Effect Of The Refrigerant Charge On The System Performance And Mass Distribution In Air-To-Water Heat Pumps

Recent regulations in the matter of climate change and environmental protection are pushing to reduce the release of greenhouse gases into the atmosphere. The overall environmental impact of a refrigeration system can be reduced by optimizing and possibly minimizing the amount of refrigerant charge in the system. Even in the case of natural and low-GWP synthetic refrigerants, due to the well-known problems with toxicity and flammability, it is required to minimize the amount of refrigerant charged into the system to reduce the associated risks. The charge minimization process requires to know the refrigerant distribution to identify and redesign the critical components in terms of charge retention. This paper analyses numerically the influence of the refrigerant charge on the system performance and on the mass distribution in an air-to-water reversible heat pump working with R32. A mathematical model has been developed to simulate the unit during the cooling mode operation. The model uses the finite volume method to predict the refrigerant charge within the heat exchangers; the amount of refrigerant dissolved in the compressor oil is also accounted for. The results show that most of the charge is stored into the condenser and highlight the existence of an optimum charge that maximizes the system COP. The same model allows to compare various refrigerants in terms of direct and indirect impact on the greenhouse effect

Experimental and numerical study on the refrigerant charge in chiller and heat pump systems

The current climate change concerns are forcing international organizations and governments to issue protocols and regulations for environmental protection. Regulation No 517 of the European Union, in 2014, has introduced a significant reduction of the number of hydrofluorocarbons (HFCs) to be placed in the European market for the following years. From a global perspective, the Kigali Amendment to the Montreal protocol, in 2016, has asked for the phase-out of the high-GWP refrigerants due to their significant environmental impact. In this context, it is fundamental to identify new alternative fluids and reduce the refrigerant charge of refrigeration and air-conditioning systems. Many low-GWP refrigerants present a lower environmental impact, but have problems with flammability and toxicity. The refrigerant charge reduction is thus essential to attenuate the risks associated with using these fluids. The main goal of this research work is to perform a numerical and experimental study on the refrigerant charge in air-conditioning and heat pumps systems. For this purpose, simulation software has been developed; the model can simulate chiller and heat pump systems and calculate the amount of refrigerant in the various components. As a first step, the heat transfer coefficient of two zeotropic mixtures (R452B and R455A, blends of R32 and R1234yf) has been measured during condensation and vaporization; the experimental data have been used to assess the prediction of heat transfer models. A comparative analysis of the heat transfer performance of refrigerant R410A and the alternative fluids R32, R452B and R455A is then carried out; the study has indicated R32 as an effective alternative to R410A for comfort applications. For this reason, a new system working with refrigerant R32 has been tested; the system is an invertible air-to-water heat pump in replacement of an R410A unit. The unit's performance has been first measured with a finned coil heat exchanger and then a microchannel heat exchanger on the airside. Moreover, measurements of refrigerant mass distribution among the system components have been carried out. The experimental data have been used to validate the software developed. The heat transfer coefficient measurements have been carried out at the two-phase laboratory at the University of Padua. The performance and mass distribution tests have been performed in the test room of Clivet S.p.A, financier of the present work. The manuscript is organized into eight chapters: • Chapter 1 presents a literature review on the available void fraction correlations and lists the previous research on refrigerant mass distribution in air conditioning and heat pump systems. • Chapter 2 describes Charge Calculator, a calculation tool developed to simulate heat pumps and air conditioning systems. • Chapter 3 presents the apparatus and the procedures used in the experimental activities. Measurements of heat transfer coefficient, performance and mass distribution have been performed in this work. • Chapter 4 presents the experimental heat transfer coefficient measured during the condensation and vaporization of the two zeotropic blends R452B and R455A. The heat transfer models' assessment is also reported in this chapter. • Chapter 5 shows the experimental results of the performance tests. • Chapter 6 reports the experimental results of the refrigerant mass distribution measurements among the system components. • Chapter 7 presents the validation of Charge Calculator. • Chapter 8 reports and discusses the numerical results obtained with Charge Calculator. The effect of the refrigerant charge and airflow velocity distribution on the system performance is studied. Finally, a TEWI-based analysis attempts to assess the substitution of refrigerant R32 with two low-GWP alternatives.The current climate change concerns are forcing international organizations and governments to issue protocols and regulations for environmental protection. Regulation No 517 of the European Union, in 2014, has introduced a significant reduction of the number of hydrofluorocarbons (HFCs) to be placed in the European market for the following years. From a global perspective, the Kigali Amendment to the Montreal protocol, in 2016, has asked for the phase-out of the high-GWP refrigerants due to their significant environmental impact. In this context, it is fundamental to identify new alternative fluids and reduce the refrigerant charge of refrigeration and air-conditioning systems. Many low-GWP refrigerants present a lower environmental impact, but have problems with flammability and toxicity. The refrigerant charge reduction is thus essential to attenuate the risks associated with using these fluids. The main goal of this research work is to perform a numerical and experimental study on the refrigerant charge in air-conditioning and heat pumps systems. For this purpose, simulation software has been developed; the model can simulate chiller and heat pump systems and calculate the amount of refrigerant in the various components. As a first step, the heat transfer coefficient of two zeotropic mixtures (R452B and R455A, blends of R32 and R1234yf) has been measured during condensation and vaporization; the experimental data have been used to assess the prediction of heat transfer models. A comparative analysis of the heat transfer performance of refrigerant R410A and the alternative fluids R32, R452B and R455A is then carried out; the study has indicated R32 as an effective alternative to R410A for comfort applications. For this reason, a new system working with refrigerant R32 has been tested; the system is an invertible air-to-water heat pump in replacement of an R410A unit. The unit's performance has been first measured with a finned coil heat exchanger and then a microchannel heat exchanger on the airside. Moreover, measurements of refrigerant mass distribution among the system components have been carried out. The experimental data have been used to validate the software developed. The heat transfer coefficient measurements have been carried out at the two-phase laboratory at the University of Padua. The performance and mass distribution tests have been performed in the test room of Clivet S.p.A, financier of the present work. The manuscript is organized into eight chapters: • Chapter 1 presents a literature review on the available void fraction correlations and lists the previous research on refrigerant mass distribution in air conditioning and heat pump systems. • Chapter 2 describes Charge Calculator, a calculation tool developed to simulate heat pumps and air conditioning systems. • Chapter 3 presents the apparatus and the procedures used in the experimental activities. Measurements of heat transfer coefficient, performance and mass distribution have been performed in this work. • Chapter 4 presents the experimental heat transfer coefficient measured during the condensation and vaporization of the two zeotropic blends R452B and R455A. The heat transfer models' assessment is also reported in this chapter. • Chapter 5 shows the experimental results of the performance tests. • Chapter 6 reports the experimental results of the refrigerant mass distribution measurements among the system components. • Chapter 7 presents the validation of Charge Calculator. • Chapter 8 reports and discusses the numerical results obtained with Charge Calculator. The effect of the refrigerant charge and airflow velocity distribution on the system performance is studied. Finally, a TEWI-based analysis attempts to assess the substitution of refrigerant R32 with two low-GWP alternatives

Refrigerant mass distribution in an invertible air-to-water heat pump: effect of the airflow velocity

This paper presents the results of refrigerant mass distribution measurements carried out in an invertible air-to-water heat pump. The unit works with refrigerant R32 and produces a nominal cooling capacity equal to 60 kW. Experimental tests have been performed during both cooling and heating operations using the quick-closing valves technique to isolate the charge within the components of the system. A mathematical model, developed to predict the refrigerant charge in the heat exchangers of the heat pump and its performance, is described and validated. The model includes a physical description of the heat transfer mechanisms in the condenser and in the evaporator. The model employs void fraction correlations to calculate the refrigerant mass in the two-phase regions of the heat exchangers. In the finned coil heat exchanger, the model can account for the effects of the airflow maldistribution, which has been also experimentally measured. When the not uniform air velocity is considered, the refrigerant charge in the heat exchangers is modelled within ±10%. The results of the model showed that the airflow maldistribution can increase the total refrigerant charge in chiller mode by 12.7% and decrease the performance of the system by 11%, compared to the case of uniform air velocity

Measurements and modelling of R455A and R452B flow boiling heat transfer inside channels

Heat transfer coefficients during flow boiling of non-azeotropic mixtures are measured inside two horizontal smooth tubes of 8.0 mm and 0.96 mm inner diameter. The experimental tests are performed with ternary mixtures R455A (R32/R1234yf/R744 at 21.5/75.5/3% by mass) and R452B (R32/R1234yf/R125 at 67/26/7% by mass), displaying respectively a temperature glide of about 11 K and 1 K in the present tests. The effects of vapour quality, saturation pressure, heat flux, mass velocity and channel diameter on the heat transfer coefficient of the mixtures are investigated. The experimental results are compared against selected flow boiling correlations. Because of the zeotropic characteristics of the mixtures, the models developed for pure fluids are modified following the Shah (2015) approach to account for the mass transfer resistance, affecting both the nucleate and the convective boiling mechanisms. Simple tools to estimate the mass transfer resistance in the prediction of the flow boiling heat transfer coefficients in the case of a high temperature-glide mixture are here presented and assessed