Technical misunderstandings affecting refrigeration engineering fall into two broad camps, electrical & mechanical.
Mechanical problems of misunderstanding most commonly concern lubrication, although Liquid Refrigerant causes far more failures in actual practice, the Oil and the tasks it performs is far less understood.
The oil in most machines performs one basic requirement: Lubricates-Protects
When any two solid surfaces bear and move against each other, friction, i.e. resistance to movement, will arise. This is due to minute imperfections upon adjacent surfaces interlocking against each other. 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, sound etc.
For any bearing surface to have a reasonable life span it becomes necessary to physically separate the two solid surfaces. By use of a suitable liquid to make the separation happen, 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 shear film. Thus to provide an effective lubrication the medium must have ‘body’ to it, this is referred to as viscosity, also the medium must not have an effect of surface tension, allowing the medium to stay in place over an extended surface without being forced to remain there mechanically. Oil provides this function.
Cooling - 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 of the oil. 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 rotation occurs the shaft within the bearing will try to climb over the adjacent oil wedge, thereby suspending the shaft in an oil film.
After several revolutions the shaft and journal settle into an equilibrium where the two are equally separated 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 by splash effect, or in larger bearings by forcing a continuous flow into the bearing through appropriate drillings and oilways using a pump.
Viscosity & Viscosity Index
By varying 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 ranges considerably overlap each other. the ISO standard measurement for viscosity as used by our industry is the Centistoke.
Common viscosity levels are ISO 32; ISO 46 and ISO 68, where the listed viscosity is the viscosity at 400C. 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). the low point temperature at which an oil thickens to a point where reasonable flow will cease is referred to as Floc Point. Viscosity Index refers to the rate of alteration in viscosity with temperature variations. This 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.
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 afridge 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.
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 ....
a. In order to maintain a balance the oil must be returning to the compressor at a similar rate to it’s leaving.
b. Build up of oil in the evaporator will reduce the effectiveness of the evaporator and system efficiency will reduce.
c. Pudding 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.
FAQ Mixing of Oils
In this regard you may regard Mineral & Alkyl Benzene’s as ‘Sugar’ and Polyolester’s as ‘Salt’. Mixing any of the same broad type will not effect the overall performance, although the resultant viscosity will charge proportional to the mix ratio. It is important is not to mix Mineral with Polyolester unless using with traditional HCFC refrigerants or blends of these gases. Do not use Mineral on any system using R134a or a derivative.
These are simple refined products made direct from crude oil. Most mineral oils utilised in refrigeration are Napthenic type oils. Viscosities are normally rated at 32; 46 & 68 for use on most systems from - 400C to + 150C. Thinner viscosities are also available for very low temperature systems.
Common Grades of Mineral Oil
|Sunisco||3GS (ISO 32); 4GS (ISO 68): 5 GS (ISO 100)|
|Texaco||Capella WF (I) 32; 68|
|Shell||Clavus 32; 46; 68|
|Castrol||Icematic 266 (ISO 32); 299 (ISO 68)|
155 (ISO 32); 300 (ISO 68)
These are synthetically produced oil with very similar characteristics to mineral oils, however they are claimed to have improved lubricative qualities.
Common Grades are
|Texaco||ROLT 46 (ISO 46 is a common mid-grade)|
|Shell||SD (this is actually a mixture of mineral and AlkylBenzene)|
|Mobil||Arctic SHC424 (ISO 46)|
Zerice S46; S68; S100
These are fully synthetic oils produced in a totally different manner to synthetic Mineral oils. These are the commonest lubricants for HFC based gases. These oils use the same ISO grading system as Mineral / AB types. however they tend to be more viscosity stable and may offer improved lubricative qualities to the compressor in extreme situations.
Typical Grades are
|Mobile||EAL Arctic 22; 32; 46; 68 (EAL - Environmental Awareness Lubricant) ThermaCom do not recommend Arctic 22 for use with metalled bearings.|
|Castrol||Icematic SW 32; 68; 100|