Bond Graph model of a vertical U-tube steam condenser coupled with a heat exchanger.

International audienceA simulation model for a vertical U-tube steam condenser in which the condensate is stored at the bottom well is developed in this paper. The U-tubes carrying the coolant are partially submerged in the stored condensate and thus the bottom well acts as a heat exchanger. The storage of hydraulic and thermal energies is represented using a coupled pseudo-bond graph model. Advection effects are modelled by considering reticulated segments of the tubes carrying coolant, over which condensation takes place. The developed model is of intermediate complexity and it is intended for use in observer based real time process supervision, which works by comparing the process behaviour to the reference model outputs. The simulation results obtained from the bond graph model are validated with experimental data from a laboratory set-up

Heat Integrated Milk Powder Production

Dairy processing is critical to New Zealand’s (NZ) economy producing NZ$13 billion in exports for 2012 while consuming 32 PJ of fossil fuels for process heat. Three quarters of NZ dairy exports are milk powders. This thesis presents methods to reduce process heat use in Milk Powder Plants (MPP) through improved heat integration and addresses key technical challenges preventing industrial implementation. My original contributions to literature include: (1) a novel design method called the Cost Derivate Method (CDM) that cost optimally allocates area in direct heat exchange networks, (2) a new design methodology for integration of semi-continuous process clusters using a Heat Recovery Loop (HRL) with a Variable Temperature Storage (VTS) system for improved heat recovery, (3) an experimentally validated deposition model for predicting critical air conditions that cause milk powder fouling, and (4) a thermo-economic assessment tool for the optimisation of industrial spray dryer exhaust heat recovery projects via a Liquid Coupled Loop Heat Exchanger (LCHE) system. By applying Pinch Analysis to an industrial MMP, this work confirms that heat must be recovered from the milk spray dryer exhaust air (~75 °C) to achieve maximum heat integration in MPPs. For stand-alone MPPs exhaust heat is best used to indirectly preheat the inlet dryer air reducing steam use by 12.7 A key barrier preventing exhaust heat recovery implementation in NZ MPPs is the possibility of milk powder fouling. Dryer exhaust air contains a low concentration of powder that when exposed to low temperatures at high humidity becomes sticky. For a heat exchanger face air velocity of 4 m/s, experimental data from milk powder fouling tests of flat plates, tubes and fins indicates particulate fouling becomes severe when the exhaust air temperature reaches 55 °C. Higher face velocities are shown to lower this critical exhaust temperature for avoiding severe fouling, which gives potential for increased heat recovery but for increased pressure drop. Lower face velocities show the opposite effect. Designing exhaust heat recovery systems entail an acute trade-off between heat transfer, pressure drop and fouling. Two important design parameters are the number of tube rows in the exhaust heat exchanger and the face velocity. The outputs of a thermo-economic spreadsheet tool suggest LCHE systems for a dryer producing 23.5 t/h is economic. With a face velocity of 4 m/s and 14 rows of finned round tube, the project had an estimated payback of 1.6 years, a net present value of NZ$3 million and internal rate of return of 71 %. This tool will empower industry with greater confidence to uptake exhaust heat recovery technology as a vital method for improving the heat integration of MPPs in NZ

High-performance heat pipes for heat recovery applications

Methods to improve the performance of reflux heat pipes for heat recovery applications were examined both analytically and experimentally. Various models for the estimation of reflux heat pipe transport capacity were surveyed in the literature and compared with experimental data. A high transport capacity reflux heat pipe was developed that provides up to a factor of 10 capacity improvement over conventional open tube designs; analytical models were developed for this device and incorporated into a computer program HPIPE. Good agreement of the model predictions with data for R-11 and benzene reflux heat pipes was obtained

Structured Mathematical Modeling of Industrial Boiler

As a major utility system in industry, boilers consume a large portion of the total energy and costs. Significant reduction of boiler cost operation can be gained through improvements in efficiency. In accomplishing such a goal, an adequate dynamic model that comprehensively reflects boiler characteristics is required. This paper outlines the idea of developing a mathematical model of a water-tube industrial boiler based on first principles guided by the bond graph method in its derivation. The model describes the temperature dynamics of the boiler subsystems such as economizer, steam drum, desuperheater, and superheater. The mathematical model was examined using industrial boiler performance test data.It can be used to build a boiler simulator or help operators run a boiler effectively

Active heat exchange system development for latent heat thermal energy storage

Five tasks to select, design, fabricate, test and evaluate candidate active heat exchanger modules for future applications to solar and conventional utility power plants were discussed. Alternative mechanizations of active heat exchange concepts were analyzed for use with heat of fusion phase change materials (PCMs) in the temperature range of 250 to 350 C. Twenty-six heat exchange concepts were reviewed, and eight were selected for detailed assessment. Two candidates were selected for small-scale experimentation: a coated tube and shell heat exchanger and a direct contact reflux boiler. A dilute eutectic mixture of sodium nitrate and sodium hydroxide was selected as the PCM from over 50 candidate inorganic salt mixtures. Based on a salt screening process, eight major component salts were selected initially for further evaluation. The most attractive major components in the temperature range of 250 to 350 C appeared to be NaNO3, NaNO2, and NaOH. Sketches of the two active heat exchange concepts selected for test are given

Experimental development and bond graph dynamic modelling of a brazed plate heat exchanger

This article is devoted to the dynamic study of a brazed plate heat exchanger (BPHE). First, an introduction to the industrial context of the current FUI THERMOFLUIDE project is proposed. A succinct presentation of the heat exchanger technology is proposed. Afterward, a state of the art discussion of BPHE modelling, heat transfer and pressure drop correlations is given. Then a detailed mathematical description of an original dynamic model is presented. The last section deals with a description of the experimental test rig and performed validation tests.FUI Thermodfluid-R

Active heat exchange system development for latent heat thermal energy storage

Active heat exchange concepts for use with thermal energy storage systems in the temperature range of 250 C to 350 C, using the heat of fusion of molten salts for storing thermal energy are described. Salt mixtures that freeze and melt in appropriate ranges are identified and are evaluated for physico-chemical, economic, corrosive and safety characteristics. Eight active heat exchange concepts for heat transfer during solidification are conceived and conceptually designed for use with selected storage media. The concepts are analyzed for their scalability, maintenance, safety, technological development and costs. A model for estimating and scaling storage system costs is developed and is used for economic evaluation of salt mixtures and heat exchange concepts for a large scale application. The importance of comparing salts and heat exchange concepts on a total system cost basis, rather than the component cost basis alone, is pointed out. The heat exchange concepts were sized and compared for 6.5 MPa/281 C steam conditions and a 1000 MW(t) heat rate for six hours. A cost sensitivity analysis for other design conditions is also carried out

LNG as the future marine fuel, Waste Heat Recovery Systems & Cold Ironing solutions for eco-friendly maritime transport

Nowadays a regulatory framework regarding the pollutant emissions abatement, both at international and local level, significantly interests the maritime sector. To comply with the latest environmental rules, a new approach to ship design and a renewed way to operate the ship are need both in navigation and in harbour. This PhD Thesis aims to investigate on the positive features offered by the LNG fuel, more eco-friendly than the traditional marine fuel oils. In the first part of the research, the performance comparison between two marine engines, fuelled by natural gas and diesel oil respectively is reported. Two different simulation codes, one for each engine, validated by means of geometrical and performance data provided by the manufacturer have been developed to extend the comparison to the whole working area of the examined engines. In the second part of the research, a LNG-repowering study of a cruise ferry is presented. The study is enhanced by the investigation on possible Waste Heat Recovery (WHR) systems aiming at the reduction of Green House Gas (GHG), pollution, and money saving. A computational code has been developed to carry out the sizing and to analyse the energetic performance and economical aspects of the several examined WHR layout systems. The more eco-friendly layout for the considered ship is proposed to comply with in force rules. The third part of the PhD Thesis is focused on studying some maritime technical solutions for the electric energy generation and delivery to ships moored in port, by means of LNG fuelled generators installed on board a floating unit. Two different scenarios, regarding the LNG supply chain, are considered and some options for producing cleaner electric energy are then investigated. The reference area examined in this study is the old port of Genoa, where the traffic of both passenger and cargo ships takes place. The analysis is carried out by means of a MATLAB numerical code to calculate the most important features of the floating unit, as dimensions and weights and the most significant characteristics of the electric generation equipment, as the average load factor, fuel consumption and energy cost. From an economical point of view, the externalities costs abatement, thanks to the technical solution proposed are investigated. The study also focuses on the estimation of governmental incentives to promote and sustain the use of the proposed power supply barge, resulting into a fully positive technical solution

HIGH-PERFORMANCE TUBULAR EVAPORATOR UTILIZING HIGH ASPECT RATIO MANIFOLD MICROCHANNELS

Heat recovery using absorption chillers has not been economical for small scale applications due to high capital requirements and heavy weight/volume as deterring factors for its expanded use in waste heat-to-cooling applications. Development of advanced, high performance heat and mass exchanger components can significantly improve the competitive edge of heat activated absorption cooling systems, particularly with respect to weight reduction and size/volume of these systems. The main contribution of this thesis is demonstration of a novel high performance micro-grooved evaporator, as well as a solution heat exchanger, for use in a small-scale ammonia-water absorption cooling system. A compact tubular evaporator was developed which uses an innovative manifold/fluid feed system combined with a micro-grooved evaporator to realize substantially higher (4 to 5 fold) overall heat transfer coefficient of the evaporator; while requiring much less refrigerant charge per ton of cooling, when compared to conventional state of the art systems. The experimentally measured heat transfer coefficients reported in this study are record high, while pressure drops for the given capacity are modest. Additional contributions of the study included a detailed numerical study of single- stage absorption cycle with multiple cycle design enhancements to identify the controlling system parameters. A single-phase numerical study for manifold microchannel design was carried out to understand the effect of important geometrical parameters in support of design and development of the evaporator. The tubular evaporator was successfully fabricated and tested to the system pressure of 500 psi on the refrigerant-side and was experimentally evaluated with several microgroove surface made of aluminum and nickel alloys, and also with different flow header enhancements using R134a/water pair. For the experiments conducted, the microchannel width was typically in the range of 30-100 µm with a maximum aspect ratio of 10. The refrigerant flow rate was varied within 5-30 g/s and water flow rate was varied within 150-600 ml/s obtaining wide range of cooling capacity between 1- 5 kW for 2-12 °C LMTDs. The overall heat transfer coefficients greater than 20,000 W/m2-K was obtained which is roughly 4-5 times higher than state of art for given application. A maximum pressure drop of 200 mbars on water-side and 100 mbars on the refrigerant-side was observed at maximum mass flow rates. An alternative method for the evaporator design was also explored in form of flat plate evaporators which can further provide improved overall heat transfer coefficients. Manifold microchannels were used on both sides of the plates, with the aim to achieve overall heat transfer coefficient greater than 50,000 W/m2-K. The new micro-grooved evaporator has the potential to introduce a game-changing evaporative surface, with precise flow delivery and high heat transfer coefficients, driven by a combination of thin film evaporation, as well as convective boiling on the heat transfer surface