The Function of Lubricating Oil in Screw Compressors

I. Core Functions and Operating Requirements of Air Compressor Oil

Air compressor oil primarily lubricates the moving components of compressor cylinders and exhaust valves, while fulfilling four critical roles: rust prevention, corrosion protection, sealing, and cooling.

Given that air compressors operate continuously under high-pressure, high-temperature conditions with condensation present within the system, the oil must meet the following core performance requirements:

Excellent high-temperature oxidation stability

Low carbon deposit tendency

Appropriate viscosity and viscosity-temperature characteristics

Superior water separation capability and rust/corrosion protection

II. Key Performance Requirements for Air Compressor Oil

1. Base Oil Quality Must Meet High Standards

Air compressor oil base oils fall into two main categories: mineral-based and synthetic. Their quality directly determines the performance of the finished oil, typically accounting for over 95% of the formulation.

Mineral-based base oils: Produced through processes such as solvent refining, solvent dewaxing, hydrogenation, or clay supplementary refining, then blended with multiple additives to form the final product. The deeper the base oil refining, the lower the heavy aromatics and gum content, the lower the carbon residue value, and the better the antioxidant sensitivity. This results in reduced carbon deposit tendency within compressor systems, superior oil-water separation, and extended service life.

Synthetic Base Oils: Formulated from chemically synthesized organic liquids through blending or additive incorporation, with core components being polymers or high-molecular-weight organic compounds. Common types include synthetic hydrocarbons (poly-α-olefins), organic esters (diesters), SNOT lubricants, polyalkylene glycols, fluorosilicates, and phosphates. Though significantly more expensive than mineral oils, they offer superior overall economic benefits—excellent oxidation stability, low carbon deposit tendency, lubrication capability beyond standard mineral oil temperature ranges, extended service life, and performance in harsh conditions beyond mineral oil tolerance.

2. Base oils must utilize narrow-distillation fractions

Analysis of air compressor operating conditions reveals that the distillation profile of base oils is critical for enhancing compressor oil quality. Using wide-distillation oils blending light and heavy fractions leads to two major issues:

Light fractions exhibit excessive volatility, causing them to prematurely separate from working surfaces after injection into cylinders and reducing lubrication effectiveness;

Heavier fractions exhibit poor volatility, failing to rapidly exit the working area after completing lubrication tasks. Over time, these residues accumulate and readily form carbon deposits under the influence of heat and oxygen.

Therefore, air compressor oils should utilize narrow-distillation-range base oils rather than multi-distillation blends. For instance, Compressor Oil No. 19, formulated with a wide-distillation-range base oil containing substantial residue components, resulted in significantly elevated carbon deposits during compressor operation. Replacing the residue-rich base oil with a narrow-distillation base oil substantially mitigates carbon buildup.

3. Viscosity Must Match Actual Operating Conditions

In power transmission lubrication, oil film thickness increases with higher viscosity, but friction also rises accordingly. Viscosity selection requires precise balancing:

Too low viscosity: Insufficient oil film strength accelerates component wear and shortens equipment lifespan;

Excessively high viscosity: Increases internal friction, elevating compressor specific power consumption, thereby raising energy and oil consumption. It may also cause deposits in piston ring grooves, valves, and exhaust passages.