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.     

Fluid temperature and velocity effect on fouling

A wide variety of industrial processes involve the transfer of heat energy between fluids in process equipment. As a result of this energy exchange unwanted deposits accumulate on the process surfaces causing a resistance to energy transfer. These deposits reduce the heat recovery and can restrict fluid flow in the exchanger by narrowing the flow area. Prevention and control of fouling is costly and time consuming. In many situations, fouling can be reduced but not necessarily eliminated. Fouling is a major unresolved problem in heat transfer.In general, the heat exchangers evaluated in this study were exposed to untreated lake water for typical conditions. After the prescribed time period the exchangers were taken off line and evaluated. Conclusions and observations regarding fouling of brazed heat exchangers, exposed to once-through lake water, are presented here. Transient observations and photographs of the exchanger surfaces are given. Results are presented that compare these heat exchangers to test plates, also exposed to lake water. The progressive change of surface appearance with increasing immersion times is presented.     

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.     

“Zero Fouling” Self-Cleaning Heat Exchanger

Conventional shell and tube heat exchangers sometimes have to use two severely fouling process streams, one in the tubes and one in the shell. This paper presents the design of a self-cleaning heat exchanger that applies the self-cleaning mechanism in the tubes of two parallel bundles handling the fouling process streams. For the transfer of heat between both bundles, a small circulating flow of conditioned water is used as an intermediate fluid, a fraction of which evaporates on the outside of the tubes of the high-temperature bundle and condenses on the outside of the tubes of the low-temperature bundle. This novel design, which consists of two parallel bundles in one shell, experiences very high film coefficients at the outside surface of both tube bundles and does not suffer from any fouling. Therefore, it is referred to as a “zero fouling” self-cleaning heat exchanger. In this paper, a conventional severely fouling crude oil preheater will be compared with a zero fouling self-cleaning heat exchanger for the same service.     

Detailed equipment design in heat exchanger networks synthesis and optimisation

In heat exchanger network synthesis, important features like pressure drop and fouling effects are usually neglected. In this work a new methodology is proposed to include these effects in grassroots as in retrofit designs. Heat exchangers are detailed designed during the heat exchanger network synthesis. Pinch analysis is used to obtain the heat exchangers network with the maximum energy recovery, and a new systematic procedure is proposed to the identification and loop breaking. Bell–Delaware method for the shell side is used to design the heat exchangers. An example of the literature was studied and the results show differences between heat exchangers, with and without the detailed design, relative to heat transfer area, fouling and pressure drop. The great contribution of this work is that individual and global heat transfer coefficients are always calculated, in despite of the current literature, where these value are assumed in the design step. Moreover, the methodology proposed to the heat exchangers design assures the minor heat exchanger according to TEMA standards, contributing to the minimisation of the heat exchanger network global annual cost. Finely, the new heat exchanger network considering pressure drops and fouling effects presents values more realistic then those one neglecting the equipment detailed design.     

Fouling Management of Thermal Cracking Units

ABSTRACT

Visbreakers and other thermal cracking units are thermal process units in crude oil refineries that upgrade heavy petroleum, usually residual oils produced from atmospheric or vacuum distillation of crude oil. The associated process streams of these units consist of heavy hydrocarbons with very high viscosities and impurities, resulting in fouling of the heat exchangers used to cool or heat these streams. This paper presents a practical fouling analysis for thermal cracking units in a refinery in Germany. Fouling management at this refinery was initiated as part of the refinery energy-saving program. Following similar analysis of the refinery's crude preheat trains, heat exchanger networks associated in the thermal cracking units were modeled by entering the plant monitoring data, network topology, and heat exchanger geometries into a commercial heat exchanger network simulator, SmartPM. Fouling behaviors of vacuum residue streams and thermal cracker residue streams were identified and quantified. Both chemical reaction fouling and particulate fouling mechanisms were identified to be responsible for the fouling in these streams. Dynamic fouling models were fitted and used to predict fouling of these heavy petroleum streams, which fouled on both the shell and tube sides of the shell-and-tube heat exchangers.     

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.     

Reconstruction of a heat exchanger network under industrial constraints — the case of a crude distillation unit

Heat exchanger network retrofit using a pinch based approach is presented. In this approach, the criterion of minimum sensitivity of heat exchanger to fouling effects is accounted for. The present paper introduces this criterion without explaining its details that are described in the literature. A summary is given of HEN reconstruction in a crude distillation unit processing 4.2 million ton crude oil per year. While the total heat quantity of hot streams is 110 MW, the heat recovery in the existing HEN is 60 MW. Using Pinch Analysis, the target value of heat recovery at ΔT_min=10 K was determined at 91 MW. Measurements were carried out on the existing HEN with the aim to determine the influence of fouling effects on the heat transfer in the exchangers. Taking local constraints including fouling into account, HEN reconstruction was proposed. The heat savings in the reconstructed HEN was estimated at 75 MW.     

Extending the Induction Period of Crystallization Fouling Through Surface Coating

To minimize the negative effects of scale formation in heat exchangers, new anti-fouling strategies are focusing on the modification of heat transfer surfaces. These modifications should lead to tailor-made surfaces for different technical applications. The aim of this surface modification is the extension of the induction period to minimize the negative effects of fouling and maximize the endurance of the heat exchanger. To achieve this, different surface coatings on stainless steel were investigated with respect to fouling tendency. The effects of flow velocity with respect to Reynolds number on the induction time of CaSO₄ crystallization fouling were tested in different test units. Diamond-like carbon (DLC) coatings extend the induction time at every measured flow velocity. At higher Reynolds numbers, the effect of different surface crystallization due to energetic modification is reduced because of the dominating effect of the low adhesive surface. Thus the induction time can be extended by the factor of 2 for low fluid velocities (DLC or SICON®) and by more than 14 for higher Reynolds numbers (DLC and SICON®). The combination of limited nucleation spots due to electro-chemical treatment of the substrate before coating can give a tailor-made surface with maximum induction time for crystallization fouling.     

Use of crude oil fouling threshold data in heat exchanger design

The existence of a `threshold' below which chemical reaction fouling of heat transfer surfaces by crude oil does not occur has been identified by Ebert and Panchal [Fouling Mitigation of Industrial Heat-Exchange Equipment, Begell House, 1997, 451–460] and clearly demonstrated by Knudsen et al. [Understanding Heat Exchanger Fouling and its Mitigation, Begell House, 1999, 265–272]. This phenomenon has important implications for the design and operation of heat exchangers in refinery pre-heat trains used for the processing of crudes. In this paper we show how a consideration of the fouling threshold condition can be incorporated into the design procedures for shell-and-tube heat exchangers. We then proceed to show how fouling can be mitigated through attention to heat exchanger design, particularly the choice of configuration. The cost of improperly designed units, based on the conventional use of `fouling factors', is demonstrated.     

Numerical investigation of fouling on cross-flow heat exchanger tubes with conjugated heat transfer approach

Although fouling on heat exchanger tubes is extensively investigated, due to the lack of energy resources, the effects of fouling on heat exchangers is still an important area of study and gaining more and more attention every day. In this study we investigated the effects of fouling on heat transfer and flow structures numerically for cross-flow heat exchanger tube geometry. The distributions of temperature, heat transfer coefficient and heat flux at the surface of fouling were obtained for single and double layer fouling cases. In the analysis, Reynolds number and the blockage ratio were fixed to 100 and 0.1 respectively. We used ANSYS software in our analyses and compared some of our results with the literature.     

Experimental investigations of frost release by hydrophilic surfaces

Frost formation occurs when water vapor in the surrounding air comes into contact with cold surfaces through heat and mass transfer. It is usually an undesirable phenomenon in most refrigeration and cryogenic systems. A few studies have shown that changing the surface energy, such as increasing the surface hydrophilicity or hydrophobicity, has significant effects on frost growth. In this paper, a kind of hydrophilic polymer paint is formulated to counteract frost deposition on cold surfaces. The coated surface can retard frost formation up to three hours under low plate temperatures (− 15.3°C) and high air humidity (72%). To test the antifrosting performance of the hydrophilic paint under more practical conditions, it is applied to a fin-and-tube heat exchanger and a domestic refrigerator at a coating thickness of 30 μm. Comparisons of frost deposition, pressure drops, and outlet temperatures are made between uncoated and coated heat exchangers. Under conditions of high air temperature (2.2°C) and relative high air humidity (90%), the paint prolongs the defrosting interval from 80 to 137 min. Experimental observations also show that the coated hydrophilic fins are free of frost deposition during the entire course of the test and that the coating has no significant additional thermal resistance.     

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.     

Pool Boiling Enhancement through Graphene and Graphene Oxide Coatings

ABSTRACT

Boiling has served as an effective means to dissipate large quantities of heat over small areas. Graphene, a two-dimensional material, has garnered significant attention of researchers due to its excellent thermal properties. In this study, copper test chips are dip coated with a solution consisting of graphene oxide and graphene and its pool boiling performance with distilled water at atmospheric pressure was investigated. The surfaces were characterized using X-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetric analysis, and scanning electron microscopy which confirmed the presence of graphene and graphene oxide. The contact angles measured on the coated surfaces indicated hydrophobic wetting behavior. Four heat transfer surfaces were prepared with dip coating durations of 120 s, 300 s, 600 s, and 1200 s, respectively. A Critical Heat Flux (CHF) of 182 W/cm² and a heat transfer coefficient (HTC) of 96 kW/m²°C was obtained with the shortest coating duration which translated to an enhancement of 42% in CHF and 47% in HTC when compared to a plain uncoated surface under similar conditions. Contact angle changes were not seen to be responsible, although roughness was seen as an influencing factor contributing to the enhancement. Further studies are needed to explain the enhancement mechanism.     

Investigation of an alternative operating procedure for fouling management in refinery crude preheat trains

Crude oil atmospheric distillation in petroleum refineries involves a heat exchanger network to heat the crude using hot side streams and pumparounds. This energy integration reduces the furnace load as well as the cold utility consumption, diminishing fuel costs and carbon emissions. During the operation, the effectiveness of the heat exchangers decreases due to fouling. This paper deals with preheat trains composed by multiple parallel branches, where it is investigated an alternative operating policy based on the optimization of stream splits, aiming to manipulate the flow rates according to the fouling status of the existent heat exchangers. The performance of the proposed approach is illustrated by three examples: two networks from the literature and one real network from a Brazilian refinery.     

Supervision of the thermal performance of heat exchanger trains

In oil refining, heat exchanger networks are employed to recover heat and therefore save energy of the plant. However, many heat exchangers in crude oil pre-heat trains are under high risk of fouling. Under fouling conditions, the thermal performance of heat exchangers is continuously reduced and its supervision becomes an important task. The large number of heat exchangers in pre-heat trains and the change of operation conditions and feedstock charges make the daily supervision a difficult task. This work applies an approach to follow the performance of heat exchangers [M.A.S. Jerónimo, L.F. Melo, A.S. Braga, P.J.B.F. Ferreira, C. Martins, Monitoring the thermal efficiency of fouled heat exchangers – A simplified method, Experimental Thermal and Fluid Science 14 (1997) 455–463] and extends it to monitor the whole train. The approach is based on the comparison of measured and predicted heat exchanger effectiveness. The measured value is computed from the four inlet and outlet temperatures of a heat exchanger unit. The predicted clean and dirty values of effectiveness are calculated from classical literature relations as a function of NTU and of heat capacity ratio (R). NTU and R are continuously adjusted according to mass flow rate changes. An index of fouling is defined for the whole network and the results show the performance degradation of the network with time. The work also suggests that Jerónimo’s index of fouling can be used to estimate the fouling thermal resistance of heat exchangers.     

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.     

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.     

Flow Boiling Heat Transfer Characteristics in Minitubes With and Without Hydrophobicity Coating

Abstract

Wettability plays an important role during flow boiling inside micro and mini channels. The present work focuses on the flow boiling heat transfer characteristics inside copper minitube (inner diameter of 3?mm) coated internally to render the inside surface nearly hydrophobic. Electroless Galvanic Deposition technique is employed for hydrophobic coating inside the copper tube. Both single phase heat transfer and two-phase flow boiling heat transfer and pressure drop characteristics were investigated in regular and internally coated hydrophobic copper minitubes. The experiments were performed with deionized water as a working fluid and the mass flux was varied from 100 to 650?kg/m²s. The two-phase heat transfer characteristics was observed to be both functions of mass flux as well as heat flux. The two phase heat transfer has been observed to be augmented due to the wettability within the tubes. The two-phase pressure drop has also been observed to increase when compared to the regular, uncoated tube; however, the proportional increment is lower than the augmentation achieved in two-phase heat transfer. The enhanced heat transfer effects observed have been explained on the basis of wetting physics.     

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.