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Alkyl Benzene As a Compressor Lubricant W Purdue University Purdue e-Pubs International Compressor Engineering Conference School of Mechanical Engineering 1974 Alkyl Benzene as a Compressor Lubricant W. D. Cooper Freon Products Division R. C. Downing Freon Products Division J. B. Gray Freon Products Division Follow this and additional works at: https://docs.lib.purdue.edu/icec Cooper, W. D.; Downing, R. C.; and Gray, J. B., "Alkyl Benzene as a Compressor Lubricant" (1974). International Compressor Engineering Conference. Paper 103. https://docs.lib.purdue.edu/icec/103 This document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact [email protected] for additional information. Complete proceedings may be acquired in print and on CD-ROM directly from the Ray W. Herrick Laboratories at https://engineering.purdue.edu/ Herrick/Events/orderlit.html ALKYL BENZENE AS A COMPRESSOR LUBRICANT W. D. Cooper, R. C. Downing, J. B. Gray, "Freon" Products Division, Du Pont Company, Wilmington, Del. Mineral oils for the lubrication of refrigeration at which the pure oil does not readily flow is compressors have been developed to a high level somewhat lower for the alkyl benzene but the of performance and have served with distinction difference is not great. Relatively small dif­ over a wide range of applications. However, in ferences are also found in the specific gravity, some cases, the demands of refrigeration systems flash point, aniline point, etc. have exceeded the ability of mineral oils to per­ form adequately. Two areas where mineral oils The greatest differences in the properties of have been deficient are in good oil return with alkyl benzene and mineral oils are found in the some refrigerants such as R-22, R-502, R-13, and floc point and thermal stability. The floc point R-503, and in high temperature stability. Good has long been used in the refrigeration industry oil return is directly related to the solubility to measure the likelihood of wax separation from of refrigerants in the oil at low temperatures. solutions of oil and refrigerant. The test is High refrigerant solubility tends to keep the made with R-12 and is not intended to indicate oil viscosity low and allow it to be moved out of general solubility relationships but rather the the low temperature areas by the f~ow of refrig­ possibility of the deposit of potentially harmful erant. A deficiency in high temperature stability higher melting constituents of the oil. Solu­ results in coking, varnishing, sludging, and bility relationships for the two oils with other other evidences of oil breakdown. refrigerants are compared later. Alkyl benzenes have been found to offer improved TABLE 1. TYPICAL PROPERTIES OF OILS performance in refrigeration systems, both with respect to better oil return and better high Alkyl Mineral temperature stability. Representatives of this Benzene Oil type of oil have been in limited use in Europe for some time (1). Sanvordenker (2) has reviewed Viscosity, SUS 100°F 150 155 the properties of alkyl benzene oils as well as 210°F 41.1 4o.8 other synthetic materials of possible interest in refrigeration. Schwing (3) has studied the Molecular Weight 320 325 properties of polyisobutyl benzenes in the labo­ Pour Point, °F -50 -45 ratory and found that they are very stable in the Floc Point, °F <-115 -68 presence of R-12. Sealed-tube tests showed Aniline Point, °F 125 160 superior performance on the basis of chloride ion Specific Gravity, 60/60 0.872 0.917 development, copper plating, and the production Dielectric Strength, KV >34 >34 of dissolved copper. SOLUBILITY RELATIONSHIPS It is the object of this paper to describe the properties of an alkyl benzene lubricating oil R-12 is miscible in all proportions with alkyl for refrigeration applications. The properties benzene down to the lowest temperature measured, and performance characteristics of a typical about -ll5°F. Since R-12 is also miscible with naphthenic oil are also included as a frame of naphthenic mineral oils to about the same low reference. temperature, little difference would be expected between the two oils as far as oil return is con­ OIL PROPERTIES cerned. Typical properties of an alkyl benzene oil are The solubilizing effect of alkyl benzene for compared with those of a mineral oil with about refrigerants is more evident with R-22. The the same viscosity in Table 1. Some properties consolute temperature is used to indicate the are quite similar while others are greatly dif­ magnitude of mutual solubility of an oil and ferent. The viscosity at the standard reference refrigerant. This point is the highest tempera­ temperatures of 100°F and 210°F is similar for ture at which separation into two liquid phases both oils. In the example shown, the change in occurs. For R-22 and a typical naphthenic viscosity with temperature seems to be slightly mineral oil, this temperature is about 35°F for less for the alkyl benzene than for the mineral a solution containing about 80% R-22 by weight. oil but this difference is not considered sig­ The consolute temperature has not been determined nificant. The molecular weight for the two oils exactly for R-22 and alkyl benzene but is some­ is about the same. The pour point or temperature where in the vicinity of -l00°F. An even greater 88 difference is found with R-502. With this refrig­ oil. It has been shown that breakdown of oil at erant and a naphthenic mineral oil, two liquid the temperatures found in refrigeration systems is phases exist at all temperatures up to the crit­ often caused by interaction with the refrigerant. ical temperature of the refrigerant (180°F) for (4). During the reaction, the refrigerant is compositions ranging from about 50% to 90% R-502. reduced and one chlorine atom is replaced by a With alkyl benzene, the consolute temperature is hydrogen atom. For example, R-12 CCl2F2 produces about -25°F at a composition of 58% R-502. At R-22, CHClF2 R-22, CHClF2, produces R-32, higher temperatures, the two liquids are com­ CH2F2 etc. The formation of reduced refrigerant pletely miscible in all proportions. has come to be used as a measure of oil/refrig­ erant stability in sealed tube tests. The test Other solubility comparisons are illustrated in is usually made with R-12 and the oil in question Table 2. The data is for saturated conditions. sealed in glass tubes in the presence of copper, For example, in the first and second columns steel, and sometimes aluminum strips. The amount showing refrigerant solubility in oil, the addi­ of R-22 produced shows the extent of reaction tion of more refrigerant would lead to the forma­ between the oil and refrigerant. The tubes are tion of a second liquid phase. On the other heated for two weeks at a temperature of 175°C hand, the addition of more oil to the conditions (34T°F). The results of such stability tests represented by the last two columns would cause summarized in Table 4 show that alkyl benzene is the formation of a second liquid phase. It can distinctly superior to mineral oil in this impor­ be seen that both R-22 and R-502 are considerably tant property. more soluble in alkyl benzene than in mineral oil. Resistance to oxidation can also be used to judge The solubility of refrigerant in oil is important the stability of lubricating oils. Of course, in dry-type evaporators where all of the refrig­ oxygen should not be present in a properly sealed erant is evaporated before reaching the evapora­ and charged system, but sometimes it is. Centrif­ tor outlet. In this condition high refrigerant ugal compressors used with high boiling refrig­ solubility tends to keep the oil thin and less erants normally have inward leakage of air. In viscous and thus enables it to more easily be one test of oxidative stability (5), the oil is returned to the compressor. held at ll5°C (239°F) in an open beaker for 96 hours in the presence of copper. The quality of Information on the solubility of oil in liquid the oil is judged by the development of color. refrigerant is of interest in flooded evaporators In this test, alkyl benzene turns from colorless or liquid re-circulation applications. Oil solu­ to pale yellow while a mineral oil of similar bility determines whether or not a separate oil viscosity becomes black. layer is formed and what methods of oil separa­ tion and return are needed. FOAMING The solubility of refrigerant gas in oil is of Oil foam is usually generated in the compressor interest in various parts of the refrigeration crankcase when the compressor begins to operate, system such as the compressor crankcase, the especially after not operating for a period suction line, discharge line, etc. Refrigerant of several hoUrs or longer. Refrigerant tends to solubility in alkyl benzene and a mineral oil is dissolve in the oil while the machine is idle summarized in Table 3 for several conditions of and to be rapidly evolved when the compressor temperature and pressure. begins to run. This withdrawal of refrigerant tends to cause foaming. Loss of oil from the Experimental measurements of the vapor-pressure crankcase is thought to be directly related to of refrigerant dissolved in alkyl benzene have the degree of foaming, but the exact mechanism demonstrated that this solution follows Raoult's is not clearly understood. To the extent that law very closely, so the values reported in foaming is the cause of oil carry-over, it is Table 3 have been calculated. Extensive measure­ bad. On the other hand there is so~e evidence ment with mineral oils of various compositions that oil foaming tends to reduce noise in operat­ have shown a positive deviation from Raoult's ing compressors and this is good.
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