Defrosting dynamic behaviors of a propane air conditioner and its rotary compressor under low ambient temperature conditions

Abstract

Due to the great potential of propane in environmental protection and thermodynamic properties, the promotion of propane room air conditioner will make a great contribution to environmental protection. The defrosting behaviors of propane room air conditioner system and rotary compressor were studied by experiments in this paper. Experimental analysis redefines and distinguishes defrosting time and frost melting time, and their relationship with condition temperature is different. The experimental results show that the lower the ambient temperature was, the longer the defrosting time was, but the shorter the frost melting time on the outdoor heat exchanger was. The state of the oil sump and foam layer in propane rotary compressor using Mineral Oil (MO) was recorded and the variation reason for liquid level was analysed during defrosting. Combined with the change of viscosity and liquid level of the oil sump, the experimental results show that the compressor has abnormal oil discharged, and the lower the ambient temperature was, the larger the oil discharge was under three conditions during defrosting process. The total mass of the mixture in the oil sump under the three working conditions is 275.18 g, 289.10 g and 276.41 g, the lubricating oil is 202.97 g, 200.23 g and 189.71 g, and the refrigerant mass is 72.21 g, 88.87 g and 86.71 g respectively when the defrosting process ended. This study will provide an experimental basis of the reliability study and application of propane air conditioners.

Introduction

CFCs and HCFCs are being gradually banned according to the Montreal Protocol and the Kigali Amendment with the destruction of the ozone layer and increasing trend of global warming. Similarly, HFCs with high global warming potential are being gradually restricted. As a viable and potential alternative for HCFCs and HFCs in the refrigeration industry, propane has obvious advantages (Park and Jung, 2007, Saravanan et al., 2016). To be specific, propane has zero ODP and an extremely low GWP of 3. Moreover, it has good thermo-physical properties, although the inflammable and explosive properties of propane limit its charge and wide application. It should be noted that, the propane charge of system can be significantly reduced by using microchannel structure, according to the research of Zhou et al. (Zhou and Gan, 2019) and Qiu et al. (Qiu et al., 2020). Ruan et al. (Ruan et al., 2018) proposed that the problem of charge limit of propane air conditioner can be solved significantly by using falling film condenser.

In addition, many risk assessments of equipment using propane have been conducted, and measures for safe use have been considered (Cai et al., 2015, Cai et al., 2017, Tang et al., 2017, Tang et al., 2018). Cai et al. (Cai et al., 2015) studied the difference in the leakage loss characteristics between propane compressors and other refrigerant compressors. Tang et al. (Tang et al., 2017) studied a new method and conducted experiments to investigate the refrigerant distribution in propane air conditioners and the leaking rate. This paper proposed that the safety of an propane room air conditioner can be guaranteed within the international refrigerant filling standard (Ghoubali et al., 2017). Liu et al. (Liu et al., 2018) studied the influence of the refrigerant charge on the system performance of propane air conditioners with different expansion devices.

Most research works on propane air conditioners focus mainly on system performance (Park and Jung, 2007, Cavallini et al., 2010, Choudhari and Sapali, 2017, Fernando et al., 2007, Gao et al., 2019, Nasution et al., 2018). Many authors have proved that propane has performance superiority over HFCs and HCFCs when applied to room air conditioners. Liang et al. (Jierong and Tingxun, 2018) studied the refrigerant migration characteristics in a propane room air conditioner. In addition, Lin et al. (Lin et al., 2017) studied the characteristics of an propane air conditioner during start-up pocess by experiments. Lubricating oil in the system also has a certain effect on the system characteristics. In this regard, Navarro et al. (Navarro et al., 2005) conducted oil circulation rate tests with POE as the oil and propane as the refrigerant under steady-state conditions to evaluate possible effects and differences from the traditionally used mineral oils. Wu et al. (Wu et al., 2018) investigated the practical solubility of propane in mineral oil (MO) and the influence of oil sump viscosity on compressor reliability. Wang et al. (Wang et al., 2021) carried out the research that the solubility of propane in two kinds of MO with different viscosity grade and calculated the enthalpy of the refrigerant and lubricating oil mixtures.

As the key component of the air conditioner system, the research on dynamic characteristics of compressor is very important. Shi et al. (Wu et al., 2020, Shi and Wu, 2020) analyzed the temperature distribution and heat balance of the compressor oil sump by simulation. The the propane compressor discharge temperature is lower than other compressors, especially the discharge superheat is smaller, which makes propane compressor easier to appear liquid hammer phenomenon in the start-up process under low-temperature environment. Lin et al. (Lin et al., 2020) proposed a newly developed model of rotary compressor to predict the pressure jump in the cylinder and investigated the two-phase compression characteristics of a propane rotary compressor. Fang et al. (Fang et al., 2021) studied the characteristics of the rotary compressor by simulation, and compared the performance of the compressor using propane, propylene, R32 and R22. Experimental data show that the compressor with propane has highest coefficient of performance under the same condition.

The reliability study of a dynamic process should include not only the dynamic characteristics of the start-up process, but also those of the defrosting process. As a typical dynamic process, defrosting has more complex variation of parameters than start-up, because frost melting is affected by many factors, and these factors are difficult to be studied by controlling variables (Amer and Wang, 2017, Song and Mao, 2020). Wei et al. (Wei et al., 2020) conducted the investigation of characteristics of a multi-split air source heat pump in defrost, and the compressor is a scroll one with vapor injection. Wang et al. (Wang et al., 2021) studied the control strategy of a carbon dioxide water heater during reverse cycle defrosting process, and the compressor is a semi-hermetic reciprocating one. Abdulla et al. (Abdulla et al., 2021) proposed a new defrosting method of a R404A air conditioner which can reduce the energy needed in defrosting process, and the compressor is a piston one. Maldonado et al. (Maldonado et al., 2018) provided a system control strategy both for defrost and fans operation, depending on the frost built up on the evaporator and the control improves the energy performance of the whole refrigeration system. Song et al. (Song et al., 2018) proposed two defrosting cases with air source heat pump unit and experimental results show that, the heating supply of indoor air thermal energy contributed about 80% of the total energy usage for defrosting, nearly 90% of energy consumed on frost melting and ambient air heating, respectively. Qu et al. (Minglu et al., 2020) investigated the heat coupling relations between the high temperature cycle and low temperature cycle during thermal energy storage based reverse cycle defrosting and conducted five cases with the same amount of frost under different outdoor conditions. In addition to the above literature, in recent years, most of the research literatures on the defrosting process of heat pump or air conditioner focus on the system capacity and defrosting efficiency. Most of the refrigerants used are non environmental refrigerants, and less attention is paid to the dynamic characteristics of the compressor in the defrosting process. Compressor is the key component of air conditioner, water heater, heat pump and so on, whether it can work stably and efficiently is very important. Especially the rotary compressor is widely used in the mass production of air conditioner, its dynamic response and working characteristics in the defrosting process are worthy of attention and research..

The migration of lubricating oil in this dynamic process directly affects the reliability of the compressor and the heat transfer coefficient of the heat exchanger. When too much lubricating oil enters the system pipeline, it may also cause oil blockage (Du et al., 2019). Due to the low charge of propane air conditioner, the mass ratio of lubricating oil to refrigerant is very large when the lubricating oil amount in the compressor oil sump is similar with other system (Du et al., 2020). At the same time, the density of propane is smaller than that of HFCs, which makes the discharge characteristics of the compressor, especially in the dynamic process, different from the other refrigerant. It is vital to understand the dynamic behaviours of propane system during defrosting because of the physical characteristics of propane and the smaller charge of propane air conditioners in comparison with HFCs and HCFCs. In addition, the dynamic characteristics of the propane compressor are worth exploring in the defrosting process due to its low discharge temperature. In this study, the dynamic characteristics of the system and the rotary compressor were experimentally investigated within an propane split room air conditioner during defrosting.