged strictly from this layer)      │
     └──────────────────────────────────────────────────┘

Mandatory Liquid Withdrawal

Follow the cylinder and refrigerant manufacturer's instructions. Zeotropic blends are generally withdrawn as liquid to preserve composition, then introduced in a manner that prevents liquid damage to a running compressor; series number alone is not a complete safe-charging instruction.

Protecting the Compressor During Liquid Charging

Compressors are positive displacement vapor pumps; drawing liquid refrigerant directly into the suction service port causes hydraulic shock ("liquid slugging"), broken reed valves, and shattered scroll wraps. To charge liquid safely into the low-pressure side while the system is running:

1. Invert cylinders without internal dip tubes (or connect to the dedicated "Liquid" valve on dip-tube cylinders).
2. Install a liquid throttle restrictor valve or use the manifold gauge hand valve to meter the liquid through an orifice.
3. The restricted orifice creates a rapid pressure drop that flashes the liquid into vapor before it passes through the service valve into the compressor suction port.