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.
For the detailed oil-return, miscibility, moisture, and in-service viscosity comparison, see CO₂ Refrigeration Lubricants: POE vs PAG for R-744 Systems
System type determines lubricant chemistry, not the refrigerant alone. Transcritical CO2 systems require a miscible lubricant because oil return depends on it. Subcritical systems do not require high miscibility and can instead use effective oil separation.
Miscibility and viscosity retention sit at opposite ends of the same axis. The dissolved CO2 that carries lubricant back to the compressor is also what reduces its viscosity. No lubricant chemistry provides both maximum miscibility and maximum viscosity retention.
Dissolved refrigerant can reduce lubricant viscosity by an order of magnitude because CO2 viscosity is two to three orders of magnitude lower than oil viscosity. Selection must target the calculated in-service viscosity, not the fresh ISO grade measured at 40 °C (104 °F).
PAG absorbs approximately four times more water than POE but does not chemically react with it. POE hydrolyzes, breaking ester bonds into organic acids. The more hygroscopic chemistry is therefore not necessarily the more vulnerable one.
In-service viscosity changes throughout the year. Above the critical point, pressure and temperature are independent, so gas-cooler pressure is set for efficiency and rises during summer. The lubricant grade must satisfy the summer design case, not the annual average.
Transcritical duty is a load case as well as a pressure case. Published data places compressor drive-gear loads at up to five times those found in HFC, HFO, and ammonia systems, with the highest load coinciding with the lowest lubricant viscosity.
CO₂ Refrigeration: Process, Applications & Compressor Role
The high-sdie condition also decides the lubricant. A transcritical system depends on the lubricant travelling with the refrigerant and returning, which requires miscibility. A subcritical or cascade system has receivers and drums where lubricant can be collected and returned deliberately, so a partly miscible lubricant with better viscosity retention becomes viable.
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.
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 both absorb atmospheric moisture during storage, handling and servicing, and both require dry, sealed handling at every stage. They differ in what happens next.
PAG holds water by hydrogen bonding to the polyether chain and drains it out with the lubricant without reacting — and absorbs roughly four times more of it than POE. POE hydrolyses: water breaks the ester bonds into organic acids and alcohols, which are implicated in corrosion and sludge. Hydrolysis requires sufficient water at elevated temperature, above roughly 80 °C, so it is a transcritical discharge-temperature concern rather than a storage one.
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 — and decide whether the lubricant must be miscible.
- 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. Discharge is the governing condition.
- Evaporating and High-Side Temperature Drive low-temperature fluidity, gas-cooler or condensing conditions, compressor loading and refrigerant circulation. In transcritical systems the high-side condition moves seasonally.
- 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
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?
Polyol ester (POE) and polyalkylene glycol (PAG), and the choice follows the system type rather than the refrigerant.
Transcritical systems depend on the lubricant returning with the refrigerant through the circuit, which requires miscibility. POE is fully miscible with liquid CO₂ and is the normal answer there.
Subcritical and industrial systems have receivers, drums and rectifiers — physical places where lubricant can be collected and returned deliberately. Where that infrastructure exists, a partly miscible PAG becomes viable and retains more viscosity under dissolved CO₂.
Some industrial systems are designed to keep the lubricant out of the circuit entirely using separation and low solubility, in which case a polyalphaolefin (PAO) is appropriate.
The compressor manufacturer’s specification governs in every case.
Why does CO₂ reduce lubricant viscosity so much more than other refrigerants?
Because of how much dissolves and how thin the dissolved phase is.
CO₂ viscosity is two to three orders of magnitude lower than lubricant viscosity, so dissolving it into the oil can reduce viscosity by as much as an order of magnitude. The amount dissolved rises with pressure, and transcritical systems run 80 to 120 bar on the high side with components designed for 130 to 140 bar.
A NEXT calculation for a PAG at 70 °C shows 23.8 cSt neat falling to around 11 cSt with CO₂ dissolved — more than half, at the point where the compressor carries its highest loads.
This is why in-service viscosity is calculated per operating point rather than read from a chart.
Does dissolved CO₂ ever help?
Yes, and the same mechanism does both jobs at opposite ends of the circuit.
A POE at −40 °C is effectively immobile as a neat fluid — a NEXT calculation puts it at 126,862 cSt. Dissolve 12 % CO₂ into it at 50 bar and it becomes a 35 cSt fluid that flows. The refrigerant is what makes oil return physically possible from a low-temperature evaporator.
At the discharge end the same dissolution is the threat. That is the central tension in CO₂ lubricant selection: maximum oil return and maximum film strength sit at opposite ends of one axis, and the system design decides which end to favour.
Is POE unsafe in CO₂ systems because it is hygroscopic?
No, but moisture control is not optional — and the common framing has it backwards.
PAG absorbs roughly four times more water than POE: approximately 10,000 ppm against 2,500 ppm at saturation, with mineral oil around 25 ppm for comparison. So PAG is the more absorbent chemistry.
The difference is what happens next. PAG holds water by hydrogen bonding to the polyether chain and drains it out with the lubricant, with no chemical reaction. POE hydrolyses: water breaks the ester bonds into organic acids and alcohols, and those acids are implicated in corrosion and sludge.
Hydrolysis is also conditional rather than automatic. It requires sufficient water at elevated temperature, above roughly 80 °C, and is insignificant at ambient conditions and low moisture content. Transcritical discharge temperatures reach that window, which is why moisture is treated more seriously in CO₂ service than in HFC service running the same chemistry.
What is density inversion, and when does it matter?
It is the point at which liquid CO₂ and a partly miscible lubricant swap positions in a vessel, because their relative densities cross over as temperature falls.
Published industrial data places it below approximately −32 °C saturated suction temperature for a PAG-68. Below that point, an oil recovery arrangement designed to draw from a particular layer draws the wrong fluid — so the recovery strategy stops working at exactly the temperatures where oil accumulation matters most.
It is relevant to low-temperature industrial and pumped systems and not to commercial medium-temperature duty. Where a plant runs below −32 °C saturated suction and uses a partly miscible lubricant, the recovery arrangement should be reviewed against the specific fluid rather than assumed.
Can POE contaminate the ammonia side of a cascade system?
Yes, and the pressure difference makes it a one-way risk.
Ammonia attacks the ester bond and depolymerises POE, producing solids, viscous residues and sludge that foul heat transfer surfaces and can block orifices. In an ammonia/CO₂ cascade the CO₂ circuit commonly runs POE and normally sits at the higher pressure, so a cascade heat exchanger leak carries ester toward the ammonia circuit rather than away from it.
That belongs on the design review checklist alongside the refrigerant leak assessment, not only on the lubricant order.
Can one CO₂ lubricant be replaced with another without flushing?
It depends on the chemistry families involved, and documentation rather than assumption should decide it.
NEXT holds a documented conversion from Fuchs Reniso C85E to NEXT 744-POE-85 supporting changeover with no flush and top-off use, based on mixture testing. Compatibility documentation for other CO₂ lubricants is available on request.
Two limits apply. PAG and hydrocarbon-based lubricants including PAO are not compatible with one another and require a full drain, flush and filter change to change between. And POE and PAO, while chemically compatible, represent different oil management strategies — switching between them is a system decision rather than a product substitution.
One point specific to CO₂: any service intervention is a moisture exposure event. On these systems the changeover procedure matters as much as the compatibility result.
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