- Email: [email protected]
Measurement of electrical properties of refrigerants and refrigerant–oil mixtures
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 5 ( 2 0 1 2 ) 1 3 6 7 e1 3 7 1
Available online at www.sciencedirect.com
w w w . i i fi i r . o r g
journal homepage: www.elsevier.com/locate/ijrefrig
Measurement of electrical properties of refrigerants and refrigeranteoil mixtures S. Feja* ILK Dresden gGmbH, Bertolt-Brecht-Allee 20, Dresden 01309, Germany
article info
abstract
Article history:
The paper deals with the description of a new pressure proof device developed at ILK
Received 4 November 2011
Dresden for measuring electrical properties of refrigerants, oils and refrigeranteoil
Received in revised form
mixtures under pressure. The determination can be performed based on DIN EN 60247.
7 March 2012
This standard is valid for the determination of the relative permittivity, the dielectric
Accepted 11 March 2012
dissipation factor and the direct current (DC) resistivity of isolating fluids at atmospheric
Available online 21 March 2012
pressure. The measurements can be made in a temperature range from 30 C to 90 C at pressures up to 80 bar under dry atmospheric conditions. The sensitivity of the method as
Keywords:
well as its considerable flexibility meets the requirements of the industrial and commercial
Air conditioning
refrigeration. Challenges regarding the development and performance of the measuring
Dielectric constant
cell as well as some preliminary experimental results are presented. The focus of the
Dielectric property
experiments is on environmentally friendly refrigerants (GWP < 150) such as R744, R1234yf
Electric resistance
and R152a, and their mixtures with lubricants. ª 2012 Elsevier Ltd and IIR. All rights reserved.
R134a R744 R152a R1234yf
Mesures des proprie´te´s e´lectriques des frigorige`nes et des me´langes de frigorige`ne / huile Mots cle´s : Conditionnement d’air ; Constante die´lectrique ; Proprie´te´ die´lectrique ; Re´sistance e´lectrique ; R134a ; R744 ; R152a ; R1234yf
1.
Introduction
Due to the increasing applications of hermetic and semihermetic compressors in refrigeration the question of the electrical properties of the refrigeranteoil mixtures used is becoming an important issue. The MAC-systems in electro
and hybrid vehicles, whose electrical systems will be designed for voltages up to 500 V, are only one example for this interest. The knowledge about the specific resistivity of the working fluid mixtures and their application in electrical refrigeration machines is becoming more and more important. Beyond that the knowledge on the dielectric permittivity leads to
* Tel.: þ49 351 4081 767; fax: þ49 351 4081 755. E-mail address: [email protected]. 0140-7007/$ e see front matter ª 2012 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/j.ijrefrig.2012.03.011
1368
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 5 ( 2 0 1 2 ) 1 3 6 7 e1 3 7 1
Nomenclature εx T w x Ca Cg Ce
dielectric constant, permittivity temperature (K) temperature ( C) concentration (wt%) idle capacity (empty cell) F correction capacity F electrode constant F
fundamental statements about the behaviour of molecules in an electrical field, which could be used for energetic optimization of air conditioning systems. Although the determination of the permittivity (dielectric constant, εx) of oils has been performed at the ILK Dresden since 1965, very little has been published on this topic (e.g. Pa¨tz et al., 1968). The analysis of the dielectric constant and the DC resistivity of refrigerants and refrigeranteoil mixtures in the relevant temperature interval between 30 C and þ90 C and at pressures up to 100 bar is quite challenging. Only a few research groups around the world have been working on this subject (Barao et al., 1995, 1996, 1998; Baustian et al., 1986; Bo¨hmer and Loid, 1988; Fellows et al., 1991; Hwang et al., 2008; Meurer et al., 2001; Ribeiro & Nieto de Castro, 2009; Tanaka et al., 1999) mostly using pure substances, but no refrigeranteoil systems. Actually, no measuring standards for the determination of the permittivity and electrical resistivity of refrigeranteoil systems are defined. Therefore, the investigations are performed with a novel test setup, which is based on DIN EN 60247 (2005), ASTM D 924 (2008) and ASTM D 1169 (2011) set for the determination of electrical properties of transformer oils. The capacity and the resistivity of a cylinder gap apparatus without protective ring have been measured directly filled with air (102 mbar), calibrations substances and test fluids in the gap. The permittivity and the specific resistivity of the test fluids are calculated from these measurements according DIN EN 60247 (2005).
2.
Experimental methodology
With respect to the nature of refrigerants and refrigerant mixtures the measurements require a pressure-tight cell, which allows investigation in a homogeneous electrical field necessitating homogeneous refrigeranteoil mixtures. However, in addition to these requirements the enormous sensitivity of high-resistance and permittivity measurements are challenging. Especially the permittivity is very sensitive to external influences (e.g. partial capacitances, impurities, evolving pressure). Furthermore, the measurement setup has to cover a high measurement range with respect to the pure working fluids, i.e. oils and refrigerants. DC resistivities ranging from 105 to 1015 Um have to be determinable at constant current. On the other hand, the examination of the capacitance used to calculate the permittivity requires alternating current. Thus, the redesign of the measurement cell had to be performed with respect to the required experimental parameters including:
Cn Cx εn r U I K
capacity with calibration liquid F capacity with measuring liquid F dielectric constant calibration liquid specific resistivity Um voltage V current A cell constant m
- thermal control (30 C to 90 C) of the test cell and both electrodes - pressure proof (up to 80 bar refrigerant pressure). Fig. 1 shows the electrode setup of the test cell. The gap between the two electrodes is 2 mm. The outer temperature and pressure sensor (Kulite HKL/T 375 M) was screwed in the visible NPT screwing in the outer electrode. Due to the high voltage of 500 V required for the resistivity measurement as well as the partial capacitance generated by the sensor this part of the cell had to be reconstructed. Therefore, the pressure sensor was fixed between the storage container and the test cell and electrically decoupled from the outer electrode by a pressure-tight synthetic hose. The temperature of the outer electrode is defined by the temperature of the surrounding thermostat bath. The test cell is embedded in a silicon bath distinguished by a low dielectric constant and an electrical conductivity below 1012 S m1. As temperature sensor a Pt100 is inserted into the inner electrode. A Precision Component Analyzer 6425 from Wayne Kerr is used as the capacitance detection unit. Additionally a TerraOhmmeter TO-3 from FISCHER ELEKTRONIK is utilized for detection of the DC resistivity. Furthermore a storage container which acts as a sample reservoir for pre-conditioning of the refrigeranteoil mixtures is connected to the test cell (Fig. 2). The capacitance measurement is performed at 1 kHz AC and 1 V. To control the values of the capacitance measurement and to get further information on the dependence of the dissipation factor from the properties of the liquid the dissipation factor and the AC resistance is measured too. After the AC measurement the cell is short-circuited for 1 min. After this the electrodes are connected with the TerraOhmmeter. After another minute the zero current is determined. This zero current should be 1000 times less than the measuring current to avoid an influence on the DC measurement. Next the DC voltage is applied to the system. The DC resistance is measured at 500 V given a load of 250 V/mm according to DIN EN 60247 (2005). If the fluids exhibit a resistivity lower than 106 Um the voltage is set to 1 V instead of 500 V. This is done because the current becomes too high and therefore is not within the measurement range of the TerraOhmmeter used. The voltage is set to 1 V so that the measurement range is as wide as possible. 1 min after turning on the voltage the resistance is measured and after a further minute the current is measured to check the influence of the zero current. The temperature range of the measurements is set above the ambient temperature because of condensing water at the electrode top surface. The resulting partial capacitances affect
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 5 ( 2 0 1 2 ) 1 3 6 7 e1 3 7 1
1369
Fig. 1 e a: Inner (left) and outer electrode (right). b: Assembled measurement cell.
the determined permittivity of the sample. In the worst case, the condensed film connects the inner and the outer electrode which prevents a precise measurement of the electrical properties. Efforts to overcome this shortcoming are being made.
temperature independent. The permittivity εx of an unknown liquid is now determined using Cx from the capacitance measurement according to Eq. (3). εx ¼
3.
Experimental results
3.1.
Calibration
(1)
The terms Cg and Ce can be determined using the calibration substance following Eqs. (1) and (2). Ce ¼
Cn Ca εn 1
(3)
For determining the specific DC resistivity the resistance of the filled cell is determined at constant voltage. In DIN EN 60247 the specific DC resistivity is calculated according to Eq. (4).
According to DIN EN 60247 the test cell has to be calibrated at room temperature in empty and liquid-filled state (calibration substance). The idle capacity is marginally affected by temperature in the range of 30 C to 90 C. The capacity of the empty cell without protective ring is divided into the parts correction capacity (Cg) and electrode constant (Ce) (DIN EN 60247 (2005)) (Eq. (1)). Ca ¼ Ce þ Cg
Cx Cg Ce
(2)
Although the capacity of the cell filled with calibration substance is temperature dependent, the terms Ce and Cg are
r¼K
U I
with K ¼ 0:113$capacity of the empty cell
(4)
Using the idle capacity Ca in this equation (Eq. (4)) is only valid for cells constructed with a protective ring. After comprehensive study using our cell, the value of the electrode constant Ce must be used as the capacity of the empty cell in Eq. (4) for cells without a protective ring. Due to the wide range of DC resistivities of insulating fluids from 105 to 1015 Um this misinterpretation of the DIN EN 60247 (2005) is not crucial using the correct oil for hermetic compressors, but should be considered comparing the results from different suppliers. The test cell was calibrated after each cleaning process with n-Heptane as calibration liquid at room temperature. The cell was also calibrated with more polar fluids such as Acetone and Fluorobenzene. These calibrations gave the same results as the calibration with n-Heptane, but it was very time
Fig. 2 e Block diagram of the test setup to determine electrical properties of refrigeranteoil mixtures.
1370
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 5 ( 2 0 1 2 ) 1 3 6 7 e1 3 7 1
Fig. 3 e Permittivity and resistivity of R744, R134a, R1234yf and R152a.
Fig. 4 e Permittivity and resistivity of R152a mixed with the POE oil ISO VG 80.
consuming to remove the water content from the substances. In fact the water content influences the measurement of the dielectric constant and so a calibration with these fluids is too complicated for daily measurements under normal laboratory conditions.
3.2. Measurements of refrigerants, oils and refrigeranteoil mixtures First the pure refrigerants R134a, R1234yf, R152a and R744 were measured (Fig. 3). These refrigerants are the most popular candidates for use in electro-mobile air conditioning systems. The results of the permittivity measurement of R134a were in agreement with the results given in Barao et al. (1995), Meurer et al. (2001), Barao et al. (1996) and Fellows et al. (1991). Also the value of the permittivity of R152a is comparable to the results presented in Barao et al. (1998) and Fellows et al. (1991) (Fig. 4). The permittivity of all four refrigerants decreases with increasing temperature whereas the resistivity doesn’t show a consistent temperature dependence. With increasing temperature the resistivity decreases (R134a), increases (R1234yf), or remains almost constant (R152a). Comparing permittivity and resistivity values, R152a exhibits the highest permittivity and the lowest resistivity in comparison to the other refrigerants. The resistivity of R134a and of R1234yf is nearly the same. The resistivity of liquid and supercritical R744 is higher than 1014 Um (data not shown).
The measurements were carried out at the saturation pressure of the fluids. After measuring the pure refrigerants, refrigeranteoil mixtures were measured using at least one commercial oil (Fig. 4). Interestingly, the permittivity of a mixture containing nearly 75 wt% oil does not differ much from the permittivity of the pure oil whereas the addition of 50 wt% oil to the refrigerant significantly decreases the permittivity compared to the pure refrigerant sample. The same could be observed in case of the resistivity measurements. No linear correlation has been observed between the concentration and the electrical properties of the pure fluids. Before and after the measurements the water contents of the oils were measured to check the influence of water on the electrical properties.
4.
Conclusion and outlook
A device to determine electrical properties of refrigeranteoil mixtures under pressure has been developed. The test device is based on the existing method of measuring the electrical properties of isolating fluids under atmospheric pressure, as described in the literature. The electrical properties direct current resistivity and permittivity of refrigerants, oils and refrigeranteoil mixtures were determined in a temperature range of 20 Ce90 C. Further efforts are being made to design
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 5 ( 2 0 1 2 ) 1 3 6 7 e1 3 7 1
a measuring cell which allows the determination of electrical properties of refrigeranteoil mixtures below ambient temperature. Apart from that, the influence of the water content and the impurities of oil and-/or refrigerant on the electrical properties (possibly monitored by sensors) will be subject of further studies.
Acknowledgments This work was supported by the grant of the Bundesministerium fu¨r Wirtschaft und Technologie “FuE-Fo¨rderung gemeinnu¨tziger externer Industrieforschungseinrichtungen in Ostdeutschland e Innovationskompetenz (INNO-KOMOst)” Modul Vorlaufforschung VF090028. The author wishes to thank Jan Hegewald (Hegewald & Peschke, Meb und Pru¨ftechnik GmbH, Nossen) for establishing and testing the apparatus and Steven Rhode (TU Dresden) for his help with the measurements. Special thanks go to Sven Heinrich (H. -P. FISCHER ELEKTRONIK GmbH & Co.) for supporting us with the development of the measuring cell.
references
ASTM D 924, 2008. Standard Test Method for Dissipation Factor (Or Power Factor) and Relative Permittivity (Dielectric Constant) of Electrical Insulating Liquids. ASTM International. ASTM D 1169, 2011. Standard Test Method for Specific Resistance (Resistivity) of Electrical Insulating Liquids. ASTM International. Barao, T., Nieta de Castro, C.A., Mardolcar, U.V., Okambawa, R., St-Arnaud, J.M., 1995. Dieelectric constant, dielectric virial coefficient and dipole moments of 1,1,1,2-Tetrafluorethane. J. Chem. Eng. Data 40, 1242e1248.
1371
Barao, M.T., Nieto de Castro, C.A., Mardolcar, U.V., 1996. The dielectric constant of liquid HFC 134a and HCFC 142b. Int. J. Thermophys. 17, 573. Barao, M.T., Mardolcar, U.V., Nieto de Castro, C.A., 1998. Dielectric constant and dipole moments of 1,1,1-Trifluoro-2,2Dichloroethane (HCFC 123) and 1,1-Difluoroethane (HFC 152a) in the liquid phase. Fluid Phase Equil. 1753, 150e151. Baustian, J.J., Pate, M.B., Bergles, A.E., 1986. Properties of oilrefrigerant liquid mixtures with applications to oil concentration measurement: part II-electrical and optical properties. ASHRAE Trans. 92 (Pt.1), 74e92. Bo¨hmer, R., Loid, A., 1988. Dielectric properties of condensed fluoromethanes and fluoromethane mixtures. J. Chem. Phys. 89 (8), 4981e4986. DIN EN 60247, 2005. Isolierflu¨ssigkeiten e Messung der Permittivita¨tszahl, des dielektrischen Verlustfaktors (tan d) und des spezifischen Gleichstrom-Widerstandes. DIN EN. http:// www.beuth.de/cn/J-650DE375B16CD477738B952469BEEDA9.2/ d29ya2Zsb3duYW1lPWV4YUJhc2ljU2VhcmNoJnJlZj10cGwta G9tZSZsYW5ndWFnZWlkPWRl.html. Fellows, B.R., Richard, R.G., Shankland, I.R., 1991. Electrical characterization of alternate refrigerants. In: Actes Congr. Int. Froid, 18th Seint-Hyacinthe, Que, vol. 2, pp. 398e402. Hwang, Y., Radermacher, R., Hirata, T., 2008. Oil mass fraction measurement of CO2/PAG mixture. Int. J. Refrigeration 31, 256e261. Meurer, C., Pietsch, G., Haacke, M., 2001. Electrical properties of CFC- and HCFC-substitutes. Int. J. Refrigeration 24, 171e175. Pa¨tz, G., Ha¨ntzschel, H., Neubert, J., 1968. Messung elektrischer Eigenschaften von Ka¨ltemaschineno¨len. Fachbericht Nr. 97/ 68. ILK Dresden. Ribeiro, A.P.C., Nieto de Castro, C.A., 2009. Dielectric properties of liquid refrigerants: facts and trends. In: IIR 3rd Conference on Thermophysical Properties and Transfer Processes of Refrigeration, Boulder, CO, paper No 108. Tanaka, Y., Matsuo, S., Sotani, T., Kondo, T., Matsuo, T., 1999. Relative permittivity and resistivity of liquid HFC refrigerants under high pressure. Int. J. Thermophys. 20 (1), 107e117.
Available online at www.sciencedirect.com
w w w . i i fi i r . o r g
journal homepage: www.elsevier.com/locate/ijrefrig
Measurement of electrical properties of refrigerants and refrigeranteoil mixtures S. Feja* ILK Dresden gGmbH, Bertolt-Brecht-Allee 20, Dresden 01309, Germany
article info
abstract
Article history:
The paper deals with the description of a new pressure proof device developed at ILK
Received 4 November 2011
Dresden for measuring electrical properties of refrigerants, oils and refrigeranteoil
Received in revised form
mixtures under pressure. The determination can be performed based on DIN EN 60247.
7 March 2012
This standard is valid for the determination of the relative permittivity, the dielectric
Accepted 11 March 2012
dissipation factor and the direct current (DC) resistivity of isolating fluids at atmospheric
Available online 21 March 2012
pressure. The measurements can be made in a temperature range from 30 C to 90 C at pressures up to 80 bar under dry atmospheric conditions. The sensitivity of the method as
Keywords:
well as its considerable flexibility meets the requirements of the industrial and commercial
Air conditioning
refrigeration. Challenges regarding the development and performance of the measuring
Dielectric constant
cell as well as some preliminary experimental results are presented. The focus of the
Dielectric property
experiments is on environmentally friendly refrigerants (GWP < 150) such as R744, R1234yf
Electric resistance
and R152a, and their mixtures with lubricants. ª 2012 Elsevier Ltd and IIR. All rights reserved.
R134a R744 R152a R1234yf
Mesures des proprie´te´s e´lectriques des frigorige`nes et des me´langes de frigorige`ne / huile Mots cle´s : Conditionnement d’air ; Constante die´lectrique ; Proprie´te´ die´lectrique ; Re´sistance e´lectrique ; R134a ; R744 ; R152a ; R1234yf
1.
Introduction
Due to the increasing applications of hermetic and semihermetic compressors in refrigeration the question of the electrical properties of the refrigeranteoil mixtures used is becoming an important issue. The MAC-systems in electro
and hybrid vehicles, whose electrical systems will be designed for voltages up to 500 V, are only one example for this interest. The knowledge about the specific resistivity of the working fluid mixtures and their application in electrical refrigeration machines is becoming more and more important. Beyond that the knowledge on the dielectric permittivity leads to
* Tel.: þ49 351 4081 767; fax: þ49 351 4081 755. E-mail address: [email protected]. 0140-7007/$ e see front matter ª 2012 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/j.ijrefrig.2012.03.011
1368
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 5 ( 2 0 1 2 ) 1 3 6 7 e1 3 7 1
Nomenclature εx T w x Ca Cg Ce
dielectric constant, permittivity temperature (K) temperature ( C) concentration (wt%) idle capacity (empty cell) F correction capacity F electrode constant F
fundamental statements about the behaviour of molecules in an electrical field, which could be used for energetic optimization of air conditioning systems. Although the determination of the permittivity (dielectric constant, εx) of oils has been performed at the ILK Dresden since 1965, very little has been published on this topic (e.g. Pa¨tz et al., 1968). The analysis of the dielectric constant and the DC resistivity of refrigerants and refrigeranteoil mixtures in the relevant temperature interval between 30 C and þ90 C and at pressures up to 100 bar is quite challenging. Only a few research groups around the world have been working on this subject (Barao et al., 1995, 1996, 1998; Baustian et al., 1986; Bo¨hmer and Loid, 1988; Fellows et al., 1991; Hwang et al., 2008; Meurer et al., 2001; Ribeiro & Nieto de Castro, 2009; Tanaka et al., 1999) mostly using pure substances, but no refrigeranteoil systems. Actually, no measuring standards for the determination of the permittivity and electrical resistivity of refrigeranteoil systems are defined. Therefore, the investigations are performed with a novel test setup, which is based on DIN EN 60247 (2005), ASTM D 924 (2008) and ASTM D 1169 (2011) set for the determination of electrical properties of transformer oils. The capacity and the resistivity of a cylinder gap apparatus without protective ring have been measured directly filled with air (102 mbar), calibrations substances and test fluids in the gap. The permittivity and the specific resistivity of the test fluids are calculated from these measurements according DIN EN 60247 (2005).
2.
Experimental methodology
With respect to the nature of refrigerants and refrigerant mixtures the measurements require a pressure-tight cell, which allows investigation in a homogeneous electrical field necessitating homogeneous refrigeranteoil mixtures. However, in addition to these requirements the enormous sensitivity of high-resistance and permittivity measurements are challenging. Especially the permittivity is very sensitive to external influences (e.g. partial capacitances, impurities, evolving pressure). Furthermore, the measurement setup has to cover a high measurement range with respect to the pure working fluids, i.e. oils and refrigerants. DC resistivities ranging from 105 to 1015 Um have to be determinable at constant current. On the other hand, the examination of the capacitance used to calculate the permittivity requires alternating current. Thus, the redesign of the measurement cell had to be performed with respect to the required experimental parameters including:
Cn Cx εn r U I K
capacity with calibration liquid F capacity with measuring liquid F dielectric constant calibration liquid specific resistivity Um voltage V current A cell constant m
- thermal control (30 C to 90 C) of the test cell and both electrodes - pressure proof (up to 80 bar refrigerant pressure). Fig. 1 shows the electrode setup of the test cell. The gap between the two electrodes is 2 mm. The outer temperature and pressure sensor (Kulite HKL/T 375 M) was screwed in the visible NPT screwing in the outer electrode. Due to the high voltage of 500 V required for the resistivity measurement as well as the partial capacitance generated by the sensor this part of the cell had to be reconstructed. Therefore, the pressure sensor was fixed between the storage container and the test cell and electrically decoupled from the outer electrode by a pressure-tight synthetic hose. The temperature of the outer electrode is defined by the temperature of the surrounding thermostat bath. The test cell is embedded in a silicon bath distinguished by a low dielectric constant and an electrical conductivity below 1012 S m1. As temperature sensor a Pt100 is inserted into the inner electrode. A Precision Component Analyzer 6425 from Wayne Kerr is used as the capacitance detection unit. Additionally a TerraOhmmeter TO-3 from FISCHER ELEKTRONIK is utilized for detection of the DC resistivity. Furthermore a storage container which acts as a sample reservoir for pre-conditioning of the refrigeranteoil mixtures is connected to the test cell (Fig. 2). The capacitance measurement is performed at 1 kHz AC and 1 V. To control the values of the capacitance measurement and to get further information on the dependence of the dissipation factor from the properties of the liquid the dissipation factor and the AC resistance is measured too. After the AC measurement the cell is short-circuited for 1 min. After this the electrodes are connected with the TerraOhmmeter. After another minute the zero current is determined. This zero current should be 1000 times less than the measuring current to avoid an influence on the DC measurement. Next the DC voltage is applied to the system. The DC resistance is measured at 500 V given a load of 250 V/mm according to DIN EN 60247 (2005). If the fluids exhibit a resistivity lower than 106 Um the voltage is set to 1 V instead of 500 V. This is done because the current becomes too high and therefore is not within the measurement range of the TerraOhmmeter used. The voltage is set to 1 V so that the measurement range is as wide as possible. 1 min after turning on the voltage the resistance is measured and after a further minute the current is measured to check the influence of the zero current. The temperature range of the measurements is set above the ambient temperature because of condensing water at the electrode top surface. The resulting partial capacitances affect
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 5 ( 2 0 1 2 ) 1 3 6 7 e1 3 7 1
1369
Fig. 1 e a: Inner (left) and outer electrode (right). b: Assembled measurement cell.
the determined permittivity of the sample. In the worst case, the condensed film connects the inner and the outer electrode which prevents a precise measurement of the electrical properties. Efforts to overcome this shortcoming are being made.
temperature independent. The permittivity εx of an unknown liquid is now determined using Cx from the capacitance measurement according to Eq. (3). εx ¼
3.
Experimental results
3.1.
Calibration
(1)
The terms Cg and Ce can be determined using the calibration substance following Eqs. (1) and (2). Ce ¼
Cn Ca εn 1
(3)
For determining the specific DC resistivity the resistance of the filled cell is determined at constant voltage. In DIN EN 60247 the specific DC resistivity is calculated according to Eq. (4).
According to DIN EN 60247 the test cell has to be calibrated at room temperature in empty and liquid-filled state (calibration substance). The idle capacity is marginally affected by temperature in the range of 30 C to 90 C. The capacity of the empty cell without protective ring is divided into the parts correction capacity (Cg) and electrode constant (Ce) (DIN EN 60247 (2005)) (Eq. (1)). Ca ¼ Ce þ Cg
Cx Cg Ce
(2)
Although the capacity of the cell filled with calibration substance is temperature dependent, the terms Ce and Cg are
r¼K
U I
with K ¼ 0:113$capacity of the empty cell
(4)
Using the idle capacity Ca in this equation (Eq. (4)) is only valid for cells constructed with a protective ring. After comprehensive study using our cell, the value of the electrode constant Ce must be used as the capacity of the empty cell in Eq. (4) for cells without a protective ring. Due to the wide range of DC resistivities of insulating fluids from 105 to 1015 Um this misinterpretation of the DIN EN 60247 (2005) is not crucial using the correct oil for hermetic compressors, but should be considered comparing the results from different suppliers. The test cell was calibrated after each cleaning process with n-Heptane as calibration liquid at room temperature. The cell was also calibrated with more polar fluids such as Acetone and Fluorobenzene. These calibrations gave the same results as the calibration with n-Heptane, but it was very time
Fig. 2 e Block diagram of the test setup to determine electrical properties of refrigeranteoil mixtures.
1370
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 5 ( 2 0 1 2 ) 1 3 6 7 e1 3 7 1
Fig. 3 e Permittivity and resistivity of R744, R134a, R1234yf and R152a.
Fig. 4 e Permittivity and resistivity of R152a mixed with the POE oil ISO VG 80.
consuming to remove the water content from the substances. In fact the water content influences the measurement of the dielectric constant and so a calibration with these fluids is too complicated for daily measurements under normal laboratory conditions.
3.2. Measurements of refrigerants, oils and refrigeranteoil mixtures First the pure refrigerants R134a, R1234yf, R152a and R744 were measured (Fig. 3). These refrigerants are the most popular candidates for use in electro-mobile air conditioning systems. The results of the permittivity measurement of R134a were in agreement with the results given in Barao et al. (1995), Meurer et al. (2001), Barao et al. (1996) and Fellows et al. (1991). Also the value of the permittivity of R152a is comparable to the results presented in Barao et al. (1998) and Fellows et al. (1991) (Fig. 4). The permittivity of all four refrigerants decreases with increasing temperature whereas the resistivity doesn’t show a consistent temperature dependence. With increasing temperature the resistivity decreases (R134a), increases (R1234yf), or remains almost constant (R152a). Comparing permittivity and resistivity values, R152a exhibits the highest permittivity and the lowest resistivity in comparison to the other refrigerants. The resistivity of R134a and of R1234yf is nearly the same. The resistivity of liquid and supercritical R744 is higher than 1014 Um (data not shown).
The measurements were carried out at the saturation pressure of the fluids. After measuring the pure refrigerants, refrigeranteoil mixtures were measured using at least one commercial oil (Fig. 4). Interestingly, the permittivity of a mixture containing nearly 75 wt% oil does not differ much from the permittivity of the pure oil whereas the addition of 50 wt% oil to the refrigerant significantly decreases the permittivity compared to the pure refrigerant sample. The same could be observed in case of the resistivity measurements. No linear correlation has been observed between the concentration and the electrical properties of the pure fluids. Before and after the measurements the water contents of the oils were measured to check the influence of water on the electrical properties.
4.
Conclusion and outlook
A device to determine electrical properties of refrigeranteoil mixtures under pressure has been developed. The test device is based on the existing method of measuring the electrical properties of isolating fluids under atmospheric pressure, as described in the literature. The electrical properties direct current resistivity and permittivity of refrigerants, oils and refrigeranteoil mixtures were determined in a temperature range of 20 Ce90 C. Further efforts are being made to design
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 5 ( 2 0 1 2 ) 1 3 6 7 e1 3 7 1
a measuring cell which allows the determination of electrical properties of refrigeranteoil mixtures below ambient temperature. Apart from that, the influence of the water content and the impurities of oil and-/or refrigerant on the electrical properties (possibly monitored by sensors) will be subject of further studies.
Acknowledgments This work was supported by the grant of the Bundesministerium fu¨r Wirtschaft und Technologie “FuE-Fo¨rderung gemeinnu¨tziger externer Industrieforschungseinrichtungen in Ostdeutschland e Innovationskompetenz (INNO-KOMOst)” Modul Vorlaufforschung VF090028. The author wishes to thank Jan Hegewald (Hegewald & Peschke, Meb und Pru¨ftechnik GmbH, Nossen) for establishing and testing the apparatus and Steven Rhode (TU Dresden) for his help with the measurements. Special thanks go to Sven Heinrich (H. -P. FISCHER ELEKTRONIK GmbH & Co.) for supporting us with the development of the measuring cell.
references
ASTM D 924, 2008. Standard Test Method for Dissipation Factor (Or Power Factor) and Relative Permittivity (Dielectric Constant) of Electrical Insulating Liquids. ASTM International. ASTM D 1169, 2011. Standard Test Method for Specific Resistance (Resistivity) of Electrical Insulating Liquids. ASTM International. Barao, T., Nieta de Castro, C.A., Mardolcar, U.V., Okambawa, R., St-Arnaud, J.M., 1995. Dieelectric constant, dielectric virial coefficient and dipole moments of 1,1,1,2-Tetrafluorethane. J. Chem. Eng. Data 40, 1242e1248.
1371
Barao, M.T., Nieto de Castro, C.A., Mardolcar, U.V., 1996. The dielectric constant of liquid HFC 134a and HCFC 142b. Int. J. Thermophys. 17, 573. Barao, M.T., Mardolcar, U.V., Nieto de Castro, C.A., 1998. Dielectric constant and dipole moments of 1,1,1-Trifluoro-2,2Dichloroethane (HCFC 123) and 1,1-Difluoroethane (HFC 152a) in the liquid phase. Fluid Phase Equil. 1753, 150e151. Baustian, J.J., Pate, M.B., Bergles, A.E., 1986. Properties of oilrefrigerant liquid mixtures with applications to oil concentration measurement: part II-electrical and optical properties. ASHRAE Trans. 92 (Pt.1), 74e92. Bo¨hmer, R., Loid, A., 1988. Dielectric properties of condensed fluoromethanes and fluoromethane mixtures. J. Chem. Phys. 89 (8), 4981e4986. DIN EN 60247, 2005. Isolierflu¨ssigkeiten e Messung der Permittivita¨tszahl, des dielektrischen Verlustfaktors (tan d) und des spezifischen Gleichstrom-Widerstandes. DIN EN. http:// www.beuth.de/cn/J-650DE375B16CD477738B952469BEEDA9.2/ d29ya2Zsb3duYW1lPWV4YUJhc2ljU2VhcmNoJnJlZj10cGwta G9tZSZsYW5ndWFnZWlkPWRl.html. Fellows, B.R., Richard, R.G., Shankland, I.R., 1991. Electrical characterization of alternate refrigerants. In: Actes Congr. Int. Froid, 18th Seint-Hyacinthe, Que, vol. 2, pp. 398e402. Hwang, Y., Radermacher, R., Hirata, T., 2008. Oil mass fraction measurement of CO2/PAG mixture. Int. J. Refrigeration 31, 256e261. Meurer, C., Pietsch, G., Haacke, M., 2001. Electrical properties of CFC- and HCFC-substitutes. Int. J. Refrigeration 24, 171e175. Pa¨tz, G., Ha¨ntzschel, H., Neubert, J., 1968. Messung elektrischer Eigenschaften von Ka¨ltemaschineno¨len. Fachbericht Nr. 97/ 68. ILK Dresden. Ribeiro, A.P.C., Nieto de Castro, C.A., 2009. Dielectric properties of liquid refrigerants: facts and trends. In: IIR 3rd Conference on Thermophysical Properties and Transfer Processes of Refrigeration, Boulder, CO, paper No 108. Tanaka, Y., Matsuo, S., Sotani, T., Kondo, T., Matsuo, T., 1999. Relative permittivity and resistivity of liquid HFC refrigerants under high pressure. Int. J. Thermophys. 20 (1), 107e117.