Blends

Refrigerant blends consist of mixtures of two or more pure refrigerants formulated to achieve specific thermophysical properties, environmental characteristics, or system performance objectives. These mixtures exhibit unique phase-change behavior that distinguishes them from pure compounds and directly impacts system design, operation, and service procedures.

Blend Classification Systems

ASHRAE 400/500-Series Nomenclature

Refrigerant blends receive numeric designations based on composition:

Letter Suffixes for Blend Variants

Different compositions of the same component refrigerants receive sequential letter designations:

• R-404A: HFC-125/143a/134a (44/52/4%)
• R-404B: Different composition, same components (alternative formulation)

Zeotropic Blends

Phase-Change Characteristics

Zeotropic mixtures exhibit temperature glide during constant-pressure phase transitions. The dew point and bubble point temperatures differ, creating a temperature range over which evaporation or condensation occurs.

Temperature Glide Definition:

Glide = T_dewpoint - T_bubblepoint

Physical Mechanisms

During phase change at constant pressure:

1. Initial Evaporation - Liquid composition at bubble point differs from vapor composition at dew point
2. Progressive Composition Shift - More volatile components preferentially evaporate first
3. Final Evaporation - Remaining liquid becomes enriched in less volatile components

This behavior arises from differences in vapor pressure among blend components at any given temperature.

Temperature Glide Magnitude

Glide values vary significantly among zeotropic blends:

R-410A exhibits near-azeotropic behavior despite being classified as zeotropic due to minimal glide.

System Design Implications

Heat Exchanger Performance:

Temperature glide affects heat transfer in evaporators and condensers:

• Counterflow Advantage - Glide enables closer temperature approach in counterflow heat exchangers
• LMTD Calculation - Logarithmic mean temperature difference calculations must account for non-isothermal phase change
• Capacity Impact - Effective heat transfer coefficient varies through phase-change region

Charging Procedures:

Zeotropic blends require liquid charging to maintain design composition:

• Vapor Charging Prohibited - Preferential vaporization alters blend composition in cylinder and system
• Liquid Phase Required - Charge as liquid through suction service port with system off or through liquid line

Fractionation Phenomena

Fractionation occurs when blend composition differs between liquid and vapor phases or between system locations.

Leakage-Induced Fractionation:

When zeotropic blends leak from a system:

1. Vapor-Phase Leak - More volatile components escape preferentially
2. Composition Drift - Remaining charge becomes enriched in less volatile components
3. Performance Degradation - System capacity, efficiency, and pressure relationships change
4. Topping-Off Prohibited - Adding refrigerant to partially leaked system creates non-homogeneous mixture

Recovery-Induced Fractionation:

During refrigerant recovery:

• Initial Recovery - Vapor phase removed first contains higher proportion of volatile components
• Progressive Shift - Recovery cylinder composition differs from original system charge
• Complete Recovery Required - Partial recovery leaves non-representative composition

Circulating Charge Distribution:

In operating systems:

• Evaporator Outlet - Vapor enriched in volatile components
• Liquid Line - Composition matches design specification
• Accumulator Risk - Trapped liquid may have altered composition after repeated cycles

Common Zeotropic Blends

Azeotropic Blends

Phase-Change Behavior

Azeotropic mixtures behave as single-component refrigerants during phase transitions. The liquid and vapor phases have identical composition at the azeotropic point, resulting in:

• Zero Temperature Glide - Single saturation temperature at given pressure
• Constant Composition - No preferential vaporization of components
• No Fractionation - Leakage does not alter remaining charge composition

Thermodynamic Basis

Azeotropy occurs when the mixture’s total vapor pressure exhibits an extremum (maximum or minimum) at a specific composition. For refrigerant applications, minimum-boiling azeotropes are typical.

At the azeotropic composition:

(∂P/∂x)_T = 0

where P is total pressure, x is liquid mole fraction, and T is temperature.

Advantages for Service

Azeotropic blends simplify system service:

• Vapor Charging Acceptable - Composition remains constant regardless of phase
• Partial Leak Tolerance - Remaining charge maintains original composition
• Topping-Off Permitted - Additional refrigerant can be added without composition concerns
• Simplified Recovery - Recovery method does not affect composition

Common Azeotropic Blends

Near-Azeotropic Blends

Some zeotropic blends exhibit temperature glide less than 0.5 K, functionally behaving as azeotropes:

• R-410A - Glide of 0.17 K allows vapor charging and simplified service
• R-508A - Glide of 0.02 K, used in ultra-low temperature applications

These blends combine the formulation flexibility of zeotropes with the service simplicity of azeotropes.

Composition Tolerance and Quality Control

ASHRAE Standard 34 specifies composition tolerances for refrigerant blends:

Typical Tolerance Ranges:

• Major components (>20%): ±1.0 to ±2.0 mass%
• Minor components (5-20%): ±0.5 to ±1.0 mass%
• Trace components (<5%): ±0.2 to ±0.5 mass%

Manufacturing precision ensures consistent:

• Thermodynamic properties
• System performance
• Safety characteristics (flammability, toxicity)
• Environmental metrics (GWP, ODP)

Blend Property Prediction

Mixture properties are calculated using:

Ideal Solution Assumptions:

For low-pressure vapor: P_total = Σ(y_i × P_i)

where y_i is mole fraction and P_i is partial pressure of component i.

Non-Ideal Behavior Corrections:

Real mixtures require equation-of-state models:

• Peng-Robinson equation
• REFPROP database (NIST)
• Activity coefficient models (UNIFAC, Wilson)

These account for molecular interactions affecting:

• Vapor pressure
• Enthalpy of vaporization
• Density
• Specific heat

Environmental and Safety Considerations

Global Warming Potential

Blend GWP is mass-weighted average:

GWP_blend = Σ(w_i × GWP_i)

where w_i is mass fraction of component i.

Flammability Classification

Blend flammability depends on composition and worst-case fractionation:

• Worst Case of Formulation (WCFF) - Tests most flammable possible composition within tolerance
• Worst Case of Fractionation (WCF) - Tests composition after leak scenarios
• ASHRAE 34 Classification - A1, A2L, A2, A3, B1, B2L, B2, B3 based on toxicity and flammability

Many low-GWP blends containing HFO-1234yf or HFO-1234ze(E) are classified A2L (mildly flammable).

Service and Handling Requirements

Leak Response

Zeotropic Blends:

1. System Evacuation - Remove entire charge
2. Leak Repair - Fix leak source
3. System Recharge - Install fresh refrigerant of correct composition
4. No Topping-Off - Never add refrigerant to partially leaked system

Azeotropic Blends:

1. Leak Repair - Fix leak source
2. Recharge to Specification - Add refrigerant as needed
3. Vapor or Liquid - Either phase acceptable for charging

Storage and Cylinder Management

All blends require:

• Dedicated Cylinders - Never mix refrigerants
• Liquid Withdrawal - Dip tube or cylinder inversion for zeotropes
• Proper Identification - ASHRAE number and composition on label
• Temperature Control - Store within manufacturer specifications

Retrofitting Considerations

When replacing existing refrigerants with blends:

1. Oil Compatibility - Verify lubricant compatibility with all blend components
2. Material Compatibility - Check elastomers, seals, and gaskets
3. Capacity Matching - Account for different volumetric capacity
4. Pressure Relationships - Verify component pressure ratings
5. Control Adjustment - Recalibrate or replace pressure controls
6. Filter-Drier Replacement - Install desiccant compatible with new refrigerant
7. System Flushing - Remove residual oil and contaminants if required

Proper blend selection and system modification ensure reliable long-term operation while meeting environmental objectives.

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