Lubricant Terminology

Proper lubrication is vital to compressor operation and requires special attention in package design. Two independent systems lubricate a compressor; the frame oil system and the force feed system. The frame oil system is a pressurized circulating system that supplies oil to the crankshaft, connecting rods, and crossheads. The force feed system is a high-pressure injection system that supplies small quantities of oil to the piston rod packings and piston rings.

In a compressor, lubrication:

1. Reduces friction - Decreases energy consumption and heat generation.

2. Reduces wear - Increases equipment life and decreases maintenance costs.

3. Removes heat from the system - Cools moving parts and maintains working clearances.

4. Prevents corrosion - Generally provided by additives rather than the base lubricant.

5. Seals and reduces contaminant buildup - Improves gas seal on piston and packing rings, and

flushes away contaminants from moving parts.

6. Dampens shock - Reduces vibration and noise and increases component life.

Many types of oils exist, some petroleum based, others synthetic. Each oil exhibits different characteristics that suit it for a specific application.

VISCOSITY - Measures fluid resistance to flow. It decreases with increasing temperature. In this document, viscosity is expressed in centistokes (cSt). Proper viscosity is the most important aspect of compressor lubrication. Fig. 6-5 illustrates viscosity differences between base stock types.

Viscosity can increase with oxidation or contamination by a liquid of higher viscosity or decrease with contamination by hydrocarbon gas condensate or other liquid of lower viscosity. Oil degradation increases viscosity, unless it is multi-viscosity oil (such as SAE 10W40). In multi-viscosity oils, the viscosity improvers degrade, not the base oil itself.

VISCOSITY INDEX - Indicates the magnitude of viscosity change with respect to temperature. The higher the viscosity index, the less viscosity decreases as temperature increases.

POUR POINT - Specifies the lowest temperature at which oil flows. It is important in cold weather applications and in cylinder and packing lubrication with cold suction temperatures.

FLASH POINT - Specifies the lowest temperature at which oil vaporizes to create a combustible mixture in air. If exposed to flame or high temperature, the mixture flashes into flame and then extinguishes itself. This is important in high temperature applications where oil may mix with air.