An experimental investigation and correlation of the viscosity refrigerant/oil solutionsÉtude expérimentale et corrélation de la viscosité des solutions frigorigène/huile
Introduction
Concerns about climate change have provided a stimulus for limiting emissions of greenhouse gases resulting from human activities (Myhre et al., 2013). At the 28th meeting of the parties to the Montreal Protocol (MOP 28) in 2016, a stepwise reduction based on high global warming potential (GWP) of hydrofluorocarbons (HFC) refrigerants was agreed globally. Currently, R134a (GWP = 1430), R404a (GWP = 3920), and R410a (GWP = 2090) are mainly used as HFC refrigerants for refrigeration system (Mota-Babiloni et al., 2015). As regional regulatory trends, F-Gas regulation in Europe requires GWP < 2500 after 2020, the Freon Emission Control Law in Japan requires GWP < 1500 after 2025, SNAP in the United States regulates the refrigerant type to be used sequentially from 2017. Under such a severe situation, refrigerants with low environmental burden and favorable properties are in high demand. Due to the carbon-carbon double bond, 2,3,3,3-tetrafluoropropylene (R1234yf) has an atmospheric lifetime of only 11 days, an ODP of 0 and a GWP of 4 relative to CO2 on a 100-year time horizon (McLinden et al., 2014); as the isomer, trans-1,3,3,3-tetrafluoropro-pylene (R1234ze(E)) has similar properties (an atmospheric lifetime of 1 day, an ODP of 0 and a GWP of 6) (Mota-Babiloni et al., 2016). Therefore, they are considered as the “next generation” refrigerants.
In a vapor-compression refrigeration cycle, a part of oil escapes from the compressor discharge area and finds its way into other parts of the system (such as the evaporator, condenser, expansion device) even with an efficient oil separator installed, and then instead of the pure refrigerant, it is a refrigerant/oil mixture that circulates in the refrigeration cycle. Some related problems might occur owing to the oil presence. For example, oil accumulation may take place on the inner surface of heat exchanger reducing heat transfer capabilities and leading to an overall decrement of the refrigeration cycle performance. Particularly in the evaporator, the temperature and pressure are the lowest in a refrigeration cycle where the phase separation may occur most probably. On the other hand, the circulating refrigerant comes into contact with the oil used in compressors and some may dissolve in the oil. The presence of dissolved refrigerant may substantially lower the viscosity of the oil-rich phase resulting in lower lubrication properties and giving rise to a potential breakdown of the compressor mechanical parts. Thus, the understanding of phase behavior and viscosity properties of refrigerant/oil mixtures is highly important for an optimal performance design (Quinones-Cisneros et al., 2005).
To the knowledge of the authors, the literature on thermodynamic properties of R1234yf and R1234ze(E) with oil has increased considerably in the recent years. Bobbo et al. (2014) measured the solubilities of R1234yf in two commercial PAG oils from 258 to 338 K and found that a partial immiscibility in the high R1234yf mass fraction region at temperatures higher than 293.15 K with PAG oil and higher than 303.15 K with DC-PAG. Marcelino et al. (2014) studied the absorption of R123yf and R134a through the free surface of a stagnant layer of POE. Their results showed that R134a/POE and R1234yf/POE mixtures had similar phase behavior; R1234yf was more soluble in the POE than that of R134a; no miscibility gaps were identified from 286 to 353 K. Zhai et al. (2017, 2019) measured the miscibility of pure R1234ze(E) and its blends with several oils. They found that R600a promoted the miscibility of R1234ze(E)/R600a with mineral oil while R1234ze(E) blocked it. For the R1234ze(E)/R32 mixture with POE, the more R32 had the better miscibility while the more R1234ze(E) had an opposite effect. Lee et al. (2016) found that R1234ze(E) was completely miscible with POE (the mass fraction of oil is below 20%) and with PVE (the mass fraction of oil is below 10%) from 238.15 to 353.15 K. Our research group (Sun et al., 2015, 2017, 2020; Jia et al., 2020) systematically investigated the solubilities of R1234yf and R1234ze(E) in PEC (precursors of POE oil) and POE, and analyzed the effect of PEC structure, temperature and pressure on the phase behavior of refrigerant/oil system.
The literature survey shows that these research mainly focused on the phase behavior of R1234yf and R1234ze(E) with oil. However, there is a lack of studies dealing with viscosities of R1234yf/oil and R1234ze(E)/oil mixtures. With these premises, the research on the thermodynamic properties of refrigerant/oil mixtures is ongoing in our laboratory. The phase equilibrium of R1234yf/POE75 and R1234ze(E)/POE75 have been reported (Jia et al., 2020). Based on our previous solubility data, this work presents the viscosities of POE75 saturated with R1234yf and R1234ze(E) from 303.15 to 343.15 K.
Section snippets
Chemicals
R1234yf and R1234ze(E) were purchased from Honeywell Corporation with a declared mass purity of 99.9%. Both refrigerants were degassed 3~5 times using "freeze-pump-thaw" to eliminate non-condensable gases before measurements. POE75 oil was supplied by Gree Electric Appliances Inc. of Zhuhai, China. The analysis of the composition and proportion (molar fraction) of the POE75 oil was described previously (Jia et al., 2020). The basic information of POE75 oil was shown in Tables 1 and 2.
Viscosity measurement
A
Experimental data
The viscosities were measured for POE75 oil saturated with R1234yf or R1234ze(E) from 303.15 to 348.15 K. The data was listed in Tables 4 and 5. According to the measured temperatures and pressures in the equilibrium state, the refrigerant/oil solubility (x1) in the tables was calculated by the NRTL model given by Jia et al. (2020). Fig. 2, Fig. 3 presented the effects of solubility and temperature on the viscosities of the refrigerant/oil mixtures. As the increase of solubility, a sharp
Conclusions
To improve the performance and reliability of compressors and refrigerators, the thermodynamic properties of refrigerant/oil mixtures must be well understood. In this work, the dynamic viscosities of POE75 oil saturated with R1234yf and R1234ze(E) were measured from 303.15 to 348.15 K by a dual-capillary method. The results show that the viscosities of the mixtures dropped dramatically as increasing mole-fraction of refrigerants in oil, especially at lower temperatures. R1234ze(E) has an impact
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
The authors are grateful for support from Opening Foundation of State Key Laboratory of Air-conditioning Equipment and System Energy Conservation (ACSKL2018KT14);Natural Science Basic Research Project of Shaanxi Province (2020JM-036); the Fundamental Research Funds for the Central Universities; the National Natural Science Foundation of China (No. 51606148).
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2021, International Journal of RefrigerationCitation Excerpt :Bobbo et al. (2014) measured the solubilities of R1234yf/PAG and R1234yf/DC-PAG between 258 and 338 K; there is a partial immiscibility when the temperature exceeds a certain point (the temperatures are higher than 293.15 K for R1234yf/PAG and higher than 303.15 K for R1234yf/DC-PAG). Our research group (Sun et al., 2015, 2017, 2020, 2021) measured the solubilities of R1234yf and R1234ze(E) in several PEC oils and analyzed the effect of pressure, temperature and oil structure on the solubility. Meanwhile, the viscosities of POE oil saturated with R1234yf or R1234ze(E) were also studied.
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