Just as every Scotch is a whiskey, but not every whiskey is a Scotch, the same applies to polyalkylene glycol (PAG) oils. PAGs come in various chemistries, each with its own advantages and disadvantages. The most common type is fully based on propylene oxide, often referred to as water-insoluble PAG (WI-PAG). This means that while every water-insoluble PAG is a PAG oil, not all PAGs are water-insoluble!
PAG oils are highly polar base oils known for their resistance to carbon and varnish formation. They have a high viscosity index (ranging from 180 to 250) and excellent anti-friction and anti-wear properties. In this article, we will explore different types of PAG oils from the perspective of refrigeration and heat pump compressors, as well as the interactions between PAGs and hydrocarbon refrigerants.
Water-Insoluble PAG (WI-PAG)
The most commonly used PAG oil in the refrigeration industry is the fully propylene oxide-based WI-PAG, also known as polypropylene glycol (PPG). WI-PAG is relatively apolar, exhibiting poor solubility in water but good solubility with heavy hydrocarbon gases. Under typical operational conditions using butane (R-600) as a refrigerant, dilutions can range between 30% and 15%. WI-PAG is suitable for hydrocarbon direct expansion (DX) systems. It is also highly miscible with ammonia, making it also a good choice for ammonia-based DX systems.
Water-Soluble PAG (WS-PAG)
Often used as gear oils, water-soluble PAGs (WS-PAG) are copolymers of propylene oxide (PO) and ethylene oxide (EO), typically in a 1:1 ratio. This composition gives WS-PAG good solubility of water but poor solubility with hydrocarbon gases. Under typical operational conditions with butane (R-600) as the refrigerant, dilutions can range from 20% to 10%. WS-PAGs are commonly used in compressor applications that involve high pressures and heavy hydrocarbon gas streams.
Polyethylene Glycol (PEG)
Polyethylene glycol (PEG) oils are fully based on ethylene oxide (EO), making them extremely polar and fully miscible with water. A key advantage of PEG oils is their resistance to dilution with hydrocarbon gases, with a maximum dilution of less than 3% when mixed with pentane (R-601). In immiscible heat pump systems using hydrocarbon refrigerants, PEGs are the oil of choice.
Other PAG Oils
In addition to the types mentioned above, there are other PAG oils worth noting, such as oil-soluble PAGs (OSP) and double-end capped PAGs. Oil-soluble PAGs are fully miscible with mineral and PAO oils and are often based on butylene oxide (BO). Double-end capped PAGs are a premium variant of water-insoluble PAGs, offering improved miscibility with certain refrigerants.
Next Lubricants provides calculation tools, PVT curves, and miscibility graphs for various PAG oils and hydrocarbon refrigerants. This enables us to make high-standard lubricant recommendations. Get in touch if you need a lubricant recommendation or want to learn more about our PAG oils
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