RACA Journal March 2022 | Page 41

Support

PROPERTIES OF LUBRICATING OILS Remember , that you should use only the lubricant recommended by the compressor manufacturer . Oil producers and compressor manufacturers go to great lengths and expense to develop and test new lubricants . There are tests for viscosity , floc point , pour point , flash point , etc . You should understand the terms and why they are important .

Lubrication is the separation of moving parts by a film of oil . The closer the parts are to each other , the tighter the fit , the more emphasis there must be on the role of lubrication . It is safe to say that all high-quality oils do lubricate satisfactorily .

Refrigerant oils are described by the following characteristics :

• viscosity

• floc point

• dielectric strength

• neutralisation number

• flash point

• fire point .

• 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 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 very small 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 of 0.1 in . between two poles immersed in the oil . Recommended dielectric strength is a minimum of 25 kV .

• Neutralisation number : 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 , 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 . These substances are known as organic acids . Mineral acids , if present , are detected by the neutralisation number test . 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 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 : º Those that mix in all proportions with oil in the range of temperature operation . These include ethane , propane , isobutane , and the other hydrocarbons . º Those that separate into two 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 . º Those that are insoluble in oil in the operating temperature range . These include R-13 , R-14 , ammonia , and carbon dioxide .

As a matter of interest : of the refrigerants listed in the last two groups , only ammonia and carbon dioxide are lighter than oil . They float on top when the oil / refrigerant mixture separates into two phases .

Gean , with regards to selection ; as I have said before , let the compressor manufacturers specify what oil to use in any application . If you are unsure of what oil to use , call the compressor manufacturer or use the specification data along with their performance curve literature .

Gean , thank you for the question , I hope that this helps you moving forward .

Thank you for all your questions . Send your problems ( and sometimes your creative solutions ) to acra @ netactive . co . za with ‘ Solutions Page ’ in the subject line . You may include pictures . RACA