CO₂ Refrigeration
CO₂ refrigeration compressor lubricants are synthetic fluids engineered for the high pressures, refrigerant solubility, and wide operating-temperature range of R744 systems.
CO₂ operates at higher pressures than conventional refrigerants. Above its critical point of 31.1 °C and 73.8 bar absolute, R744 enters a supercritical state and does not condense. Lubricant viscosity, CO₂ solubility, oil return, and OEM requirements govern product selection.
NEXT Lubricants produces polyol ester (POE) and polyalkylene glycol (PAG) formulations for transcritical, subcritical, and cascade CO₂ systems in food retail, cold storage, food processing, and industrial refrigeration.
- CO₂ systems operate at higher pressures than conventional systems. Transcritical operation raises high-side pressure further.
- CO₂ dissolves into the lubricant under operating conditions and reduces in-service viscosity. Selection targets expected in-service viscosity, not the fresh-oil ISO grade.
- Compressor design, system configuration, suction and discharge pressure, oil temperature, and OEM requirements determine the correct lubricant.
- Low-temperature fluidity, reliable oil return, and moisture control protect the compressor and hold system operation stable.
- POE and PAG are the lubricant chemistries for CO₂ refrigeration. NEXT 744-POE covers ISO 55 to 170. NEXT 744-PAG covers ISO 46 to 150.
CO₂ Refrigeration: Process, Applications & Compressor Role
CO₂ systems remove heat by circulating R744 through compression, heat rejection, expansion, and evaporation. High-side conditions set the cycle. A subcritical cycle rejects heat by condensation. A transcritical cycle rejects heat through a gas cooler above the critical point.
The compressor raises CO₂ vapor pressure and temperature and drives heat rejection. In transcritical operation, high-side pressure varies with gas-cooler conditions, ambient temperature, and system load. The compressor works across a wider pressure range than conventional refrigeration. The lubricant maintains protection across the full range.
Supermarket & Food Retail
Transcritical CO₂ booster systems serve supermarkets and food-retail refrigeration in both medium-temperature display cases and low-temperature freezer applications.
Cold Storage & Food Processing
CO₂ serves industrial cold storage, food-processing facilities, and freezing applications in transcritical systems and in subcritical cascade configurations.
Cascade Refrigeration
In cascade systems, CO₂ operates as the low-temperature refrigerant and rejects heat to a separate high-temperature refrigeration stage. This configuration suits low-temperature freezing and industrial refrigeration.
Ice Rinks
CO₂ serves direct and secondary refrigeration systems for ice-rink cooling, where its thermophysical properties support efficient heat transfer and compact system design.
Combined Cooling & Heat Recovery
Transcritical CO₂ systems recover heat from the high-pressure side of the refrigeration cycle for space heating and hot-water production.
Lubrication Considerations for CO₂ Refrigeration
CO₂ refrigeration imposes different lubricant demands than HFC, HFO, and ammonia systems. The primary factors are CO₂ solubility, operating viscosity, high pressure, low-temperature performance, oil return, and moisture control.
CO₂ Solubility and Operating Viscosity
CO₂ dissolves into the compressor lubricant under operating conditions. The dissolved refrigerant reduces the viscosity of the lubricant-refrigerant mixture. Lower viscosity reduces film thickness and component protection. Lubricant chemistry, refrigerant pressure, and oil temperature set the magnitude of the reduction.
Selection targets expected in-service viscosity, not the fresh-oil ISO grade. The ISO grade classifies the lubricant at 40 °C before refrigerant dilution.
High Operating Pressure
CO₂ systems operate at higher pressures than conventional refrigeration. The lubricant maintains operating viscosity, film strength, and wear protection across the pressure and temperature envelope of the compressor.
High pressure alone does not set the ISO viscosity grade. Compressor design, CO₂ solubility, oil temperature, and OEM requirements set it together.
Low-Temperature Fluidity and Oil Return
Low-temperature CO₂ systems require adequate lubricant fluidity and reliable oil circulation. Pour point, viscosity-temperature behavior, refrigerant interaction, and system design determine whether oil returns from the low-temperature side.side.
Inadequate oil return lowers the lubricant level in the compressor and allows oil to accumulate in heat exchangers and elsewhere in the refrigeration circuit. The result is reduced heat-transfer performance in the affected components and loss of lubricant supply at the compressor.
Moisture Control and Lubricant Stability
POE and PAG lubricants are hygroscopic. They absorb atmospheric moisture during storage, handling, and servicing. Excess moisture degrades lubricant stability, promotes corrosion, and causes system damage. In POE lubricants, moisture drives ester hydrolysis and acid formation. Containers, charging equipment, and systems require dry, sealed handling at every stage.
Operating Conditions Affecting CO₂ Lubricant Selection
CO₂ lubricant selection depends on the complete operating envelope of the refrigeration system. The nominal viscosity grade does not indicate how the lubricant performs after exposure to CO₂ at actual pressure and temperature. The following conditions define that envelope.
- System Configuration Transcritical, subcritical and cascade systems create different pressure, temperature, dilution and oil-return conditions.
- Compressor Type and OEM Requirements Compressor design and manufacturer specification determine the permitted lubricant chemistry, viscosity and performance requirements.
- Suction and Discharge Pressure Set CO₂ solubility, lubricant dilution, pressure ratio and operating viscosity inside the compressor.
- Evaporating and High-Side Temperature Drive low-temperature fluidity, gas-cooler or condensing conditions, compressor loading and refrigerant circulation.
- Oil Temperature and Oil Return Determine lubricant viscosity, dissolved CO₂ content and reliable oil circulation to the compressor.
Operational Benefits of Correct CO₂ Lubricant Selection
CO₂ lubricant selection depends on the complete operating envelope of the refrigeration system. The nominal viscosity grade alone does not show how the lubricant will perform after exposure to CO₂ at the actual pressure and temperature.
- Stable Operating Viscosity Maintains film strength after dissolved CO₂ reduces mixture viscosity. High-viscosity-index formulations hold film protection from low-temperature evaporators to high-pressure transcritical discharge.
- Clean Running and Deposit Control Limits varnish, sludge and deposit formation. Clean components maintain reliable oil circulation.
- Reduced Unplanned Downtime Prevents lubrication-related wear, overheating, oil-return failures and unplanned compressor shutdowns.
- Extended Component Life Protects bearings, scrolls, pistons, rotors and other lubricated compressor components.
- Reliable Oil Return and Lower Carryover Maintains oil circulation and reduces lubricant accumulation and loss across the system.
- Consistent Compressor Efficiency Maintains sealing and lubrication without adding viscous drag inside the compressor.
CO₂ Refrigeration Compressor Lubricants
The following lubricants are selected for CO₂ refrigeration applications across transcritical, subcritical and cascade system configurations.
Our technical team can help identify the right product.
Lubricant Selection, Technical Support and Compatibility Documentation
- Application-Specific Product Recommendations Lubricant recommendations based on gas or refrigerant composition, compressor design, operating conditions, and current performance issues..
- Lubricant Cross-Referencing Identifies suitable NEXT alternatives by comparing base-oil chemistry, viscosity, application, specifications and operating requirements.
- Compatibility and Conversion Documentation Provides chemistry comparisons, mixture-test data, material compatibility, flushing requirements and top-off or changeover guidance.
- Dilution Data and PVT Graphs Shows how gas or refrigerant concentration, pressure and temperature affect lubricant dilution, density and operating viscosity.
- Troubleshooting and Root-Cause Support Supports investigations into foaming, oil carryover, dilution, deposits, corrosion, high lubricant consumption and reduced oil life.
What type of oil is used in CO₂ refrigeration compressors?
CO₂ (R744) refrigeration compressors commonly use synthetic POE or PAG lubricants, although the correct chemistry depends on the compressor manufacturer, system configuration and operating conditions.
POE is widely used in both subcritical and transcritical CO₂ refrigeration. PAG is also specified by certain compressor manufacturers and can be used for particular high-pressure or compressor-specific applications.
The OEM lubricant specification should always be checked before selecting a replacement oil.
Why does CO₂ reduce compressor oil viscosity?
CO₂ can dissolve into the compressor lubricant under operating conditions.
The dissolved refrigerant changes the properties of the oil-refrigerant mixture and can reduce its effective viscosity.
The amount of viscosity reduction depends on lubricant chemistry, pressure and temperature, which is why CO₂ lubricant selection should be based on expected in-service viscosity rather than fresh-oil viscosity alone.
What is the difference between POE and PAG for CO₂ refrigeration?
POE and PAG have different viscosity-temperature, refrigerant-solubility and miscibility characteristics.
POE is commonly specified for many conventional subcritical and transcritical R744 compressors, while PAG is specified for some compressor designs and particularly demanding pressure conditions.
Neither chemistry should be selected solely because a system uses CO₂. The compressor manufacturer’s requirements and actual operating envelope determine the appropriate lubricant.
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