Synthesis of Cost-Optimal Shell-and-Tube Heat Exchangers

Abstract

Synthesis of cost-optimal shell-and-tube heat exchangers is a difficult task since it involves a large number of parameters. An attempt is made in this article to simplify the process of choosing the parameter values that will minimize the cost of any heat exchanger satisfying a given heat duty and a particular set of constraints. The simplification is based on decoupling of the geometric and the thermal aspects of the problem. The concept of curves for cost-optimal design is introduced and is shown to simplify the synthesis process for shell-and-tube heat exchangers.     

Computer Techniques to Analyze Shell-and-Tube Heat Exchangers

This paper outlines computer analyses to predict thermal-hydraulics, flow-induced vibration, and fretting-wear damage in shell-and-tube heat exchangers. The analytical techniques are briefly described and the results arc illustrated by example. It is concluded that computer techniques can do much to improve the reliability and performance of heat exchangers.     

Exergetic Optimization of Shell-and-Tube Heat Exchangers Using NSGA-II

In this article, a multi-objective exergy-based optimization through a genetic algorithm method is conducted to study and improve the performance of shell-and-tube type heat recovery heat exchangers, by considering two key parameters, such as exergy efficiency and cost. The total cost includes the capital investment for equipment (heat exchanger surface area) and operating cost (energy expenditures related to pumping). The design parameters of this study are chosen as tube arrangement, tube diameters, tube pitch ratio, tube length, tube number, baffle spacing ratio, and baffle cut ratio. In addition, for optimal design of a shell-and-tube heat exchanger, the method and Bell–Delaware procedure are followed to estimate its pressure drop and heat transfer coefficient. A fast and elitist nondominated sorting genetic algorithm (NSGA-II) with continuous and discrete variables is applied to obtain maximum exergy efficiency with minimum exergy destruction and minimum total cost as two objective functions. The results of optimal designs are a set of multiple optimum solutions, called “Pareto optimal solutions.” The results clearly reveal the conflict between two objective functions and also any geometrical changes that increase the exergy efficiency (decrease the exergy destruction) lead to an increase in the total cost and vice versa. In addition, optimization of the heat exchanger based on exergy analysis revealed that irreversibility like pressure drop and high temperature differences between the hot and cold stream play a key role in exergy destruction. Therefore, increasing the component efficiency of a shell-and-tube heat exchanger increases the cost of heat exchanger. Finally, the sensitivity analysis of change in optimum exergy efficiency, exergy destruction, and total cost with change in decision variables of the shell-and-tube heat exchanger is also performed.     

A Comparative Assessment of RODbaffle Shell-and-Tube Heat Exchangers

The shell-and-tube heat exchanger (SBE), with its tubes held in plate baffles to produce cross flow of the shell-side fluid, has recently been modified to produce a RODbaffle heat exchanger (RBE) free from tube failure due to vibration. The results showed slightly enhanced heat transfer coefficients with significant reductions in pressure loss, leading to reduced cost of exchangers and in some instances smaller exchangers.     

Heat Transfer Enhancement in Shell-and-Tube Heat Exchangers Using Porous Media

A numerical simulation has been carried out to investigate the heat transfer enhancement in a shell-and-tube heat exchanger using a porous medium inside its shell and tubes, separately. A three-dimensional geometry with k-? turbulent model is used to predict the heat transfer and pressure drop characteristics of the flow. The effects of porosity and dimensions of these media on the heat exchanger's thermal performance and pressure drop are analyzed. Inside the shell, the entire tube bundle is wrapped by the porous medium, whereas inside the tubes the porous media are located in two different ways: (1) at the center of the tubes, and (2) attached to the inner wall of the tubes. The results showed that this method can improve the heat transfer at the expense of higher pressure drop. Evaluating the method showed that using porous media inside the shell, with particular dimension and porosity can increase the heat transfer rate better than pressure drop. Using this method inside the tubes leads to two diverse results: In the first configuration, pressure loss prevails over the heat transfer augmentation and it causes energy loss, whereas in the second configuration a great performance enhancement is observed.     

Thermal-Hydraulic Characteristics of Helical Baffle Shell-and-Tube Heat Exchangers

Abstract

Shell-and-tube heat exchangers are frequently used in several industrial applications. A model was developed using engineering equations solver software to predict the performance of various baffle configurations of shell-and-tube heat exchangers. The model is based on the Bell-Delaware method for the segmental baffle, while mathematical correlations for the helical baffle are provided for design and analysis purpose. The accuracy of the present model is validated against the experimental data available in the literature. The results showed that helical baffles offer much higher performance compared to the segmental baffled heat exchanger. It was found that the performance of the helical baffle increases with increasing baffle angle until it reaches an optimum value; it then starts to decrease at an angle of 42°.     

Flow-Induced Vibration in Shell-and-Tube Heat Exchangers with Double-Segmental Baffles

One of the techniques used by designers of shell-and-tube heat exchangers when they encounter a potential flow-induced vibration problem is to shift from a tube bundle with segmental baffles to one with double-segmental baffles. This results in a split of the flow into either half of the shell with lower velocities and makes it possible to reduce the unsupported tube span length while keeping below a given allowable pressure drop. Tests were performed as a part of a systematic study of water flow-induced vibration in industrial-size heat exchangers. Results for nine different double-segmental baffled bundle configurations are presented. Comparison of the results with those for similar segmental baffled bundles shows that higher flows can be tolerated without developing damaging flow-induced vibration.     

Tube Vibration and Flow Distribution in Shell-and-Tube Heat Exchangers

Avoiding flow-induced vibration damage is a major concern to the designers and operators of shell-and-tube heat exchangers. This paper discusses the characteristics of tube vibration as determined from a research program featuring tests of an industrial-size exchanger. The state of the art in understanding and predicting tube vibration and shell-side flow distribution is briefly reviewed. Finally, research needs are identified and discussed.     

Review of Improvements on Shell-and-Tube Heat Exchangers With Helical Baffles

Helical baffles are employed increasingly in shell-and-tube heat exchangers (helixchangers) for their significant advantages in reducing pressure drop, vibration, and fouling while maintaining a higher heat transfer performance. In order to make good use of helical baffles, serial improvements have been made by many researchers. In this paper, a general review is provided of developments and improvements on helixchangers, which includes the discontinuous helical baffles, continuous or combined helical baffles, and the combined multiple shell-pass helixchangers. Extensive results from experiments and numerical simulations indicate that these helixchangers have better flow and heat transfer performance than the conventional segmental baffled heat exchangers. Based on these new improvements, the conventional heat exchangers with segmental baffles might be replaced by helixchangers in industrial applications to save energy, reduce cost, and prolong the service life and operation time.     

Influence of Scaling on the Performance of Shell-and-Tube Heat Exchangers

Fouling in shell-and-tube heat exchangers was modeled by combining Hasson's ionic diffusion model for scaling from CaCO₃ solutions with a model for predicting the temperature distribution developed by Gaddis and Schlünder. Using the computed results, clean heat exchanger design rules were tested for fouling conditions. The effects of fouling on the efficiency of heat exchanger configurations were determined.     

Modeling and Prediction of Shell-Side Fouling in Shell-and-Tube Heat Exchangers

Fouling is a challenging, longstanding, and costly problem affecting a variety of heat transfer applications in industry. Mathematical models that aim at capturing and predicting fouling trends in shell-and-tube heat exchangers typically focus on fouling inside the tubes, while fouling on the shell side has generally been neglected. However, fouling deposition on the shell side may be significant in practice, impairing heat transfer, increasing pressure drops, and modifying flow paths. In this paper, a new model formulation is presented that enables capturing fouling on the shell side of shell-and-tube heat exchangers including the effect of occlusion of the shell-side clearances. It is demonstrated by means of an industrial case study in a crude oil refinery application. The model, implemented in an advanced simulation environment, is fitted to plant data. It is shown to capture the complex thermal and hydraulic interactions between fouling growth inside and outside of the tubes, the effect of fouling on the occlusion of the shell-side construction clearances, and to unveil the impact on shell-side flow patterns, heat transfer coefficient, pressure drops, and overall exchanger performance. The model is shown to predict the fouling behavior in a seamless dynamic simulation of both deposition and cleaning operations, with excellent results.     

改进粒子群算法在管壳式换热器优化设计中的应用

以管壳式换热器每年的总费用作为目标函数,采用含随机扰动算子的改进粒子群算法(IPOS)对其进行了优化.在优化设计模型中,采用Bell-Delaware法描述壳侧流体,优化变量选择管程数、换热管内径和外径及间距、管布置方式、封头类型、流体分配方式、密封条数、壳程压降和管程压降.对采用IPOS算法得到的优化结果与相关文献的结果进行了比较.结果表明:IPOS算法具有全局收敛、计算精度高、稳定性好的特点,并能获得约束条件下管壳式换热器的最优设计方案.     

Temperature Distribution and Heat Exchange in Multipass Shell-and-Tube Exchangers with Baffles

A model predicting the temperature distribution and the mean temperature difference in multipass shell-and-tube heat exchangers with baffles is presented. The exchanger is treated as a cascade of cells with mixing taking place in each fluid. From the computed results, design rules are deduced to enable the choice of the construction that leads to the highest efficiency.

The effect of leakage currents on exchanger effectiveness is ignored in this analysis and is treated separately in another paper.     

Helical Baffles in Shell-and-Tube Heat Exchangers,Part I: Experimental Verification

Performance of heat exchangers with helical baffles, or helixchangers, is discussed using the results of tests conducted on units with uarious baffle geometries. An optimum helix angle is identified at which the conversion efficiency for converting pressure drop to heat transfer on the shell side of helixchangers is maximized. Designs for standard industry applications are optimized using the analysis of test results.     

Experimental Performance Comparison of Shell-Side Heat Transfer for Shell-and-Tube Heat Exchangers with Different Helical Baffles

In this study, experiments were carried out to study the effects of baffle overlap proportion on the shell-side flow resistance and heat transfer performance of the shell-and-tube heat exchangers with helical baffles (STHXsHB). Three STHXsHB with an overlap proportion of 10% and helix angles of 20°, 30°, and 40° were tested. Comparisons were made of the experimental data of the STHXsHB with the same helix angles but 50% overlap proportion. The theory of entransy dissipation was employed to evaluate the irreversible loss in STHXsHB with different helix angles and overlap proportions. The results indicated that both the baffle overlap proportion and the helix angle have a great effect on the shell-side flow resistance and heat transfer. For a given helix angle, the comprehensive performance of STHXsHB with small overlap proportion is always better than that with large overlap proportion at the same mass flow rate or Reynolds number on the shell side. However, for the same heat transfer area, working conditions, and helix angle, the STHXsHB with large baffle overlap proportion has less irreversibility in the heat exchange process, according to the theory of entransy dissipation. In addition, experimental results demonstrated that the configuration of the relatively large helix angle and baffle overlap proportion is the preferred alternative in STHXsHB.     

The Effect of a Number of Baffles on the Performance of Shell-and-Tube Heat Exchangers

The number of baffles has an impact on the thermal-hydraulic performance of a shell-and-tube heat exchanger (STHX), thus a model was developed using Engineering Equations Solver software to solve the governing equations. The program uses Kern, Bell-Delaware, and flow-stream analysis (Wills Johnston) methods to predict both the heat-transfer coefficient and pressure drop on the shell side of an STHX. It was found that Bell-Delaware method is the most accurate method when compared with the experimental results. The effect of a number of baffles, mass flow rate, tube layout, fluid properties and baffle cut were investigated. The analysis revealed that an increase in the number of baffles increases both the heat-transfer coefficient and pressure drop on the shell-side. Increasing the mass flow rate, the heat transfer coefficient increases; however, the pressure drop increases at a higher rate. For a large number of baffles, the pressure drop decreases with an increase in the baffle cut. It also shows that the heat transfer coefficient increases at a higher rate with the square tube layout, whereas the rotated square and triangular layouts have approximately the same behavior.     

Design of Shell-and-Tube Heat Exchangers to Achieve a Specified Operating Period in Refinery Preheat Trains

Fouling dominates the design of heat exchangers used in crude oil preheat trains. It also dominates the lifetime cost of the trains, where the most important cost factor is lost profit through reduced production. Thus, the design objective should be the identification of geometries that provide acceptable performance throughout a desired operating period. This paper suggests a new design approach for shell-and-tube heat exchangers in refinery preheat trains that uses dynamic crude oil fouling models rather than conventional fouling factors to yield designs that are capable of achieving a specified operating period between cleaning operations.     

Extruded Microchannel-Structured Heat Exchangers

Abstract

In the search for more compact air/liquid heat exchangers, one possibility is to increase the heat transfer coefficient and surface area by a decrease of the size of the fluid channels. A practical example could be seen in the air/water cross-flow heat exchangers used in cars. For such exchangers, minimization of the total volume leads to a very thin structure, with a lot of small and short air channels. We have designed and patented a cross-flow heat transfer surface with microchannels that has such a structure and can be manufactured industrially at a reasonable cost by extrusion either in aluminum or in polymers. The thermo-hydraulic performance of the structure has been simulated using standard correlations and CFD codes, and prototypic structures are under investigation to validate simulations. Compared to classical heat exchangers, our design is superior in flexibility and compactness for air/liquid applications.     

Double Perforated Impingement Plate in Shell-and-Tube Heat Exchanger

This article presents a solution to a chronic problem causing repeated tube failure at shell-and-tube heat exchangers. The problem is related to the fouling process on the tubes' surface, which accumulates downstream from the impingement plate at the exchanger inlet nozzle within the first tube rows due to low velocity and vortices production. In fouling services, the suspended deposits, fouling, accumulates on the tubes' surface downstream from the impingement plate, causing under-deposit corrosion, raising the tubes' surface temperature due to lack of cooling, and accelerating fouling process. Under-deposit corrosion attacks tubes and causes repeated tube failure, costing a lot of money in terms of material, maintenance, and production losses. Normal practice of extending tube life and delaying their failure is to upgrade the tubes' metallurgy. So the article objective is to present an economical solution option through modifying the impingement plate in the shell-and-tube heat exchangers where the impingement plate is recommended by the Tubular Exchanger Manufacturers Association (TEMA). The impingement modification is to replace the solid conventional impingement plate with double spaced plates having offset holes, called double perforated impingement plates (DPIP). The objective of this work can be met by comparing the simulation of the shell-side inlet flow distribution around the conventional and modified (DPIP) impingement plates and ensuring enhancement of the flow pattern distribution at the area behind the impingement plates. Since experimental work in flow investigation is time-consuming and costly, computational fluid dynamics (CFD) Fluent software was implemented as a cost-effective helpful tool to conduct the simulation for comparison purposes. The modified impingement plate, DPIP, will destroy vortices created behind the conventional plate, retarding fouling accumulation. DPIP will enhance shell-side flow distribution downstream from the impingement plate and will stop fouling accumulation on the tubes to prevent under-deposit corrosion.