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The Truth About Viscosity For Refrigerator Compressor Fluids

Checking all the ratings on a compressor oil for ammonia refrigeration systems before you use it is the smart thing to do – as we all know, using the wrong fluid can have some serious consequences. But anyone could be forgiven for being overly focused on the ISO grade when the pour point is equally or more important……

ISO Grade – What is it? Why do we use it?

The ISO VG (International Standards Organisation Viscosity Grade) is the standard viscosity rating used across all oils worldwide. Viscosity refers to a lubricant’s thickness at a given temperature.

The Capillary Tube Viscometer Test is used determine the ISO VG grade of a lubricant. It measures a fluid’s flow under the force of gravity (kinematic viscosity) and is reported in centistokes (cSt). For ISO VG the test is done by placing oil in a glass capillary U-tube at a controlled temperature of 40°C.

Why 40°C? That’s what is considered to be the optimal operating temperature for most equipment….so it’s understandable this number isn’t likely to line up with the operational requirements of an industrial ammonia refrigeration system.

So you might be wondering how the ISO grading is relevant to refrigeration compressor lubricants that operate under a lot cooler conditions? It is still relevant of that the rating is used to indicate where the oil sits in terms of industry standards – so for refrigeration fluids you’re still likely to go for a ‘68’ – like the REFLO™68A, but when you look at the pour point the number will be different.

Pour Point – Why it’s more important than ISO

The pour point is the minimum temperature at which the oil can still pour down from a beaker, or more importantly, the lowest temperature the oil can maintain its liquidity. In terms of refrigerator compressor oils this is very important for two reasons:

1. This rating recognises the extremely cold operating temperatures of a ammonia refrigeration system and the need to maintain liquidity to lubricate; and

2. It’s extremely important the oil used in a refrigeration compressor unit maintains its characteristics under both hot and cold operating conditions in order to keep the machine functioning properly. These oils are designed with excellent thermal properties and oxidative stability to be able to deal with the ammonia gas running through the compressors with minimal carry over. Failure of these properties will result in too much oil making its way into the refrigerator itself, and if the pour point is not low enough the oil will become too thick and heavy and sludge up causing valves and components in the ammonia refrigeration system to block.

So what number should you be looking at?

Both – although the pour point tells you the extreme cold end that the oil will still flow so you want to make sure this number is colder than your evaporator temperature. Realistically you should be able to look at a refrigeration compressor oil labelled ‘Refrigerator Compressor Oil 68’ and know that it is a ‘standard’ oil for that use. Further inspection of the pour point should confirm that the oil will remain liquid at cold temperatures, and this will differ between different brands.



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