What is Lubrication? How to Perform?

The rotating parts in the cooling system, which come contact with each other create friction forces that may be detrimental. The friction also increases the temperature of the mating parts. Correct lubrication is important and indispensible in reducing the friction created between mating mechanical parts. The compressors should be provided with the lubrication at the mating parts like bearings, pistons and gears.

For piston compressors, the gap between the piston and cylinder wall should be too small to prevent ingress of the refrigerant into the cylinder during the compression stage of the gas by the pistons.

In a way, the lubrication oil is carried along the compressor walls in the compressor for the sake of obtaining such tightness. Any failure in ensuring the tightness during up and down reciprocating motion of the pistons, part of the gas will leak into compressor’s crankcase, resulting loss of efficiency.

As previously stated, the lubrication oil for the cooling systems mixes with the refrigerant and circulates in the system together. It is indispensible to prevent pumping the oil out of the compressor into the condenser. The lubrication oil should be set into full circulation in the system with the refrigerant to come back to compressor, in order to ensure smooth lubrication of rotating parts, without depleting the oil level in the oil pan of the compressor.

In its course of circulation, the lubrication oil reaches to the evaporator, where the presence of oil creates problem. The oil will be contained and built up in the evaporator, if not carried away to the discharge line. This end up with the decrease in heat transfer efficiency of coil.

In order to ensure the oil to complete its cycle through the system and turn back to the compressor, piping should be sized accordingly to increase the gas line speed. If the oil does not turn back to the compressor, it will then run out of the necessary oil. In such case, the necessary oil to ensure tightness will not be supplied and compressor’s efficiency will be reduced. If this is not corrected, compressor might be damaged.

There are mainly two methods to lubricate the compressor properly:

1.    Splashing method

2.    Forced or pressurized lubrication method

In first method, the lubrication is performed by operating the crankshaft submerged in the oil. The scoops or skimmers on the crankpin carries the oil and splashes it onto the grooves leading to the bearings. The oil is also fed onto the pistons and cylinder walls. By this way, the tightness against the refrigerant gas is ensured. It is however more important to circulate the lubrication oil in the system together with the refrigerant rather than keeping the oil level in the oil pan.

In forced or pressurized lubrication, the oil is pumped to bearings, piston pins, piston rings and cylinder wall via a small pump. This lubrication system is more expensive when compared to the splash type lubrication; however, the first method also provides sufficient lubrication and better protection so long as the oil pan is filled with the oil.

Some compressors operate like natural oil pumps. That is, the oil as mixed with the refrigerant is circulated through the system at a faster rate than usual as if the oil is continuously pumped. In most cases, the manufacturer includes an oil separator on the condenser. If the compressor is used in system consisted of separate parts, the manufacturer recommends installing such a separator.

It is important to make the return back to the compressor again immediately and thus the separator is installed between the compressor and condenser. The pressurized gas at elevated temperature is forced out of the compressor mixed with the oil and passes through discharge piping to the oil separator. Here, the direction of the gas is changed; its flowrate is decreased, since sectional area of the separator is greater than discharge piping. Besides, the oil separator might be complete with additional parts for effective oil separation. The oil is trapped, but the gas is conveyed to circulate further in the system.

In most separators, there are floats or valve arrangements to ensure the return of the oil back to the compressor. If oil level rises, the buoyancy force exerted on the float will raise the lever connected to it and open the valve that opens to the compressor. The cooler output (discharge) pressure is bigger than the pressure in the oil pan of the compressor and oil will flow back to the compressor. Meanwhile, the oil level in the separator will drop, the float descent and cause closing the valve to ensure trapping the oil in the separator.

The oil separator is insulated to prevent the oil temperature to drop; otherwise condensation may occur if the system is out of operation. In some cases, it might be necessary to include electrical heater on or inside the separator to keep the refrigerant in gas state, if the system frequently idles or the oil separator is cooler portion of the system.