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Abstract In this work, the use of R1234yf as a drop-in replacement for R134a in an air to water refrigeration system used to produce cool water at 7 °C is studied. This equipment was constructed in our laboratory with a minichannel heat exchanger working as condenser and a plate heat exchanger working as evaporator resulting in a 2.5 kW water chiller, which initially was working with R134a as refrigerant. A brief description of the refrigeration system and the installation used to test its behaviour as well as the uncertainty analysis is presented. Then, the test matrix defined in order to evaluate the effect of the different variables of interest on the performance of the refrigeration cycle is also presented. Air inlet temperature in the condenser was varied in order to analyse the influence of a variation in the ambient temperature. Other variables were varied in order to analyse the influence of different operating conditions as a variation in the compressor frequency or in the condenser fan velocity (by varying air inlet velocity). Finally, the subcooling degree was varied in order to compare a system with refrigerant receiver (no subcooling) and a system without refrigerant receiver and different refrigerant charge (different subcooling). According to the results obtained, R1234yf performs worse than R134a, presenting lower cooling capacity and higher compressor consumption in all tests, with a global efficiency around 25% lower. This can be explained by its lower isentropic and volumetric efficiency, which could be related to the type of lubricant used. [ABSTRACT FROM AUTHOR]

This paper analyses the performance of an air/water chiller typically used for residential air conditioning when a minichannel condenser is used in replacement of a conventional fin-and-tube condenser. The experimental facility, the data reduction process and the uncertainty analysis are briefly described and then the experimental results are presented and discussed, comparing the results obtained with both, the minichannel and the fin-and-tube condenser. Additionally, the chiller is numerically modelled using a commercial refrigeration system modelling software (IMST-ART). The good agreement between experimental and numerical results allows validating the model that is then used to calculate the mass of refrigerant contained in all the components of the system, as well as other operational parameters of interest not measured experimentally. The analysis of the results shows that replacing a conventional fin-and-tube condenser by a minichannel condenser allows, under almost any operating condition, reducing the mass of refrigerant increasing efficiency and cooling capacity. [ABSTRACT FROM AUTHOR]

This paper reports the results of an experimental and numerical study of the use of a minichannel coil and a fin-and-tube coil in an air/water heat pump working as evaporator when the system is working in heating mode. The experimental installation developed to test the heat pump unit is briefly described and then the experimental results are presented. Alternatively, the heat pump is modelled using a refrigeration system design program. The results obtained using this model are compared to experimental results allowing the validation of the model. Once the model is validated, it is used to numerically determine the distribution of refrigeration charge and other interesting parameters not experimentally measured. Finally, an analysis of the numerically obtained results is presented in order to study the influence of several operating conditions on refrigerant charge, system COP and heating power for both types of evaporators tested. [ABSTRACT FROM AUTHOR]

In this work, a method for the characterization of a plate heat exchanger working as evaporator is presented. It is based on a one-dimensional discretization of the exchanger, which solves the heat transfer balance equations by means of an iterative methodology based on a heat transfer area converging method. The inputs of the method are the flow rates of the fluids, the inlet enthalpy of the refrigerant, the superheating, and the inlet temperature and pressure of the secondary fluid. Once the inlet pressure of the refrigerant is assumed, pressure drop is calculated in each cell and then enthalpy. The consideration of the proper heat transfer coefficient (HTC) correlations allows the calculation of the heat transfer area, which is after compared to the actual one. The method has been validated by means of a database of 366 experimental data obtained for eight plate heat exchangers working as evaporators by using six different refrigerants, namely R134a, R1234yf, R513A, R744, R290, and R507A. As the method requires suitable correlations for the calculation of the HTC and pressure drop, several correlations for the HTC and Δ p found in the literature are studied and the results obtained by using them are presented in terms of the maximum absolute relative deviation (MARD). The results corresponding to the correlation which yields the best results are graphically represented. Finally, the method is used to predict the evaporator performance operating in transient conditions. The results obtained show an excellent agreement with the experimental results collected during the transient operation of a transcritical CO 2 water heater coupled to a storage tank. • A method for the simulation of boiling heat transfer and pressure drop is presented. • The model predicts evaporation pressure with less than 4% average error. • The average error in the heat transfer rate prediction is lower than 2%. • Three heat transfer models are compared and all provide similar results. • The method provides excellent results for the simulation of long transient process. [ABSTRACT FROM AUTHOR]

The present paper reports condensing two-phase flow pressure drop gradient and heat transfer coefficient (HTC) inside a mini-channel multiport tube with R1234yf and R134a. Several models available in the literature are used to compare predictions of these two fluids. Experimental data are analysed to get the influence of saturation temperature, mass flux, vapour quality and fluid properties. HTC values of R1234yf seem to be lower than R134a under similar conditions. Two-phase flow pressure drops are also lower in the case of the new refrigerant R1234yf. [ABSTRACT FROM AUTHOR]

• A reduction of vertical filling velocity a factor ∼ 3 increases the system COP ∼ 12%. • The increase of evaporator inlet water temperature from 5 to 20 °C increases COP 59%. • Richardson number (Ri) is defined based on m ̇ , heat pump COP & tank characteristics. • Ri ∼ 40 in the DHW system is recommended to ensure stratification. This work presents an experimental study of the dynamic performance of a CO 2 water-to-water heat pump in a domestic hot water production system. A facility was developed and used to characterize the time evolution of the COP of this heat pump, the heating and stratification processes of the hot-water storage tank, and the global COP of the system. Results showed that, when heating the water storage tank, strategies based on promoting stratification to reach Ri ∼40, such as the use of vertical tank filling velocities v ∼ 10-4 m·s−1 with low water flow rates between the tank and the heat pump gas cooler, permits an increase of ∼12.4% in the system global COP and a reduction of ∼16% of the compressor energy consumption compared to other strategies. However, strategies based on considering higher water flow rates (i.e. Ri ∼1) increase the thermal energy available in the tank (∼6% when flow rate and v increases a factor 3.6) but enhance the water mixing and extend the heating time which reduces the global COP of the system. Besides, an increase of the evaporator inlet water temperature from 5 °C to 20 °C increases the system global COP by 59% and reduces the heating time ∼40%. [ABSTRACT FROM AUTHOR]