Understanding Polyalkylene Glycols (PAG)
PAG lubricants are a family of synthetic base lubricants used across refrigeration, heat pump, and hydrocarbon gas compression applications. Unlike mineral lubricants, PAGs come in several distinct chemistries — each with different solubility, miscibility, and dilution behaviour with both refrigerants and process gases. Selecting the right type depends on the gas composition, system design, and operating conditions.
Common PAG Chemistries Used in Compressor Lubricants
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Water-Insoluble PAG (WI-PAG)
Fully propylene oxide. Relatively apolar, poor water solubility, good solubility with heavy hydrocarbon gases. -
Water-Soluble PAG (WS-PAG)
PO/EO copolymer, typically 1:1. Good water solubility, lower hydrocarbon dilution than WI-PAG. -
Polyethylene Glycol (PEG)
Fully ethylene oxide. Extremely polar, fully miscible with water, essentially immune to hydrocarbon dilution.
Technical Background
MONOMER CHEMISTRY AND POLARITY
180–250
Typical PAG viscosity index
< 3%
PEG dilution with pentane (R-601)
15–30%
WI-PAG dilution with butane (R-600)
DILUTION BEHAVIOUR WITH HYDROCARBON GASES
MODELLING IN-SERVICE DILUTION
Predicting how much a PAG will dilute in real service requires solving the thermodynamics of gas solubility in the specific lubricant at the actual operating pressure and temperature. NEXT uses a PVT (pressure–volume–temperature) model calibrated against measured solubility data for each base chemistry — WI-PAG, WS-PAG, and PEG, with PAO and mineral baselines for comparison. The model takes the gas composition in mol % (methane, ethane, propane, butane, pentane, CO₂, H₂S, N₂), the suction and discharge pressure, and the expected sump temperature as inputs, and returns the in-service viscosity at those conditions.
That in-service number is what should drive nominal grade selection. For heavy hydrocarbon streams or high-pressure duty, the gap between nominal and in-service viscosity is typically one to two ISO grades — large enough to move a compressor from correctly lubricated to under-lubricated if selection is made from the data sheet alone. The PVT model removes that gap from the decision.
MISCIBILITY WITH REFRIGERANTS
Certain PAG formulations are highly miscible with ammonia (R717), making them suitable for ammonia DX systems where oil return depends on refrigerant/lubricant circulationWith CO₂ (R-744), miscibility is partial and formulation-specific — dedicated CO₂ PAGs are built to manage this. PAGs are not standard with HFC or HFO refrigerants, where POE is the dominant chemistry.
LUBRICITY AND FILM STRENGTH
PAGs are naturally high-lubricity. The polar backbone adsorbs onto metal surfaces, giving boundary protection that synthetic hydrocarbons cannot match without additive support. Combined with viscosity indices of 180 to 250, a PAG typically outperforms a mineral or PAO of the same nominal grade across a wider temperature range.
CARBON AND VARNISH BEHAVIOUR
PAGs are known for low deposit tendency and clean burn-off behaviour compared with many hydrocarbon-based oils, helping reduce carbon and varnish risk in suitable applications. For high-ratio gas compressors and heat pumps running at elevated discharge temperatures, this behaviour is often the deciding factor over base-stock cost.
Key Factors
Four variables determine whether a given PAG will perform in a given system. They interact — a change in one usually shifts the requirements on the others.
Base Lubricant Chemistry
The monomer type (PO, EO, BO, or copolymer) determines polarity, water solubility, and solubility with both refrigerants and hydrocarbon gases.
Gas Composition
Light hydrocarbons (methane, ethane) cause limited dilution; medium fractions (propane, butane) cause moderate dilution; heavy fractions (pentane+) cause significant viscosity reduction, particularly in WI-PAG.
Operating Pressure
Higher discharge pressures increase gas solubility in the lubricant. This effect is most pronounced with medium and heavy hydrocarbon gases and must be factored into viscosity grade selection.
Operating Viscosity
In-situ viscosity can drop significantly under gas dilution. Nominal grade selection must account for expected dilution across the full range of operating pressures and gas compositions.
Applications
PAG Selection by Application
PAG selection starts with what the compressor is moving. Gas composition and refrigerant chemistry determine which backbone works. Viscosity grade is the second decision, made after the chemistry is fixed.
Hydrocarbon Gas Compression
WS-PAG or PEG for medium-to-heavy streams (propane, butane, pentane) where dilution must be controlled. WI-PAG is acceptable for lighter gas or lower-pressure duty.
CO₂ Refrigeration
Dedicated CO₂ PAGs formulated for partial miscibility. Selection depends on whether the system is transcritical or subcritical.
Immiscible Hydrocarbon Heat Pumps
PEG is the default. The lubricant stays in the sump rather than circulating with the refrigerant, so dilution resistance and thermal stability dominate.
High-Pressure Natural Gas
WS-PAG for streams with significant C3+ content. WI-PAG loses too much viscosity at pressure under these conditions.
Lubrication Considerations
Four practical rules that separate a reliable PAG installation from a problem one.
Match the Backbone to the Gas Stream
Dilution behaviour comes from the base chemistry, not the additive package. A WI-PAG in heavy hydrocarbon service will never hold grade, regardless of the starting viscosity.
Select Viscosity for In-Service Conditions, Not Nominal
Data-sheet viscosity is measured in a beaker, not at pressure. A VG 220 WI-PAG at 25% dilution behaves like a VG 150 in service. Use the NEXT PVT dilution model to calculate in-service viscosity for your actual gas composition and operating envelope, and select the nominal grade that lands on the target in-service value — not above it, not below it.
Never Mix PAG with Mineral, PAO, or POE
Most conventional PAGs should not be mixed with mineral oil, PAO or other lubricant chemistries unless compatibility has been confirmed. Conversions normally require thorough draining and flushing.
Manage Moisture in WS-PAG and PEG
Water solubility is a feature in service but a liability in storage. Keep drums sealed, dedicate transfer equipment, and monitor water content in operation.
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Frequently Asked Questions
Which PAG type should I use for my application?
It depends on what the compressor is moving. For ammonia refrigeration and heat pumps, WI-PAG is standard because of its high miscibility with ammonia. For medium-to-heavy hydrocarbon gas compression, WS-PAG is the usual choice as it dilutes less than WI-PAG at pressure. For immiscible heat pump systems running on hydrocarbon refrigerants, PEG is the default because dilution is negligible. For CO₂ systems, dedicated CO₂ PAGs are formulated specifically for that refrigerant and should not be substituted with general WI-PAG or WS-PAG.
Can I convert a compressor from mineral lubricant to PAG without flushing?
No. PAG is chemically incompatible with mineral, PAO, and POE lubricants and forms sludge on contact. A proper conversion requires draining the system, flushing with a compatible fluid, and confirming that residual contamination is below 1% before charging the new lubricant. Skipping the flush is the most common cause of premature PAG failure after a base-stock change.
How does the NEXT PVT dilution model work?
The model is a thermodynamic tool that calculates how much of each gas component dissolves into the lubricant at the system’s actual operating pressure and temperature. It is calibrated against measured solubility data for WI-PAG, WS-PAG, and PEG, with PAO and mineral baselines included for comparison. Inputs are gas composition in mol %, suction and discharge pressure, and expected sump temperature. Outputs are total dilution percentage and the resulting in-service viscosity. For compressors running on hydrocarbon streams or at elevated pressures, this in-service viscosity is the number that matters for grade selection — and it can differ from the nominal data-sheet value by one or more ISO grades. To run your case through the model, use the Request Lubricant Recommendation form or contact the NEXT technical team directly.
What is the difference between WI-PAG and polypropylene glycol (PPG)?
They are the same material. PPG is the chemical name for a polymer built entirely from propylene oxide; WI-PAG is the industry designation used in the lubricant market. The terms are used interchangeably in technical literature and in NEXT product documentation.
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