Use of high performance plate heat exchangers in chemical and process industries

At present, plate heat exchangers constantly open up new application fields in the chemical, process, and allied industries due to their numerous advantages. The channel flow between individual plates is characterized by high turbulence induced at low flow velocities. Heat transfer coefficients are generally higher in plate heat exchangers than in conventional shell-and-tube heat exchangers. According to the nature of the process, physical properties of the media, and allowable pressure drops, plates with a variety of patterns are available to adapt the equipment optimally to the specific process conditions. For handling aggressive media the module welded plate heat exchanger was developed. The laser welded modular design keeps the inherent advantages of plate type heat exchanger. It can be disassembled and mechanically cleaned outside the modules. The capacity can also be subsequently modified by changing the number of plates, or the plate patterns can be altered as it can be with the gasketed units. Typical applications of module welded plate heat exchangers in the chemical industry are acid coolers, thermal oil coolers, or condensers for hydrocarbon mixtures.     

Thermal design of multi-stream heat exchangers

The thermal design of multi-stream heat exchangers of the plate and fin type is presented. Although originally used in low temperature processes, their application is extrapolated to above temperature processes and it is shown that, conceptually, multi-stream exchangers could replace whole heat recovery networks. The approach is based on the use of temperature vs. enthalpy diagrams or composite curves, which show how a multi-stream exchanger can be subdivided into block sections that correspond to enthalpy intervals and indicate the entry and exit points of the streams. A design methodology for plate and fin exchangers in countercurrent arrangement, characterized by the maximization of allowable pressure as a design objective is extended to the design of multi-fluid exchangers. The methodology uses a thermo-hydraulic model which relates pressure drop, heat transfer coefficient and exchanger volume. The potential applicability of the methodology is demonstrated on a case study.     

Asymmetrically corrugated plate heat exchanger plates

Design flexibility for plate heat exchangers is a measure of the ability to adjust the characteristics of individual channels (semi-)independently to suit a particular application. Conventional plate heat exchangers with approximately sinusoidal corrugations have limited design flexibility since the channels for the two heat- exchanging streams are generally geometrically identical. Corrugations that are assymetrical in profile make it possible to construct physically different configurations from a set of identical plates: channels of different cross-sectional free flow area are feasible. Such plates used in various orientations with respect to each other were found to be less efficient than plates with sinusoidal corrugations because a lower heat transfer rate is obtained per unit of pumping power. However, when the flow rates or allowable pressure drops of the two streams exchanging heat differ greatly, such plates may be advantageous.     

A general calculation method for plate heat exchangers

The possibilities of process stream guidance within plate heat exchangers are very complex. There exist constructional solutions, in which the process-streams are led through stream-passages connected in series or parallel. Other constructions realize the heat transfer between more than two process streams. Known are also constructions, where the stream passages are spirals. All these constructions deviate more or less from the ideal principle of uniflow or counterflow guidance. While the approximate interpretation of plate heat exchangers is solved methodically, the only methods used for the calculation of existing apparatus are either mathematically extremely exacting or not generally valid. To analyse the behaviour of plate heat exchangers, exact and easy to apply calculation methods are required. This article presents a general calculation method for the stationary simulation of plate heat exchangers of any type required. The method allows a mathematically exact determination of the temperature profiles of types of apparatus mentioned above. In a further step the proposed method can also be used for the simulation of the heat flux along the walls of the plates as well as the dispersion in the passages. The basic idea of the presented method is the simulation of a heat exchanger network with generalized operating characteristics. The “classic formulas” of this operating characteristic are given in this method as special cases. The general calculation method for plate heat exchangers can be used without any iteration. Only the consideration of temperature dependent properties as well as phase transitions require iterations. This method is characterized by a great numerical stability.     

A general solution for one-dimensional multistream heat exchangers and their networks

A mathematical model for predicting the steady-state thermal performance of one-dimensional (cocurrent and countercurrent) multistream heat exchangers and their networks is developed and is solved analytically for constant physical properties of streams. By introducing three matching matrices, the general solution can be applied to various types of one-dimensional multistream heat exchangers such as shell-and-tube heat exchangers, plate heat exchangers and plate-fin heat exchangers as well as their networks. The general solution is applied to the calculation and design of multistream heat exchangers. Examples are given to illustrate the procedures in detail. Based on this solution the superstructure model is developed for synthesis of heat exchanger networks.     

Theoretical prediction of longitudinal heat conduction effect in cross-corrugated heat exchanger

In the elementary heat exchanger design theory, the longitudinal heat conduction through the heat transfer plate separating hot and cold fluid streams is neglected, and only the transverse heat conduction is taken into account for the conjugate heat transfer problem. In the cross-corrugated heat exchanger, the corrugated primary surface naturally leads to the highly non-uniform convective heat transfer coefficient distribution on opposite sides of the plate. In such a case, the longitudinal heat conduction may play a significant role in the thermal coupling between high heat transfer regions located on opposite sides of the plate. In the present study CFD is used to perform a quantitative assessment of the thermal performance of a cross-corrugated heat exchanger including the longitudinal heat conduction effect for various design options such as different plate thickness and corrugation geometry for a typical operating condition. The longitudinal heat conduction effect is then predicted by the theoretical method using the ‘network-of-resistance’ in the wide range of the heat exchanger design space.     

Thermal Design of Multistream Plate Fin Heat Exchangers—A State-of-the-Art Review

Multistream plate fin heat exchangers have replaced two-stream heat exchangers in diverse applications due to their compactness, capacity of handling multiple fluid streams in a single unit, and possibilities of having intermediate entry and exit of the streams. Unique features of such heat exchangers like direct/indirect crossover in temperatures due to several thermal communications among the fluid streams and the dependence of the thermal performance on “stacking pattern” have no equivalent in two-stream modules. As a consequence, an extension of the commonly used design/simulation techniques like ?-NTU or the LMTD method, applicable for two-stream exchangers, fails miserably in the case of multistream units. Though several techniques have been suggested over the years, in reality, no universally accepted methodology exists for the “thermal design” of multistream plate fin heat exchangers to date. In this communication, a state-of-the-art review of the thermal design of multistream plate fin heat exchanger is provided. Reported techniques based on heuristics, extension of the analysis applicable for two-stream heat exchangers, differential analysis, network analysis, and rigorous numerical solutions are briefly reviewed. Advantages and limitations of such techniques are also critically judged. The method of “area splitting” and “successive partitioning” proposed by the present research group is also elaborated. Apart from the basic design methodology, the techniques adopted for accounting for variable fluid properties, axial heat conduction in the solid matrix, and thermal communication with the environment have been discussed. Further, the suggested methodologies for optimizing the thermal design are reviewed. Finally, comments have been made indicating the future need of research in this topic.     

Evaluation of heat exchanger surface coatings

Compact heat exchangers are very popular due to their effectiveness, small footprint and low cost. In order to protect heat exchangers in dirty applications, coatings can be applied to the heat transfer surfaces to extend effectiveness and minimize fouling. Coating selection is extremely important since the wrong coating can decrease unit effectiveness, cause more fouling, and/or erode the surface.An experimental investigation of coating effectiveness in compact plate heat exchangers is presented. New, cleaned and coated plate heat exchangers are considered in this study. Heat exchangers have been exposed to untreated lake water for various time periods. Transient effectiveness results compare the rate of fouling for coated and uncoated heat exchangers. Additional results compare deposit weight gain at the end of the test period and transient observations of heat transfer surface appearance. All heat exchanger combinations showed some deposit accumulation for the period considered.Results indicate that the thermal performance of the unit decreases with time, resulting in an undersized heat exchanger. For the conditions considered here, uncoated plates accumulate deposits up to 50% faster than coated plates and show a decrease in performance of up to 40%. Surface coating, exposure time, fluid velocity and concentration of particles can affect fouling.     

Design optimization of shell-and-tube heat exchanger using particle swarm optimization technique

Shell-and-tube heat exchangers (STHEs) are the most common type of heat exchangers that find widespread use in numerous industrial applications. Cost minimization of these heat exchangers is a key objective for both designer and users. Heat exchanger design involves complex processes, including selection of geometrical parameters and operating parameters. The traditional design approach for shell-and-tube heat exchangers involves rating a large number of different exchanger geometries to identify those that satisfy a given heat duty and a set of geometric and operational constraints. However, this approach is time-consuming and does not assure an optimal solution. Hence the present study explores the use of a non-traditional optimization technique; called particle swarm optimization (PSO), for design optimization of shell-and-tube heat exchangers from economic view point. Minimization of total annual cost is considered as an objective function. Three design variables such as shell internal diameter, outer tube diameter and baffle spacing are considered for optimization. Two tube layouts viz. triangle and square are also considered for optimization. Four different case studies are presented to demonstrate the effectiveness and accuracy of the proposed algorithm. The results of optimization using PSO technique are compared with those obtained by using genetic algorithm (GA).     

Alternative Sizing Methodology for Compact Heat Exchangers of the Spiral Type

A rapid sizing methodology for compact heat exchangers of the spiral plate type is presented. The methodology allows for the determination of the exchanger geometry such that full pressure drop utilization is achieved on both streams. This is done by considering plate width and plate spacing as continuous variables. The resulting values are the basis for selecting the final exchanger dimensions according to standard dimensions. The design approach makes use of empirical correlations for the calculation of heat transfer coefficient and friction factor based on average curvature. The approach is demonstrated using two case studies.     

Air side heat transfer coefficients of discrete plate finned-tube heat exchangers with large fin pitch

The objective of this study is to investigate the heat transfer characteristics of discrete plate finned-tube heat exchangers with large fin pitches. Thirty-four heat exchangers were tested with variations of fin pitches, the number of tube rows, fin alignment, and vertical fin space. The j-factor of the discrete plate finned-tube exchanger was analyzed as a function of coil geometry and then compared with that of the continuous plate finned-tube heat exchanger. For fin pitches of 7.5–15 mm, the j-factors of the discrete plate finned-tube heat exchangers were 6.0–11.6% higher than those of the continuous plate finned-tube heat exchangers. Two separate correlations for the j-factor were developed for the inline and the staggered fin alignment in the discrete plate finned-tube heat exchangers to predict the measured data within a relative deviation of 2.9%.     

A comparative analysis of different heat exchangers containing phase change materials

针对日益严重的环境问题,“煤改电”已经成为实现北方清洁供暖的有效手段。此外,我国每年在余热利用,尤其是在中低温品质热能利用上还相当不充分。在此大背景下,以相变储热供热技术为切入点,着重对目前相变储热换热器进行了比较,定性分析了板式、管壳式、热管式及其他异形（储热砖/球）换热器的优缺点。并通过数值模拟的方式,定量比较了相同换热面积及边界条件下,管壳式和板式相变换热的二维相变材料熔化模型,管壳式换热器需6 h完全熔化,板式换热器需8.5 h完全熔化,主要原因在于二者在换热管/板在排布上差异导致。但考虑到相较于管壳式换热器,板式换热器结构紧凑、加工工艺简单、拆卸方便,未来可形成通过制成储热砖的方式实现模块化运行,为后期维护提供了很大便利,因此具有巨大发展潜力。     

Revamp study of crude distillation unit heat exchanger network: Energy integration potential of delayed coking unit free hot streams

This work addresses the revamp study of the crude distillation unit (CDU) heat exchanger network (HEN) of a typical refinery with and without the consideration of the free hot streams available in the delayed coking unit (DCU). Based on pinch design method, two sub-cases of revamp study have been considered namely (a) installation of new heat exchangers for the entire network and (b) reutilization of existing heat exchangers. Based on the study, it has been evaluated that the revamp design of existing CDU HEN without considering the DCU free hot streams allows the enhancement of heat integration by 4.73% with respect to that available for the base case. On the other hand, the heat integration potential of DCU free hot streams is evaluated to enhance energy integration by 15.66% (with respect to the base case) with a simultaneous reduction of furnace duty by 37.1% and cooling water duties by 89.8%. Of various cases considered, the most attractive option corresponds to the partial revamp of CDU HEN along with DCU free hot streams that involve the reutilization of existing heat exchangers. The profitability analysis of this option concludes that the revamp design needs an additional investment of 2.68 M$ to enhance annual profit by 1.58 M$ with a payback period of 1.9 years. Thereby, the heat integration potential of DCU free hot streams is inferred to be significant and marks an important choice amongst different key revamp parameters associated to heat exchanger networks.     

Investigation of heat transfer and pressure drop in plate heat exchangers having different surface profiles

It would be misleading to consider only cost aspect of the design of a heat exchanger. High maintenance costs increase total cost during the services life of heat exchanger. Therefore exergy analysis and energy saving are very important parameters in the heat exchanger design. In this study, the effects of surface geometries of three different type heat exchangers called as PHE_flat (Flat plate heat exchanger), PHE_corrugated (Corrugated plate heat exchanger) and PHE_asteriks (Asterisk plate heat exchanger) on heat transfer, friction factor and exergy loss were investigated experimentally. The experiments were carried out for a heat exchanger with single pass under condition of parallel and counter flow. In this study, experiments were conducted for laminar flow conditions. Reynolds number and Prandtl number were in the range of 50 ? Re ? 1000 and 3 ? Pr ? 7, respectively. Heat transfer, friction factor and exergy loss correlations were obtained according to the experimental results.     

Optimal design of plate heat exchangers with and without pressure drop specifications

The traditional design method for plate heat exchangers (PHEs), either ε-number of transfer units (ε-NTU) or logarithmic mean temperature difference method, involves many trials in order to meet the pressure drop constraints. This can be avoided through the developed design method, which takes the full utilization of the allowable pressure drops as a design objective. The proposed method is valid for the design situations with and without pressure drop specifications. In the case of the design with pressure drop specification, only one stream can fully utilize the allowable pressure drop. In the case of no pressure drop specification, allowable pressure drops can be determined through economical optimization. Compared to the previous design method, the proposed method does not require many trial iterations. Instead, all heat exchanger parameters, including plate size, number of passes, path, fluid velocity, etc., are determined in a straightforward way. Moreover, the suggested method can guarantee that the optimized values of allowable pressure drops can be fully utilized simultaneously by the two streams. In addition, the optimal corrugation angle is discussed for the most common chevron-type PHEs.     

Investigations of the pressure drop and flow distribution in a chevron-type plate heat exchanger

In this work the hydrodynamic characteristics and distribution of flow in two cross-corrugated channels of plate heat exchangers have been investigated. A three-dimensional model with the real-size geometry of the two cross-corrugated channels provided by chevron plates and taking into account the inlet and outlet ports has been conducted for the numerical study. The numerical results have been validated with the measurements taken by laboratory experiments. The local flow characteristics around the contact points have been discussed. The velocity, pressure and flow distribution of the fluid among the two channels of the plate heat exchanger have also been presented.     

Experimental investigation of single phase convective heat transfer coefficient in a corrugated plate heat exchanger for multiple plate configurations

Corrugated plate heat exchangers have larger heat transfer surface area and increased turbulence level due to the corrugations. In this study, experimental heat transfer data are obtained for single phase flow (water-to-water) configurations in a commercial plate heat exchanger for symmetric 30°/30°, 60°/60°, and mixed 30°/60° chevron angle plates. Experiments were carried out for Reynolds number ranging from 500 to 2500 and Prandtl number from 3.5 to 6.5. Experimental results show significant effect of chevron angle and Reynolds number on the heat transfer coefficient. Based on the experimental data, a correlation to estimate Nusselt number as a function of Reynolds number, Prandtl number and chevron angle has been proposed.     

Optimal heat extraction from an updraft biomass reactor

This paper presents the design of gas-water and gas-air heat exchangers for extraction of thermal energy from updraft biomass gasifiers in the firing rate range 10–120 kg/h. Mathematical models are developed to study the sensitivity of the heat exchanger efficiency and effectiveness to geometric and flow variables. Optimal parameters to suit the biomass reactor have been evolved. The calculated heat transfer coefficients have been compared with experimental results obtained in test gas-water and gas-air heat exchangers with an observed deviation between −25 and + 17%. In conclusion, system efficiencies of about 75–80% can be achieved by choice of appropriate operating flow regimes and heat exchanger sizes.     

Steady state simulation for the de-bottlenecking of heat recovery networks

This work presents the use of a steady state simulator for the de-bottlenecking of heat recovery networks. It is shown how a heat exchanger network designed for fixed conditions can be de-bottlenecked when process streams undergo changes in operating conditions such as flow rate and supply temperature. A network is said to be flexible if it is capable of maintaining acceptable operation either during normal or under modified conditions. The de-bottlenecking of heat recovery networks can be considered as a special case of the design for improved flexibility. A simulation model for a single phase network of heat exchangers is presented. The model is based on the use of the thermal effectiveness (ε) parameter for heat exchangers. Any type of exchanger configuration and flow arrangement can be modeled by using the appropriate ε–number of transfer units relationships. A general methodology for improving network flexibility is proposed.     

Numerical study of heat pipe application in heat recovery systems

Heat pipes are two-phase heat transfer devices with extremely high effective thermal conductivity. They can be cylindrical or planar in structure. Heat pipes can be embedded in a metal cooling plate, which is attached to the heat source, and can also be assembled with a fin stack for fluid heat transfer. Due to the high heat transport capacity, heat exchangers with heat pipes have become much smaller than traditional heat exchangers in handling high heat fluxes. With the working fluid in a heat pipe, heat can be absorbed on the evaporator region and transported to the condenser region where the vapour condenses releasing the heat to the cooling media. Heat pipe technology has found increasing applications in enhancing the thermal performance of heat exchangers in microelectronics, energy and other industrial sectors.Utilisation of a heat pipe fin stack in the drying cycle of domestic appliances for heat recovery may lead to a significant energy saving in the domestic sector. However, the design of the heat pipe heat exchanger will meet a number of challenges. This paper presents a design method by using CFD simulation of the dehumidification process with heat pipe heat exchangers. The strategies of simulating the process with heat pipes are presented. The calculated results show that the method can be further used to optimise the design of the heat pipe fin stack. The study suggests that CFD modelling is able to predict thermal performance of the dehumidification solution with heat pipe heat exchangers.