Fouling Thermal Resistance Research Articles

A Screening Apparatus for Comparing the Fouling Resistance of Heat Exchanger Surfaces

AbstractFouling is a challenge in many processes, especially in heat exchangers and evaporators. Even though many treatments have been developed to mitigate fouling on metallic surfaces, no standardized method exists to compare equipment based on its antifouling performance. In this work, a screening apparatus is presented, which allows to compare samples of treated metallic surfaces based on thermal fouling resistance. A parameter screening and a ranking of three differently treated metallic surfaces using two model substances, namely, whey protein concentrate (WPC) and calcium sulfate, are demonstrated.

Characterization of Water-Based Anti-Corrosion and Anti-Fouling Coating in the Heat Exchanger Tube of Gas Water Heater

Due to the fouling and corrosion issues with gas water heater heat exchangers during use, a water-based coating is created using the sol-gel method and applied to the inner surface of the copper tube of the heat exchangers. The water-based coating can reduce the surface energy of copper that is oxygen-free by 82.40 % and has outstanding hydrophobicity. The coating's adhesion strength was 8.46 MPa, and its wet adhesion reduction rate was only 0.0375 MPa·d-1 when submerged in a bath of water at a constant temperature. Its water contact angle remained above 110° even after being abraded by sandpaper, demonstrating its excellent wear resistance. At a constant temperature of 45 °C, the corrosion experiment was carried out in salt, acid, and alkali solutions. The maximum corrosion rate of the coating was 3.24 × 10-2mg∙(cm2∙h)-1, which was only 20.85 % of that of the oxygen-free copper substrate. The dynamic fouling experiment shows that the coating can effectively inhibit fouling growth, prolong the fouling induction period and reduce the average fouling rate. By factoring in the fouling thermal resistance and the coating thermal resistance, the overall heat transfer coefficient of the copper tube was computed. The results demonstrate that after applying this water-based coating, the copper tube can continue to have a high heat transfer coefficient for an extended period, ensuring the continuous and effective operation of the heat exchanger and having practical application value.

Determination of Heat Transfer Correlations for Fluids Flowing through Plate Heat Exchangers Needed for Online Monitoring of District Heat Exchanger Fouling

This article deals with the problem of estimating the degree of fouling of plate heat exchangers (PHEs) used in district heating substations (where the working medium is water). A method for calculating the thermal resistance of fouling is proposed based on a comparison of the thermal resistance of a fouled and clean heat exchanger. The thermal resistance of the heat exchanger for both fouled and clean apparatuses is determined as the inverse of their overall heat transfer coefficient. In the method, the heat transfer coefficients necessary to determine the overall heat transfer coefficient of the clean exchanger are calculated using a modified Wilson method. Moreover, the heat transfer coefficients on the clean heat exchanger plates’ cold water side are determined based on experimental tests. The computational algorithm presented in this paper will make it possible to develop software to monitor and thus optimise the operation of district heating substations.

A Prognosis Method for Condenser Fouling Based on Differential Modeling

Fouling in heat exchanger tubes is a common problem in the operation of condensers. The deposition of fouling can affect the thermal efficiency and safety of the condenser. Therefore, it is necessary to predict the impact of fouling on time and carry out scientific treatment. Firstly, fault prognosis methods require a significant amount of historical fault data, which is often lacking in practical applications. This paper proposes a method based on dynamically adjusting parameters of the fouling thermal resistance empirical equation to establish a fouling thermal resistance digital twin model. It is combined with simulation tools to rapidly generate a large amount of fault data for the research of prognosis methods. Secondly, in the research of fault prognosis methods, prognosis accuracy relies on establishing a reliable and accurate model that describes the behavior of faults. The uncertainty in the modeling process significantly affects the results. Classic modeling methods do not effectively quantify uncertainty. Therefore, this paper proposes a method that applies differential modeling to predict fouling faults in condensers, automatically obtaining uncertain parameters while establishing a reliable model. By calculating the performance evaluation indicator, the accuracy error indicator of the differential modeling-based prognosis method is further reduced to 0.35. The results demonstrate that this method can provide effective reference opinions for handling fouling faults in condensers.

Study of Particulate Fouling Inhibition Characteristics on a Novel Composite Coating

Particulate fouling is a common fouling in heat exchange equipment, it causes tube corrosion and increases flow resistance. Particulate fouling increases the hidden danger of equipment and requires high treatment costs. In this paper, a novel Ni−P−TiO2 composite coating is prepared on 316 stainless steel using electroless plating and the fouling inhibition characteristics of the novel composite coating are studied using a dynamic monitoring experimental system. The experimental results show that the fouling thermal resistance of the Ni−P−TiO2 composite coating is obviously lower than that of 316 stainless steel under the same working conditions. With the increase in cooling water velocity and inlet temperature, the surface fouling thermal resistance decreases, while, with the increase in particle concentration, the fouling thermal resistance increases. Based on DLVO theory, it is found that the surface energy of Ni−P−TiO2 composite coating is close to the best surface energy for inhibiting particulate fouling deposition, which can significantly inhibit particulate fouling deposition. Compared with the stainless-steel surface of a conventional plate heat exchanger, the Ni−P−TiO2 composite coating not only inhibits the accumulation of particulate fouling, but also reduces the adhesion strength of particulate fouling; additionally, the fouling is easier to strip off the heat exchange surface, which realizes the lasting and efficient fouling inhibition on the heat exchange surface. The research results can provide a data reference for the fouling inhibition design and daily efficient operation of heat exchangers.

Determination of the Fouling Degree of the Heat Substation Based on Tests of Plate Heat Exchangers

The subject of the following considerations is research on plate heat exchangers. The research concerns the determination of the degree of fouling of plate heat exchangers with mineral deposits in the water. The method proposed in this publication for determining the thermal resistance of fouling is based on the determination of the overall heat transfer coefficient for a clean heat exchanger (operating under laboratory conditions) and a fouled one (operating in a heat substation). With the overall heat transfer coefficients determined, the difference in thermal resistance for the fouled and clean heat exchanger and thus the total thermal resistance of the fouling can be determined. The calculation algorithm presented in the work will allow the creation of software that enables monitoring, thus optimizing work heating centers.

Dynamic modeling and improvement on ash deposition and thermal–hydraulic characteristics in a waste heat boiler of cement kiln

Ash deposition is the key factor restricting the heat utilization efficiency of a waste heat boiler in the cement kiln. The reasonable design of heat-transfer surfaces is a crucial issue for reducing ash deposition. In this paper, in order to overcome the manufacturing difficulty or unpractical problems of most heat-transfer surfaces, different designs based on the characteristics of the elliptical tube were proposed. To explore the overall performance of these designs, a detailed CFD model is established to study the ash deposition and thermal–hydraulic characteristics, and the dynamic mesh method is adopted to simulate the dynamic growth process of the ash deposited layer. Compared with the reference scheme (case 1), the performance evaluation criteria PEC of the novel design (case 3) in the clean state is improved by 13%, and the fouling thermal resistance and thermal attenuation coefficient are significantly reduced. Therefore, the novel designs that can effectively reduce particle deposition while maintaining easy manufacturing and acceptable thermal–hydraulic performance are applicable to waste heat utilization of dust-laden exhaust gas.

Migration–deposition coupling characteristics and influence of corrosion products on heat transfer in steam generators

• A coupled model of impurity migration-deposition considering boiling is developed. • A critical time step is proposed to realize the simulation time to the real scale. • The whole lifetime fouling characteristics are predicted in steam generators. • The effects of fouling on heat transfer of steam generator is investigated. Fouling on tube surfaces degrades the heat transfer characteristics of heat exchangers. The objective of this study was to predict the distribution of deposits on the surface of heat transfer tubes and assess its influence on heat transfer characteristics. For this purpose, a comprehensive model of impurity migration and deposition, which considered deposition and removal processes under boiling conditions, was established to simulate the fouling characteristics of steam generators. A critical time step was determined to solve the problems at a large time-scale span between the fluid flow and long-term deposition time in actual operation. Then, the deposit distribution was used to propose a fouling thermal resistance model based on the Maxwell model to study the influence of fouling on the heat transfer performance of steam generators. The results showed that the corrosion products were mainly deposited on the hot side and in the U-bend region. The deposition rate on the hot side gradually decreased with time, whereas the deposition mass on the cold side linearly increased with time. Fouling had an evident effect on the overall heat transfer efficiency and distribution of steam generators. This study is relevant for the prediction of operating conditions during the life cycle of heat exchangers in industry.

Properties of Ceramic Coating on Heating Surface of Waste Incineration Boiler Prepared by Slurry Method.

In order to alleviate the problem of high-temperature fly ash corrosion and slag on the heating surface of a high-parameter waste incinerator, a ceramic coating material that can be prepared in situ on the heating surface by the slurry method was studied. The ceramic coating can be formed by sintering at a lower temperature of 750 °C. Its surface and profile are very dense, and the porosity is less than 1%. The mechanical properties test results show that the ceramic coating can withstand 60 cycles of water-cooled thermal shock at 700 °C, and the bonding strength is 25.14 ± 2.21 MPa. It will not fall off in a large area when subjected to pressure load, and it has a certain degree of processable plasticity. High-temperature wettability experiments show that the ceramic coating has lower liquid-bridge force, smaller adhesion area, and shorter fouling cycle for molten corrosive fouling, and potential self-cleaning properties. Its practical mechanical properties make the coating valuable for production applications and meet expectations, and excellent antifouling properties to reduce average fouling thermal resistance and corrosion.

Generalized Analysis of Dynamic Flow Fouling on Heat Transfer Surfaces

Several energy applications involve flow of untreated heat transfer fluids that contain trace amounts of mineral salts, such as calcium carbonate and calcium sulfate, which show reverse solubility with temperature. The gradual accretion of these salts onto the heat transfer surfaces causes fouling that degrades thermal performance over time. While fouling of bare surfaces has been extensively studied, fouling of nonwetting superhydrophobic or liquid-infused surfaces is less understood, especially under dynamic flow conditions. This study presents a systematic experimental analysis to elucidate dynamic flow fouling of calcium carbonate and calcium sulfate on smooth, superhydrophobic and liquid-infused surfaces in a generalized manner. Copper tubes with modified inner surface wettability are fabricated and subjected to forced convection at different flow rates and fouling salt concentrations. Fouling characteristics of the two salts on the different surfaces are examined through microstructural analysis, and quantified in terms of asymptotic fouling resistance that measures the total extent of fouling, and a threshold time, which correlates to the onset of significant fouling. Using Taguchi design of experiments, the study reports, for the first time, closed-form analytical relationships for the asymptotic fouling thermal resistance and threshold time on the Reynolds number, dimensionless concentration of fouling agent and dimensionless infused liquid viscosity that represents the different surface types in a unified manner. It is shown that the analytical model accurately predicts the fouling characteristics in up to about 97% of the data, based on which contour maps of optimum surface designs for minimizing fouling resistance are presented. The results provide information on designing surfaces for reduced fouling or for estimating the fouling characteristics for a given surface in applications.

Effects of variable magnetic field on particle fouling properties of magnetic nanofluids in a novel thermal exchanger system

Magnetic nanofluids, as a new type of heat transfer media, propose a possible way to achieve low energy consumption and carbon neutrality, which has attracted a lot of attention in the heat transfer field. To comprehensively discuss the feasibility of magnetic nanofluids application in heat exchange systems, this paper conducted an experimental study of the particle fouling properties based on Fe3O4–H2O-AG magnetic nanofluids. The effects of variables including Reynolds number, magnetic field form, and magnetic flux density B on the fouling properties were analyzed. The anti-fouling rate η was introduced to evaluate the anti-fouling properties. The results showed that high Reynolds number (Rf (Re = 9000) = 2.83 × 10−6 m2 W K−1), constant parallel magnetic field with high magnetic flux density (Rf (B = 18mT) = 3.86 × 10−6 m2 W K−1), and corrugated tube with twisted turbulator (Rf (with turbulator) = 3.74 × 10−6 m2 W K−1) demonstrated less fouling thermal resistance Rf. Meanwhile, constant parallel magnetic field with high magnetic flux density also exhibited better anti-fouling properties (η(B = 18mT) = 1.35), which indicated that the parallel magnetic field has a positive effect on preventing the deposition of magnetic nanoparticles.

Application of experimental data to model local fouling resistances

Fouling is a problem that varies in space and time, but a fouling situation is commonly quantified using the integral thermal fouling resistance based on the integral heat balance and an area-averaged heat transfer assessment. However, current modeling does not take into consideration the local differences, such as constrictions, that can affect the integral fluid dynamic behavior. This work shows experimental and analytical results of local investigations of a fouled counterflow double-pipe heat exchanger. The determined local parameters are used to calculate different local fouling resistances as part of a holistic modeling approach with the aim of modeling and linking local and integral fouling resistances as well as the thermal and the mass based approaches. Therefore, mass based parameters are utilized to recalculate the obtained local thermal fouling resistances as a way to account for the heat transfer increases caused by local surface roughness and/or local constrictions. The aim of this procedure is to explain and eliminate apparent negative fouling resistances on a local basis. The local thermal fouling resistances are determined by measuring the local temperatures and then used for quantification of the local overall heat transfer coefficients. Modeling of the local mass based fouling resistances requires knowledge of the density and thermal conductivity of the local material, as well as the local layer thickness and/or the local fouling mass. The local thermal fouling resistances are recalculated using the local friction coefficients and local flow velocities resulting from local constrictions. All experimental and theoretical approaches are merged into the model presented here for the determination of local fouling resistances.

On-site experimental study on fouling and heat transfer characteristics of flue gas heat exchanger for waste heat recovery

A large number of fly ash particles in flue gas easily cause the fouling problem of heat exchangers, which seriously affects the efficient utilization of flue gas waste heat. In this study, experimental and numerical investigations on fouling and heat transfer characteristics in cross flow heat exchangers were carried out. The on-site experimental system was established, and the heat transfer coefficients for different heat-transfer surfaces at the fouling state were monitored in real time. The weakened degree of heat transfer coefficient and fouling thermal resistance are adopted to assess the effect of fouling on the heat transfer performance and compare the practicality of four different heat-transfer surfaces under dusty conditions. Numerical investigation was performed with the fouling model established in previous work to model the thermal-hydraulic and fouling processes. Numerical results are compared with experimental measurements and a good agreement is achieved. Results showed that compared with the aligned circular tube bundle, using H-type fins or honeycomb tube bundle both improve the anti-fouling performance, especially for the honeycomb H-type finned tube bundle, which has the lowest fouling thermal resistance. Therefore, due to the characteristics of easily soot-blowing and lower fouling thermal resistance, honeycomb heat exchangers are suitable for waste heat utilization of dusty flue gas.

Heat exchanger failure analysis in the simulated marine environment: Prediction of the fouling removal temperature

The present work aims to analyze the failure resulting from temperature change and its influence on the corrosion and fouling resistance of the cupronickel Cu-Ni 90/10 in simulated marine solution to predict the removal fouling temperature in the desalination heat exchanger and how affects fouling composition and morphology. The Cu-Ni 90/10 electrochemical behavior has been studied through: polarization tests, following the evolution of open circuit potential (OCP) and electrochemical impedance spectra (EIS) with immersion time under thermal and hydrodynamic conditions close to those prevailing in the heat exchanger of desalination industry. Electrochemical measurements show that the corrosion rate of Cu-Ni 90/10 alloy change with temperature, due to a competition between the formation of stable complexes on the metal surface and the dissolution of salts to generate aggressive ions in the corrosion medium. 60 °C ensures an equilibrium between the complexation reactions that promotes fouling stability, and fouling mitigation that leads to metal oxidation. In contrast, at high temperatures (80 °C), the dissolution of salts predominates over the complexation process. It can be deduced that 60 °C is a critical point for heat exchanger that promotes the formation of thin layer with low thermal fouling resistance that protects the installation against damage, maintain a good heat transfer and extend the cleaning scheduling of the machine. Surface analyses were performed by means Raman spectroscopy, SEM and EDS quantification, reveals the reinforcement of fouling by iron compound which make deposit more resistant at 60 °C.

Fouling characterization of calcium carbonate on heat transfer surfaces with sodium carboxymethyl cellulose as an inhibitor

Fouling via the deposition of calcium carbonate (CaCO3) on heat transfer surface reduces the heat transfer efficiency and compromises the stable operation of the heat exchanger. Sodium carboxymethyl cellulose (SCMC) as a fouling inhibitor effectively prevents the growth of CaCO3. In this work, the fouling characteristics of a heat transfer surface by CaCO3 with the SCMC inhibitor were studied experimentally; the effects of the SCMC concentration, inlet temperature of the solution, mass flow, SCMC degree of substitution, and SCMC viscosity on fouling inhibition were investigated. Moreover, the asymptotic fouling thermal resistance and fouling inhibition efficiency were analyzed by varying the operation conditions and the physical properties of SCMC. The results show that the fouling thermal resistance decreases with increasing SCMC concentration, degree of substitution, and solution temperature, and increases with increasing mass flow of the solution. However, the effects of the SCMC viscosity varying from 600 mpa⋅s to 15000 mpa⋅s on fouling thermal resistance were observed to be negligible. Additionally, there was significant variation of the CaCO3 fouling morphology with a change from calcite particle to an irregular sphere after the addition of inhibitor.

Introducing a holistic approach to model and link fouling resistances

Fouling is the unwanted deposition of soils on heat transfer surfaces and is a major challenge for industry and has been s subject to scientific investigations for decades, still being an unsolved problem for many applications. A fouling situation is commonly quantified with the thermal fouling resistance describing the integral fouling behavior of an apparatus. Modeling of this quantity is a permanent subject to research. This contribution presents the basics of an expanded consideration by introducing a holistic approach to model and link fouling resistances based on the extension of previous work in this field. A thermal and a mass based approach to calculate fouling resistances are considered integrally and locally. This will provide a detailed knowledge of the fouling behavior. Various variables are needed for modeling the different fouling resistances. Therefore, both experimental and analytical methods have to be applied to obtain the required data regarding local differences of crystallization deposits within double-pipe heat exchangers. Here the planned experimental and analytical approaches to receive all the required input data are described, also presenting the required test equipment briefly. Core equipment is a test rig equipped with double pipe heat exchangers, which allows the measurement of thermal and fluid flow related values and provides samples for the analysis of the fouling deposits. Furthermore, the aim of the new modeling concept is to link integral and local fouling resistances by taking into account locally varying parameters regarding the fouling layer. In order to allow for that, a recalculation of the thermal fouling resistance into a corrected version by considering heat transfer enhancing effects attempts to correlate with the mass based approach in a first step. In the end, the holistic modelling approach is presented.

Effect of transverse triangular cross-sectional wedges on the asymptotic fouling deposition behavior of the flow through an inline tube bundle

The deposition on a superheater and economizer may come from loose ash particles adhering to their tube surfaces. It causes a fouling thermal resistance, an increase in the flow pressure drop, and hence a higher operating cost. An active approach such as using a high-grade fuel can diminish the effect. However, it is costly. The deposition may be predicted using sophisticated computational fluid dynamics software. Alternatively, for the asymptotic mode of deposition, it might be predicted by the lumped models proposed by various researchers. However, they can be used to predict only the saturated time and the overall saturated thermal resistance. In this work, a novel passive approach is proposed. Isosceles triangle cross section wedges were installed parallel to the tubes to increase the wall shear stress due to the flow (t) and hence decrease the deposition. In addition, by modeling the tube bundle as a lump, a mathematical formulation for predicting the deposition behavior of the tube bundle was developed. The experiment was carried out on an inline 6 rows – 3 columns tube bundle with constant air velocity and ash feed rate. Four isosceles triangular wedges with the same height but with different base (b) to height (e) ratio or aspect ratio (α), with α= 0.275, 0.550, 0.758 and 1 being used. Three wedge positions from the tube bundle inlet in the direction of the flow (x) to the tube bundle length (L) ratios (x/L) are considered: at the middle between the second row and the third row (x/L=1/3), at the middle between the fourth row and the fifth row (x/L=2/3), and at the tube bundle outlet (x/L=1). The regression approach was used to analyze the overall thermal resistance obtained from the experiment, according to the obtained mathematical relation. The saturated time and the overall saturated thermal resistance obtained from the analysis were used to calculate the deposition rate on the tubes (fd), the removal rate of the deposition, deposition growth rate, coefficient of the deposition (kd) and coefficient of the removal (kr). The result showed that the overall saturated thermal resistance and deposition were inversely proportional to t. fd, the removal rate, the deposition growth rate, kd and kr increase with the wedge distance. Except for fd and kd, those parameters were sensitive to a. The tube bundle with wedges has higher values for those parameters. For the bundle with wedges, those parameters were inversely proportional to t. The product of kr and t is the governing parameter of the saturated time.

Investigation of fouling mechanisms for diesel engine exhaust heat recovery

The effect of diesel engine exhaust fouling on heat and mass exchangers for waste heat recovery is investigated. Experiments are conducted for a range of operating conditions relevant to the use of the waste heat from a diesel engine for powering a 2.71 kW capacity absorption heat pump. Thermal resistances of fouling layers as well as their effects on pressure drop are determined, and the underlying mechanisms for deposition and removal are investigated. Fouling layer effects are most severe for the lowest coolant temperatures and 100% generator load. At these conditions, the fouling thermal resistance is approximately 70% of the total resistance, and the ratio of the steady-state pressure drop to the baseline pressure drop is 3.25. Flow induced shear is found to be the primary growth inhibiting mechanism at high exhaust velocities. Results from this study can guide heat exchanger design for a variety of waste heat recovery systems.

Boiling flow of graphene nanoplatelets nano-suspension on a small copper disk

In the present work, an attempt was made to experimentally quantify the boiling heat transfer coefficient (BHTC) of graphene oxide-water nano-suspension (NS) inflow boiling heat transfer regime. The NS was prepared at weight fractions of 0.025, 0.05, and 0.1% using the two-step method and further stabilized for 17 days (at wt% = 0.1). Results showed that the presence of graphene oxide nanoplatelets (GNPs) imposed an extreme fouling thermal resistance (FTR) to the surface, which caused a reduction in the BHTC over 1000 min of continuous operation after the CHF point. This was mainly due to the presence of the graphene oxide on the surface, which created a surficial fouling layer and heat accumulation on the surface. Instead, the sedimentation layer promoted the critical heat flux (CHF) point such that the point for water was 1370 kW/m2 reaching 1640 kW/m2 for NS at wt% = 0.1. Likewise, the highest BHTC of 17.4 kW/(m2K) at Re = 10,950 was obtained. Also, with increasing the heat flux and flow rate, the BHTC increased. The same trend was also identified with a mass fraction of GNPs up to CHF point. The increase in the BHTC was attributed to the intensification of the Brownian motion and thermophoresis effect in the boiling micro-layer close to the surface.

Pool boiling heat transfer to CuO-H2O nanofluid on finned surfaces

In the present research, the general aim is to understand further the potential effect of the surface shape and geometrical specification of rectangular parallel fins developed on the surface on the heat transfer coefficient, bubble formation, and fouling of the nanoparticles. To achieve this, the boiling thermal performance of the copper oxide nano-suspension (NS) was quantified on the modified surfaces with different geometrical specifications, including the width and height of the fins and space between fins. Results showed that the designed fins reduce the rate of the fouling of the nanoparticles on the boiling surface such that the best thermal performance was achieved for the surface modified with the fins with more towering height and smaller width. Also, the fouling thermal resistance was found to follow an asymptotic behaviour while developing three regions of inception, growth, and equilibrium. During the inception of fouling, negative values were measured for the fouling thermal resistance, which was attributed to the enhancement in the specific surface area and thermal performance of the system. Overall, the presence of the fins improved the thermal performance of the system in comparison with the plain surface.