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.     

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.     

Modeling and Coupling Particle Scale Heat Transfer with DEM through Heat Transfer Mechanisms

The detailed heat transfer mechanisms particle interior, gas film around particles, gas gap between contact surfaces, and rough surface are considered to model heat transfer between particles. The validation of the heat transfer model is accomplished and the predicted results show good agreement with other experiments. From the quantitative comparison of four heat transfer paths, it is revealed that the heat transfer through gas gap and rough surface could be neglected for a particle diameter larger than 2 mm. Furthermore, the detailed heat transfer model is coupled with the discrete element method (DEM) to calculate macro effective thermal conductivity (ETC) of fixed beds, and the accuracy and applicability is verified by comparing with other estimated and experimental results. The influence of particle diameter, density, specific thermal capacity, and thermal conductivity on ETC is investigated. Results show that the proposed heat transfer model provides an effective and accurate way to couple with DEM in the particle system.     

Use of C-factor for monitoring of fouling in a shell and tube heat exchanger

Heat exchangers operating in process industries are fouled during operations and results in decrease in the thermal efficiency of a heat exchanger. Once the thermal efficiency decreases to a minimum acceptable level, cleaning of the equipment becomes necessary to restore the performance. This paper uses C-factor as a tool for investigation of the performance of a heat exchanger due to fouling which consequently gives information regarding the extent of fouling developed on the heat transfer surfaces. The fouling parameters are predicted by measurements of flow rate and pressure drop. In contrast to most conventional methods, the extent of fouling can be detected considering the flow rate and pressure drop when the heat exchanger operates in transient states. The C-Factor is first calculated through out cleaning period and then compared with the clean and the design value. The results show that the proposed tool is very effective in detecting the fouling developed and the corresponding degradation in heat transfer efficiency of a heat exchanger. Hence the results of this work can find applications in predicting the reduction in heat transfer efficiency due to fouling in heat exchangers that are in operation and assist the exchanger operators to plan cleaning schedules.     

Study of Scale-Removal Methods in a Double-Pipe Heat Exchanger

The objective of the present study was to examine the effectiveness of four cleaning methods for the removal of tube-side fouling in a double-pipe heat exchanger. The four cleaning methods are (1) hydrolazing with 10,000-psi head pressure, (2) hydrolazing with 20,000-psi head pressure, (3) chemical cleaning, and (4) brush punching. Fouled tubes were prepared using water from a cooling tower. Each test started with a brand new tube. When the overall heat transfer coefficient dropped by 40% from the initial peak value, scales in the fouled tubes were removed using one of the above four methods. Both the overall heat transfer coefficient and inside diameter were measured before and after fouling, by which the effectiveness of a particular cleaning method was evaluated. The chemical cleaning method was found to be most effective, whereas the brush punching was least effective. When a fouled tube was kept dry, brush punching completely removed scales, indicating that surface wetness plays an important role in scale removal.     

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.     

椭圆形换热单管积灰特性的数值模拟研究

椭圆形换热管作为一种强化换热元件,在抗积灰性能方面具有一定优势.本文基于ANSYS FLUENT软件平台建立了一套模拟程序,针对椭圆形换热单管的积灰特性展开了数值模拟研究.重点研究了换热管的椭圆度、烟气流速以及飞灰颗粒粒径对飞灰沉积特性的影响.研究表明,当换热管的椭圆度在1～2之间变化时,5～100μm粒径的颗粒沉积率...     

Heat Exchanger Fouling in Phosphoric Acid Evaporators

Multistage shell and tube evaporators are frequently used in phosphoric acid plants to increase the concentration of dilute phosphoric acid to 52–55 wt% P₂O_5. The concentrated phosphoric acid solution is supersaturated with respect to calcium sulfate. As a result, part of the calcium sulfate in the liquor deposits on the heat exchanger tube walls. Because the thermal conductivity of these scales is very low, thin deposits can create a significant resistance to heat transfer. Therefore, regular cleaning of heat exchangers is required, frequently at shorter than biweekly intervals. As the major costs in modern phosphoric acid plants are the cost of energy, a thorough understanding of the fouling kinetics and of the effects of various operational parameters on the behavior of calcium sulfate is required to improve operation and design of the shell and tube heat exchangers, which are extensively used. In this investigation, a large set of heat exchanger data was collected from shell and tube heat exchangers of the phosphoric acid plant of the Razi Petrochemical Complex in Iran, and the fouling deposits were analyzed with respect to appearance and composition. The overall heat transfer coefficients and fouling resistances were evaluated at different times, and a kinetic model for the crystallization fouling was developed. It is shown that the crystallization rate constant obeys an Arrhenius relationship with an activation energy of 57 kJ/mol. The average absolute error of 12.4% shows that the predictions of the suggested model are in good agreement with the original plant data.     

Study of the Effect of Fin-and-Tube Heat Exchanger Fouling on its Structural Performance

This paper presents the thermal and structural analysis of high temperature fin-and-tube heat exchanger. Water flowing in tubular space and flue-gas flowing in the intertubular space, were considered as working fluids. The effect of limescale fouling on thermal and structural performance of heat exchanger was studied. The analysis considered an industrial heat exchanger, which failure occur from time to time. The expert inspection, after the failure indicated the existence of fouling layer within the heat exchanger tubes. In order to understand the reasons of heat exchanger failure, a detailed fluid flow analysis (both in the tubular and intertubular spaces) was performed. The analysis indicated that the silicate limescale fouling layer with thermal conductivity of 0.35 W/(mK) and thickness up to 1.5 mm existing in the tube, may increase the tube wall temperature even more than 150°C. The study also includes the impact of outer tube wall surface fouling with thickness of 0.2 mm and heat transfer coefficient of 2 W/(mK). As a result, the compressible stresses may increase over three times compared to the situation where the tube wall fouling does not exist.     

Heat recovery from hot solid particles in a shallow fluidized bed

A heat exchanger with a shallow gas–solid fluidized bed was experimentally studied in order to analyze energy recovery from solid particles leaving a combustion process. The experiments were carried out with and without vertical baffles in a fluidized bed with immersed horizontal tubes filled with water, in a counter flow arrangement. Two particle diameters (254 and 385 μm), two solid flow rates (50 and 80 kg/h) and two gas flow rates (46 and 50 kg/h) were tested. The bed temperature along the equipment length, the mass flow rate and the inlet and outlet temperatures of solid particles, air and water were measured in order to obtain the bed-tube heat transfer coefficient and the heat exchanger effectiveness. An increment of about 55% in the heat transfer coefficient and higher values of the heat exchanger effectiveness, in experiments with the presence of baffles, was verified. The experimental results also showed that the suspension-wall heat transfer coefficient increased considerably with the solid flow rate and also when the particle diameter decreased.     

Heat transfer and pressure loss characteristics of a heat exchanger for recovering latent heat (effects of straight fin length and fin pitch on the heat transfer and pressure loss characteristics)

In recent years, the requirement for the reduction of energy consumption has been increasing to solve the problems of global warming and the shortage of petroleum resources. A latent heat recovery type heat exchanger is one of the effective methods for improving thermal efficiency by recovering latent heat. This paper describes the heat transfer and pressure loss characteristics of a latent heat recovery type heat exchanger having straight fins (fin length: 65 mm or 100 mm, fin pitch: 2.5 mm or 4 mm). These were clarified by measuring the exchange heat quantity, the pressure loss of the heat exchanger, and the heat transfer coefficient between the outer fin surface and gas. The effects of fin length and fin pitch on heat transfer and pressure loss characteristics were clarified. Furthermore, equations for predicting the heat transfer coefficient and pressure loss which are necessary for heat exchanger design were proposed. ©2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(4): 230– 247, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20153 Copyright © 2004 Wiley Periodicals, Inc.     

一种监测换热器污垢的新方法

在考虑污垢对换热器传热性能影响的基础上,提出换热器当量总污垢热阻和污垢函数的概念,并给出换热器当量总污垢热阻的监测方法,讨论了换热流型、传热有效度ε和冷热流体热容量率比R对换热器当量总污垢热阻的影响。     

Air-Side Fouling of Compact Heat Exchangers for Discrete Particle Size Ranges

The industrial problem of the air-side fouling of compact heat exchangers has been studied in a laboratory wind tunnel for particles in the μm to mm range. The measurements of pressure drop across the exchanger as well as the quantification and classification of the particles blocked by and passing through it were taken for discrete ranges of particle sizes. Observation showed that the blocked particles either drop to the wind tunnel floor, remain on the outside surface of the exchanger (falling to the floor once the wind velocity drops), or penetrate a short distance into it according to the relative values of fin spacing and particle size. These last are the most detrimental to exchanger performance. There is a critical particle size for which this penetration is maximal. A brief non-dimensional geometrical analysis allows one to predict this critical size range for any finned exchanger. It has been found to be between 0.5–0.7 times the diameter of the largest sphere that can be inscribed between the fins. Confirmation of this was found with a second exchanger. The addition of humid conditions within the tunnel or on the exchanger itself did not modify these values. Subtracting the pressure drop due to the clean exchanger from the total measured value confirms that the foulant acts like an extra mechanical filter in series with the exchanger. This is quite understandable given the short penetration length of the particles (up to 3 mm). Finally, the effects of a closed wind tunnel test section on the measurements for non-isothermal conditions in the exchanger are discussed.     

Comparative study of mechanical and chemical methods for surface cleaning of a marine shell‐and‐tube heat exchanger

In this article, experimental analysis is done on shell‐and‐tube heat exchanger of a marine vessel for removal of fouling using optimum surface‐cleaning techniques. The main objective is to compare the performance of the heat exchanger before and after maintenance. Two identical deteriorated systems of heat exchangers are taken and real‐time analysis is conducted. The log data are taken before and after undergoing maintenance for the two systems. Two different cleaning techniques are used, namely, chemical cleaning and mechanical cleaning. Detailed calculations are made for the shell‐and‐tube heat exchanger. From the obtained data, comparisons are made for different parameters on the tube side such as friction factor, heat transfer coefficient and pressure drop, as well as total heat transfer rate on the shell side. From the analysis and comparison, it was found that greater heat transfer takes place for the tubes cleaned using the chemical cleaning method than for tubes cleaned by the mechanical cleaning method. Pressure drop is found to be less for chemical cleaning method than mechanical cleaning method. This indicates that the fouling effect is reduced for tubes cleaned by the chemical cleaning method, and furthermore these tubes remain corrosion‐resistant for longer periods of time.     

考虑污垢时换热器热力学性能的评价

在分析污垢对换热器传热性能影响的基础上,在考虑污垢时采用Ｙｏｎｇ损率这一指标对换热器的热力学性能进行了评价,讨论了传热数和冷热流体热容量率比对其性能的影响,并把结果与不考虑污垢时的情况进行了比较,得到了一些有益的结论。     

壳管式海水换热器污垢状况的火用评价方法研究

分析了壳管式海水换热器管程结垢后换热强度及流动压降变化对换热器火用损失的影响,提出了一种利用(火用)损失系数评价换热器污垢状况的方法.该方法比通过检测污垢热阻评价换热器污垢状况的方法更全面,更简捷.     

Influence of gas velocity on particulate fouling of exhaust gas recirculation coolers

The objective of this research is to study the influence of gas flow velocity on particulate fouling of exhaust gas recirculation (EGR) coolers. An experimental setup has been designed and constructed to simulate particulate fouling in EGR coolers in diesel engines. The setup consists of soot generator, gas/particle flow heater, testing section for EGR coolers and finally an exhaust system. Two sets of fouling experiments have been performed with and without water injection, and the gas velocity in each set has varied between 30, 70 and 120 m/s. The concentration of soot particles in the gas flow is 100 mg/m³, and the average diameter of the particles is 130 nm with a standard deviation of 55 nm. It has been found that the thermal resistance and thickness of the fouling layer and the fouling rate decrease as the gas velocity in the EGR cooler increases. If EGR coolers are operated with a gas velocity, which is just lower than the critical flow velocity for the largest particle in the flow, quick deterioration of the thermal performance of the heat exchanger will nevertheless occur. This strongly indicates that the gas velocity should exceed a certain critical flow velocity in order to prevent particulate fouling. In addition, the presence of water vapour in the gas flow improves the thermal performance of the cooler and decreases the fouling rate, and its influence decreases as the gas velocity increases.     

惰性粒子流化床蒸发器强化传热性能的研究

本文对惰性粒子流化床蒸发器的强化传热进行了研究，并对惰性粒子流化床蒸发器的防、除垢性能进行了探讨。     

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.     

Effect of Geometry and Flow Conditions on Particulate Fouling in Plate Heat Exchangers

This article describes particulate fouling experiments performed on small-scale and full-scale plate heat exchangers for three different corrugation angles (30 deg, 45 deg and 60 deg). The velocity effect has been studied as well as the particle type and concentration effects. The test duration ranges between 20 and 1,500 h in order to reach asymptotic behavior. The results clearly indicate that the corrugation angle has a major influence on the asymptotic fouling resistance. Increasing the corrugation angle leads to lower values for the fouling resistance. Furthermore, for a given corrugation angle, the asymptotic fouling resistance is inversely proportional to the velocity squared. Finally, the asymptotic fouling resistance is proportional to the particle concentration. Fouling mitigation can be obtained by taking into account at the design stage the heat exchanger geometry and fluid velocity.