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VISCOSITY Viscosity means how thick or thin an oil is at a given temperature , and how readily it flows at that temperature . The oil becomes more ‘ fluid ’ ( it flows more easily ) as the temperature increases , and more ‘ viscous ’ ( thick ) at low temperatures . A standard of 37.7 ° C ( 100 ° F ) has been set as the temperature at which most oil viscosities are measured .
The importance of viscosity lies in the selection of a range that provides proper lubrication under all conditions . It must also allow for the diluting effect of the refrigerant . Refineries can supply viscosity ranges to meet any specification . When in doubt about proper oil viscosity , consult the manufacturer ' s recommendations .
FLOC POINT All mineral oils contain paraffin wax - some more than others . Even those that are called ‘ wax-free ’ contain minute amounts of wax . The floc test determines the measurable temperature at which wax separates from an oil . The small amounts of wax in ‘ wax-free ’ oils will settle out in a mass ( flocculate ) only when oil / refrigerant mixtures are at very low temperatures .
In general , the solubility of a material ( its ability to be dissolved in a solvent ) decreases as the temperature of the solvent decreases . In the same way , as the temperature of an oil / refrigerant mixture is lowered , the solubility of the paraffin wax decreases . If there is more wax present in an oil / refrigerant mixture than can remain in solution at a given temperature , precipitation will take place .
DIELECTRIC STRENGTH Dielectric strength is a measure of the resistance of an oil to an electric current . It is expressed in thousands of volts ( kilovolts , or kV ) of electricity required to jump a gap between two poles immersed in the oil . Recommended dielectric strength is a minimum of 25kV .
NEUTRALISATION NUMBER Neutrality is a difficult thing to visualise . Chemically , it is something that is neither alkaline nor acidic . The concept of neutrality may take on more meaning if you think of it in terms of drinking water . Household ammonia is alkaline and vinegar is acidic . By contrast , ordinary drinking water is neutral . Nearly all lubricating oils contain materials of varied and uncertain chemical composition , which react with alkali . A low neutralisation number indicates the absence of corrosive mineral acids .
FLASH AND FIRE POINT If you heat an oil in an open container and pass a small flame over the surface of the oil , at some point ( temperature ) there will be a flash of fire on the surface as the oil vapour burns . The temperature at which this occurs is called the flash point . If you continue to heat the oil further , it will reach a point ( temperature ) which it can be ignited and will continue to burn . This temperature is called the fire point .
SOLUBILITY Oil in a refrigeration system must do more than simply meet the normal requirements of lubrication . It must be able to mix with the refrigerant and travel through the system . Therefore , you must consider the solubility relationships between refrigerants and oils .
These relationships fall into three groups :
• Those that mix in all proportions with oil in the range of temperature operation . These include ethane , propane , isobutane and the other hydrocarbons .
• Those that separate into phases in the operating range , depending on temperature , pressure , and type base stock ( chemical category ) of the oil . These include R-21 , R-22 , R-114 , and R-115 .
• Those that are insoluble in oil in the operating temperature range . These include ammonia , and carbon dioxide .
“ Oil producers and compressor manufacturers go to great lengths and expense to develop and test new lubricants .”
Of the refrigerants listed in the last two groups , only ammonia and carbon dioxide are lighter than oil . and float on top when the oil / refrigerant mixture separates into two phases .
Ryan , I hope that this helps with your understanding of refrigerant oils . I will continue with lubricants in the next issue . Incorrect oil will almost certainly cost you a compressor . RACA
Grant Laidlaw
REFERENCES : 1 . ACRA
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RACA Journal I September 2024 www . refrigerationandaircon . co . za