Lubricating Oils and Heat Transfer Oils: The Differences
January 1, 2009
There is sometimes a fine line between the Thermal Oil and Lubricating Oil product lines marketed by oil companies. Both are slippery and transfer heat, but beyond that, they are formulated for very different functions. Certain terms and test results provided in product data sheets are more common to lube oils than heat transfer oils.
- Viscosity Index. Most lube oils are formulated for as little change in the viscosity as possible whereas for heat transfer you want as much change as you can get. Viscosity Index, for heat transfer fluids, is much less useful to engineers than the actual viscosity at a given temperature.
- SAE Grade/ISO Grade. General lube terms where the lower the number, the lower the viscosity. Used by manufacturers to distinguish between product grades. Works for car oil but again provides no usable data for heat transfer systems.
- SUS (Saybolt Universal Seconds). This is a viscosity unit that makes sense only for lubrication. Heat transfer oils use centistokes, centipoise or lbs/ft-hr for sizing pumps and heat exchangers.
- Copper Corrosion. Copper is seldom used in heat transfer systems so why worry about corrosion?
- Resistance to Water/Non-foaming/Demulsibility. Only lube oils retain water long enough to cause system corrosion or foam. Anybody who has seen a geyser of thermal fluid coming out of an expansion tank vent knows that water doesn't typically remain inside a thermal fluid system for very long.
- Conradson Carbon Residue/Ramsbottom Carbon Residue/Carbon Residue (Micro Method). While theoretically an indication of the tendency of an oil to form deposits, this test really measures the amount of inorganic additives left after pyrolysis destroys most of the carbon in the residue.
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