Conference Year
2021
Keywords
R1234ze(E), Evaporator, Shell-and-tube, Drop-in, Low GWP
Abstract
For the planned phase-down of the hydrofluorocarbon (HFC) R134a in vapor compression cycles, a reasonable and accepted long-term substitute is the hydrofluoro-olefin (HFO) R1234ze(E): due to similar operating pressures of the two fluids, an existing plant can be easily adapted to work with R1234ze(E) as a drop-in replacement for R134a. This works presents a direct comparison of the experimental and calculated performances obtained by using R134a and R1234ze(E) in the same water-cooled chiller at the same operating conditions, focusing on the evaporator, a direct expansion shell-and-tube heat exchanger with micro-fin copper tubes. The working conditions are set to typical air conditioning values: the evaporating temperature of the refrigerant ranges from 3.5 to 5.0 °C, and the outlet water temperature is kept at a constant value of 7 °C. As the refrigerant -2 -1 flows inside the tubes with one passage only, the mass flux values are quite low, ranging from 30 to 100 kg m s , with a constant inlet quality of 0.25 and an outlet superheating of 5 K. The average heat flux (referred to the whole heat exchanger) varies from 2 to 10 kW m-2. The compressor capacity is progressively varied from 30 % to 100 % at constant water flow rate in a first data set, and at constant inlet water temperature (12 °C) in a second one, running experimental tests in the Onda Heat Transfer Laboratory. The maximum heat fluxes obtained for the two different fluids differ each other reflecting the same difference in the volumetric capacity, being 25 % lower for R1234ze(E) than for R134a. Considering the evaporator, at a given capacity the value of the water-to-saturation temperature approach for R1234ze(E) results very similar to that of R134a. The experimental data are compared to the results of a software simulation developed specifically for the evaporator. The critical parameters for the model are the characteristics of the new HFO fluid and the low mass fluxes. Specific literature models were chosen for the water heat transfer coefficient, for the superheated vapor heat transfer coefficient and pressure drop (concentrated and distributed), for the dry-out inception quality, for the two-phase concentrated and distributed pressure drop. The evaporating heat transfer coefficient, instead, was calculated with two different literature models, respectively Cavallini et al. (2006) and Mehendale (2018). For both the models used, the simulation results are in good agreement with the experimental data.