TECHNOLOGICAL INNOVATIONS IN REFRIGERATION, IN AIR CONDITIONING AND IN THE FOOD INDUSTRY

POLITECNICO DI MILANO

29th-30th June 2001

DEVELOPMENT OF LUBRICANTS FOR

HYDROCARBON REFRIGERATION

Castrol International
Pangbourne
Reading
Berkshire
RG8 7QR
England
Tel + 44 1189 765280
Fax + 44 1189 845516
Email Swalloa@Castrol.com

DEVELOPMENT OF LUBRICANTS FOR

HYDROCARBON REFRIGERATION

P M Banfi - Castrol Italiana
A P Swallow - Castrol International

Abstract

With the phase out of CFC refrigerants, covered by the Montreal Protocol and by separate governmental legislations within several signatory countries, the refrigeration and air conditioning industry is required to adopt non-ozone depleting alternatives. For household refrigeration R134a was adopted initially as the refrigerant of choice by some major US, European and Japanese compressor manufacturers in favour of other alternatives.

As time has progressed and other alternatives have been fully evaluated and become commercially available household compressor manufacturers are now adopting hydrocarbons, as a more efficient, more environmentally friendly, solution to the domestic refrigeration needs of the 21st Century.

Mineral lubricants have been used for many decades in the refrigeration compressor industry, indeed since the days of sulphur dioxide, carbon dioxide, and CFC's. A great deal of new lubricant products have been proposed to compressor manufacturers when the new HFC's were developed and it is widely accepted that polyol esters are the lubricant of choice with these refrigerants.

With these same manufacturers now using hydrocarbons as a refrigerant of choice they again are looking for a better solution to their lubrication needs that is both cost and performance effective.

This paper details some standard work undertaken for the general development and testing of refrigeration lubricants and demonstrates some performance parameters of candidates for hydrocarbon refrigeration.

1. Introduction.

The phase out of CFC refrigerants, covered by the Montreal Protocol and by separate governmental legislation within several signatory countries, signified significant changes in the Refrigeration and Air Conditioning Industry. The decades of expert knowledge and experience that we had built up on those materials and systems now had to be built up again with a new set of operating parameters, and a new set of test regimes.

Household refrigeration was adopted R134a initially as the refrigerant of choice by some major US, European and Japanese compressor manufacturers in favour of other candidates offered.

As time has progressed and other alternatives have been more fully evaluated and become commercially viable household compressor manufacturers are now adopting hydrocarbons, as a more efficient, more cost effective, and more environmentally friendly solution to the domestic refrigeration needs of the 21st Century.

Mineral lubricants have been used for many decades in the refrigeration compressor industry, indeed since the days of sulphur dioxide, carbon dioxide, and CFC's. A great deal of new lubricant products were proposed to compressor manufacturers when the new HFC's were developed and it is widely accepted that polyol esters are the lubricant of choice with this type of refrigerant in domestic applications.

Adopting HFC 134a has presented problems, initially relating to the incompatibility of conventional mineral oils with the refrigerant. An answer to this was the development of synthetic polyol ester based lubricants, which are fully compatible with HFC 134a and formulated for suitable use. In addition a great number of manufacturing processes have been altered to accommodate the intolerance of HFC's for contamination from some of the products used in compressor manufacturing and assembly processes.

With these same manufacturers now using hydrocarbons as a refrigerant of choice they again are looking for a better solution to their lubrication needs that is both cost and performance effective.

2. Lubricant Development

2.1 Base fluid

Household refrigeration systems are renowned for their reliability and all the component parts, including the lubricant, are expected to have a useful life in excess of 15 years operation without maintenance. This can be compared to the life of an automotive lubricant that is around several hundred hours operation. The reliability of the refrigeration system is determined by the reliability of the compressor. The ability of the lubricant to provide good lubrication to the internal moving parts within the compressor and be compatible with the working refrigerant is crucial for both the system performance and long-term durability.

The first formulation decision to be made is the choice of the base fluid or base fluid mix that will be used. There are three fundamental types of basefluid: - a) mineral, b) modified mineral, and c) synthetic.

A) Mineral base fluids are essentially a selected fraction of crude oil with some components removed in order to improve performance. Every molecule present in the base stock is present in the crude oil. Typically this will be a mixture of several hundred different molecules.

B) Modified mineral oils, such as hydrocracked or hydrotreated basestocks, are produced from selected fractions of the refining process that undergo a severe treatment causing some of the molecules to rearrange. The resultant base stock will still contain many of the molecules that were present in the original crude oil. This type of material is likely to be more chemically and thermally stable than A).

C) Synthetic base stocks do not use molecules that were present in original crude oil. Instead the base stocks are synthesised by the chemical reaction of a very limited number of well-defined components. For example PAO's (polyalphaolefins) are derived from alpha-decene, and POE's (polyol esters) are synthesised from acids and alcohols. In this way the manufacture of synthetic base stocks can be tightly controlled and can provide performance that is equally tightly controlled to meet targeted performance. Synthetics can provide improved performance such as low temperature behaviour or high temperature cleanliness. This improved performance is obtained because of the nature of the synthetic molecule or the absence of unwanted components that are present in mineral oils.

Base stock selection from a formulatory perspective will depend upon the targets set from both a performance required and cost desired

Polyol ester lubricants have been successfully employed in several applications, usually where high temperature performance is required. Screening of polyol ester basefluids revealed several candidates that possessed the correct qualities for Hydrocarbon refrigeration, however the cost of an ester may be many times that of a mineral oil and it was felt that a more cost effective solution to the compressor manufacturer could be found from within the available portfolio of mineral base fluids.

2.2 Additives

The most appropriate lubricant to use is a mineral oil with some anti-wear / EP additives. The reason for this is that the mineral oil is the most cost effective option to the consumer (as compared to synthetic products) and the antiwear / EP additives are required to provide the antiwear protection needed. When there was a prevalence of use of CFC's with mineral oil the antiwear protection arose from the chlorine that was present in the refrigerant. For current applications of HFC's we have ensured that the antiwear / EP performance is met by both the selection of the base fluid as well as the use, where necessary, of specific additives.

It is equally true of lubricants as of many purchases in that the performance of a product will depend upon the price paid for it. In applying this to mineral oils, apart from the underlying raw material cost, the level of refining that the product has undertaken will ultimately dictate the end price. The lower priced product is usually as a result of a minimal refining process.
This refining process will reduce the presence of nitrogen-, sulphur- and oxygen containing heterocycles, together with mercaptans, thioethers and disulphides. Some of these materials will provide a beneficial effect to the overall lubricant composition, whilst others will be deleterious to performance. In the mineral base fluids selected for hydrocarbon refrigeration development we have taken a highly refined mineral oil in order to reduce the number of variable resulting from general base oil quality and then added in known additives to achieve effective wear protection.
The additives may not be required as the discharge pressures (and resultant compressor stress) are reduced when a hydrocarbon refrigerant is used as compared to an HFC, but the ever-increasing duties demanded from the same compressor mean that it is preferable to provide a product that is slightly above specification for the duty rather than one that may cause failure under excessive conditions as long as the overall product cost is kept to a minimum.

The ever-increasingly important component of compressor design is the lubricant. This is in itself a designed component with a selection available from many different base stocks and additives to formulate a final product. The overall lubricant formulation is a balance of many different aspects of performance that need to be considered as a whole and not as single, separable component selection effects.

3. Physical Properties

3.1 Kinematic Viscosity

The viscosity of a fluid is its resistance to flow and is directly affected by the temperature of the liquid. Viscosity decreases as temperature increases and increases as temperature decreases. Kinematic Viscosity is normally expressed in centistokes (cSt) and is normally determined at 40°C and 100°C. These two values can then be used to determine the Viscosity Index (VI) of the lubricant. The measurement is made by timing a defined volume reservoir of the fluid to pass through a narrow aperture in a glass tube. The time taken for this fluid volume to pass through this aperture is directly related to the viscosity of the fluid. A photograph of a viscometer, together with a representative schematic of the tubing is illustrated below.

Figure 1. Automatic CAV-4 Viscometer

Figure 2. U-Tube Viscometer Assembly

Original Equipment Manufacturers (OEMs) normally specify the required viscosity grade for use in their equipment with a given refrigerant. The grade for refrigeration lubricants is usually defined as the ISO viscosity of the lubricant at 40°C.

The VI is a measurement of the change of viscosity with respect to temperature. The higher this number then the less the lubricant viscosity changes with temperature. An illustration is shown below to compare a mineral oil, PAO, and ester. It can be clearly seen from this illustration that although the viscosity at, say, 40°C, may be identical, the viscosity index value will determine the behaviour with respect to change in temperature.

Figure 3. Viscosity Index Comparison

As the lubricant in a refrigeration system is in intimate contact with the refrigerant there will always be an amount of refrigerant dissolved in the lubricant. As the viscosity of a refrigerant is very low, the effect is to cause a significant reduction in the viscosity of the lubricant/refrigerant mixture. This subject is more fully discussed in other papers.
Reduced viscosity is a concern as the lubricant may not be able to provide sufficient film strength for separation of the moving parts, leading to increased wear and failure.

3.2 Floc point

This is an industry standard test that is used to provide a guide to the lowest operating temperature that a lubricant should be used at with refrigerant R12. The test uses the fact that the R12 acts as a solvent to the mineral oil. As the temperature is reduced, the waxes present in a mineral oil can no longer be held in solution and will 'flocculate'. The temperature at which this starts to occur for a mixture of 10% lubricant in R12 is known as the floc point. The waxes in a mineral oil can, if not fully solubilised in the refrigerant, cause capillary tubes to block and expansion valves to stick. The test is really only valid for mineral oil lubricants and to a large extent has been superseded by refrigerant miscibility testing.

3.3 Refrigerant Miscibility

This is an extension of the ideas used in the Floc Point test detailed above but it can be applied to any refrigerant/lubricant mixture, thus giving minimum recommended operating temperatures for specific combinations.

3.4 Pour Point

The pour point of a lubricant is the lowest temperature at which a lubricant will flow. Although it is deemed desirable to use a lubricant that has a pour point that is lower than the lowest system temperature it is not essential provided the lubricant is at least slightly soluble with the refrigerant. The addition of the very low viscosity refrigerant, acting as a solvent, to the lubricant, will cause a significant depression of the pour point. This will usually provide sufficient mobility of the lubricant for system requirements.
Common types of contamination that cause a variation in the pour point can be a maintenance error in which the system is charged or topped off with an incorrectly specified lubricant, high wax content residues from metal working or build fluids, or fine wear debris that act as the nuclei for coagulation. These sources can normally be avoided by following good maintenance practices.
The pour point of used lubricants can be affected by the onset of oxidation in the lubricant base fluid and overall contamination of the lubricant in use.

3.5 Thermal Stability

Oxidation is caused by exposure of the lubricant to high temperatures in the presence of air. The result of oxidation is the formation of sludge, particulates and deposits that will increase the viscosity of the lubricant. In addition to the increase in viscosity these oxidation products also increase the pour point of the degraded lubricant.
A common test to evaluate the overall thermal stability of the lubricant / refrigerant mixture is the ASHRAE 97 test. This is where metal coupons are placed in intimate contact with the lubricant / refrigerant mixture and then heated at various test temperatures and durations. The ISO 15 product was tested as detailed below for 2 weeks with isobutane (R600a).
It can be seen that there is some lubricant degradation at the elevated temperatures, but this is expected with mineral oil type lubricants and not considered an issue with R600a where the operating conditions in respect of temperature and pressure are less severe than with HFC's.

Table 1. ASHRAE 97 Tests

4. Friction and lubricating regimes

4.1 Friction

Friction is defined as 'the resistance a body meets in moving over another body in respect of transmitting motion'. Friction coefficient is defined as friction force / normal force. In the lubricated surface situation the coefficient of friction will be determined by the lubrication regime. In simple terms there are two lubricant regimes, hydrodynamic regime (thick lubricant film) and boundary regime (thin lubricant film). Friction modifiers act most successfully in the boundary regime. The friction co-efficient of hydrodynamic lubrication is between 0.001 and 0.01. This is effectively hydroplaning.
Friction co-efficient as determined via the pin-on-disc or Cameron-Plint machine will depend on the friction between two nominally smooth metal surfaces. The friction co-efficient can be modified by a change in: -

1. Pressure
2. Temperature
3. Surface finish
4. Surface material identity
5. Chemistry of the friction modifier additive.

In terms of lubricant development the only one that we have a direct influence over is the chemistry of the friction modifier, the other parameters are determined by the compressor OEM's. Work is undertaken in the lubricants field but also consideration must be given to the other aspects listed above.

There is a constant striving for improved efficiency within the domestic refrigeration market and this has focused a lot of attention when R134a was first developed as a suitable alternative for R12. The mission to constantly improve the performance of both the compressor and the overall unit continues, both in the light of general competitive nature as well as environmental legislation. In order to improve the efficiency there have been many technological developments, but from a tribologists perspective the above items are of most interest.

4.2 Surface Treatments

One area that is increasingly utilised is that of surface coatings. These are employed because although the general materials used in compressor construction are of importance, the surface is the most important part. The desired properties of the material surface may not be required of the material in totality and the most cost effective solution is to utilise minimal material on the surface to provide the desired properties. Surface coatings generally utilised are of gaseous state (Chemical Vapour Deposition, Physical Vapour Deposition, Ion implantation), solution state (Chemical solution deposition, Electrochemical deposition), or molten state (Laser surface treatment).
Even with modern manufacturing techniques the surface can have many defects and surface coating can be the only way to remove the asperities caused in the manufacturing process.

4.3 Dry Film

The utilisation of Bonded Solid Film Lubricant products started with the aviation industry many decades ago where performance under extreme operating conditions was critical. This has now progressed to many areas of industrial, automotive, and aerospace industries. The bonded solid film lubricants contain materials with inherent lubricating properties that are then bonded firmly to the underlying material surface. The methods employed include resin bonding, burnishing, mechanical impingement, and sputtering.
It is important in the application of these films to consider the end cost implications to the consumer. It is far better usually to manufacture a fit-for-purpose unit that will be commercially viable rather than an extremely high performance piece of machinery that will not sustain a commercially viable business.

5. Design Partnership

There is an obvious cost involved in development of efficient and reliable compressor units and this is more effectively served from a design partnership between engineers and tribologists in order to provide acceptable performance within defined costs.

5.1 Laboratory Wear Testing

Modern manufacturing techniques have enabled compressors to be built with ever finer tolerance, which will increase the potential for lubricant failure from a reduced film thickness providing the same or better lubrication.

In order to develop the lubricant it is necessary to consider the strengths and weaknesses of the selected basestocks and additives relative to the performance that is required. One method for identifying these strengths and weaknesses is to use screening tests. Screening tests are typically small-scale laboratory tests that are inexpensive to run and can be carried out at either a high rate or be able to be completed in a short period of time. It is important that the screening tests mimic as closely as possible the conditions within the compressor or that they can be relied upon to differentiate the fluid performance within the compressor. The more close to real life the screening test becomes then usually the more expensive it also becomes.

The wear tests employed as a screening test to evaluate these lubricants are a combination of 4-Ball (single ball rotated in contact with three fixed balls - IP239), Falex (pin rotating between two fixed V-Blocks - ASTM D2670) and an in-house test, SRV (Oscillating ball on disc).

The results of the 4-Ball and Falex tests are shown below for a typical refrigeration lubricant.

Table 2. Wear Test Results

5.2 Compressor Testing

As we have stated earlier there are a great number of modifications that can be made to a compressor in order to achieve improved performance, the lubricant being only one piece of this complex jigsaw.
For instance a reduction in the viscosity of the lubricant will result in improved efficiency simply by reducing the viscous drag when the compressor is started up. There is a limit to the minimum viscosity of lubricant required in the operation of compressors without significant investment from the manufacturer in respect to the overall manufacturing process, some parts of which may not be under their direct control. These detrimental effects may only be observed when validation testing is scaled up to full scale compressor testing.
This means that although the screening tests can be used to identify lubricants strengths and weaknesses an important and integral part of the testing lies within the compressor test.
A compressor test programme is usually performed in conjunction with an OEM in order that maximum benefit can be gained from this relatively expensive part of the product validation process, be it from failure or pass.

The compressor tests performed on the candidates for hydrocarbon refrigeration were as detailed: -

Test Duration 168 Hours
Suction Pressure 2.25 Bar
Discharge Pressure 23 Bar
Winding Temperature 105 Deg C
Compressor Shell Temperature 86.9 Deg C
Compressor Oil Temperature 84.4 Deg C

The compressors were disassembled after test and all the parts examined.
There was negligible wear reported on the moving parts and the surface coating on the critical parts was undamaged.

OEM tests were carried out to compare the Coefficient of Performance (COP) and comparison of the Noise Level with a typical R134a / POE compressor and these proved acceptable.

6. Conclusion

This paper has discussed some detail in respect of the design considerations for lubricants for hydrocarbon refrigeration and demonstrated some key areas of performance validation.
It is important to appreciate that it is essential to match up the different components to give efficient and reliable performance in the final refrigeration system to be presented to the end consumer.

These components include all the compressor parts that are in motion in addition to the base fluid and additive formulation. It is not just a principle of selecting the finest materials available and combining them. We have seen this in the evolution of food preparation where it is the overall balance of the final ingredients in the meal that is critical rather than the single individual components.

7. References

R.M. Mortier & S.T. Orszulik, Chemistry & Technology of Lubricants, Blackie 1992.
W.S. Robertson, Lubrication in practice, Macmillan Education Ltd 1987.
R M Gresham, Bonded Solid Film Lubricant, STLE Basic Lubrication Course, London 1997.