Patent Publication Number: US-2002007640-A1

Title: Flushing composition

Description:
[0001] This invention relates to a flushing composition and in particular to a flushing composition having a deposit removal effect which is especially useful in removing deposits from refrigeration systems. The invention also relates to the method of reducing or removing deposits in a refrigeration system.  
       [0002] Conventional refrigeration systems typically have a compressor, a condenser, an expansion device and an evaporator linked to form a loop in which a refrigerant circulates and is successively condensed and evaporated so as to provide a refrigeration effect. Various types of compressor are employed in refrigeration systems including reciprocating, scroll, rotary and screw compressors and are selected according to the particular application. The compressor contains moving parts which are lubricated during use. The expansion device in refrigeration systems generally contains an area of constricted flow of refrigerant and may be, for example a capillary tube which is typically arranged in a serpentine path or an expansion valve.  
       [0003] A range of different materials are used in the construction of the components of a refrigeration system including metals and plastics materials. Other materials such as oils may be used in the assembly of the hardware of such systems and the components of the refrigerant working fluid especially additives may be susceptible to thermal or hydrolytic decomposition. During use and through wear, some of these materials may be present in the refrigeration loop and be carried around the system by the flow of refrigerant as unwanted residues. Other unwanted residues may also be introduced through servicing or the repair of refrigeration systems or in retrofilling new refrigerant or lubricant to the system once it has been used. In particular plastics materials, paraffinic materials, poly alpha-olefins, silicone oils and carbonaceous materials especially high molecular weight and non polar materials may be found as unwanted residues in the refrigeration loop. Such materials may be deposited in the refrigeration system especially in areas of constriction, and cause blockages and trap additional materials, for example particulate matter. Deterioration in performance and in extreme cases, system failure may occur due to such blockages.  
       [0004] In general, there are two types of refrigeration system, first, systems in which the lubricant and refrigerant are present as a mixture and circulate around the refrigeration system as such, for example in automotive refrigeration systems, and secondly, systems in which the refrigerant circulates in the system and the lubricant is present in a sump in the compressor, for example open and closed hermetic compressors and so-called industrial and commercial compressors. In the second case the system is designed to avoid or at least minimise the lubricant being carried from the compressor sump into the refrigeration loop although in practice, this typically occurs to a certain extent due to the entrainment of the lubricant into the refrigerant gas. Once lubricant is carried into the refrigeration loop it is necessary that it be transported around the system and deposited back in the sump otherwise blockage and fouling may occur in the loop and problems due to a reduced level of lubricant in the sump may be encountered.  
       [0005] For many years, chlorofluorocarbons, for example dichlorodifluoromethane (R-12), have been used as refrigerants but have been implicated in the destruction of the ozone layer. Following the Montreal Protocol of 1987, such materials are being phased out and are being replaced by hydrochlorofluorocarbons on a temporary basis and also by hydrofluorocarbons. In particular, 1,1,1,2-tetrafluoroethane (R-134a) has found widespread use as a replacement refrigerant for R-12. Accordingly, many refrigeration systems, which contain chlorofluorocarbon refrigerants, have been refitted with a different refrigerant which is accepted under the Montreal Protocol in a procedure known in the art as retrofilling.  
       [0006] The problem of blockage due to the presence of foreign bodies in the recirculating refrigerant has hitherto been addressed by modifying the mechanical design of the expansion device, for example capillary tubes in which the cooler part of the device has a larger diameter so as to reduce the likelihood of deposition of foreign bodies. Attempts have also been made to reduce the level of foreign bodies which may be incorporated into the system during manufacture. Refrigeration systems having hermetic compressors may be especially prone to these problems due to the level of foreign material present in the compressor motor. These approaches have the general drawback of increasing the costs of production of the refrigeration system, as new materials of construction may need to be employed and have met with only limited success.  
       [0007] EP-A-468759 (Castrol) describes a process in which a refrigeration system containing an old refrigerant and a mineral oil lubricant is retrofilled with R-134a refrigerant and a synthetic lubricant ester having a molecular weight in excess of 250. In this process, the old material is drained from the system but a residue is left in the system and the new refrigerant and lubricant is then charged to the system which is then suitable for use.  
       [0008] In retrofilling processes, it is known to remove the old refrigerant and lubricant from the system and then to flush the empty system with a flushing agent. As noted above, unwanted deposits may have formed in the system for a variety of reasons and impair performance. Indeed such deposits may be formed in part due to the servicing of refrigeration systems. In known flushing compositions, the material employed to flush the refrigeration system may not have a beneficial lubricating effect. In these circumstances, residual flushing material left in the system may act to impair the lubrication of the compressor during subsequent performance.  
       [0009] We have now found that by flushing the system with a novel flushing composition problems associated with unwanted deposits may be ameliorated. In particular, the presence of a deposit removal component in the flushing compositions may reduce or avoids problems associated with the presence of unwanted residues such as capillary blockage. Further, we have found that a component having amphiphilic properties provides a suitable deposit removal effect.  
       [0010] Accordingly a first aspect of the invention provides a refrigeration system flushing composition comprising a lubricant and an amphiphilic deposit removal component.  
       [0011] A second aspect of the invention comprises a use of a flushing composition according to the first aspect of the invention in a refrigeration system in the removal of deposits in the refrigeration system.  
       [0012] The invention also provides a method of removing deposits from a refrigeration system which comprises charging a flushing composition as described according to the first aspect of the invention to the refrigeration system, circulating the flushing composition through the refrigeration system for a period of time sufficient to remove at least some deposits and removing the flushing composition containing at least some deposits from the refrigeration system.  
       [0013] Suitably the refrigeration system comprises a compressor, a condenser, an expansion device and an evaporator linked to form a loop in which a refrigerant circulates and is successively condensed and evaporated so as to provide a refrigeration effect.  
       [0014] The invention is especially applicable to automotive air-conditioning systems and also in domestic and industrial and commercial refrigeration systems.  
       [0015] In a preferred embodiment, the method of removing deposits (flushing process) further comprises the steps of removing the old refrigerant and lubricant contained in a refrigeration system prior to charging the flushing composition to the refrigeration system and charging the refrigeration system with a new refrigerant and lubricant and optionally other components after the flushing composition has been removed from the refrigeration system.  
       [0016] The flushing composition may be charged to the refrigeration system while the system is charged with old refrigerant and lubricant and the system operated for a period of time, for example 1 or more days so the flushing composition acts to reduce or remove unwanted residues during refrigeration use. Under such circumstances, the refrigeration system may be retrofilled or serviced with a new refrigerant and lubricant without conducting a separate flushing procedure between operation with the old refrigerant/lubricant and the new refrigerant/lubricant although a separate flushing procedure may be conducted if desired. Flushing the refrigeration system during use is especially preferred where the scale of the system is large such as in commercial or industrial applications, for example supermarket refrigeration and air-conditioning units.  
       [0017] In the flushing process, the flushing composition is typically circulated for a period the same or similar to that employed in known retrofilling processes, for example as described in EP-A-468759. Other process conditions to be employed are also the same or similar to those employed in known retrofilling processes.  
       [0018] The flushing composition is suitably employed at a level of 1 to 15, preferably 2 to 10 liters per tonne of refrigerant capacity of the refrigeration system. Suitably, the flushing composition is present in the refrigeration system for at least 10 minutes and preferably at least 20 minutes when used to flush a system during retrofilling.  
       [0019] Suitably, the old refrigerant and/or lubricant may be expelled from the system using a pressurised gas for example air or nitrogen. Similarly, if desired, the flushing composiiton may be removed from the system using a pressurised gas.  
       [0020] The old refrigerant and lubricant may typically comprise a chlorofluorocarbon refrigerant and a mineral oil lubricant although the method of removing deposits is also applicable to refrigeration systems in which the old refrigerant was a hydrofluorocarbon and/or a hydrochiorofluorocarbon refrigerant and the old lubricant was a synthetic material, for example a polyalkylene glycol lubricant, an alkyl benzene, a polyvinyl ether and a polyol ester, especially neopentyl polyol ester lubricant.  
       [0021] We have found that compositions according to the invention aid removal of unwanted deposits and believe, without being bound by any theory, that the mechanism may be by solubilising or dispersing the deposits in the flow of the flushing composition. Furthermore, as the flushing composition according to the invention comprises and may act as a lubricant, the risk of impairment of lubrication of the compressor during subsequent use is reduced.  
       [0022] The refrigerant which is charged to the system after the flushing process is suitably a hydrofluorocarbon (HCFC) refrigerant, a hydrofluorocarbon (HFC) refrigerant, or a blend of refrigerants containing at least one HFC, HCFC or both. Suitably the refrigerant does not contain chlorine atoms, thus the refrigerant is preferably consists essentially of only HFC refrigerant. HCFC&#39;s and HFC&#39;s contain at least one atom of carbon, hydrogen and fluorine and, in the case of HCFC&#39;s, one or more chlorine atoms. Other known refrigerant gasses including carbon dioxide, hydrocarbons, for example pentane and isobutane, and ammonia may also be employed as the refrigerant gas.  
       [0023] Examples of HCFC&#39;s include chloro difluoromethane (R22) and dichloro trifluoro ethane (R123a).  
       [0024] Examples of HFC&#39;s include 1,1,1,2-tetrofluoroethane (R134a), 1,1,1,2,2-pentafluoroethane (R125), difluoromethane (R-32), 1,1,1-trifluoroethane (R143a) and 1,1-difluoroethane (R-152a). Other components typically found in refrigerant blends may also be included including hydrocarbons, especially hydrocarbons having from 1 to 6 carbon atoms for example propane, isobutane, butane and pentane, fluorinated hydrocarbons and other refrigerants, for example carbon dioxide.  
       [0025] A HFC, for example 1,1,1,2-tetrofluoroethane (R134a) is especially preferred as the refrigerant.  
       [0026] The flushing composition comprises a lubricant. The lubricant is suitably a synthetic lubricant for example, a polyalkylene glycol (PAG), a polyvinyl ether, an alkyl benzene and a polyol ester.  
       [0027] In an especially preferred embodiment of the invention, the refrigerant comprises R134a and optionally a further refrigerant and the lubricant comprises a PAG and/or a polyol ester, especially a neopentyl polyol ester of one or more monocarboxylic acids.  
       [0028] In one embodiment of the invention, the same lubricant is employed in the flushing composition as is to be charged to the refrigeration system after the flushing process. If desired, the flushing composition may be employed as a lubricant composition in the subsequent operation of the refrigeration system optionally together with additional components as are conventional in refrigeration lubrication compositions. However, the lubricant to be charged to the system for subsequent use will typically be selected having regard to known criteria pertaining to the desired application such as viscosity. Selection of the lubricant for use in the flushing composition need not be limited by considerations which are relevant to the lubricant which is to be subsequently used in the system.  
       [0029] The amphiphilic deposit removal component must have an optimum balance of amphiphilicity and solubility in order to provide the desired effect of removing at least some of the unwanted deposits in the refrigeration system. A measure of the amphiphilicity of the component may be obtained by observing the effect of the component in a standard test as hereinafter defined.  
       [0030] In this test, referred to as the “Dispersibility Test” for convenience, a mixture of 3GS mineral oil, available from Suniso, a neopentyl polyol ester and the amphiphilic component is dispersed in 1,1,1,2-tetrafluoroethane (R134a) and the time for full phase separation of the mixture from R134a is recorded. 50% by weight of 3GS mineral oil is mixed with 50% by weight of a pentaerythritol ester sold under the trade name EMKARATE RL (grade 32H) available from ICI to form a test oil mixture (TOM). To this TOM, the amphiphilic component is added at a level of 1% by weight based on the weight of the oil mixture to form a homogeneous mixture. The TOM with the amphiphilic component and liquid R134a are then mixed in a ratio of 100 parts TOM to 100 parts R134a and 1 part deposit removal component by weight at approximately 20° C. and agitated vigorously to form a dispersion of R134a with the TOM. The time from which agitation ceases to the formation of 2 distinct clear liquid phases is then measured visually. The time for the distinct phases to form provides an accurate measure of the efficacy of the additive in providing an deposit removal effect, a longer time for the formation of the distinct phases relative to a sample without the component being indicative of greater efficacy. It is preferred in the present invention that the phases separate to form two distinct clear liquid phases only after at least 10 seconds, more preferably 30 seconds and even more preferably after at least 1 minute. Especially preferred are those components that delay separation of the phases for at least 3 minutes and most desirably at least 5 minutes. As a reference, a mixture of TOM and R134a without the deposit removal component separates almost immediately and in any event in less than 5 seconds. It is an essential requirement of the invention that the component does not precipitate from the test mixture and at the concentration employed in the test, at any point during the test.  
       [0031] The deposit removal component may be any material which meets the criterion of the Dispersibility Test. The component typically has several moieties within the molecule, at least one of which is oleophilic and one of which has a greater affinity for R134a than the oleophilic moiety and which is referred to as a polar moiety.  
       [0032] The deposit removal component may be cationic, amphoteric, nonionic or anionic. It is especially preferred that the component be anionic and contain a non-polar part to the molecule.  
       [0033] It is preferred that the component contains, as a polar moiety, an ionisable moiety desirably in ionised form and especially an anionic moiety, or a moiety containing a fluorocarbon group or both an ionisable moiety and a moiety containing a fluorocarbon group. Suitable anionic moieties include sulphate, sulphonate, phosphate and carboxylate and moieties having an active hydrogen. for example anionic fluorosurfactants including compounds available under the ZONYL trade name available from Aldrich. Anionic sulphates and carboxylates are less preferred due to stability and performance reasons. The fluorocarbon group may be any group which contains a carbon atom and a fluorine atom including, by way of example, a hydrocarbyl group wherein at least one hydrogen atom is substituted by a fluorine atom, and optionally all hydrogen atoms have been substituted by fluorine atoms, in other words a group containing exclusively carbon and fluorine atoms for example trifluoromethyl, pentafluoroethyl heptafluoropropyl. Preferably the fluorocarbon group has from 1 to 8 carbon atoms, more preferably from 1 to 6 carbon atoms and especially from 1 to 3 carbon atoms. The fluorocarbon group may be linear or branched. Especially preferred materials include alkyl succinates, for example dioctyl sulphosuccinate and aromatic sulphonic acids and petroleum sulphonates. Ionic species may be employed as salts or preferably in acid form.  
       [0034] Suitable nonionic components include alkyl alkoxylates derived from an alkylene oxide and a moiety derivable from a compound having an active hydrogen atom and an oleophilic moiety, for example a long chain alcohol. Preferred oleophilic moieties include moieties having an aliphatic hydrocarbyl group, for example a hydrocarbyl group having from 6 to 22 carbon atoms, an aromatic hydrocarbyl group and mixtures thereof. Suitable moieties having an active hydrogen atom include an alcohol group, an amine group, a carboxylic group whether derived from an acid, ester or anhydride.  
       [0035] Other suitable nonionic components include esters of polyalkylene glycols and fluorinated polyethers.  
       [0036] Examples of especially preferred deposit removal components include those listed in Table 1 below and the classes of compounds to which they belong. Particularly preferred examples include dialkylsulphosuccinates and salts thereof, fluoroaliphatic polymeric esters, alkyl aromatic sulphonic acids and salts thereof and comb graft copolymers of methyl methacrylate methacrylic acid and methoxy polyethylene oxide methacrylate and solutions of acrylic graft copolymers, for example.  
       [0037] The deposit removal component is suitably present in the composition at a level of 0.001 to 5%, preferably 0.001% to 3%, more preferably 0.01 to 3%, especially 0.01 to 0.5% and optimally 0.01 to 0.1% by weight of the lubricant in the flushing composition.  
       [0038] Where the flushing composition is added to the system while the system is charged with the old refrigerant and lubricant, it is preferred that the flushing composition be in the form of a concentrate comprising the lubricant and the deposit removal component. In a concentrate form, it is preferred that the component preferably be present at a level of the order of at least 10 times and optimally 20 times the level in the above ranges by weight of the flushing composition, preferably 0.02 to 10% and more preferably 0.2 to 5% by weight of the lubricant in the flushing concentrate composition. In some cases, it may be desirable for the deposit removal component to be present at a level of at least 20% by weight of he flushing composition. The level of the deposit removal component in the flushing composition should not however be so high that a separate phase of material forms.  
       [0039] for example 0.5% by weight by weight of the lubricant. The component is suitably mixed with the lubricant prior to charging to a refrigeration system. A single deposit removal component or a mixture of such components for example a mixture of an anionic component and a nonionic component may be employed as desired.  
       [0040] It is required that the deposit removal component be employed at a level at which it remains soluble in the refrigerant/lubricant mixture in the refrigeration system. If the component does not remain soluble at the dose-rate employed, it may itself cause undesirable blockage in the system.  
       [0041] The deposit removal component is suitably soluble with all the materials which may be charged to the refrigeration system including the refrigerant and the lubricant, in the refrigeration system, the evaporation of refrigerant at the exit to the expansion device is likely to present the most severe conditions under which the component must remain soluble due to the low temperature, typically at or around the boiling point of the refrigerant and desirably the deposit removal component remains soluble at this point in the refrigeration system.  
       [0042] Desirably, the level of and type of deposit removal component is selected so that the component is soluble in a mixture of the refrigerant and lubricant, at a level of 10% by weight of lubricant to the refrigerant/lubricant mixture, to be used at the boiling point of the refrigerant.  
       [0043] Synthetic lubricants preferred for use in the flushing composition of the invention are those selected from the class known as the polyol esters and especially neopentyl polyol esters which have, inter alia, a relatively high level of thermal stability. Other suitable synthetic lubricants include complex esters, being esters of polyacids and polyalcohols with suitable terminal end groups, alkyl benzenes and esters of diacids, preferably esters of a diacid having 2 to 10 carbon atoms, for example adipic acid and azelaic acid, and an alcohol, preferably branched, having from 7 to 13 carbon atoms.  
       [0044] Suitable neopentyl polyol esters include the esters of pentaerythritol, polypentaerythritols such as di- and tripentaerythritol, trimethylol alkanes such as trimethylol propane, and neopentyl glycol. Such esters may be formed with linear and or branched aliphatic carboxylic acids, such as linear and/or branched alkanoic acids, or esterifiable derivatives thereof, for example anhydrides. A minor proportion of an aliphatic polycarboxylic acid, for example an aliphatic dicarboxylic acid, or an esterifiable derivative thereof may be also used in the synthesis of the ester lubricant in order to increase the viscosity thereof. However, where such an aliphatic polycarboxylic acid (or esterifiable derivative thereof) is employed in the synthesis, it will preferably constitute no more than 50 mole %, more preferably no more than 30 mole %, especially no more than 10 mole % of the total amount of carboxylic acid used in the synthesis. For convenience, the term “carboxylic acid” when employed herein is to be taken to include “esterifiable derivatives” of that acid unless the context clearly precludes this meaning. Usually, the amount of the carboxylic acid(s) used in the synthesis will be sufficient to esterify all of the hydroxyl groups contained in the polyol, but in certain circumstances residual hydroxyl functionality may be acceptable.  
       [0045] A preferred neopentyl polyol ester lubricant is one comprising one or more compounds of the general formula II:  
       R (0 C (O) R 1 ) n   II 
       [0046] wherein  
       [0047] R 1  is a hydrocarbon radical remaining after removing the hydroxyl groups from pentaerythritol, dipentaerythritol, tripentaerythritol, trimethylol ethane, trimethylol propane or neopentyl glycol, or the hydroxyl containing hydrocarbon radical remaining after removing a proportion of the hydroxyl groups from pentaerythritol, dipentaerythritol, tripentaerythritol, trimethylol ethane, trimethylol propane or neopentyl glycol;  
       [0048] Each R 1  is, independently, H, a straight chain aliphatic hydrocarbyl group, a branched chain aliphatic hydrocarbyl group, an aliphatic hydrocarbyl group (linear or branched) containing a carboxylic acid or carboxylic acid ester substituent, provided that at least one R 1  group is a linear aliphatic hydrocarbyl group or branched aliphatic hydrocarbyl group; and  
       [0049] n is an integer.  
       [0050] The aliphatic hydrocarbyl groups specified for R 1  above may be substituted, for example by chloro, fluoro and bromo, and/or may include hetero atoms for example oxygen and nitrogen which may be pendant to the carbon chain or part of the carbon chain of the hydrocarbyl group. Preferably, however, the hydrocarbyl groups contain hydrogen, carbon and optionally oxygen for example in the case where R 1  is an aliphatic hydrocarbyl group containing a carboxylic acid of carboxylic acid ester substituent. It is especially preferred that the hydrocarbyl group contains only carbon and hydrogen atoms.  
       [0051] The ester lubricants of Formula II may be prepared by reacting the appropriate polyol or mixture of polyols with the appropriate carboxylic acid or mixture of acids. Esterifiable derivatives of the carboxylic acids may also be used in synthesis, such as the acyl halides, anhydrides and lower alkyl esters thereof. Suitable acyl halides are the acyl chlorides and suitable lower alkyl esters are the methyl esters. Aliphatic polycarboxylic acids or esterifiable derivatives thereof, may also be used in the synthesis of the ester lubricant. Where an aliphatic polycarboxylic acid is used in the synthesis of the ester lubricant, the resulting lubricant will comprise one or more compounds of Formula II in which at least one of the R1 groups is an aliphatic hydrocarbyl group (linear or branched) containing a carboxylic acid or carboxylic acid ester substituent. The ability of polycarboxylic acids to react with two or more alcohol molecules provides a means of increasing the molecular weight of the ester formed and so a means of increasing the viscosity of the lubricant. Examples of such polycarboxylic acids include maleic acid, adipic acid and succinic acid, especially adipic acid. Generally, however, only monocarboxylic acids will be used in the synthesis of ester lubricant, and where polycarboxylic acids are used they will be used together with one or more monocarboxylic acids and will constitute only a minor proportion of the total amount of carboxylic acids used in the synthesis. Where an aliphatic polycarboxylic acid is employed in the synthesis, it will preferably constitute no more than 50 mole %, more preferably no more than 30 mole %, and especially no more than 10 mole % of the total amount of carboxylic acids used in the synthesis, with one or more monocarboxylic acids constituting the remainder.  
       [0052] The amount of the carboxylic acid(s) (or esterifiable derivatives thereof) which is used in the synthesis suitably is sufficient to esterify all of the hydroxyl groups contained in the polyol(s), in which case the resulting lubricant will comprise one or more compounds of Formula II in which R is the hydrocarbon radical remaining after removing the hydroxyl groups from pentaerythritol, dipentaerythritol, tripentaerythritol, trimethylol ethane, trimethylol propane or neopentyl glycol. However, in certain circumstances ester lubricants which comprise residual hydroxyl functionality may be acceptable. Such lubricants comprise one or more ester compounds of Formula II in which R is the hydroxyl containing hydrocarbon radical remaining after removing a proportion of the hydroxyl groups from pentaerythritol, dipentaerythritol, tripentaerythritol, trimethylol ethane, trimethylol propane or neopentyl glycol. Esters containing residual (unreacted) hydroxyl functionality are often termed partial esters, and lubricants containing them may be prepared by utilising an amount of the carboxylic acid or acids which is insufficient to esterify all of the hydroxyl groups contained in the polyol or polyols.  
       [0053] The neopentyl polyol ester lubricants may comprise a single compound of Formula II, i.e. the reaction product that is formed between a single polyol and a single monocarboxylic acid. However, the ester lubricant may also comprise a mixed ester composition comprising two or more single compounds of Formula II. Such mixed ester compositions may be prepared by preparing two or more single esters and then blending those esters together. Esters utilising two or more carboxylic acids in the synthesis of the ester will produce an ester having two or more acids within a single compound. These materials are also suitable for use alone or in combination with other single esters or mixed esters. Thus, different mixed ester compositions, each of which ester has been prepared by utilising two or more polyols and/or two or more carboxylic acids in their synthesis, may also be blended together.  
       [0054] The preferred neopentyl polyol ester lubricants comprise one or more compounds of Formula II in which R is the hydrocarbon radical remaining after removing the hydroxyl groups from pentaerythritol, dipentaerythritol, trimethylol propane or neopentyl glycol. Particularly preferred alcohols for the synthesis of the ester are pentaerythritol, dipentaerythritol and trimethylol propane.  
       [0055] Preferably, each R 1  in Formula II is, independently, a linear aliphatic hydrocarbyl group or a branched aliphatic hydrocarbyl group.  
       [0056] Preferred linear aliphatic hydrocarbyl groups for R 1  are the linear alkyl groups, particularly the C 3-12  linear alkyl groups, more particularly the C 5-10  linear alkyl groups and especially the C 5-8  linear alkyl groups. Examples of suitable linear alkyl groups include n-pentyl, n-hexyl, n-heptyl, n-octyl, n-nonyl and n-decyl. Esters containing such alkyl groups can be prepared by utilising a linear alkanoic acid (or esterifiabie derivative thereof) in the synthesis of the ester.  
       [0057] Preferred branched aliphatic hydrocarbyl groups for R 1  are the branched alkyl groups, particularly the C 4-14  branched alkyl groups, more particularly the C 6-12  branched alkyl groups and especially the C 8-10  branched alkyl groups. Examples of suitable branched alkyl groups include isopentyl, isohexyl, isoheptyl, isooctyl, isononyl, isodecyl, 2-ethylbutyl, 2-methylhexyl, 2-ethylhexyl, 3,3,5-trimethylhexyl, neopentyl, neoheptyl and neodecyl. Esters containing such alkyl groups can be prepared by utilising a branched alkanoic acid (or esterifiable derivative thereof) in the synthesis of the ester.  
       [0058] In a particular preferred embodiment of the present invention, the lubricant in the flushing composition comprises one or more esters of general Formula III  
                 
 
       [0059] wherein  
       [0060] R 2  is the hydrocarbon radical remaining after removing the hydroxyl groups from the pentaerythritol, dipentaerythritol or trimethylol propane;  
       [0061] each R 3  is, independently, a linear alkyl group or branched alkyl group;  
       [0062] and  
       [0063] p is an integer of 3, 4 or 6,  
       [0064] wherein one or more of the named polyols, one or more linear alkanoic acids or esterifiable derivatives thereof and optionally one or more branched alkanoic acids, or esterifiable derivatives thereof, are utilised in the synthesis of the ester lubricant.  
       [0065] Preferably, a mixture of two or more linear alkanoic acids, in particular two, or esterifiable derivatives thereof, are utilised in the synthesis of the ester. More preferably, a mixture of one or more linear alkanoic acids, or esterifiable derivatives thereof, and one or more branched alkanoic acids, or esterifiable derivatives thereof, are utilised in the synthesis. Thus, particularly preferred ester lubricants of the invention are mixed ester compositions which comprise a plurality of compounds of Formula III.  
       [0066] Where a mixture of linear and branched alkanoic acids, are utilised in the synthesis with the ester, as is preferred, the linear alkanoic acid(s) preferably constitutes at least 25 mole % e.g. from 25 to 25 mole %, of the total amount of carboxylic acids used. In this way, at least 25 mole % e.g. from 25 to 75 mole % of the hydroxyl groups contained in the polyol or mixture of polyols may be reacted with the said linear alkanoic acid(s).  
       [0067] Ester based lubricants comprising one or more compounds of Formula III exhibit good thermal stability, good hydrolytic stability and acceptable solubility and miscibility with the refrigerant. Refrigeration systems that contain replacements for R-22 and R-502 typically operate at temperatures above those before using R-134a as the sole replacement refrigerant. Thus, it is particular desirable that the lubricant which is used in a working fluid composition designed to replace the existing compositions based on R-22 and R-502 exhibits good thermal stability.  
       [0068] Preferably; R 2  is the hydrocarbon radical remaining after removing the hydroxyl groups from pentaerythritol or dipentaerythritol.  
       [0069] Preferred linear and branched alkyl groups for R 3  are those described above in connection with R 1  and are derived by utilising the corresponding alkanoic acids, or esterifiable derivatives thereof.  
       [0070] An especially preferred flushing composition contains an ester based lubricant comprises a plurality of esters of Formula III and which is the reaction product of pentaerythritol, heptanoic acid and a mixture of branched C 8-10  alkanoic acids. Preferably, the heptanoic acid will constitute from 25 to 75 mole % of the total amount of acids utilised in the synthesis, with the branched C 8-10  acids constituting the remainder. Another especially preferred ester lubricant comprises an ester of pentaerythritol, optionally with up to 10 mole % of dipentaerythritol (together known as technical grade pentaerythritol) and a mixture of i) linear aliphatic acid having from 5 to 7 carbon atoms, in particular a mixture of pentanoic and heptanoic acids together with ii) a branched aliphatic acid, especially a 9 carbon branched acid. Esterifiable derivatives of the acids may also be used in the synthesis of the ester.  
       [0071] Suitable polyoxyalkylene glycol lubricants for use in the flushing composition include hydroxyl group initiated polyoxyalkylene glycols, for example ethylene and/or propylene oxide oligomers initiated on mono alcohols for example methanol and butanol, or polyhydric alcohols, for example, pentaerythritol and glycerol. Such polyoxyalkylene glycols may also be end-capped with suitable terminal groups including alkyl, for example methyl groups.  
       [0072] A preferred polyoxyalkylene glycol lubricant has an average molecular weight in the range of from about 150 to about 3000 and comprises one or more compounds of general formula I:  
       A[—O—(CH 2 CH(CH 3 )O) 1 (CH 2 CH 2 O) m —Q] x   I 
       [0073] wherein  
       [0074] A is the residue remaining after removing the hydroxyl groups from a hydroxyl containing organic compound;  
       [0075] Q represents hydrogen, an optionally substituted alkyl, acyl, aralkyl or aryl group;  
       [0076] l and m are independently 0 or an integer provided that at least one of 1 or m is an integer, and x is an integer.  
       [0077] The polyoxyalkylene glycol lubricant may be prepared using conventional techniques that are known to those skilled in the art. Thus, in one method, a hydroxyl containing organic compound is reacted with ethylene oxide and/or propylene oxide to form an ethylene oxide and/or propylene oxide oligomer/polymer containing terminal hydroxyl groups. Optionally, this material may then be etherified to produce a polyoxyalkylene glycol of Formula I. The polyoxyalkylene glycol lubricant which is finally formed may comprise a mixture of such compounds which vary from one another in respect of the degree of polymerisation, i.e. the number of ethylene and/or propylene oxide residues. Moreover, a mixture of alcohols and/or phenols may be used as initiators in the formation of the polyoxyalkylene glycol lubricant, and a mixture of etherifying agents which provide different Q groups may also be used. The molecular weight of a polyoxyalkylene glycol lubricant comprising a mixture of compounds of Formula I will represent the average molecular weight of all the compounds present, so that a given lubricant may contain specific polyoxyalkylene glycols which have a molecular weight outside the range quoted above, providing the average molecular weight of all the compounds is within that range.  
       [0078] The moiety A in the polyoxyalkylene glycol of Formula I is the residue remaining after removing the hydroxyl groups from a hydroxyl containing organic compound. It is to be understood that this in no way implies that the moiety A need be produced by removing the hydroxyl group. Such compounds include the mono- and polyhydric alcohols and phenols. Where the hydroxyl containing compound which is used as an initiator in the formation of the polyoxyalkylene glycol is a monohydric alcohol or phenol, A is preferably a hydrocarbyl group and more preferably is an alkyl, aryl, alkaryl or aralkyl group, especially alkyl. Suitably alkyl groups for A may be selected from the straight chain (linear), branched or cyclic alkyl groups. Preferably, A is a C 1-5  alkyl group, more preferably a C 1-12,  particularly a C 1-10  and especially the C 1-6  alkyl groups. The alkyl group may be linear or branched and straight chain C 1-6  alkyl groups are especially preferred. Specific examples of alkyl groups include methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, the various pentyl groups, the various hexyl groups, cyclopentyl, cyclohexyl and the like. An especially preferred alkyl group for A is methyl or n-butyl.  
       [0079] Other suitable hydrocarbyl groups for A are those which remain after removing the hydroxyl group(s) from benzyl alcohol and phenols such as phenol, cresol, nonylphenol, resorcinol and bisphenol A.  
       [0080] Where a polyhydric alcohol is used in the formation of the polyoxyalkylene glycol, A is preferably a hydrocarbon radical. Suitable hydrocarbon radicals for A are those which remain after removing the hydroxyl groups from polyhydric alcohols such as ethylene glycol, propylene glycol, 1,4-butanediol, 1,6-hexanediol, diethylene glycol, dipropylene glycol, neopentyl glycol, cyclohexane dimethanol, glycerol, 1,2,6-hexane triol, trimethylolpropane, pentaerythritol, dipentaerythritol and sorbitol. A particularly preferred hydrocarbon radical for A is that remaining after removing the hydroxyl groups from glycerol.  
       [0081] The moiety Q in the polyoxyalkylene glycol of Formula I is H, an optionally substituted alkyl, aralkyl or aryl group. A preferred optionally substituted aralkyl group for Q is an optionally substituted benzyl group. Preferred optionally substituted aryl groups for Q include phenyl and alkyl substituted phenyl groups. Preferably, Q is an optionally substituted, for example halogen substituted, alkyl group, particularly an optionally substituted C 1-12  alkyl group and more particularly an optionally substituted C 1-4  alkyl group. Suitable alkyl groups for Q may be selected from, the straight chain (linear), branched or cyclic alkyl groups especially the linear alkyl groups. Although the alkyl groups for Q may be optionally substituted, they are preferably unsubstituted. Accordingly, particularly preferred alkyl groups for Q are selected from methyl, ethyl, propyl, isopropyl and the various butyl groups. An especially preferred alkyl group for Q is methyl.  
       [0082] The polyoxyalkylene glycol of Formula I may be a polyoxyethylene glycol, a polyoxypropylene glycol or a poly(oxyethylene/oxypropylene) glycol in the latter case, the ethylene oxide and propylene oxide residues may be arranged randomly or in blocks along the polymer chain Preferred polyoxyalkylene glycols are polyoxypropylene glycols and the poly(oxyethylene/oxypropylene) glycols.  
       [0083] The flushing composition is preferably also employed as the lubricant composition in the subsequent operation of the refrigeration system.  
       [0084] The flushing composition and the lubricant composition may also comprise one of more of the additives which are conventional in the refrigeration lubricants art. Specific mention may be made of oxidation resistance and thermal stability improvers, corrosion inhibitors, metal deactivators, viscosity index improvers, anti-wear agents and extreme pressure resistance additives. Such additives are well known to those skilled in the art.  
       [0085] Especially preferred additives include acid-catchers, preferably epoxy compounds, for example as disclosed in EP-A-475751 and EP-A-496937 and in particular cyclohexyl epoxides. Other acid-catchers may also be employed including imides. An acid catcher is suitably employed at a level of 0.05 to 0.5% by weight of the lubricant in the flushing composition.  
       [0086] Another especially preferred additive is an anti-corrosion agent including sulphonates, for example barium sulphonate and ammonium sulphonate, and succinates. An anti-corrosion agent is suitably employed at a level of 0.05 to 0.5% by weight of the lubricant in the flushing composition.  
       [0087] Where the lubricant is part of a lubricant composition containing one or more additives, such additives may be present in the amounts conventional in the art. Preferably, the cumulative weight of all the additives will not be more than 8%, e.g. 5% of the total weight of the lubricant composition.  
       [0088] Suitable oxidation resistance and thermal stability improvers may be selected from the diphenyl-, dinaphthyl-, and phenylnaphthyl-amines, the phenyl and naphthyl groups of which may be substituted. Specific examples include N,N′-diphenyl phenylenediamine, p-octyldiphenylamine, p,p-dioctyldiphenylamine, N-phenyl-1-naphthyl amine, N-phenyl-2-naphthyl amine, N-(p-dodecyl)-phenyl-2-naphthyl amine, di-1-naphthyl amine, and di-2-naphthyl amine. Other suitable oxidation resistance and thermal stability improvers may be selected from the phenothiazines such as N-alkylphenothiazines, and the hindered phenols such as 6-(t-butyl) phenol, 2,6-di-(t-butyl) phenol, 4-methyl-2,6-di-(t-butyl) phenol and 4,4′-methylenebis(-2,6-di-[t-butyl] phenol).  
       [0089] Suitable cuprous metal deactivators may be selected from imidazole, benzamidazole, 2-mercaptobenzthiazole, 2,5-dimercaptothiadiazole, salicylidine-propylenediamine, pyrazole, benzotriazole, tolutriazole, 2-methylbenzamidazole, 3,5-dimethyl pyrazole, and methylene bis-benzotriazole. Examples of more general deactivators and/or corrosion inhibitors include organic acids and the esters, metal salts and anhydrides thereof, such as N-oleyl-sarcosine, sorbitan monooleate, lead naphthenate, dodecenyl-succinic acid and its partial esters and amides, and 4-nonylphenoxy acetic acid; primary, secondary and tertiary aliphatic and cycloaliphatic amines and amine salts of organic and inorganic acids, such as oil soluble alkylammonium carboxylates; heterocyclic nitrogen containing compounds, such as thiadiazoles, substituted imidazolines, and oxazolines: quinones and anthraquinones; ester and amide derivatives of alkenyl succinic anhydrides or acids, dithiocarbamates, dithiophosphates; and amine salts of alkyl acid phosphates and their derivatives.  
       [0090] Suitable viscosity index improvers include polymethacrylate polymers, copolymers of vinyl pyrrolidone and methacrylates, polybutene polymers, and copolymers of styrene and acrylates.  
       [0091] Examples of suitable anti-wear and extreme pressure resistance agents include suiphurised fatty acids and fatty acid esters, such as sulphurised octyl tallate; sulphurised terpenes; sulphurised olefin; organopolysulphides; organo phosphorous derivatives including amine phosphates, alkyl acid phosphates, dialkyl phosphates, aminedithiophosphates, trialkyl and triaryl phosphorothionates, trialkly and triaryl phosphines, and dialkylphoaphites, e.g. amine salts of phosphoric acid and monohexyl ester, amine salts of dinonylnaphthalene sulphonate, triphenyl phosphate, trinaphthyl phosphate, diphenyl cresyl and dicresyl phenyl phosphates, tricresyl phosphate, naphthyl diphenyl phosphate, triphenylphosphorothionate; dithiocarbamates, such as an antimony dialkyl dithiocarbamate; chlorinated and/or fluorinated hydrocarbons and xanthates.  
       [0092] The invention is now described by way of non-limiting example. 
     
    
    
     EXAMPLE 1  
     [0093] A series of test mixtures were produced by mixing 10 g of EMKARATE RL (supplied by ICI) Grade 32H with 10 g of 3GS mineral oil available from Suniso and 0.2 g of the deposit removal component as listed in Table 1 below. This mixture was then added to 20 g R134a and subjected to the Dispersibility Test set out above. The time for the materials to separate was then measured and the results are shown in Table 1.  
                       TABLE 1                       Deposit Removal               Component   Supplier   Separation Time                                                SPAN 85   ICI   15   s       SPAN 80   ICI   25   s       FC430   3M   90   s       FC431   3M   10   s       ZONYL FSJ   Aldrich   30   s       ZONYL FSP   Aldrich   10   s       ZONYL FSA   Aldrich   10   s       TRITON X-100   BDH   20   s       TWEEN 20   ICI   10   s       TWEEN 60   ICI   25   s       SURFYNOL SE   Lancaster   15   s       Dioctylsulfosuccinate   Lancaster   &gt;5   mins       HYPERMER CG6   ICI   10   s       TWEEN 80   ICI   20   s       SPAN 80   ICI   10   s       SYNPERIONIC 91/6   ICI   10   s       ATLAS G1284   ICI   30   s       SYNPERONIC A7   ICI   10   s       ZONYL FSE   Aldrich   15   s       Dodecylbenzenesulphonic acid   Aldrich   &gt;5   mins       Dodecylsulphate   Aldrich   25   s       Lauryl acrylate   Lancaster   15   s       Allyl stearate   Lancaster 20 s   20   s       2-hydroxyhexadecanoic acid   Lancaster   35   s                  
 
     EXAMPLE 2  
     [0094] A simulated automotive air-conditioning unit was operated with various refrigerant/lubricant materials to provide a reference point for the cooling capacity of the system (Runs A and B below). The system was then operated using a degraded material to simulate operation after prolonged system use (Run C). The system was then emptied and treated with a flushing composition according to the present invention. A new charge of HFC refrigerant and polyol ester lubricant was then charged to the system and the cooling capacity was recorded after the flushing operation (Run D).  
     [0095] In the first reference run, the system was operated with a charge of R134a as the refrigerant and a commercially available polyol ester as the lubricant to determine the cooling capacity of the unit. Five tests were conducted and the average cooling capacity was recorded and is shown as Run A in Table 2.  
     [0096] This test was then repeated using dichlorodifluoromethane (R12), a CFC refrigerant and 5GS mineral oil and the average cooling capacity over five tests was recorded and is shown as Run B in Table 2.  
     [0097] The cooling capacity of the system was then intentionally degraded to simulate operation after a period of prolonged use. In this test, degraded 5GS mineral oil and ARI-700 grade R12 were charged to the system and the system was operated for approximately 70 hours. The average cooling capacity of the system after this test was recorded and is shown as Run C in Table 2.  
     [0098] Degraded 5GS was obtained by heating at 100 C., 1 liter of 5GS oil with steel strips and copper coupons and bubbling oxygen through the oil for 1 month. The oxygen flow was then stopped and the oil was heated for a further month at 160 to 175 C. 2 drops of HCl were then added to the oil which was heated for a further month. 2 further drops of HCl were added and the oil heated for a final month at the same temperature to produce the degraded oil.  
     [0099] A flushing composition according to the present invention was prepared containing EMKARATE RL 22H lubricant (polyol ester available from ICI) FLUORAD FC430 (fluorocarbon ester available from 3M) at a level of 250 ppm based on the lubricant and butylated hydroxy toluene at a level of 500 ppm based on the lubricant.  
     [0100] After Run C, the refrigerant and degraded oil were removed from the system and 1.2 kg of the flushing composition was flushed through the system. The composition was then removed from the system by blowing through with compressed nitrogen and about 70 g of composition remained in the system.  
     [0101] The system was then charged with R134a and a commercially available polyol ester and operated to determine the average cooling capacity of the retrofilled and flushed system after five runs. The results were recorded and are shown as Run D in Table 2.  
                   TABLE 2                       Run   Cooling Capacity (BTU/h)                  A   2974 (52300 Joules/min)       (pre-flush, R134a/polyol ester)       B   2816 (49410 J/m)       (pre-flush, R12/mineral oil)       C   2724 (47900 J/m)       (pre-flush, R12/old mineral oil)       D   2998 (52720 J/m)       (post-flush, R134a/polyol ester                  
 
     [0102] The results in Table 2 are given in BTU/hour which were the units used to record the results. The data is also presented in Joules/minute but the data in BTU/h shall prevail in the event of discrepancy.  
     [0103] The figures in Table 2 illustrate that after flushing the system with a flushing composition according to the present invention, the cooling capacity of the system was restored to a level comparable with that observed in Run A. In particular, after flushing, the cooling capacity of the system was improved by 6% as compared to that observed in Run B and by 9% as compared to the simulated “used” system tested in Run C.