A dynamic mathematical model of a shell-and-tube evaporator. Validation with pure and blend refrigerants

[EN] This work presents a mathematical model of a shell-and-tube evaporator based on mass continuity, energy conservation and heat transfer physical fundamentals. The model is formulated as a control volume combination that represents the different refrigerant states along the evaporator. Since the model is based on refrigerant and secondary fluid states prediction, it can be easily adapted for modelling any type of evaporator. The strategy of working with physical fundamentals allows the steady- and dynamic-state analysis of any of its performance variables. The paper presents a steady-state validation made with two pure refrigerants (HCFC-22, HFC-134a) and with a zeotropic blend (HFC-407C), and a dynamic validation with transient experimental tests using HCFC-22. The model prediction error is lower than 5% and it is well in accordance with actual dynamic behaviour.Llopis, R.; Cabello, R.; Navarro-Esbrí, J.; Torrella Alcaraz, E. (2007). A dynamic mathematical model of a shell-and-tube evaporator. Validation with pure and blend refrigerants. International Journal of Energy Research. 31(3):232-244. doi:10.1002/er.1243S23224431

Experimental drop-in replacement of R404A for warm countries using the low GWP mixtures R454C and R455A

[EN] This article presents an analysis of the feasibility of R454C and R455A, two new low global warming potential (GWP of 148) refrigerants, in vapor compression refrigeration systems as alternatives to R404A for warm countries. R454C and R455A can be the most viable low GWP options to perform a direct replacement of R404A because of the similar characteristics. They only present meaningful differences in flammability, critical temperature, temperature glide and vapor density. The analysis is based on an experimental comparison of R404A with R454C and R455A, using a fully instrumented experimental setup equipped with Internal Heat Exchanger (IHX) at condensation temperatures that represent operating conditions of warm countries. The experimental results show that cooling capacity of the replacements is slightly lower than R404A, being the Coefficient of Performance (COP) of the new mixtures 10-15% higher than that of R404A, especially at higher condensation temperatures. The results also show that the adoption of an IHX is not recommended with the alternatives due to the discharge temperature increase and the low energy performance benefit.Dr. Adrian Mota-Babiloni would like to acknowledge the funding received from the Plan for the promotion of research of the University Jaume I for the year 2016 (Grant number POS-DOC/2016/23).Mota-Babiloni, A.; Haro-Ortuño, JR.; Navarro-Esbrí, J.; Barragán Cervera, Á. (2018). Experimental drop-in replacement of R404A for warm countries using the low GWP mixtures R454C and R455A. International Journal of Refrigeration. 91:136-145. https://doi.org/10.1016/j.ijrefrig.2018.05.018S1361459

Variable speed liquid chiller drop-in modeling for predicting energy performance of R1234yf as low-GWP refrigerant

This paper presents a model for a variable-speed liquid chiller integrating a compressor model based on Buckingham π-theorem to accurately predict the system performance when R134a is replaced with R1234yf, using a wide range of data obtained from an experimental setup. Relevant variables such as temperature, pressure, mass and volumetric flow rates, compressor power consumption and rotation speed were measured at several positions along the refrigeration and secondary circuits and were used to validate the developed model. Model results show that cooling capacity and power consumption predicted values are in good agreement with experimental data, within ±5%, being slightly higher for the deviation obtained for R134a than for R1234yf. Moreover, model results indicate that R1234yf has a reduction of coefficient of performance (COP) compared with R134a (between 2 and 11.3%), and that R1234yf COP reduction is diminished at intermediate volumetric flow rate and higher inlet temperature for the evaporator secondary fluid, respectively. On the other hand, an environmental analysis based on TEWI (total equivalent warming impact) method showed that direct emissions are almost negligible for R1234yf. However, there are no environmental benefits in terms of indirect greenhouse gas emissions using R1234yf without system modifications (as for instance the addition of internal heat exchanger or R1234yf new design components), which are required to reduce the liquid chiller climate change contribution using it as low GWP alternative in comparison with the typically used R134a refrigerant

Comparative evaluation of R1234yf, R1234ze(E) and R450A as alternatives to R134a in a variable speed reciprocating compressor

A comparative energetic evaluation of R1234yf, R1234ze(E) and R450A as alternatives to R134a in a variable speed compressor is carried out. A compressor model based on dimensionless numbers was obtained using the Buckingham p -theorem, which was validated with experimental data; showing that the prediction error of the model is lower than ± 10% and ± 2 K for temperature. The experimental data were obtained by testing R134a, R1234yf, R1234ze(E) and R450A for a wide range of operating condi- tions. Results obtained with the validated model, show that the dimensionless approach provides a similar estimation of energy parameters compared with the experimental results, such as power con- sumption, refrigerant mass fl ow rate, cooling capacity, COP, discharge temperature and compressor ef- fi ciencies for each refrigerant tested using the dimensionless approach proposed. The comparative evaluation of the compressor predictions shows a reduction in the cooling capacity obtained with R1234yf, R450A and R1234ze(E), in comparison with R134a. Also, COP values for R1234yf, R450A, and R1234ze(E) are lower than those obtained from R134a. Finally, results shows that the dimensionless correlation compressor model can be used to predict the performance of other reciprocating compres- sors, at similar operating conditions for a wide range of compressor rotation speed, with a reasonable accuracy.The authors thankfully acknowledge to the "Consejo Nacional deCiencia y Tecnología (CONACYT)” for their support to this study, and to the “Ministerio de Educación, Cultura y Deporte” (Grant number FPU12/02841) for sponsoring this work through “Becas y Contratosde Formación de Profesorado Universitario del Programa Nacional de Formación de Recursos Humanos de Investigación del ejercicio 2012”

An approach for evaluating the bioavailability and risk assessment of potentially toxic elements using edible and inedible plants—the Remance (Panama) mining area as a model

Mining affects the environment, particularly through the persistence of accumulation of tailings materials; this is aggravated under tropical climatic conditions, which favours the release of potentially toxic elements (PTEs) bioavailable to the local flora and fauna and supposing a risk to human health. The Remance gold mine (Panamá), exploited intermittently for more than 100 years, and has remained derelict for over 20 years. Within the area live farmers who carry out subsistence agriculture and livestock activities. The objective of this study has been to study the transference of PTEs in the local agricultural soil-plants system, with the goal of identifying their bioavailability to perform a human risk assessment. The results obtained of the Bioaccumulation coefficient in local plants show very weak to strong absorption of As (< 0.001–1.50), Hg (< 0.001–2.38), Sb (0.01–7.83), Cu (0.02–2.89), and Zn (0.06–5.32). In the case of Cu in grass (18.3 mg kg−1) and plants (16.9 mg kg−1) the concentrations exceed the maximum authorised value in animal nutrition for ruminants (10 mg kg−1). The risk to human health for edible plants exceeds the non-carcinogenic risk for rice, corn, cassava, and tea leaves for Sb (HQ 19.450, 18.304, 6.075, 1.830, respectively), the carcinogenic risk for Cu (CR = 2.3 × 10–3, 7.7 × 10 −4, 1.1 × 10–3, 1.0 × 10–3, respectively), and the carcinogenic risk for As in rice, corn and tea leaves (CR = 8 × 10–5, 3 × 10–5, 3 × 10–5, respectively). Urgent measures are needed to alleviate these effects.Depto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUECRUE-CSICNational Secretary of Science and Technology (Panamá)Institute for the Training and Use of Human Resources (Panamá)Universidad de Castilla La Manchapu

Evaluación probabilística del riesgo para la salud por exposición a elementos potencialmente tóxicos en sedimentos afectados por actividades mineras en Panamá (Remance)

Depto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEpu

Riesgos ambientales asociados a la gestión inadecuada de residuos mineros en una explotación minera de oro, Remance, Panamá

Depto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEpu

Dynamic model of a shell-and-tube condenser. Analysis of the meanvoid fraction correlation influence on the model performance

A moving-boundary dynamic model of a shell-and-tube condenser is presented. Within this approach, the mean void fraction is a relevant parameter which is obtained, in this work, using different correlations proposed in the literature for the flow pattern analyzed. In order to evaluate the performance of the model with each different mean void fraction correlation, a set of experimental tests using R134a as working fluid, varying the main operating variables (refrigerant mass flow rate, secondary fluid mass flow rate and inlet temperature), are performed. The model performance is analyzed from the system model outputs, namely, condensing pressure and refrigerant and secondary fluid outlet temperatures. The results, comparing model predictions and experimental data, show the great influence of the mean void fraction correlation on the model predictions with noticeable discrepancies depending on the correlation used. It is also observed that the model using the homogeneous correlation frequently provides acceptable results in all the tests analyzed, although the most appropriate correlation depends on the transient characteristics.Milián Sánchez, V.; Navarro Esbrí, J.; Ginestar Peiro, D.; Molés, F.; Peris, B. (2013). Dynamic model of a shell-and-tube condenser. Analysis of the meanvoid fraction correlation influence on the model performance. Energy. 59:521-533. doi:10.1016/j.energy.2013.07.053S5215335

Développement et validation d'un modèle d'évaporateur à tubes à micro-ailettes utilisant le R134a et le R1234 yf comme fluides actifs

This paper presents a model of shell and tube evaporator with micro-fin tubes using R1234yf and R134a. The model developed for this evaporator uses the ε-NTU method to predict the evaporating pressure, the refrigerant outlet enthalpy and the outlet temperature of the secondary fluid. The model accuracy is evaluated using different two-phase flow boiling correlations for micro-fin tubes and comparing predicted and experimental data. The experimental tests were carried out for a wide range of operating conditions using R134a and R1234yf as working fluids. The predicted parameter with maximum deviations, between the predicted and experimental data, is the evaporating pressure. The correlation of Akhavan– Behabadi et al. was used to predict flow boiling heat transfer, with an error on cooling capacity prediction below 5%. Simulations, carried out with this validated model, show that the overall heat transfer coefficient of R1234yf has a maximum decrease of 10% compared with R134a.The authors are grateful to University of Guanajuato, the ISTENER research group of University Jaume I, and the Consejo Nacional de Ciencia y Tecnologia (CONACYT) for their support to this study. Finally, the linguistic support of Dr. Carlos Montoro is appreciated

Experimental characterization of an Organic Rankine Cycle (ORC) for micro-scale CHP applications

[EN] This work conducts an experimental characterization of an Organic Rankine Cycle (ORC) module along the operating conditions expected for micro Combined Heat and Power (CHP) applications. For this, a monitored test bench has been used and adapted to the planned test procedure, which consisted of varying the condensing conditions for a fixed low grade heat source. Thereby, 10 steady state points are achieved and analyzed according to thermal power input, useful heat production, gross and net electrical powers, electrical and thermal cycle efficiencies and expander effectiveness parameters. The results show the importance, for all the cited parameters, that takes to operate with a suited pressure ratio, associated with the expander built-in volume ratio, to each specific application. So, if the useful heat temperature requirements allow to operate with a moderate under-expansion in the expander, the module is optimized.The authors want to acknowledge all the invaluable cooperation of Rank (R), the ORC manufacturer, for its support in this project. Also to thank greatly the Jaume I University for its financial support under the PhD grant PREDOC/2013/28 of "Convocatoria d'ajudes predoctorals per a la formacio de personal investigador del Pla de promocio de la investigacio de la Universitat jaume I de Castello (Spain)".Peris, B.; Navarro-Esbrí, J.; Molés, F.; Martí, JP.; Mota-Babiloni, A. (2015). Experimental characterization of an Organic Rankine Cycle (ORC) for micro-scale CHP applications. Applied Thermal Engineering. 79:1-8. https://doi.org/10.1016/j.applthermaleng.2015.01.020S187