Azeotropic Mixtures

Azeotrope Definition and Thermodynamic Behavior

An azeotropic mixture is a blend of two or more refrigerants that exhibits constant boiling point behavior at a specific composition. The vapor and liquid phases maintain identical compositions during phase change processes.

Fundamental Characteristics

Thermodynamic Properties:

• Constant boiling point at fixed pressure
• No temperature glide during evaporation or condensation
• Vapor composition equals liquid composition at equilibrium
• Behaves as a single-component refrigerant
• Consistent properties throughout phase change

Gibbs Phase Rule Application:

For an azeotropic mixture at equilibrium:

F = C - P + 2

Where:

• F = degrees of freedom
• C = number of components
• P = number of phases

During phase change (P = 2), a binary azeotrope (C = 2) has two degrees of freedom (pressure and temperature are dependent).

Phase Behavior

Vapor-Liquid Equilibrium:

At the azeotropic point:

• x₁ = y₁ (liquid mole fraction equals vapor mole fraction)
• No composition change during boiling or condensation
• Single saturation temperature at given pressure
• P-T diagram identical to pure refrigerant behavior

Deviation from Raoult’s Law:

Azeotropes exhibit non-ideal behavior described by:

P = x₁γ₁P₁ˢᵃᵗ + x₂γ₂P₂ˢᵃᵗ

Where:

• γ = activity coefficient (deviates from unity)
• Pˢᵃᵗ = saturation pressure of pure component
• x = liquid mole fraction

Positive azeotropes (lower boiling point than components) and negative azeotropes (higher boiling point) exist based on activity coefficient behavior.

Common Azeotropic Refrigerants

R-500 (R-12/152a Azeotrope)

Composition:

• 73.8% R-12 (CCl₂F₂) by weight
• 26.2% R-152a (CHF₂CH₃) by weight
• Azeotropic at atmospheric pressure

Thermodynamic Properties:

Performance Characteristics:

• Higher capacity than R-12 (approximately 15% increase)
• Lower discharge temperature than R-12
• Better energy efficiency in many applications
• Suitable for direct R-12 replacement in many systems

Status:

• Phase-out under Montreal Protocol (contains CFC-12)
• No longer manufactured in developed countries
• Historically used in air conditioning and refrigeration
• Replaced by HFC and HFO alternatives

R-502 (R-22/115 Azeotrope)

Composition:

• 48.8% R-22 (CHClF₂) by weight
• 51.2% R-115 (CClF₂CF₃) by weight
• Near-azeotropic behavior (temperature glide < 0.3°C)

Thermodynamic Properties:

Applications:

• Low-temperature refrigeration systems
• Commercial freezers and walk-in coolers
• Supermarket display cases
• Ice-making equipment
• Transport refrigeration

Operational Advantages:

• Lower discharge temperatures than R-22 alone
• Suitable for evaporator temperatures to -40°C
• Good volumetric capacity
• Compatible with mineral oil lubricants
• Lower compression ratios at low temperatures

Phase-Out Status:

• Banned under Montreal Protocol (contains HCFC-22 and CFC-115)
• Production ceased in developed countries (2020)
• Replaced by R-404A, R-507A, and newer alternatives
• Servicing allowed with reclaimed refrigerant

R-507A (R-125/143a Azeotrope)

Composition:

• 50% R-125 (CHF₂CF₃) by weight
• 50% R-143a (CH₃CF₃) by weight
• True azeotropic blend (zero temperature glide)

Thermodynamic Properties:

Applications:

• R-502 retrofit and replacement
• Low-temperature commercial refrigeration
• Supermarket refrigeration systems
• Cold storage facilities
• Process cooling applications

Performance Characteristics:

• Capacity similar to R-502
• Energy efficiency comparable to R-502
• Higher discharge temperatures than R-502
• Requires POE or PVE synthetic lubricants
• Non-flammable (ASHRAE A1 classification)

System Considerations:

• Incompatible with mineral oil
• Higher operating pressures than R-502
• Requires different expansion device sizing
• Low-GWP alternatives now preferred for new installations

R-508A and R-508B

R-508A Composition:

• 39% R-23 (CHF₃) by weight
• 61% R-116 (CF₃CF₃) by weight

R-508B Composition:

• 46% R-23 (CHF₃) by weight
• 54% R-116 (CF₃CF₃) by weight

Properties:

Applications:

• Ultra-low temperature refrigeration (-70°C to -90°C)
• Cascade refrigeration systems (low stage)
• Laboratory freezers
• Medical storage equipment
• Cryogenic applications

Constant Composition Behavior

Phase Change Characteristics

Evaporation Process:

Unlike zeotropic blends, azeotropes maintain constant composition:

1. Initial Liquid State: Composition = 50% A / 50% B
2. Partial Evaporation: Vapor and liquid both = 50% A / 50% B
3. Complete Evaporation: Final vapor = 50% A / 50% B

No Fractionation:

• All molecules evaporate proportionally
• No preferential boiling of lighter components
• Liquid composition remains constant
• Vapor composition equals liquid composition

Temperature Profile

Saturation Behavior:

During phase change at constant pressure:

• T_evap = constant (no glide)
• Single saturation point on P-T diagram
• Superheat and subcooling clearly defined
• Log P-h diagram appearance identical to pure refrigerant

Pressure-Temperature Relationship:

For azeotropic mixtures:

ln(P) = A - B/(T + C)

Where A, B, C are mixture-specific constants, and the relationship follows the same form as pure refrigerants.

Thermodynamic Advantages

Single Boiling Point Benefits:

1. Predictable Performance:

   • Constant evaporator temperature
   • Consistent superheat control
   • No glide compensation required in controls

2. Heat Transfer Efficiency:

   • Isothermal evaporation and condensation
   • Maximum LMTD (Log Mean Temperature Difference)
   • No mass transfer resistance within blend

3. System Design:

   • Standard expansion device sizing
   • Conventional superheat settings (5-7°C typical)
   • Simple pressure-temperature charts

4. Control System Simplicity:

   • Single saturation temperature at given pressure
   • Standard thermostatic expansion valve operation
   • Conventional pressure control strategies

Applications and System Selection

Commercial Refrigeration

Low-Temperature Applications:

• Display freezers: R-507A
• Walk-in freezers: R-507A, R-404A
• Ice cream cabinets: R-507A
• Frozen food storage: R-507A

Advantages in Commercial Systems:

• Simplified inventory management (single blend)
• Consistent performance across equipment
• Standardized service procedures
• Compatible with liquid level control

Industrial Refrigeration

Process Cooling:

• Food processing: R-507A for low temperatures
• Chemical manufacturing: R-508A for ultra-low temperatures
• Pharmaceutical production: Clean azeotropes (HFC-based)

Cascade Systems:

• High stage: R-134a, R-404A
• Low stage: R-508A, R-508B
• Ultra-low stage: R-23 or azeotropic blends

HVAC Applications

Limited Use in Comfort Cooling:

• Most HVAC systems use pure refrigerants or zeotropes
• R-410A (near-azeotropic) dominant in residential/light commercial
• Azeotropes historically used in centrifugal chillers (R-500)

Retrofit Applications

R-502 to R-507A Conversion:

Retrofit Procedure:

1. Recover existing R-502 refrigerant
2. Replace mineral oil with POE lubricant
3. Replace filter-drier with compatible core
4. Check elastomer compatibility (seals, gaskets)
5. Adjust expansion device if necessary
6. Evacuate to 500 microns or lower
7. Charge with R-507A to specifications
8. Verify superheat and subcooling

Charging Considerations

Liquid Charging Requirement

Phase Independence:

• Azeotropes can be charged as liquid or vapor
• Composition remains constant regardless of charging method
• No fractionation during cylinder discharge
• Tank pressure unchanged by liquid vs. vapor draw

Charging Methods:

1. Liquid Charging (Preferred):

   • Faster charging process
   • More accurate weight measurements
   • Suitable for large systems
   • Use liquid service valve on cylinder

2. Vapor Charging (Acceptable):

   • Slower process
   • Prevents liquid slugging into compressor
   • Required for small charges
   • Safe for operating systems

Quantity Determination

Charging by Weight:

Calculate required charge:

W = V × ρ_liquid × FF

Where:

• W = refrigerant weight (kg)
• V = system liquid volume (L)
• ρ_liquid = liquid density (kg/L)
• FF = fill factor (typically 0.80-0.85)

Charging by Subcooling:

1. Charge to nominal weight (80-90% of calculated)
2. Operate system at design conditions
3. Measure liquid line temperature
4. Measure liquid line pressure
5. Calculate subcooling = T_sat(P) - T_actual
6. Target subcooling: 4-7°C for typical systems
7. Add refrigerant if subcooling < target

Charging by Superheat:

For systems without receiver:

1. Measure suction line temperature
2. Measure suction line pressure
3. Calculate superheat = T_actual - T_sat(P)
4. Target superheat: 5-10°C at design conditions
5. Add refrigerant if superheat > target
6. Remove refrigerant if superheat < target

System Fill Limits

Safety Considerations:

Leak Behavior and Servicing

Composition Stability After Leaks

Azeotropic Advantage:

When system leaks occur:

• Vapor and liquid leak at identical composition
• Remaining refrigerant maintains original blend ratio
• No change in thermodynamic properties
• No performance degradation from composition shift

Comparison with Zeotropic Blends:

Service Procedures

Leak Detection:

• Electronic leak detectors (heated diode or infrared)
• Bubble solution for visual confirmation
• Ultrasonic leak detectors for large leaks
• Fluorescent dye with UV light

Topping Off Charge:

For azeotropic blends:

1. Locate and repair leak
2. Add refrigerant (liquid or vapor acceptable)
3. No composition correction needed
4. Verify charge by subcooling or superheat
5. Document refrigerant added

Recovery and Recycling:

Azeotropes can be:

• Recovered using standard equipment
• Recycled on-site (filtered and dehydrated)
• Reclaimed to ARI-700 standards
• Mixed with same refrigerant designation

Contamination Handling:

If contaminated (acid, moisture, air):

1. Complete refrigerant recovery required
2. System cleanup (flush or component replacement)
3. Oil change and filter-drier replacement
4. Evacuate to deep vacuum (500 microns)
5. Recharge with virgin or reclaimed refrigerant

Comparison with Zeotropic Blends

Thermodynamic Differences

Phase Change Behavior:

Performance Implications:

Azeotropic Advantages:

• Isothermal heat transfer (higher effectiveness)
• Predictable saturation temperature
• Simple P-T charts
• No glide considerations in system design
• Standard control algorithms

Zeotropic Advantages:

• Temperature glide can improve heat exchanger performance
• Better match to temperature profiles in some applications
• Countercurrent heat exchange benefits
• Potentially higher efficiency in specific designs

System Design Differences

Expansion Device Sizing:

Azeotropes:

• Standard orifice sizing methods
• Single saturation temperature assumed
• Conventional superheat control
• TXV selection per manufacturer tables

Zeotropes:

• Must account for bubble point temperature
• Glide affects available subcooling
• Superheat measured from bubble point
• Modified selection procedures

Heat Exchanger Design:

Evaporator:

Azeotropes:

• Constant evaporation temperature
• LMTD = (ΔT₁ - ΔT₂) / ln(ΔT₁/ΔT₂)
• Standard approach temperature design

Zeotropes:

• Temperature increases during evaporation
• Can improve LMTD in counterflow arrangements
• Requires integration of temperature profile

Condenser:

Azeotropes:

• Constant condensing temperature
• Simple subcooling section design
• Standard approach calculations

Zeotropes:

• Temperature decreases during condensation
• Glide zone requires separate analysis
• Potential for closer approach with cooling medium

Service and Maintenance Differences

Charging Procedures:

Leak Management:

Azeotrope Protocol:

• Repair leak
• Add refrigerant as needed
• Verify charge
• Return to service

Zeotrope Protocol:

• Repair leak
• Assess leak magnitude
• Consider recovery if >10% lost
• Add liquid refrigerant
• Verify charge and performance
• May require complete recovery/recharge if significant vapor leak

Equipment Considerations

Compressor Selection

Displacement Requirements:

For azeotropic refrigerants, calculate required displacement:

V_disp = (Q_evap × v_g) / (h_1 - h_4) / η_v

Where:

• V_disp = compressor displacement (m³/h)
• Q_evap = evaporator capacity (kW)
• v_g = specific volume of vapor (m³/kg)
• h_1 - h_4 = refrigeration effect (kJ/kg)
• η_v = volumetric efficiency

Discharge Temperature Considerations:

Temperature Control:

• Liquid injection for R-507A if T_discharge > 90°C
• Economizer cycles reduce discharge temperature
• Oversized condensers lower condensing pressure and temperature

Lubricant Compatibility

Oil Selection by Refrigerant:

POE Lubricant Characteristics:

• Miscible with HFC refrigerants at all temperatures
• Hygroscopic (absorbs moisture readily)
• Requires handling in dry environment
• Oil return similar to mineral oil systems
• Acid number < 0.05 mg KOH/g required

System Component Materials

Elastomer Compatibility:

HFC-based azeotropes affect elastomers differently than CFCs:

Recommended Materials for HFC Azeotropes:

• Seals: HNBR, Viton
• Gaskets: EPDM, Neoprene
• O-rings: HNBR, Viton
• Hoses: Barrier-type with HFC-compatible liner

Filter-Drier Selection

Desiccant Types:

For azeotropic HFC refrigerants:

• Molecular sieve (Type 3A or 4A preferred)
• Activated alumina (secondary)
• Silica gel (not recommended for HFCs)

Sizing Criteria:

• Water removal capacity ≥ 30% by weight
• Acid removal capacity considered
• Pressure drop < 2 psi at design flow
• Replaceable core for large systems

Moisture Limits:

Expansion Devices

TXV Selection:

Azeotropic refrigerants use standard TXV sizing:

1. Calculate refrigerant mass flow rate
2. Select valve for application refrigerant
3. Use single saturation temperature
4. Standard superheat settings (5-7°C)
5. No glide correction factors needed

Electronic Expansion Valves:

Benefits for azeotropic systems:

• Precise superheat control (±0.5°C)
• Adaptive control algorithms
• Fast response to load changes
• Integrated with building management systems

Capillary Tube Sizing:

For fixed-orifice applications:

L = (π² × d⁴ × ΔP) / (128 × μ × ṁ)

Where:

• L = tube length (m)
• d = inside diameter (m)
• ΔP = pressure drop (Pa)
• μ = dynamic viscosity (Pa·s)
• ṁ = mass flow rate (kg/s)

Azeotropes allow standard capillary sizing methods without glide considerations.

Safety and Environmental Considerations

Safety Classification

ASHRAE Standard 34 Classifications:

Class A1 Characteristics:

• Non-flammable at all concentrations
• Low toxicity (>400 ppm exposure limit)
• Suitable for occupied spaces
• Minimal safety restrictions

Environmental Impact

Ozone Depletion Potential (ODP):

Global Warming Potential (GWP):

Future Trends

Low-GWP Alternatives:

Replacing azeotropic blends:

• R-448A, R-449A replacing R-507A (GWP ~1400)
• R-454B replacing R-410A (GWP ~466)
• R-513A as R-134a alternative (GWP ~631)
• Natural refrigerants (CO₂, ammonia) for specific applications

Note: New alternatives are typically zeotropic blends, sacrificing composition stability for environmental benefits.

References:

• ASHRAE Handbook - Fundamentals, Chapter on Thermodynamics
• ASHRAE Standard 34: Designation and Safety Classification of Refrigerants
• ASHRAE Handbook - Refrigeration, Chapter on Refrigerants
• ARI Standard 700: Specifications for Refrigerants