Optimising oil performance

Lily Harper of Climalife looks at the considerations needed to bring optimal oil performance to end-users in support of today’s environmental targets.


ollowing the Montreal Protocol which banned CFCs and HCFCs, and the Kyoto Protocol which sought to establish commitments for the reduction of greenhouse gases, there has been an increased push towards refrigerants with low ozone depleting potential and low global warming potential. This has indirectly promoted the use of ammonia and carbon dioxide as refrigerant options once again. As products are being phased out or restricted by quotas, the requirements for oils are also changing to suit the current market.

The use of correct lubrication in a refrigeration system is imperative in its successful operation and optimum performance. The use of oil can facilitate the movement of parts, control wear, reduce friction, absorb and dissipate heat, and seal and protect systems from corrosion. The choice of oil will depend on the installation or application, but, more importantly, the refrigerant and compressor type. Oil can be defi ned by several key properties:

 The viscosity of oil can defi ne its operating capability at high or low temperatures and its ability to reduce friction within a system and minimise wear and tear. The correct viscosity oil for the type of compressor is very important.

 The solvency of oil can help to create a seal within a compressor and also measures its compatibility with any additives. The oxidisation stability of oil will aff ect the amount of acid and sludge that could form within a system.

 Residue could form depending on the solvency, volatility and oxidation stability of oil. Chemical activity within the oil will determine its corrosion properties. Any tendency to foam or demulsify will depend on any surface activity of the oil.

 Miscibility with the refrigerant will depend on the system requirements.

 All of these properties can have an infl uence over oil return 36 July 2018

to the compressor; this is why it is critical that high quality oil, fi t for purpose, is always used. The diff erent types of oils can be defi ned as below:-  Mineral oil is a wax-free hydrocarbon mixture specifi cally chosen for its excellent fl uidity at low temperatures in air conditioning and refrigeration compressors. It is compatible with CFCs, HCFCs, ammonia, and hydrocarbon refrigerants.

 Alkyl benzene oil is a synthetic base oil which is a viable alternative to mineral oil.

 Polyol ester synthetic oil (POE) is a synthetic and wax- free oil used within refrigeration compressors. It has the fl exibility of use with HCFC, HFC, CO2

, HC and HFO refrigerants. It also has a hydroscopic tendency.

 Poly alkylene glycol (PAG) is a synthetic oil typically used in automotive air conditioning compressors. It also has a hydroscopic tendency.

 Poly-alpha-olefi n (PAO) is a synthetic lubricant which contains only poly-alpha-olefi n type hydrocarbons. A very high-quality oil, some of which are food grade.

 Poly vinyl ether (PVE) is a refrigerant oil specially formulated for hydrofl uorocarbon refrigeration systems.

Synthetic oils are polymers made up of specifi c chemical monomers such as esters. The fi rst synthetic oils were developed for HCFCs (alkyl benzene oil and poly-alpha-olefi n oil). Alongside the arrival of HFCs in the 1990s, polyol ester synthetic oil appeared in refrigeration compressors as the mineral, alkyl benzene oil and poly-alpha-olefi n oil were not compatible with HFCs. Naturally, the use of such oils in refrigeration systems has decreased.

Semi-synthetic oils are mixtures of mineral and synthetic oils and can be designed for refrigeration installations that work at low temperatures with limited miscibility. The mineral/alkyl-benzene blends that were typically used with R22 are no longer available, but with the demise of R22 this

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