Mineral Oils

Mineral oils represent the original and most widely used lubricant class for refrigeration compressors from the 1930s through the 1990s. These petroleum-derived lubricants consist of complex hydrocarbon mixtures refined from crude oil through distillation and treatment processes. While mineral oils demonstrate excellent compatibility with CFC and HCFC refrigerants, their immiscibility with HFC refrigerants has driven the transition to synthetic lubricants in modern systems.

Composition and Refining

Base Stock Origins

Mineral oils derive from paraffinic, naphthenic, or mixed-base crude petroleum sources. The refining process determines the final lubricant characteristics:

Refining Steps:

1. Atmospheric distillation - Initial separation at 350-400°C
2. Vacuum distillation - Further fractionation at 10-50 mmHg
3. Solvent extraction - Removal of aromatics using furfural or phenol
4. Dewaxing - Crystallization and filtration at -15 to -30°C
5. Hydrotreating - Catalytic hydrogenation for stability improvement
6. Blending - Viscosity adjustment and additive incorporation

The degree of refining affects color (water-white to amber), stability, and performance characteristics.

Hydrocarbon Structure

Mineral oils contain three primary hydrocarbon classes:

The ratio of these components fundamentally determines lubricant performance in refrigeration applications.

Naphthenic vs Paraffinic Mineral Oils

Naphthenic Base Oils

Naphthenic mineral oils contain predominantly cycloparaffin (naphthene) structures with 5-6 carbon rings. These oils originated from Gulf Coast and California crude sources.

Advantages:

• Low pour point - Typically -40 to -50°C without additives
• Excellent refrigerant miscibility - Complete miscibility with CFCs/HCFCs across operating range
• Superior low-temperature fluidity - Maintain pumpability at evaporator conditions
• Natural wax-free composition - Minimal wax separation concerns
• Consistent viscosity-temperature relationship - Predictable behavior across temperature range

Disadvantages:

• Lower viscosity index - VI typically 40-60
• Greater volatility - Higher vapor pressure at elevated temperatures
• Reduced oxidation stability - More susceptible to thermal degradation
• Limited availability - Declining crude sources

Typical Properties:

Paraffinic Base Oils

Paraffinic oils consist primarily of straight-chain and branched alkanes. Pennsylvania and Mid-Continent crude oils provide paraffinic base stocks.

Advantages:

• High viscosity index - VI typically 90-110
• Excellent oxidation stability - Superior resistance to thermal breakdown
• Lower volatility - Reduced oil carryover in discharge gas
• Better lubricity - Enhanced boundary lubrication characteristics
• Wide availability - Abundant crude sources

Disadvantages:

• Higher pour point - Typically -15 to -30°C
• Wax separation tendency - Paraffin crystallization at low temperatures
• Reduced miscibility - Partial miscibility with some refrigerant/temperature combinations
• Dewaxing required - Additional processing for low-temperature applications

Typical Properties:

Selection Criteria

Application requirements determine the appropriate base oil type:

Naphthenic oils preferred for:

• Low-temperature applications (evaporator < -25°C)
• Wide temperature range systems
• Maximum refrigerant miscibility requirements
• Flooded evaporator designs

Paraffinic oils preferred for:

• High-temperature applications (discharge > 100°C)
• Screw and reciprocating compressors
• Systems requiring extended oil life
• Moderate temperature refrigeration (0 to -20°C)

Viscosity Characteristics

Viscosity Grading System

Refrigeration mineral oils follow ISO viscosity grade (VG) classifications based on kinematic viscosity at 40°C:

Viscosity-Temperature Relationship

The Walther equation describes viscosity variation with temperature:

log(log(ν + 0.8)) = A - B·log(T)

Where:

• ν = kinematic viscosity (cSt)
• T = absolute temperature (K)
• A, B = empirical constants

Viscosity Index (VI) Calculation:

VI quantifies the rate of viscosity change with temperature. Higher VI indicates less viscosity variation.

VI = ((L - U) / (L - H)) × 100

Where:

• L = viscosity at 40°C of 0 VI oil having same viscosity at 100°C
• U = viscosity at 40°C of test oil
• H = viscosity at 40°C of 100 VI oil having same viscosity at 100°C

Practical Implications:

Low VI oils (naphthenic) experience greater viscosity increase at low temperatures, potentially hindering oil return. High VI oils (paraffinic) maintain more consistent viscosity but may exhibit wax separation.

Dilution Effects

Refrigerant dissolution reduces effective lubricant viscosity. The dilution factor depends on:

Concentration relationship:

ν_mix = ν_oil × (1 - X_ref)^2.5

Where:

• ν_mix = mixture viscosity
• ν_oil = pure oil viscosity
• X_ref = refrigerant mass fraction

At 25% refrigerant concentration (typical crankcase condition), effective viscosity drops to approximately 40% of pure oil viscosity. This necessitates selecting higher viscosity base oils to maintain adequate lubrication with refrigerant present.

CFC and HCFC Compatibility

Miscibility Behavior

Mineral oils demonstrate excellent miscibility with CFC (R-12, R-502) and HCFC (R-22, R-123) refrigerants due to similar molecular polarity. This miscibility varies with temperature and concentration.

Phase Behavior:

The refrigerant-oil mixture exhibits either:

• Complete miscibility - Single liquid phase at all concentrations
• Partial miscibility - Two-phase region (upper critical solution temperature)
• Immiscibility - Complete phase separation

Miscibility temperature relationship:

T_misc = T_crit - K × X_oil × (1 - X_oil)

Where:

• T_misc = miscibility temperature
• T_crit = critical solution temperature
• K = interaction parameter
• X_oil = oil mass fraction

CFC Applications (R-12, R-502)

R-12 and R-502 show complete miscibility with mineral oils across typical refrigeration operating ranges.

R-12/Mineral Oil:

System Advantages:

• Oil returns from evaporator in solution with refrigerant
• No special oil return provisions required
• Simplified system design
• Proven long-term reliability

Maintenance Considerations:

• Oil analysis every 12-24 months
• Typical oil life 3-5 years
• Moisture limit <50 ppm
• Acid number limit <0.05 mg KOH/g

HCFC Applications (R-22, R-123)

R-22 exhibits partial miscibility with mineral oils, requiring careful system design.

R-22/Mineral Oil Miscibility:

Phase Separation Concerns:

Below the critical solution temperature, the mixture separates into:

• Oil-rich phase (bottom) - High viscosity, poor refrigerant content
• Refrigerant-rich phase (top) - Low viscosity, minimal oil content

This separation can trap oil in evaporators, causing:

• Reduced heat transfer (oil film on tubes)
• Compressor oil starvation
• Capacity loss (5-15% typical)

Design Requirements for R-22/Mineral Oil:

1. Minimum evaporator velocity - 7-10 m/s for horizontal runs
2. Suction line sizing - Velocity >15 m/s at minimum load
3. Oil return provisions - Traps, P-traps with proper dimensions
4. Suction line heat exchangers - Increase refrigerant superheat
5. Oil separator efficiency - >95% for low-temperature applications

R-123/Mineral Oil:

R-123 (low-pressure centrifugal chiller refrigerant) shows excellent miscibility with mineral oils at chiller operating conditions (2-10°C evaporator, 35-45°C condenser).

Limitations with HFC Refrigerants

Immiscibility Problems

HFC refrigerants (R-134a, R-404A, R-407C, R-410A) demonstrate poor miscibility with mineral oils due to fundamental molecular incompatibility. HFCs lack chlorine atoms, eliminating the weak polar interactions that provided CFC/HCFC miscibility.

Solubility Comparison:

System Failure Mechanisms

Oil Trapping:

Without miscibility, oil cannot return to the compressor dissolved in refrigerant vapor. Oil droplets must be mechanically entrained, requiring:

• Minimum vapor velocity calculation:

V_min = 4.5 × √(ρ_oil / ρ_vapor)

Where:

• V_min = minimum velocity (m/s)
• ρ_oil = oil density (kg/m³)
• ρ_vapor = refrigerant vapor density (kg/m³)

For R-134a/mineral oil at -25°C: V_min ≈ 18-22 m/s (vs. 7-10 m/s for R-12/mineral oil)

This high velocity requirement becomes impractical for:

• Variable capacity systems (velocity drops at part load)
• Large-diameter piping (oversized for full load)
• Low-temperature evaporators (low vapor density)

Evaporator Fouling:

Oil accumulation in evaporators creates:

• Heat transfer degradation - 0.1 mm oil film reduces U-value by 20-30%
• Capacity loss - 5% oil accumulation → 15-20% capacity reduction
• Refrigerant charge increase - Oil displaces refrigerant in evaporator
• Pressure drop increase - Oil restricts flow passages

Compressor Starvation:

Inadequate oil return causes:

• Bearing failure - Within 100-500 operating hours
• Cylinder scoring - Rapid wear of pistons and cylinders
• Seizure - Complete compressor failure
• Valve damage - Inadequate valve lubrication

Chemical Incompatibility

Beyond miscibility issues, mineral oils exhibit chemical problems with HFCs:

Hydrolysis Reactions:

Moisture in HFC systems forms hydrofluoric acid:

HFC + H₂O ⇄ HF + organic products

HF attacks mineral oil additives and causes:

• Additive depletion (antioxidants, anti-wear)
• Acid formation (TAN increase >0.1 mg KOH/g per 1000 hrs)
• Metal corrosion (copper plating, iron oxide)
• Sludge formation (polymerization products)

Copper Plating:

The combination of HFCs, moisture, and mineral oil accelerates copper plating:

• Copper dissolution from condenser tubes
• Transportation through refrigerant circuit
• Deposition on compressor bearing surfaces
• Bearing failure from excessive clearances

Wax Separation Concerns

Paraffin Wax Formation

Paraffinic mineral oils contain high-molecular-weight normal paraffins (C₂₀-C₄₀) that crystallize at low temperatures. This wax formation impairs system operation.

Crystallization Thermodynamics:

Wax solubility in oil decreases with temperature according to:

ln(X_wax) = -ΔH_fus/R × (1/T - 1/T_m)

Where:

• X_wax = wax solubility (mole fraction)
• ΔH_fus = heat of fusion (≈200-250 kJ/kg for paraffin wax)
• R = gas constant
• T = system temperature (K)
• T_m = wax melting point (K)

Wax Appearance Temperature (WAT):

The temperature at which wax crystals first appear depends on:

• Oil composition (paraffin content)
• Cooling rate (1-5°C/hr typical)
• Refrigerant dilution (depresses WAT)
• Pressure (minimal effect)

System Effects

Evaporator Plugging:

Wax crystals accumulate in evaporators causing:

• Flow restriction - Gradual pressure drop increase
• Heat transfer loss - Wax deposits on tube walls
• Oil drainage blockage - Prevents oil return
• Expansion device clogging - Wax plugs TXV or capillary tube

Oil Return Impediment:

Wax formation increases oil viscosity by 10-100× at evaporator conditions:

μ_wax = μ_oil × (1 + 2.5φ + 10.05φ²)

Where:

• μ_wax = viscosity with wax present
• μ_oil = base oil viscosity
• φ = wax volume fraction

At 10% wax concentration, viscosity increases approximately 3×, preventing oil return through suction lines.

Prevention and Mitigation

Oil Selection:

• Use naphthenic oils for evaporator <-20°C
• Specify dewaxed paraffinic oils (pour point <-30°C)
• Verify floc point at least 10°C below minimum evaporator temperature

System Design:

• Install oil separators (>90% efficiency) on low-temperature systems
• Provide suction line heat exchangers to warm returning oil
• Use oil return solenoids with hot gas bypass for oil heating
• Design for adequate refrigerant velocities (>7 m/s horizontal, >2 m/s vertical)

Operational Practices:

• Maintain crankcase heaters to prevent refrigerant migration
• Implement periodic hot gas defrost cycles
• Monitor oil level and return rates
• Analyze oil for wax content annually

Pour Point and Floc Point

Pour Point Definition

Pour point represents the lowest temperature at which oil flows under gravity (ASTM D97). The test procedure:

1. Cool sample in 3°C increments
2. Check fluidity after 5 seconds at each step
3. Pour point = lowest flow temperature + 3°C

Significance for Refrigeration:

Pour point indicates:

• Minimum pumpability temperature
• Crankcase starting capability
• Oil return potential

However, pour point has limitations:

• Measured under no-shear conditions
• Does not account for refrigerant dilution
• 3°C increment provides coarse resolution

Typical Pour Points:

Floc Point Test

Floc point (ASHRAE Standard 86) measures the temperature at which wax separates in a refrigerant-oil mixture. This test provides more relevant data for refrigeration applications than pour point.

Test Procedure:

1. Mix oil and refrigerant in specified ratio (90% oil, 10% refrigerant)
2. Cool mixture at 1-2°C/hr in observation tube
3. Illuminate sample with focused light
4. Record temperature when wax crystals first appear (Tyndall effect)

Interpretation:

Floc point represents the maximum evaporator temperature for that oil-refrigerant combination. System design requires:

T_evap > T_floc + 10°C (minimum safety margin)

Floc Point Data (R-12/Mineral Oil):

Floc Point Data (R-22/Mineral Oil):

R-22 miscibility elevates floc point by 5-10°C compared to R-12 due to partial solubility effects.

Refrigerant Dilution Effects

Refrigerant dissolution in oil affects low-temperature properties:

Pour Point Depression:

ΔT_pp = -K_pp × X_ref

Where:

• ΔT_pp = pour point depression (°C)
• K_pp = depression constant (15-25°C per 10% refrigerant)
• X_ref = refrigerant mass fraction

At 20% refrigerant concentration (typical crankcase), pour point may decrease 30-50°C, improving low-temperature fluidity.

Practical Application:

• Use floc point for refrigerant-oil selection
• Pour point serves as secondary indicator for crankcase conditions
• Always verify manufacturer’s refrigerant-oil compatibility data
• Test actual oil samples when operating near limits

Equipment Specifications and Applications

Compressor Type Requirements

Different compressor designs impose specific lubricant demands:

Reciprocating Compressors:

Rotary Screw Compressors:

Scroll Compressors:

System Design Considerations

Oil Separator Selection:

High-efficiency oil separators (>95% removal) are essential for:

• Low-temperature applications (evaporator <-20°C)
• Long vertical refrigerant risers (>10 m)
• Screw compressor installations (high oil circulation rate)

Oil separator efficiency equation:

η_sep = (m_oil,in - m_oil,out) / m_oil,in × 100%

Target efficiency based on evaporator temperature:

Piping Sizing:

Suction line sizing must ensure adequate oil return velocity:

Horizontal suction lines:

V_min = 7 m/s (for complete oil miscibility)
V_min = 15 m/s (for partial miscibility, R-22)

Vertical suction risers:

V_min = 2.5 m/s (up-flow)

Discharge lines:

V = 10-15 m/s (minimize pressure drop while ensuring oil transport)

Oil Management Systems

Oil Level Control:

Maintain crankcase oil level within manufacturer specifications:

• Reciprocating: 1/2 to 3/4 full sight glass
• Screw: Variable based on separator design
• Scroll: Factory-sealed charge

Oil Charging:

Initial oil charge calculation:

M_oil = V_crank × ρ_oil × Fill_fraction + V_system × C_oil

Where:

• M_oil = total oil charge (kg)
• V_crank = crankcase volume (L)
• ρ_oil = oil density (~0.88 kg/L)
• Fill_fraction = fill level (0.5-0.75)
• V_system = system volume (L)
• C_oil = oil concentration in system (0.01-0.05 kg/L)

Oil Logging:

For multi-compressor systems with long refrigerant lines:

• Calculate total system oil retention capacity
• Provide oil level equalization between compressors
• Install oil return solenoids for evaporator oil management
• Monitor individual compressor oil levels

Transition Considerations

Retrofit from CFC/HCFC to HFC

Converting existing mineral oil systems to HFC refrigerants requires complete oil change due to immiscibility.

Retrofit Procedure:

1. System Evaluation:

   • Document existing refrigerant and oil type
   • Measure oil charge quantity
   • Check system cleanliness (filter-drier condition)
   • Verify compressor compatibility with HFC refrigerants

2. Oil Removal Process:

Target: Reduce mineral oil residual to <5% of total lubricant

Flushing effectiveness calculation:

R_remaining = (1 - E_flush)^n

Where:

• R_remaining = residual oil fraction
• E_flush = single-flush removal efficiency (0.70-0.85)
• n = number of flush cycles

To achieve <5% residual with 80% per-cycle efficiency: n ≥ 3 flushes required

3. Flushing Methods:

4. Synthetic Oil Charging:

   • Use POE oil for R-134a, R-404A, R-407C
   • Use POE or PVE oil for R-410A
   • Charge 80% of original mineral oil quantity initially
   • Operate and verify oil return before final adjustment

5. System Cleanup:

   • Install oversized filter-drier (3× normal capacity)
   • Replace filter-drier after 24 hours operation
   • Install second filter-drier after 168 hours
   • Monitor acid levels (TAN <0.05 mg KOH/g)

Compatibility Issues:

Residual mineral oil causes problems:

• Miscibility degradation - Even 5% mineral oil significantly reduces POE miscibility
• Stability reduction - Mineral oil accelerates POE hydrolysis
• Copper plating - Mixed oils promote metal transport
• Elastomer incompatibility - Different swell characteristics

Cost-Benefit Analysis:

Retrofit costs typically include:

• Synthetic oil: 3-5× mineral oil cost
• Flushing labor: 8-16 hours
• Filter-driers: 2-3× normal quantity
• Refrigerant: Complete charge replacement
• Component replacement: Expansion device, gaskets

For systems >15 years old, replacement often proves more economical than retrofit.

Mineral Oil Disposal

Environmental regulations govern used mineral oil disposal:

Classification:

Used refrigeration oil may be classified as:

• Hazardous waste - If contaminated with CFCs (>50 ppm)
• Universal waste - If meeting TCLP limits
• Recyclable petroleum product - If clean and CFC-free

Disposal Options:

1. Re-refining - Vacuum distillation and reprocessing (preferred)
2. Fuel blending - Mixing with fuel oils for industrial combustion
3. Incineration - High-temperature destruction (>850°C)
4. Landfill - Only if solidified and meeting leachate requirements (last resort)

Documentation Requirements:

• Waste manifest for quantities >55 gallons
• EPA ID numbers for generator and receiver
• Material Safety Data Sheet (MSDS)
• Oil analysis showing contaminant levels

Oil Analysis and Condition Monitoring

Recommended Test Schedule:

Critical Test Parameters:

Interpretation:

• TAN increase - Oxidation, acid formation, thermal breakdown
• Viscosity increase - Oxidation, contamination, refrigerant loss
• Viscosity decrease - Excessive refrigerant dilution, shear breakdown
• Moisture increase - System leak, poor maintenance, hygroscopic contamination
• Metal increase - Wear, corrosion, copper plating

Oil Life Expectancy:

Properly maintained mineral oil in CFC/HCFC systems:

• Reciprocating - 5-7 years typical
• Screw - 3-5 years (higher thermal stress)
• Scroll - 7-10 years (sealed, lower stress)

Extend oil life through:

• Maintaining clean, dry refrigerant
• Preventing oxidation (minimize air exposure)
• Operating within design temperature limits
• Regular filter-drier maintenance
• Periodic oil analysis

Summary: Mineral oils served as the standard refrigeration lubricant for over 60 years, providing excellent compatibility with CFC and HCFC refrigerants. Their naphthenic and paraffinic variants offer distinct advantages for specific applications. However, complete immiscibility with modern HFC refrigerants has largely eliminated mineral oils from new equipment, except for ammonia (NH₃) and hydrocarbon refrigerant applications where they continue to excel. Understanding mineral oil characteristics remains essential for maintaining legacy systems and properly executing refrigerant conversion projects.