Industrial heat exchangers evaluation using thermal design and optimization

Esta tesis de Maestría en Ingeniería contribuye al área de eficiencia energética mediante la aplicación de metodologías de “Diseño y optimización térmica” para evaluar intercambiadores de calor industriales viables a través de dos estudios de caso. En un primer enfoque, se propuso una mejora del proceso de producción de biodiésel de aceite de ricino estándar mediante la adición de un precalentador de aceite en un lugar justo antes de bombear la materia prima al proceso de transesterificación. Se desarrolló una metodología para seleccionar este precalentador comparando la efectividad de la transferencia de exergía y la generación de entropía en los arreglos de fluidos habituales para los intercambiadores de calor. Debido al mejor rendimiento de la Segunda Ley, se seleccionó la disposición de carcasa y tubo y se analizaron algunas configuraciones mediante el método de Bell-Delaware para elegir los parámetros geométricos que funcionan mejor. Es de destacar que la configuración que tiene las relaciones geométricas recomendadas de fabricación estándar está muy de acuerdo con la exergía y el análisis de la segunda ley. Por otro lado, los intercambiadores de calor de placa y marco tienen su principal aplicación en la industria de procesamiento de alimentos cuando se requiere una situación líquido - líquido. Debido a esto, se simularon 40 intercambiadores de calor agua-agua de placas empaquetadas factibles para mostrar su comportamiento, mediante un análisis de desempeño, utilizando índices de Primera y Segunda Leyes Termodinámicas en algunas configuraciones operativas de los mismos. Los parámetros de las variables operativas fueron el área del intercambiador de calor, la asignación de la conexión del puerto de fluido frío y la temperatura ambiente. La conclusión principal es que las configuraciones a contracorriente tienen un mejor rendimiento que las configuraciones de flujo paralelo debido a la influencia de la diferencia de temperatura finita. Además, la diferencia de temperatura ambiente es un parámetro que se debe tener en cuenta para seleccionar este tipo de intercambiador de calor.This Engineering Master dissertation contributes to the energy efficiency area by applying “Thermal design and optimization” methodologies to evaluate industrial feasible heat exchangers via two case studies. In a first approach, an enhancement to standard castor oil bio-diesel production process was proposed by adding an oil preheater in a place just before pumping the raw material to the transesterification process. A methodology to select this preheater was developed by comparing exergy transfer effectiveness and entropy generation in the usual fluid arrangements for heat exchangers. Due to the best Second Law performance, Shell-&-Tube arrangement was selected and some configurations analyzed by Bell-Delaware method to choose those geometric parameters that perform best. It is to highlight that configuration that has the standard manufacturing recommended geometry relations is in very good agreement with exergy and second law analysis. On the other hand, Plate-&-Frame heat exchangers have their main application in food processing industry when a liquid - liquid situation is required. Due to this, 40 feasible gasketed-plate water-water heat exchangers were simulated to show their behavior, by a performance analysis, using First and Second Thermodynamic Laws indexes on some operational configurations of them. Operational variable parameters were heat exchanger area, cold fluid port connection allocation and room temperature. Main conclusion is that counter-current configurations have better performance than parallel flow configurations due to influence of finite temperature difference. Additionally, room temperature difference is a parameter that must be taken into account to select this kind of heat exchanger

Energetic and exergetic study for cross-corrugated membrane-based total recovery exchanger for ventilation

Indoor air quality is an important component of the air conditioning of buildings due to its major effect on the health of the occupants, thus the air supplied to these buildings by the ventilation system should be sufficient, clean and healthy. A most promising development was the heat recovery system which offers better thermal energy efficiency and comfort with adequate fresh air. An energetic and exergetic analysis has been conducted on a cross-corrugated membrane based total heat exchanger core for ventilation of single dwellings. In order to enhance the sensible and latent effectiveness of the heat and mass transfer intensification was achieved by selecting Polyethersulfone for the membrane material, and a cross-corrugation arrangement of different dimensions for the primary surface exchanger. The design was tested against a ventilation air volume flow rate for an individual household; from 85 to 100 m³/hr. The dimensions of the exchanger were based on the polymer core being developed by Redring-Xpelair, Peterborough UK, with core dimensions of width and length both 250 mm, and a range of heights 100 – 500 mm. The cross-corrugated design of the test core had triangular openings with pitch lengths of 5, 10 and 25 mm. The ambient conditions were for a cold and humid winter in the UK. The ambient temperature test values were 2, 4, 6, 8 and 10 °C, and the inlet air velocities in the core were 0.5, 1.0, 1.5 and 2 m/s, with Reynolds numbers not exceeding 2200. CFD studies were conducted to investigate the thermal-fluid performance of the core, the Transition-SST model was used in the simulations within ANSYS Fluent 17.1 software and was validated using experimental data in the literature. The proposed model performed successfully in this study and proved that it was compatible with the test conditions. The exergetic analysis was conducted using the IPSEpro modelling software, by creating a system consisting of membrane core, a domestic dwelling, fresh air and exhaust fans. The energetic analysis results were the basis of the IPSEpro modelling to determine the exergy, the exergetic efficiency and exergy destruction in the system. The study concluded from both the energetic and the exergetic analysis that the membrane based exchanger core showed promising performance as a total heat and moisture recovery application with sensible and latent effectiveness values varying from 65% to 82%; and exergetic efficiency values varying from 30% to 60%, depending on core geometry and ambient conditions. The chemical exergy was the dominant factor in the performance in all cases, and the membrane core had the highest exergy destruction percentage comparing to the other system components. Decreasing the pitch length of the exchanger core intensified its performance, the 5 mm case showed the best performance, but there are likely to be difficulties in manufacturing such a compact core. But, and more directly, its use would mean unpleasant compromises due to the extremely higher pressure drop across such a core even at low Reynolds numbers. The 10 mm case gave a better performance than the 25 mm, but not substantially different, therefore, the optimum choice lies between the better heat and mass transfer performance of the 10 mm case and the lower pressure drop and relative ease of manufacture of the 25 mm

Evaluation of frost prevention strategies for membrane energy exchangers

In cold climates, the application of heat recovery is restricted by the issue of frost, which causes potential damage to heat exchangers and degrades their effectiveness. Membrane energy exchangers (MEEs), which enable simultaneous heat and moisture transfer, can reduce and delay frost formation and accumulation in cold climates. MEEs are recognized as the essential component for the new generation of Heating, Ventilation, and Air Conditioning (HVAC) systems. Despite of extensive studies on heat and mass transfer characterising and increased use of MEEs, the evaluation of suitable frost control strategies for the emerging MEEs in cold climates are still missing. This study presents numerical models of a quasi-counter-flow membrane energy exchanger (QCFMEE) and a quasi-counter-flow heat exchanger (QCFHE). Three different frost prevention strategies are examined: preheating outdoor air, heating room air and bypassing outdoor air. These strategies’ threshold values to prevent frost are calculated numerically and validated against experimental measurements. The results show that QCFMEE has lower threshold values and thus better frost tolerance ability compared with QCFHE because of mass transfer through the membranes. Moreover, the frost prevention strategies are evaluated based on annual energy consumption, energy saving ratio (ESR), and complexity of control for real-life applications. The simulated results show that among the discussed frost prevention strategies, preheating outdoor air has the advantage of the lowest energy consumption and highest ESR. Meanwhile, heating room air consumes the most energy and faces the problem of overheating outdoor air. Finally, concerning the bypassing outdoor air strategy, the significant fluctuation of its threshold values increases the complexity of control for real-life applications.publishedVersio

Design, fabrication and performance evaluation of a compact regenerative evaporative cooler: towards low energy cooling for buildings

© 2017 Elsevier Ltd The urges of reducing energy use and carbon footprint in buildings have prompted the developments of regenerative evaporative coolers (RECs). However, the physical dimensions of RECs have to be designed enormous in order to deliver a large amount of supply airflow rate and cooling capacity. To tackle the issue, this paper develops a large-scale counter-flow REC with compact heat exchanger through dedicated numerical modelling, optimal design, fabrication and experimentation. Using modified ε-NTU method, a finite element model is established in Engineering Equation Solver environment to optimise the cooler's geometric and operating parameters. Based on modelling predictions, the cooler's experimental prototype was optimally designed and constructed to evaluate operating performance. The experiment results show that the cooler's attained wet-bulb effectiveness ranges from 0.96 to 1.07, the cooling capacity and energy efficiency ratio from 3.9 to 8.5 kW and 10.6 to 19.7 respectively. It can provide sub-wet bulb cooling while operating at high intake channel air velocities of 3.04–3.60 m/s. The superior performance of proposed cooler is disclosed by comparing with different RECs under similar operating conditions. Both the cooler's cooling capacity per unit of volume and per unit of airflow rate are found to be 62–108% and 21.6% higher respectively

Evaluation of advanced regenerator systems

The major considerations are discussed which will affect the selection of a ceramic regenerative heat exchanger for an improved 100 HP automotive gas turbine engine. The regenerator considered for this application is about 36cm in diameter. Regenerator comparisons are made on the basis of material, method of fabrication, cost, and performance. A regenerator inlet temperature of 1000 C is assumed for performance comparisons, and laboratory test results are discussed for material comparisons at 1100 and 1200 C. Engine test results using the Ford 707 industrial gas turbine engine are also discussed

Estimation of Thermal and Hydraulic Characteristics of Compact Brazed Plate Heat Exchangers

This thesis work presents various performance estimation methods of compact brazed plate heat exchangers (BPHE) operating in single phase, condenser, evaporator, cascaded and transcritical applications. Such methods play a vital role in development of heat exchanger selection software and during geometry parameter estimation in the new product development process. The suitability of employing commercial computational fluid dynamics (CFD) codes for estimating single phase thermal and hydraulic performance is investigated. Parametric studies are conducted on geometries of single phase fluid sections to isolate and quantify the influence of individual geometric parameters. The influence of mesh characteristics, choice of boundary conditions and turbulent flow modeling on the accuracy of the thermal and hydraulic predictions is presented. Benefits of simulation of fluid flow in entire channels and characteristics of channel flow for different geometric patterns are also presented. A computationally light, general, robust and continuous rating calculation method is developed for implementation in BPHE selection software. The pressure-enthalpy based method provides a generic rating core for various types of applications and provides extensive post processing information of the heat transfer process. General single phase thermal and hydraulic empirical correlations are developed as functions of plate geometric parameters. For facilitating better integration of the developed calculation method with other refrigeration system simulation software, first or higher order continuity is maintained in the sub-routines used for calculating local heat transfer coefficients and refrigerant properties. A new finite grid interpolation method is developed for fast and accurate retrieval of refrigerant properties. The developed method is currently implemented in SSPG7 (BPHE selection software of SWEP International AB) for supporting transcritical CO2 calculations and cascaded heat exchanger calculations. Additionally, the methods developed for single phase and two phase test data evaluation based on meta-heuristic optimization routines is also presented. The application and results of using the developed rating models for various types of calculations is summarized. Other topics such as influence of variable fluid properties on BPHE rating calculations, influence of multi-pass flow arrangement on lumped BPHE rating calculations are briefly presented