Tide , of Invention: WOlXING, FLUID COMPOSIT~'ON FOR . . , * REFRIGERATOR . i j Technical Fikld. . 5 [0001] The present invention relates to a working fluid composition for a refrigerating machine, and more specifically relates to a working fluid combosition for a refrigerating . machine that contains a mixed refrigerant which contains a hydrofluoroethane, difluoromethane (also j . . referred to as "HFC-32" or "R32") and 2,3,3,3-tetrafluoropropene (also 1 10 referred to as "HFO- 123 hyf I). , i Background Art I . . [0002] CFC (chlorofluorocarbon) ' ' and HCFC , I @ydrochlorofluorocarbon), which have been convei~tionally used as I . refiigerants for refrigeration equipment, have been subject to regulation 15 due to the problem of recent ozone depletion, and HFC (hydrofluorocarbon) has come to be used as a &tigerant instead of them. [0003] Among HFC refiigerants, HFC- 134a, R407C, and R410A ate . . normally used as refrigerants for car air-conditioners, cold 'storage . ,. , 20 chambers, or roorn air-conditioners. Altliough the ozone depletion ' . potential (ODP) of.these HFC refi-igerant is zero, these come to be subject to, regulation, because the global warming pbtential (G*) . thereof is high. While difluoromethane has been studied as one of' alternate candidates of these refrigerants, difluoromethane -has the 25 following problems: the global warming thereof is not . sufliciently low; the boiling pointthereof is so low that thermodynamic J characteristics cannot be applied to a current rekigeration system directly; and difluoromethane is not easily compatible with lubricating oils (reMgerating machine oils) used for conventional HFC refrigerants, such as polyol esters and polyvinyl ethers. On the other l~and, ' . &aturated hydrofluoroc,arbo~~hsa ve been proposed to beu sed as a refrigerant due to the following reasons; both of its ODP and G W are very low; unsaturated hydrofluorocarbons are non-flammable depending on structures; and in particular with respect to HFO- 123 4yf, thermodynamic characteristics as measures of refiigerant performances 10 are comparable with or better than those of HFC-134a (Patent . ' Literatures I to 3): Citation List [0004] [patent Literature 11 International Publication W02004/03,79 1 3, 15. patent Literature 21 International Publication ~02b05/105947 [Patent ~iteratuce31 International Publication W02009/057475 Summary of Invention . Tech~icaPl roblem [0005] In tl~ec ase where a refrigerant and a ref?igerating,macline oil. 26 are applied to a refi.igeration system, it is demanded for the refrigerant that the refige~ant does not. have any adverse influences on the .. . environment and that the~modynamicsc haracteristics are adapted to the refrigeration system. In addition, in a circumstance of coexistence of the re~igeranat nd the refrigerating machine oil it is demanded that they 25 are soluble in each other (compatibility) and are excellent in thermal/chemical stability, and that an oil film is maintained in order to suppress wear of a sliding inember (lubricity). [0006] With respect to a refrigeration system using unsaturated hydrofluorocarbons, it has been considered that since refiigerating ' machine oils used in FEC, such as polyol esters and polyvinyl ethers, & . . 5 . exhibit compatibility with %nsaturated hydrofluorocarbons, these . . refrigerating machine oil can be applied. According to the studies by the present invelltors, however, the following problem has been revealed: unsaturated hydrofluorocarbons have unstable double bonds in their molecules and thus are poor in thermal/chemical stability. I 10 I [0007] In addition, one reason why the compatibility between the refi-igerant and the refigerating machine oil (hereinafter, sometimes simply referred to as "comnpatibility".) is demanded is because a refrigerating machme oil for lubricating a refiigerant compressor w circulates together with a refiigerant in the refrigerant circulation cycle 15 of refrigeration equipment. Tliat is, in the refrigeration equipment, sufficient compatibility between the refiigerant and the refiigerating ; macline oil is not achieved depending on the selection of the refrigerating machine oil used to the refrigerant, and the refrigerating . . machine oil, discharged ' from the refrigerant compressor easily remains 2G in the cycle. As a result, thcre occur problems of wear due to lubrication failure by the reduction in ,amount of the oil in the refiigerant , ' compressor and of blockage of an expansion mechanism such as a capillary. ~herefore,iits important to select the rei5igerating machine . . oil adapted to the refiigerant. In a refrigeration system using 25 difluorornethane, however, a .problem is that difluoromethane is not easily compatible with the refrigerating inacliine oil, and there is also the problem of ihemodyfiamics characteristics. [0008] The present invention has been made in view of such problems, and an object thereof is to provide a working fluid composition for a refiigerating machine that can achieve compatibility and themal/chernical stability in a refrigeration system using a re~gerant which contains a hydrofluoroethane, difluoromethane and an unsaturated h.y dr.o fluorocarbon without requiring for a significant modification of an existing system. ' Solution to Problem 10 ' [0009] The present inventors have made intensive studies in order to achieve the above object, and as a result, have found that by using a ' rekigerant containing a specific hydrofluoroethane, difluorolnetl~ane and a specific ~lnsaturated hydrofluorocarbon, and a refrigerating machine oil of a specific ester or ether as a base oil, both of s~ifficientl~ 15 high compatibility and thermaYchemica1 stability can be achieved at , highlevels, leading to the completion of The present invention. [0010] That is, the present invention provides a working fluid composition for a refrigerating machine, comprising: . a refrigerant comprising a hydrofluoro'ethane represented by the 20 following formula (A), difluorotnethane and 2,3,3,3-tetrafluor~pro~ene; and a refrigerating machine oil comprising at, least one s e l e c t e d h , a polyol ester, a polyvinyl ether and a polyalkylene glycol compound as a base oil, wherein a carbonloxygen molar ratio of the base oil is 2.5 or ' 25 more &d 5.8 or less. .. .. .. . [n represents 1 or 2.5 . . ,. [00 11 j It is preferable that the refigera& comprise 5 to 40 parts by mass of' the hydrofluoroethane, 20 to 60 parts by mass o f , . difluor-omethane and 20 to 70 parts .by mass , ' of 5 2,3,3,3-tetrafluoropropene based on 100 parts by mass of the refrigerant. [00123 It is preferable that a mass ratio of the reGerant' to the refrigerating machine oil be 90 : 10 to'30 : 70. [0013] It is preferable that the hydrofluoroethane be 1,1,1,2-tetrafluoroetl~ane and' that a global warming potential of the 4.0 refrigerant be 500 or less. [0014] In the case where the base oil comprises a polyol ester having a carbodoxygen molar ratio of 2.5 or more and 5.8' or less, preferable examples of the polyol ester include polyol esters obtainable by synthesis from fatty acids having 4 to 9 carbon atoms and polyhydric 15 alcohols having 4 to 22 carbon atoms. [0015] In the case where the base oil comprises a polyalkylene glycol having a carbodoxygen molar ratio of 2.5 or more and 5.a or less, . preferable examples of the polyallqlene glycol compound include a compound having a homopolymerization chain of propylene oxide or a 20 copolyme~izationc hain of propylene oxide and ethylene oxide, iil wl-ich at least one of both ends of the chain is blocked by an ether bond. [0016] In the case where the base oil compllses a polyvinyl ether having a carbodoxygen molar ratio of 2.5 or more and 5.8 or less, preferable examples of the polyvinyl ether include polyvinyl ethers 25 having a structural unit represented by the following formula (1). [Chemical Formula 11 ml,IX 2 and d may be the same or different and each represent a hydrogen atom or a hydrocarbongroup having 1 to 8 carboh atoms, , represents a divalent hydrocarbon group having i to 10 carbon atoms or 3 ' . 5 an ether bond oxygen-contsining divalent hydrocarbon group having 2 to 20 carbon atoms, R' represents a hydrocarbon group having 1 to 20 - carbon atoms, m represents a number such that an average value of rn in the polyvinyl ether is 0 to 10, R' to R' may be the same or different in each occurrence of the structural units, and when m represents 2 or 18 more in one structural unit, a plurality of ~~0 may be the same or different .] [0017] The present invention can provide a working fluid composition for a refiiigerating machine that can achieve compatibility and theqnal/chernical stability in a refrigeration system using a refrigerant which contains a hydrofluoroethane, difluo'romethane and an unsaturated hydroflumocarbon without requiring for a significant modification of an existing system. . . Description of Embodiments [0018] Hereinafter, a preferable embodiment of the present invention is described in detail. . . [0019] A workmg fluid composition for a refrigerating' machine . . according to an embodiment of the present invention compsises I : I ' a refrigerant comprising a hydro£hqroethane represented by the 1 foliowing formula (A), difluoromethane and 2,3,3,3 -tetrafluoropropene, 1 . , and J I a refrigerating machine oil comprising at least one selected fi-om I : .5 a polyol ester, a polyvinyl ither and a polyallqlene glycol'compound as , . . a base oil, wherein a carbon/oxyg&n molar ratio of the base oil is 2.5 or more and 5.8 or less. C2%F6-II (A) [n represents 1 or 2.1 * , 10 [0020] In the worlcing fluid composition for a refrigerating machine according to the' present embodiment, the proportions of the refrigerant , , and the refrigerating machine oil blended are not particularly limited, but the mass ratio of the refrigerant to the refrigerating machine oil is preferably 90 : 10 to 30 : 70 and more preferably 80 : 20 to 40 : 60. 15 [0021] Then, the components contained in . the worlchg fluid composition for a refiiigerathg .machme are described in detail. . [0022] [Refi-igerant] The ref?igerant in the present embodiment contains the hydrofluoroethane represented by the above formula (A), 20 difluorornethane and 2,3,3,3 -tetrafluoropropene. [0023] Herein, the hydrofluoroethae represented by the above formula (A) is non-flammable; Thus, as the ratio thereof blended is higher, the flammability is lower but the global wanning potential is higher. While no test method for evaluating the boundary between 25 "non-flainmable" and "low flammable" is established and is being . studied internationally, the non-flammability of the mixed refrigerant is T - p - ~ r - - - z - -- - 7 I increased as the proportion of a non-flammable compound is increased. [0024] ~ifl~oroðan(Re 32), whose pressure is high increases the :.I efficiency of the reflligerating machine when the ratio thereof blended is . 8 high. B u t the GWP is increased and also the compatibility of the 5 refrigerant with the refrigerating machine oil is deteriorated, when the ,ratio thereof blended is high. [00?5] On the other hand, 2,3,3,3 -tetrafluoropropene (HFO- 1234~0, 1 whose global warming potential is extremely low, is thus enviromentally-fiiendly when the ratio thereof blended is high. But it 10 is poor in stability, and thus the stability of the working fluid is deteriorated when the ratio thereof blended is high. . . [0026] Then, in the present embodiment, the hydrofluoroethane, I represented by the above formula (A), difluoromethane and . 2,3,3,3 -tetrafluoropropene are mixed, and the resulting mixed 15 refrigerant can be used as a suitable refrigerant, in which disadvantages . of the each refligerants are compensated and advantages thereof are utilized. [0027] As the hydrofluoroethane represented by the above formula (A), 20 is preferable in tams of thermodynamics characteristics. Furthemore, I whe'n the global warning potentials thereof are compared, that of WC-125 is 3400 while that of &~-134a is 1300, and thus HFC-134a ' having a lower GWP is more preferable. [0028] The content of each of the rekiierants in the above refiigerant based on 100 parts by mass of the rehigerant is preferably as follows: the content of the hydrofluoroethane represented by the above formula , . . , I (A) is 5 to 40 pa-ts by mass, the content of hifluoromethane is 20 to 60 , parts by mass, and the content of 2,3,3,3 -tetrafluoropropene is 20 to 70 parts by mass; and more preferably as follows; the content of the hydrofluoroethane represented by the above formula (A) is 10 to 30 parts by mass, the content of difluoromethane is 20 to 60 parts by mass,' and the content of 2,3,3,3-tetrafluoropropene is 20 to 70 parts by mass. Berein, difluosomethane is a high-pressure gas, and therefore when the content thereof is high, the efficiency of the refrigerating machine is , increased. But when the content thereof is high, the GWP is increased, 1 and also the compatibility between the refrigerant and the refrigerating machine oil is deteriorated. On the other h'and, 2,3,3,3-tetrafluoropropene, whose GWP is low, contributes , environmental-friendliness when the content thereof is high. But it is poor in stability and thus the stability of the working fluid is deteriorated when the coiltent thereof is li&, [0029] The global warming potential of the refrigerant . . (mixed refrigerant) in the present embodiment is preferably 500 or less and , . more preferably 300 or less. Herein, the global warming potential can also be reduced b y mixing only difluoromethane and 2,3,3,3-tetrafluoropropene witkout using the hydrofluoroethane, but in such a case, the content of 2,3,?,3