Gas & Refrigerant Dilution in Compressor Lubricants
Process gas or refrigerant can dissolve into compressor lubricant under pressure — and the dissolved gas or refrigerant can change the lubricant’s operating viscosity, density and film-forming behaviour. The lubricant inside the running machine is not always the same lubricant that was poured in.
Gas and refrigerant dilution are often discussed in gas compression, but the same principle can also matter in refrigeration and heat pump systems. CO₂ systems, hydrocarbon refrigeration and high-temperature heat pumps can expose the lubricant to refrigerant-rich conditions where pressure, temperature and solubility influence the working viscosity inside the compressor.
What Gas or Refrigerant Dilution Changes
Gas or refrigerant dilution is not a contaminant in the conventional sense. It does not produce wear particles and it does not necessarily change the colour of the oil. Standard used-oil analysis often does not show the full effect, because the sample is no longer at operating pressure when it reaches the laboratory.
What dilution changes is the lubricant’s working condition inside the compressor — and through that, the conditions the bearings, rotors and seals actually run on.
Matching a lubricant to a compressor application means considering the operating viscosity inside the machine — not only the rated viscosity on the data sheet.
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Operating Viscosity
Dissolved gas or refrigerant can lower the in-service viscosity below the rated grade — especially in applications with high solubility, elevated pressure and substantially in hydrocarbon service. -
Bearing Film Thickness
Can reduce the hydrodynamic film at journal bearings and rotor contacts when the working viscosity falls below the expected level. -
Density and Carryover
Can change lubricant density and separation behaviour, affecting separator efficiency, oil return and oil consumption. -
Foaming and Air Release
Dissolved gas or refrigerant can influence foam stability and air-release behaviour. Additives may help, but they cannot fully correct an unsuitable lubricant/refrigerant or lubricant/gas match.
Technical Background
WHAT GAS DILUTION IS
How much process gas or refrigerant dissolves into the lubricant depends on gas or refrigerant composition, operating pressure, operating temperature and lubricant chemistry. In general, solubility increases with pressure and is influenced by temperature. In a screw compressor running at elevated pressure on a hydrocarbon-rich feed, several percent of the oil mass in the sump can be dissolved gas.
Similar behaviour can also be relevant in refrigeration and heat pump systems, where refrigerant may dissolve into the compressor oil depending on the refrigerant, lubricant chemistry and operating envelope.
A lubricant with an ISO VG 68 grade has that viscosity measured at 40 °C, at atmospheric pressure, in clean condition. Inside the compressor, the same fluid is exposed to operating temperature, operating pressure and process gas or refrigerant. The combination can reduce the effective working viscosity, and that working viscosity is what determines film formation — not only the data-sheet value.
THE BASE OIL CHEMISTRY EFFECT
Different base oils dissolve different gases and refrigerants to different extents. For hydrocarbon process gas, the spread between chemistries can be wide. Mineral oils and PAOs generally show stronger mutual solubility with hydrocarbons, while selected polyalkylene glycol chemistries can show lower hydrocarbon solubility and therefore better viscosity retention in the same service.
In refrigeration and heat pump systems, the picture depends strongly on the refrigerant. CO₂, ammonia, hydrocarbons and HFO/HFC refrigerants do not behave the same way, and lubricant selection must balance viscosity, miscibility, oil return, separation and compressor design.
This is one of the engineering reasons PAG chemistries are often considered in heavy hydrocarbon, sour gas and natural gas pipeline service. The chemistry is not selected for additive load or thermal margin alone — it is also selected for the working viscosity that remains after gas has dissolved into the lubricant.
Variables That Drive Gas Dilution
Four variables strongly influence the operating viscosity inside the compressor. The NEXT PVT dilution tool can use these inputs to estimate the resulting operating viscosity.
Gas Composition or Refrigerant
Different gas species dissolve into base oils to very different extents. Heavy hydrocarbons dissolve far more than methane; CO₂ behaves differently again.
Operating Pressure
Solubility generally increases with pressure. Operating pressure is often one of the main drivers of dilution magnitude.
Operating Temperature
Temperature influences how much gas or refrigerant remains dissolved in the lubricant. Sump temperature, discharge temperature and system design all affect the lubricant’s working condition.
Base Oil Chemistry
Often one of the largest variables. PAG, PAO, mineral, POE and ester-based lubricants can interact very differently with the same gas or refrigerant under the same conditions.
Polarity and Lubricant Selection
Polarity is one of the factors that determines how strongly a refrigerant or gas interacts with the lubricant.
A useful rule of thumb is: like dissolves like. Refrigerants and lubricants with similar polarity generally have higher affinity and miscibility. Refrigerants and lubricants with different polarity generally show lower miscibility and lower solubility.
For hydrocarbon refrigerants such as propane, butane and pentane, dilution can be significant with non-polar lubricants such as mineral oil and PAO. This is one reason why PAG chemistries are often used in immiscible hydrocarbon refrigeration systems, where maintaining working viscosity and oil separation is important. The same principle can also apply in hydrocarbon gas compression and hydrocarbon heat pump systems, where gas composition, pressure and temperature can strongly influence dilution and working viscosity.
For ammonia refrigeration, the lubricant is commonly selected to remain largely immiscible with the refrigerant. In these systems, mineral oil and PAO are commonly used because they support separation from ammonia and stable lubricant return through the oil management system.
For CO₂ refrigeration, miscibility is often required for oil return. In those systems, POE lubricants are commonly used because they provide suitable miscibility with CO₂.
In immiscible CO₂ applications, such as certain CO₂ compression duties rather than CO₂ refrigeration systems, PAG or PAO chemistries may also be considered depending on pressure, temperature, dilution risk and compressor design.
The right base oil therefore depends on the full application: refrigerant or gas, polarity, miscibility requirement, pressure, temperature, oil return and compressor type.
BASE OIL CHEMISTRIES
How Base Oils Behave in Compression Service
Base oil chemistry is the largest variable in gas dilution behaviour. The ranges below reflect general engineering expectation — exact behaviour depends on the gas, pressure and temperature.
Hydrotreated Mineral
Generally higher mutual solubility with hydrocarbon gas. Suitable in applications where dilution remains manageable and the overall duty profile fits mineral-oil chemistry.
Polyalphaolefin (PAO)
Moderate-to-high hydrocarbon solubility in many process-gas applications. Strong thermal and oxidative stability; widely used where the gas or refrigerant interaction remains manageable for the duty profile.
Water Insoluble Polyalkylene Glycol (PAG-WI)
Often lower hydrocarbon solubility than mineral oil or PAO. Long-established in selected light hydrocarbon and pipeline gas applications for that reason.
Water Soluble Polyalkylene Glycol (PAG-WS)
Selected PAG-WS chemistries can show low solubility with hydrocarbon-rich gases. Used where dilution would otherwise reduce operating viscosity below application requirements.
Ethylene Oxide Polyalkylene Glycol (PAG-EO or PEG)
Pure ethylene oxide chemistry with very low hydrocarbon solubility. Strong choice for heavy hydrocarbon, sour gas and petrochemical service where holding operating viscosity is the priority.
Polyol Ester (POE)
Used in selected compressor applications where the full duty profile requires ester-based chemistry. Widely used in refrigeration systems using CO₂ as a refrigerant
Di-ester
Used in selected compressor applications where the full duty profile requires ester-based chemistry. Solubility behaviour depends on the gas or refrigerant, formulation and operating conditions.
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Frequently Asked Questions
What is gas or refrigerant dilution in a compressor lubricant?
Gas or refrigerant dilution occurs when process gas or refrigerant dissolves into the compressor lubricant under pressure. This can reduce the lubricant’s working viscosity inside the compressor and influence film strength, oil return, separation and foaming behaviour.
How much can dilution reduce operating viscosity?
The reduction depends on gas or refrigerant composition, pressure, temperature and lubricant chemistry. In hydrocarbon-rich gas service, reductions can be significant and may reach 30% or more under certain conditions. Other refrigerant and lubricant combinations may behave very differently.
Why doesn’t standard oil analysis always show dilution?
Standard used-oil analysis often does not show the full effect of gas or refrigerant dilution because the sample is depressurised before laboratory testing. The measured viscosity may therefore not represent the lubricant’s working viscosity inside the running compressor.
Which base oil chemistry handles dilution best?
There is no universal best chemistry. For hydrocarbon-rich gas service, selected PAG chemistries can offer lower gas solubility than mineral oil or PAO. Within PAG chemistries, a pure ethylene oxide PAG resists dilution the most. In refrigeration and heat pump systems, the correct chemistry depends on the refrigerant, miscibility requirements, compressor type, oil return, separation and operating temperature.
How do I calculate gas dilution for my system?
Enter the gas composition, operating pressure and sump temperature into the NEXT Lubricants PVT dilution tool. The tool returns the predicted gas dilution and the resulting operating viscosity for the lubricant chemistry you select, and supports side-by-side comparison between chemistries.