The reciprocating compressor is by far the most usual type employed in refrigeration installations. Such compressors may be vertical or horizontal, single-acting or double - acting and the crankcase may be opened or enclosed. Lubrications is by the method generally used for reciprocating machinery. In trunk piston types the cylinders are splash lubricated, with supplementary direct feed in some instances; in crosshead type oil is supplied separately to the stuffing glands and to the cylinders. In the latter case the oil may be led into the gas suction pipe or, more commonly, delivered direct to the cylinders and glands.

Bearing  lubrication of open-crankcase machines presents no feature peculiar to refrigerators and the following notes, are therefore, concerned with cylinder lubrication and with the bearing lubrication of trunk piston machines, where the oil comes into contact with the refrigerant. Although the functions of a refrigerator compressor lubricant are the same as those of compressor lubricants in general, conditions of service in refrigerators impose certain additional demands on the oil. The reason for this is the close association between the refrigerant and lubricant. Oil is unavoidably carried into the circuit with refrigerant discharging from the compressor. In many installations provision is made for the removal of this oil, nevertheless, some oil is liable to reach the cold parts of the system. On the other hand, with many of the Freons-especially Freon 12, oil which enters the system is difficult to separate and is commonly allowed to circulate with the refrigerant, such refrigerants can be regarded as oil - miscible. In either case the behaviour of oil in cold parts of the system is important. The properties that have to be considered have are low pour point and low wax-forming tendencies. 

It is essential to use an oil that will not congeal in the evaporator and in other cold parts of the system. Low pour point is important principally where the refrigerant and oil are not completely miscible in the low temperature region. This is the case, for example with ammonia or carbon dioxide, which for practical purposes may be regarded as immiscible with mineral oil. Even so, experience has shown that refrigerators with these refrigerants will function satisfactorily at temperatures below the pour point of oil. With a few exceptions, including Freon 22, the halogen refrigerants may be regarded as completely miscible with the oil in the normal working range and the pour point of the mixture is considerably below that of the oil. Under such conditions the pour point of the undiluted oil is of relatively little importance. However, mineral oil is a complex mixture of hydrocarbons which have different solubility characteristics; accordingly, whilst the bulk of the oil may remain in solution, certain constituents may separate out at low temperatures and form wax-like deposits. A further requirement of a refrigerator oil therefore is low tendency to wax formation.

Because of the efforts made to eliminate air from the system, refrigerator oil is unlikely to be subjected to severe oxidation. Nevertheless, the presence of some small quantity of air is unavoidable & this may provide a starting point for deterioration. Moreover, deterioration of the lubricant is bound up with chemical reactions which may involve moisture and other impurities, construction materials and the refrigerant itself. For example, some refrigerants are liable to form corrosive acids in the presence of moisture. Under such conditions unsuitable oils are liable to thicken and form gummy deposits and sludge. Adherent deposits are liable to cause excessive wear and inefficient operation of the compressor, particularly through piston ring - sticking and interference with operation and seating of valves. Seizure may occur in extreme cases. 

The foregoing are instances of the undesirable effects that may occur. It is found in practice that under normal conditions these effects can largely be eliminated by using oils processing chemical stability of a high order and avoiding contamination.

In all cases the rate of supply of oil to the compressor cylinders must be the minimum required for proper lubrication. Where the oil and refrigerant are mutually immiscible all excess oil must be removed from the system to reduce the accumulations of congealed oil in the system. Where oil-miscible refrigerants are use, dilution of the oil reduces the viscosity of the resultant mixture and the viscosity of the oil must be high enough to afford satisfactory lubrication under such conditions. In all cases, however, the viscosity of the oil used must not be so high that the oil unable to penetrate fine working clearances to all surfaces where it is required. A further factor limiting viscosity is that some increase of viscosity at lower temperatures is inevitable and removal of oil from cold parts of the system becomes increasingly difficult with oils of greater viscosity. In general, refrigeration compressor cylinders require relatively light oils.

Consideration must be given to the requirement of all refrigerators that the refrigerants and the installation in general must be free from moisture. The presence of water leads to obstruction of the system by accumulations of ice and apart from the possible formation of corrosive acids, with some refrigerants it may form solid refrigerant hydrates.

- Low tendency to wax deposition at low temperatures.

- High chemical stability.

- Appropriate viscosity

In operation the following points should be remembered:

- The rate of oil supply to the cylinders should be kept to a minimum

- Oil should be removed from the oil separators regularly.

- The system should be kept entirely free from air and moisture.

- In the case of trunk piston compressors, the oil should be withdrawn and filtered regularly to remove foreign matter which may have gained access.

-Discharge gas temperature should be carefully watched as an index of efficient operation.

Conditions which may lead to high temperature are:

- Faulty valve operation causing wire-drawing.

-Worn rings or cylinder liners, in the case of double acting compressors causing recompression.

-Gas entering the suction valve in superheated condition.

- Too much refrigerant in the system.

-Presence of foreign gases in the system

- Inefficiency of the condenser.

Summarizing, oils for 'internal' lubrication of refrigerators require the following characteristics:

-Ability to maintain a lubricating film at high temperatures and remain fluid at low temperatures.