©Copyright CPI Engineering Services, Inc. 1995,1996
By Glenn D. Short and Thomas E. Rajewski
CPI Engineering Services, Inc., PO Box 1666, Midland, MI 48641-1666
Every manufacturer of refrigeration and air-conditioning compressors and systems has had to re-evaluate the requirements for lubricants. The HFC (hydrofluorocarbon) refrigerants that are replacing CFC (chlorofluorocarbon) refrigerants have a different influence on lubricants which effects both compressor durability and system performance. Other types of refrigerants such as ammonia and hydrocarbons are also alternatives to CFCs, and new lubricants are being evaluated to improve performance in these systems.
This paper provides an overview of several types of synthetic lubricants currently supported by equipment manufactures. Practical examples are provided to improve the understanding of these new lubricants, how they are used, and suggested application guidelines. Also described are lubricants currently being developed.
An increased awareness that chlorine containing refrigerants are depleting the ozone layer has resulted in the increased use of HFC (hydrofluorocarbon), ammonia, and hydrocarbon refrigerants. Synthetic lubricants improve performance with these refrigerants.
Polyalkylene glycol (PAG), polyol ester (POE), alkyl benzene (AB) and other new lubricants have been developed for commercial application with HFC and blends of HFC with other refrigerants. Mineral oils have generally been found to be unacceptable with these refrigerants due to poor miscibility. Major development considerations for the synthetic lubricants include: miscibility, solubility, stability, electrical properties, lubricity and retrofitting requirements.
PAG and some new types of synthetic lubricants are soluble or miscible with ammonia. This allows the use of ammonia in refrigeration systems with direct expansion (DX) evaporators. Immiscible types of synthetic oils, such as polyalphaolefin (PAO), are used in traditional ammonia systems where their good low temperature properties allow operation at very low temperatures.
The efficient use of hydrocarbon refrigerants may require lubricants that are higher in viscosity or less soluble than available refrigeration grade mineral oils. Synthetic oils such as PAO and PAG are currently being used.
HFC refrigerants and suitable lubricant types
Table I provides an overview of synthetic lubricants which are used with many of the most common types of halocarbon refrigerants. Each type will be described.
In a 1985 publication, Kussi provided a good review of the chemistry of PAG lubricants . The current PAGs being used for HFC applications are of three distinct groups: polypropylene glycol, polypropylene-polyethylene glycol and polypropylene-polyethylene ether. The glycols can be classified as mono, di or tri-functional. This is an indication of the number of terminal hydroxyl groups that are involved in the manufacture of the product.
Polyol esters (POE) include a wide variety of materials . These lubricants are made by combining an organic neopentyl alcohol with an organic acid. Some of the alcohols used are: neopentyl glycol (NPG), trimethylol propane (TMP), pentaerythritol (PE) and di-pentaerythritol (Di-PE).
The major difference between the POEs currently being used for HFC refrigeration is the type of acids attached to the alcohols through a chemical reaction (esterification). Acids are classified as “linear” or “branched” or “mixed.” For HFC applications, the acids used usually contain from four to ten carbons (C4 to C10), but can have up to twenty carbons if the initiating alcohol is neopentyl glycol.
Currently there are three other types of synthetic lubricants entering the market. Carbonates are esters or diesters of carbonic acid, usually made through transesterification of dimethyl carbonate . Complex esters utilize malonate-acrylate chemistry . Modified PAGs combine short chain acids with the standard type of PAG to form an ester .
Table 1. Common Types of Lubricants and Refrigerants.
KEY: 1.) Recommended 2.) Alt. Recom. 3.) Not Recom. E.) Experimental
Miscibility relationships with HFC refrigerants
Refrigeration system designers are interested in how the lubricant behaves in the system so that they can design piping and other components to best manage lubricant return to the compressor. The behavior of a refrigerant on a lubricant entering the system can affect film characteristics on heat transfer surfaces, and thus energy efficiency performance. Generally, the first property considered is miscibility (lubricant with liquid refrigerant).
Fig 1. Miscibility Limits of Difunctional Polyalkylene glycols of Varying Molecular Weights with HFC-134a ( AMU = average molecular weight )
Generally lower viscosity or lower molecular weight lubricants have better miscibility.
With PAG lubricants, higher functionality improves miscibility for a given viscosity grade. The inclusion of ethylene improves miscibility with HFCs but also increases water solubility. Replacing the hydroxyl group, making the product an ether, lowers water solubility and reduces miscibility with HFCs. The PAGs have interesting miscibility characteristics called “inverse solubility.” They become less soluble at increased temperatures. This may improve compression efficiency in rotary compressors . Figure 1 shows these general relationships with HFC-134a.
POE lubricants based on lower molecular weight alcohols, such a neopentyl glycols, tend to be more miscible than those based on the higher molecular weight alcohols as shown in Figure 2. POE lubricants using linear acids become less miscible when manufactured in higher viscosity grades, usually due to the use of higher molecular weight acids. Using branched acids helps to improve miscibility.
Fig 2. Miscibility Changes of Branched Chain Polyol Esters with HFC-134a, Based on Starting Alcohol
Replacements for R-22 and R-502 have taken two courses of development. One is to use HFC blends (these sometimes contain R-22, for short term “retrofit” replacement refrigerants), another is to include a hydrocarbon such as butane in a blend with HFCs. Including hydrocarbon gasses or R-22 in a refrigerant blend improves the miscibility with hydrocarbon oils. The improvement may not be adequate to allow the use of hydrocarbon lubricants. Figure 3 compares miscibility relationships for various types of POE with HFC refrigerants.
Fig 3. Miscibility with HFC Blends Compared to Non-Blended HFCs 10% Lubricant Concentration
The use of lower cost alkyl benzene oils where possible is desirable. AB oils are miscible with some less polar HFC, HFC/HCFC blends and HFC/hydrocarbon blends. Even so, some OEMs have suggested that at least fifty percent POE be blended with alkyl benzenes for use with the HFC/HCFC blended refrigerants in lower temperature applications.
It is possible to use an oil with less miscibility provided the amount of lubricant entering the evaporator is equal to the amount leaving. Our experience with a very low pour point (-90oC) synthetic hydrocarbon oil with HFC-23 has shown excellent results in systems with direct expansion evaporators. At least one OEM has recommended low viscosity hydrocarbon oils for use in air-conditioning systems. Another has suggested that very low viscosity dialkylbenzes be used. With this lower viscoity, miscibility is no longer a critical issue in oil return. This has proven suitable for applications with temperatures of -40oC and greater . Some mineral oil manufacturers are testing additives to form oil-HFC mixtures or emulsions. Our own laboratory has developed a new type of hydrocarbon oil that has a few percent of miscibility with HFC-134a. These lubricants are not likely to be OEM approved for several years due to extensive testing requirements.
Solubility and viscosity with HFC refrigerants
OEMs examine viscosity-solubility information at various temperatures and pressures to determine the optimum viscosity to lubricate and seal compressors, and to provide adequate fluidity to return from evaporators. Figures 4 and 5 provide various examples.
Fig 4. PVT Relationship of 100 ISO
Fig 5 : PVT Relationship of 100 ISO
Polyol Ester and R-410A Polyol Ester and R-404A
The ideal case is to have a lubricant that has a minimum reduction in viscosity due to dilution by the gas (solubility) in the compressor but is miscible in the evaporator.
Figure 6 shows the relative solubility of three ISO VG 32 POE lubricants with an HFC blend, R-404A at saturation conditions. . Excessive solubility will lower the capacity of the compressor due to expansion of the dissolved refrigerant in the compressor and reduced compression sealing efficiency. The cooling effect of this expansion may also result in low compressor discharge temperatures. It is noteworthy that the S-31-HE has less solubility but was previously shown to have improved miscibility compared to the other POE lubricants.
Fig 6. Relative dilution at saturation pressure with R- 404A
Stability and system cleanliness with HFC refrigerants
Tests for lubricant stability predict the possible interaction of the lubricant and refrigerant at high temperatures. The sealed tube method most often uses steel, copper and aluminum metal strips. Metals act as a catalyst and also provide a measure of the effect of the lubricant/refrigerant pair on the metal. Visual results and chemical analysis indicate the breakdown of the refrigerant or the lubricant. This type of test is also useful in examining the effects of contaminants, such as moisture, and for screening additives.
POEs, PAGs and carbonates all have a tendency to accumulate moisture during storage. Some POEs may break down rapidly in the presence of small amounts of water. More than 100 ppm for POE lubricants and 250 ppm for PAG lubricants is unacceptable for these new lubricants. Specially designed additives such as antioxidants and hydrolysis stabilizers are effective in reducing problems with residual moisture and air.
Contaminant metals can cause deterioration of POEs in refrigerant systems. Metal catalysts, such as tin, must be avoided or completely removed during the lubricant manufacturing process. Some types of brazing flux can result in rapid acid number increase . The authors have found that certain plated components can cause similar results. These contaminants may result in copper transfer within the system. The presence of moisture or chlorine containing refrigerants increases the potential for these types of reactions.
POEs made with a high percentage of branched acids are more stable in the presence of water than those with a high proportion of linear acids. A more important factor is the final processing factors of the ester, such as the degree of esterification and removal of excess acids and of catalysts after esterification. The carbon length of the acids used for manufacturing POEs should also be considered. One investigator found that the POE made from a linear pentanoic (C5) acid produced a Total Acid Number four times greater than that made with a branched octanoic (C8) acid in ambient air . Acids produced may have a rapid effect on the system, such as corrosion or copper plating. The properly synthesized POE lubricant S-31-HE has about forty percent less acid forming tendency compared to three other commercial POE lubricants.
Carbonates were thought to be more stable as they did not produce unwanted acids upon decomposition. It was later found that these lubricants were breaking down to form significant amounts of carbon dioxide. More recent efforts have been toward developing more stable carbonates.
Electrical properties and moisture with HFC refrigerants
Tests such as dielectric strength and motor winding and insulation compatibility are necessary for hermetic and semi-hermetic systems. The lubricant is in intimate contact with the motor windings in these systems and therefore must possess good electrical insulation, thermal stability, and material compatibility. Careful handling of new lubricants and special procedures while conducting compressor and system maintenance must be followed to keep moisture below 100 ppm. Many manufacturers suggest that the compressor not be left open for more than 15 minutes. The use of properly sized filter-dryers is important to keep moisture levels low and reduce contaminant levels.
Wear and compressor durability tests with HFC refrigerants
Compressor and Lubricant manufacturers are spending millions of dollars testing new lubricant/refrigerant combinations. These tests generally are a series of compressor bench tests beginning with short term (100 to 400 hours) followed by long term (4000 to 8000 hours). Each model type and refrigerant/lubricant pair are tested in the course of the program.
Some laboratory test methods are: Falex pin and vee block , ring and disk , Falex block and ring test, and a custom made High Pressure Tribometer . More specific tests have also been conducted with roller bearings  with R-134a. Falex pin and vee block results in HFC-134a atmosphere produced the following results.
Once again, the S-31-HE and S-68 formulas showed superior results.
There are some common results of lubricity investigations with HFCs. All investigators note that refrigerants such as HFC-134a do not themselves have lubricating properties. CFC and HCFC refrigerants were shown to have relatively better lubrication properties, particularly boundary lubrication, most likely due to the anti-seizure properties of chlorine. Small amounts of oxygen (equivalent to 0.75% air) results in reduced friction and wear. Small amounts of moisture also decreases friction and wear but may affect the reactivity of refrigerants such as HFC-134a . Lubricity additives such as phosphorus may require oxygen in the system to be effective. Most important, the HFC lubricants tend to have more erratic lubricating behavior at higher refrigerant dilution concentrations. This is the opposite of CFCs, such as CFC-12, which had higher seizure loads with increasing refrigerant concentration.
Controlling lubricant solubility (less solubility) is more effective in reducing wear than the use of lubricity or EP additives . The S-31-HE and S-68 described were formulated a balance of molecular components to achieve desired lubrication properties, stability, solubility and miscibility.
Retrofitting with HFC refrigerants
Retrofitting calls for draining as much of the mineral oil from the system as possible and replacing it with the new, POE lubricant. The POE is then used with the existing refrigerant for a period of time, drained and replaced with new POE. The second lubricant replacement is generally accompanied by replacement with the new refrigerant. The object is to remove as much of the mineral oil from the system as possible, using the old refrigerant to help make the mineral oil soluble. Additional “flushing” with new lubricant may be necessary until the mineral oil level drops below about three percent or lower. The cost of flushing systems is recovered quickly through evaporator efficiency improvements.
Lubricants for Ammonia as an alternative to CFC and HCFC refrigerants
Ammonia, which has no ozone depletion effect, has been a choice for efficient refrigeration for a hundred years. Limitations include a strong odor and limited range of flammability in air. Even with these limitations, ammonia is being considered for applications with limited exposure to dense populations. Such applications include rooftop air-conditioning, water chillers, secondary and remote locations. Synthetic lubricants are now available that improve efficiency and expand application opportunities.
Mineral oils and synthetic hydrocarbon lubricants have low solubility and miscibility with ammonia. This is a benefit in flooded evaporator systems as these oils are heavier than ammonia and can be easily removed by draining from the bottom of evaporator vessels and returned to the compressor. Synthetic and semi-synthetic oils have been developed for optimum performance in these systems .
The solubility and miscibility of hydrocarbon oil limits applications in systems with direct exchange (DX) evaporators. PAO lubricants with improved low temperature fluidity offer some advantage over mineral oils and alkyl benzene oils. Other types of synthetics have been developed which are soluble with ammonia and provide more efficient heat transfer.
It has been known for over twenty years that PAG and PAG ether lubricants are soluble with ammonia. Increasing the ethylene content in PAGs increases miscibility with both ammonia and water. Since ammonia systems often contain water, this could be a problem. PAG copolymers of ethylene and propylene have inverse solubility with water. Keeping compressor discharge temperatures at 70oC or higher can prevent water accumulation. The PAG-ether lubricants limit water solubility to a few percent. A restriction of the use of PAG lubricants is that the miscible varieties have very low solubility with hydrocarbon oils. Compressors and systems must be flushed to remove any mineral that may be present. Both types of lubricants are currently providing good results in field tests.
We have developed a new type of synthetic that has a high degree of solubility with ammonia, good mixing properties with mineral oil and limited solubility with water. This new synthetic is currently being used in several water chiller applications equipped with rotary screw compressors and aluminum DX evaporators. Conversion from the mineral oil is not difficult, the mineral oil is drained and the new lubricant installed. This new product has a high degree of solubility, but limited miscibility. This property improves oil transport properties and heat transfer. Initial performance tests resulted in a twenty percent improvement in heat transfer efficiency when compared to mineral oil. Another, more miscible version of this lubricant was developed for low temperature applications.
Lubricants for hydrocarbon refrigerants
Propane, butane and other hydrocarbon refrigerants are being considered as substitutes for CFC and HCFC refrigerants due to their low ozone depletion potential. The major limitation is extreme flammability. These refrigerants have long been used in refinery applications. PAG lubricants have been used very successfully in these applications [15, 16]. They resist dilution by the refrigerant during compression, even at high pressures, and thus provide improved compression efficiency. Careful selection of the PAG lubricant will also result in adequate oil transport properties. PAO lubricants have also been selected for these applications. These lubricants are available in high viscosity grades, which helps to provide more efficient compression. PAO lubricants are completely miscible with hydrocarbon refrigerants.
Several types of lubricants have been developed over the past several years. While many of these lubricant types are not new, many are modified to enhance their suitability for today’s refrigerants. The performance of these lubricants has been tested in the laboratory and through collaboration with industry, compressor system and component manufacture experts throughout the world. Several lubricants for HFC applications are available for HFC-134a, HFC-23, as well as several blends. New lubricants for ammonia and hydrocarbon refrigerants that offer greatly improved efficiency. Research will continue to improve these lubricants and provide additional choices.
- Kussi S. “Polyethers as Base Fluids to Formulate High Performance Lubricants,” Lubrication Engineering Magazine , L47, 11, 926-933,.STLE, 1991.
- Shubkin, R.L. editor. “Synthetic Lubricants and High-Performance Functional Fluids,” Marcel Dekker, Inc., NY, USA, 1993.
- Takahata K. Et al., “New Lube Oil For Stationary Air Conditioner”, International Refrigeration Conference at Purdue Proceedings, 1994.
- Lilje and Sabahi ,”A Novel Class of Synthetic Lubricants Designed for HFC Compressors”, International CFC and Halon Alternatives Conference 1994, pp. 145-153.
- McGraw P. ASHRAE Symposium, Chicago, Jan. 1993.
- Sjoholm, L., and G.D. Short. “Twin Screw Compressor Performance and Suitable Lubricants with HFC-134a”, International Refrigeration Conference at Purdue Proceedings, 1990.
- Sunami, M. Et al., “Optimization of POE Type Refrigeration Lubricants”, International Refrgieration Conference at Purdue Procedings, 1994.
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- Komatsuzaki, S. Et al “Polyol Esters as HFC-134a Compressor Lubricants”, Lubrication Engineering, Volume 50, 10, 801-807,1993.
- Mizuhara K. Et al, “The Friction and Wear Behavior In Controlled Alternative Refrigerant Atmosphere” Tribology Transactions, Volume 37(1994), 1, 120-128, 1992.
- Cusano, C. Et al , “Tribological Evaluation of Contacts Lubricated by Oil-Refrigerant Mixtures”, ACRC Project 04, University of Illinois, ACRC TR-16, 1992.
- Wardle, F.P. Et al, “The Effects of Refrigerants on The Lubrication of Rolling Element Bearings Used in Screw Compressors”, International Refrigeration Conference at Purdue Proceedings, 1992.
- Short, G. ” Refrigeration Lubricants Update: Synthetic and Semi-Synthetic Oil are Solving Problems with Ammonia and Alternative Refrigerants”, IIAR Annual Meeting, 1990.