Refrigerants

Refrigerants are working fluids used in vapor compression refrigeration cycles to absorb heat at low temperature and pressure, then reject heat at higher temperature and pressure. The selection of refrigerant fundamentally determines system performance, efficiency, safety characteristics, and environmental impact. Modern refrigerant selection involves balancing thermodynamic properties, environmental considerations, safety requirements, regulatory compliance, and economic factors.

Fundamental Properties

Refrigerant performance depends on thermophysical properties that govern heat transfer, pressure-volume relationships, and transport characteristics.

Thermodynamic Properties

Vapor Pressure Characteristics

• Evaporator pressure must remain above atmospheric to prevent air infiltration
• Condenser pressure should not exceed equipment pressure ratings
• Pressure ratio affects compressor efficiency and discharge temperature
• Critical temperature must exceed maximum condensing temperature

Latent Heat of Vaporization

• Higher latent heat reduces refrigerant mass flow rate for given capacity
• Affects compressor displacement requirements
• Influences heat exchanger sizing
• Varies with saturation temperature

Specific Heat and Density

• Liquid specific heat affects subcooling effectiveness
• Vapor specific heat determines superheat temperature rise
• Liquid density impacts pump sizing and pipe friction losses
• Vapor density affects compressor displacement and pressure drop

Transport Properties

Thermal Conductivity

• Affects heat transfer coefficients in evaporators and condensers
• Influences nucleate boiling performance
• Important for low-temperature applications

Viscosity

• Liquid viscosity affects pressure drop and film heat transfer
• Vapor viscosity influences compressor efficiency
• Critical for oil return in direct expansion systems

Surface Tension

• Influences nucleate boiling heat transfer
• Affects refrigerant-oil miscibility
• Important for flooded evaporator design

Refrigerant Classification Systems

ASHRAE Numbering System

The ASHRAE Standard 34 numbering system provides systematic identification:

Methane-Series Compounds (R-10 through R-50)

• First digit: (number of carbon atoms - 1)
• Second digit: (number of hydrogen atoms + 1)
• Third digit: number of fluorine atoms
• Example: R-134a has 2 carbons, 2 hydrogens, 4 fluorines

Ethane-Series Compounds (R-100 through R-200)

• Follows same pattern with first digit representing carbon atoms
• Example: R-125 has 2 carbons, 1 hydrogen, 5 fluorines

Isomer Designation

• Lowercase letters (a, b, c) denote increasingly asymmetric isomers
• Most symmetric structure has no letter designation

Zeotropic Blends (R-400 series)

• Temperature glide during phase change
• Composition changes with leakage
• Requires special charging and service procedures

Azeotropic and Near-Azeotropic Blends (R-500 series)

• No temperature glide or minimal glide
• Constant composition during phase change
• Can be charged as vapor or liquid

Organic Compounds (R-600 series)

• Hydrocarbons and other organic refrigerants
• Example: R-600a (isobutane), R-290 (propane)

Refrigerant Categories by Chemical Composition

Environmental Properties

Ozone Depletion Potential (ODP)

ODP quantifies the relative capacity of a refrigerant to destroy stratospheric ozone, referenced to R-11 (ODP = 1.0).

Mechanism

• Chlorine and bromine atoms catalytically destroy ozone molecules
• One chlorine atom can destroy thousands of ozone molecules
• Stratospheric lifetime determines total ozone depletion

Regulatory Impact

• Montreal Protocol (1987) mandated CFC phaseout
• Copenhagen Amendment accelerated schedule
• HCFCs subject to progressive reduction and elimination

Global Warming Potential (GWP)

GWP measures the heat-trapping capacity of a refrigerant over 100 years relative to CO₂ (GWP = 1).

Total Equivalent Warming Impact (TEWI) TEWI = Direct Emissions + Indirect Emissions

Direct emissions result from refrigerant leakage and end-of-life losses. Indirect emissions result from energy consumption and associated CO₂ production.

Life Cycle Climate Performance (LCCP) More comprehensive than TEWI, including manufacturing, transport, and disposal impacts.

Safety Classifications

ASHRAE Standard 34 and ISO 817 establish refrigerant safety classifications based on toxicity and flammability.

Classification Matrix

Toxicity Criteria

Class A (Lower Toxicity)

• Occupational Exposure Limit (OEL) ≥ 400 ppm
• Suitable for occupied spaces with proper design
• Standard ventilation requirements

Class B (Higher Toxicity)

• OEL < 400 ppm
• Requires refrigerant detection systems
• Machinery room requirements more stringent
• Ammonia (R-717): OEL = 25 ppm

Flammability Criteria

Class 1 (No Flame Propagation)

• Will not propagate flame under ASHRAE 34 test conditions
• No special flammability precautions required

Class 2L (Lower Flammability)

• Burning velocity < 10 cm/s
• Heat of combustion < 19 kJ/kg
• Limited charge size regulations
• Requires ignition source risk assessment

Class 2 (Flammable)

• Lower flammability limit (LFL) > 0.10 kg/m³
• Heat of combustion ≥ 19 kJ/kg
• Significant safety design requirements

Class 3 (Higher Flammability)

• LFL ≤ 0.10 kg/m³ or burning velocity ≥ 10 cm/s
• Stringent safety requirements
• Machinery room isolation mandatory
• Explosion-proof electrical equipment

Refrigerant Selection Criteria

Performance Factors

Thermodynamic Efficiency

• Coefficient of Performance (COP) in intended operating range
• Pressure ratio across compressor
• Discharge temperature limits
• Volumetric capacity (refrigeration effect per unit volume)

Compatibility

• Materials compatibility (metals, elastomers, plastics)
• Lubricant miscibility and solubility
• Moisture tolerance
• Chemical stability at operating temperatures

Heat Transfer Characteristics

• Evaporation and condensation coefficients
• Pressure drop characteristics
• Nucleate boiling performance
• Transport properties

Regulatory Compliance

International Agreements

• Montreal Protocol: ODP restrictions
• Kigali Amendment: HFC phasedown schedule
• F-Gas Regulation (EU): Progressive reduction quotas
• AIM Act (USA): HFC production and consumption reduction

Building Codes and Standards

• International Mechanical Code (IMC)
• Uniform Mechanical Code (UMC)
• ASHRAE Standard 15: Safety Standard for Refrigeration Systems
• ASHRAE Standard 34: Designation and Safety Classification

Charge Limits

• Refrigerant quantity restrictions based on safety class
• Occupied space vs. machinery room requirements
• Probability of exposure calculations
• Ventilation and detection requirements

Economic Considerations

Refrigerant Cost

• Initial charge cost
• Replacement and service costs
• Price volatility and availability
• Reclamation value

System Cost Impact

• Pressure ratings and materials
• Safety equipment requirements
• Efficiency-related energy costs
• Maintenance and service complexity

Common Refrigerant Properties

*Sublimation temperature at atmospheric pressure

Pressure-Temperature Relationships

Refrigerant vapor pressure varies exponentially with temperature according to the Clausius-Clapeyron relation. System designers must ensure:

• Evaporator pressure remains sufficiently above atmospheric pressure
• Condenser pressure does not exceed equipment ratings
• Pressure ratio remains within compressor design limits
• Discharge temperature stays below decomposition limits

Representative Saturation Pressures (kPa absolute)

Refrigerant Transitions and Replacements

Historical Evolution

First Generation (Pre-1990s)

• R-12, R-22, R-502 dominated
• CFCs widely used
• No environmental regulations

Second Generation (1990s-2010s)

• HCFCs as transitional refrigerants
• HFCs as long-term replacements
• Montreal Protocol implementation

Third Generation (2010s-Present)

• Low-GWP HFOs and HFO blends
• Natural refrigerant resurgence
• Kigali Amendment phasedown

Drop-In and Retrofit Considerations

True Drop-In Requirements

• Same safety classification
• Compatible with existing lubricant
• Similar operating pressures
• No equipment modifications required

Practical Retrofit

• May require lubricant change
• Pressure transducer recalibration
• Control algorithm adjustments
• Performance verification

Common Replacement Paths

Charging and Handling Requirements

Charging Methods

Vapor Charging

• Required for zeotropic blends during system operation
• Prevents composition shift
• Slower process
• Used for final topping-up

Liquid Charging

• Acceptable for pure refrigerants and azeotropes
• Must charge into liquid line or through restriction
• Faster bulk charging
• Risk of compressor liquid slugging if improper

Leak Detection

Detection Methods by Sensitivity

• Electronic leak detectors: 0.1-0.5 oz/year
• Ultrasonic detectors: larger leaks, non-specific
• Fluorescent dyes: visual identification
• Soap bubbles: qualitative, low-cost

Regulatory Leak Rate Thresholds

• Commercial refrigeration: 35% annually (EPA)
• Industrial process: 35% annually
• Comfort cooling: 20% annually
• Trigger mandatory repair requirements

Recovery and Reclamation

Recovery

• Removal from system to external storage
• Required before servicing or disposal
• Equipment must meet EPA/AHRI standards

Recycling

• Oil separation and filtration
• Reduces moisture and particulates
• Can be returned to same system

Reclamation

• Reprocessing to ARI 700 specifications
• Chemical analysis required
• Returns refrigerant to virgin specification

This overview establishes the foundation for understanding modern refrigerant selection, application, and management in HVAC and refrigeration systems. Subsequent sections detail specific refrigerant types, system applications, and regulatory compliance strategies.

Sections