Oil retention and its mass distribution within a R290 room air conditioner using miscible or partially miscible oils

International Journal of Refrigeration 111 (2019) 20–28

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International Journal of Refrigeration journal homepage: www.elsevier.com/locate/ijrefrig

Oil retention and its mass distribution within a R290 room air conditioner using miscible or partially miscible oils Jianhua Wu, Zhenhua Chen, Rui Chen∗, Yanjun Du, Jvyuan Duan, Xiaoyang Li School of Energy and Power Engineering, Xi’an Jiaotong University, No. 28, Xian-ning West Rd., Xi’an 710049, Shaanxi, PR China

a r t i c l e

i n f o

Article history: Received 26 July 2019 Revised 14 October 2019 Accepted 24 November 2019 Available online 27 November 2019 Keywords: R290 Oil retention Oil mass distribution Lubricating oils Miscibility Room air conditioner

a b s t r a c t At present, a variety of lubricating oils are available for R290 room air conditioners (RACs). R290 is highly miscible with mineral oils (MOs), while it is partially miscible with some alkylene oils (PAGs). In this paper, the oil retention behavior of a 1.5 HP R290 RAC with two miscible MOs and two partially miscible PAGs was studied. Firstly, the OCR of a R290 rotary compressor with PAG VG60 or MO VG100 was measured in compressor performance test platform, and it was 0.66–0.88%. Then the oil retention and its mass distribution within the RAC using PAG VG60 or MO VG100 were measured by Remove and Weight Technique (RWT) and Mix and Sample Technique (MST). And then, the oil retention when using PAG VG40 or MO VG56 was measured by RWT for further comparison. Results showed that the oil retention was 1.9–8.2 g, only 0.79%–3.9% of the total oil quantity. The oil mass in heat exchangers was nearly proportional to the local refrigerant mass, while that in gas line was greatly affected by the viscosity grade of oil. It was verified that the miscibility effect of the R290/PAG mixture was mild on this R290 RAC, and the viscosity grade of oil was the most critical parameter for the oil retention in the gas line. © 2019 Elsevier Ltd and IIR. All rights reserved.

Rétention d’huile et sa distribution massique dans un climatiseur résidentiel au R290 utilisant des huiles miscibles ou partiellement miscibles Mots clés: R290; Rétention d’huile; Distribution massique d’huile; Huiles lubrifiantes; Miscibilité; Climatiseur résidentiel

1. Introduction According to the Montreal Protocol on Substances that Deplete the Ozone Layer and the recently gone-into-effect Kigali amendment, R22 and R410A currently used in RACs are now being eliminated. R290 is a promising alternative because of its zero ODP and low GWP of 3, and show high energy efficiency in refrigeration appliance (Liu et al., 2018; Ruan et al., 2018; Nasution et al., 2019; Zhao et al., 2019) Flammability is one of the disadvantages of R290. Relevant standards about limiting the charge mass of R290 in refrigeration equipment have been published (IEC 60335-2-40, 2018). Nevertheless, studies about reducing the charge mass within R290 refrigeration systems have been conducted (Ghoubali et al., 2017; Zhou and Gan, 2019). In addition, papers about the safety of R290 ∗

Corresponding author. E-mail address: [email protected] (R. Chen).

https://doi.org/10.1016/j.ijrefrig.2019.11.027 0140-7007/© 2019 Elsevier Ltd and IIR. All rights reserved.

refrigeration systems are also ample (Zhang et al., 2016; Tang et al., 2018a,b). During the operation of refrigeration compressors, the lubricating oil is inevitably discharged, influencing system performance and reliability. The oil supply depends closely on the oil level in the compressor (Wu and Wang, 2013), and it may be insufficient because of the low oil level if too much oil is retained in the system. In heat exchangers, oil affects the flow pattern, heat transfer coefficient and pressure drop (Lottin et al., 2003; Wetzel et al., 2014; Riberto and Barbosa, 2016; Weise et al., 2017). The presence of oil is generally associated with an increase in pressure drop and decrease in system performance. The oil retention in components or overall unit of refrigeration system depends on various parameters such as OCR, fluid properties, component configurations and operating conditions. Zhu et al. (2018) investigated the OCR of a R744 trans-critical ejector cooling cycle. Xu and Hrnjak (2016, 2017, 2018a,b, 2019) vi-

J. Wu, Z. Chen and R. Chen et al. / International Journal of Refrigeration 111 (2019) 20–28

Nomenclature RAC ODP GWP HCFC HFC OCR MO AB POE PAG RWT MST MSD EEV Comp. OUHE IUHE LL GL W1 W2 W3 W4 W5 W6 OR ORsection ORsec,tot Goil Gref SD

room air conditioner ozone depletion potential global warming potential hydrochlorofluorocarbon hydrofluorocarbon oil circulation rate (%) mineral oil alkylbenzene synthetic oil polyolester synthetic oil polyalkylene glycol synthetic oil remove and Weight Technique mix and Sample Technique mix and sample device electronic expansion valve compressor outdoor unit heat exchanger indoor unit heat exchanger liquid line gas line mass of compressor before running (g) mass of compressor after running (g) mass of vacuumed sampling vessel (g) mass of HCFC-141b in section (g) mass of sampled sampling vessel (g) mass of sampling vessel without HCFC-141b (g) oil retention measured by RWT (g) oil mass in a section (g) total oil mass in OUHE, IUHE, LL, GL (g) mass flow rate of oil (kg/s) mass flow rate of refrigerant (kg/s) standard deviation (g)

sualized the oil flow in the compressor plenum and measured the OCR as well as oil retention in the compressor discharge line and oil separator. A vertical suction line is the most critical situation for oil transport, and relative researches have been largely conducted (Mehendale and Radermacher, 20 0 0; Sethi and Hrnjak, 2014; Kim et al., 2014). In this case, the refrigerant vapor velocity and viscosity of the oil film are thought to be the major parameters. As for heat exchanger, the oil retention (Jin and Hrnjak, 2016; Yatim et al., 2014; Cremaschi et al., 2018) as well as the heat transfer and pressure drop characteristics (Li and Hrnjak, 2015; Cremaschi et al., 2016) of microchannel heat exchangers were also studied in recent years. The oil retention was large in refrigeration systems with scroll compressors (Cremaschi et al., 2005; Hwang et al., 2007). For example, Peuker (2009) adopted Remove and Weight Technique (RWT) and Mix and Sample Technique (MST) to measure the oil mass distribution within a R134a automotive air conditioner, and the oil mass in the compressor was only 11% of the total oil quantity at steady state. Wujek et al. (2014) measured the refrigerant and oil mass distribution within a 10.5 kW R32 heat pump air conditioner, and the oil mass outside the compressor accounted for 50% of the total oil quantity under heating mode. However, no related studies about the oil retention behavior in R290 RACs were found. At present, lubricating oils used in R290 compressors include mineral oils (MO), alkylbenzene oils (AB), ester oils (POE) and alkylene oils (PAG). Among them, R290 is miscible and highly soluble with MO, thus a high viscosity grade for MO such as VG100 is needed to maintain the R290/oil mixture viscosity in oil sump when rotary compressors are used (Wu et al., 2018; Du et al., 2019;

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Wang et al., 2019). In contrast, the miscibility of PAG with R290 can be adjusted from completely miscible to partially miscible by changing the molecular structure of the PAG. For R290 RACs with rotary compressors, some PAGs partially miscible with R290 are now being concerned because of its low pour point, low solubility with R290 and fine performance in failure load test and wear test. It was proved that the heating capacity of a R290 RAC was 1.8%–2.3% higher and the refrigerant mass in the compressor was 10–13 g lower when a PAG was used, compared with that when a MO was used (Chen et al., 2019). However, the use of the immiscible or partially miscible refrigerant/oil pairs needs to be evaluated in terms of the oil return characteristics, especially under the conditions with low evaporating temperatures. In the previous refrigerant replacement process, relative studies about the oil return characteristics of miscible and immiscible refrigerant/oil pairs were carried out. Some of them found oil return failure or a large oil retention for immiscible refrigerant/oil pairs (Fung and Sundaresan, 1994; Sundaresan et al., 1996), while some other found the use of immiscible refrigerant/oil pairs with low viscosity grade of oil did not exhibit greater oil retention (Sunami et al., 1994; Sumida et al., 1998; Gopalnarayanan and Rolotti, 20 0 0). In addition, some researchers proved that the heat transfer coefficient or system performance would decrease when using immiscible refrigerant/oil pairs, since thick oil film would be formed along the pipes, or much oil was trapped at the inner surface structure of heat exchangers (Popovic et al., 20 0 0; Crompton et al., 2004; Dang et al., 2012). From above literature review, it is apparent that previous study never considered the oil retention behavior within R290 RACs, especially those with immiscible or partially miscible lubricating oils. Indeed, the use of some partially miscible PAGs in R290 RACs has its advantages, such as reducing the refrigerant charge mass and improving system performance. Hence, in this study, the oil retention behavior of two MOs (VG100, 56) and two partially miscible PAGs (VG60, 40) within a 1.5 HP R290 RAC was experimentally investigated. 2. Experimental setup The experimental work can be divided into two parts: the OCR of a R290 rotary compressor was measured in a compressor performance test platform, while the oil retention and oil mass distribution of a R290 RAC with the R290 rotary compressor were measured in a psychrometric chamber. 2.1. OCR measurement of the R290 rotary compressor Fig. 1 is the schematic diagram of the compressor performance test platform. As shown in Fig. 1, the test system was mainly composed of the compressor, a condenser, a throttling valve and a calorimeter. Detailed parameters of the compressor can be found in Table 1. The oil quantity was 239 g and 210 g for the PAGs and MOs respectively, as recommended. In this part, the OCR with the PAG VG60 and MO VG100 was measured. Characteristics of the PAG VG60 and MO VG100 are shown in Table 2. It is worth noting that the PAG VG60 are partially miscible with R290. For example, as shown in Fig. 2, the R290/PAG VG60 mixture may separate into two layers at a given temperature, namely an oil-rich layer and a refrigerant-rich layer, in condition of a certain R290/oil concentration. The operating conditions of the OCR test, namely the condensing pressure, evaporating pressure, subcooling degree and suction superheat degree could be precisely set by the test system based on the RAC cycle parameters results presented in Chen et al. (2019). A sampling device was installed parallel to

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J. Wu, Z. Chen and R. Chen et al. / International Journal of Refrigeration 111 (2019) 20–28

Fig. 1. Scheme of the compressor test platform.

Table 1 Specifications of the test apparatus. Components

Specifications

Compressor

Type: Rotary DC inverter compressor Cylinder diameter: 50 mm Cylinder height: 24 mm Crankshaft eccentricity: 5.3 mm Stroke volume: 17.9 cm3 Oil quantity for PAG VG60: 239 g Oil quantity for MO VG100: 210 g Oil quantity for PAG VG40: 239 g Oil quantity for MO VG56: 210 g Condenser Type: Finned-tube Tube material: Copper Tube type: Rifled U-tube Total tube length: 20880 mm Tube outer diameter: 7 mm Tube inner diameter: 6.52 mm Number of rows: 1 Number of tubes: 24 Volume: 697 cm3 Length: 3 m Inner diameter: 5 mm Volume: 59 cm3 Volume: 310 cm3

Heat exchangers

Liquid line

Gas line

Evaporator Type: Finned tube Tube material: Copper Tube type: Rifled U-tube Total tube length: 18630 mm Tube outer diameter: 7 mm Tube inner diameter: 6.52 mm Number of rows: 2 Number of tubes: 28 Volume: 622 cm3

Table 2 Characteristics of the PAG VG60 and MO VG100. Oils

Kinematic viscosity at 40 °C (mm2 /s)

Kinematic viscosity at 100 °C (mm2 /s)

Viscosity Index

Acid value (mg·KOH/g)

Pour point (°C)

PAG VG60 MO VG100

63.9 97.4

10.28 10.9

148 95

0.01 0

−37.5 −15

the liquid line to sample the refrigerant and oil mixture after the compressor had reached steady state for about 2 h. Then, the refrigerant was evaporated to obtain the mass of both refrigerant and oil inside the sampling device, and the OCR could be calculated based on the assumption that R290 and oils would form homogenous mixtures or that there was no slip between the liquid

Ignition point (°C) >200 > 200

phases. The OCR is defined as the ratio of the oil mass flow rate to the refrigerant and oil mixture mass flow rate, presented as follows:

OCR =

Goil Goil + Gref

(1)

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condition. It was found that the optimum R290 charge mass was 365 g and EEV opening was 250° for both the system with the PAG VG60 and MO VG100. Then, the EEV opening was optimized for a maximum COP under heating mode with the 365 g R290 charge mass, while the compressor frequency was 79 Hz and 90 Hz under RH and LH condition, respectively, also the same as the default setting. The optimum EEV opening was also found to be the same when the PAG VG60 or MO VG100 was used under heating mode. Finally, the operating conditions and the corresponding control logic of the RAC could be decided, as shown in Table 3. Then, when the PAG VG40 or MO VG56 was used, the R290 charge mass as well as the control logic of the RAC followed the settings in Table 3. For detailed performance results, authors can refer to Chen et al. (2019). 2.3. Measurement of the oil retention and oil mass distribution within the R290 RAC Fig. 2. Miscibility sketch map of R290/PAG VG60.

Firstly, the compressor was detached from the test apparatus, and the heat exchangers and pipes were flushed by HCFC-141b to obliterate the oil. Then, a proper amount of oil was sealed into the compressor as defined in Table 1. The compressor with oil was then weighed as W1 . The RAC was turned on and after it had reached steady state for about 2 h, the RAC and six ball valves were shut off simultaneously. The hand valves were opened slowly and the refrigerant was removed. RWT was then used to measure the oil retention: the compressor was detached from the test apparatus and evacuated for about 1 h, and then weighed as W2 . The oil retention can be calculated by the difference of compressor weight before and after running, as shown in the following equation:

OR = W1 − W2

Fig. 3. Schematic diagram of the experimental apparatus.

2.2. Modifications and settings of the tested R290 RAC A 1.5HP R290 heat pump RAC was selected as test apparatus for the measurement of oil retention and oil mass distribution. The throttling device of the RAC was an electronic expansion valve (EEV), and the compressor was the one mentioned in the OCR test method. The specifications of the RAC are shown in Table 1. As shown in Fig. 3, some modifications were made: ball valves and hand valves were installed at both sides of the compressor, condenser and evaporator. Thus, the system was divided into five sections, namely compressor (Comp.), outdoor unit heat exchanger (OUHE), indoor unit heat exchanger (IUHE), liquid line (LL) and gas line (GL). The test unit was set in a psychrometric chamber, consisting of an indoor side and an outdoor side, and the temperature and humidity of air in both sides could be adjusted independently. For the PAG VG60 and MO VG100, the oil retention and its mass distribution were measured under three operating conditions, namely rated cooling (RC), rated heating (RH) and low ambient temperature heating (LH) conditions. For the PAG VG40 and MO VG56, only the oil retention was measured for further comparison. Before the measurement of oil retention and oil mass distribution, the R290 charge mass of the RAC should be obtained and the EEV opening should be determined under each operating condition. Thus, the charge optimization tests were performed under RC condition when the PAG VG60 or MO VG100 was used. The R290 charge mass and EEV opening was adjusted to reach a maximum EER, with the compressor frequency fixed at 65 Hz which was the company’s default setting of the prototype of this RAC under RC

(2)

MST was adopted to determine the oil mass in heat exchangers and pipes. A Mix and Sample Device (MSD) was fabricated to fulfill the MST. As shown in Fig. 4, the MSD comprised a gear pump, a mixing tank, a sampling vessel, two sight glasses and some hand valves. The operating steps of the MST were as follows: (1) Vacuum the sampling vessel and weigh it as W3 ; (2) Connect the MSD and the tested section with hoses and vacuum them through hand valve 1, with hand valve 1–3 opened and hand valve 4–5 closed; (3) Fill the MSD and the tested section with liquid HCFC-141b, close the hand valve 1 and then record the mass of HCFC-141b as W4 ; (4) Run the gear pump for about 1 h; (5) Turn off the gear pump, and then open the hand valve 4–5 to sample the HCFC141b/oil mixture; (6) Close the hand valve 4–5, detach the sampling vessel from MSD, and then weigh it as W5 ; (7) Place the sampling vessel in a water bath with a constant temperature of about 60 °C for 30 min to remove the HCFC-141b, then vacuum the sampling vessel for another 30 min, and then weigh it as W6 . The oil mass in the tested section could be calculated by the following equation:

ORsection =

W6 − W3 W4 W5 − W3

(3)

3. Result and discussions The oil retention and its mass distribution within the RAC using the PAG VG60 or MO VG100 are shown in Table 4. The theoretical test error of the RWT can be estimated by the following equation:

ε OR =

δ (W1 − W2 ) W1 − W2

=

δW1 + δW2 W1 − W2

(4)

A balance with an accuracy of 0.1 g was used to weigh the compressor, thus δ W1 , δ W2 were 0.1 g. From Table 4, OR was about 3.2–8.2 g, so that εOR was 2.44%–6.25%.

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J. Wu, Z. Chen and R. Chen et al. / International Journal of Refrigeration 111 (2019) 20–28 Table 3 Specifications of operating conditions. Conditions

Outdoor unit

RC RH LH

Freq.

EEV Opening

Charge

DBT/°C

WBT/°C

Indoor unit DBT/°C

WBT/°C

Hz

degree

g

35 7 −10

24 6 −11.2

27 20 20

19 15 15

65 79 90

250 200 100

365 365 365

Table 4 Oil retention and its mass distribution at steady state for PAG VG60 and MO VG100. Test

Oils

Cond.

No. 1.

PAG VG60 (SD) PAG VG60 (SD) PAG VG60 (SD) MO VG100 (SD) MO VG100 (SD) MO VG100 (SD)

2. 3. 4. 5. 6.

RC RH LH RC RH LH

Oil mass distribution (g)

Oil retention (g)

OUHE

IUHE

LL

GL

Collected

OR = W1 –W2

1.24 (0.05) 0.47 (0.08) 0.54 (0.04) 0.85 (0.06) 0.44 (0.07) 1.19 (0.04)

0.60 (0.1) 1.62 (0.05) 1.57 (0.04) 0.54 (0.05) 1.13 (0.03) 4.23 (1.81)

0.40 (0.02) 0.60 (0.02) 0.83 (0.03) 0.73 (0.02) 0.57 (0.03) 0.90 (0.03)

1.96 (0.05) 0.48 (0.03) 0.77 (0.05) 2.04 (0.02) 1.44 (0.04) 2.11 (0.01)

4.2 (0.2) 3.16 (0.16) 3.71 (0.13) 4.17 (0.13) 3.58 (0.14) 8.43 (1.75)

4.0 (0.17) 3.2 (0.09) 3.7 (0.1) 4.0 (0.14) 3.5 (0.17) 8.2 (0.13)

Deviation (%)

5.0 1.25 0.27 4.25 2.29 2.80

Fig. 4. Scheme and photo of oil circulation device.

The theoretical test error of the oil mass in a component (excepted compressor) measured by MST can be calculated by Eq. (5), which could represent the credibility of the MST and the oil mass distribution results:

ε ORsec =

δ ORsection

(5)

ORsection

Furthermore, δ ORsection can be calculated by the following equation:

δ ORsection = δ

W − W  6 3

W − W  6 3

W5 − W3

W5 − W3



= W4 +

W4 +

δ (W6 −W3 )

W − W  6 3 W5 − W3

δW4

1 W6 − W3 + δ (W5 − W3 ) W5 −W3 (W5 − W3 )2

δW4



(6)

The HCFC-141b tank was weighed by the balance with 0.1 g accuracy, while the sampling vessel was weighed by a more accurate one with 0.01 g interval, so that δ W4 was 0.1 g, and δ W3 , δ W5 , δ W6 were all 0.01 g. Consequently, δ ORsection was about 0.04–0.12 g, and εORsection was approximately 2.34%–10.07%. The measurement of both the oil retention and oil mass distribution was repeated 3 times, and the standard deviation (SD) for each data was also provided along with the average results in Table 4. Moreover, the deviation between the oil retention measured by RWT and the total oil mass determined by MST can be obtained by Eq. (7). As shown in Table 4, the deviation was about 0.27%–5%, which proved the rationality of the measurement.

Deviation =

ORsec,tot − OR × 100% OR

(7)

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Table 5 Steady-state OCR measured in R290 compressor performance test platform. Oils

PAG VG60

Cond.

RC

RH

LH

MO VG100 RC

RH

LH

OCR (%)

0.79

0.86

0.88

0.66

0.81

0.88

Fig. 6. Correlation of the measured and estimated oil mass in heat exchangers and LL.

Fig. 5. Oil mass distribution in the four sections.

3.1. Oil retention and OCR at steady state for PAG VG60 and MO VG100 As shown in Table 4, the oil retention for the RAC with the PAG VG60 or MO VG100 was 3.2–8.2 g, 1.34%–3.9% of the total oil quantity. The oil retention was evidently lower compared with that in other refrigerant systems mentioned above (Cremaschi et al., 2005; Hwang et al., 2007; Peuker, 2009; Wujek et al., 2014), and such low oil retention might not cause compressor failure. The reason was probably that the OCR was low for this R290 RAC. The OCR of the R290 rotary compressor with the PAG VG60 or MO VG100 is shown in Table 5. From Table 5, the OCR was within 0.66%–0.88%, lower than that of other compressors (Wu et al., 2015; Yan et al., 2015) measured under similar operating conditions. It seemed that the OCR was larger under heating mode than cooling mode because of the higher compressor frequency. However, the OCR of the R290 rotary compressor did not go up quickly as its frequency increased from 65 Hz to 90 Hz as shown in Table 5, while that of the R22 or rotary compressor went up quickly as the frequency increased (Sarntichartsak et al., 2006; Wu et al., 2015). The reason might be that the refrigerant vapor/oil density ratio was lower and the surface tension of the oil droplet was higher in the R290 compressor compared with those in R22 or R410A compressors, so that the oil was more likely to be separated under the high motor speed. Therefore, the oil retention was still lower than 4% even when the evaporating temperature was very low as under LH condition. 3.2. Oil mass distribution among heat exchangers and pipes for PAG VG60 and MO VG100 Fig. 5 shows the oil mass distribution among heat exchangers and pipes. The condenser (OUHE for cooling mode and IUHE for heating mode) and the GL held most of the oil, accounting for 63%–77% of the total oil retention. The total oil mass in OUHE, IUHE and LL can be estimated by multiplying the OCR and the

local refrigerant mass. Table 6 shows the comparison of the measured and estimated total oil mass in OUHE, IUHE and LL. The total refrigerant mass was measured in Chen et al., 2019). Actually, along with the calculation of the estimated oil mass, an assumption has been made: the refrigerant and oil formed homogenous flow and all of the oil circulated with refrigerant at same speed. This assumption would be proved only if the estimated oil mass equaled to the measured one. However, from Table 6, the measured oil mass was slightly larger, and the assumption here was not validated. The reason was speculated to be: some oil was retained along the internal thread of the heat exchangers tubes or other places with large structural change such as the EEV and the inlet/outlet of heat exchangers, just like that mentioned in (Popovic et al., 20 0 0; Crompton et al., 2004; Dang et al., 2012). Furthermore, from Table 6, the discrepancies between the measured and estimated oil mass were mostly within 30%, so that such kind of oil retained was of small amount. However, for the MO VG100 under LH condition, the measured oil mass was evidently larger. The most probable reason was: ((1) a large amount of oil was retained at the end of the evaporator because of the high R290/oil mixture viscosity at the end of the evaporator due to the low R290 concentration in oil, high viscosity grade of oil and low evaporating temperature; (2) a large amount of oil was retained nearby the EEV because the EEV opening was small under LH condition (Table 3). As shown in Fig. 6, the results of measured oil mass and calculated oil mass in OUHE, IUHE and LL (excepted for MO VG100, LH) could be linearly correlated as the line y = 0.95x + 0.414 with a correlation coefficient of 0.906, and the results fell in a ± 8% error range. Thus, the measured oil mass was highly positively correlated with the estimated oil mass. Since the estimated oil mass was the product of the OCR and refrigerant mass, it could be seen that the oil mass was nearly proportional to the refrigerant mass, and ratio of the oil mass to the refrigerant mass was close to the OCR. At steady state, the OCR was identical in each section of the RAC, thus the condenser held more oil compared with the evaporator (IUHE for cooling mode and OUHE for heating mode) and LL because most of the refrigerant mass was in the condenser. From Fig. 5, The oil mass in the evaporator was only 12%–15% of the total oil retention, and using the partially miscible PAG VG60 did not exhibited too much oil retention in the evaporator compared with the miscible MO VG100, as shown in Fig. 7. The miscibility effect was mild in this case, probably because of the lower viscosity

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J. Wu, Z. Chen and R. Chen et al. / International Journal of Refrigeration 111 (2019) 20–28 Table 6 Comparison of the measured and estimated oil mass in OUHE, IUHE and LL. Oils

PAG VG60

Cond.

RC

RH

LH

MO VG100 RC

RH

LH

Total refrigerant mass (g) Estimated oil mass (g) Measured oil mass (g) Discrepancy (%)

259.8 2.05 2.24 9.27

265.2 2.28 2.69 17.98

294.1 2.59 2.94 13.51

247.4 1.63 2.12 29.82

251 2.03 2.14 5.42

285.4 2.51 6.32 252

Fig. 7. Oil mass in evaporator for PAG VG60 and MO VG100.

Fig. 8. Oil mass in Gas line for PAG VG60 and MO VG100.

grade of the PAG VG60. From Fig. 2, when the evaporating temperature was 12 °C, layer separation might happen when the oil concentration in R290 was within the range of 14%–63%, which was typical at the middle or the end of the evaporator flow path. At this time, the oil concentration was 63% for the oil-rich layer and 14% for the refrigerant-rich layer. The viscosity of pure PAG VG60 at 12 °C was larger than 500 mPa s, but that for the oil-rich layer was only 0.88 mPa s. Thus, even if layer separation appeared, the viscosity of the oil-rich layer was not so high. In addition, the total tube length of the heat exchangers was evidently shorter in this 1.5 HP RAC compared with that of other refrigeration systems with larger capacity. Therefore, the corresponding tube length where the oil concentration was within the layer separation range of the refrigerant/oil mixture was shorter. This might be another reason for the mild influence of the miscibility effect of R290/PAG VG60 on the oil retention.

impact of the viscosity grade of oil was greater than the influence of solubility in this case. From Fig. 8, the oil retention in the GL under heating mode was less than that under cooling mode for the PAG VG60. This was due to the pipe arrangement and the specific control logic of this heat pump RAC. The suction line was shorter for heating mode (Fig. 3). In addition, the compressor frequency was 65 Hz under RC condition, and it was 79 Hz and 90 Hz under RH and LH condition, respectively (Table 3). The higher vapor velocity under heating mode provided a higher shear stress between the vapor refrigerant and oil film and therefore resulted in a lower oil mass in the GL for the PAG VG60. However, for the MO VG100, the influence of the compressor frequency was mild compared with the influence of the viscosity grade, and the oil mass in the GL was still higher under LH condition than under RC condition. 3.4. Oil retention for various oils

3.3. Oil mass in gas line for PAG VG60 and MO VG100 Fig. 8 shows the oil mass in the GL. It was about 2 g under cooling mode, almost the same when using the PAG VG60 and MO VG100. However, under RH and LH condition, the oil mass in the GL for the MO VG100 was 1.44 g and 2.11 g, respectively, evidently larger than that for the PAG VG60, which was 0.48 g and 0.77 g. The oil mass in the GL was mostly in the suction line, and the oil mass in the suction line was mainly related to the oil film thickness since the oil concentration in R290 was located within the miscible region (Fig. 2). Furthermore, the oil film thickness depended on the refrigerant vapor velocity and the oil film viscosity, and the oil film viscosity was determined by the viscosity grade of oil and the refrigerant concentration in oil. The refrigerant vapor velocity was almost the same for the two oils because of the similar suction temperature and pressure (Chen et al., 2019). Thus, the miscibility of the refrigerant/oil mixtures made no effect, and the

Fig. 9 shows the oil retention measured only by RWT for the systems with MO VG100, VG56 and PAG VG60, VG40 under RC and LH condition. From Fig. 9, the oil retention was 1.9–4.0 g, 0.79%– 1.67% of the total oil quantity for the system with the PAGs, and 3.2–8.2 g, 1.52%–3.9% of the total oil quantity for the system with the MOs. In this case, using oils with lower viscosity grade performed less oil retention. It was speculated that the miscible MOs used in this study were easier to stay in the suction line due to the high viscosity grade, especially under the conditions with low ambient temperature. In addition, the miscibility effect of the R290/PAGs on steady-state oil retention was mild in this R290 RAC. Among the four oils, PAG VG40 showed the best oil return behavior at steady state. Nevertheless, if other partially miscible oils are used, the layer separation region in the miscibility map of the refrigerant/oil pairs should be highly concerned, and the oil retention behavior should still be evaluated.

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Fig. 9. Oil retention for various oils measured by RWT.

4. Conclusion The oil retention behavior of a 1.5 HP R290 heat pump RAC using miscible MOs (VG100, VG56) or partially miscible PAGs (VG60, VG40) was experimentally studied. Conclusions are as follows: (1) The oil retention within the R290 RAC was only 1.9–8.2 g, 0.79%–3.9% of the total oil quantity when the four oils were used. The OCR for the system with the PAG VG60 or MO VG100 was only 0.66%–0.88%. (2) When the PAG VG60 or MO VG100 was used, the condenser and the gas line held most of the oil among the four sections, accounting for 63%–77% of the total oil retention. In heat exchangers and liquid line, the oil mass was nearly proportional to the local refrigerant mass, indicating that the enhanced surface of the heat exchangers was slightly occupied by oil for both the systems with the PAG VG60 and MO VG100. In gas line, the oil mass was greatly affected by the viscosity grade of oil. The miscibility effect of the R290/PAG VG60 was mild in this R290 RAC. (3) The oil retention of the MO VG100 was larger than that of the PAG VG60, and the difference was greater under low ambient temperature conditions because much more oil was retained in the suction line for the MO VG100, mostly due to the high viscosity grade. (4) Under heating mode, the viscosity of oil was large and it was bad for oil return. On the contrary, the higher compressor frequency and the shorter suction line were good for oil return. The pipe arrangement and control logic should be carefully considered during system design of R290 RACs. Declaration of Competing Interest None. Acknowledgment This study is supported by Guangdong Meizhi Compressor Co. Ltd. The author would like to thank the Midea Group for the sponsorship. References Chen, R., Wu, J., Duan, J., 2019. Performance and refrigerant mass distribution of a R290 split air conditioner with different lubricating oils. Appl. Therm. Eng. 162 (5), 114225.

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