Heat Exchanger Materials Research Articles

열교환기용 스테인레스강 열화 용접부의 고온 인장 및 피로특성

Purpose: This study aims to determine the most suitable material for heat exchangers by analyzing the tensile and fatigue strengths of different types of stainless steel, i.e., AISI 310S, AISI 316L, and AISI 347 after welding and aging at both room and high temperatures. BRMethods: Tensile and fatigue tests were performed on three types of stainless steel for heat exchangers using samples aged at 800℃ for 1000 hours and samples that were welded and aged at both room temperature and 800℃. The microstructures of the base material and aged samples were observed using an optical microscope and SEM. The fatigue tests were conducted using a stress ratio of 0.1 and a stress constant test; the fracture surface was then observed using SEM.BRResults: Chromium carbides tend to precipitate more in aged stainless steel samples than in the base material. It was found that AISI 316L stainless steel has the best elongation and fatigue strength at room temperature and 800℃.BRConclusion: In terms of aged and welded samples, AISI 316L stainless steel has the highest elongation and endurance limit. In particular, it has the smallest amount of chromium carbide precipitation at 800℃. In conclusion, it is determined to be an excellent heat exchanger material in terms of weldability, formability, and durability.

Fireside performance of different coatings in biomass power plant

ABSTRACT The energy sector will need to employ novel strategies to reduce greenhouse gas emissions, such as the increase of steam temperatures/pressures or the use of low carbon fuels (i.e. biomass). Both cause heat exchanger materials’ degradation issues, due to the formation of more/different corrosive deposits, which requires the use of expensive nickel-based materials or coatings. This paper focuses on the behaviour of three different coatings (HVOF NiCrFeSi, laser clad FeCrAl and Laser Clad NiCrFeSi) deposited on TP347HFG, at 700°C (up to 1000 h exposure). Tests were performed using the ‘deposit recoat’ method (KCl deposit) in simulated biomass combustion environments. Cross-sections were analysed using dimensional metrology, to determine distributions of metal loss and internal damage. Intergranular damage and pits were identified using SEM/EDX. A ‘diffusion cell’ behaviour was observed, which led to depletion of alloying elements from the coating and consequent increase in damage. The results suggested a severe degradation of all coatings.

Comprehensive analysis of thermophysical properties of a binary stearyl alcohol and benzamide eutectic phase change material and determination of thermal performance of a solar dryer integrated with phase change material

AbstractA thermal energy storage system must integrate with a solar thermal collector for continuous energy delivery. This article focuses on the identification, preparation, thermo‐physical characteristic studies, and experimental investigation of a novel eutectic hot storage material for solar dryer applications. The identified organic novel eutectic mixture comprises stearyl alcohol and benzamide (SA‐B) with a molar ratio of 70:30. From the differential scanning calorimetry analysis, the melting point and enthalpy of the developed eutectic mixture were found to be 54. 26°C and 157.45 kJ kg−1 respectively. The melting temperature of the eutectic phase‐change material (PCM) fits the drying application. The corrosion analysis test confirms that the prepared eutectic mixture is compact with stainless steel (SS 316) heat exchanger material. Further, the eutectic PCM were characterized using Fourier transform infrared spectroscopy and thermo‐gravimetric analysis, which confirms the material is chemically and thermally stable. The thermal reliability of the eutectic PCM was evaluated through 1000 thermal charge and discharge cycling. In addition, the thermal conductivity of the identified material was found to be 0.18 W m−1 K−1. Thus, the specified material ascertained its suitability for thermal application. Further, 5 kg of eutectic PCM‐filled rectangular heat exchanger was integrated into the solar dryer. The use of eutectic PCM extended the drying time by more than 4 h, which can be helpful to run continuously on intermittent solar days and in the evening after sunset. Hence, the prepared eutectic SA‐B material is irresistible for hot storage dryer application.

Numerical modeling of the effects of the radial and axial location of added micro-encapsulated phase change materials in vertical borehole heat exchangers

A numerical model of a vertical borehole heat exchanger (BHE) with micro-encapsulated phase-change material (PCM) was used to examine the thermal effect of the radial and axial location of the added PCM within the borehole. In the radial direction, PCM could be placed at the center of the borehole, at the borehole periphery, or interspersed within the grout. In the axial direction, PCM could be concentrated either in the top, bottom, or mid portions of the borehole grout, or again interspersed evenly. PCM could also be added to the borehole heat transfer fluid. Each location has its own thermal, and therefore economic, advantages and disadvantages, and thus this present work investigated these effects toward decreasing borehole length. Analysis of results showed that the best radial location for the PCM is intermixed within the grout, even though that configuration has a disadvantage of increasing the overall borehole thermal resistance. In the axial direction, it was found that the thermal behavior of the PCM varied over the borehole length, and thus adding the PCM-grout mixture to only the bottom half-depth of the borehole reduced the thermal benefit by only about 19%. Thus, it is concluded that strategic placement of PCM within borehole grout can significantly improve the economics of its use in vertical GCHP systems, but at current costs of PCM, its use in borehole grouts remains an economic challenge.

Surface durability during high-speed spray impingement onto heat-exchanger materials and modified surfaces

Surface durability was investigated for materials used in electronics and heat exchangers during water spray impingement at ambient conditions. Smooth, rough, and modified copper and aluminium samples were tested with surface hardness, roughness and contact angle ranging from 52–100 HV, 0.53–79.28 μm and 48-153°, respectively. Although material loss and hardness alterations were insignificant after the high-speed droplet collisions, changes in surface morphology were observed. Both smooth and rough surfaces were resistant to 36-hour sprays with droplet velocities from 11–22 m s−1. However, sprays did cause changes in surface roughness and static contact angle, and the water repellent coatings were damaged, losing their low wettability.

Recent Progress on High Temperature and High Pressure Heat Exchangers for Supercritical CO2 Power Generation and Conversion Systems

Heat exchangers for supercritical CO2 power generation and waste heat to power conversion systems have a significant impact on the overall cycle efficiency and system footprint. Key challenges for supercritical CO2 heat exchangers include ability to withstand high temperature and high pressure (typical temperature range of heat source 350 to 800 °C and typical required operating pressure range 150 to 300 bars), and large pressure differential between fluid streams. Other requirements are low pressure drop, high effectiveness and high reliability under thermal cycling. This paper presents recent developments in supercritical CO2 heat exchangers in terms of material selection, design, manufacture, and operation. Since heat exchangers represent a significant portion of the total system cost, another key challenge is to find a compromise between the heat exchanger type, cost, durability, and performance. This paper explores heat exchanger technologies, manufacturing techniques and materials for high temperature and high pressure heat exchangers for supercritical CO2 applications. It also identifies technology gaps and research needs to accelerate the development of effective designs to facilitate the commercialization of both supercritical CO2 heat exchanger technologies and power cycles.

Experimental Study on the Transient Behaviors of Mechanically Pumped Two-Phase Loop with a Phase Change Energy Storage Device for Short Time and Large Heat Power Dissipation of Spacecraft

For the purpose of dissipating large heat power with cyclical operating modes of satellite, one mechanically pumped two-phase loop (MPTL) coupled with a novel phase change energy storage device was designed and constructed. The phase change energy storage device integrating with filament tube heat exchanger and form-stable phase change material (PCM) with expanded graphite (EG) was designed and employed to increase the conductivity. The two-phase change behavior of liquid-vapor change for MPTL and solid-liquid transition for PCM was used to acquire, transport and store the heat. Results indicated that the time of heat storage for PCM device was more than 598.0 s, and the temperature at the outlet of the device increased from −2.0 °C to 15.0 °C under the heating power of 6.7 kW. The flowrate decreased from 0.059 kg/s to 0.034 kg/s due to the length increase of two-phase section during the melting process of PCM device. Lowering the rotation speed of pump increased the oscillating amplitude of flowrate, and its maximum value reached to 0.008 kg/s. The total working period for PCM device for lower rotation speed of 8000 r/min was 48.0 s longer than that for higher speed of 10000 r/min due to decreased heat transfer coefficient.

Residual stresses of explosively welded bimetal studied by hard X-ray diffraction

Geothermal heat from the Earth`s crust is a source of natural and renewable energy. This energy can be extracted and used for generating electricity and heating of houses in the winter months. However, in order to extract energy from a well, we need to use material that can sustain contact with geothermal steam and is resistant to corrosion of the geothermal fluid and non-condensing gases such as hydrogen sulfide (H2S) and carbon dioxide (CO2), chloride ions (Cl−), and hydrogen fluoride (HF). An interesting alternative to today's materials are bimetals, composed of two different materials where the layer in contact with the aggressive environment is made of a noble material, while the outer layer (typically low-carbon steel) strengthens the composite and additionally provides good weldability.This paper presents the microstructure, phase composition, and distribution of residual stresses of the bimetallic system nickel-chromium-molybdenum alloy (Alloy 625) cladded on the ferritic pressure vessel steel P355NH base material.The bimetal has been prepared by explosion welding and is its use is geared for transport of highly corrosive media and as a material for heat exchangers, condensers, etc.

Nanosecond fiber laser machining of A4SS under different environments: A comparative study

A4 stainless steel (SS) is a suitable metallic material for heat exchangers due to its inherent resistance to harsh environments and a usually difficult to process due to its high strength and superior plasticity. However, many sophisticated machining strategies exist to compete with the material's responsiveness while generating features in the micro-domain. Out of those, short-pulsed fiber laser (SPFL) is the most eminent choice due to its less arduousness. In this research laser micro-machining process (LMMP) is adopted for producing micro-channels on A4SS with the assistance of SPFL. The designed micro-channel structure holds its applicability in heat exchanger applications. Being a thermal process, the current LMMP may have high machining capability regardless of material properties, but at the same time, it has some adverse effects on machined surfaces. This research attempts to scrutinize the impact of SPFL variables on the channel surface’s characteristics by utilizing different characterization equipment. The SPFL-processed surfaces were investigated as a distinct industrial potential in dry, liquid, active, and inactive gas environments by adjusting the crucial variables of the SPFL. Further, the mechanical property, surface, and sub-surface analysis (including the study of functional groups, phases, morphology, and chemical elements) have also been performed to learn more about the varied environmental implications during LMMP-A4SS interactions.

Dimensioning and efficiency evaluation of a hybrid photovoltaic thermal system in a tropical climate region

This paper analyses the performance of photovoltaic thermal (PVT) panels in tropical climate regions. In PVT systems, a heat exchanger is coupled to conventional photovoltaic (PV) panels, making possible to improve the efficiency of the PV panels and to provide heat: The decrease in the PV cell temperature increases the electrical power produced, whereas the heat absorbed can be used in low temperature applications, such as water heating or air conditioning. In this research paper, the main factors that affect PVT systems performance are deeply studied in order to choose the most suitable set of parameters for a tropical climate operation based on technical and economic aspects. The combined use of Digital logic and merit index methods are applied to the heat exchanger material selection. The performance analysis of the other constructive aspects and operational variables are conducted through a numerical simulation using the thermal resistance model solved by trust region method via a class of dogleg paths and validated by experimental results. Using real weather data, the performance is compared with respect to a conventional PV system over a full day period. The results have shown a significant improvement in efficiency: In the worst case, the 12.53% average electrical efficiency of the PV system is increased to 29.06% for a PVT system (12.55% electrical plus 16.51% thermal efficiency). In the best case, an average overall efficiency of 41.83% was achieved by PVT, demonstrating the ability of PVT to increase the energy capture from solar radiation and showing that the performance is improved by selecting carefully those factors.

Comparison of the Efficiency of Cross-Flow Plate Heat Exchangers Made of Varied Materials

This paper discusses a mathematical model for airflow through a cross-flow plate heat exchanger. The exhaust air is used to heat the supply air. Three kinds of plates are considered: made of aluminium, copper, and steel. The purpose of this research was to verify which material used to build the plate heat exchangers uses the exhaust air heat more efficiently. The method of the Trefftz function was used to determine approximate solutions to the analysed problem. The results obtained for 1.2 mm-thick plates and for external winter, summer, and spring–autumn temperatures are discussed. The results indicate that if the efficiency and price of the metals are considered, then steel is the best material for the plate heat exchanger. Thanks to the use of thin steel plates and the reduction in air exchange time to a few minutes, a cheap and efficient cross-flow heat exchanger can be obtained.

Enhancement of thermal energy storage in a phase change material heat exchanger having elliptical and circular tubes with & without fins

The objective of this study is to examine the thermal performance of a heat exchanger having multiple elliptical tubes and a phase change material (PCM) filled in the cylindrical shell of a heat exchanger. To improve the heat transmission from elliptical tubes to PCM, two and four fins are incorporated on the circumference of the elliptical tubes. The use of elliptical tubes with fins in the enhancement of heat transfer is an innovative approach in thermal performance studies of heat storage systems. The geometry of heat exchangers having elliptical tubes without fin, with two fins, and four fins are compared with the geometry of heat exchanger with circular tubes having no fin, two fins, and four fins respectively. The current investigation is carried out with the use of a 2D CFD model using a physical enthalpy-porosity technique to explore the phase transition of the solid PCM. The effect of integrating a number of fins on heat transfer has also been explored, and outcomes of elliptical and circular tube geometries in terms of PCM mean temperature and liquid percentage at two different temperatures of 50 °C and 60 °C have been compared. Results show, that PCM melting time is being reduced to 550 s in PCM heat exchanger geometry having elliptical tubes with four fins compared to 585 s for the same heat exchanger with two fins and 665 s for heat exchanger without fin at 50 °C resulting 12 % & 17 % reduction in melting time by applying two and four fins respectively. Whereas PCM melting time is 515 s in PCM heat exchanger geometry having circular tubes with four fins compared to 560 s for heat exchanger with two fins and 655 s for heat exchanger without fin resulting 14.5 % and 21 % reduction in melting time by applying two and four fins respectively. It has been concluded that adding fins on the circumference of elliptical and circular tubes enhances the heat transmission and reduces melting time as well as more heat can be transferred by circular tube geometry compared to elliptical tube heat exchanger geometries. The findings of the investigations are validated with the available simulation data and show good agreement, suggesting a 3.8 percent deviation.

Temperature-Dependent Molten Chloride Salt Corrosion of Nickel Alloys

As next-generation clean and renewable energy technologies like concentrating solar power (CSP) systems and molten salt-cooled reactors (MSR) operate at ever higher temperatures (HT) to increase efficiency, their structural material and molten salt selection becomes increasingly important. To understand the corrosion behavior of candidate alloys to be used as materials for storage tanks, heat exchangers, and piping, seven alloys (Hastelloy N, Haynes 242, Haynes 214, Hastelloy C-276, Haynes 230, Haynes 224, Haynes 244) are immersed in purified 45.8 MgCl2- 38.5 KCl- 15.7 NaCl wt% chloride salt for 100 hours at 650°, 725°, and 800°C. After the exposure tests, the alloys are analyzed using SEM and EDS to determine the extent of salt infiltration and primary corrosion mechanisms. In addition, room-temperature XRD analysis provides quantitative information about the alloys’ corrosion products. These findings contribute important insight for molten salt energy system communities.

Measuring the maximum capacity and thermal resistances in phase-change thermal storage devices

Thermal energy storage can increase the efficiency of the electric grid by adding flexibility to thermal systems. The value of thermal storage is a function of its energy and power density, which are driven by the capacity and thermal resistances in the storage device. Measuring these properties in-situ at the device level is an important step to understanding the performance and improving the design of thermal storage systems. In this paper, we present methods to measure the total capacity and thermal resistances in heat exchangers with integrated phase change materials. These methods are demonstrated on two thermal storage devices—a 570-kWh ice-based storage tank and a 0.35-kWh graphite-tetradecane composite device. The results show how thermal resistances evolve with the state of charge and discharge rate in these devices and quantify the impact of applied pressure on the contact resistance in composite phase change material heat exchangers. The proposed method allows for easy comparison between different systems and provides information on the thermal bottlenecks limiting performance. Ultimately, these measurements will allow designers to make robust, high-performance thermal storage devices for next-generation thermal systems.

Design for energy flexibility in smart buildings through solar based and thermal storage systems: Modelling, simulation and control for the system optimization

The present study investigates the use and implementation of energy efficient measures and strategies for building applications, toward the Nearly Zero Energy Buildings target. Specifically, objective of the study is to implement building integrated photovoltaic thermal devices coupled with a phase change materials heat exchanger acting as an active thermal storage building component, with the aim to add flexibility to the building while still maintaining indoor comfort conditions. To show the potentials of the novel configuration proposed in this paper, a multi-zone grey-box model is developed and validated to capture the thermal dynamics of a building, and a control strategy applied to the whole system is developed for energy management purpose. The whole simulation model, including thermophysical properties of the building-system and the control features, is implemented in a MATLAB environment. To assess the model and application potentials toward the optimal design and operation of the proposed system for energy efficiency and flexibility goals, a suitable case study analysis is conducted. Thus, a sensitivity analysis, using an evolutionary algorithm, is performed by considering economic and energy objective functions which focuses on the reduction of the building energy demand, load variability and economic aspects. In this regard, the optimal design configuration is underlined in a way that the operation of the components can be maximized to provide flexibility to the building: in average working conditions one single layer of PCM can provide around 186.3 Wh/K per unit of temperature and width. A rule-based management strategy is proposed to prove the possibility to shift and shave the energy peaks during high energy request periods, demand response events. Finally, by considering an approximate economic calculation, the simple payback, taking into account only the positive effects on the winter management, is around 13.5 years.

The Effect of Microencapsulated PCM Slurry Coolant on the Efficiency of a Shell and Tube Heat Exchanger

This paper describes the results of experimental studies on heat transfer in a shell and tube heat exchanger during the phase changes of the HFE 7000 refrigerant. The studies were performed using a mixture of water and a microencapsulated phase change material as a coolant. HFE 7000 refrigerant condenses on the external surface of the copper tube, while a mixture of water and phase change materials flows through the channels as coolant. Currently, there is a lack of research describing cooling using phase change materials in heat exchangers. There are a number of publications describing the heat exchange in heat exchangers during phase changes under air or water cooling. Therefore, the research hypothesis was adopted that the use of mixed water and microencapsulated material as a heat transfer fluid would increase the heat capacity and contribute to the enhancement of the heat exchange in the heat exchanger. This will enable an increase in the total heat transfer coefficient and the heat efficiency of the exchanger. Experimental studies describe the process of heat transfer intensification in the above conditions by using the phase transformation of the cooling medium melting. The test results were compared with the results of an experiment in which pure water was used as the reference liquid. The research was carried out in a wide range of refrigerant and coolant parameters: ṁr = 0.0014–0.0015 kg·s−1, ṁc = 0.014–0.016 kg·s−1, refrigerant saturation temperature Ts = 55–60 °C, coolant temperature at the inlet Tcin = 20–32 °C, and heat flux density q = 7000–7450 W·m−1. The obtained results confirmed the research hypothesis. There was an average of a 13% increase in the coolant heat transfer coefficient, and the peak increase in αc was over 24%. The average value of the heat transfer coefficient k increased by 5%, and the highest increases in the value of k were noted at Tin = 27 °C and amounted to 9% in relation to the reference liquid.

A thermodynamic model for high temperature corrosion applications: The (Na2SO4 + K2SO4 + ZnSO4 + PbSO4) system

The (Na2SO4 + K2SO4 + ZnSO4 + PbSO4) salt system is important for deposit buildup, fouling and associated corrosion of metallic structural and heat exchanger materials of municipal solid waste incinerators, low-grade fuel firing boilers, as well as metallurgical heat recovery boilers. In this study, a complete critical evaluation of all available phase diagram and thermodynamic data has been performed for all condensed phases of the (Na2SO4 + K2SO4 + ZnSO4 + PbSO4) system, and optimized model parameters have been found. The Modified Quasichemical Model for short-range ordering was used for the molten salt phase while most of the relevant solid solutions were modeled using the Compound Energy Formalism. The (Na2SO4 + K2SO4) sub-system has been critically evaluated in a previous article. A (60 mol% PbSO4 + 40 mol% ZnSO4) binary mixture was investigated by DTA-TGA since no thermodynamic data were available in the literature for this particular sub-system. The model parameters obtained for the binary and ternary sub-systems of the (Na2SO4 + K2SO4 + ZnSO4 + PbSO4) system can be used to predict thermodynamic properties and phase equilibria for the multicomponent system. In particular, ternary and quaternary eutectics (corresponding to the formation of a liquid phase at low temperatures) may be predicted, thus permitting to avoid “catastrophic” corrosion. This work is part of a large thermodynamic model for the Na+, K+, Zn2+, Pb2+ // Cl−, SO42− system.

The deeper the better? A thermogeological analysis of medium-deep borehole heat exchangers in low-enthalpy crystalline rocks

The energy sector is undergoing a fundamental transformation, with a significant investment in low-carbon technologies to replace fossil-based systems. In densely populated urban areas, deep boreholes offer an alternative over shallow geothermal systems, which demand extensive surface areas to attain large-scale heat production. This paper presents numerical calculations of the thermal energy that can be extracted from the medium-deep borehole heat exchangers in the low-enthalpy geothermal setting at depths ranging from 600 to 3000 m. We applied the thermogeological parameters of three locations across Finland and tested two types of coaxial borehole heat exchangers to understand better the variables that affect heat production in low-permeability crystalline rocks. For each depth, location, and heat collector type, we used a range of fluid flow rates to examine the correlation between thermal energy production and resulting outlet temperature. Our results indicate a trade-off between thermal energy production and outlet fluid temperature depending on the fluid flow rate, and that the vacuum-insulated tubing outperforms a high-density polyethylene pipe in energy and temperature production. In addition, the results suggest that the local thermogeological factors impact heat production. Maximum energy production from a 600-m-deep well achieved 170 MWh/a, increasing to 330 MWh/a from a 1000-m-deep well, 980 MWh/a from a 2-km-deep well, and up to 1880 MWh/a from a 3-km-deep well. We demonstrate that understanding the interplay of the local geology, heat exchanger materials, and fluid circulation rates is necessary to maximize the potential of medium-deep geothermal boreholes as a reliable long-term baseload energy source.

Integrating phase change material in building envelopes combined with the earth-to-air heat exchanger for indoor thermal environment regulation

To utilize the combined effects of earth-to-air heat exchanger (EAHE) and phase change material (PCM) in indoor thermal environment regulation during summer, this study proposes a combination mode, i.e., EAHE–PCM. In this method, EAHE continuously provides cool air to indoor spaces. PCM integrated into building envelopes stores excess cold provided by EAHE at night and transfers it indoors across time. We performed two full-scale comparative experiments (EAHE–PCM combined operation mode and PCM alone operation mode) on typical summer days in hot-summer and cold-winter regions. The results showed that the maximum reduction in the EAHE outlet air temperature and moisture content reached 21.6 °C and 4.55 g/kg, respectively, inducing an average cooling capacity of approximately 1186.5 W. The use of EAHE decreased the time-averaged PCM internal temperature by 6.0 °C in 24 h compared to the PCM alone operation mode, thereby activating the phase change process of PCM between 19:54 and 9:45. Therefore, PCM can store excess cold provided by the EAHE via latent heat and transfer it to indoor air through the melting process, resulting in a 157 min lag in the PCM internal temperature rise. When the ambient air temperature varied between 26.3 and 45.2 °C, the EAHE–PCM combined operation mode decreased the time-averaged indoor air temperature by 6.6 °C compared to the PCM operation alone, which helped maintain the indoor air temperature within the comfort temperature range of 22–28 °C for 73% in 24 h, with a peak temperature of 30.6 °C.

Tritium permeation from tritiated water to water through Inconel

From a safety standpoint, it is important to understand the behavior of tritium in the coolant of DT fusion reactors. The high-temperature and high-pressure water is expected to be used as a coolant in JA DEMO. Therefore, it is necessary to deepen the understanding of the tritium transfer from the primary cooling water to the secondary cooling water in a heat exchanger. In this study, a double-tube permeation device consisting of Inconel 600 tube, which is a heat exchanger material, and SS316 tube, was assembled and the tritium permeation from tritiated water to water at 300 °C and 17 MPa was observed. Remarkable tritium permeation was observed after around 17 days as the cumulative heating time. Gaseous tritium (HT and T2) was detected in the gas phase of the tritiated water side after the permeation experiments. This suggests that a part of tritium generated in the metal oxidation reaction dissolved in the Inconel and diffused and permeated. With assuming that the diffusion process of tritium through the Inconel was the rate controlling step of permeation, the permeability of tritium through the Inconel tube from the tritiated water derived from the experimental data was 3 orders of magnitude smaller than that obtained from the hydrogen isotope permeation experiments from gas phase to gas phase.