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In this paper, aiming at resolving the contradictions of heat transfer enhancement, resistance reduction and soot-blowing effect of common heat transfer surfaces, aligned elliptical tube bundle (AETB) and honeycomb tube bundle (HCTB) were proposed for the high dust content conditions, and the fouling characteristics and thermal-hydraulic performance before and after fouling were investigated. Firstly, the thermal-hydraulic performance of aligned circular tube bundle (ACTB), AETB, conventional staggered circular tube bundle (SCTB) and HCTB in the clean state were compared, and advantages of AETB and HCTB were verified. Secondly, based on the ash fouling model and prediction method established by the author’s group, fouling distribution and morphology of AETB and HCTB were obtained, and ash fouling and thermal-hydraulic characteristics were examined by parameterization. Finally, in the range of geometry parameters studied in this paper, the correlations of the asymptotic value of fouling thermal resistance Rf∗ in fouling state, Nusselt number Nu and friction factor f in clean state were developed. Results show that the relative deviations between the predicted results and the numerical values is within 4%, which indicates that the proposed correlation is of sufficient accuracy for engineering purposes, and can accurately predict the thermal-hydraulic characteristics of flue gas heat exchanger. The results of this paper can provide a reference for development and engineering application of the heat-transfer surface for the efficient recovery of industrial flue gas waste heat.

An on line experimental system was set up in a heating boiler so as to reproduce dusty working environments in heat recovery systems and to investigate heat transfer and fouling performance of different kinds of finned tube heat exchangers. Measurements are reported for the heat transfer coefficients, fouling and the effect of fouling on heat transfer performance of five different H-type finned tube heat exchangers. Fouling resistance and the weakened degree of heat transfer coefficient are employed to evaluate the effect of fouling on the heat transfer performance. Numerical simulations are also conducted on the fouling performance of each heat exchanger bundle and compared to the measurements. Double H-type fins can slightly reduce the heat transfer performance but also reduce fouling. Elliptical finned tubes with the same tube pitch decrease heat transfer performance and increase fouling, and with the same relative tube pitch, significantly increase the heat transfer performance before fouling, and can effectively reduce fouling.

In this paper, the fouling and convection heat transfer characteristics of various flue gas heat exchangers are investigated, and the weakening degrees of fouling on the heat transfer performance of different heat exchangers are compared. The anti-fouling performance of novel H-type finned tube heat exchangers are discussed. The results show that the elliptical tube arrangement can effectively reduce the effect of fouling on heat transfer performance. Overall, SHETB has the best heat transfer and anti-fouling performance. The results of this paper are helpful for the efficient recovery and utilization of dusty flue gas waste heat.

Particulate fouling on flue gas heat exchanger surfaces reduces heat transfer efficiency and increases the instability of equipment operation. It is necessary to develop a better understanding of how fouling occurs, aiming to find strategies to predict and reduce it. In this paper, a multiple mean cycle method is proposed based on a comprehensive fouling model considering the deposition and removal mechanisms, combined with the discrete phase model and dynamic mesh method, to predict the fouling morphology. Then the effects of transverse pitch, longitudinal pitch, tube shape and arrangement on fly-ash fouling and heat transfer characteristics are examined. It is found that, for aligned arrangement, the fly-ash particles are mainly deposited at the upwind stagnation region at the first row and whole windward side of the rest rows; for staggered arrangement, the fly-ash particles are deposited at the upwind stagnation region. The multiple mean cycle method can predict the fouling morphology efficiently and keep the characteristic of non-uniform fouling distribution. The fouling mass increases with the increase of relative transverse and longitudinal pitches. The elliptical tube bundle with small relative transverse and longitudinal pitches can obviously reduce the fouling mass especially for the staggered elliptical tube. Compared with the aligned circular tube arrangement, the fouling mass m f is decreased by 81.1% at 10 min. In addition, the weaken degree of Nusselt number after fouling Δ Nu for the staggered elliptical tube is only about 49.8% of the aligned circular tube arrangement. Consequently, using the staggered elliptical tube contributes to reduce the fly-ash fouling, decrease the soot-blowing frequency and ensure the efficiency, economy and safety of flue gas heat exchangers.

The issues of fouling, erosion and corrosion are commonly occurring phenomena on the flue gas heat exchangers for middle/low temperature waste heat utilization due to the special properties of high ash content, viscous and corrosive components. How to solve these problems effectively has been the subject of many researches in recent years. This paper summarized the development of the simulations and experimental studies for the fouling, erosion and corrosion of heat exchangers. To begin with, the fundamental mechanisms, the prediction models and methods, the simulations with these models and relevant experiments of fouling, erosion and corrosion were introduced. In addition, the prediction processes of the fouling, erosion and corrosion rate were introduced by taking authors’ studies as illustrations among them. Finally, it should be noted that for the fouling, erosion and corrosion issues, there are still some key works to be done to better understand the fouling, erosion and corrosion mechanisms, and propose the novel heat exchangers for anti-fouling, anti-erosion and anti-corrosion. It would be desirable that the future heat exchanger designs can overcome the problems, and therefore to promote the development of technologies that can efficiently recovery the flue gas waste heat to improve the overall energy utilization efficiency.

Fouling of heat exchangers in industrial waste heat recovery is a problem which needs to be solved urgently. In this paper, a numerical investigation on fly-ash deposition and removal processes in tube bundle heat exchangers was conducted for industrial waste heat recovery. This study using a comprehensive fouling model integrated with the CFD framework was carried out with a discrete phase model and a dynamic mesh model in ANSYS FLUENT. First of all, the fly-ash deposition and removal algorithms were developed to establish the judgment criterion of particulate fouling based on the energy balance model and the critical moment theory. Based on this model, then, fouling characteristics with and without removal mechanism were compared. Then fouling processes for different particle diameters and flue gas velocities were simulated. At last, fouling morphology and the effect of fouling on heat transfer performance were predicted. The results indicated that the fouling mass increased almost linearly with time without removal mechanism, but an asymptotic behavior of fouling mass was observed when the removal mechanism was considered. When the number of particles injected from the inlet was specified, smaller particles showed a higher fouling rate. Also, the deposit growth rate decreased with the increase of flue gas velocity. For the particle diameter of 5 μm and the flue gas velocity of 6 m/s, the thermal effectiveness number was reduced by about 10% after 15 min. The paper highlights the fly-ash deposition and removal model and the application of the dynamic mesh method.

In this paper, numerical simulations were performed for the fouling characteristics and heat transfer performance after fouling on the H-type finned tube heat exchangers, and the optimum parameters studies were also conducted according to the orthogonal array design method. Firstly, the effect of row number on the fouling results were discussed. It was found that there was a critical row number Ncr = 6, and the fouling results will reach a constant value when the row number is larger than the critical row number. Then, the orthogonal array design method was employed to design the test arrangements, and the effects of six geometric parameters and two operating conditions on the fouling results as well as heat transfer performance after fouling were investigated. The order of the effects of each factor and the optimum parameters corresponding to the least fouling and best heat transfer performance after fouling were also determined. At last, a correlation of the Nusselt number after fouling Nuf∗ with the geometric parameters and the operating conditions was obtained. The results showed that the optimum conditions we determined using the orthogonal array design method do have less fouling or have better heat transfer performance after fouling than the reference cases in the test arrangements. The correlation of Nuf∗ has good prediction accuracy, and it can be useful in the design and evaluation of heat exchanger in dusty environment.

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.

The problem of high exhaust temperature universally exists around the industrial boilers, which causes enormous loss of energy. Different kinds of heat exchangers are widely used in the waste heat recovery systems to recover waste energy from different heat sources. However, considering the heat exchangers mainly work in dusty environment, such as in the tail of large boiler, fly ash fouling on heat exchangers is a major problem in waste heat recovery systems, which can lead to significant efficiency deterioration. Besides, the fouling may also lead to failure of the systems with serious economic consequence. The H-type finned tube heat exchanger is a new type of heat exchanger which have been widely used in waste heat recovery systems in recent years, because of their excellent anti-wear and anti-fouling performance. In this paper, a numerical model was developed to investigate the fouling characteristics of two kinds of H-type finned tube heat exchangers used in the waste heat recovery systems. Firstly, a fouling model which contains a deposition process and a removal process is developed. The deposition process is determined by the energy balance on the particles and the removal process is determined by the the local wall shear stress and the fouling thickness on the fin surface. As the flow field directly affects the particle motion, the fluid flow characteristics of the two kinds of heat exchangers are calculated, and the discrete phase model (DPM) was employed to trace the fly ash particles in the flow field during the fouling process. Since the simulation time is much smaller in magnitude than the real time in fouling process, it is impossible to simulate the whole real fouling time period (hours) with a small time step size. In order to obtain a reliable result, it is necessary to enlarge the simulation time and results to the real time scale. In this paper, a fouling time ratio is proposed by considering the volume difference between the simulated deposit layer and the real deposited particles and the concentration difference of injected particles between the simulation and real situation. According to the fouling time ratio, we can enlarge the simulation time and results to the real time scale. Then the real-time fouling characteristics of the two kind of H-type finned tube heat exchangers are investigated and the effects of inlet velocity, particle diameter on the fouling results are also studied. The results show that, the fouling mainly occurred in the flow stagnation region and the recirculation region of the H-type finned tube. The net fouling mass increases with the operating time, showing an asymptotic characteristic of growth, and finally will reach a stable value, which corresponds to the asymptotic fouling resistance. The increase in inlet velocity and particle diameter would lead to a rapidly decrease in fouling results. The anti-fouling performance of double H-type finned tubes shows slightly better than that of single ones, but the advantages decreases with the increase in inlet velocity and particle diameter.

A numerical method was developed to investigate the fouling characteristics of a typical heat exchanger. Firstly, an integrated fouling model which consists of a deposition process and a removal process was developed to simulate the fouling characteristics. The deposition criterion was determined by the energy balance during the interaction of particle and surface, and the removal process was based on the local wall shear stress and the fouling thickness. Then, the fluid flow characteristics were calculated and the discrete phase model (DPM) was employed to trace the fly ash particles during the fouling process. Since the simulation time is much smaller in magnitude than the real time in fouling process, a fouling time ratio was proposed to enlarge the simulation time and results to the real time scale. The flow characteristics, real-time fouling characteristics and the effects of inlet velocity, particle diameter on the fouling resistance were investigated. At last, the comparison with experimental results was also conducted. The results show that the particle motion was easily controlled by different vortex structures and the fouling mainly occurred in the flow stagnation region and the recirculation region. The net fouling results shows an asymptotic characteristic of growth and the increase in inlet velocity and particle diameter would lead to a decrease in fouling results.

With the ongoing emphasis on the efficient use of energy, much effort is put on the development and application of various heat recovery technologies. Heat exchangers play an important role in energy transfer and utilization and are widely used to recover waste energy from different kinds of heat sources in many fields such as energy and power engineering, petroleum and chemical engineering, air conditioning and refrigeration, aviation and aerospace, etc. In the heat exchangers for waste heat recovery, the gas-side fluid mediums usually are unclean and corrosive. The problems of fouling, erosion and corrosion of heat exchangers in the middle and low temperature flue gas-side caused by the high ash content, viscous and corrosive components are inevitable, and how to effectively solve the problems has been the subject of much research in recent years. In this paper, the researches of fouling, erosion and corrosion characteristics in middle and low temperature flue gas heat exchangers are reviewed. The causes and influence factors are discussed. Firstly, the fouling mechanisms of heat exchangers are introduced and the existing theoretical models, empirical models, and numerical methods of particle transport, impact and deposition are reviewed. The ash fouling characteristics of 6-row tube heat exchangers are presented and the effects of flue gas velocity, particle diameter, spanwise tube pitch, longitudinal tube pitch, tube shape and geometry conditions on fouling rate are discussed. Both decrease of the longitudinal tube spacing and increase of spanwise spacing can effectively reduce the deposition of particles. The geometry of elliptical tubes can fulfill this requirement for fouling reduction with smaller longitudinal spacing and larger spanswise pitch. The feasibility of using oval tube is explored for the fouling reduction. Compared with the circular tube arrangement, the deposition rate of the elliptical tube was decreased by 68.8%. Secondly, the existing erosion mechanisms, theories and methods are introduced. Different erosion models, including the model advantages and disadvantages are also summarized. The dynamics behaviour of the entrained solid particles in the flow and the deformation of the tube surface due to erosion are presented. The effects of particle diameter, particle mass flow rate and fluid Reynolds number are discussed. Among the three parameters, fluid Reynolds number has the most important effect on the erosion of tube surface. Erosion mainly occurs at the first part of the single tube surface. Thirdly, the existing mechanisms, theories and methods of corrosion characteristics of heat exchangers are introduced. Then a coupling numerical model, developed in author’s previous research, is discussed to predict the condensation characteristics of sulfuric acid vapor on heat exchanger surfaces. The corrosion risk can be reduced by decreasing the water vapor concentration and increasing the flue gas temperature. The effects of operating parameters and geometry conditions on the sulfuric acid condensation and corrosion characteristics are also discussed. A correlation of Sh number of sulfuric acid versus fin geometries for a 10-row tube bundle is provided. In order to reveal the anti-corrosion characteristics of different materials and the coupling mechanisms between the fouling and dewpoint corrosion, the online water-cooled test system and laboratory static sample test system on the corrosion characteristics of heat exchangers are introduced. Some dewpoint corrosion resistant steels are presented. The mechanism of dewpoint corrosion is revealed, the relationship between the thickness of corrosion layer and wall temperature is discussed and the coupling mechanisms between the fouling and dewpoint corrosion are also discussed. The corrosion products are composed of the ash and acid reaction products in the outer layer, iron sulfate in the middle layer, and iron oxide in the inner layer. The innermost layer is the main corrosion layer. Finally, the research needs and prospect of fouling, erosion and corrosion research are discussed. Hope to solve the problems of designing the anti-fouling, anti-erosion and anti-corrosion heat exchangers, and promote the development of efficient utilization technologies in middle and low temperature flue gas heat exchangers.

The fouling problem of flue gas heat exchangers is one of the most critical problems to be solved in industrial waste heat recovery applications. In this paper, the fouling potential prediction and efficient design method of a flue gas heat exchanger were proposed. Firstly, the evaluation index of the fouling factor indicating the fouling degree of the heat exchanger was defined. Then, the fouling factors under different coal ash types and structure parameters were numerically predicted, forming the fouling factor database. Finally, the Pareto optimal solution set is obtained by the neural network and genetic algorithm. The results show that fouling factor index FFI effectively reflects the effect of coal ash type and structural parameters on the fouling degree, which can make up for the deficiency of existing fouling evaluation index B/A. The thermal-hydraulic performance of flue gas heat exchanger was optimized with multi-objective optimization method, and the fouling degree of Pareto optimal solutions were compared. Case A (S1/D = 3.00 and S2/D = 1.50) and case B (S1/D = 2.04 and S2/D = 2.19) are applicable to the waste heat recovery of dusty flue gas with and without considering the space size constraint, respectively. Compared with case E (S1/D = 1.50 and S2/D = 3.00) considering only heat transfer performance, thermal-hydraulic and anti-fouling performance of case A and case B are significantly improved.