Lubricant 101

Compressor Types

Types of Fluids Used in Compressors

 
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Hydrocarbons

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Hydrocarbons are the least expensive lubricants on a “cost per gallon” basis, but the most expensive on a “cost per year” basis. Because they are inexpensive, they are useful for very contaminated chemical environments where contamination means that frequent changes will be required.

Hydrocarbons are subject to varnish formation if run beyond the recommended life, which is typically about 1000 hours. Both flash point and carry over are linked to volatility.  The higher volatility of hydrocarbon oils also results in higher carry over. Typical examples of this type of fluid include ATF(Dexron), motor oil, turbine oils, R & O oils.

Benefits

►     Inexpensive (per gallon, not per year).
►     Good for very contaminated environments.

Disadvantages

►     Varnish Formation

►     Very Limited Life

►     Lower Flashpoint

►     Higher Carry Over into the Air System

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PAO / Synthetic Hydrocarbons

PAO’s are also known as Synthetic Hydrocarbons.  They are also a significant improvement over standard petroleum oils boasting a longer life and has good compatibility with petroleum oils. 

PAO’s are subject to varnish when the antioxidant in them is depleted.  Compressor service technicians have reported that the varnish left by PAO’s is sometimes even more difficult to remove from a compressor than that left by petroleum oils.

Synthetic Hydrocarbons are the only fluids which are eligible for the USDA H-1 rating for incidental food contact. The H-1 rated PAO’s typically have a life span in the range of 2000

hours. 

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Other PAO’s are rated from 4000- 8000 hours.

Nearly all of the automotive synthetics on the market are PAO fluids.  They are regarded highly for this application.

Benefits

►     Longer life than petroleum hydrocarbons
►     Also used in contaminated environments
►     Good compatibility with elastomers.
►     Best Food Grade option (H-1)

Disadvantages

►     Varnish Formation is not very predictable

►     Moderate Carryover

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Diesters

Diesters were one of the earlier types of synthetics used in compressor, automotive, and other applications.  They are still used extensively in reciprocating compressors today. 

 

Diesters are the most aggressive to some of the common types of rubber and elastomers found in compressors and compressed air systems.  In addition, diesters in the presence of water hydrolyze causing sludge formations.

 

Diesters have been around for a long time, but other types of synthetic fluids are gaining much more popularity in rotary compressors.

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Benefits

► Used as reciprocating compressor fluid

Disadvantages

►     Deposit formation

►     Attacks many solenoids, seals, hoses, and plastics.

►     Moderate carry over.

►     Some implicated as endocrine disruptors

 

Polyglycol/POE Blend

Polyglycol/POE lubricant fits the needs of rotary compressors very well. In rotary compressors, the lubricant cools, lubricates, seals rotors and protects against corrosion. 

Benefits

►     No varnish / Removes existing varnish
►     High VI for temperature extremes (151)
►     Very low carryover into air system
►     High flashpoint >500°F  (260°C) typical
►     Cools compressor more efficiently
►     Good compatibility with air system.
►     Now used by leading compressor OEM’s

Disadvantages

►     As with other synthetics, avoid chlorine and acids.

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Rotary Compressors 

Rotary Compressor Basics

►     Air flows in through an inlet port is trapped in between the lobes and the stator of two helical screws.

►     As air travels along the two helical screws, the space between the screws decreases causing an increase in air pressure. 

►     Air continues to travel down the screw until it reaches the discharge port where the compressed air is discharged.

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Rotary Compressor Lubricant Needs

►     In rotary screw compressors, lubricants main purpose is to cool, lubricate, seal rotors, and protect against corrosion.

►     The lubricated parts are bearings, contacting surfaces of rotors, as well as gears if present. 

 

Centrifugal Compressors 

Centrifugal Compressor Basics

►     Inside a casing, an impeller rotates at high velocity causing air to be thrown outward at a high speed. 

►     Air enters the diffuser ring where the speed is reduced, resulting in increased pressure.

►     Centrifugal compressors are used in plants with air capacities of 100 to 15,000 cfm at pressures up 125 PSI.

►     Impeller Speeds can reach up to 50,000 RPM.

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Centrifugal Compressor Lubrication Needs
►     Centrifugal air compressors provide oil free air, as there is no lubricant in the compression chambers. 
►     Bearings and shaft seals are lubricated away from the compression chamber via shaft seals.

 

Reciprocating Compressors 

Reciprocating Compressor Basics

►     Reciprocating compressors use a piston, driven through a connecting rod from the crankshaft. 

►     As the piston moves along the crank shaft, air is compressed, then discharged. 

►     Commonly compared to a manual bicycle pump. 

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Reciprocating Compressor Lubrication Needs
►     Lubricant in reciprocating compressors is used to lubricate the main bearings, connecting rod bearings, cross head bearings, motor bearings. Inside the cylinder, rider bands, compression rings, packing and valves are lubricated. 
►     Cylinders and frame bearings can be lubricated from the same oil supply or separately.
►     Cylinders that are open to crankcase are lubricated by oil thrown by scoops, or other projections, on connecting rods or cranks.
►     Cylinders that are not connected to the crankcase are lubricated by mechanical force feed lubricators.
►     Frame lubrication can be from wet sump or pressure circulation system.
►     Many times, different oils are used in separate systems.