Recuperative Heat Exchanger Research Articles

Effects of geometrical parameters on thermo-hydraulic characteristics of perforated plates

Perforated plates interspaced with spacers are effectively used in recuperative heat exchangers. In this paper, we present generalized correlations for Colburn factor and friction factor of perforated plates as functions of all geometrical variables and Reynolds number. Heat transfer and flow friction data are generated through CFD modelling on a stack of perforated plates. The model is validated with the published experimental data. Correlations are developed through regression analysis over a large number of computed data generated under varying parametric conditions. Effects of various geometrical parameters on the thermos-hydraulic characteristics of perforated plates are discussed in terms of Colburn factor, friction factor and modified area goodness factor.

Boiling of multicomponent working fluids used in refrigeration and cryogenic systems

Working fluids based on mixtures are widely used in cryogenic and refrigeration engineering. One of the main elements of low-temperature units is a recuperative heat exchanger where the return flow cools the direct (cold regeneration is carrying out) resulting in continuous boiling and condensation of the multicomponent working fluid in the channels. The temperature difference between the inlet and outlet of the heat exchanger can be more than 100K, which leads to a strong change in thermophysical properties along its length. In addition, the fraction of the liquid and vapor phases in the flow varies very much, which affects the observed flow regimes in the heat exchanger channels. At the moment there are not so many experimental data and analytical correlations that would allow to estimate the heat transfer coefficient during the flow of a two-phase mixture flow at low temperatures. The work is devoted to the study of the boiling process of multicomponent working fluids used in refrigeration and cryogenic engineering. The description of the method of determination of heat transfer coefficient during boiling of mixtures in horizontal heated channel is given as well as the design of the experimental stand allowing to make such measurements. This stand is designed on the basis of a refrigeration unit operating on the Joule-Thomson throttle cycle and makes it possible to measure the heat transfer coefficient with a good accuracy. Also, the calculated values of the heat transfer coefficient, obtained with the use of various correlations, are compared with the existing experimental data. Knowing of the heat transfer coefficient will be very useful in the design of heat exchangers for low-temperature units operating on a mixture refrigerant.

CAE methods for plate heat exchanger design

The heat recovery can substantially reduce energy consumption needful for heating or air conditioning of buildings. The aim of work is to develop a simple CAE method for rapid design and optimization of the dimensions of plate heat exchangers for heat recovery. Flow and heat transfer in an air-to-air recuperative counter-flow plate heat exchangers were investigated numerically. Pressure drop and effectiveness were evaluated as functions of inlet velocity for three sizes of real heat exchangers. Obtained data were analyzed, inlet/outlet cross-flow and middle counter-flow parts were investigated independently and data were substituted by suggested functions for Nusselt number and loss coefficients. These functions were used to illustrate the effect of heat exchanger dimensions. It proved that bigger heat exchangers provide higher effectiveness and lower pressure drop for the same flow rate, but also the heat transfer surface is larger. Derived equations which also take the effect of material thickness and plate pitch into account represent a useful tool for design and optimization of recuperative heat exchangers for use in HVAC systems.

Cost comparison of printed circuit heat exchanger to low cost periodic flow regenerator for use as recuperator in a s-CO2 Brayton cycle

Supercritical Carbon Dioxide (sCO2) power cycles have the potential to deliver high efficiency at low cost. However, in order for an sCO2 cycle to reach high efficiency, highly effective recuperators are needed. These recuperative heat exchangers must transfer heat at a rate that is substantially larger than the heat transfer to the cycle itself and can therefore represent a significant portion of the power block costs. Regenerators are proposed asa cost saving alternative to high cost printed circuit recuperators for this application. A regenerator is an indirect heat exchanger which periodically stores and releases heat to the working fluid. The simple design of a regenerator can be made more inexpensively compared to current options.The objective of this paper is a detailed evaluation of regenerators asa competing technology for recuperators within an sCO2 Brayton cycle. The level of the analysis presented here is sufficient to identify issues with the regenerator system in order to direct future work and also to clarify the potential advantage of pursuing this technology. A reduced order model of a regenerator is implemented into acycle model of an sCO2 Brayton cycle. An economic analysis investigates the cost savings that is possible by switching from recuperative heat exchangers to switched-bed regenerators. The cost of the regenerators was estimated using the amount of material required if the pressure vessel is sized using ASME Boiler Pressure Vessel Code (BPVC) requirements. The cost of the associated valves is found to be substantial for the regenerator system and is estimated in collaboration with an industrial valve supplier. The result of this analysis suggests that a 21.2% reduction in the contribution to the Levelized Cost of Electricity (LCoE) from the power block can be realized by switching to a regenerator-based system.

Modeling and design of a high efficiency hybrid heat pump clothes dryer

Computational modeling is used to design a hybrid heat pump clothes dryer capable of saving 50% of the energy used by residential clothes dryers with comparable drying times. The model represents the various stages of a drying cycle from warm-up through constant drying rate and falling drying rate phases and finishing with a cooldown phase. The model is fit to data acquired from a commercially available U.S. standard vented electric clothes dryer, and when a hybrid heat pump system is added, the energy factor increases from 3.0lbs/kWh to 5.7–6.0lbs/kWh, depending on the increase in blower motor power. The hybrid heat pump system is designed from off-the-shelf components and includes a recuperative heat exchanger, an electric element, and an R-134a vapor compression heat pump. Parametric studies of element power and heating element use show a trade-off between energy savings and cycle time. Results show a step-change in energy savings from heat pump dryers currently marketed in the U.S. based on performance represented by Energy Star from standardized DOE testing.

Dynamic behavior of the gas turbine engine regenerator with complanar channels

This paper expounds a technique for calculating unsteady gas temperatures, air and heat transfer elements of the corrugated plate of a recuperative heat exchanger with complanar channels for start and shut-off modes of the regenerative turbine engine. Under consideration are the distribution of thermal stresses in the plate, heat exchanger service life under stationary conditions, and the allowable number of dynamic “start–shut-off” cycles.

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.

Rotary reverse flow reactor vs. adiabatic reactor with regenerative preheating - Design and comparison

The autothermal catalytic-combustion systems are commonly used for the purification of waste air streams contaminated with low concentrations of volatile organic compounds (VOC). Within this type of devices, the reverse flow reactors (RFR) are known to be more efficient than systems employing recuperative (surface) heat exchangers to preheat the waste air stream with the lean air effluent from the catalytic incinerator. The advantage of the RFR is basically due to the regenerative heat-exchange mechanism, provided by the inert and catalytic solids inside the unit.As an alternative, the regenerative mechanism of preheating can be achieved by an independent heat exchanger, which coupled to a catalytic reactor could be expected to produce similar performance as the RFR.In this context, this contribution is devoted to analyse comparatively the performances of a rotary reverse flow reactor (RRFR) and a system comprising a rotary regenerative heat-exchanger and a catalytic reactor (RHE-SR system) for the treatment of a waste air stream contaminated with ethanol and ethyl acetate, by means of mathematical simulation. Both alternatives are assumed to be composed of monoliths with square channels. A strategy of design for both systems suitable for their comparison is proposed, attending to a range of VOC concentration in the waste stream. Both alternatives can be regarded as being suitable options to carry out the target. However, the resulting designs show clear advantages in favour to the RRFR, as this alternative requires a significantly more compact equipment than the RHE-SR does and, besides, it allows to be operated under a wider range of the rotational speed, which is the main control variable once the systems are operating.

New text comparison between CO2 and other supercritical working fluids (ethane, Xe, CH4 and N2) in line- focusing solar power plants coupled to supercritical Brayton power cycles

This study is focused on comparing four supercritical fluids: Ethane, Xenon, Methane and Nitrogen, as possible alternative to supercritical Carbon Dioxide (s-CO2) in Brayton power cycles coupled to line- focusing solar power plants with Solar Salt (60% NaNO3; 40% KNO3) as heat transfer fluid. The Simple Brayton cycle with heat recuperation and reheating is the configuration selected in this paper, providing a balance of plant design with reduced number of equipment and cost. The gross plant efficiency is calculated fixing the recuperator conductance (UA) for different Turbine Inlet Temperatures (TIT), confirming the maximum plant gross efficiency is related with the minimum allowable recuperator pinch point temperature. The reheating pressure and compressor inlet temperature are optimized with the mathematical algorithms SUBPLEX, UOBYQA and NEWOUA. According to the REFPROP database ranges of applicability, the maximum TIT limits are established for the supercritical fluids (N2 TIT = 550 °C, CO2 TIT = 550 °C, C2H6 TIT = 400 °C, Xe TIT = 450 °C and CH4 TIT = 350 °C). The reference scenario considered for calculating the thermosolar plant energy balances and simulations is the wet-cooling system with a Compressor Inlet Temperature (CIT = 32 °C). The gross efficiency results with the wet-cooling system are: N2 (45.8%), CO2 (44.37%), C2H6 (40.74%), Xe (39.88%), CH4 (32.15%). The plant efficiency is also translated into solar field effective aperture area and estimated cost, for a fixed power output. For optimizing the solar collector aperture area and cost, the Primary Heat Exchanger (PHX) and the ReHeating Heat Exchanger (RHX) capacity ratio (CR) are fixed (CR = 1). The dry-cooling system scenario (CIT = 47 °C) is alto estimated: N2 (43.34%), CO2 (42.42%), C2H6 (37.34%), Xe (37.26%), CH4 (29.53%).For predicting the recuperator heat exchanger dimensions for a fixed conductance (UA), the heat transfer coefficient (HTC) is calculated with the Dittus–Boelter correlation and compared with the CO2 as reference. The C2H6, and CH4 have relative higher HTC in relation with CO2. Also is calculated the recuperator pressure drop. The C2H6, CH4 and N2 pressure drop is lower in comparison with the CO2 for the same operating conditions.The energy efficiency in solar power station coupled to Brayton cycle is very constrained by the ambient temperature variation, impacting directly in the dry-cooling system performance. For this reason a Compressor Inlet Temperature (CIT) sensing analysis is carried out ranging from 32 °C to 57 °C, and also varying TIT from 400 °C to 550 °C. A sensing analysis is also developed varying the Turbine Inlet Pressure (TIP) from 200 bar to 375 bar. The CO2 improves the plant efficiency when increasing the TIP from 250 bar to 350 bar, however the rest of fluids (Ethane, Methane, Nitrogen and Xenon) nearly not suffered any impact in the plant efficiency when increasing the TIP.

Evaluation of CAE Methods Used for Plate Heat Exchanger Design

Abstract Flow and heat transfer in a plastic recuperative counter flow plate air-to-air heat exchanger were investigated experimentally and numerically using Ansys Fluent software. It was employed previously developed methods to generate a computational mesh and assumed nonzero thickness of the plates to calculate the flow in an air-to-air heat exchanger. Pressure drop and effectiveness were evaluated as functions of inlet velocity. Obtained numerical data were substituted by suggested functions dependent on the Reynolds number and later modified to take change in plate pitch and material thickness into account. It was found that change in plate pitch influence only friction losses due to change of turbulence of the flow. The shape of mini-channels, which are defined by the shape of pressing molds that are constant during our investigation, determines local loses. Increasing the plate pitch has the same effect on Nusselt number as increasing the Reynolds number. Modified functions were used to illustrate the effect of material thickness, plate pitch and heat conductivity of material on pressure drop and effectiveness. Derived equations witch take the effect of material thickness and plate pitch into account represent a useful tool for design and optimization of recuperative heat exchanger for use in HVAC systems in sustainability buildings.

Numerical simulation of tubes-in-tube heat exchanger in a mixed refrigerant Joule–Thomson cryocooler

Mixed refrigerant Joule-Thomson (MRJT) cryocoolers can produce cryogenic temperatures with high efficiency and low operating pressures. As compared to the high system pressures of around 150–200 bar with nitrogen, the operational pressures with non-azeotropic mixtures (e.g., nitrogen-hydrocarbons) come down to 10–25 bar. With mixtures, the heat transfer in the recuperative heat exchanger takes place in the two-phase region. The simultaneous boiling and condensation of the cold and hot gas streams lead to higher heat transfer coefficients as compared to single phase heat exchange. The two-phase heat transfer in the recuperative heat exchanger drastically affects the performance of a MRJT cryocooler. In this work, a previously reported numerical model for a simple tube-in-tube heat exchanger is extended to a multi tubes-in-tube heat exchanger with a transient formulation. Additionally, the J-T expansion process is also considered to simulate the cooling process of the heat exchanger from ambient temperature conditions. A tubes-in-tube heat exchanger offers more heat transfer area per unit volume resulting in a compact design. Also, the division of flow in multiple tubes reduces the pressure drop in the heat exchanger. Simulations with different mixtures of nitrogen-hydrocarbons are carried out and the numerical results are compared with the experimental data.

Analysis of efficiency and reliability of blast–furnace process waste heat recovery systems

The object of research is a hot blast generating system, which consists of three hot blast stoves with sequential mode of operation. One of the factors that reduce the hot blast stove block efficiency is the existence of losses with the waste gases, the heat of which can be recovered and used for combustion air preheating. In order to improve the efficiency of a hot blast generating system the possibility of using of recuperative heat exchanger for waste heat recovery is observed. The process of initial parameters determining (the required level of combustion air preheating, waste gases temperature and flow rate at the inlet of the heat exchanger) is described. Software based on using of an original mathematical model and used for calculating of the parameters of the waste heat recovery heat exchanger was created. These data provide tools for refined calculation of heat recovery systems based on recuperative heat exchangers. It is shown that the combustion air preheating results in a reduction of the coke oven gas flow rate. The calculations results in building of temperature distribution diagrams that allow to define the areas of corrosion. Such areas in the observed recuperative heat exchanger model appear at temperatures below 26 oC. Thus, the set approaches can be used to refine the calculation of heat-transfer equipment of waste heat recovery systems to improve their reliability, long life, analyze their technical and economic parameters. This will improve the energy efficiency of the hot blast stoves block and reduce the cost of iron production.

Анализ применимости методик расчета коэффициента теплоотдачи при кипении многокомпонентных рабочих тел

Multicomponent working fluids are widely used in refrigeration and cryogenic technologies. A recuperative heat exchanger, the device influencing the efficiency of the entire low-temperature installation, is one of the main elements of these systems. It serves to regenerate the cold: the reverse (low-pressure) flow cools the direct (high-pressure) flow. In the direct flow, the flow of working fluid condenses, and in the reverse flow it boils. Thus, it is important to know how the design of this heat exchanger shall be determined in designing low-temperature systems. However, the boiling and condensation of mixtures still remain poorly studied due to a small amount of available experimental data and, accordingly, due to a limited number of empirical correlations describing these processes. As a result, those who develop heat exchangers for low-temperature systems operating on mixed working fluids encounter certain difficulties in designing such devices. The article describes various existing correlations describing the boiling of multicomponent working fluids at low temperatures and compares the results obtained using these correlations with the existing experimental data on the boiling of mixtures. The values of the heat transfer coefficient calculated using the Mishra method have better agreement with the experimental data in comparison with the results obtained from other correlations. However, since the experimental data are available for a small number of experiments, in which mixtures having the same composition, with the same operating pressures and mass flowrates are used, it would be incorrect to make a final conclusion about the applicability of the correlations. The analysis carried out in this article paper is a step toward detailed understanding of the applicability of correlations for multicomponent mixtures. More complete experimental data obtained for the same composition under different conditions are needed in order to understand the effect of each parameter on the boiling heat transfer coefficient. Reliable data on the heat transfer coefficient are necessary in designing heat exchangers for low-temperature installations operating on a mixed refrigerant.

The use of energy saving technology as the basis for the state program implementation

The article views prospects of application of energy-saving technology in heat sources and plants that consume thermal energy in the form of steam and hot water. This technology involves creation of a two-phase medium in the transonic jet nozzle apparatus. The effectiveness of the transonic jet nozzle apparatus is compared with the recuperative heat exchangers effectiveness. The use of these devices allows to obtain economic benefit by reducing the required building volume to accommodate the transonic jet nozzle apparatus, increasing performance coefficient compared with conventional recuperative heat exchangers and reducing operating costs. Two operation modes are considered: “pump-boiler” and “boiler”. The most effective is “pump-boiler” mode, because, in this case, more energy saving is obtained through acting powerful network pumps only as launchers. An example of the use of transonic jet nozzle devices in the Samara region is given in the article. The trends and opportunities to solve the task controlling thermobaric parameters by changing the volume fraction of steam in the two-phase medium are studied.

Optimisation of a small solar organic Rankine cycle based on the exergetic analysis

A mathematical model is written for a solar organic Rankine cycle (ORC) taking into account the Pinch temperature differences in the system heat exchangers. The (exergetic factor)-(variation in enthalpy) diagram of the hot and cold composite curves of the heater-evaporator and the superheater of the ORC gives information about the strategy to be followed in order to improve the structure of the system. The introduction of an internal recuperative heat exchanger to preheat the ORC fluid and the splitting of the solar fluid to balance the heat capacities in the heater and the superheater are also considered. An exergetic analysis reveals the magnitude of the exergy destruction in the key pieces of the system. A comparative simulation of the way, the ORC operates with different organic fluids points out that R245fa gives the highest exergetic efficiency.

SELECTING THE SCHEME OF TECHNOLOGICAL CONNECTION OF SUBSCRIBERS TO THE HEAT NETWORK UNDER CONDITIONS OF TRANSITION TO PULSE CIRCULATION OF THE HEATER IN THE EQUIPMENT OF HEAT SUPPLY UNITS

Objectives. The aim is to justify the expediency of a transition en masse to independent connection of heat consumption systems under the conditions of pulse circulation of the coolant through the equipment of a heat supply unit in order to increase the overall energy efficiency of district heat supply systems. Methods. The research is based on methodologies for the effective use of technologies providing a means of effecting a pulsed motion of the coolant in the heat supply system. The advantages and disadvantages of known technical solutions are identified and summarised. Hydraulic accumulators or any other devices for damping a hydraulic shock can be used as devices for preventing the propagation of a wave of hydraulic shock into the transport mainline. Results. An analysis of approaches to heat consumption system connection is carried out in order to assess power efficiency; the most effective ways for improving the efficiency of heat and power devices are identified; a synthesis of the technical solution for the implementation of a technological connection of heat consumption systems is performed. The expediency of transition to independent connection of heat consumption systems under the conditions of pulse circulation of coolant in heat supply units is demonstrated. The independent scheme of connection of subscribers to the heat network is indicated as the most appropriate. The factors constraining the en masse transition to independent connection of the heat load are determined. Based on the analysis and scientific research in the field of increasing the energy efficiency of heat and power systems, the transition to pulse circulation of the coolant in the heat supply unit of the independent heat consumption system is selected. The presented technical solutions are confirmed by Russian Federation patents of inventions and utility models. Conclusion . It is shown that the use of pulse coolant circulation in the equipment of heat supply units for independent connection of heat consumption systems will increase the reliability and long-term operation of the recuperative heat exchanger, as well as increase the overall energy efficiency of the heat supply system.

Modelling, simulation and geometric optimization of cross flow recuperative heat exchanger based on controllability condition number

This paper presents the algebraic mathematical model of cross - flow heat exchanger derived on the basis of transport approach. Theirs operation in the face of variable loads is usually controlled by manipulating inlet fluid temperatures or mass flow rates, where the controlled variable is usually the output temperature of principal flow. The aim of this paper is to optimize the geometry of a tube with the inlet flow of principal incompressible fluid and an external cross - country flow of compressible fluid, based on performance index expressed throughout its controllability characteristics. Thus the condition number has been used to provide the necessary information on the best situation for control of the exchanger under consideration. This concept can also provide us with information about the easiest operating condition to control a particular output. A transient model of a cross-flow heat exchanger is developed, where an implicit formulation is used for transient numerical solutions. The condition number performed throughout the ratio of geometric parameters of tube is optimized, subject to volume constraints, based on the optimum operation in terms of output controllability. The reported optimized aspect ratio, water mass flow rate and output controllability are studied for different external properties of the tube.

Assessment of MDEA absorption process for sequential H2S removal and CO2 capture in air-blown IGCC plants

This work deals with pre-combustion CO2 capture by Methyldiethanolamine (MDEA) scrubbing in air-blown integrated gasification combined cycles (IGCCs). Two types of coal, with low- and high-sulphur content, are considered as fuel input in power plants, as well as two combustion turbines, with different turbine inlet temperature, representative of state-of-the-art and advanced technologies.The gasification section and the power island are simulated by means of the proprietary code GS. Acid gas removal (AGR), consisting in the sequential H2S removal and CO2 capture from the syngas by MDEA solvent, is calculated with Aspen Plus®. MDEA concentration and solvent circulation are varied in each of the assessed cases to comply with the target CO2 capture efficiency and with the CO2 and H2S purity specifications. The resulting heat duties for H2S and CO2 stripping, the consumption of the auxiliaries in the AGR plants, as well as the CO2 compression work are used to calculate the energy and mass balances of the integrated gasification combined cycles. Sensitivity analysis on the temperature approach in the recuperative heat exchanger of the CO2 capture section and on the pressure of the CO2 stripping column have been performed to assess potential energy savings. Results are compared with benchmark IGCC plants without CO2 capture.Net electric efficiencies between 36.6% and 40.4%, with 95% of CO2 capture efficiency, are achieved depending on the coal quality (i.e. the sulphur content), the combustion turbine technology and the MDEA regeneration pressure and heat exchanger temperature difference. Correspondingly, a specific primary energy consumption for CO2 avoided (SPECCA) between 2.85 and 3.2MJ/kgCO2 for the low-sulphur coal cases and between 3.2 and 3.7MJ/kgCO2 for the high-sulphur coal cases have been calculated.

Energy Consumption Minimization of an Industrial Furnace by Optimization of Recuperative Heat Exchange

AbstractA mathematical model has been developed to determine the best geometry of a compact heat exchanger in this research study. Accordingly, an objective function was introduced to define the optimized structure of the exchanger. Two main targets were pursued in this regard. First was the rate of existing discrepancy between the possible heat transfer rate in the exchanger and the maximum rate. Second was possible heat transfer consideration between exchanging gasses and incoming air of the furnace. A sample shell and tube heat exchanger of existing tube in a processing industry has been studied and the calculations have been performed to solve the relevant equations. In addition, a comprehensive code to design an optimized compact heat exchanger for heat recovery of the furnaces has been presented. Then, data gathering and model synchronization caused to a preliminary evaluation of model quality. Thus, for increasing accuracy level of mentioned research, along with measurements’ correction program, model modification has been done and new results were calculated. Optimization model analysis showed that by using this approach, not only significant heat conservation can be achieved, but also suggested procedure might be completely economical.

Numerical investigation of transient behaviour of the recuperative heat exchanger in a MR J–T cryocooler using different heat transfer correlations

In J–T cryocoolers operating with mixed refrigerants (nitrogen-hydrocarbons), the recuperative heat exchange takes place under two-phase conditions. Simultaneous boiling of the low pressure stream and condensation of the high pressure stream results in higher heat transfer coefficients. The mixture composition, operating conditions and the heat exchanger design are crucial for obtaining the required cryogenic temperature. In this work, a one-dimensional transient algorithm is developed for the simulation of the two-phase heat transfer in the recuperative heat exchanger of a mixed refrigerant J–T cryocooler. Modified correlation is used for flow boiling of the high pressure fluid while different condensation correlations are employed with and without the correction for the low pressure fluid. Simulations are carried out for different mixture compositions and numerical predictions are compared with the experimental data. The overall heat transfer is predicted reasonably well and the qualitative trends of the temperature profiles are also captured by the developed numerical model.