Applied Thermal Engineering 21 (2001) 863±870

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Vapor pressure of R-410A/oil and R-407C/oil mixtures Yeau-Ren Jeng a, Cheng-Shion Chang a, Chi-Chuan Wang b,* b

a Department of Mechanical Engineering, National Chung Cheng University, Chia-Yi, Taiwan, ROC Energy and Resources Laboratories, Industrial Technology Research Institute, D400 ERL/ITRI, Building 64, 195-6 Section 4, Chung Hsing Road, Chutung, Hsinchu 310, Taiwan, ROC

Received 16 March 2000; accepted 20 July 2000

Abstract An experimental study was carried out to examine the vapor pressure of R-410A and R-407C in the presence of lubricant oil. The grades of the tested lubricants are ISO-32 and ISO-100. For R-410A refrigerant, the vapor pressure decreases with the increase of oil concentration. In addition, it is found that there are no signi®cant changes of vapor pressures for the presence of lubricant oils for Ts 6 25°C. For R407C refrigerant, the change of vapor pressure with oil concentration is comparatively small. It is likely that this phenomenon is related to the zeotropic nature of R-407C. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Vapor pressures; R-410A; R-407C

1. Introduction Despite the Montreal Protocol ban the use of HCFC-22 by the year 2020, some nations are, however, in favor of complete phase-out of HCFC-22 by the year 2000. Consequently, extensive search for potential replacements has been made during the past few years. There are no singlecomponent HFCs that have closer thermodynamic properties compared to HCFC-22. Consequently, this led to the introduction of the binary or ternary refrigerant mixtures. Presently, R-407C and R-410A are considered as the major substitutes of HCFC-22, which is used in both household air-conditioners and commercial air-conditioning apparatus. In US, R-410A is the major substitute of R-22 in room air-conditioners and commercial package system. In Japan, R-410A is considered as the substitute to room air conditioners (RAC) *

Corresponding author. Tel.: +886-3-591-6294; fax: +886-3-582-0250. E-mail address: [email protected] (C.-C. Wang).

1359-4311/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 4 3 1 1 ( 0 0 ) 0 0 0 8 6 - 7

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Nomenclature C D L P T d

oil concentration diameter, m length, m pressure, kPa temperature, K wall thickness, m

and R-407C as the replacement of R-22 in package air-conditioners (PAC). Hence, R-410A is regarded as the major substitute to R-22. Table 1 shows the thermophysical properties of R-410A, and R-407C in comparison with R-22 at Ts ˆ 2°C. As seen, the latent heat for R-410A is about 11% higher than that of R-22. The vapor and heat capacity of R-410A exceeds those of R-22 by approximately 20% and 15%, respectively. Unlike the R-407C refrigerant mixtures, R-410A is a near azeotropic mixture having 50 wt.% R-32 and 50% of R-125. Typical temperature glide for R410A between 0.5 and 2.5 MPa is about 0.09±0.12°C. Hence, in practice, the basic design philosophy of R-410A is analogous to that of R-22. Unfortunately, when compared to R-407C, its 50±60% higher working pressure relative to R-22 makes its use as a ``drop-in'' replacement less likely. In practical air-conditioning or refrigeration systems, in addition to the working refrigerant, a small amount of lubricant oil is necessary for lubricating the sliding parts in compressors. However, part of the lubricant oil may migrate from the compressor to other parts of the system, such as the evaporator, condenser, expansion device, and piping. Note that the viscosity of lubricant oil is about three orders higher than that of the refrigerant, and the corresponding surface tension is approximately one order higher. Hence, the presence of the lubricant oil would considerably a€ect the transport properties of refrigerant and may have a signi®cant impact on the associated heat transfer characteristics. Table 1 Comparison of thermophysical properties of R-22 and R-410A at Ts ˆ 2°C Property

R-22

R-407C

R-410A

Pressure (kPa) ql (kg/m3 ) qv (kg/m3 ) CPl (kJ/kg) CPv (kJ/kg) ifg (kJ/kg) ll (lPa s) lv (lPa s) kl (W/m) Molecular weight Critical pressure (kPa)

528.3 1276 22.66 1.201 0.77 201.2 216.2 11.82 0.1021 86.5 4978

605 1229 26.2 1.416 0.993 208.8 204 11.8 0.0999 86.2 4653

850.6 1186 31.33 1.44 0.8923 223.6 168.5 12.04 0.1105 72.6 4949.6

Calculations were based on REFPROP [3].

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In a conventional R-22 system, mineral oils were often used as lubricant. Mineral oils are generally complex mixtures consisting mainly of hydrocarbons. For HFC refrigerants, synthetic oils are usually employed due to the miscibility problem. Synthetic oils are usually composed of molecules of similar type with a more limited spread of molecular weights. Potential advantages of synthetic oils are the extremely wide range of operating temperatures, ®re resistance, and resistance to oxidation and nuclear radiation [1]. In this connection, it is essential to report the relevant characteristics of the HFC-synthetic oil mixtures. The objective of the present study is focused upon the pressure±temperature±concentration relation of R-407C and R-410A refrigerants mixed with polyol ester oils.

2. Experimental apparatus The schematic of the present test facility is shown in Fig. 1. It consists of a test vessel, measuring devices, vacuum pump, and relevant connecting pipes made of stainless steel. The test vessel is made of stainless steel cylinder (L ˆ 15 cm, D ˆ 5 cm, d ˆ 0:3 cm). Because of the signi®cant di€erences in the working pressure of the test refrigerants, three pressure gauge calibrated with an accuracy of 0:2% of the measuring span were used in the study. The pressure gauge is placed in the piping connected to the test vessel to measure the system pressure. Note that the pressure gauge is changed once the working pressure is dramatically changed. The liquid refrigerant temperature is recorded by two resistance temperature devices (RTDs). The thermocouples and RTDs were precalibrated by a quartz thermometer with a calibrated accuracy of 0.1°C. Initially, the test section was cleaned and made leak free before it was evacuated. Then, the system was evacuated using a turbo-molecular vacuum pump. The vacuum pump continued working for another 2 h after the vacuum gauge manometer reached 10 4 Torr to ensure that it contained no non-condensable gases. Before obtaining a series of experimental data with di€erent oil compositions, the experimental setup was cleaned thoroughly prior to ®lling the vessel with the mixture. The lubricant oils tested in this study are SW-32 and SW-100 from Castrol [2]. Typical properties of the tested lubricant oils are tabulated in Table 2.

Fig. 1. Schematic of the test apparatus.

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Table 2 Physical properties of SW-32 and SW-100 Property

SW-32

SW-100

Speci®c gravity at 20°C Speci®c gravity at 80°C Kinematic viscosity at 40°C (cSt) Kinematic viscosity at 100°C (cSt) Viscosity index Closed ¯ash point (°C) Pour point (°C) Total acid number (mg KOH/g)

0.993 0.949 32 5.7 118 252 54 0.15

0.967 0.926 100 11.4 98 258 30 0.15

3. Results and discussion Results of measured vapor pressures of pure R-410A and R-407C are plotted in Fig. 2. For comparison purpose, the experimental data are compared with the calculated results by REFPROP [3]. As seen in the ®gure, the measured data are in excellent agreement with the calculations for temperature less than 40°C. However, in the range of 40±50°C, saturation temperatures calculated by REFPROP are about 3±7% lower than the test data. The good agreement of the measured data with the calculations substantiated the applicability of the present test facility. Vapor pressure of R-410A/SW-32, R-410A/SW-100, R-407C/SW-32, and R-407C/SW-100 refrigerant±oil mixtures in the range of 0±50% are plotted in Figs. 3±6. As shown in Figs. 3 and 4, for R-410A/oil mixtures, the pressure drops decrease with the increase of oil concentration. However, the pressure drops are relatively small for Ts 6 25°C. For Ts ˆ 50°C, a pronounced decrease of vapor pressure is seen with higher oil concentration. The corresponding pressure drops are about 27% for SW-32 and 20% for SW-100, respectively. Converse to the results of R410A, as seen in Figs. 5 and 6, the e€ect of lubricant oil on the vapor pressure of R-407C is

Fig. 2. Comparison of the measured vapor pressures of R-410A and R-407C with results from REFPROP [3].

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Fig. 3. Vapor pressures of R-410A with SW-32 lubricant oil at various temperatures.

Fig. 4. Vapor pressures of R-410A with SW-100 lubricant oil at various temperatures.

comparatively small even at Ts ˆ 50°C. Apparently, this phenomenon is associated with the zeotropic nature of R-407C. Explanation of this phenomenon can be described as follows. The composition of R-407C is 23 wt.% R-32, 25 wt.% of R-125 and 52 wt.% of R-134a. The R-32 and R-125 are the more volatile

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Fig. 5. Vapor pressures of R-407C with SW-32 lubricant oil at various temperatures.

Fig. 6. Vapor pressures of R-407C with SW-100 lubricant oil at various temperatures.

components. As a result, the vapor phase may contain more of R-32 and R-125 compared to the initial concentration of the liquid phase. Table 3 tabulates, vapor pressure of R-22, R-125, R134a, and R-22 obtained from the ASHRAE [4]. One can easily ®nd that at 20°C, the vapor pressures of R-32 and R-125 are about 3 and 2.5 times higher than those of R-134a. With the presence of lubricant oils, the lubricant oils become the least volatile components. Note that the

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Table 3 Vapor pressure of R-32, R-125, R-134a, and R-22 (kPa) Refrigerants R-32 R-125 R-134a R-22

20 405.8 337.9 132.68 245.29

0

20

40

812.9 672.02 292.69 498.11

1473.5 1206.6 571.59 910.41

2477.5 2009.8 1016.5 1534.1

vapor pressure of R-407C, and lubricant mixture is mainly composed of R-32, R-125, and R-134a due to the negligible vapor pressure of lubricant oils. The presence of lubricant oil will certainly decrease the vapor pressure. However, its non-volatile nature may result in a greater percentage of R-32 and R-125 appearing in the vapor phase that contribute to an increase of the vapor pressure. Hence, the summation of these two e€ects shows relatively small change of vapor pressure in the range of 0±50%. For the near-azeotropic refrigerant of R-410A refrigerant, one can expect that the related vapor pressure characteristic is analogous to that of a pure substance. The presence of oil will a€ect the corresponding vapor pressure of refrigerant±oil mixtures. Van Gaalen et al. [5,6] had presented the following empirical correlation applicable to R-22 refrigerants with two kinds of lubricant oils (naphthenic and alkylbenzene). Their equation valid for Naphthenic is Pˆ

39:0311C ‡ 42:921CT ‡ 338:2758C 2

575:6515C 2 T ‡ 234:8328C 2 T 2

…1†

The corresponding equation valid for Alkylbenzene is Pˆ

38:6515C ‡ 40:111CT ‡ 245:7157C 2

406:7188C 2 T ‡ 162:9384C 2 T 2

…2†

Based on the above-mentioned approach, a multi-regression is proposed to correlate the present test results. The ®nal correlations are given as follows: For R-410A, 1. With SW-32 P ˆ 104:979 ‡ 15:6746C

0:99809T

12:4965C 2

2:3  10 4 CT 2

‡ 2:35  10 3 T 2 ‡ 1:4  10 4 C 2 T 2

…3†

2. With SW-100 P ˆ 158:646 ‡ 6:04  10 3 C ‡ 2:7  10 5 T 2

1:302  10 2 T ‡ 0:0201C 2

5:3  10 8 C 2 T

3:3  10 7 C 2 T 2

…4†

For R-407C, 1. With SW-32 P ˆ 159:307 ‡ 0:307C 3:9  10 5 C 2 T 2 2. With SW-100

1:306T ‡ 2:723C 2

3:4  10 6 CT 2 ‡ 2:7  10 3 T 2 …5†

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P ˆ 160:974 ‡ 1:773C ‡ 1:1  10 5 C 2 T 2

1:318T

1:426C 2

2  10 5 CT 2 ‡ 2:7  10 3 T 2 …6†

The units used in Eqs. (3)±(6) are P: 100 kPa, C: oil concentration (wt. oil/refrigerant mixtures), and T: temperature (K). The applicable range of Eqs. (3)±(6) is 20±50°C with an oil concentration less than 50%. Compared to the test data, the deviations of the empirical correlations are less than 3.0%.

4. Conclusions An experimental study was carried out to examine the vapor pressure of R-410A and R-407C in the presence of lubricant oil. For R-410A refrigerant, the vapor pressure decreased with the increase of oil concentration. In addition, it is found that there are no signi®cant changes of vapor pressures for the present lubricant oils at Ts 6 25°C. For R-407C refrigerant, the change of vapor pressure with oil concentration is comparatively small. It is likely that this phenomenon is related to the zeotropic nature of R-407C. Empirical correlations of the associated vapor pressures of R410A and R-407C with the lubricant oils are proposed. These correlations can satisfactorily predict the present test results.

Acknowledgements The present study was supported ®nancially by Energy R&D foundation funding from the Energy Commission of the Ministry of Economic A€airs, Taiwan.

References [1] [2] [3] [4]

ESDU 94020, Selection of synthetic oils, Engineering Science Data Unit 94020, 1994. Castrol catalog, Synthetic ester refrigeration lubricants, Castrol Catalog Icematic SW, 1994. NIST, REFPROP 5.01, National Institute of Standards and Technology, Gaithersburg, MD, 1996. ASHRAE handbook, Fundamentals, American Society of Heating, Refrigerating, and Air-conditioning Engineers Inc., Atlanta, USA, 1997. [5] N.A. Van Gaalen, M.B. Pate, S.C. Zoz, The measurement of solubility and viscosity of solutions of oil/refrigerant mixtures at high pressures and temperatures: test facility and initial results of R-22/naphthenic oil mixtures, ASHRAE Trans. 96 (2) (1990) 100±108. [6] N.A. Van Gaalen, S.C. Zoz, M.B. Pate, The measurement of solubility and viscosity of solutions of R-22 in a Naphthenic oil and in an Alkylbenzene at high pressure and temperatures, ASHRAE Trans. 97 (1) (1991) 100±108.