High Temperature Heat Exchanger Research Articles

Performance Evaluation of an Industrial Absorption System

The Absorption systems have a wide range of the potential employing thermal processes to obtain both chilled water and hot water for cooling and heating applications. The performance of absorption chiller system has been extensively investigated. The absorption chiller type is (Li-Br/H2O) double-effect, parallel-flow and it is capable of providing a cooling capacity [500 R.T], [1750 kW] using natural gas as a fuel to separate refrigerant in the high pressure generator. A thermodynamic analysis for an absorption system has been analyzed based on the principle of mass conservation and the first and the second laws of thermodynamics of each component of the absorption system. The fouling effect of low and high temperature heat exchangers on the performance of absorption system has been investigated. Measurements has been recorded in two years (2015 and 2016).The temperature sensors and infrared device is used to measure specific points on the cycle components which permits to predict the COP and the second law efficiency behavior for a long time of operation. Results showed that the COP of the absorption chiller system in 2015 is higher than obtained in 2016 by average percentage 22%. The effectiveness of the low and high temperature heat exchangers decreased by 2.89% and 0.5%, respectively. Consequently, it was clear that the second law efficiency decreased by 14.04% from 2015 to 2016. Due to the fouling effect of low and high temperature heat exchangers it is recommended that doing periodic maintenance to these components of the absorption chiller systems.

Thermal design, optimization and additive manufacturing of ceramic regular structures to maximize the radiative heat transfer

The present study is focused on the application of a ceramic tubular high temperature heat exchanger with engineered cellular architectures. Thermal design and optimization to maximise the radiative heat transfer has been investigated both experimentally and computationally. Numerical models were designed involving various arrangements of cells and their different sizes (while the total heat transfer area remains constant). They were 3D-printed by Stereolithography (SLA) and subsequently sintered. Heat transfer tests were performed both with a high temperature pressure drop test and by CFD simulations on 2D and 3D models. The computational results agree with the experimental data. We found that radial heat transfer in a tube increases by 160% to 280%, if a ceramic lattice is inserted, in respect of an empty tube. Moreover, the arrangement of cells and their size significantly influences the radiative heat transfer showing (for a given array) its top performances above 773 K. Geometries with large cells outside and small cells inside in the radial direction allow radiation to penetrate better through the core of the porous body. With this engineered ceramic lattices it is possible to reduce the tube length by one third to obtain more compact heat exchangers than an empty tubular solution.

Cermet material could aid the development of future power plants

A remarkable metal–ceramic composite material has been produced that could aid the development of the next generation of power plants — and might even have a role in curing the world of its addiction to fossil fuels. A cermet designed for use in high-temperature heat exchangers.

Design and optimization of a novel high temperature heat exchanger for waste heat cascade recovery from exhaust flue gases

The waste heat of high temperature exhaust flue gases is widely distributed in many industrial processes. Recovery of waste heat is of great significance to energy saving and sustainability. In this paper, a novel high temperature heat exchanger with hybrid enhancement technologies is proposed to improve waste heat recovery efficiency based on the cascade recovery and utilization method. Algorithm for HTHE structural design and optimization is developed and verified according to the experimental results. Heat transfer and pressure drop performance of the proposed HTHE are estimated by using the algorithm. The results show that the effectiveness of the proposed HTHE increases as the gas temperature increases and mass flow rate decreases. Average effectiveness of the proposed HTHE and temperature of preheated air are 12.5% and 85.8 °C higher than those of traditional HTHE with additional 70.0% and 22.0% pressure drop on air and gas sides, respectively. The structural optimization of the proposed HTHE is carried out and it shows that the optimized HTHE has better heat transfer capacity and comprehensive performance under identical pressure drop, increasing effectiveness by 12.6% without enlarging pressure drop compared with the non-optimized HTHE.

Assessment of the Superheater Ash Fouling Using a Numerical Model of the Superheater

ABSTRACTSuperheaters are high-temperature cross-flow heat exchangers. Steam flows inside the tubes, and the flue gas outside in a direction perpendicular to the axes of the tubes. However, they differ very substantially from the other heat exchangers operating at low temperatures. Superheaters are characterized by complex flow system and high tube walls temperature. Superheaters are among the most exposed to damaging pressure elements of steam boilers. Damage to the superheater causes about 40% of emergency shutdowns of boilers. The paper presents the boiler superheater model with distributed parameters, which is used to determine on-line the degree of superheater fouling by ash.

Thermodynamic analysis of small-scale externally fired gas turbines and combined cycles using turbo-compound components for energy generation from solid biomass

This paper proposes the implementation of cost-effective commercially available automotive components in small-scale power plants for the energy generation from carbon-neutral biomass. Specifically, a turbocharger and a power turbine of turbo-compound systems are proposed to be coupled with an external combustor and a high temperature heat exchanger in order to obtain a cheap externally fired gas turbine capable of producing about 30 kW of electrical power from the combustion of pruning residues. The externally fired gas turbine cycle can be combined either with a final heat exchanger to generate useful thermal power or with a bottoming cycle to generate useful thermal power and an additional electrical power of about 15 kW. Two plant configurations are proposed for the bottoming cycle: the first is a water Rankine cycle employing the “green steam turbine” as the steam expander, whereas the second is an organic Rankine cycle using an axial turbine and toluene as the working fluid. The results of the simulations, obtained through a detailed thermodynamic model, show that the use of a combined cycle is fundamental to maximize the primary energy savings of the power plant. In the case of negligible pressure losses, the use of a bottoming water Rankine cycle leads to a maximum second law efficiency of about 0.25 and maximum primary energy savings of about 0.23. Instead, a bottoming organic Rankine cycle employing a single stage turbine can increase the second law efficiency and the primary energy savings up to about 0.27 and 0.26, respectively. It is also demonstrated that the use of a two-stage turbine for the organic Rankine cycle can further enhance the plant performance. The effects of the pressure drops in the system are investigated in detail to point out that the minimization of the pressure losses is fundamental to improve the performance parameters of all the proposed configurations.

Effect of operating variables on performance of an absorption chiller driven by heat from municipal solid waste incineration

This study presents an experimental analysis for a lithium-bromide/water waste steam driven parallel flow double effect absorption chiller evaluation. The absorption chiller analyzed is allocated at Bali refuse incineration plant for air conditioning system. This unit can supply a cooling capacity [400 USRT], [1406 kW]. A set of mathematical equations have been developed to estimate coefficient of performance (COP) of the chiller for each component. Theoretical analysis shows that increasing of the low temperature and high temperature heat exchangers effectiveness, decreasing temperature of absorber and condenser, increasing temperature of high pressure generator, and increase loading factor of the system improved the COP. The entire cycle is optimized by Engineering Equation Solver (EES) software from the standpoint of maximizing the COP via applying the direct experimental method. The optimization result of COP is obtained by the thermodynamic cycle.

Numerical simulation of convective superheaters in steam boilers

Superheaters are high-temperature cross-flow heat exchangers. Steam flows inside the tubes, and the flue gas outside in a direction perpendicular to the axes of the tubes. However, they differ very substantially from the other heat exchangers operating at low temperatures because of the high dependence of steam heat capacity on temperature and radiation heat transfer on the flue gas side. Also, the flow tube arrangement is usually complicated. Due to the work of superheaters in a harsh environment, they are prone to damage. About 40% of the emergency shutdowns of boilers are caused by overheating of the tube material which in turn causes them to break. The paper presents the mathematical model of the superheater. The finite difference model with distributed parameters is based on the solution of energy conservation equations for steam, tube wall, and flue gas. The CFD superheater model was also developed to verify the finite-difference model. The model developed in the paper can be used to assess the fouling degree of the superheater tubes by ash.

Integration of solid oxide fuel cell (SOFC) and chemical looping combustion (CLC) for ultra-high efficiency power generation and CO2 production

This work presents a thermodynamic analysis of the integration of solid oxide fuel cells (SOFCs) with chemical looping combustion (CLC) in natural gas power plants. The fundamental idea of the proposed process integration is to use a dual fluidized-bed CLC process to complete the oxidation of the H2-CO-rich anode exhausts from the SOFC in the CLC fuel reactor while preheating the air stream to the cathode inlet temperature in the CLC air reactor. Thus, fuel oxidation can be completed in N2-free environment without the high energy and economic costs associated to O2 production, avoiding at the same time the high temperature and high cost heat exchanger needed in conventional SOFC plants for air preheating. In the proposed configurations, the CLC plant is operated at mild conditions (atmospheric pressure and temperature in the range of 700–800 °C), already demonstrated in several pilot plants. Two different scenarios have been investigated: in the first one, the SOFC is designed for large-scale power generation (100 MWLHV of heat input), featuring a heat recovery steam cycle and CO2 capture for subsequent storage. In the second scenario, the system is designed for a small-scale plant, producing 145 kg/h of pure CO2 for industrial utilization, as a possible early market application.The main parameters affecting the plant performance, i.e. SOFC voltage (V) and S/C ratio at SOFC inlet, have been varied in a sensitivity analysis. Three different materials (Ni, Fe and Cu-based) are also compared as oxygen carriers (OCs) in the CLC unit. The integrated plant shows very high electric efficiency, exceeding 66%LHV at both small and large scale with a carbon capture ratio (CCR) of nearly 100%. It was found that, except for the cell voltage, the other operating parameters do not affect significantly the efficiency of the plant. Compared to the benchmark SOFC-based hybrid cycles using conventional CO2 capture technologies, the SOFC-CLC power plant showed an electric efficiency ∼2 percentage points higher, without requiring high temperature heat exchangers and with a simplified process configuration.

An approach of adaptive effective cycles to couple fretting wear and creep in finite-element modeling

Fretting wear and creep can occur simultaneously when two surfaces in contact vibrate against each other at relatively high temperatures. An ability to analyze combined creep and wear is important for applications involving engines, contacting surfaces within fluids of high-temperature heat exchangers, such as fuel rods in nuclear power stations, and contact with polymers close to their glass-transition temperatures. The two phenomena of creep and wear interact with each other through their roles in affecting the shape and stress fields at contacts. Wear is a phenomenon that is generally considered to have no inherent time scale associated with it, beyond the period of the fretting oscillations. Traditional computational models for wear do not require a specific reference to time, but creep models do. Currently, a reliable and efficient numerical algorithm to couple the relatively long time scales of creep to the, generally, short periods of vibrations is not available. In this paper, we present such an algorithm, using an approach of adaptive effective vibration cycles. We demonstrate how to ensure the efficiency and reliability of the modeling by bounding the magnitude of the stress redistribution along the interface during the effective cycle. This approach enables the simulation of problems involving coupled wear and creep. We show that this approach is robust, and that it can be used during full-slip and partial-slip fretting for geometries with smooth or discontinuous interfaces at the edge of the contact. In particular, we use these examples to illustrate some of the crucial roles of creep in wear, such as transitions between partial and full slip.

Convection heat transfer of alkali liquid metals and LBE in hexagonal bundles of uniformly heated tubes with helical spacers

High temperature heat exchangers for solar thermal power and alkali liquid metals and lead–bismuth eutectic (LBE) cooled small modular nuclear reactors (SMRs) employ bundles of heated tubes or fuel rods with and without helical spacers for enhancing convection heat transfer. However, applicable Nusselt number correlations are not available and the reported data is relatively scarce. This work reviews, compiles, and correlates the reported experimental data and numerical analysis results for convection heat transfer of sodium, sodium–potassium (NaK) alloys, and lead–bismuth eutectic (LBE) in hexagonal bundles of uniformly heated tubes with helical wire-wrap and finned spacers. The compiled experimental data of fully developed Nusselt number for NaK alloys and LBE is for bundles of 19 and 37 uniformly heated tubes with pitch-to-diameter ratio (P/D) = 1.053–1.2824. It includes 103 Nusselt number values for a wide range of Peclet number (Pe) (9–2560). In addition, the compiled database includes 42 numerical Nusselt number values for liquid sodium and LBE at Pe = 30–1070 in bundles of 7–217 uniformly heated tubes with P/D = 1.194–1.31. The complied database for the bundle average Nusselt Number, Nub,ww, and the Nusselt number in central subchannels, Nuww, is correlated, respectively, as:Nub,ww=[4.387+0.00878(P/D)20.6]+[0.0306-1.647×10-4(P/D)14.03]Pe0.85Nuww=1.515[4.387+0.00878(P/D)20.6]+1.364[0.0306-1.647×10-4(P/D)14.03]Pe0.85Both correlations agree with the majority of the compiled experimental and numerical data to within ±15%. Nuww is consistently lower than Nub,ww, and the difference is independent of P/D, but increases from 38% at low Pe (<100) to as much as 52% at Pe = 1100. Nuww, is also higher than that reported earlier by the authors for bare heated tubes, Nus, with the difference increasing from 42% to 111% with increased Pe from 300 to 1100, and decreased P/D from 1.3 to 1.1.

The Turbo-Fuel-Cell 1.0 – family concept

The “Turbo-Fuel-Cell-Technology” has been described as a MGT-SOFC hybrid system consisting of a recuperated micro gas turbine (MGT) process with an embedded solid oxide fuel cell (SOFC) subsystem. SOFC stacks are connected to “SOFC stack grapes” and are equipped with the so called HEXAR-Module. This module is composed of a high-temperature heat exchanger (HEX), an afterburner (A) and a steam reformer (R). The MGT-concept is based on a generator driven directly by the turbomachine and a recuperator, which returns the exhaust heat to the pressurized compressor outlet air. This provides the necessary base for a highly effective, pure MGT process and the “MGT-SOFC-high-efficiency process”. This paper describes the concept and the thermodynamic background of a highly effective and compact design of the “Turbo-Fuel-Cell 1.0-Family” in the electrical performance class from 100 to 500 kW. The technological state of the system is shown and a rating of the system with comparative parameters is discussed. It becomes visible that all necessary basic technologies should be available and that the technology (for stationary applications) can have the “entry into services (E.I.S.)” in the next 10 years. The MGT-SOFC performance map under different operation conditions is discussed. This article also provides an overview of the research on MGT-SOFC-Systems and the scenario of an energy supply network and a mobile energy conversion of the future introduction.

Numerical simulation of porous media combustion for high temperature heat exchanger

The purpose of this work is developing the numerical 1D model of porous media combustion for investigating porous media burner systems. The software is used to solve energy, mass transfer and chemical reaction equation of the combustion. The operating condition and property parameters, which mainly affect the functions and quality of the industrial burner design, such as the inlet velocity of the reactants, the equivalence ratio, the extinction coefficient and the thermal conductivity of porous media, will be investigated and validated with experimental data. For developing the procedure of experiment, three diameter sizes of porous media materials (5 mm, 10 mm, and 15 mm.) were used. As a result, the developed model will be used as a tool to explore temperature distribution of heat exchange to improve thermal performance and overall efficiency system. Moreover, this knowledge can be applied to design porous media burner systems for uniform temperature distribution operation.

Simulation and Optimization of Metal-foam Tube Banks for Heat Transfer Enhancement of Exhaust Heat Exchangers

An essential issue of waste heat recovery (WHR) from engine’s exhaust gas, which could be effectively purified by modern after-treatment techniques, is the development of high-efficient, compact and lightweight high-temperature gas heat exchanger. Due to its multi-functional advantages, open-cell metal foam shows promising potentials in gas heat transfer enhancement. Thus, it is meaningful to explore the feasibility of this novel structure’s application on engine exhaust gas, by replacing bulky structures outside tubes with metal foam layers. In this paper, the heat transfer enhancement of forced convection in metal foam-wrapped tube banks was numerically studied. The Darcy-Forchheimer-Brinkman momentum model considering viscous loss and inertial loss, and the local thermal non-equilibrium energy model which considers the interstitial heat transfer between fluid and solid were employed for the porous zone. Effects of foam thickness and free stream velocity were analyzed to examine the heat transfer and pressure drop characteristics. A comparative analysis was also conducted with a bare tube bank. Results showed that the heat transfer performance of metal foam tube bank can enhance 1.8 to 2.0 times, measured in terms of the area goodness factor. In an array with fixed foam thickness, the better heat transfer performance is obtained at lower free stream velocities. It was also found that, within the designated fluid velocity range, tube banks with thicker foam layers show higher heat transfer rates and larger pressure drops as well. However, there exists an optimal dimensionless thickness of foam layer at 0.15 which gives the highest area goodness factor for the configuration considered in this study.

Effect of high temperature oxidation on the radiative properties of HfC-based ceramics

Hafnium carbide (HfC) is a promising material for high-temperature heat exchangers due to its high melting point (4170K). Nevertheless, its fast oxidation into fragile hafnia prevents it from being used for open-air solar receivers. The incorporation of silicon-based compounds would reduce its oxidation kinetics and extend the lifetime of HfC-based elements. The oxidation behavior of HfC sintered with the addition of 10vol% TaSi2 or ZrSi2 is here investigated. The optical properties on the as-received surfaces and on the oxidized ones were measured, especially the ratio of the 0.6–2.8μm emissivity (close to the solar absorptivity α) to the total hemispherical emissivity ε. These optical properties are important parameters for applications using concentrated solar radiation.

Novel, cost-effective configurations of combined power plants for small-scale cogeneration from biomass: Design of the immersed particle heat exchanger

This paper aims at proposing a general design procedure for the application of the Immersed Particle Heat Exchanger to a novel, small scale, externally fired combined cycle capable of generating electrical and thermal power from carbon-neutral biomass. The Immersed Particle Heat Exchanger serves as the high temperature heat exchanger needed to couple the Brayton cycle with an external combustor of biomass; it is composed of either one module or more modules, with each module being constituted by two heat exchange columns. The combustion gases and the working fluid (clean air) flow separately in the two columns, and ceramic particles are employed as solid intermediate medium to transfer heat between the two columns. Three particle-handling systems, mainly composed of rotary valves coupled with guillotine valves, are conceived in this paper to move the particles within the heat exchanger; analytical models are developed for the design of these systems and for the evaluation of the associated energy losses. A new architecture employing internal cooling channels is also proposed for the two heat exchange columns. In addition, an optimization design procedure, based on the coupling between a computational fluid dynamic model and a genetic algorithm, is developed to correctly select the number of heat exchanger modules as well as the main project parameters, with application to a combined cycle of 110kWe with an overall efficiency of about 70%. The numerical results show that the project is highly viable. Only a negligible part of the compressed air is lost because of the particle handling systems (about 0.2%); furthermore, the size of the heat exchanger results to be compact, as the selected optimum is characterized by two heat exchanger modules, with each having an overall height, necessary for the heat exchange, of about 4m and a maximum diameter of 1.2m.

Manufacturing of a high-temperature resistojet heat exchanger by selective laser melting

The paper presents the design, manufacturing and postproduction analysis of a novel high-temperature spacecraft resistojet heat exchanger manufactured through selective laser melting to validate the manufacturing approach. The work includes the analysis of critical features of a heat exchanger with integrated converging-diverging nozzle as a single piece element. The metrology of the component is investigated using optical analysis and profilometry to verify the integrity of components. High-resolution micro-Computed Tomography (CT) is applied as a tool for volumetric non-destructive inspection and conformity since the complex geometry of the thruster does not allow internal examination. The CT volume data is utilised to determine a surface mesh on which a novel perform coordinate measurement technique is applied for nominal/actual comparison and wall thickness analysis. A thin-wall concentric tubular heat exchanger design is determined to meet dimensional accuracy requirements. The work indicates the production of fine structures with feature sizes below 200 μm in 316L stainless via selective laser melting is feasible and opens up new possibilities for the future developments in multiple industries.

Cycle analysis of solid oxide fuel cell-gas turbine hybrid systems integrated ethanol steam reformer: Energy management

A solid oxide fuel cell-gas turbine (SOFC-GT) hybrid system that uses such liquid fuels as ethanol is attractive for distributed power generation for applications in remote rural areas or as an auxiliary power unit. The SOFC system includes units that require and generate heat; thus, its energy management is important to improve its efficiency. In this study, a SOFC-GT integrated system with the external steam reforming of ethanol to produce hydrogen for the SOFC is proposed. Two SOFC-GT hybrid systems using a high-temperature heat exchanger and cathode exhaust gas recirculation are considered under isothermal conditions. The effects of key operating parameters, such as pressure, fuel use and turbomachinery efficiency, on the SOFC-GT hybrid system performance are discussed. The simulation results indicate that recycling the cathode exhaust gas from the SOFC-GT system requires less fresh air from the compressor, to maintain the SOFC stack temperature, and the heat recovered from the SOFC system is sufficient to supply both the fuel processor and air pre-heater. In contrast, an external heat is needed for the SOFC-GT system coupled to a recuperative heat exchanger.

Thermodynamic simulation of a multi-step externally fired gas turbine powered by biomass

A thermodynamic model for a realistic Brayton cycle, working as an externally fired gas turbine fueled with biomass is presented. The use of an external combustion chamber, allows to burn dirty fuels to preheat pure air, which is the working fluid for the turbine. It also avoids direct contact of ashes with the turbine blades, resulting in a higher life cycle for the turbine. The model incorporates a high temperature heat exchanger and an arbitrary number of turbines and compressors, with the corresponding number of intercoolers and reheaters. It considers irreversibilities such as non-isentropic compressions and expansions, and pressure losses in heat input and release. The composition and temperature of the combustion gases, as well as the variable flow rate of air and combustion gases, are calculated for specific biomasses. The numerical model for a single stage configuration has been validated by comparing its predictions with the data sheets of two commercial turbines. Results are in good agreement. Curves on the dependence of thermal efficiency and power output with the overall pressure ratio will be shown for several plant configurations with variable number of compression/expansion stages. Also the influence of different types of biomasses and their moisture will be analyzed on parameters such as fuel consumption and exhaust gases temperature. For a single step plant layout fueled with eucalyptus wood an efficiency of 23% is predicted, whereas for a configuration with two compressors and one turbine efficiency increases up to 25%. But it is remarkable that the latter leads to a 29% increase in power output.

Heat transfer, pressure drop and structural analysis of a finned plate ceramic heat exchanger

High temperature heat exchangers (HTHE) constructed with ceramics can achieve higher temperatures of operation. Resistance to oxidation is the great advantage of using ceramics for this application. This paper presents experimental characterization of one ceramic heat exchanger composed of finned plates operating at temperatures as high as 800 °C and Reynolds number between 170 and 2000. The heat exchanger operates in counter-flow with air in both sides. The plates were constructed using alumina (Al2O3) with the Gelcasting technique. Thermal performance was obtained in the form of Colburn and friction factors as a function of Reynolds number. The heat exchanger effectiveness varied between 0.620 and 0.901. Progressively higher temperature was imposed to the heat exchanger prototype to cause structural failure. In addition, the design and structural integrity assessments were carried out using refined finite element computations based on real experiments regarding fracture resistance of the employed ceramic. Thermal performance of the ceramic heat exchanger is adequate and predictable using CFD simulations, but guarantee structural integrity remains challenging.