José Fernández-Seara

Study and control of the optimal generation temperature in NH3–H2O absorption refrigeration systems

Abstract This paper presents the results of a study on the optimal generator temperature (OGT) in single stage NH3–H2O Absorption Refrigeration Systems (AARS). It is well known that the generation temperature affects the AARS’s Coefficient of Performance (COP) and that there is a temperature value, called optimal generation temperature, for which the COP is maximum. Therefore, to develop new control strategies designed to maintain the optimal temperature in the system generator, it is necessary to study the dependency of this temperature on thermal operating conditions and system design parameters. One such type of study has been carried out here by means of a parametric analysis, using a simple model implemented in a computer program. Based on the results obtained a novel control system that works on two separated control-loops has been designed. The proposed control system maintains a constant temperature in the space to refrigerate as well as the optimal temperature in the system generator.

Heat recovery system to power an onboard NH3-H2O absorption refrigeration plant in trawler chiller fishing vessels

Abstract This paper is concerned with the design, modelling and parametric analysis of a gas-to-thermal fluid heat recovery system from engine exhausts in a trawler chiller fishing vessel to power an NH3-H2O absorption refrigeration plant for onboard cooling production. Synthetic oil was used as heat transfer fluid and recirculated. The major components of the system are fluid-to-solution and gas-to-fluid heat exchangers. Both heat exchangers and the complete system have been modelled. Models are implemented in several computer programs. These models have been used to study the influence of geometric design parameters and thermal operating conditions on heat exchangers and system thermal performance. The analysis of the results allowed us to find the optimum thermal operating conditions that minimise total heat transfer area. Optimal design based on real data was performed and the operating function of exhaust gases by-pass control was obtained and is presented.

Distillation column configurations in ammonia-water absorption refrigeration systems

Abstract In ammonia–water absorption refrigeration systems a purification process to reduce the water content in the vapour leaving the generator is required. During this process the water content in the vapour must be reduced to a minimum, otherwise it tends to accumulate in the evaporator and strongly deteriorates the efficiency of the system. The vapour purification can be carried out by partial condensation, by establishing a liquid–vapour counter flow or by combining both methods. In systems with partial condensation, the distillation column can be composed of one or more rectifiers using different cooling mediums, and the rectifying and stripping sections. In complete condensation systems only the rectifying and stripping sections can be used. Therefore different distillation column arrangements should be considered. This paper presents a study of several distillation column configurations for single stage ammonia–water absorption refrigeration systems with partial and complete condensation. In order to evaluate and compare the different configurations, a parameter that indicates the ratio of the ammonia vapour concentration increase in each part of the column to the total ammonia purification has been defined. The analysis has been based on the system COP. Finally the efficiency in each part of the column has been calculated to estimate its design requirements.

Laboratory practices with the wilson plot method

The Wilson plot method is a well-known tool used in practical applications and research activities that involve the analysis of different heat exchanger devices. Therefore, it is of interest to researchers and engineers working on heat transfer and related fields. Moreover, the application of this method is simple enough to be taught in laboratory practices with students. Taking into account the reasons cited above, a simple experimental apparatus has been designed and built at our laboratory. In this article, the calculation procedure to apply the Wilson plot method is explained and characteristic experimental results for a smooth tube and a spirally corrugated tube are reported. Moreover, the experimental results are compared with different correlations proposed in the literature.

Heat and mass transfer analysis of a helical coil rectifier in an ammonia–water absorption system

Abstract This paper presents a detailed study on the ammonia–water vapour rectification process in absorption systems using a helical coil rectifier. A differential mathematical model has been developed on the basis of mass and energy balances and heat and mass transfer equations. The differential volume has been defined in each coil turn by a differential angle on the turn and a second differential angle on the coiled tube cross section. It contains the corresponding differential portion of coolant, coiled tube wall, condensate film and vapour. Simultaneous heat and mass transfer processes have been taken into account in the vapour and liquid phases. The model equations have been solved using the finite-difference method. Results have been obtained for characteristic data from an ammonia–water absorption refrigeration system. Most significant calculated variable profiles along the coil height as well as in the coiled tube cross section are presented and discussed. The influence of the heat and mass transfer coefficients on the rectifier performance has also been considered.

Experimental apparatus for measuring heat transfer coefficients by the wilson plot method

The Wilson plot is a technique to estimate the film coefficients in several types of heat transfer processes and to obtain general heat transfer correlations. This method is an outstanding tool in practical applications and in laboratory research activities that involve analysis of heat exchangers. Moreover, the application of this method is simple enough to be taught in laboratory practices for students at university and doctoral level of physics and engineering. Therefore, an experimental apparatus has been designed and built in our laboratory that allows the students to carry out experiments based on the application of the Wilson plot method. In this note, the principles of the method are explained, the experimental apparatus is described and representative results of the experimental data taken from the apparatus and the application of the Wilson plot method are shown.

Simultaneous heat and mass transfer of a packed distillation column for ammonia–water absorption refrigeration systems

Abstract This paper presents a study on the NH3–H2O distillation process using a packed column with liquid reflux from the condenser in an absorption refrigeration system. A differential mathematical model has been developed on the basis of mass and energy balances and the heat and mass transfer equations. A net molar flux between the liquid and vapour phases has been considered in the mass transfer equation, which obviates the need to assume equimolar counter-diffusion. The model equations have been solved using the finite-difference method. Results obtained for a specific application are shown, including parameter distributions along the column length. The influence of rectifying and stripping lengths, mass and heat transfer coefficients and volumetric heat rejection from the column, on the distillate ammonia concentration has been analysed.

Heat Transfer Performance of Functionalized Graphene Nanoplatelet Aqueous Nanofluids

The low thermal conductivity of fluids used in many industrial applications is one of the primary limitations in the development of more efficient heat transfer systems. A promising solution to this problem is the suspension of nanoparticles with high thermal conductivities in a base fluid. These suspensions, known as nanofluids, have great potential for enhancing heat transfer. The heat transfer enhancement of sulfonic acid-functionalized graphene nanoplatelet water-based nanofluids is addressed in this work. A new experimental setup was designed for this purpose. Convection coefficients, pressure drops, and thermophysical properties of various nanofluids at different concentrations were measured for several operational conditions and the results are compared with those of pure water. Enhancements in thermal conductivity and in convection heat transfer coefficient reach 12% (1 wt %) and 32% (0.5 wt %), respectively. New correlations capable of predicting the Nusselt number and the friction factor of this kind of nanofluid as a function of other dimensionless quantities are developed. In addition, thermal performance factors are obtained from the experimental convection coefficient and pressure drop data in order to assess the convenience of replacing the base fluid with designed nanofluids.

Simulation of compression refrigeration systems

This study presents the main features of a software for simulating vapor compression refrigeration systems that are self designed by the user. A library of 10 different components is available: compressor, expansion device, condenser, evaporator, heat exchanger, flash tank, direct intercooler flash tank, indirect intercooler flash tank, mixer, and splitter. With these components and a library of different refrigerants many different refrigeration systems may be solved. By a user‐friendly interface, the user can draw the system scheme by adding different components, connecting them and entering different input data. Results are presented in the form of tables and the cycle diagram of the system is drawn on the logP–h and T–s thermodynamic charts.

Experimental Analysis of the Freezing Process in a Horizontal Plate Freezer With CO2 as Refrigerant in a Cascade Refrigeration System

An experimental evaluation of a cascade refrigeration system prototype using NH3 and CO2 as refrigerants has been performed. The prototype is used to supply a 9-kW refrigeration capacity to a horizontal plate freezer at an evaporation temperature of –50°C as design conditions. The prototype includes a specific control system and a data acquisition system. The article describes the prototype and the experimental program carried out to study the freezing process in the horizontal plate freezer. The experimental results include the freezing time, the temperature evolution in the product to be frozen, the refrigeration capacity of the facility, the electric power of the compressors, the coefficient of performance (COP) of the low- and high-temperature systems, and the overall system COP. The results obtained would confirm that the use of CO2 in plate freezers represents a viable alternative to refrigerants currently in use.