Refrigeracion

This paper describes an Excel add-in that gives the thermo-physical properties of various refrigerants used in vapour-compression refrigeration (VCR) systems. The add-in functions that determine the properties of saturated refrigerants do not use equations, but interpolate the data published by ASHRAE. Its functions for superheated refrigerants adopt modified ideal-gas relationships in which the specific heat of the refrigerant is taken from ASHRAE data for saturated vapour refrigerants at a skewed pressure. The paper verifies these functions by comparison with the values determined with the NIST REFPROP software for two ozone-friendly refrigerants, R410A and R1234yf, and two natural refrigerants, R290 and R744. The paper also shows how the add-in can be used for thermodynamic analysis of cascade VCR systems.

Calculation intensive courses lead to the need to integrate computer technology into the classroom, especially in courses such as The Refrigeration and air conditioning at then Dronacharya College of Engineering Gurugram (Haryana). Therefore the opportunity arises for the implementation of interactive tools for ease of calculations. Once the students have mastered the concepts and ability to perform the necessary manual calculations, computer programs can be used to allow the students to study more advanced topics in the material without being bogged down in the calculations. To ease the considerable calculations involved in solving vapor-compression refrigeration (VCR) cycle. This program has recently been revised to be compatible with the Microsoft Windows operating environment prevalent today in engineering software. In addition, a fourth refrigerant, R-134a, was added to account for the addition of new refrigerants in use today. Another modification made to the program was the addition of a tutorial for the thermal system analysis of a VCR cycle. This tutorial emulates the general solution methodology used in the course and reinforces the concepts with the students. The program is available via current web pages for the described course.

RESUMEN
Para la investigación tecnológica de sistemas criogénicos es común proponer diseños experimentales para que sean evaluados en diferentes circunstancias a partir de la simulación computarizada de las propiedades termodinámicas del proceso. En la preocupación mundial creciente por el ambiente ha aumentado la exigencia respecto a la eficiencia de cualquier sistema mecánico. El coeficiente de funcionamiento integral del sistema criogénico resulta la medida natural como criterio de desempeño para para responder la referida exigencia, no obstante, recientes trabajos experimentales han demostrado ciertos inconvenientes al descansar en una medida unidimensional de desempeño. Éste trabajo tiene como objetivo explorar y proponer múltiples criterios para construir un indicador más confiable y adecuado para la selección de arreglos experimentales promisorios en la evaluación tecnológica de sistemas criogénicos. Al evaluar mediante diferentes criterios los resultados pueden ser y muchas veces son contradictorios, con lo cual el requerimiento consiste en determinar la ponderación de cada criterio dentro del indicador global. Se presentan los resultados de combinar dos criterios: el coeficiente integral de funcionamiento y la cantidad total de metano líquido obtenido, para evaluar la combinación óptima de los componentes de un refrigerante ternario en un sistema criogénico y se discutirán los aspectos metodológicos para obtener la combinación de ambos criterios contradictorios sin establecer de antemano una ponderación específica.

Palabras claves: Gas Natural Líquido (GNL), Procesos Criogénicos, Coeficiente Integral de Funcionamiento, Evaluación Multiatributo, Mezclas Refrigerantes.

ABSTRACT
In technological research of cryogenic systems it is a common practice to propose experimental designs in order to evaluate different circumstances by means of computer-assisted estimation of the cryogenic system's thermodynamic properties. All around the world are increasing concern for the environment. A natural consequence is upward pressures demanding mechanical-systems efficiencies improvement. To this respect in cryogenic systems the integral coefficient of performance turns out to be the natural measure as criterion of performance to deal with the above-mentioned pressures; nevertheless recent experimental works have demonstrated certain disadvantages on having rested in a one-dimensional measure of performance. This work aims to explore and to propose multiple criteria to build a more reliable and suitable indicator for selecting between promissory experimental arrays when cryogenic systems are being technologically-evaluated. Outcomes can be and often they are contradictory when cryogenic systems are being evaluated by means of different criteria thus the challenge is to determine the best weighting of each criterion to build a global indicator. This work will present the results of combining two criteria: the integral coefficient of performance and the total quantity of liquid methane obtained, in order to evaluate the ideal proportion of components in a ternary refrigerant-mixture operating a cryogenic system. The methodological aspects will be discussed to obtain the combination of both contradictory criteria without establishing in advance specific weights.

Keywords: Liquid Natural Gas (GNL), Cryogenic Processes, Integral Coefficient of Performance, Multi-attribute Evaluation, Mixed Refrigerants.