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Lubricating Oil: A Key Lifeline Of Refrigeration Compressor

The lubricating oil is a key constituent of refrigeration compressor and plays a key role in maintaining and extending life cycle of compressor. The life of the compressor depends on the quality and properties of lubricating oil. It helps operate it smoothly, carries away the heat on account of compression process.

In view of its significance on the overall performance of system and compressor in particular, it is desired that it has the right qualities that are maintained throughout:

  • Chemical stability
  • Pour & floc point
  • Dielectric strength
  • Viscosity

In an operating system, properties of lubricating oil are always under stress and are under threats due to various factors. Among these are:

  • Moisture
  • System contaminants
  • Excessive heat

Sometimes, a single or a combination of these together, if go un-noticed over a period of time, will result in damage to the compressor. Lubrication oil is friend of compressor but drags down the system performance and efficiency if gets logged up in condenser and/or evaporator. Now let us examine how these factors affect the overall system efficiency in general and affect the compressor in particular.

Loss of oil properties Viscosity

Viscosity

By definition, it is the resistance that a fluid, in this case the lubricating oil, offers to the flow. Function is to maintain a protective film between mechanical rubbing parts and facilitate smooth movement of compressor. Viscosity of lubricating oil changes with temperature and alters viscosity. It increases as temperature lowers and vice-a-versa.
In simple language, the oil becomes thicker as temperature reduces and therefore the fluid friction & power consumption of compressor increases. The lubrication of rubbing parts will not be adequate and will result in high temperatures. When oil temp rises, lubrication oil will become thinner and will flow faster but in turn lubrication suffers. With a view to providing an adequate lubrication to the compressor, it is very essential to maintain a viscosity within permissible limit is for optimum performance.

Sealed / semi-sealed stator winding failures are interpreted as an electrical problem many times. However, deeper understanding of the failure root cause points out to an overheated compressor. In a nutshell, overheating due a variety of reasons leads to stator failure.

Chemical deterioration of lub oil 

It occurs due to elevated temperatures, and accelerates in the presence of moisture or air (non-condensable gases). The combined effect of moisture and air in system on compressor life is devastating. Excessive heat generated due to compression process accelerates in the presence of air and is responsible for the oil degradation. Base material of lubricating oil, a polymer, first becomes black, sludge and powder as further aggression of high temperature continues. As a result of polymer dis-integration, oil becomes contaminated and loss of oil property entails a devastating effect on the fine drilled passages in crank-shaft, bearing journals & elsewhere. Interrupted oil supply to these locations results in damage to its rubbing surfaces & accelerates mechanical failure. Besides, the contamination in the form of powdered sludge of polymer can remain in system undetected for a very long time and which can plug fine passages in TEX valves, solenoid valves, oil return path of oil separator, accumulator aspiration holes etc.

All in all, heat damages the lubricating oil and initiates oil contamination, and therefore calls for a constant monitoring of sources & conditions which finally are attributed.

Some of the key factors that result in overheating are :

  • High condensing temperature
  • Low evaporating temperature
  • A combination of the above two
  • Under-charged system
  • Presence of air/non-condensable gases
  • High superheat
  • Refrigerant overcharge

The above attributes can result in compressor heat up, individually or in combination. When sludge is deposited on the seat of discharge valve reed, the high pressure high temperature refrigerant enters the cylinder on the return/downward stroke of piston. The refrigerant, which is at high temperature heats up the cylinder walls of compressor. Besides, the re-expanded refrigerant occupies space in cylinder whereby the pumping capacity reduces and in turn, the system suffers capacity reduction.

The re-entered high pressure refrigerant exerts pressure on the top of piston continuously, whereby the lower part of the gudgeon pin is tightly pressed on to the connecting rod as the piston reciprocates. Since the gudgeon pin seats tight at a ppoint of contact with con-rod, that particular portion of the pin is deprived of its vital ingredient, lubricating oil. The phenomenon results in wear & tear and failure of the connecting rod. A close observation of the failed connecting rods will reveal this; needless to add the root cause is high discharge pressure.

In order that we save the compressor, which is precision engineered, expensive component of the refrigeration system, from failure, always look for:

  • Problems associated with contaminated/underperforming condensers
  • Carbon deposits on valve reeds, leaky valve reeds
  • Ineffective jacket cooling system, if provided on the compressor
  • High super-heat resulting from dislodged/weak insulation of suction lines.

It is of utmost importance that the issues which potentially cause the high condensing pressures are regularly monitored and addressed promptly.

The probable reasons discussed above for compressor heat up are not exhaustive but are indicative – and therefore individual systems should be looked into carefully and conclusively – to identify causes. And high discharge end issue can be addressed by providing compressor head cooling fan, liquid injection method etc. and some of the precautions would be system dependent.

This would ensure a consistent system performance – and help maintain low repair and service costs over a long period of time. All in all, a failed compressor has its roots somewhere else and in several cases; high discharge end temperature would be one of its major contributing factors.


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