How is or why are oils used in compressors?

Function Technical misunderstandings affecting refrigeration engineering fall into two broad camps, electrical (dealt with later) and mechanical. Mechanical problems of misunderstanding most commonly concern Oil, although Liquid Refrigerant causes far more failures in actual practice, the oil and the tasks it performs is far less understood. Here we will try to break down some common misconceptions and misunderstandings ... The oil in most machines performs three distinct functions ... Lubricates Cools Protects In considering refrigeration systems the third function can conveniently be ignored because most refrigerants are chemically inert to the materials used to contain them and thus corrosion or other degradation by chemical action is an insignificant effect (please note this statement does not take account of the deleterious effects of contaminated systrems). The other two functions are of equal importance to the continued correct running of the machines we are interested in - Refrigeration compressors.

Lubrication When any two solid surfaces bear and move against each other, friction, i.e. resistance to movement, will arise. This is due to imperfections upon adjacent surfaces interlocking against each other. Even highly polished surfaces will demonstrate this effect. This rubbing action will produce debris which can further interfere with the smooth running of adjacent surfaces, and in generating the debris a quantity of energy is absorbed and converted to heat. For any bearing surface to have a reasonable longevity it becomes necessary to physically separate the two solid surfaces. By use of a suitable liquid to make the separation the friction, tearing and grinding action is contained within the natural flow of the liquid and thus damaging friction is grossly reduced. The simplest solution therefore would be to take a common form of liquid and pump it in between the adjacent surfaces to provide a liquid film, a shear film. Water would provide an excellent lubricant except for two reasons ...

1. Water is very thin, as soon as water is squeezed in it will run out of all but the smallest crevices.

2. Water naturally forms a surface tension over itself which reduces it’s ability to spread easily over a surface without forming into distinct puddles and globules, leaving voids where the surfaces will meet, leading to a breakdown of lubrication shear film.

Thus to provide an effective lubrication the above need to be considered ... The medium must have body to it, this is referred to as viscosity The medium must not have an effect of surface tension, allowing the medium to remain in place over an extended surface without being forced to remain there. Oil provides this function. Cooling The shear effect of the lubricant itself absorbs energy which manifests also as heat. In addition the very effect of a compressor is to produce a temperature increase in the refrigerant fluid being compressed, the heat energy of one mass volume is reduced to a second mass volume, concentrating the heat into a smaller space and causing an effective rise in temperature. This effect leads to conductive heating of the adjacent machine parts. By allowing the lubricating oil to flow through the bearing parts the oil absorbs locally generated heat and carries it away to a suitable point where it can be rejected. In refrigeration systems the cold refrigerant entering the compressor provides the secondary cooling to the oil within the compressor.

How Lubrication Happens When the viscous oil is introduced between two bearing surfaces the forces propelling the solid parts together will attempt to force the lubricant away, this force is resisted by the viscosity or thickness of the oil. By allowing some flow away by controlling the effective clearance then a balance can be reached where a sensible amount of oil is bled away to prevent excessive heat build up, but the bleed rate is low enough to leave sufficient behind to perform the lubricative function.

Close attention to the working clearances and allowable tolerances within the bearings is vital to controlling the effective lubrication. In rotating bearings the lubrication occurs locally by formation of an ‘Oil Wedge’ at the point where the two surfaces come closest. When the bearing is static this would be the bottom area due to gravity.

When rotation occurs the shaft within the bearing will try to climb over the adjacent oil wedge, however this causes a locally increased oil pressure at the point of close contact, this increased pressure forces the surfaces apart, moving the point of closest contact around the circumference of the bearing. After several revolutions the shaft and journal settle into an equilibrium where the two are equally separate around the whole bearing. Lubrication and cooling supply balance is maintained by admitting a continuous supply of lubricant to the running bearing. In smaller bearings this can simply be by splash effect where sufficient flow is provided by droplets settling adjacent to the bearing, or in larger bearings by forcing a continuous flow into the bearing through appropriate drillings and oilways.

Viscosity & Viscosity Index By adjusting the thickness of the oil used in a particular machine the balance of oil flow against effective pressure can be altered. Generally, larger bearings will require a more viscous oil to reduce excessive flow through relatively large clearance areas at the end of bearings, whereas smaller bearings require thinner fluids to maintain acceptable cooling rates. Conveniently most lubricants considerably overlap each other. Viscosity is measured by timing the rate at which either a small object passes through a specific fluid under gravity, or by rate of flow through a known orifice.

The commonest measurements of viscosity are ‘Centistokes’ and ‘Redwood Seconds’.

Fuel oil uses the latter, lubricants tend to use the former, which are referred to by ISO numbers. Further consideration must be given to Viscosity Index. This refers to the rate of alteration in viscosity with variations in temperature. Viscosity index is also referred to as stability and is an important consideration for machines which have wide temperature differentials adjacent to their rotating bearings.

In particular screw compressors all have rotating bearings at their hottest ends, lubricated by the same oil as that at the cold end. Common viscosity levels are ISO 32; ISO 46 and ISO 68, where the listed viscosity is the viscosity at 40 C. Polyolester oils have higher viscosity indexes (more stable) and thus it is common to use thinner oils on R134a based systems, where use of a thinner oil marginally improves systems efficiency by reduced lubricative energy absorption. Oil will tend to thicken at low temperature, therefore due consideration must be given to ensure the lubricant chosen for a particular application will not thicken too much in the cold regions of the system otherwise it will stop flowing around the system and back to the compressor (see oil containment below). The low point temperature at which an oil thickens to a point where reasonable flow will cease is referred to as Floc Point. Oil Containment Retaining the oil within a fast rotating machine creates numerous engineering problems, particularly in machines designed to pump gas in close confinement with the lubricant oil. However, as fridge systems are closed loop systems, development technology has exploited this. Why contain the oil within the compressor, simply allow it to flow around with the refrigerant. For this effect to work efficiently the oil must be able to be carried effectively and this requires an easy mixing relationship (miscibility) between the oil and the refrigerant. This was a convenient relationship between traditional CFC refrigerants and simple mineral based lubricating oils.

The advent of CFC replacements posed a problem because the new HFC gases do not mix with the simple mineral type lubricants. However, it was found another group of lubricants, synthetic (man-made) Polyolester oils do mix well with these gases and these are now the commonest used oils for the new gases. These oils are known as Polyolester or simply ‘Ester’ oils. It is important to understand that miscibility problems occur only at relatively low temperatures when the oil tends to thicken (as described in the previous section), thus the greatest problems of separation occur at the coldest point of the system, i.e. the evaporator. An excess of oil build up here could cause several problems ... In order to maintain a balanced supply of oil to the bearings the oil must be returning to the compressor at a similar rate to it’s leaving. A build up of oil in the evaporator will reduce the effectiveness of the evaporator and thus the efficiency of the system will drop. Puddling of the oil can lead to mass return of oil at the suction of the compressor where it will attempt to compress the incompressible oil which can cause hydraulic failure of the mechanical parts. Procedures to adopt to effect refrigerant renewal procedures have been well documented elsewhere and will not be covered further here. Finally the commonest questions we are asked regarding oil concerns mixing of differing makes and grades. Mixing of Oils In this regard you may regard Mineral and Alkyl Benzenes as ‘Sugar’ and Polyolesters as ‘Salt’. Mixing any of the same broad type will not affect the overall performance, although the resultant viscosity will change proportional to the mix ratio. It is important is not to mix Mineral with Polyolester unless using with traditional (CFC / HCFC) refrigerants. Do not use Mineral on any system using R134a or a derivative.