18 August 2011

Improved oil analysis method for refrigeration compressors

The current, standard oil analysis method involves observing only the lubrication qualities of unused oil and then compares these results with a data sheet. The results are based on:

  • Water content
  • Particle counting
  • Viscosity at 40°C

However, we would like to introduce an improved method of oil analysis that also considers other important details, such as the effects of interaction between the oil and the refrigerant. This method provides more comprehensive results upon which we can then base our recommendations.

During the refrigeration process, the refrigerant comes into direct contact with the lubricant.
Possible effects that the refrigerant can have on the oil include:

  • Water absorption by the refrigerant
  • Water absorption by modern oils, such as synthetic and mineral oils
  • Formation of a catalytic agent from water and additives
  • Oil oxidation
  • Alteration of the oil composition during the oil separation process (with multi-blended oils)
  • Oil film wash-out

Guidelines for optimal compressor performance:

  • Always use pure oil with a viscosity of at least 8 cSt
  • The necessary amount of oil film should completely accumulate within one minute
  • Only dry refrigerant gas should enter the compressor

For a complete oil analysis, the following additional information is needed regarding:

  • The refrigerant used
  • Oil viscosity over the operational temperature range
  • The discharge pressure
  • Whether the oil is pure, or composed of several base-oils (multi-blended)

In addition, our experience has taught us that testing should be performed on three separate oil samples:

  • Unused oil
  • The oil from the oil filter

This shows us the functional properties of the oil, in addition to its properties in its original, pure form.

The testing method described above has given us more insight into the following issues:

  • Foaming during capacity changes
  • Dry-running
  • Chemical reactions
  • Capacity losses
  • Oil carryover

We recommend that oil analysis be carried out not only considering the lubrication qualities of unused oil by itself, but also with regard to the influences that the refrigeration and oil separation methods can have on the oil.

The next step in complete compressor analysis should include the influence of the refrigerant on the lubricant. Do not hesitate to contact us for more advice!

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