What are Lubricants in the refrigeration system?

The necessity of providing lubrication in refrigeration systems is to prevent frictional contact on metal surfaces in compressors and motor & fan bearings.

Methods of lubrication:

It varies depending upon the type and size of the compressor and the individual manufacturer. However, the lubrication methods can be grouped into two general types:

  1. Splash lubrication:

The compressor crankcase acts as an oil sump and is filled with oil with each revolution of the crankshaft, connecting rod and crankshaft dip into the oil causing the oil to be splashed up on the cylinder walls, bearings, and other rubbing surfaces.

2. Forced feed lubrication:

The oil is circulated under pressure developed by a small oil pump, through oil tubes or drilled passages in the crankshaft and connecting rods to the various rubbing surfaces.

Lubricants:

Lubricants or refrigeration oils are classified into four main groups such as:

  • Animal oils
  • Vegetable oils
  • Mineral oils
  • Synthetic oils

Animal and vegetable oils are called fixed oil because they cannot be refined without decomposing. They are unstable and tend to form acids and gums which make them unsuitable for refrigeration purposes.

Mineral oils are the common lubricants. It contains paraffin, naphthenes, aromatics, and non-hydrocarbons. Paraffinic and naphthenic oils have excellent chemical stability but poor solubility aromatics have good solubility and lubrication properties.

Synthetic oils such as talk; benzenes and phosphate esters have desirable properties for use with some refrigerants.

Properties of good lubricant:

For a lubricant to be effective and efficient, it must have

  1. very low acidity
  2. Free from wax separation
  3. Minimum oil foaming tendency
  4. High miscibility with refrigerant used
  5. High chemical stability
  6. High flash point
  7. Low pour point
  8. High dielectric strength
  9. Free from dissolved air
  10. The viscosity must be maintained within reasonable limits as recommended by the manufacturers.
  11. Low solubility in water
  12. Low sulfur content

Applications of lubricants:

The only components which require lubrication in a refrigerant system are:

  1. Compressors
  2. Bearings of motor and fans

The compatibility of selected lubricants with various refrigerant groups is given below.

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