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Lubricating Oils and Their Effects

It is known that the lubricating oil contained in the crankcase compressor as a rule, in contact with the refrigerant. When the oil is dissolved in the refrigerant, it affects the thermodynamic properties of the refrigerant. The main effect is to reduce the pressure of steam in the amount depending on the nature of the oil and refrigerant and how much oil dissolves. It is important to note that the refrigerants is expected to be chemically and physically stable in the presence of oil, so that neither any refrigerant oil this negatively affects the relations.

For example, in ammonia systems the amount of oil in the solution of ammonia, extremely low to cause any effect. However, with HC refrigerants volume of oil in the solution much more, and some of hydrocarbon refrigerants, therefore respond oil to some extent. The magnitude of this effect depends on the conditions of operation under normal operating conditions with the high oil quality in dry and clean system, the reaction becomes insignificant to cause any effect.

However, if pollutants such as air and moisture present in a system with a low quality oils, various problems can occur including oil decomposition and formation of corrosive acids and sludge. Another aspect is that high discharge temperatures accelerate such reasons tremendously.

How lubricating oil and refrigerant relations are concerned, one of the distinguishing characteristics for different refrigerants oil compatibility, which is defined as the ability of refrigerant dissolved in oil, and Vice versa. With reference to the oil compatibility, refrigerants, can be divided into three groups as follows (Dossat, 1997):

  • those, combined with oil all proportions, in conditions of a cooling system,
  • those mixed in conditions typically found in the condensation section but separate from the oil in conditions typically found in the evaporator section, and
  • those that is not mixed with oil at all (or very little) in the conditions found in the system.

viscosity oil is one of the most significant and thermophysical aspects should be maintained within certain limits, in order to form a protective film between different friction surfaces and keep them separately from each other. For example, if too low viscosity oil can't do it; if it's too high oil lacking fluidity to make the necessary penetration. In both cases, compressor lubrication is not adequate. It is important to note that in order to minimize the oil circulation in the refrigerant, oil separator or the trap sometimes installed in the discharge line of a compressor.

One of the latest works (Kramer, 1999) concerns the fact that HFC refrigerants and blends not miscible with mineral oil. He explores the factors that favour or by retaining mineral oil at modernization of existing systems discussion of potential problems arising from such use or storage. In practice one of the main questions is why do we leave mineral oil? The following factors favour of maintaining or using mineral oil in the systems of refrigerants:

  • lower cost of grease,
  • a direct replacement refrigerant,
  • below the solubility of refrigerant
  • improvement of the working viscosity,
  • reduction of the refrigerant
  • faster cooling start,
  • reduction of traffic jams, and entrainment of oil at the start (less need for pumping cycles or electric heaters),
  • less distortion composition of the refrigerant blends,
  • reduction of oil separator floods,
  • reduced water absorption,
  • reduced chemical reactivity,
  • reduction of electric resistance, and
  • reduction of pollution transfer.
The use of mineral oils with HFCs is preferable to a polyol ester lubricants due to a number of benefits, including (Kramer, 1999)
  • nonsticking suction necessary,
  • best visual detection
  • Solubility in water and reducing the load on the environment, and
  • improvement of foaming characteristics (to promote lubrication of the bearings and reduce the noise of the compressor).
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