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Oil concentration in liquid refrigerants: in situ measurement
International Journal of Refrigeration 22 (1999) 499±508
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Oil concentration in liquid refrigerants: in situ measurement Eduardo Navarro de Andrade a, Eric Skowron b, Victor W. Goldschmidt c, Eckhard A. Groll c,* a Carrier Corporation, One Carrier Parkway, PO Box 4808, Syracuse, NY 13221, USA Hughes Space and Communications Company, PO Box 92919, Los Angeles, CA 90009-2919, USA c Ray W. Herrick Laboratories, School of Mechanical Engineering, Purdue University, West Lafayette, IN 47907-1077, USA b
Received 15 September 1998; accepted 6 November 1998
Abstract The concentration of oil in refrigerants (while in liquid state) can be measured with an acoustic velocity sensor. The transit time for an acoustic signal can be related to the oil concentration and temperature of the liquid mixture. The performance of the sensor is dependent on the properties of the oil and refrigerant, and their miscibility. In general, a thorough calibration becomes necessary. It is shown in this paper that for concentrations less than 10%, an approximation can be made for the estimate of concentration hence eliminating the need for an elaborate calibration procedure. # 1999 Elsevier Science Ltd and IIR. All rights reserved. Keywords: Refrigerant; Lubricant; Concentration; Measurements; Acoustics
Concentration d'huile dans les frigorigeÁnes liquides : mesures sur le terrain ReÂsume La concentration d'huile dans les frigorigeÁnes (aÁ l'eÂtat liquide) peut eÃtre mesureÂe aÁ l'aide d'un capteur de vitesse acoustique. Le temps de transit d'un signal acoustique varie en fonction de la concentration d'huile et la tempeÂrature du meÂlange liquide. La performance du capteur est fonction des proprieÂteÂs de l'huile et du frigorigeÁne ainsi que de leur miscibiliteÂ. De facËon geÂneÂral, on doit eectuer un calibrage approfondi. Cette communication montre que pour les concentrations infeÂriures aÁ 10%, il est possible d'utiliser une approximation a®n d'eÂvaluer la concentration ; de cette manieÁre, on n'a plus besoin de faire un calibrage complexe. # 1999 Elsevier Science Ltd and IIR. All rights reserved. Mots cleÂs: FrigorigeÁne; Lubri®ant; Concentration; Mesure; Acoustique
* Corresponding author. Tel.: +1-765-496-2201; fax: +1-765-494-0787. E-mail addresses: [email protected] (E. Navarro de Andrade), [email protected] (E. Skowron), [email protected] (V.W. Goldschmidt), [email protected] (E.A. Groll) 0140-7007/99/$20.00 # 1999 Elsevier Science Ltd and IIR. All rights reserved. PII: S0140-7007(99)00008-0
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E. Navarro de Andrade et al. / International Journal of Refrigeration 22 (1999) 499±508
Nomenclature
A C L m mix oil ref t !
contact area between liquid layers (m,ft) speed of sound through ¯uid (m/s,ft/s) ¯uid layer thickness (m,ft) mass of ¯uid (kg,lbm) property of refrigerant/oil mixture property of pure lubricant property of pure refrigerant ¯ight time of pressure wave through ¯uid (s) density (kg/m3, lbm/ft3) concentration of element in mixture (±)
1. Introduction The ``standard'' method of measuring oil concentration is based on ASHRAE Standard 41.4±1996. This standard calls for removing a sample and measuring the quantity of refrigerant and oil in that sample. In many instances, it is desirable to measure concentrations ``in situ''. Table 1 reviews some of the alternatives open to the user, while Table 2 compares some of the advantages and disadvantages of the potential methods. For a current study of in-line, real-time oil concentration of refrigerant/oil mixtures in a vapor-compression system,
a sensor based on the measurement of acoustic velocity in the liquid refrigerant/oil mixture was selected. Compared to the other methods and sensors, the acoustic velocity method consists of the best compromise between cost and accuracy. 2. Method description The values of temperature and acoustic velocity of a liquid oil-refrigerant mixture can be used to determine the oil content of the mixture. Baustian et al. [3] described the basic technique for refrigerant±oil combinations such as R-12/mineral oil, R-22/mineral oil, and R-502/ synthetic oil. The method makes use of the large dierence between the acoustic velocities of pure refrigerants and oils. Typical data show that the speed of sound in oil is some 2±3 times greater than in a liquid refrigerant. The acoustic velocity in a medium can be calculated by measuring the transit time of a pressure wave across a known distance. This was accomplished in this study by using a pulse generator and a pair of ultrasonic immersion transducers. A block diagram of the electronic components is shown in Fig. 1 . The pulse generator was used to generate electric pulses to excite the transducers. The ®rst transducer received these electric pulses, and converted them, sending pressure waves across the liquid oil±refrigerant mixture. The second transducer detected the pressure waves and emitted electric pulses in response. The two signals were monitored using an oscilloscope. The transit time was then the interval between the initiation of each signal.
Table 1 Oil concentration measurement methods Tableau 1 Concentration d'huile : meÂthodes de mesure Method
Description
ASHRAE Standard 41.4±1996
Fluid sample is removed from the liquid line of the system and weighed. The refrigerant component is boiled o, and the remaining oil is weighed. The density of the ¯owing liquid is formed using a vibrating U-tube or a more accurate straight tube densimeter. Calibration is required to correlate the density of the refrigerant/oil mixture in use to temperature and oil concentration. Calibration can be avoided for mixtures known to follow the ideal solution assumption. Kutsuna et al. [1] correlated the amount of ultra-violet light absorbance in a liquid sample to the oil concentration. While Suzuki et al. [2] related the ratio of the transmittance of infrared light through a refrigerant/oil sample to that of a standard solution of known attenuation. Calibration with samples of known concentration is necessary. The viscosity of the refrigerant/oil mixture can be measured with a bypass viscometer. The known predictive relationships are very inaccurate. Therefore, it is necessary to calibrate the instrument by correlating the viscosity of known samples to temperature and oil concentration. The speed of sound through a refrigerant/oil mixture can be determined by measuring the transit time of an ultrasonic signal across a known distance. This acoustic velocity varies with temperature and oil concentration. Either a lengthy calibration procedure is required, or the predictive relationship development in this work can be used.
Density based
Light absorption based
Viscosity based Acoustic velocity based
E. Navarro de Andrade et al. / International Journal of Refrigeration 22 (1999) 499±508
501
Table 2 Comparison of oil concentration sensors Tableau 2 Comparaison des capteurs utiliseÂs pour mesurer la concentration Method
Advantages
Disadvantages
Reference
ASHRAE Standard
Low cost
ASHRAE [4]
Density based Light absorption based
In-line, real time Accuracy, real time
Viscosity based Acoustic velocity based
Real time In-line, real time, cost
Not real-time, time consuming, charge removal Accuracy Cost, complexity, requires bypass, calibration, fragile Response, requires bypass calibration Calibration
Baustian et al. [5] Bayani et al. [6] Kutsuna et al. [1] Suzuki et al. [2] Baustian et al. [7] Baustian et al. [8] Meyer and Saiz Jabardo [9]
Fig. 1. Schematic diagram of oil concentration measurement method. Fig. 1. ScheÂma montrant la meÂthode utiliseÂe pur mesurer la concentration d'huile.
The ultrasonic transducers were held in position by stainless steel jackets that were in turn inserted into a brass housing. Fig. 2 illustrates the housing arrangement. All parts were built speci®cally for this application. The stainless steel jackets were cylindrical blocks with circular holes bored into them to match the outer diameter of the transducers. The transducers slid tightly into these openings, and were ®xed in place by setscrews. The jackets and the transducers were then ®xed to a brass block by a series of screws. This brass block had a circular channel drilled through it where the liquid mixture was to ¯ow. The channel in the brass housing was dimensioned in order that the sensor would ®t directly into the piping of the liquid line of the system. The transducers selected for this study had an epoxy layer separating the active element of the transducers from the environment (wear plate). In this application some concern existed regarding the resistance of the epoxy wear plate to corrosive attacks from the refrigerant used (R-22). To avoid problems of this nature the holes in the steel jackets were bored to a depth of 0.25
mm (0.01 inches) smaller than the total height of the jacket, thus leaving a thin protective layer of stainless steel across the face of the transducer. 3. Sample calibration results Sample data are now presented for R-22/alkyl benzene mixtures. Due to the lack of published data on acoustic velocity in oil±refrigerant mixtures, calibration curves had to be generated experimentally for the oil± refrigerant combination of interest. This was done by measuring the propagation time of the pressure waves across the known path length of the sensor using mixture samples of known concentration and under controlled temperature conditions. For this research the working pair was R-22 and an alkyl benzene synthetic lubricant of the 300 SUS viscosity classi®cation. The oil used was fully miscible with R-22. The required experimental data were obtained using a calibration vessel arrangement depicted in Fig. 3. A
502
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Fig. 2. Ultrasonic transducer housing schematic. Fig. 2. ScheÂma du boõÃtier du transducteur ultrasonique.
Fig. 3. Schematic of sensor calibration vessel. Fig. 3. ScheÂma du dispositif de calibrage du capteur.
copper tube was brazed to each end of the refrigerant ¯ow path in the sensor housing. To one side, an immersed thermocouple was attached for temperature measurement. On the other side, a sight glass was attached to visually insure that the sensor was ®lled with liquid. In addition, this side had a pressure relief valve and a service port through which the ¯uids were inserted into the bottle. Tests were carried out for concentrations of 2, 5, 7.5, 10, 15, 20, and 25%, over a temperature range varying from 15.6 to 40.6 C (60 to 105 F) at 2.8 C (5 F) intervals.
The calibration vessel was thoroughly cleaned with heptane before each test. The desired amount of oil was then inserted through the service port using a syringe. The mass of oil to be inserted was weighed using a scale with an accuracy of 0.5 g (0.018 oz). After the oil was introduced the cylinder was evacuated using a vacuum pump. The next step was to submerge the calibration vessel in an ice bath to reduce its temperature. The cylinder was then charged with the appropriate mass of R-22 to produce the desired concentration in
E. Navarro de Andrade et al. / International Journal of Refrigeration 22 (1999) 499±508
503
Fig. 4. Acoustic velocity as a function of oil concentration. Fig. 4. Vitesse acoustique en fonction de la concentration d'huile.
the liquid phase of the mixture. The R-22 was fed into the calibration bottle, from the reservoir tank, as a vapor, to guarantee it entered free of lubricant. The mass of refrigerant in the bottle was weighed with an electronic scale with a precision of 0.5 g (0.018 oz). The calibration vessel was then placed in a constant temperature environment where the ambient temperature was controlled. Transit time measurements were then taken and the concentration and speed of sound were related for particular mixture temperatures. Fig. 4 presents the results. Details and polynomial ®ts to the calibration are given in Navarro de Andrade [10].
acoustic velocity sensor to measure oil concentration has to include the experimental generation of calibration acoustic velocity data for the mixture of interest. A signi®cant amount of time and eort can be saved if the acoustic velocity of the refrigerant/oil mixture can be predicted from pure ¯uid properties. In trying to accomplish this goal a simple model was created in which the speed of sound of a liquid mixture is approximated using the acoustic velocities of the pure ¯uids. Fig. 5 illustrates schematically the model applied.
4. An approximation The fundamental question now being addressed is whether the results of Fig. 4 could have been predicted, and hence eliminate the need for the lengthy calibration procedure. One of the disadvantages of the acoustic velocity sensor, when compared to the standard method of measurement of oil concentration by sample removal, is the necessity of property data (acoustic velocity) for the speci®c refrigerant/oil mixture being used. A literature review encountered no published predictive relationships for the acoustic velocities of liquid mixtures. The experimental data available are also extremely limited, including only acoustic velocity data on mixtures of R-12 and R-22 with a naphthenic mineral oil, and mixtures of R-502 with an alkyl benzene synthetic lubricant [3]. This means any attempt to employ an
Fig. 5. Acoustic velocity predictive model schematic. Fig. 5. ScheÂma d'un modeÁle pour la vitesse acoustique.
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The refrigerant/oil mixture is treated as being made up of two distinct layers, one of pure refrigerant and the other of pure lubricant. Based on this separation of the mixture's components the acoustic velocity of the individual substances is given by: Loil Coil 1 toil and Cref
Lref tref
2
where toil and tref are the ¯ight times of the acoustic pressure wave across the layers of oil and refrigerant. These two equations can then be used to obtain the acoustic velocity of the mixture: Cmix
Loil Lref toil tref
3
Substituting for the unknown individual transit times from Eqs. (1) and (2) in the above equations yields:
Loil Lref Loil Lref Coil Cref
Cmix
4
In order to eliminate the individual layer thicknesses, Lref and Loil , the following substitutions are used: Loil
moil oil Ac
5
Lref
mref ref Ac
6
where Ac is the contact area between the two liquid layers. After employing the substitutions above and multiplying both numerator and denominator by oil , the following relationship is obtained: oil moil mref ref Cmix 7 1 oil 1 moil mref Coil Cref ref
Fig. 6. Comparison of predicted and experimental values of acoustic velocity of R-22/alkyl benzene mixtures, 16 to 24 C (60 to 75 F). Fig. 6. Comparaison des valeurs preÂvues et expeÂrimentales de vitesse acoustique des meÂlanges de R22 et d'alkylbenzeÁne aÁ 16 aÁ 24 C.
E. Navarro de Andrade et al. / International Journal of Refrigeration 22 (1999) 499±508
This equation can be made more useful by dividing its terms by the total mass of the mixture, mmix . The resulting form of the equation is an expression for the
505
mixture acoustic velocity as a function of the acoustic velocities of the individual components, their densities, and the concentration of oil in the mixture, !oil :
Fig. 7. Comparison of predicted and experimental values of acoustic velocity of R-22/alkyl benzene mixtures, 27 to 41 C (80 to 105 F). Fig. 7. Comparaison des valeurs preÂvues et expeÂrimentales de vitesse acoustique des meÂlanges de R22 et d'alkylbenzeÁne aÁ 27 aÁ 41 C.
506
Cmix
E. Navarro de Andrade et al. / International Journal of Refrigeration 22 (1999) 499±508
oil !oil 1 ÿ !oil ref 1 oil 1 !oil 1 ÿ !oil Coil Cref ref
8
In order to use this equation, only density and acoustic velocity information on the pure ¯uids are required. Density data of pure ¯uids are widely available in the open literature. Acoustic velocity data for liquid refrigerants are also plentiful. ASHRAE [11], for instance, contains a comprehensive list of tabulated properties for most refrigerants presently in use, including acoustic velocity data for the majority of them. REFPROP, the NIST Thermodynamic Properties of Refrigerants and Refrigerant Mixture Database, also oers the possibility of computing the speed of sound in liquid refrigerants or blends of liquid refrigerants. As far as the lubricant is concerned acoustic velocity data are not so easily found for speci®c types. However, this information can be
obtained by ®lling the acoustic sensor cavity with the oil and making measurements over the temperature range of interest. Eq. (8) was solved for the temperature and concentration range of interest, for the sample R22/alkyl benzene oil mixture. The theoretical results were compared with the acoustic velocities obtained from the calibration test. Figs. 6 and 7 show the theoretical and experimental speed of sound as a function of oil concentration. The deviations between the predicted and experimental speed of sound results as a function of oil concentration can be seen in Figs. 8 and 9. The results show that the simpli®ed model can predict the acoustic velocity in the mixture with an accuracy of higher than 98% for concentrations up to 10 weight-%, and temperatures below 29.4 C (85 F). For higher temperatures and concentrations the model systematically under-predicts the values of acoustic velocity, but remains higher than 95% accurate for concentrations as
Fig. 8. Deviations between predicted and experimental values of acoustic velocity of R-22/alkyl benzene mixtures, 16 to 24 C (60 to 75 F). Fig. 8. Ecarts entre les valeurs preÂvues et expeÂrimentales de vitesse acoustique des meÂlanges de R22 et d'alkylbenzeÁne aÁ 16 aÁ 24 C.
E. Navarro de Andrade et al. / International Journal of Refrigeration 22 (1999) 499±508
high as 15%, over the entire temperature range. These deviations between the theoretical and experimental values can be reduced with the addition of an empirical temperature, and concentration dependent parameter to
507
the model shown in Eq. (8). The departure from the measured values is most likely caused by the use of the ideal solution assumption, which is implicit in the derivation of the model. The progression of the deviations
Fig. 9. Deviations between predicted and experimental values of acoustic velocity of R-22/alkyl benzene mixtures, 27 to 41 C (80 to 105 F). Fig. 9. Ecarts entre les valeurs preÂvues et expeÂrimentales de vitesse acoustique des meÂlanges de R22 et d'alkylbenzeÁne aÁ 27 aÁ 41 C.
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between the predicted values, with respect to temperature and concentration, is consistent with the behavior of the density of real mixtures as compared to that of ideal mixtures. According to published data [12], real mixtures tend to increasingly depart from the ideal solution theory as temperature and oil concentration increase (for the ranges of interest here). Since most real mixtures become denser than ideal, the model will under-predict the speed of sound of mixtures in the higher temperature and concentration range. Due to the lack of available data on the speci®c mixture being used no correction to account for this problem was attempted. 5. Conclusions The method of operation of an acoustic velocity sensor used as an oil concentration measurement device was outlined. The sensor is intended to be installed in the liquid line of refrigeration systems, and is capable of providing in-line, real time estimates of the oil concentration of the ¯owing mixture. Use of this device is limited to situations in which the mixture is in the liquid phase, and the components are fully miscible. A practical, but time-consuming, calibration procedure was noted and applied to obtain speed of sound data for an R-22/alkyl benzene synthetic lubricant mixture. Prompted by the lack of acoustic velocity data for refrigerant/oil mixtures, and the desire of eliminating the time consuming calibration process, a simpli®ed model for the prediction of acoustic velocities in liquid mixtures was suggested. The model provided very good estimates for low temperature and low oil concentration cases, but systematically under-predicted the experimental results for the higher temperatures and oil concentrations. These errors were attributed to departure from the ideal solution theory. No correction was possible due to the lack of available data on the speci®c mixture investigated. Further examination, possibly including a temperature and concentration dependent term based on a mixture interaction parameter, is suggested.
Acknowledgements The research results reported in this paper were part of a broader scope research program sponsored by the United Technologies Carrier Corporation. Their valuable technical input and support are acknowledged.
References [1] Kutsuna K, Inoue Y, Mizutani T. Real time oil concentration measurement in automotive air conditioning by ultraviolet light absorption. SAE paper no. 910222, 1991. [2] Suzuki S, Fujisawa Y, Nakasawa S, Matsuoka M. Measuring method of oil circulation ratio using light absorption. ASHRAE Transactions 1993;99(1):413±21. [3] Baustian JJ, Pate MB, Bergles AE. Measuring the concentration of a ¯owing oil±refrigerant mixture: instrument test facility and initial results. ASHRAE Transactions 1988;94(1):167±77. [4] ASHRAE. Standard 41.4. Standard method for measurement of proportion of lubricant in liquid refrigerant. Atlanta (GA): American Society of Heating, Refrigerating, and Air Conditioning Engineers, 1996. [5] Baustian JJ, Pate MB, Bergles AE. Measuring the concentration of a ¯owing oil±refrigerant mixture with a vibrating utube densimeter. ASHRAE Transactions 1988;94(2):571±87. [6] Bayani A, Thome JR, Favrat D. Online measurement of oil concentrations of R-134a/oil mixtures with a density ¯owmeter. HVAC&R Research 1995;1(3):232±41. [7] Baustian JJ, Pate MB, Bergles AE. Measuring the concentration of a ¯owing oil±refrigerant mixture with a bypass viscometer. ASHRAE Transactions 1988;94(2):588±601. [8] Baustian JJ, Pate MB, Bergles AE. Measuring the concentration of a ¯owing oil±refrigerant mixture with an acoustic velocity sensor. ASHRAE Transactions 1988;94(2):602±15. [9] Meyer JJ, Saiz Jabardo JM. An ultrasonic device for measuring the oil concentration in ¯owing liquid refrigerant. International Journal of Refrigeration 1994;17(7):481±6. [10] Navarro de Andrade JE. Investigation of rotary compressor oil carry-over. Master's thesis, Purdue University (IN), 1995. [11] ASHRAE. ASHRAE handbookÐfundamentals. Atlanta (GA): American Society of Heating, Refrigerating, and Air Conditioning Engineers, 1993. [12] ASHRAE. ASHRAE handbookÐrefrigeration. Atlanta (GA): American Society of Heating, Refrigerating, and Air Conditioning Engineers, 1994.
www.elsevier.com/locate/ijrefrig
Oil concentration in liquid refrigerants: in situ measurement Eduardo Navarro de Andrade a, Eric Skowron b, Victor W. Goldschmidt c, Eckhard A. Groll c,* a Carrier Corporation, One Carrier Parkway, PO Box 4808, Syracuse, NY 13221, USA Hughes Space and Communications Company, PO Box 92919, Los Angeles, CA 90009-2919, USA c Ray W. Herrick Laboratories, School of Mechanical Engineering, Purdue University, West Lafayette, IN 47907-1077, USA b
Received 15 September 1998; accepted 6 November 1998
Abstract The concentration of oil in refrigerants (while in liquid state) can be measured with an acoustic velocity sensor. The transit time for an acoustic signal can be related to the oil concentration and temperature of the liquid mixture. The performance of the sensor is dependent on the properties of the oil and refrigerant, and their miscibility. In general, a thorough calibration becomes necessary. It is shown in this paper that for concentrations less than 10%, an approximation can be made for the estimate of concentration hence eliminating the need for an elaborate calibration procedure. # 1999 Elsevier Science Ltd and IIR. All rights reserved. Keywords: Refrigerant; Lubricant; Concentration; Measurements; Acoustics
Concentration d'huile dans les frigorigeÁnes liquides : mesures sur le terrain ReÂsume La concentration d'huile dans les frigorigeÁnes (aÁ l'eÂtat liquide) peut eÃtre mesureÂe aÁ l'aide d'un capteur de vitesse acoustique. Le temps de transit d'un signal acoustique varie en fonction de la concentration d'huile et la tempeÂrature du meÂlange liquide. La performance du capteur est fonction des proprieÂteÂs de l'huile et du frigorigeÁne ainsi que de leur miscibiliteÂ. De facËon geÂneÂral, on doit eectuer un calibrage approfondi. Cette communication montre que pour les concentrations infeÂriures aÁ 10%, il est possible d'utiliser une approximation a®n d'eÂvaluer la concentration ; de cette manieÁre, on n'a plus besoin de faire un calibrage complexe. # 1999 Elsevier Science Ltd and IIR. All rights reserved. Mots cleÂs: FrigorigeÁne; Lubri®ant; Concentration; Mesure; Acoustique
* Corresponding author. Tel.: +1-765-496-2201; fax: +1-765-494-0787. E-mail addresses: [email protected] (E. Navarro de Andrade), [email protected] (E. Skowron), [email protected] (V.W. Goldschmidt), [email protected] (E.A. Groll) 0140-7007/99/$20.00 # 1999 Elsevier Science Ltd and IIR. All rights reserved. PII: S0140-7007(99)00008-0
500
E. Navarro de Andrade et al. / International Journal of Refrigeration 22 (1999) 499±508
Nomenclature
A C L m mix oil ref t !
contact area between liquid layers (m,ft) speed of sound through ¯uid (m/s,ft/s) ¯uid layer thickness (m,ft) mass of ¯uid (kg,lbm) property of refrigerant/oil mixture property of pure lubricant property of pure refrigerant ¯ight time of pressure wave through ¯uid (s) density (kg/m3, lbm/ft3) concentration of element in mixture (±)
1. Introduction The ``standard'' method of measuring oil concentration is based on ASHRAE Standard 41.4±1996. This standard calls for removing a sample and measuring the quantity of refrigerant and oil in that sample. In many instances, it is desirable to measure concentrations ``in situ''. Table 1 reviews some of the alternatives open to the user, while Table 2 compares some of the advantages and disadvantages of the potential methods. For a current study of in-line, real-time oil concentration of refrigerant/oil mixtures in a vapor-compression system,
a sensor based on the measurement of acoustic velocity in the liquid refrigerant/oil mixture was selected. Compared to the other methods and sensors, the acoustic velocity method consists of the best compromise between cost and accuracy. 2. Method description The values of temperature and acoustic velocity of a liquid oil-refrigerant mixture can be used to determine the oil content of the mixture. Baustian et al. [3] described the basic technique for refrigerant±oil combinations such as R-12/mineral oil, R-22/mineral oil, and R-502/ synthetic oil. The method makes use of the large dierence between the acoustic velocities of pure refrigerants and oils. Typical data show that the speed of sound in oil is some 2±3 times greater than in a liquid refrigerant. The acoustic velocity in a medium can be calculated by measuring the transit time of a pressure wave across a known distance. This was accomplished in this study by using a pulse generator and a pair of ultrasonic immersion transducers. A block diagram of the electronic components is shown in Fig. 1 . The pulse generator was used to generate electric pulses to excite the transducers. The ®rst transducer received these electric pulses, and converted them, sending pressure waves across the liquid oil±refrigerant mixture. The second transducer detected the pressure waves and emitted electric pulses in response. The two signals were monitored using an oscilloscope. The transit time was then the interval between the initiation of each signal.
Table 1 Oil concentration measurement methods Tableau 1 Concentration d'huile : meÂthodes de mesure Method
Description
ASHRAE Standard 41.4±1996
Fluid sample is removed from the liquid line of the system and weighed. The refrigerant component is boiled o, and the remaining oil is weighed. The density of the ¯owing liquid is formed using a vibrating U-tube or a more accurate straight tube densimeter. Calibration is required to correlate the density of the refrigerant/oil mixture in use to temperature and oil concentration. Calibration can be avoided for mixtures known to follow the ideal solution assumption. Kutsuna et al. [1] correlated the amount of ultra-violet light absorbance in a liquid sample to the oil concentration. While Suzuki et al. [2] related the ratio of the transmittance of infrared light through a refrigerant/oil sample to that of a standard solution of known attenuation. Calibration with samples of known concentration is necessary. The viscosity of the refrigerant/oil mixture can be measured with a bypass viscometer. The known predictive relationships are very inaccurate. Therefore, it is necessary to calibrate the instrument by correlating the viscosity of known samples to temperature and oil concentration. The speed of sound through a refrigerant/oil mixture can be determined by measuring the transit time of an ultrasonic signal across a known distance. This acoustic velocity varies with temperature and oil concentration. Either a lengthy calibration procedure is required, or the predictive relationship development in this work can be used.
Density based
Light absorption based
Viscosity based Acoustic velocity based
E. Navarro de Andrade et al. / International Journal of Refrigeration 22 (1999) 499±508
501
Table 2 Comparison of oil concentration sensors Tableau 2 Comparaison des capteurs utiliseÂs pour mesurer la concentration Method
Advantages
Disadvantages
Reference
ASHRAE Standard
Low cost
ASHRAE [4]
Density based Light absorption based
In-line, real time Accuracy, real time
Viscosity based Acoustic velocity based
Real time In-line, real time, cost
Not real-time, time consuming, charge removal Accuracy Cost, complexity, requires bypass, calibration, fragile Response, requires bypass calibration Calibration
Baustian et al. [5] Bayani et al. [6] Kutsuna et al. [1] Suzuki et al. [2] Baustian et al. [7] Baustian et al. [8] Meyer and Saiz Jabardo [9]
Fig. 1. Schematic diagram of oil concentration measurement method. Fig. 1. ScheÂma montrant la meÂthode utiliseÂe pur mesurer la concentration d'huile.
The ultrasonic transducers were held in position by stainless steel jackets that were in turn inserted into a brass housing. Fig. 2 illustrates the housing arrangement. All parts were built speci®cally for this application. The stainless steel jackets were cylindrical blocks with circular holes bored into them to match the outer diameter of the transducers. The transducers slid tightly into these openings, and were ®xed in place by setscrews. The jackets and the transducers were then ®xed to a brass block by a series of screws. This brass block had a circular channel drilled through it where the liquid mixture was to ¯ow. The channel in the brass housing was dimensioned in order that the sensor would ®t directly into the piping of the liquid line of the system. The transducers selected for this study had an epoxy layer separating the active element of the transducers from the environment (wear plate). In this application some concern existed regarding the resistance of the epoxy wear plate to corrosive attacks from the refrigerant used (R-22). To avoid problems of this nature the holes in the steel jackets were bored to a depth of 0.25
mm (0.01 inches) smaller than the total height of the jacket, thus leaving a thin protective layer of stainless steel across the face of the transducer. 3. Sample calibration results Sample data are now presented for R-22/alkyl benzene mixtures. Due to the lack of published data on acoustic velocity in oil±refrigerant mixtures, calibration curves had to be generated experimentally for the oil± refrigerant combination of interest. This was done by measuring the propagation time of the pressure waves across the known path length of the sensor using mixture samples of known concentration and under controlled temperature conditions. For this research the working pair was R-22 and an alkyl benzene synthetic lubricant of the 300 SUS viscosity classi®cation. The oil used was fully miscible with R-22. The required experimental data were obtained using a calibration vessel arrangement depicted in Fig. 3. A
502
E. Navarro de Andrade et al. / International Journal of Refrigeration 22 (1999) 499±508
Fig. 2. Ultrasonic transducer housing schematic. Fig. 2. ScheÂma du boõÃtier du transducteur ultrasonique.
Fig. 3. Schematic of sensor calibration vessel. Fig. 3. ScheÂma du dispositif de calibrage du capteur.
copper tube was brazed to each end of the refrigerant ¯ow path in the sensor housing. To one side, an immersed thermocouple was attached for temperature measurement. On the other side, a sight glass was attached to visually insure that the sensor was ®lled with liquid. In addition, this side had a pressure relief valve and a service port through which the ¯uids were inserted into the bottle. Tests were carried out for concentrations of 2, 5, 7.5, 10, 15, 20, and 25%, over a temperature range varying from 15.6 to 40.6 C (60 to 105 F) at 2.8 C (5 F) intervals.
The calibration vessel was thoroughly cleaned with heptane before each test. The desired amount of oil was then inserted through the service port using a syringe. The mass of oil to be inserted was weighed using a scale with an accuracy of 0.5 g (0.018 oz). After the oil was introduced the cylinder was evacuated using a vacuum pump. The next step was to submerge the calibration vessel in an ice bath to reduce its temperature. The cylinder was then charged with the appropriate mass of R-22 to produce the desired concentration in
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Fig. 4. Acoustic velocity as a function of oil concentration. Fig. 4. Vitesse acoustique en fonction de la concentration d'huile.
the liquid phase of the mixture. The R-22 was fed into the calibration bottle, from the reservoir tank, as a vapor, to guarantee it entered free of lubricant. The mass of refrigerant in the bottle was weighed with an electronic scale with a precision of 0.5 g (0.018 oz). The calibration vessel was then placed in a constant temperature environment where the ambient temperature was controlled. Transit time measurements were then taken and the concentration and speed of sound were related for particular mixture temperatures. Fig. 4 presents the results. Details and polynomial ®ts to the calibration are given in Navarro de Andrade [10].
acoustic velocity sensor to measure oil concentration has to include the experimental generation of calibration acoustic velocity data for the mixture of interest. A signi®cant amount of time and eort can be saved if the acoustic velocity of the refrigerant/oil mixture can be predicted from pure ¯uid properties. In trying to accomplish this goal a simple model was created in which the speed of sound of a liquid mixture is approximated using the acoustic velocities of the pure ¯uids. Fig. 5 illustrates schematically the model applied.
4. An approximation The fundamental question now being addressed is whether the results of Fig. 4 could have been predicted, and hence eliminate the need for the lengthy calibration procedure. One of the disadvantages of the acoustic velocity sensor, when compared to the standard method of measurement of oil concentration by sample removal, is the necessity of property data (acoustic velocity) for the speci®c refrigerant/oil mixture being used. A literature review encountered no published predictive relationships for the acoustic velocities of liquid mixtures. The experimental data available are also extremely limited, including only acoustic velocity data on mixtures of R-12 and R-22 with a naphthenic mineral oil, and mixtures of R-502 with an alkyl benzene synthetic lubricant [3]. This means any attempt to employ an
Fig. 5. Acoustic velocity predictive model schematic. Fig. 5. ScheÂma d'un modeÁle pour la vitesse acoustique.
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The refrigerant/oil mixture is treated as being made up of two distinct layers, one of pure refrigerant and the other of pure lubricant. Based on this separation of the mixture's components the acoustic velocity of the individual substances is given by: Loil Coil 1 toil and Cref
Lref tref
2
where toil and tref are the ¯ight times of the acoustic pressure wave across the layers of oil and refrigerant. These two equations can then be used to obtain the acoustic velocity of the mixture: Cmix
Loil Lref toil tref
3
Substituting for the unknown individual transit times from Eqs. (1) and (2) in the above equations yields:
Loil Lref Loil Lref Coil Cref
Cmix
4
In order to eliminate the individual layer thicknesses, Lref and Loil , the following substitutions are used: Loil
moil oil Ac
5
Lref
mref ref Ac
6
where Ac is the contact area between the two liquid layers. After employing the substitutions above and multiplying both numerator and denominator by oil , the following relationship is obtained: oil moil mref ref Cmix 7 1 oil 1 moil mref Coil Cref ref
Fig. 6. Comparison of predicted and experimental values of acoustic velocity of R-22/alkyl benzene mixtures, 16 to 24 C (60 to 75 F). Fig. 6. Comparaison des valeurs preÂvues et expeÂrimentales de vitesse acoustique des meÂlanges de R22 et d'alkylbenzeÁne aÁ 16 aÁ 24 C.
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This equation can be made more useful by dividing its terms by the total mass of the mixture, mmix . The resulting form of the equation is an expression for the
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mixture acoustic velocity as a function of the acoustic velocities of the individual components, their densities, and the concentration of oil in the mixture, !oil :
Fig. 7. Comparison of predicted and experimental values of acoustic velocity of R-22/alkyl benzene mixtures, 27 to 41 C (80 to 105 F). Fig. 7. Comparaison des valeurs preÂvues et expeÂrimentales de vitesse acoustique des meÂlanges de R22 et d'alkylbenzeÁne aÁ 27 aÁ 41 C.
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Cmix
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oil !oil 1 ÿ !oil ref 1 oil 1 !oil 1 ÿ !oil Coil Cref ref
8
In order to use this equation, only density and acoustic velocity information on the pure ¯uids are required. Density data of pure ¯uids are widely available in the open literature. Acoustic velocity data for liquid refrigerants are also plentiful. ASHRAE [11], for instance, contains a comprehensive list of tabulated properties for most refrigerants presently in use, including acoustic velocity data for the majority of them. REFPROP, the NIST Thermodynamic Properties of Refrigerants and Refrigerant Mixture Database, also oers the possibility of computing the speed of sound in liquid refrigerants or blends of liquid refrigerants. As far as the lubricant is concerned acoustic velocity data are not so easily found for speci®c types. However, this information can be
obtained by ®lling the acoustic sensor cavity with the oil and making measurements over the temperature range of interest. Eq. (8) was solved for the temperature and concentration range of interest, for the sample R22/alkyl benzene oil mixture. The theoretical results were compared with the acoustic velocities obtained from the calibration test. Figs. 6 and 7 show the theoretical and experimental speed of sound as a function of oil concentration. The deviations between the predicted and experimental speed of sound results as a function of oil concentration can be seen in Figs. 8 and 9. The results show that the simpli®ed model can predict the acoustic velocity in the mixture with an accuracy of higher than 98% for concentrations up to 10 weight-%, and temperatures below 29.4 C (85 F). For higher temperatures and concentrations the model systematically under-predicts the values of acoustic velocity, but remains higher than 95% accurate for concentrations as
Fig. 8. Deviations between predicted and experimental values of acoustic velocity of R-22/alkyl benzene mixtures, 16 to 24 C (60 to 75 F). Fig. 8. Ecarts entre les valeurs preÂvues et expeÂrimentales de vitesse acoustique des meÂlanges de R22 et d'alkylbenzeÁne aÁ 16 aÁ 24 C.
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high as 15%, over the entire temperature range. These deviations between the theoretical and experimental values can be reduced with the addition of an empirical temperature, and concentration dependent parameter to
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the model shown in Eq. (8). The departure from the measured values is most likely caused by the use of the ideal solution assumption, which is implicit in the derivation of the model. The progression of the deviations
Fig. 9. Deviations between predicted and experimental values of acoustic velocity of R-22/alkyl benzene mixtures, 27 to 41 C (80 to 105 F). Fig. 9. Ecarts entre les valeurs preÂvues et expeÂrimentales de vitesse acoustique des meÂlanges de R22 et d'alkylbenzeÁne aÁ 27 aÁ 41 C.
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between the predicted values, with respect to temperature and concentration, is consistent with the behavior of the density of real mixtures as compared to that of ideal mixtures. According to published data [12], real mixtures tend to increasingly depart from the ideal solution theory as temperature and oil concentration increase (for the ranges of interest here). Since most real mixtures become denser than ideal, the model will under-predict the speed of sound of mixtures in the higher temperature and concentration range. Due to the lack of available data on the speci®c mixture being used no correction to account for this problem was attempted. 5. Conclusions The method of operation of an acoustic velocity sensor used as an oil concentration measurement device was outlined. The sensor is intended to be installed in the liquid line of refrigeration systems, and is capable of providing in-line, real time estimates of the oil concentration of the ¯owing mixture. Use of this device is limited to situations in which the mixture is in the liquid phase, and the components are fully miscible. A practical, but time-consuming, calibration procedure was noted and applied to obtain speed of sound data for an R-22/alkyl benzene synthetic lubricant mixture. Prompted by the lack of acoustic velocity data for refrigerant/oil mixtures, and the desire of eliminating the time consuming calibration process, a simpli®ed model for the prediction of acoustic velocities in liquid mixtures was suggested. The model provided very good estimates for low temperature and low oil concentration cases, but systematically under-predicted the experimental results for the higher temperatures and oil concentrations. These errors were attributed to departure from the ideal solution theory. No correction was possible due to the lack of available data on the speci®c mixture investigated. Further examination, possibly including a temperature and concentration dependent term based on a mixture interaction parameter, is suggested.
Acknowledgements The research results reported in this paper were part of a broader scope research program sponsored by the United Technologies Carrier Corporation. Their valuable technical input and support are acknowledged.
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