Zeotropic Refrigerant Mixtures

Definition and Fundamental Behavior

Zeotropic refrigerant mixtures are blends of two or more refrigerants that do not exhibit azeotropic behavior—meaning their components have different volatilities and the mixture changes composition during phase change processes. The term “zeotropic” derives from the Greek “zeo” (to boil) and “tropos” (change), indicating that the mixture’s boiling point changes during evaporation.

Key Distinguishing Characteristics:

• Components have different boiling points at the same pressure
• Composition varies between liquid and vapor phases during phase change
• Exhibits temperature glide during constant-pressure phase change
• Fractionation occurs during leaks or incomplete charging
• Dew point and bubble point differ at constant pressure
• Vapor-liquid equilibrium shifts with composition changes

Unlike pure refrigerants or azeotropic mixtures that change phase at constant temperature and constant pressure, zeotropic blends undergo phase change over a temperature range. This fundamental thermodynamic behavior affects system design, performance, charging procedures, and service practices.

Temperature Glide Phenomenon

Temperature glide is the temperature difference between the bubble point (saturated liquid temperature) and dew point (saturated vapor temperature) at constant pressure during phase change. This characteristic defines zeotropic mixture behavior and distinguishes these refrigerants from pure compounds.

Thermodynamic Definition

At constant pressure during evaporation or condensation:

Temperature Glide (ΔT_glide) = T_dew - T_bubble

Where:

• T_dew = temperature at which vaporization completes (100% vapor)
• T_bubble = temperature at which vaporization begins (0% vapor, 100% liquid)
• Both temperatures measured at constant system pressure

Magnitude and Classification

Zeotropic mixtures are categorized by glide magnitude:

Note: Temperature glide values vary with pressure. The glide typically increases at lower pressures (lower temperatures) and decreases at higher pressures.

Physical Mechanism

During evaporation of a zeotropic mixture:

1. Initial vaporization (bubble point): The most volatile component vaporizes preferentially
2. Progressive evaporation: Liquid becomes progressively richer in less volatile components
3. Changing equilibrium: As liquid composition shifts, saturation temperature increases
4. Complete vaporization (dew point): Final liquid contains highest proportion of heavy components

The reverse occurs during condensation, with the least volatile component condensing first.

Pressure-Temperature Relationships

For zeotropic mixtures, pressure-temperature tables must specify:

• Bubble point temperature (saturated liquid line)
• Dew point temperature (saturated vapor line)
• Quality lines (intermediate vapor fractions)

Example: R-407C at 100 psig (115 psia, 792 kPa):

Common Zeotropic Refrigerants

R-410A (ARI 700 Designation)

Composition: 50% R-32 / 50% R-125 (by mass)

Properties:

• Temperature glide: 0.3°F (0.2°C) at typical conditions
• Classification: Near-azeotropic zeotrope
• ODP: 0
• GWP: 2,088
• Critical temperature: 158.3°F (70.2°C)
• Critical pressure: 714 psia (4,925 kPa)

Applications:

• Residential air conditioning systems
• Light commercial air conditioning
• Heat pumps
• Direct replacement for R-22 in new equipment

Charging: Liquid charging required, though small glide minimizes fractionation risk.

Performance Characteristics:

• Higher operating pressures than R-22 (approximately 50-60% higher)
• Excellent capacity and efficiency
• Requires polyolester (POE) lubricants
• Minimal glide effect on heat exchanger performance

R-407C (ARI 700 Designation)

Composition: 23% R-32 / 25% R-125 / 52% R-134a (by mass)

Properties:

• Temperature glide: 5-7°F (2.8-3.9°C) depending on pressure
• ODP: 0
• GWP: 1,774
• Critical temperature: 187.2°F (86.2°C)
• Critical pressure: 669 psia (4,610 kPa)

Applications:

• Retrofit replacement for R-22 in existing systems
• New commercial air conditioning equipment
• Medium-temperature refrigeration
• Heat pump applications

Charging: Liquid charging mandatory due to moderate glide.

Performance Characteristics:

• Similar operating pressures to R-22
• 5-10% lower capacity than R-22 in retrofit applications
• Requires POE lubricants
• Glide affects counterflow heat exchanger performance
• Composition shift affects system performance after leaks

R-404A (ARI 700 Designation)

Composition: 44% R-125 / 52% R-143a / 4% R-134a (by mass)

Properties:

• Temperature glide: 0.5-1.0°F (0.3-0.6°C) at typical conditions
• Classification: Near-azeotropic zeotrope
• ODP: 0
• GWP: 3,922 (being phased down due to high GWP)
• Critical temperature: 162.3°F (72.4°C)
• Critical pressure: 535 psia (3,690 kPa)

Applications:

• Low and medium-temperature commercial refrigeration
• Supermarket refrigeration systems
• Cold storage facilities
• Transport refrigeration

Charging: Liquid charging required.

Performance Characteristics:

• Higher pressures than R-502 replacement application
• Excellent low-temperature capacity
• Requires POE lubricants
• Phase-out pending due to high GWP

R-407A

Composition: 20% R-32 / 40% R-125 / 40% R-134a (by mass)

Properties:

• Temperature glide: 8-10°F (4.4-5.6°C)
• Higher glide than R-407C
• ODP: 0
• GWP: 2,107

Applications:

• Medium and low-temperature refrigeration
• Replacement for R-502 in existing systems

Performance Characteristics:

• Significant glide requires careful heat exchanger design
• Composition shift more problematic than R-407C

R-507A (Near-Azeotropic)

Composition: 50% R-125 / 50% R-143a (by mass)

Properties:

• Temperature glide: 0.1°F (0.06°C)
• Effectively azeotropic behavior
• ODP: 0
• GWP: 3,985

Note: While technically zeotropic, R-507A behaves almost identically to an azeotrope due to minimal glide.

Fractionation Behavior

Fractionation is the preferential separation of refrigerant components during phase change processes or system leaks. This phenomenon is unique to zeotropic mixtures and creates significant service challenges.

Mechanisms of Fractionation

1. Leak-Induced Fractionation

When a zeotropic mixture leaks from a system:

• Vapor-phase leaks: Most volatile components escape preferentially
• Liquid-phase leaks: Composition closer to original blend
• Two-phase leaks: Composition between vapor and liquid extremes

Result: Remaining refrigerant composition shifts from original specification.

Example: R-407C Vapor Leak

The leaked vapor was rich in R-32, leaving the remaining charge rich in R-134a.

2. Charging Fractionation

Charging vapor from a cylinder results in composition shift:

• First refrigerant out: Rich in volatile components
• Remaining refrigerant: Rich in heavy components
• System receives non-specification mixture

3. Operating Fractionation

During normal operation, temporary composition differences exist:

• Evaporator inlet liquid: Near original composition
• Evaporator outlet vapor: Rich in volatile components
• Condenser inlet vapor: Composition from evaporator
• Condenser outlet liquid: Returns to original composition (closed system)

In properly functioning closed systems, operating fractionation does not cause permanent composition change.

Consequences of Fractionation

Performance Degradation:

• Altered pressure-temperature relationships
• Changed capacity and efficiency
• Modified superheat and subcooling behavior
• Potential compressor lubrication issues

Diagnostic Challenges:

• Pressure-temperature charts become inaccurate
• Superheat calculations unreliable
• Charging calculations compromised

Service Complications:

• Cannot determine proper charge by pressure alone
• Adding refrigerant to fractionated charge worsens problem
• Often requires complete refrigerant recovery and recharge

Fractionation Prevention

Primary Prevention Methods:

1. Liquid Charging Only: Always charge zeotropic mixtures as liquid to maintain composition
2. Leak Prevention: Minimize system leaks through proper installation and maintenance
3. Complete Recharge: After significant leaks, recover entire charge and recharge with fresh refrigerant
4. Proper Cylinder Handling: Maintain cylinders upright; use liquid valve or dip tube

Charging Requirements and Procedures

Proper charging of zeotropic refrigerants is critical to maintain specified composition and system performance.

Liquid Charging Mandate

Fundamental Rule: All zeotropic mixtures must be charged as liquid to prevent fractionation.

Rationale: Liquid maintains original blend composition, while vapor charging extracts components in non-proportional amounts.

Charging Methods

Method 1: Liquid Charging into Suction Line

Procedure:

1. Connect charging hose to cylinder liquid valve or use cylinder with dip tube
2. Invert cylinder if no liquid valve (only for approved cylinders)
3. Connect to suction line service port with system running
4. Throttle liquid flow to allow complete vaporization before compressor
5. Monitor superheat and system performance

Cautions:

• Never introduce liquid directly into compressor
• Ensure complete vaporization in suction line
• Use charging device with metering capability
• Monitor compressor for liquid slugging

Method 2: Liquid Charging into Liquid Line

Procedure:

1. Connect to high-side service port with system off
2. Charge liquid directly when pressure differential is sufficient
3. Alternatively, use charging cylinder and heat source for pressure boost
4. Monitor charge by subcooling, sight glass, and system performance

Advantages:

• No risk of liquid slugging compressor
• Faster charging for large systems
• More precise charge measurement

Method 3: Charging Devices with Vaporization

Specialized charging equipment can meter liquid refrigerant and vaporize it in controlled manner:

• Electronic charging stations
• Refrigerant scales with vapor dispensing
• Controlled vaporization chambers

Charge Verification

Methods for Zeotropic Refrigerants:

1. Subcooling Method (Preferred):

   • Measure liquid line temperature and pressure
   • Determine bubble point temperature from pressure
   • Calculate subcooling: ΔT_sub = T_bubble - T_liquid
   • Compare to manufacturer specifications (typically 8-15°F)

2. Superheat Method:

   • Measure suction line temperature and pressure
   • Determine dew point temperature from pressure (critical for zeotropes)
   • Calculate superheat: ΔT_super = T_suction - T_dew
   • Compare to target (10-15°F for fixed orifice, 8-12°F for TXV)

3. Charging Charts:

   • Use manufacturer-specific charging charts
   • Account for ambient conditions
   • Verify multiple parameters simultaneously

Important: For zeotropic refrigerants, always use dew point temperature for superheat calculations and bubble point temperature for subcooling calculations.

Cylinder Handling

Proper Practices:

• Store cylinders upright to maintain liquid orientation
• Use liquid service valves or dip tubes
• Never invert cylinders not designed for inversion
• Check cylinder labeling for orientation instructions
• Mark cylinders after partial use to track composition risk

Cylinder Depletion:

• Avoid using last 10-20% of refrigerant in cylinder
• Final cylinder contents may have composition drift
• Use fresh cylinder for critical charging operations

Heat Exchanger Design Implications

Temperature glide significantly affects heat exchanger design and performance, requiring modifications to conventional design approaches developed for pure refrigerants.

Counterflow vs. Parallel Flow

Counterflow Configuration (Preferred for Zeotropes):

In counterflow heat exchangers, the temperature glide of zeotropic refrigerants can improve heat transfer effectiveness:

• Refrigerant temperature profile better matches secondary fluid temperature profile
• Reduced approach temperature differences throughout heat exchanger
• Improved LMTD (Log Mean Temperature Difference)
• Higher heat transfer effectiveness

Parallel Flow Configuration:

• Temperature profiles mismatched
• Pinch point issues more severe
• Generally avoided for zeotropic refrigerants

Temperature Profile Matching

Evaporator Design:

For zeotropic refrigerant evaporating (bubble point to dew point):

Refrigerant Side:           Air/Water Side:
Inlet (bubble): 35°F        Outlet: 55°F
↓ (gliding up)              ↓ (cooling)
Outlet (dew): 42°F          Inlet: 85°F
(Glide: 7°F)                (Change: 30°F)

Advantages:

• Temperature glide partially offsets changing secondary fluid temperature
• Reduces refrigerant-side temperature difference at evaporator inlet
• Can improve system efficiency by 2-5% compared to same system with pure refrigerant

Design Modifications:

• Increase evaporator length to accommodate glide
• Optimize circuitry for counterflow arrangement
• Consider refrigerant distribution effects on local composition

Condenser Design:

Similar benefits occur in condensers where refrigerant temperature glides from dew point to bubble point:

Refrigerant Side:           Air/Water Side:
Inlet (dew): 120°F          Outlet: 105°F
↓ (gliding down)            ↓ (heating)
Outlet (bubble): 113°F      Inlet: 95°F
(Glide: 7°F)                (Change: 10°F)

LMTD Corrections

For zeotropic refrigerants, LMTD calculations require modification:

Conventional LMTD (Pure Refrigerant):

LMTD = (ΔT₁ - ΔT₂) / ln(ΔT₁/ΔT₂)

Where ΔT is constant along heat exchanger.

Modified Approach for Zeotropes:

Account for changing refrigerant temperature:

• Divide heat exchanger into segments
• Calculate LMTD for each segment
• Integrate total heat transfer
• Or use effectiveness-NTU method

Simplified Correction Factor:

For counterflow with zeotrope:

• Calculate LMTD using average refrigerant temperatures
• Apply correction factor (typically 1.02-1.08)
• Correction factor depends on glide magnitude and secondary fluid temperature change

Heat Transfer Coefficients

Mass Transfer Effects:

During phase change, composition gradients create mass transfer resistance:

Effective heat transfer coefficient:

1/U_effective = 1/U_heat + 1/U_mass

Where:

• U_heat = conventional heat transfer coefficient
• U_mass = mass transfer coefficient (composition diffusion)

Practical Impact:

• Heat transfer coefficients 5-15% lower than pure refrigerants
• Greater effect in low-velocity regions
• Mitigated by turbulent flow and proper circuitry

Design Compensations:

• Increase heat exchanger surface area by 5-10%
• Optimize refrigerant velocity
• Enhanced surface geometries more beneficial

Refrigerant Distribution

Challenges:

• Composition varies between vapor and liquid phases
• Improper distribution creates local composition differences
• Some circuits may operate with non-standard composition

Solutions:

• High-quality distributor design
• Proper feeder tube sizing
• Minimize vertical height differences between circuits
• Consider composition effects in multi-circuit designs

Leak Considerations and Management

Leaks in systems using zeotropic refrigerants create unique challenges due to fractionation and composition shift.

Leak Detection

Conventional Methods Apply:

• Electronic leak detectors
• Ultrasonic leak detection
• Bubble solutions
• Fluorescent dye

Additional Considerations:

• Detect leaks early before significant fractionation
• More critical than for pure refrigerants
• Small leaks have disproportionate effect on composition

Post-Leak System Assessment

After Detecting Leak:

1. Quantify Refrigerant Loss:

   • Estimate percentage of charge lost
   • Determine if loss was vapor, liquid, or two-phase

2. Evaluate Composition Shift:

   • If > 10% charge lost as vapor: composition significantly shifted
   • If > 20% charge lost: composition shift severe
   • If > 30% charge lost: complete recharge recommended

3. Decision Matrix:

Topping Off Procedures

When Acceptable (Small Liquid Leaks):

1. Repair leak completely
2. Verify leak repair
3. Add refrigerant as liquid only
4. Charge to manufacturer specifications
5. Monitor system performance closely
6. Document refrigerant addition

When Not Acceptable:

• Large vapor losses
• Unknown leak phase
• Multiple leak incidents
• Performance degradation observed

Complete Recharge Procedures

When Required:

1. Recover existing refrigerant:

   • Use approved recovery equipment
   • Recover to appropriate cylinder
   • Label cylinder as “contaminated” or “unknown composition”
   • Do not reuse in other systems

2. Repair all leaks:

   • Identify and repair all leak points
   • Pressure test with dry nitrogen
   • Verify leak-free operation

3. Evacuate system:

   • Deep vacuum to 500 microns or below
   • Remove moisture and non-condensables
   • Critical for POE lubricant systems

4. Recharge with fresh refrigerant:

   • Use virgin refrigerant from sealed cylinder
   • Charge as liquid per manufacturer specifications
   • Verify proper charge by multiple methods

5. Performance verification:

   • Monitor operating pressures and temperatures
   • Verify capacity and efficiency
   • Document baseline performance

Leak Prevention

Design Phase:

• Minimize refrigerant connections
• Use brazed joints over mechanical fittings where possible
• Specify high-quality components
• Design for accessibility for maintenance

Installation Phase:

• Proper brazing techniques and materials
• Pressure testing before charging
• Nitrogen purging during brazing
• Quality flare and swage connections

Operation Phase:

• Regular leak inspections
• Vibration management
• Corrosion prevention
• Proper service procedures

Comparison with Azeotropic Mixtures

Understanding differences between zeotropic and azeotropic refrigerant mixtures is essential for proper application and service.

Fundamental Differences

Thermodynamic Behavior

Zeotropic Mixture P-T Diagram:

• Two saturation curves (bubble and dew point)
• Quality lines between curves
• Temperature glide visible as curve separation
• Pressure-temperature relationship depends on composition

Azeotropic Mixture P-T Diagram:

• Single saturation curve (like pure refrigerant)
• No temperature glide
• Quality lines collapse to single curve
• Pressure-temperature relationship independent of composition (at azeotropic composition)

Service Implications

Zeotropic Advantages:

• Temperature glide can improve heat transfer in counterflow heat exchangers
• Broader range of formulations possible
• Can optimize component selection for performance

Zeotropic Disadvantages:

• Liquid charging required
• Cannot top off after vapor leaks without composition issues
• More complex pressure-temperature relationships
• Heat exchanger design more complex
• Service technician training more critical

Azeotropic Advantages:

• Service like pure refrigerants
• Vapor or liquid charging acceptable
• No fractionation concerns
• Simpler pressure-temperature relationships
• Easier for field service

Azeotropic Disadvantages:

• Azeotropic behavior only at specific composition
• Limited component choices
• Cannot optimize glide for heat transfer enhancement

Application Selection Guidelines

Choose Zeotropic When:

• Performance benefits from temperature glide justify added complexity
• System designed specifically for zeotrope characteristics
• Service infrastructure supports proper zeotrope handling
• Long-term refrigerant composition monitoring feasible

Choose Azeotropic When:

• Simplicity and ease of service are priorities
• Wide service network with varying skill levels
• Retrofit applications in existing systems
• Performance benefits of glide are minimal

Application Guidelines and Best Practices

System Design Guidelines

1. Refrigerant Selection:

• Match refrigerant glide characteristics to application requirements
• Consider service infrastructure capabilities
• Evaluate environmental regulations (GWP limits)
• Assess long-term refrigerant availability

2. Heat Exchanger Design:

• Design for counterflow configuration where possible
• Increase surface area 5-10% compared to pure refrigerant designs
• Account for temperature glide in thermal calculations
• Optimize refrigerant circuitry for composition uniformity

3. Metering Device Selection:

• Thermostatic expansion valves (TXVs) preferred over fixed orifices
• TXV sensing bulb must account for glide (measure dew point)
• Electronic expansion valves provide best control
• Consider composition effects on valve capacity

4. Control Strategy:

• Use dew point for superheat control
• Use bubble point for subcooling control
• Implement compensation for glide in control algorithms
• Monitor multiple parameters for charge verification

Installation Best Practices

1. Leak Prevention:

• Minimize field-made connections
• Use proper brazing procedures with nitrogen purging
• Pressure test to 150% of design pressure minimum
• Hold test pressure for 24 hours minimum

2. System Cleanup:

• Evacuate to 500 microns or lower
• Use micron gauge at system, not at pump
• Break vacuum and re-evacuate if moisture present
• Verify evacuation with standing vacuum test

3. Initial Charging:

• Charge with liquid refrigerant only
• Use manufacturer’s specified charge amount
• Verify charge by subcooling and superheat
• Document baseline system performance

4. Documentation:

• Record refrigerant type and composition
• Document initial charge amount
• Note all refrigerant additions
• Maintain service history log

Maintenance and Service Guidelines

1. Routine Maintenance:

• Inspect for leaks at every service visit
• Monitor superheat and subcooling trends
• Check for performance degradation
• Maintain detailed service records

2. Leak Response:

• Immediately identify and repair leaks
• Assess charge loss and composition shift
• Follow decision matrix for topping off vs. complete recharge
• Use liquid charging exclusively

3. Performance Troubleshooting:

• Compare current performance to baseline documentation
• Consider composition shift if performance degraded after leaks
• Use dew point for suction pressure evaluation
• Use bubble point for liquid line pressure evaluation

4. Refrigerant Handling:

• Store in upright position
• Use liquid service valves or dip tubes
• Never mix recovered refrigerant with virgin refrigerant
• Label partially used or recovered cylinders clearly

Training Requirements

Technician Competencies:

• Understanding of zeotrope thermodynamics
• Proper use of bubble and dew point temperatures
• Liquid charging techniques
• Fractionation recognition and mitigation
• Heat exchanger operation with glide
• Proper use of pressure-temperature tables

Certification Considerations:

• EPA Section 608 certification required (USA)
• Additional training on zeotrope-specific procedures
• Manufacturer-specific training recommended
• Regular updates as refrigerant technology evolves

Future Trends

Lower-GWP Zeotropes:

• Transition to A2L (mildly flammable) refrigerants
• New zeotropic formulations: R-454B, R-455A, R-457A
• Enhanced glide characteristics for improved performance
• Additional safety considerations with flammable components

Design Evolution:

• Optimized heat exchangers for specific zeotrope glides
• Advanced control systems with composition monitoring
• Integrated leak detection and composition verification
• Smart charging systems with fractionation prevention

Service Technology:

• Portable refrigerant analyzers for composition verification
• Electronic charging systems with automatic liquid dispensing
• Cloud-based performance monitoring and diagnostics
• Predictive maintenance for leak prevention

References:

• ASHRAE Handbook - Fundamentals, Chapter 30: Thermophysical Properties of Refrigerants
• ASHRAE Standard 34: Designation and Safety Classification of Refrigerants
• AHRI Standard 700: Specifications for Refrigerants
• Calm, J.M., “Refrigerant Transitions… Again,” ASHRAE Journal, 2008