Metal Heat Exchangers Research Articles

An additively manufactured metallic manifold-microchannel heat exchanger for high temperature applications

This work presents an additively manufactured manifold-microchannel heat exchanger made of Inconel 718 and experimentally tested for high temperature aerospace applications. The heat exchanger core with a size of 66 mm × 74 mm × 27 mm was fabricated as a single piece through the direct metal laser sintering process. A minimum fin thickness of 180 μm was achieved. Successful welding of additively manufactured headers with the heat exchanger core and conventionally manufactured flanges was demonstrated through the fabrication of the unit. The heat exchanger was tested using nitrogen (N2) on the hot-side and air on the cold-side as the working fluids. The experimental tests were conducted at 600 °C on the hot-side and 38 °C on the cold-side. A maximum heat duty of 2.78 kW and a maximum overall heat transfer coefficient of 1000 W/m2K were achieved during the experiments. The decent agreement between the experimental and the numerical results demonstrates the validity of the numerical analysis model used for heat transfer and pressure drop prediction of the additively manufactured manifold-microchannel heat exchanger. Compared to conventional plate fin heat exchangers, nearly 25% improvement in heat transfer density— the ratio between heat duty and mass (Q/m)—was noted at a coefficient of performance (COP) of 62.

Review of Heat Exchangers Enabled by Polymer and Polymer Composite Additive Manufacturing

ABSTRACTThis paper presents an overview of the most common polymer additive manufacturing processes, including vat photopolymerization, material jetting, sheet lamination, powder bed fusion, and fused filament fabrication. Next, the general strengths and challenges of the common methods are discussed and are followed by discussions on efforts to increase thermal performance of polymers used with the various manufacturing methods. Finally, heat exchangers enabled by polymer additive manufacturing are reviewed to show that novel designs in metal, ceramic, and polymer heat exchangers can be made possible by the unique properties of polymers and the advantages offered by additive manufacturing. By reporting what has been proven possible—and the associated challenges—we hope to stimulate the community and further development in a burgeoning field enabled by polymer additive manufacturing.

Effect of Sulphur on the Oxidation Behaviour of Possible Construction Materials for Heat Exchangers in Oxyfuel Plants in the Temperature Range 550–700 °C

During oxyfuel combustion metallic heat exchangers are subjected to service environments which substantially differ from those prevailing during the conventional air firing process. In the present study the behaviour of three selected construction materials (P92, super S304HCu and alloy 617) during exposure in simulated oxyfuel gas with and without addition of SO2 at temperatures between 550 and 700 °C has been investigated. The alloy microstructure and the corrosion products formed during exposures up to 1000 h were studied by SEM/EDX and correlated with gravimetric data collected during the discontinuous exposures. It was found that the behaviour of the martensitic steel was hardly affected by the presence of SO2; however, in the case of the austenitic steel S304HCu the SO2 suppressed internal oxidation occurring at 650 °C in the SO2-free gas, thus promoting formation of a protective chromium-rich oxide. In the case of the nickel base alloy 617 the SO2 addition increased the corrosion rates at 550 and 650 °C due to replacement of the external chromia scale by a multiphase scale with sulphur-containing surface nodules. At 700 °C the alloy formed a chromia base surface scale and SO2 addition suppressed the formation of volatile Cr species. The results are explained using classical oxidation theory related to conditions for external scale formation in combination with thermodynamic considerations of phase stability as well as relative rates of adsorption of various gas species.

Study of Conjugate Heat Transfer in Electromagnetic Liquid Metal Dream Pipe

Abstract The combined effect of conjugation, external magnetic field and oscillation on the enhancement of heat transfer in the laminar flow of liquid metals between parallel plate channels is analyzed. In order to make our results useful to the design engineers, we have considered here only the wall materials that are widely employed in liquid metal heat exchangers. Indeed, all the results obtained through this mathematical investigation are in excellent agreement with the available experimental results. The effective thermal diffusivity κe is increased by 3 · 106 times due to oscillation and that the heat flux as high as 1.5 · 1010 (W/m2) can be achieved. Based on our investigation, we have recommended the best choice of liquid metal heat carrier, wall material and its optimum thickness along with the optimum value of the frequency to maximize the heat transfer rate. At the optimum frequency, by choosing a wall of high thermal conductivity and optimum thickness, an increase of 19.98% in κe can be achieved. Our results are directly relevant to the design of a heat transfer device known as electromagnetic dream pipe which is a very recent development.

Pilot-scale study on water and latent heat recovery from flue gas using fluorine plastic heat exchangers

Over the years, in order to enhance the thermal efficiency, coal-fired power plants have witnessed great improvements in water and waste heat recovery. This paper aims to investigate the water vapor and latent heat recovery from the low temperature flue gases exiting a limestone-gypsum wet desulfurization system. A pilot-scale testing system mainly composed of a two-stage fluorine plastic heat exchanger (FHE) was built in a 660 MW lignite-fired power plant in northern China. The additional FHE configuration was first developed and used for water and latent heat recovery based on the condensation mechanism of water vapor. Results showed that the FHE can overcome the shortcomings of the conventional metal heat exchanger, especially when flue gas temperature is below the acid dew point. The recovered latent heat accounted for more than 80% of the total heat recovered from low temperature flue gases, and the heat supply of 92 MWth was achieved. The overall heat transfer coefficient of the FHE reached up to 275 W/(m2•K). The water recovery efficiency increased with an increase in cooling water velocity and a decrease in flue gas velocity. The water recovery efficiency higher than 70% was reached when the flue gas velocity was less than 5 m/s and the cooling water velocity was greater than 0.24 m/s. The highest water recovery efficiency of the two-stage FHE was 85%, suggesting that this technology has a greater application potential in all industrial processes. The recovered water mass from the flue gas by condensation was nearly equal to, or even exceeded, the equivalent mass of supplemental water for the desulfurization system. The good water recovery performance of the FHE device implied that zero water consumption of the desulfurization system could be achieved, which is of great significance for power plants, especially in water scarce regions. This technology will offer an innovative and feasible solution to resolve the issue of water and heat recovery in the highly corrosive conditions.

Performances of MOF vs. zeolite coatings in adsorption cooling applications

The performances of zeolite and metal-organic framework (MOF) coatings on metal heat exchanger surfaces were determined for various operating conditions of adsorption cooling systems. A mathematical model was used to calculate the optimum coating thicknesses of the adsorbents, maximizing the power of the system. The highest optimum coating thickness and power values were generally obtained for the hydrophilic triazolyl phosphonate MOF, which seemed to be superior to the different types of zeolites investigated. The power provided by zeolite LiX coatings was about 10–20% higher than that obtained for zeolite NaX coatings. Although zeolite X has quite high water adsorption capacity similar to that of triazolyl phosphonate MOF, its coatings did not perform so well. This seemed to originate mainly from the relatively slow diffusion in zeolite X, especially at the lower temperatures. The coating texture should also be taken into account in the selection of suitable materials for adsorption heat pumps/cooling systems since open coatings may lead to significant improvements in the performances of these devices.

Enameled Glass Panels for Solar Thermal Building Envelopes

The paper presents a novel concept of solar thermal panel specifically intended for building integration, aiming at a higher architectural quality and at a reduction of installation costs. The panel consists of a low-emissivity enameled flat glass as solar absorber and a metallic heat exchanger, which are glued together by an adhesive layer. It features high design flexibility and can be used as roof or façade cladding in combination with common frames and profiles. We analyze the potential of the panel both as uncovered and covered collector by means of efficiency measurements on large-sized prototypes according to ISO 9806. Our results show that panels equipped with black enameled glass can achieve performance values competitive with those of commercial available products (uncovered panel: η0 = 0.75, b1 = 8.05W/m2K, b2 = 1.64J/m3K, bu = 0.043 s/m; covered panel: η0 = 0.74, a1 = 4.26W/m2K, a2 = 0.013W/m2K2). As reported by our optical measurements on small samples, colored glass can exhibit solar absorptance up to 0.93, thus representing an aesthetically appealing alternative to black panels. For its implementation, system integration, operating conditions and design aspects have to be taken into consideration.

Experimental investigations of polymer hollow fibre heat exchangers for building heat recovery application

Due to low cost, light weight and corrosion resistant features, polymer heat exchangers have been extensively studied by researchers with the aim to replace metallic heat exchangers in a wide range of applications. Although the thermal conductivity of polymer material is generally lower than the metallic counterparts, the large specific surface area provided by the polymer hollow fibre heat exchanger (PHFHE) offers the same or even better heat transfer performance with smaller volume and lighter weight compared with the metallic shell-and-tube heat exchangers. This paper presents the construction and experimental investigations of polypropylene based polymer hollow fibre heat exchangers in the form of shell-and-tube. The measured overall heat transfer coefficients of such PHFHEs are in the range of 258–1675W/m2K for water to water application. The effects of various parameters on the overall heat transfer coefficient including flow rates and numbers of fibres, the effectiveness of heat exchanger, the number of heat transfer unit (NTU), and the height of transfer unit (HTU) are also discussed in this paper. The results indicate that the PHFHEs could offer a conductance per unit volume of 4×106W/m3K, which is 2–8 times higher than the conventional metal heat exchangers. This superior thermal performance together with its low cost, corrosive resistant and light weight features make PHFHEs potentially very good substitutes for metallic heat recovery system for building application.

Analytical solutions for heat transfer efficiency in metallic honeycombs using two-equation method

An analytical model based on corrugated wall using two-equation method is proposed to evaluate the heat transfer performance in the sandwiched metallic honeycomb heat exchanger under forced convection conditions. The local thermal non-equilibrium between cooling fluid and solid honeycombs is taken into account compared with corrugated wall model, effective medium model, and transfer matrix model. A general analytical solution carried out using introduced effective specific surface area to overcome the fin efficiency underestimation and corrugated wall overestimation shows extensive applicability in predicting the thermal performance index, which can be restated to represent corrugated wall model, transfer matrix model, and the effective medium model, respectively.

Design and Investigation into the Thermal and Mechanical Performance of a Polymer Composite Prototype Gas-Liquid Heat Exchanger

Seawater-cooled metallic heat exchangers used in natural gas processing are prone to corrosion and fouling, resulting in increased operational and maintenance costs. A lab-scale polymer composite gas-liquid webbed tube bank heat exchanger is designed and evaluated for application in conditions representative of a fielded natural gas processing application. The heat exchanger thermal performance and structural integrity are investigated numerically using computational fluid dynamics (CFD) and finite element (FE) models, respectively. For polymer composite thermal conductivities above 20 W/m-K, in forced gas-side convection, the exchanger heat transfer rate is comparable to that of a high conductivity conventional metallic heat exchanger having the same geometry, at reduced materials, manufacturing and operational costs. In addition, the prototype heat exchanger would be structurally reliable at the maximum envisaged gas-side operating pressure for the application considered.

Recent Experience with Metallic Heaters for Phosphoric Acid Evaporation

The material used for phosphoric acid heaters is in many cases graphite, despite the many advantages of choosing a metallic material. The main reasons for selecting graphite is the material's good corrosion resistance and the tradition of using graphite. The corrosiveness of phosphoric acid during the wet-process concentration is complex and is dependent on several factors such as the presence of chlorine, fluorine, and diluted sulphuric acid. Metallic materials such as Sandvik Sanicro™ 28 can be used for this process with similar or even better results than graphite. This paper refers to the advantages of using metallic heaters in phosphoric acid production. It explains the important aspects of process design to take full advantage of the benefits available with a metallic material. It also includes the recent experience with process design, fabrication and performance of a metallic heat exchanger using Sandvik Sanicro 28 which was installed in a phosphoric acid plant in Tunis.

Performance investigation of thermally enhanced polymer composite materials for microelectronics cooling

Driven by the low density, corrosion resistance, manufacturability, and low raw material and manufacturing costs of polymer composite materials, significant attention is being devoted to the innovation, characterization, and implementation of such materials. In this study the thermal and mechanical properties of a broad range of commercially available, injection moldable, thermally enhanced polymer composite materials are reviewed to help identify candidate materials that could replace conventional metal alloys in microelectronics cooling heat sink and heat exchanger applications. The material property characterization data reviewed consists of vendor data generated in accordance with applicable characterization standards. From twenty seven commercially-available polymeric composite materials, two promising groups of materials, namely polyphenylene sulfide (PPS) and polyimide 66 (PA66), are identified.A preliminary investigation of exchanger heat transfer rate is undertaken using computational fluid dynamics (CFD) to identify the envelope of thermally enhanced composite material thermal conductivities required for effective heat transfer in gas–liquid heat exchanger applications. The thermal performance of a thermally enhanced PPS, parallel plate cross-flow air–water heat exchanger prototype is shown numerically and experimentally to be comparable to that of a conventional aluminum exchanger having the same geometry, demonstrating the potential feasibility of replacing conventional metallic heat exchangers with thermally enhanced polymeric composite heat exchangers in microelectronic air–liquid cooling applications.

Application of PTFE Heat Exchanger in Low Temperature Waste Heat of Big Coal-Fired Power Plants

With the rapid development of the national economy, the use of low-temperature heat in thermal power plant boiler can not be ignored.Although low temperature economizer is widely used in low-temperature waste heat recovery of thermal power plant boiler, the problems of corrosion and fouling are very significant.New type PTFE heat exchanger filled with high thermal conductivity properties can replace the existing metal heat exchanger, fundamentally solve the problems of corrosion and fouling, meet the future development of the thermal power plant, and realize the energy recycling to maximize the benefit of energy.

Evaluation of ORC modules performance adopting commercial plastic heat exchangers

Abstract In this paper the possible replacement of conventional metallic heat exchangers with plastic components is investigated with reference to low size Organic Rankine Cycles, aiming at a reduction of the plant investment cost. A thermodynamic optimization of a 20 kW regenerative ORC plant, representative of a low temperature ( N-heptane was selected as the working fluid, thanks to the capability to operate within the pressure limits for evaporation and condensation processes imposed by the adoption of plastic components. Finally, the potential economic benefit of the plastic solution in comparison with conventional heat exchangers made of carbon steel was evaluated for the whole plant; the case of enhanced materials adoption, which is mandatory for the evaporator in presence of corrosive heat source media, was also considered. It turns out that advantages of the proposed solution become appreciable whenever the presence of corrosive heat source media requires the use of materials other than carbon steel. For instance, for a plant availability of 5000 h/year and discount rate of 10%, we obtain a cost of the produced electricity of 94.8 $/MW h, 95.4 $/MW h, 101.5 $/MW h, and 118.9 $/MW h respectively for plastic, carbon steel, stainless steel and titanium solutions.

Corrosion of laser joined silicon carbide in gasification environment

The production of synthesis gas from renewable fuels is an emerging technology. The aggressive process environment in fuel production systems poses a major challenge, potentially causing corrosion within currently used metallic heat exchangers. To improve the reliability and stability of silicon carbide heat pipes, which belong to an innovative heat exchanger design, the corrosion of involved materials due to contact with steam, ash and alkaline (earth) metals needs to be investigated. Exposures of laser joined SSiC connections with wood ash caused the formation of gas inclusions and an infiltration of the joint with alkaline (earth) metals that lead to crack formations, as revealed by energy dispersive X-ray spectroscopy analysis. It was shown that the use of silicon carbide heat pipes joined with solder materials of both the Mg–Al–Si and the Y–Al–Si systems is possible. A further result was that the gasification temperature has to be limited to < 850°C.

Utilization of orthotropic graphite plates in plate heat exchangers, analytical modeling

Recently developed graphite plates with large in-plane thermal conductivity are considered as promising alternative to conventional metallic plate heat exchangers (PHE). A new analytical model is developed to study the impact, and the potentials, of the emerging orthotropic graphite-based plates in PHE under various convective regimes. Closed-form relationships are obtained for temperature and heat flux distributions, and applied to perform a comprehensive parametric study on the orthotropic conductivity effects. Our results show that increasing the in-plane thermal conductivity leads to significant changes in heat flow pattern and reduction in temperature variation along the plate. In spite of the remarkable effects of in-plane thermal conductivity on the heat flow pattern, through-plane thermal conductivity plays the key role in controlling the total heat transfer between the hot and cold fluid streams through the plate. Moreover, a new critical through-plane conductivity is proposed to calculate the maximum value of thermal conductivity that provides the highest heat transfer rate through orthotropic slabs. The critical value also includes convective heat transfer resistance of the fluid side and the plate thickness effects. To verify the present model, an independent numerical study is conducted using COMSOL Multiphysics. The analytical results are compared with the obtained numerical data as well as an existing data set in the literature and show a great agreement with less than 5% relative difference.

High-Thermal Conductive Coating Used on Metal Heat Exchanger

Based on modified silicon polyester resin in addition to several functional fillers such as corrosion-resistant fillers, heat-resistant fillers and thermal conductive fillers, a high thermal conductive coating can be made. On the basis of boronnitride (BN) and aluminum nitride (AIN) used as thermal conductive fillers and by means of the testing system of hot disk and heat transfer experiment, researches on the varieties of thermal conductive fillers and the effects of the contents of high-thermal conductive coating have been done, which shows that the thermal conductivity of coating increases with the increase of the quality fraction and the coefficient of thermal conductivity of the thermal conductive fillers of coating. With guaranteeing better heat resistance, stronger corrosion resistance and adhesive force, the coefficient of coating can reach a level as high as 3 W·m−1·K−1.

A New Type of Heat Exchanger for Ventilation in Buildings with Nearly-Zero Energy Consumption

This article describes a new type of hollow-fiber heat exchanger that provides similar heating performance to conventional metal heat exchangers commonly used in air conditioning systems but at lower cost and greater simplicity. While factors such as fragility and element fouling have yet to be optimized, the exchanger shows great promise for application in buildings aiming at close-to-zero energy consumption.

Lessons learned from fire in air supply ducts

In 2012, loss of air flow caused activation of interlocks to shut off steam and electrical heat inputs to an air heating system that was part of a drying/milling operation. Shortly thereafter, upstream filters and air ducts ignited causing more than US$500K in damage and business interruption. Investigations concluded that the interlocks functioned as designed on loss‐of‐airflow but were not sufficient to prevent overheating the inlet air filters (including HEPA filters) by the residual heat in the air heaters. The two main lessons learned from this fire incident apply to a broad range of operations in the chemical process industries (CPI): Mundane unit operations such as air filters that have combustible components must be evaluated as thoroughly as the rest of the chemical process. Designers and PHA leaders must recognize the potential of thermal inertia of heaters (and coolers) to heat (or cool) a stagnant volume of fluid when flow is interrupted. Gas streams are particularly susceptible due to the low heat capacity relative to the heat capacity of a metallic heat exchanger. Interlocking the supplies (to steam and electrical heaters in this incident) is often necessary but may not be sufficient. Other lessons “relearned” include the importance of learning from a previous near miss, the importance of limiting escalation aspects of this fire, and the importance of preventative maintenance. © 2014 American Institute of Chemical Engineers Process Saf Prog, 2014

Fischer-Tropsch synthesis reaction in a novel reactor system of cobalt catalyst coated metallic foam and heat-exchanger type reactor

Fischer-Tropsch synthesis reaction in a novel reactor system of cobalt catalyst coated metallic foam and heat-exchanger type reactor