LUBRICATION OIL REQUIREMENTS
As it has already been discussed, the lubrication oil should have acceptable lubrication features, in addition to tightness provided between the low and high pressure regions. The lubrication oil also provides cooling effect in the compressor in addition to lubricating the bearings and carries the heat generated in the rotating parts away during the operation of the compressor.
Until now, it has not been possible to develop lubrication oil to suitable for all kind of the refrigerant and operational conditions. There are pros and cons of commercially available lubrication oils, which require close attention to be paid to the operational conditions of the system in question.
With respect to the necessary requirements of the lubrication oil should;
1. Maintain its viscosity at low temperatures,
2. Maintain its stability at elevated temperatures,
3. Unlikely react chemically with the refrigerant, metals, coil isolation (for hermetic compressors), air and other pollutants,
4. Not dissociate to form carbon at unexpected operational conditions,
5. Not leave residue in the form of wax when facet with the expected low operation temperatures, and
6. Be clean as much as possible.
For practical uses, the commercially available lubrication oils are generally mineral-based. The lubrication oil might be classified under three groups as paraffin-based, napthene-based and mixture-(napthene and paraffin) based. The oils featuring different properties might be produced from the crude oil from different areas. The crude oil is purified to extract the paraffin and napthene. The properties of the lubrication oil are as follows:
1. Viscosity
2. Pour point
3. Sedimentation point
4. Flushing point
5. Dialectical stability
6. Burning point
7. Corrosion tendency
8. Oxidation resistance
9. Color
The viscosity of a lubrication oil refrigerant is defined as its resistance to flow under specific conditions. In practice, this is also defined as its consistency, i.e., how far it is thick or thin. When the sample liquid is passed through a calibrated opening at certain temperature, the time spent in terms of seconds yields its viscosity.
The pouring point of oil is the lowest temperature that it can still flow. Usually, it is accepted as the lowest temperature where the oil cannot flow, plus 3°C, The pouring point is an indication how far the oil can still provide lubrication before it cannot as a result of freezing. All lubrication oils are determined as containing certain portion of wax. When the temperature of the lubrication oil is decreased sufficiently, the wax settles.
Testing is performed to determine the temperature that will lead the lubrication oil to settle as the wax completely (usually partial settling is observed at low temperatures). When settled, the lubrication oil or refrigerant becomes turbid. If the temperature of oil/refrigerant and wax is further reduced, the wax lumps begin to grow like a bunch of grapes. The temperature of such a formation of wax is considered as settling point of the lubrication oil/refrigerant. During cooling process, wax formation will be concentrated at the coolest part of the system (expansion valve and evaporator), the heat transfer efficiency of the evaporator will decrease and expansion valve or similar flow equipment will be restricted and plugged.
The lubrication oils suitable for cooling at elevated temperature or in the comfort air conditioners may not be suitable at low temperatures. It is therefore, pouring temperature is taken into account in selection for the lubrication oil at specific applications.
The lubrication oil does not usually give rise to the fire hazard; however, the flushing point of the lubrication oil is an important parameter. The flushing point is the temperature at which the lubrication oil vapor catches fire when ignited. This is the case at some point of increase in the oil temperature, where the oil stability is lost and its components tend to dissociate. It is therefore, operation close to the flushing point is avoided.
Most of the compressors and motors are hermetically (tightly) coupled with each other in the housing or casing. The refrigerant gas in the evaporator may pass through the motor windings. In such case, the lubrication oil should resist to the flow of the electrical current (isolation). The dialectical stability of the lubrication oil in the system is the measure of this resistance. The burning point of the lubrication oil is related with the flushing point of the refrigerant defined previously. During testing, if the temperature increases in excess the flushing point of the oil vapor or if it is burning, that it means that the flushing point is reached.
The presence of the sulfur components is not desirable in the lubrication oil. This is because; the sulfuric acid is formed, if they react with the water. The formation of acid in the cooling systems is extremely hazardous on the cooling system components, because of resulting corrosion of the metallic parts. The corrosive effect of the lubrication oil is tested under with the polished copper rod. The corrosive effect on the copper rod should be minimum when it is submerged into the lubrication oil at 93°C. The copper rod is kept in the lubrication oil for 3 to 4 hours. If wear or discoloration observed on the surface, the lubrication oil is said to contain sulfur.
The flushing point is not single factor to define the stability of the lubrication oil. The resistance of the lubrication oil to enter into a chemical reaction is another indicator. Majority of the lubrication oils should be purified to get rid of the unsaturated hydrocarbons. On the other hand, purification deteriorates lubricating quality of the oils. Previously, the oils were undergone the purification process to so that they almost lost their color. The good lubrication oils are those with a color of light yellow, indicating that majority of its free hydrocarbons is purified.
