Heat exchanger fouling model and preventive maintenance scheduling tool

The crude preheat train (CPT) in a petroleum refinery consists of a set of large heat exchangers which recovers the waste heat from product streams to preheat the crude oil. In these exchangers the overall heat transfer coefficient reduces significantly during operation due to fouling. The rate of fouling is highly dependent on the properties of the crude blends being processed as well as the operating temperature and flow conditions. The objective of this paper is to develop a predictive model using statistical methods which can a priori predict the rate of the fouling and the decrease in heat transfer efficiency in a heat exchanger. A neural network based fouling model has been developed using historical plant operating data. Root mean square error (RMSE) of the predictions in tube- and shell-side outlet temperatures of 1.83% and 0.93%, respectively, with a correlation coefficient, R², of 0.98 and correct directional change (CDC) values of more than 92% show that the model is adequately accurate. A case study illustrates the methodology by which the predictive model can be used to develop a preventive maintenance scheduling tool.     

The impact of fouling on performance evaluation of evaporative coolers and condensers

Fouling of evaporative cooler and condenser tubes is one of the most important factors affecting their thermal performance, which reduces effectiveness and heat transfer capability with time. In this paper, the experimental data on fouling reported in the literature are used to develop a fouling model for this class of heat exchangers. The model predicts the decrease in heat transfer rate with the growth of fouling. A detailed model of evaporative coolers and condensers, in conjunction with the fouling model, is used to study the effect of fouling on the thermal performance of these heat exchangers at different air inlet wet bulb temperatures. The results demonstrate that fouling of tubes reduces gains in performance resulting from decreasing values of air inlet wet bulb temperature. It is found that the maximum decrease in effectiveness due to fouling is about 55 and 78% for the evaporative coolers and condensers, respectively, investigated in this study. For the evaporative cooler, the value of process fluid outlet temperature T_p,out varies by 0.66% only at the clean condition for the ambient wet bulb temperatures considered. Copyright © 2005 John Wiley & Sons, Ltd.     

Fouling Reduction Characteristics of a No-Distributor-Fluidized-Bed Heat Exchanger for Flue Gas Heat Recovery

Heat exchangers and heat exchanger networks are extensively used for the purpose of recovering energy. In conventional flue gas heat recovery systems, the fouling by fly ashes and the related problems such as corrosion and cleaning are known to be major drawbacks. To overcome these problems, a single-riser no-distributor-fluidized-bed heat exchanger is devised and studied. Fouling and cleaning tests are performed for a uniquely designed fluidized bed-type heat exchanger to demonstrate the effect of particles on the fouling reduction and heat transfer enhancement. The tested heat exchanger model (1 m high and 54 mm internal diameter) is a gas-to-water type and composed of a main vertical tube and four auxiliary tubes through which particles circulate and transfer heat. Through the present study, the fouling on the heat transfer surface could successfully be simulated by controlling air-to-fuel ratios rather than introducing particles through an external feeder, which produced soft deposit layers with 1 to 1.5 mm thickness on the inside pipe wall. Flue gas temperature at the inlet of heat exchanger was maintained at 450°C at the gas volume rate of 0.738 to 0.768 CMM (0.0123 to 0.0128 m³/sec). From the analyses of the measured data, heat transfer performances of the heat exchanger before and after fouling and with and without particles were evaluated. Results showed that soft deposits were easily removed by introducing glass bead particles, and also heat transfer performance increased two times by the particle circulation. In addition, it was found that this type of heat exchanger had high potential to recover heat of waste gases from furnaces, boilers, and incinerators effectively and to reduce fouling related problems.     

Analysis of the influence of operating conditions on fouling rates in fired heaters

Fouling due to chemical reaction in preheat trains for the processing of crude oil plays a key role in the operation and maintenance costs and on greenhouse emissions to atmosphere in crude processing plants. A preheat train consists of a set of heat transfer units that provide the crude oil stream the required amount of thermal energy to reach its target temperature either by heat recovery or by direct firing. Fired heaters supply external high temperature heating through the burning of fuel which result in complex heat transfer processes due to the large temperature and pressure changes and vaporization that takes place inside the unit. In this work, a thermo-hydraulic analysis of the performance of fired heaters is carried out through the application of commercial software to solve the mathematical models using finite difference methods; the analysis is applied to the crude side of a vertical fired heater in order to evaluate the impact of process conditions such as throughput and crude inlet temperature (CIT) on the fouling that take place at the early stages of operation. Using a fouling rate model based on thermo-hydraulic parameters, fouling rates are predicted assuming steady state operation and clean conditions. Although variations in process conditions are known to influence fouling rates, little work has been done on the subject. In this work excess air and steam injection are studied as a means to mitigate fouling. Results show that throughput reduction brings about a marked increase in the fouling rates. A decrease in CIT affects only the convection zone and it is found that this effect is negligible. In terms of excess air, it is found that although it affects negatively the heater efficiency it can be used to balance heat transfer between the convection and radiation zone in a way that fouling rates are reduced; however this strategy should be considered right from the design stage. Finally it is observed that steam injection is an effective method to reduce fouling rates since it results in lower film temperatures and larger shear stress.     

运行工况对多向扰流强化管CaSO4污垢特性的影响

为更深入并准确研究运行工况条件对多向扰流强化管CaSO_4污垢特性的影响,基于FLUENT软件的UDF功能构建了恒壁温条件下结垢传质过程与温度场的耦合作用关系,进一步采用田口法对运行工况致垢的贡献率进行了模拟比较,分析了贡献率较大的运行工况对污垢特性的影响。结果表明:溶液溶度致垢的贡献率占53.2%,而壁面温度、进口流速和进口温度的贡献率分别为22.2%、19.3%和5.3%;溶液溶度在4.0～2.5 kg/m~3,污垢热阻降低达90.47%,并且随溶度降低其相邻溶度间降低比例基本不变;壁面温度在340.0～315.0 K时,污垢热阻降低了65.22%,在前一阶段相邻温度间降低比例基本上不变,当达到320.0 K后降低明显;流速在1.0～2.5 m/s时,随流速的增加,污垢热阻降低68.65%,且随流速的增加,相邻流速间降低的速度明显减缓。     

Numerical analysis of heat transfer and flow field around cross-flow heat exchanger tube with fouling

Fouling is one of the main problems of heat transfer which can be described as the accumulation on the heat exchanger tubes, i.e.; ash deposits on the heat exchanger unit of the boiler. A decrease in heat transfer rate by this deposition causes loss in system efficiency and leads to increasing in operating and maintenance costs. This problem concerns with the coupling among conduction heat transfer mode between solid of different types, conjugate heat transfer at the interface of solid and fluid, and the conduction/convection heat transfer mode in the fluid which can not be solved analytically. In this paper, fouling effect on heat transfer around a cylinder in cross flow has been studied numerically by using conjugate heat transfer approach. Unlike other numerical techniques in existing literatures, an unstructured control volume finite element method (CVFEM) has been developed in this present work. The study deals with laminar flow where the Reynolds number is limited in the range that the flow field over the cylinder is laminar and steady. We concern the fouling shape as an eccentric annulus with constant thermal properties. The local heat transfer coefficient, temperature distribution and mean heat transfer coefficient along the fouling surface are given for concentric and eccentric cases. From the results, we have found that the heat transfer rate of cross-flow heat exchanger depends on the eccentricity and thermal conductivity ratio between the fouling material and fluid. The effect of eccentric is dominant in the region near the front stagnation point due to high temperature and velocity gradients. The mean Nusselt number varies in asymptotic fashion with the thermal conductivity ratio. Fluid Prandtl number has a prominent effect on the distribution of local Nusselt number and the temperature along the fouling surface.     

污垢对管内层流换热性能影响的热力学分析和评价

采用热力学第二定律，分别在恒壁温和恒热流两种典型工况下分析了污垢对管内层流换热性能的影响；引入单位传热量的熵增率对污垢管道的热力学性能进行了评价；讨论了管内流体雷诺数(无污垢时)、量纲为1的入口换热温差、量纲为1的热流密度和污垢层厚度等参数对单位传热量熵增率的影响；并把结果和紊流时的对应工况进行了比较。结果可为工程上换热设备的优化设计提供依据。     

Mathematical simulation of lithium bromide solution laminar falling film evaporation in vertical tube

For utilization of the residual heat of flue gas to drive the absorption chillers,a lithium-bromide
falling film in vertical tube type generator is presented.A mathematical model was developed to
simulate the heat and mass coupled problem of laminar falling film evaporation in vertical tube.In the
model,the factor of mass transfer was taken into account in heat transfer performance calculation.The
temperature and concentration fields were calculated.Some tests were conducted for the factors
such as Re number,heating flux,the inlet concentration and operating pressure which can affect the
heat and mass transfer performance in laminar falling film evaporation.The heat transfer performance is
enhanced with the increasing of heat flux.An increasing inlet concentration can weaken the heat
transfer performance.The operating pressure hardly affects on heat and mass transfer.The bigger inlet
Re number means weaker heat transfer effects and stronger mass transfer.The mass transfer
obviously restrains the heat transfer in the falling film solution.The relation between dimensionless
heat transfer coefficient and the inlet Re number is obtained.     

An advanced model to assess fouling and slagging in coal fired boilers

The assessment of the influence of fouling and slagging on the heat transfer in utility boilers has obtained significant interest both during boiler design and operation. This paper presents a strategy to investigate this influence by introducing heat resistance to represent fouling and slagging on furnace walls. The evaluation of this strategy was performed using the plant heat flux reading data together with a 3‐D computational fluid dynamics code. The model considers the influence of the operating conditions to the incident heat fluxes and these are used to assess the heat transfer resistance from the measured absorbed heat fluxes. Copyright © 2002 John Wiley & Sons, Ltd.     

Evaluation and optimization of water source heat pump for district heating: A case study in steel plant

Using high-temperature heat pump technology to recover waste heat of circulating cooling water in a steel plant for central heating system not can only reduce the temperature of circulating cooling water to meet the needs of smelting process but also can save energy and protect environment as well as bring great economic benefits to steel plant that can sell heat to the heat users. The energy consumption equation of heat pump central heating system was established based on the energy consumption of heat pump, energy consumption of water pump, and heat loss. The optimal inlet water temperature, inlet flow rate, and the number of operating heat pump modules at different outdoor temperatures were calculated by genetic algorithm. The superiority and operating control strategy of heat pump central heating system were discussed. The results show that with the increase of outdoor temperature, the optimal inlet flow rate and the number of operating heat pump module decrease. However, the inlet water temperature almost does not change. It is more suitable for the heat pump central heating system to change the inlet flow and the number of operating heat pump modules. The operating control strategy equation was established by linear fitting, which provides guidance for the engineering application of heat pump central heating system.     

Impact of Nonuniform Fouling on Operating Temperatures in Heat Exchanger Networks

ABSTRACT

Investigations of fouling in heat exchangers are mainly focused on two factors: commercial impact due to energy losses, and environmental impact manifested through higher CO₂ emissions. The purpose of this paper is to introduce a third factor relating to safety in operations. This paper presents two case studies, one for a hydroprocessing unit with feed/effluent heat exchangers and another for preheat train exchangers installed upstream of the atmospheric furnace in a refinery crude unit. Due to a wide range of process temperatures examined in both case studies, the heat exchangers in the network are subject to various fouling mechanisms. As illustrated in the pictures of actual tube bundles, some of the exchangers within the network are heavily fouled, while the other exchangers operate in nearly clean conditions. Detailed simulations indicate that nonuniform fouling results in heat exchanger operating temperatures that are significantly higher than those predicted by conventional analyses using uniform fouling. Higher than anticipated process fluid temperatures may result in exceeding the threshold limits for certain corrosion mechanisms and/or significantly higher than expected rates of corrosion.     

Effect of Flow Distribution in Parallel Heat Exchanger Networks: Use of Thermo-Hydraulic Channeling Model in Refinery Operation

Abstract

Parallel branches are commonly observed in industrial heat exchanger networks (HENs). Despite the important relationship between flow distribution and network efficiency, not all parallel branches comprise of flow controllers or not least, flow measurements. When the network is subject to fouling, uncontrolled flow branches can introduce undesired phenomenon such as thermo-hydraulic channeling (THC) [presented at the 2007 HEFC conference; Ishiyama et al., Effect of fouling on heat transfer, pressure drop and throughput in refinery preheat trains]. Recent analysis of crude preheat train heat exchangers has shown the need to use THC models, in particular, for situations where there is insufficient flow measurement data, especially in nonsymmetric branches. This paper revisits the THC model and highlight practical importance of the THC phenomenon through analysis of plant data. The hydraulic aspect of the analysis is strongly linked to the knowledge of deposit thermal conductivity. A case study of a section of a crude refinery HEN is used to illustrate the use of thermo-hydraulic models in data reconciliation to understand flow imbalances caused due to differences in operating conditions and fouling of heat exchangers in each branch of a parallel network.     

Aging is Important: Closing the Fouling–Cleaning Loop

Process units subject to fouling often require regular cleaning, giving rise to repeated cycles of fouling and cleaning. The initial stages of fouling are strongly influenced by the effectiveness of the most recent cleaning step and, similarly, the effectiveness and rate of cleaning are determined by the extent and nature of the deposit layer present on the surface. The optimal operating cycle will therefore be determined by fouling–cleaning interactions. Deposit aging is an important factor in this, as an aged deposit is usually more difficult to clean. Aging therefore introduces an element of choice into fouling–cleaning operating cycles, between in situ “chemical” methods and ex situ “mechanical” methods, with associated differences in effectiveness, time, and cost. This paper reports a reformulation of the cleaning scheduling problem to consider the choice of cleaning method, as well as the timing of cleaning. A case study based on a shell-and-tube heat exchanger processing crude oil is used to illustrate the concepts and scope of application of this approach. A novel and more general formulation of the problem, linking design, fouling, and cost aspects via dimensionless groups, is then presented and illustrated with a second case study based on a simpler exchanger model.     

Performance analysis of organic Rankine cycle based on location of heat transfer pinch point in evaporator

The location of heat transfer pinch point in evaporator is the base of determining operating parameters of organic Rankine cycle (ORC). The physical mathematical model seeking the location of pinch point is established, by which, the temperature variations both of heat source and working fluid with UA can be obtained. Taking heat source with inlet temperature of 160 °C as example, the matching potentials between heat source and working fluid are revealed for subcritical and supercritical cycles with the determined temperature difference of pinch point. Thermal efficiency, exergy efficiency, work output per unit area and maximum work outputs are compared and analyzed based on the locations of heat transfer pinch point either. The results indicate that supercritical ORC has a better performance in thermal efficiency, exergy efficiency and work output while outlet temperature of heat source is low. Otherwise, subcritical performs better. Small heat transfer coefficient results in low value of work output per unit area for supercritical ORC. Introduction of IHX may reduce the optimal evaporating pressure, which has a great influence on heat source outlet temperature and superheat degree. The analysis may benefit the selection of operating parameters and control strategy of ORC.     

editorial

Fouling of heat exchangers is a chronic problem in processing industries. In addition to the appropriate selection of operating conditions and exchanger geometry, there are numerous chemical and mechanical methods to mitigate fouling and to remove deposits from the heat transfer surfaces. However, all methods to reduce fouling require some understanding of the mechanisms of the deposition process and of the structure and adhesion of deposits on the heat transfer surfaces. Almost exactly 50 years ago, D. Q. Kern and his co-author, R. E. Seaton, published a paper attempting to describe the growth of fouling deposits in terms of an unsteady-state heat and mass balance for the heat transfer surface. More or less at the same time, the TEMA fouling resistances were published based on operational and anecdotal evidence of fouling for a range of heat exchanger applications. These two approaches have since formed the basis for most heat transfer fouling models and heat exchanger designs. Increased costs of energy, raw materials, and production downtime have contributed to the growing interest in heat transfer fouling. More recently, environmental legislation has put additional pressure on fouling-related CO₂ emissions and disposal of cleaning chemicals. Despite these efforts, fouling of heat exchangers is still far from been understood in its whole complexity. The present paper documents the 2009 D. Q. Kern Award Lecture in which some selected aspects of fouling research to date have been presented and areas have been identified where significant research and development activities are still required.     

Evaluation of Crude Oil Heat Exchanger Network Fouling Behavior Under Aging Conditions for Scheduled Cleaning

Heat exchanger network (HEN) fouling is an endemic operational challenge prevalent in many process industries. Its impact on both plant operating cost and productivity is significant and can be compounded by aging effects of the foulant. In this paper, we model and simulate the effect of aging on tube-side fouling and cleaning dynamics in a crude oil refinery preheat train (PHT) comprising a 14-unit HEN. A prescient, HEN modeling and dynamic simulation were performed wherein the transients of fouling and aging as well as the interactions between individual units were captured. To assess the temporal effects, different crude oil deposit (gel) aging scenarios (no aging vs. slow, medial, and fast aging) in the downstream units were considered for the PHTs’ overall heat recovery, cleaning options, and operability. The results show that the deleterious impact of fouling and concomitant aging, quantified in terms of thermal resistances, was significantly reduced by fast aging as opposed to medial, slow, or no aging of the gel deposit. Faster aging rate reflected improved heat recovery and a lesser demand for and lower cost of PHT cleaning. The concomitant higher growth of coke deposit due to aging, however, resulted in greater hydraulic resistance, which is inimical to operability.     

换热表面结垢对U型管蒸汽发生器传热性能的影响分析

U型管蒸汽发生器的壳侧沉积了来自二回路系统中的腐蚀产物,结垢导致热量聚积在金属换热管上,容易造成垢下热点腐蚀,危害设备安全。为了明确结垢对蒸汽发生器传热性能的影响,本研究基于仿真平台APROS建立了U型管蒸汽发生器的分布式模型,并根据已公开论文中的数据进行了模型准确性验证;推导了污垢热阻与表面换热系数之间的关系式,分析了不同结垢厚度、位置对U型管蒸汽发生器换热区域的传热管壁面温度、流体温度、传热系数、热流密度等的影响程度。研究结果表明：随着结垢程度的加剧,蒸汽发生器的换热效率不断降低,出口蒸汽品质不断下降;结垢对沸腾段换热效率的影响比对过冷段换热效率的影响更大。     

Online Fouling Detection of Domestic Hot Water Heat Exchangers

Due to hardness of cold water supply in many countries, there is a risk of fouling in domestic hot water (DHW) counterflow plate heat exchangers. The scaling will result in increased resistance to heat transfer, which has negative effects on the economics of the district heating network. A common approach is to clean or change the heat exchanger periodically, which can be expensive if only limited fouling has occurred (unnecessary) or if a higher than expected scaling layer has formed (inefficiency). A better approach is to monitor the state of the heat exchangers and clean them when actually required. This would result in more energy-efficient operation and provide an optimum schedule for heat exchanger cleaning. This can be simple if the heat exchangers are operating under steady-state conditions; however, if large variations in the inlets are experienced, as is the case with the mass flows in DHW heat exchangers, it quickly becomes impossible with standard methods. In this paper it is proposed to monitor the state of the heat exchanger online by using measurements that are easily obtainable under normal operation and applying fast mathematical models to estimate the overall heat transfer coefficient of the heat exchanger. The results show that the methods proposed can be used to detect fouling in DHW heat exchangers.     

海水淡化系统水平管降膜蒸发器传热系数研究

针对海水淡化系统水平管降膜蒸发器,总结和分析管内冷凝侧与管外蒸发侧的换热系数关联式,比较管内径、入口蒸汽流速、蒸汽冷凝温度、出口蒸汽干度对管内蒸汽冷凝侧换热系数的影响;研究传热温差以及喷淋密度对管外蒸发侧换热系数的影响。结合不同的污垢系数,进行了总传热系数的影响因素分析,为海水淡化系统的工程设计提供依据。     

Multi-physics modeling of a symmetric flat-tube solid oxide fuel cell with internal methane steam reforming

Methane is regarded as one of the ideal fuels for solid oxide fuel cells (SOFCs) due to its huge reserves and transportation properties. In this study, a 3D numerical model coupling with chemical reaction, electrochemical reaction, mass transfer, charge transfer, and heat transfer is developed to understand the heat and mass transfer processes of methane steam direct internal reforming based on double-sided cathodes (DSC) SOFC. After the model verification, the parametric simulations are performed to study the effects of operating voltage, inlet temperature, and steam to carbon (S/C) ratio on the performance of a DSC. It is found that the non-uniform distribution of flow rate among channels results in the non-uniform distribution of each physical field. Increasing the inlet temperature significantly enhances the performance of DSC, however, when the temperature is above 1073 K, the concentration loss and the temperature gradient of DSC increase, which is not conducive to the long-term operation of the DSC. In addition, we revealed the effect of the S/C ratios on the heat and mass transfer process. This study provides an insight into the heat and mass transfer process of SOFC with a mixture of steam and methane and operating conditions for enhancing the performance.