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Component separation in refrigerant equipment using HFC mixtures
Fluid Phase Equilibria 174 (2000) 133–141
Component separation in refrigerant equipment using HFC mixtures J.T. McMullan, N.J. Hewitt, M.G. McNerlin, B. Mongey∗ Energy Research Centre, University of Ulster at Coleraine, Cromore Road, Londonderry, BT52 1SA, N. Ireland, UK
Abstract An experiment was initiated to examine the extent of component separation in refrigeration systems using HFC mixtures. The working fluid used was R407C, a ternary mixture of R32, R125 and R134a which has attracted attention as a replacement for R22 in a variety of applications. The test was designed to examine the two basic system configurations, namely dry-expansion and flooded evaporator, by incorporating an accumulator at the exit of the evaporator. A non-soluble lubricant was used to ensure that changes in composition were not attributable to differential solubilities of the component fluids. The composition of the circulating fluid was observed to be close to that of the charge composition while the system was operating in dry-expansion mode. Where the evaporation process was incomplete, an accumulation of fluid in the reservoir between the evaporator and the compressor caused the circulating fluid to become enriched in its more volatile components. The extent of composition change is dependent on a number of parameters, the most important being the charged mass, the internal volumes of the system and the compositional relationships of the component fluids. © 2000 Elsevier Science B.V. All rights reserved. Keywords: R407C; Azeotropic mixture; Composition change; Vapour–liquid equilibrium
1. Composition change mechanisms There are three primary mechanisms by which a change in the composition of the working fluid may occur, the most important being due to composition distribution in the two phase region. Except in the case of azeotropic mixtures, the compositions of the liquid and vapour phases are different during the phase change process. The phase change process is at the heart of the vapour compression refrigeration cycle, with phase transformation (whether evaporation or condensation) being the means by which heat is transferred between the working fluid and environment. The variation in composition between the two phases can be most conveniently represented on a lens diagram, with temperature or pressure plotted against composition. The other significant feature of phase transformation is the accompanying change in temperature where the phase change occurs at constant pressure. ∗ Corresponding author. E-mail address: [email protected] (B. Mongey).
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The second possible mechanism for composition change is variation in the solubility of the component fluids in the lubrication oil. A polyolester (POE) or polyalkyl glycol (PAG) lubricant is most commonly used in conjunction with HFC refrigerants, whether as single or multi-component working fluids. In the case of R407C the individual component fluids display differing levels of solubility in POE oils, with R125 being the most soluble [1]. R32 is the least soluble and R134a demonstrates a solubility relationship between these extremes. The greater the level of solubility, the greater the amount of refrigerant that becomes dissolved in the lubricant, and where the component fluids have different solubility levels the composition of the working fluid dissolved in the oil will be different to that of the charged composition. This in turn causes a change in the composition of the working fluid circulating in the system. From the solubility relationship of the individual fluids, the expectation is of an increase in the fraction of R32 in circulation where the R407C/POE combination is utilised. The third mechanism which may influence the composition of the working fluid is leakage or incorrect charging procedures. Since the compositions of the vapour and liquid phases are different in the two phase region, fluid leakage from the system will have a different composition to the charged fluid if the leakage occurs from the evaporator or condenser. While liquid leakage will not unduly affect the composition of the fluid within the system, vapour leakage will be enriched in the more volatile components and the preferential loss of these will result in a working fluid enriched in the least volatile component(s). In the case of R407C, vapour leakage will be rich in R32 and R125, while the fluid remaining within the system will be enriched in R134a. To ensure that the composition of the working fluid matches the mixture specification, charging a system with a multi-component working fluid must be done from the liquid phase of the supply cylinder. Vapour phase charging would result in a charged composition enriched in the more volatile components. Some minor degree of compositional change can be expected when charging from a supply cylinder as it approaches the empty state, since the residual liquid is enriched in the least volatile components.
2. Statement of the aims of the experiment Of the mechanisms by which changes in the composition of the working fluid occur, compositional difference between the two phases is the most significant since it applies to all system/working fluid combinations. Vapour compression refrigeration systems can be classified as one of the two types, dry-expansion or flooded evaporator. In a dry-expansion system, all of the fluid entering the evaporator is vaporised, with charge storage in a reservoir located between the condenser and the expansion device. Only a small fraction of the working fluid is in the evaporator at any time thus limiting the extent of compositional variation and its influence on the composition of working fluid in circulation. In a system employing a flooded evaporator and a multi-component working fluid, the composition of the liquid stored in the evaporator can differ very significantly from that circulating in the remainder of the system. In this work, the operational characteristics of both types of system were examined by modifying an existing dry-expansion system. This was achieved by incorporating an accumulator located at the exit of the evaporator. For the purposes of this experimental work, it was decided to use a non-soluble alkylbenzene/mineral oil mixture as lubricant, to eliminate the contribution of the differential solubility mechanism to changes in the composition of the working fluid.
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Fig. 1. Temperature–pressure relationship of R407C and R407C/oil combination.
3. Refrigerant/oil interaction In order to confirm the non-soluble nature of the refrigerant and lubricant a sample of refrigerant was prepared and the pressure temperature relationship of the fluid was verified. The experimental measurements were compared with those predicted by a model using R407C fluid properties routines. The mass of fluid and the internal volume of the sample cylinder were known (49 g and 0.29 dm3 ), and from these the mass fractions of the individual phases were determined and the pressure at any given temperature was calculated. The results of this test are shown in Fig. 1, and the level of agreement between the experimental results and calculations is consistent with the experimental accuracy of the measurements. A known mass of oil (20.9 g) was added to the sample of R407C and the equilibrium pressure of the refrigerant/oil pair was measured over the same temperature range (0–50◦ C). There was no observable change in the pressure temperature relationship, confirming the non-soluble characteristics of the oil and refrigerant. 4. Description of the test facility A hermetic compressor of 8.11 cm3 displacement was used in conjunction with a thermostatic expansion valve, an air cooled cross-flow condenser and an evaporator coil immersed in a glycol–water bath. The system also included a liquid receiver of 1.1 dm3 positioned between the condenser and the expansion valve and an accumulator of 0.8 dm3 located at the exit of the evaporator. A schematic diagram of the test facility is shown in Fig. 2. Composition analysis of the working fluid was determined by removing samples of the circulating fluid. Sample points were positioned in the compressor suction and discharge lines and in the liquid line prior to the expansion valve. The samples of the circulating working fluid withdrawn from the test facility were analysed by means of gas chromatography (GC). A flame ionizing detector (FID) was used in conjunction with a capillary column. The response characteristics of the individual components were determined using prepared binary combinations of the component fluids. The results of the binary
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Fig. 2. Schematic diagram of test facility with composition sampling points (䊉).
calibration routines are shown in Fig. 3. Variations in response characteristics of the detector over the duration of the test were accounted for by modification of the relative response factors by reference to a prepared calibration standard analysed each day during the course of the experimental work. 5. Liquid accumulation simulation The internal volume of the system was determined by repeated volumetric expansions of nitrogen from a vessel of known volume. Using this procedure, the total internal volume of the system was estimated to be 4.3 dm3 , of which the liquid receiver and accumulator account for 1.1 and 0.8 dm3 , respectively. The internal volumes of the evaporator and condenser are 0.25 and 0.3 dm3 , respectively. The remainder of the volume (1.85 dm3 ) is contained within the shell of the hermetic compressor, and this volumetric distribution would be typical of conventional hermetic installations. Knowing the internal volumes and the mass of charge in the system, a simple model was devised to relate the change in composition of the working fluid to the combined mass of fluid in the evaporator and accumulator. For the purpose of this paper the working fluid is characterised by the mass fraction of R32 as defined in Eq. (1). mR32 =
Mass R32 Mass (R32 + R125 + R134a)
(1)
In the case of R407C, mR32 = 0.23. The relationship between the accumulation of liquid refrigerant in the accumulator and the composition of the working fluid, expressed in terms of the mass fraction of R32, is demonstrated in Fig. 4. The model
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Fig. 3. Gas chromatograph response characteristics of the component fluids of R407C as binary combinations.
is characterised by Eq. (2). mR32 = 0.223 + 0.000075 MAcc where MAcc is the mass of fluid (in grammes) in the evaporator/accumulator.
Fig. 4. Mass fraction of R32 in the circulating fluid as a function of charge accumulation in the liquid accumulator.
(2)
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Fig. 5. Experimental temperature profiles.
6. Experimental results The test facility was charged with approximately 500 g of oil and 800 g of R407C. GC analysis of the charge composition established that the mixture was close to the 0.23/0.25/0.52 mass fraction specification of R32/R125/R134a. The test facility was configured to reduce the temperature of a water–glycol bath from 20◦ C to the point where heat transfer from the environment to the bath is balanced by heat transfer to the working fluid. The temperature profiles of the bath, and the working fluid at the evaporator outlet and compressor suction port are shown in Fig. 5. The fact that the temperature at the evaporator outlet is higher than the source temperature can be accounted for by heat pick-up from the surroundings, with this mechanism also driving the temperature change in the suction line. The measured composition of R407C, expressed by the mass fraction of the R32 component in circulation is plotted against elapsed time in Fig. 6. Problems were encountered with composition samples taken in the compressor discharge line due to the onset of the phase change process at the sampling point. Because of this, the measured compositions at only two of the sampling points have been presented. The disparity between the composition at the two sampling points can be explained by the likelihood of
Fig. 6. Mass fraction of R32 in the circulating fluid during experiment.
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error in the sampling process. Sample handling errors will lead to an overestimation of the component mass fractions of the more volatile components where the sample is taken in the liquid line prior to the expansion valve, while the opposite applies to samples taken in the suction line. A clear pattern is obvious, with the composition of the working fluid being close to the charge composition for the first 100 min of the test. The system was operating in a dry-expansion mode, with complete vaporisation of the working fluid. Although the operational temperatures and pressures declined as the temperature of the source was reduced from 20◦ C to approximately −10◦ C, the composition of the working fluid remained close to the 0.23/0.25/0.52 charge composition. Other authors [2] have reported R32 enrichment of the circulating fluid for dry-expansion systems. It is likely that differential solubility of the component fluids in the POE lubricant used by these authors is a contributing factor to the observed composition shift. Another possible explanation for the reported R32 enrichment in the circulating fluid is the relationship between the mass of charge and the internal volume of the system. Where a significant proportion of the working fluid is held in the evaporator, the fact that the liquid is richer in the least volatile component (R134a), will lead to R32 enrichment in circulation. A second phase of operation began to occur between 120 and 150 min after start-up. The mass fractions of R32 and R125 in the circulating fluid started to increase. There was insufficient heat transfer from the source to the working fluid to complete the evaporation process, with liquid spill-over into the accumulator. A consistent enrichment of the circulating fluid in its more volatile components was observed over time, which is consistent with a steady accumulation of liquid in the accumulator. Using the model described earlier, the estimated rate of liquid accumulation was calculated to be 4.7 g per minute, while the estimated mass flow rate of the working fluid during this period was approximately 80 g per minute. Thus the rate of liquid spill-over represents approximately 6% of the total flow rate. A maximum mass fraction of R32 in the circulating fluid of 0.28 was measured during the experiment. Using the simulation described earlier, the accumulation of liquid in the evaporator and accumulator under these conditions is approximately 700 g. Since the initial charged mass of R407C was 800 g, there is a limit to the extent of fluid accumulation in the evaporator/accumulator combination. A point must be reached where there was insufficient
Fig. 7. Compositional relationship of the component fluids of R407C.
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fluid in the condenser and the liquid line to permit the system to operate effectively. Correct operation of the expansion device depends on the working fluid being in the liquid state prior to the expansion valve. The extent of the composition variation encountered in this test is presented in Fig. 7, where the mass fractions of R125 and R134a are plotted against the mass fraction of R32. As the mixture in circulation becomes enriched in R32, there is an accompanying increase in the R125 component, while the fraction of the least volatile component (R134a) present in the mixture is reduced. Thus at the end of the experiment the circulating composition was 0.28/0.29/0.43 R32/R134a/R125 by mass. This is very different from the original charge composition of 0.23/0.25/0.52.
7. Conclusions Two distinct operational phases have been observed. In the earliest stages of the test, the evaporation process was complete and the measured circulating composition differed only marginally from the charged composition. Refrigerant hold-up occurred only in the liquid receiver located between the condenser and the expansion valve, and since this is representative of the circulating composition there is little scope for compositional change in the system as a whole. This result is in contrast to that of Kruse and Wieschollek [2], but the discrepancy can be accounted for by their use of a soluble oil which selectively removed R125 and R134a from the circulating fluid. The second operational phase was characterized by incomplete evaporation, with liquid spill-over into an accumulator positioned between the evaporator and the compressor. The composition of the liquid filling the accumulator was richer in R134a (compared to the charge composition), while the vapour passing to the compressor, which became the circulating composition, was enriched in the more volatile components. A consistent enrichment of the working fluid in its R32 and R125 components was evident during the second stage of the test. At the end of the test the circulating composition was 0.28/0.29/0.43 R32/R125/R134a by mass, compared to a charged composition of 0.23/0.25/0.52. The liquid stored in the accumulator under these conditions was estimated to be 0.22/0.24/0.54 R32/R125/R134a by mass. Further change in the circulating composition was constrained by insufficient charge, since the liquid in the accumulator can be regarded as removed from the system. This meant that there was insufficient liquid refrigerant left in the condenser and liquid line to adequately feed the expansion valve. List of symbols A GC area fraction (–) M mass (g) m mass fraction (–) T temperature (◦ C) Subscripts R32R125R134a Acc
component identifiers accumulator
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References [1] M. Burke, S. Carre, H. Kruse, Oil behaviour of the HFCs R32, R125, R134a and one of their mixtures, in: Proceedings of the IIR/IIF Conference on CFC, The Day After, Padova, 1994, pp. 89–98. [2] H. Kruse, F. Wieschollek, Concentration shift when using refrigerant mixtures, Ashrae Transactions: Symposia, 1997, pp. 747–755.
Component separation in refrigerant equipment using HFC mixtures J.T. McMullan, N.J. Hewitt, M.G. McNerlin, B. Mongey∗ Energy Research Centre, University of Ulster at Coleraine, Cromore Road, Londonderry, BT52 1SA, N. Ireland, UK
Abstract An experiment was initiated to examine the extent of component separation in refrigeration systems using HFC mixtures. The working fluid used was R407C, a ternary mixture of R32, R125 and R134a which has attracted attention as a replacement for R22 in a variety of applications. The test was designed to examine the two basic system configurations, namely dry-expansion and flooded evaporator, by incorporating an accumulator at the exit of the evaporator. A non-soluble lubricant was used to ensure that changes in composition were not attributable to differential solubilities of the component fluids. The composition of the circulating fluid was observed to be close to that of the charge composition while the system was operating in dry-expansion mode. Where the evaporation process was incomplete, an accumulation of fluid in the reservoir between the evaporator and the compressor caused the circulating fluid to become enriched in its more volatile components. The extent of composition change is dependent on a number of parameters, the most important being the charged mass, the internal volumes of the system and the compositional relationships of the component fluids. © 2000 Elsevier Science B.V. All rights reserved. Keywords: R407C; Azeotropic mixture; Composition change; Vapour–liquid equilibrium
1. Composition change mechanisms There are three primary mechanisms by which a change in the composition of the working fluid may occur, the most important being due to composition distribution in the two phase region. Except in the case of azeotropic mixtures, the compositions of the liquid and vapour phases are different during the phase change process. The phase change process is at the heart of the vapour compression refrigeration cycle, with phase transformation (whether evaporation or condensation) being the means by which heat is transferred between the working fluid and environment. The variation in composition between the two phases can be most conveniently represented on a lens diagram, with temperature or pressure plotted against composition. The other significant feature of phase transformation is the accompanying change in temperature where the phase change occurs at constant pressure. ∗ Corresponding author. E-mail address: [email protected] (B. Mongey).
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The second possible mechanism for composition change is variation in the solubility of the component fluids in the lubrication oil. A polyolester (POE) or polyalkyl glycol (PAG) lubricant is most commonly used in conjunction with HFC refrigerants, whether as single or multi-component working fluids. In the case of R407C the individual component fluids display differing levels of solubility in POE oils, with R125 being the most soluble [1]. R32 is the least soluble and R134a demonstrates a solubility relationship between these extremes. The greater the level of solubility, the greater the amount of refrigerant that becomes dissolved in the lubricant, and where the component fluids have different solubility levels the composition of the working fluid dissolved in the oil will be different to that of the charged composition. This in turn causes a change in the composition of the working fluid circulating in the system. From the solubility relationship of the individual fluids, the expectation is of an increase in the fraction of R32 in circulation where the R407C/POE combination is utilised. The third mechanism which may influence the composition of the working fluid is leakage or incorrect charging procedures. Since the compositions of the vapour and liquid phases are different in the two phase region, fluid leakage from the system will have a different composition to the charged fluid if the leakage occurs from the evaporator or condenser. While liquid leakage will not unduly affect the composition of the fluid within the system, vapour leakage will be enriched in the more volatile components and the preferential loss of these will result in a working fluid enriched in the least volatile component(s). In the case of R407C, vapour leakage will be rich in R32 and R125, while the fluid remaining within the system will be enriched in R134a. To ensure that the composition of the working fluid matches the mixture specification, charging a system with a multi-component working fluid must be done from the liquid phase of the supply cylinder. Vapour phase charging would result in a charged composition enriched in the more volatile components. Some minor degree of compositional change can be expected when charging from a supply cylinder as it approaches the empty state, since the residual liquid is enriched in the least volatile components.
2. Statement of the aims of the experiment Of the mechanisms by which changes in the composition of the working fluid occur, compositional difference between the two phases is the most significant since it applies to all system/working fluid combinations. Vapour compression refrigeration systems can be classified as one of the two types, dry-expansion or flooded evaporator. In a dry-expansion system, all of the fluid entering the evaporator is vaporised, with charge storage in a reservoir located between the condenser and the expansion device. Only a small fraction of the working fluid is in the evaporator at any time thus limiting the extent of compositional variation and its influence on the composition of working fluid in circulation. In a system employing a flooded evaporator and a multi-component working fluid, the composition of the liquid stored in the evaporator can differ very significantly from that circulating in the remainder of the system. In this work, the operational characteristics of both types of system were examined by modifying an existing dry-expansion system. This was achieved by incorporating an accumulator located at the exit of the evaporator. For the purposes of this experimental work, it was decided to use a non-soluble alkylbenzene/mineral oil mixture as lubricant, to eliminate the contribution of the differential solubility mechanism to changes in the composition of the working fluid.
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Fig. 1. Temperature–pressure relationship of R407C and R407C/oil combination.
3. Refrigerant/oil interaction In order to confirm the non-soluble nature of the refrigerant and lubricant a sample of refrigerant was prepared and the pressure temperature relationship of the fluid was verified. The experimental measurements were compared with those predicted by a model using R407C fluid properties routines. The mass of fluid and the internal volume of the sample cylinder were known (49 g and 0.29 dm3 ), and from these the mass fractions of the individual phases were determined and the pressure at any given temperature was calculated. The results of this test are shown in Fig. 1, and the level of agreement between the experimental results and calculations is consistent with the experimental accuracy of the measurements. A known mass of oil (20.9 g) was added to the sample of R407C and the equilibrium pressure of the refrigerant/oil pair was measured over the same temperature range (0–50◦ C). There was no observable change in the pressure temperature relationship, confirming the non-soluble characteristics of the oil and refrigerant. 4. Description of the test facility A hermetic compressor of 8.11 cm3 displacement was used in conjunction with a thermostatic expansion valve, an air cooled cross-flow condenser and an evaporator coil immersed in a glycol–water bath. The system also included a liquid receiver of 1.1 dm3 positioned between the condenser and the expansion valve and an accumulator of 0.8 dm3 located at the exit of the evaporator. A schematic diagram of the test facility is shown in Fig. 2. Composition analysis of the working fluid was determined by removing samples of the circulating fluid. Sample points were positioned in the compressor suction and discharge lines and in the liquid line prior to the expansion valve. The samples of the circulating working fluid withdrawn from the test facility were analysed by means of gas chromatography (GC). A flame ionizing detector (FID) was used in conjunction with a capillary column. The response characteristics of the individual components were determined using prepared binary combinations of the component fluids. The results of the binary
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Fig. 2. Schematic diagram of test facility with composition sampling points (䊉).
calibration routines are shown in Fig. 3. Variations in response characteristics of the detector over the duration of the test were accounted for by modification of the relative response factors by reference to a prepared calibration standard analysed each day during the course of the experimental work. 5. Liquid accumulation simulation The internal volume of the system was determined by repeated volumetric expansions of nitrogen from a vessel of known volume. Using this procedure, the total internal volume of the system was estimated to be 4.3 dm3 , of which the liquid receiver and accumulator account for 1.1 and 0.8 dm3 , respectively. The internal volumes of the evaporator and condenser are 0.25 and 0.3 dm3 , respectively. The remainder of the volume (1.85 dm3 ) is contained within the shell of the hermetic compressor, and this volumetric distribution would be typical of conventional hermetic installations. Knowing the internal volumes and the mass of charge in the system, a simple model was devised to relate the change in composition of the working fluid to the combined mass of fluid in the evaporator and accumulator. For the purpose of this paper the working fluid is characterised by the mass fraction of R32 as defined in Eq. (1). mR32 =
Mass R32 Mass (R32 + R125 + R134a)
(1)
In the case of R407C, mR32 = 0.23. The relationship between the accumulation of liquid refrigerant in the accumulator and the composition of the working fluid, expressed in terms of the mass fraction of R32, is demonstrated in Fig. 4. The model
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Fig. 3. Gas chromatograph response characteristics of the component fluids of R407C as binary combinations.
is characterised by Eq. (2). mR32 = 0.223 + 0.000075 MAcc where MAcc is the mass of fluid (in grammes) in the evaporator/accumulator.
Fig. 4. Mass fraction of R32 in the circulating fluid as a function of charge accumulation in the liquid accumulator.
(2)
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Fig. 5. Experimental temperature profiles.
6. Experimental results The test facility was charged with approximately 500 g of oil and 800 g of R407C. GC analysis of the charge composition established that the mixture was close to the 0.23/0.25/0.52 mass fraction specification of R32/R125/R134a. The test facility was configured to reduce the temperature of a water–glycol bath from 20◦ C to the point where heat transfer from the environment to the bath is balanced by heat transfer to the working fluid. The temperature profiles of the bath, and the working fluid at the evaporator outlet and compressor suction port are shown in Fig. 5. The fact that the temperature at the evaporator outlet is higher than the source temperature can be accounted for by heat pick-up from the surroundings, with this mechanism also driving the temperature change in the suction line. The measured composition of R407C, expressed by the mass fraction of the R32 component in circulation is plotted against elapsed time in Fig. 6. Problems were encountered with composition samples taken in the compressor discharge line due to the onset of the phase change process at the sampling point. Because of this, the measured compositions at only two of the sampling points have been presented. The disparity between the composition at the two sampling points can be explained by the likelihood of
Fig. 6. Mass fraction of R32 in the circulating fluid during experiment.
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error in the sampling process. Sample handling errors will lead to an overestimation of the component mass fractions of the more volatile components where the sample is taken in the liquid line prior to the expansion valve, while the opposite applies to samples taken in the suction line. A clear pattern is obvious, with the composition of the working fluid being close to the charge composition for the first 100 min of the test. The system was operating in a dry-expansion mode, with complete vaporisation of the working fluid. Although the operational temperatures and pressures declined as the temperature of the source was reduced from 20◦ C to approximately −10◦ C, the composition of the working fluid remained close to the 0.23/0.25/0.52 charge composition. Other authors [2] have reported R32 enrichment of the circulating fluid for dry-expansion systems. It is likely that differential solubility of the component fluids in the POE lubricant used by these authors is a contributing factor to the observed composition shift. Another possible explanation for the reported R32 enrichment in the circulating fluid is the relationship between the mass of charge and the internal volume of the system. Where a significant proportion of the working fluid is held in the evaporator, the fact that the liquid is richer in the least volatile component (R134a), will lead to R32 enrichment in circulation. A second phase of operation began to occur between 120 and 150 min after start-up. The mass fractions of R32 and R125 in the circulating fluid started to increase. There was insufficient heat transfer from the source to the working fluid to complete the evaporation process, with liquid spill-over into the accumulator. A consistent enrichment of the circulating fluid in its more volatile components was observed over time, which is consistent with a steady accumulation of liquid in the accumulator. Using the model described earlier, the estimated rate of liquid accumulation was calculated to be 4.7 g per minute, while the estimated mass flow rate of the working fluid during this period was approximately 80 g per minute. Thus the rate of liquid spill-over represents approximately 6% of the total flow rate. A maximum mass fraction of R32 in the circulating fluid of 0.28 was measured during the experiment. Using the simulation described earlier, the accumulation of liquid in the evaporator and accumulator under these conditions is approximately 700 g. Since the initial charged mass of R407C was 800 g, there is a limit to the extent of fluid accumulation in the evaporator/accumulator combination. A point must be reached where there was insufficient
Fig. 7. Compositional relationship of the component fluids of R407C.
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fluid in the condenser and the liquid line to permit the system to operate effectively. Correct operation of the expansion device depends on the working fluid being in the liquid state prior to the expansion valve. The extent of the composition variation encountered in this test is presented in Fig. 7, where the mass fractions of R125 and R134a are plotted against the mass fraction of R32. As the mixture in circulation becomes enriched in R32, there is an accompanying increase in the R125 component, while the fraction of the least volatile component (R134a) present in the mixture is reduced. Thus at the end of the experiment the circulating composition was 0.28/0.29/0.43 R32/R134a/R125 by mass. This is very different from the original charge composition of 0.23/0.25/0.52.
7. Conclusions Two distinct operational phases have been observed. In the earliest stages of the test, the evaporation process was complete and the measured circulating composition differed only marginally from the charged composition. Refrigerant hold-up occurred only in the liquid receiver located between the condenser and the expansion valve, and since this is representative of the circulating composition there is little scope for compositional change in the system as a whole. This result is in contrast to that of Kruse and Wieschollek [2], but the discrepancy can be accounted for by their use of a soluble oil which selectively removed R125 and R134a from the circulating fluid. The second operational phase was characterized by incomplete evaporation, with liquid spill-over into an accumulator positioned between the evaporator and the compressor. The composition of the liquid filling the accumulator was richer in R134a (compared to the charge composition), while the vapour passing to the compressor, which became the circulating composition, was enriched in the more volatile components. A consistent enrichment of the working fluid in its R32 and R125 components was evident during the second stage of the test. At the end of the test the circulating composition was 0.28/0.29/0.43 R32/R125/R134a by mass, compared to a charged composition of 0.23/0.25/0.52. The liquid stored in the accumulator under these conditions was estimated to be 0.22/0.24/0.54 R32/R125/R134a by mass. Further change in the circulating composition was constrained by insufficient charge, since the liquid in the accumulator can be regarded as removed from the system. This meant that there was insufficient liquid refrigerant left in the condenser and liquid line to adequately feed the expansion valve. List of symbols A GC area fraction (–) M mass (g) m mass fraction (–) T temperature (◦ C) Subscripts R32R125R134a Acc
component identifiers accumulator
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References [1] M. Burke, S. Carre, H. Kruse, Oil behaviour of the HFCs R32, R125, R134a and one of their mixtures, in: Proceedings of the IIR/IIF Conference on CFC, The Day After, Padova, 1994, pp. 89–98. [2] H. Kruse, F. Wieschollek, Concentration shift when using refrigerant mixtures, Ashrae Transactions: Symposia, 1997, pp. 747–755.