Bundle Heat Exchanger Research Articles

Numerical investigation on prevention of fouling in the horizontal tube heat exchanger: Particle distribution and pressure drop

To improve the performance of fouling prevention in liquid–solid fluidized bed heat exchanger with the horizontal tube bundle, parametric study of particle distribution and pressure drop in the heat exchanger was numerically conducted. Eulerian multiphase fluid model was used to calculate the flow of liquid–solid two phase in heat exchanger by means of the commercial software Fluent. In the simulation result, it was demonstrated that more particles occurred in the lower tubes of the horizontal tube bundle in low velocity due to the lack of full fluidization. Relative to that in low velocity, uniformity distribution of particle was obtained in high velocity which was accompanied with greater dynamic pressure drop. The particle with the greater density and diameter was inclined to sediment in the lower tubes due to larger sediment velocity and difficulty in the realization of full fluidization. Meanwhile an increase in size and density of particle would bring about greater pressure drop originating from more particle kinetic energy consumed. Effect of volume fraction of solid phase on particle distribution and pressure was discussed. It showed that an increase in volume fraction of solid phase led to significant rise of pressure drop. It was because consumed kinetic energy and collision energy of particles were greatly amplified with the increasing volume fraction of solid phase. Therefore greater volume fraction of solid phase, especially for the particle with the greater diameter and density, would not be recommended in the performance of fouling prevention in the horizontal tube bundle heat exchanger.

Influences of height to diameter ratios of dry-cooling tower upon thermo-flow characteristics of indirect dry cooling system

Ambient winds may reduce the cooling efficiency of a nature draft dry-cooling tower, so it is of benefit to the design and operation of indirect dry cooling system to determine the preferred tower shape to restrain the adverse impacts of ambient winds. Based on the dry-cooling tower with vertically arranged heat exchanger bundles around the circumference, air-side flow and heat transfer models are developed for three typical height to diameter ratios of tower. The velocity, pressure and temperature fields of cooling air in the absence and presence of ambient winds are presented, and the mass flow rate, heat rejection and outlet water temperature of each sector and air-cooled heat exchangers are calculated. The results show that the dry-cooling tower with a low height to diameter ratio is superior to that with a high ratio in thermo-flow performances of indirect dry cooling system at high wind speeds on condition that the air-cooled heat exchangers have the same heat transfer surface areas. At low wind speeds, the mass flow rate and heat rejection of air-cooled heat exchanger vary little with height to diameter ratios of tower due to almost the same buoyancy forces. It is recommended to take a low height to diameter ratio of tower, especially at strong ambient winds for better thermo-flow performances of indirect dry cooling system.

A Non-Dimensional Lattice Boltzmann Method for direct and porous medium model simulations of 240-tube bundle heat exchangers in a solar storage tank

An improved Non-Dimensional Lattice Boltzmann Method (NDLBM) is developed and a uniform computational code is compiled to fit into both direct and porous medium model simulations. Comparison studies based on natural convection of 240 cylindrical tube bundle heat exchangers immersed in a thin rectangular solar storage tank show the computational efficiency and accuracy of both direct simulations and porous medium model simulations achieved by using NDLBM. The governing parameters in the macroscopic, microscopic, and mesoscopic length scales corresponding to the enclosure width, tube diameter, and mesh size are obtained. Transient isotherms, streamlines, Nusselt numbers, and CPU time of nine models have been simulated for various pitch, porosity, and distributions of tubes. Given the same grid number and simulation time, the CPU time of the porous medium model simulations by using NDLBM is about 1/60 of that of porous medium simulations by using finite difference based on projection method, and 1/20 of that of the direct simulations with the uniform code based on NDLBM. The porous medium simulations can only obtain Darcy velocity and volume averaged temperature, while the direct simulations can obtain both macroscopic and microscopic velocity and temperature.

Flow Optimization of Gas Quenching Processes Using Perforated Plates

Abstract Industrial gas quenching is a cost-effective, environmental-friendly alternative to conventional fluid-based quenching processes used for the heat treatment of workpieces mainly in the automotive industry. The influence of the heat exchanger geometry in a novel gas quenching chamber on the gas flow was investigated through measurement in parallel to CFD-simulations. With reduced quenching chamber dimensions, a flow guiding system utilizing a perforated plate placed between the heat exchanger and the batch has been found to play a significant role on the velocity and turbulence distribution of the gas flow. The use of various perforated plates in this gas quenching process focuses on the exploration of two main parameters: plate porosity and perforation hole diameters. By combining in situ flow and CFD-simulations of heat and fluid flow, optimized parameters have been determined indicating improved uniformity and intensity of the velocity distribution in the quenching chamber, thus obtaining the potential of an overall improvement of the gas quenching process. Geometric parameters of perforated plates introduced into the gas quenching chamber facility are investigated as, e.g., the distance between the plate and the batch. More generally, the use of a perforated plate system to optimize gas flows may be extended to other applications where tube bundle heat exchangers in gas flow processes are involved.

Parametric study of particle distribution in tube bundle heat exchanger

Fluidized particles in liquid–solid fluidized bed exchangers are able to remove deposits from walls and thus are able to prevent fouling or scaling. Particle distribution is an important parameter for the performance of fouling prevention. In this paper, the Eulerian multiphase fluid model was adopted to simulate the particle distribution in the fluidized bed heat exchanger incorporating tube bundle arranged in parallel. The porous media model was applied in the zone of the tube bundle to analyze the pressure loss through the vertical tube. Factors influencing the particle distribution including the velocity, volume fraction of the solid phase and the property of the particle were discussed respectively. Particle distribution became more uniform in the high velocity due to the full fluidization of particle in each tube. With the increase in density or diameter of particle, particle maldistribution took place because the sedimentation velocity of these particles was so great that particles could not run into the tubes on both sides. With the increasing of volume fraction of solid phase, particle distribution became slightly more uniform. However increment of particle would bring about greater flow resistance and reduce the usage life of particle and devices. Therefore amount of injected particle should be adjusted according to parameters such as effect of fouling prevention, pressure drop etc. A comparison of the simulation results with the experimental measurements was carried out, showing good agreement.

Effect of turbulence intensity on the pressure drop and heat transfer in a staggered tube bundle heat exchanger

This paper investigates experimentally the correlation and the effect of the turbulence intensity on the pressure drop and the heat transfer mechanism of a heat exchanger with elliptic tubes in a staggered arrangement. The heat exchanger studied in this work is an air–water cross flow heat exchanger with air being the external fluid and water the internal one. The heat exchanger consists of 144 elliptic tubes placed in a staggered arrangement. The experiments were carried out for two setups. The first setup was referring to isothermal conditions for which only air was used, flowing around the tubes. The second setup was referring to non-isothermal conditions with air as the external fluid and water as the internal working fluid, flowing inside the tubes. The Reynolds number for the external air for the isothermal experiments ranged between 3100 and 5200 based on the maximum velocity between the elliptic tubes. The turbulence intensity values varied between 0.9% and 3%. For the non-isothermal measurements the Reynolds number took values from 3100 to 7700 and the turbulent intensity ranged from 0.9% to 3%. The measurements showed that the increase of turbulent intensity led to a decrease in the total pressure drop of the external flow of the heat exchanger together with an enhancement of the heat transfer mechanism.

Experimental investigation on powder conductivity for the application to double wall heat exchanger (NACIE-UP)

The double wall tube bundle heat exchanger is generally constituted by two or three (called double wall bayonet) concentric tubes. It is based on the concept to provide a double physical separation between two fluids: the coolant (i.e. water) and the hot fluid (i.e. lead).There are two primary reasons for the separation of the fluids. The first is to maintain a given temperature drop between the hot fluid and the coolant. The second is to increase the safety margin of the unit by reducing the probability of interaction between the coolant and the hot fluid. Furthermore, this configuration allows the possibility to monitor eventual leakages from the coolant or from the hot fluid by pressuring the separation region. On the other hand, if it is necessary to achieve high thermal performance of the unit, the annular space that separates the fluids should be filled with a heat transfer enhancer (i.e. sintetic diamond powder, stainless steel-SS powder).Several applications of the double wall and double wall bayonet tube heat exchangers have been designed and constructed at ENEA CR Brasimone. In particular, there are four facilities (NACIE and NACIE-UP, HELENA and HERO) whose heat exchanger is based on this concept and make use of SS powder (AISI-304 or AISI-316) to provide a temperature drop or to accommodate monitoring and heat transfer enhancement.In this framework, the Tubes for Powders (TxP) facility has been designed and constructed during 2012 and has been operating since the beginning of 2013 to investigate the conductivity of porous materials for their application in double wall heat exchangers. The facility consists of three concentric tubes and it allows to estimate the conductivity of a porous material by measuring the temperature drop across its borders and the removed power. It has the capability to introduce helium and to pressurize it up to 5bar.The present paper focuses on the experimental campaigns carried out in TxP to characterize AISI-316 powder in support to the qualification of the NACIE-UP heat exchanger. The main goals of the experiments are to assess the conductivity of AISI-316 powder, to highlight the effects of the procedure adopted to load the powder in the facility on its conductivity and to investigate the influence of thermal cycling. The experimental findings are finally compared to the results obtained from experimental campaigns performed in NACIE and applied to NACIE-UP to develop and discuss the pre-tests calculations carried out by means of computational fluid dynamic code.

Operational experience in the use of 18 kA HTS current leads for Edipo

In spring 2013, the Edipo facility of CRPP was commissioned. The dipole is powered via two 18 kA HTS current leads, designed and manufactured at CRPP. As part of the Edipo commissioning framework, the operational parameters of the leads were implemented in the control system. The in-situ tests were found to be in good agreement with the tests performed without a background field in 2011. The leads consist of a conduction cooled HTS module, made of AgMgAu/Bi-2223 stacks, and a wire bundle heat exchanger. The heat exchanger is cooled by forced flow helium gas, the inlet temperature of which was measured to vary between 65 K and 85 K. During operation with field, the mass flow rate is a function of current (2.05 g/s per lead at full field, 12.35 T, 17.2 kA). Reduced cooling investigations showed that 0.31 g/s per lead is suitable for overnight standby and 0.2 g/s per lead for longer periods. For detection of and protection against quench in the HTS module, a threshold of 10 mV was found to be appropriate. The heat exchanger has a voltage protection threshold of 120 mV. The temperatures of the heat exchanger, the HTS, and the helium inlet temperature were monitored in order to provide a further layer of protection.

Development of a thermal oil operated waste heat exchanger within the off-gas of an electric arc furnace at steel mills

A thermal oil operated tube bundle heat exchanger within the off-gas from an electric arc furnace (EAF) was developed for supplying heat to an electricity generating system by empirical methods. In this process both the unsteady heat flux and the dust load of the off-gas were considered.A reference heat exchanger configuration was designed and optimized by empirical methods. Therefore in-situ experiments within the off-gas channel as well as parametric studies were undertaken. It was revealed that variations of geometric parameters have a positive impact on heat exchanger performance. The impact of the dust layer thickness on heat transfer and pressure drop was quantified.A CFD model of a section of the reference heat exchanger configuration was generated using ANSYS CFX. Thus representative results for the complete heat exchanger were calculated and compared with the analytical results. The comparison of characteristic numerical results revealed a slight underestimation of heat exchange and pressure drop compared to analytical values.

Flow and heat transfer characteristics of indirect dry cooling system with horizontal heat exchanger A-frames at ambient winds

Ambient winds are key issues in the operation of indirect dry cooling system in power plants, so it is of use to make clear the thermo-hydraulic performances of indirect dry cooling system at ambient winds. On the basis of two indirect dry cooling systems with horizontally arranged heat exchanger bundles in the patterns of radial and rectangular A-frames, the computational models of air-side flow and heat transfer coupled with the performances of the circulating water and exhaust steam are developed. The velocity, pressure and temperature fields of cooling air in the absence and presence of winds are presented and the average mass flow rate of cooling air, inlet air temperature as well as the heat rejection for the A-frames of each cooling sector of the air-cooled heat exchanger are calculated, by which the outlet water temperature of heat exchanger and the back pressure of turbine are obtained. The results show that the wind effects on the thermo-hydraulic performances of indirect dry cooling system differ completely from those with vertically arranged air-cooled heat exchanger outside. The velocity and temperature fields present central symmetric characteristics for the heat exchanger in the radial pattern, however, the differences are observed for the heat exchanger A-frames in the rectangular pattern. The thermo-hydraulic performances of the upwind A-frames are most deteriorated by the adverse impacts of ambient winds, but they are improved for the downwind ones, just similar to those of direct dry cooling system. As the wind speed increases, the mass flow rate and heat rejection of the downwind A-frames increase, but they are reduced for the upwind ones. The outlet water temperature of the heat exchanger and back pressure of turbine increase with increasing wind speed. The investigation of wind effects on the flow and heat transfer characteristics of indirect dry cooling system with horizontal heat exchanger A-frames is of benefit to the design and operation of air-cooled heat exchangers and cooling towers.

A parameter study of tube bundle heat exchangers for fouling rate reduction

The formation of particulate deposits on flue gas heat exchanger surfaces will reduce heat transfer efficiency, increase the instability of equipment operation and introduce a major uncertainty into the heat exchanger design. In this paper, a numerical model was developed to predict the flue-ash particle deposition rate by considering particles transport, sticking and rebound behaviors based on the software FLUENT, extended by user-defined functions (UDFs). The numerical model was applied to cross-flow tube bundle heat exchangers with a 6-row tube arrangement. The effects of six parameters (particle diameter, flow velocity, spanwise tube pitch, longitudinal tube pitch, tube geometry shape, and arrangement) on fouling rate, as well as on the heat transfer and hydrodynamics performance, were examined. It was found that particle deposits accumulated primarily in the flow stagnation region, recirculation region, the vortex separation and reattachment regions. Increasing particle diameter moved the deposition zones towards the windward side of tubes. Using both oval tubes and staggered arrangements can reduce the fouling rate. With the increase in longitudinal tube pitch, both the particulate deposit rate and the heat transfer performance increased. To account for fouling and heat transfer performance, a tube spacing value of 2 was recommended.

Dimensional characteristics of wind effects on the performance of indirect dry cooling system with vertically arranged heat exchanger bundles

Ambient winds play a crucial role in the operation of indirect dry cooling system in power plants, so it is of use to study the wind effects on the thermo-hydraulic performances of various cooling deltas and sectors in the air-cooled heat exchanger and overall indirect dry cooling system. On the basis of a typical indirect dry cooling system with heat exchanger bundles outside, the computational models of air-side fluid and heat flows coupled with the performances of the circulating water and exhaust steam are developed. The velocity, pressure and temperature fields of cooling air at various wind speeds are presented and the air mass flow rate, inlet air temperature as well as the heat rejection for each cooling delta, each sector are calculated, by which the outlet water temperature of the air-cooled heat exchanger and the back pressure of turbine are obtained. The results show that the cooling deltas suffer from different actions of ambient winds and the conspicuous dimensional characteristics of wind effects on the thermo-hydraulic performances of indirect dry cooling system are presented. The performances of upwind cooling deltas are superior to those at the rear, and they are all better than the side ones. At high wind speeds, the side cooling deltas and sectors get improved in thermo-hydraulic performances to an extent as the wind speed increases, but the performances of the backward ones are deteriorated seriously due to the obstacle of high pressure converting from the strong ambient winds. There exists a critical wind speed, at which the air flow rate is lowest and the outlet water temperature of air-cooled heat exchanger reaches its maximum, the indirect dry cooling system performance is most deteriorated, resulting in an unfavorable turbine back pressure. The study of wind effects on the indirect dry cooling system performances is helpful to the design of air-cooled heat exchangers and dry-cooling towers.

Case Studies on Field Repairs of Stainless Steel Components in Refinery

Stainless steels (SS) possess excellent corrosion, creep and high temperature oxidation resistance and are invariably used in refinery for construction of heater tubes, tube supports, Heat exchanger bundles, piping and internal lining of pressure vessels. Ferritic stainless steel type 405 is used for column strip-lining, martensitic stainless steel type 410 is used for column trays and heater tubes and austenitic stainless steel family is used very extensively for lining, piping, heat exchanger, heater tubes and tube supports. On-stream and turnaround condition monitoring of plant and equipment are carried out for health assessment and mitigation of premature failure. However, catastrophic failures of stainless steel due to stress corrosion cracking, thermal fatigue and stress relaxation cracking are encountered in addition to bulging and cracking of strip-lining. Field repairs of these components are required to be done. Stainless steels are difficult to weld due to low thermal conductivity, higher coefficient of thermal expansion, fissuring and solidification cracking problem during welding. Lower heat input and fast cooling facilitate the welding process. Welding of service exposed stainless steels is more challenging, as it has already undergone metallurgical degradation. Welding of stainless steels is carried out using TIG and SMAW process with matching electrode after establishing the welding specification procedures and welders qualification. Field repairs of stainless steels components are also attempted with original procedures and in case of difficulties, a buttering layer of inconel (ERNiCr3) or ER 309Mo is provided on the welding surface before using matching electrodes. Quality assurance of weld joint is ensured by stage-wise inspection and non-destructive testing. Dye penetrant test of root run and radiographic examination of final weld joint are most common. Post weld heat treatment is done as per code requirement. This Paper highlights three case studies on field repairs of stainless steel components in refinery. 1. Welding procedure followed for repair of bulged and cracked SS 316 strip-lining and cladding on carbon steel backing material. It is a dissimilar welding of SS 316L with degraded carbon steel. 2. Field welding of SS 347 Piping components, which has undergone thermal relaxation cracking at fillet joints. 3. Welding repair of SS 310 cast heater tube support conforming to A 297 Gr HK 40. The Paper also presents brief failure analysis with reasons and remedies.

A geometry factor for natural convection in open cell metal foam

A dimensionless geometry factor, defined as the ratio of the product of the microscopic length scale and the solid fluid interface area to the solid volume in a representative elementary volume (REV), connects the macroscopic and microscopic drag and heat flux between the solid and fluid phases in a porous medium. Unlike the conventional porosity based geometry factor (1-porosity), the geometry factor represents how widely the solid is distributed in the REV. Based on this geometry factor, the microscopic drag coefficients and heat transfer correlations are presented in a uniform form for porous media of arbitrary microscopic geometry, such as spheres, tubes, and metal foams. Also relationships between the microscopic drag coefficients and permeability, Forcheimer coefficient, and Ergun constants are obtained based on the geometry factor. Numerical simulations based on this model agree with previous studies on sphere packed spheres, tube bundle heat exchangers, and natural convection in saturated metal foams including a metal foam with a superposed fluid layer. The effects of Rayleigh number, geometry factor, and ratio of macroscopic to microscopic length scale on overall heat transfer in natural convection in a saturated metal foam are presented. Transient isotherms and streamlines provide a full map of the overall Nusselt number versus Rayleigh number relation and show the relation between the oscillation of Nusselt number and the shedding of the thermal plumes at large Rayleigh number.

Twisted bundle heat exchangers performance evaluation by CFD (CJ12/5054)

Shell and tube heat exchanger with single twisted tube bundle in five different twist angles, are studied using computational fluid dynamics (CFD) and compared to the conventional shell and tube heat exchanger with single segmental baffles. Effect of shell-side nozzles configurations on heat exchanger performance is studied as well. Heat transfer rate and pressure drop are the main issues investigated in the paper. The results show that, for the same shell-side flow rate, the heat transfer coefficient of heat exchanger with twisted tube bundle is lower than that of the heat exchanger with segmental baffles while shell-side pressure drop of the former is even much lower than that of the latter. The comparison of heat transfer rate per unit pressure drop versus shell-side mass flow rate shows that heat exchanger with twisted tube bundle in both cases of perpendicular and tangential shell-side nozzles, has significant performance advantages over the segmental baffled heat exchanger. Optimum bundle twist angles for such exchangers are found to be 65 and 55° for all shell side flow rates.

Solar enhanced natural draft dry cooling tower for geothermal power applications

This paper presents a new concept of hybrid cooling, named solar enhanced natural draft dry cooling tower (SENDDCT), in which solar collectors are added to traditional natural draft dry cooling towers to increase their performance. The purpose of using solar energy in this new cooling system is to increase the suction through the tower so that more air flow is achieved through the compact heat exchangers that cool condensers of a geothermal power plant. For the same size of the cooling tower, more air flow across the heat exchangers means more heat can be rejected by the system. The governing equations for the SENDDCT are similar to those of a conventional natural draft dry cooling tower except that solar heating is added after the heat exchanger bundles. Performance comparisons show that SENDDCT has substantial advantages over conventional natural draft dry cooling towers for geothermal power plants as well as standalone solar chimney power plants.

Numerical evaluation of a heat exchanger with inline tubes of different size for reduced fouling rates

This paper describes the numerical evaluation of a novel cross flow tube bundle heat exchanger that combines tubes of different diameter in an inline arrangement for the purpose of reducing gas side particulate fouling rates while preserving acceptable levels of heat transfer and pressure drop performance. Three arrangements are compared: a common inline tube bundle heat exchanger with cylinders of equal diameter and two other arrangements that consist of alternately placed cylinders with a diameter ratio of d/D=0.5, at two different transverse spacings. Numerical calculations are performed in order to study heat transfer, pressure drop and fouling rates from flue gases with suspended ash particles. The alternating tube sizes achieve a suppression of the vortex shedding mechanism that has previously been shown to enhance downstream particle deposition. Results show that, compared to the standard arrangement, the tube bundle with unequal cylinders placed at the largest transverse spacing achieves a significant (∼30%) reduction in particle deposition rate without sacrificing acceptable values of heat transfer per unit volume and low pressure drop.

Heat transfer and resistance characteristics of conical spiral tube bundle based on field synergy principle

Conical spiral tube bundle are universally used in heat transfer enhancement in heat exchangers. The heat transfer and resistance of the tube bundle are affected greatly by the conical structure, so the analysis of it is necessary. In order to a further evaluation, the heat transfer and resistance characteristics of conical spiral tube bundle are investigated with regression analysis based on numerical simulation data. The correlations of heat transfer and pressure drop of conical spiral tube bundle are proposed both in laminar and turbulent fluid flow. On the based of the field synergy principle, the synergy of four vectors, the velocity, the velocity gradient, the temperature gradient and the pressure gradient, are calculated and discussed via the user defined function (UDF) program. The synergy angles β and θ, which respectively denote the performance of heat transfer enhancement and pressure drop of the conical spiral tube bundle, are analyzed. Finally, the comprehensive performance of the conical spiral tube is evaluated by the synergy angle γ and all of the three synergy angles of conical spiral tube bundle are compared to both bare tube and thin cylinder-interpolated tube. The analysis of the synergy angles shows that the heat transfer enhancement and pressure drop of conical spiral tube bundle are smaller than that of the thin cylinder-interpolated tube, while the comprehensive performance of conical spiral tube bundle is greater. The analysis of the heat transfer and pressure drop of conical spiral tube is valuable and instructional on the design and optimum of conical spiral tube bundle heat exchangers.

Study on the Conical Spiral Tube Bundle Heat Exchangers with Pulsation Flow Generator Structure

Elastic tube bundles are universally used in heat transfer enhancement by flow-induced vibration in heat exchangers, and the study of the heat transfer performance is of importance. The structure of conical spiral tube bundle heat exchanger was introduced first, and the structure of pulsation flow generator was also introduced. The frequency of pulsation flow was discussed. Finally, in condition of same shell side diameter, the heat transfer and natural frequency of the conical spiral tube bundle were compared with the planar elastic tube bundle. The results show that the natural frequency of conical spiral tube bundle was smaller, the heat transfer performance of conical spiral tube bundle was better than the planar elastic tube bundle.

Space characteristics of the thermal performance for air-cooled condensers at ambient winds

Ambient winds may lead to poor fan performance, exhaust air recirculation and mal-distribution of the air across the tube bundles of the air-cooled condensers in a power plant. Investigations of the impacts of the ambient winds on the air-cooled condensers are key area of focus. Based on a representative 2 × 600 MW direct dry cooling power plant, the physical and mathematical models of the air-side fluid and heat flow in the air-cooled condensers at various ambient wind speeds and directions are set up by introducing the radiator model to the fin-tube bundles. The volumetric flow rate, inlet air temperature and heat rejection for different air-cooled condensers as a whole, condenser cells and fin-tube bundles are obtained by using CFD simulation. The results show that the thermo-flow performances for the air-cooled condenser as a whole, condenser cells and heat exchanger bundles vary widely in space. The thermal performances of the air-cooled condensers, condenser cells and fin-tube bundles at the downstream are generally superior to those at the upwind. It is of use for the upwind fan regulations and the A-frame condenser cell geometric optimization to investigate the space characteristics of the thermal performance for the air-cooled condensers in a power plant.