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.     

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。     

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

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

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.     

The impact of heat exchanger fouling on the optimum operation and maintenance of the Stirling engine

This paper focuses on the effect of heat exchanger fouling on the performance of the Stirling engine in combined heat and power (CHP) application. Fouling results from using biomass fuels and affects the heat exchanger that transfers heat into the engine. This heat exchanger is referred to as the heater. The heat exchanger that recovers heat from the flue gases is also affected by fouling. To determine the performance of the Stirling engine, a commercial Stirling analysis tool is applied together with models that have been developed for the heat transfer in the heater, regenerator and cooler of the engine. The Stirling engine model uses constant temperatures for the heat addition and rejection, with the theory of displacement engine as a basis. The fouling in the heat exchanger is taken into account by using a fouling factor that corresponds with the degradation in the total heat transfer coefficient. The Stirling engine model together with the model for heat exchanger fouling makes it possible to estimate the effect of fouling on the performance of the Stirling engine. A cost model is developed for the engine to translate changes in performance into economy in CHP operation. In the studied application, the Stirling engine is operated by the heat demand. Together with the selected control method, performance and cost models compose a tool for the simulation and optimization of the system. The use of the models to determine the optimal cleaning interval of the heat exchanger surfaces is considered.     

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.     

Estimation of thermophysical properties of fouling using inverse problem and its impact on heat transfer efficiency

At high temperature, the circulation of fluid in heat exchangers provides a tendency for fouling accumulation to take place on the internal surface of tubes. In brief, the deposits on heat exchanger tubes are caused by the presence of inorganic salts, of small quantities of organic materials and products of corrosion in the water. From thermophysical point of view, the deposited fouling has harmful effects on the heat exchanger efficiency. Indeed, it increases the thermal resistance which can raise the energy consumption. This study shows an experimental and a theoretical process of estimation of thermophysical properties of the fouling deposited on a section of a heat exchanger and its effects on the heat transfer efficiency. The estimation method is based on the Gauss-Newton algorithm that minimizes the ordinary least squares function comparing a measured temperature and a theoretical one. The temperature response is measured on the rear face of a bi-layer system composed of a section of a heat exchanger and the fouling deposited on during and after a finite width pulse heat flux on its front face. The theoretical temperature, that is a function of the unknown thermophysical properties of the bi-layer system, is calculated by the resolution of the one-dimensional linear inverse conduction problem, and by the use of the quadrupole formalism.The results of the estimation procedure show, on the one hand the efficiency and the stability of the optimization algorithm to estimate the thermophysical properties of the fouling. On the other hand they underline the necessity of the maintenance of fluid circulating tubes at high temperature.     

Oil fouling in double‐pipe heat exchanger under liquid‐liquid dispersion and the influence of copper oxide nanofluid

Dispersions of oil in water are encountered in a variety of industrial processes leading to a reduction in the performance of the heat exchangers when thermally treating such two phase fluids. This reduction is mainly due to changes in the thermal and hydrodynamical behavior of the two phase fluid. In the present work, an experimental investigation was performed to study the effects of light oil fouling on the heat transfer coefficient in a double‐pipe heat exchanger under turbulent flow conditions. The effects of different operating conditions on the fouling rate were investigated including: hot fluid Reynolds number (the dispersion), cold fluid Reynolds number, and time. The oil fouling rate was analyzed by determining the growth of fouling resistance with time and through pressure drop measurements. The influence of copper oxide (CuO) nanofluid on the fouling rate in the dispersion was also determined. It was found that the presence of dispersed oil causes a reduction in heat transfer coefficient by percentages depending on the Reynolds number of both cold and hot fluids and the concentration of oil. In addition, the time history of fouling resistance exhibited different trends with the flow rates of both fluids and its trend was influenced appreciably by the presence of CuO nanofluid.     

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 heat transfer and friction characteristics of biofouled evaporator under wet conditions

The effects of biofouling on air-side heat transfer and friction characteristics under wet conditions of three biofouled finned tube heat exchangers and one clean finned tube heat exchanger were investigated experimentally. Experimental results indicate that the biofouled fin efficiency of the evaporator decreases by 15.5% compared with the clean evaporator under the condition of the biofouled area ratio of 60% at the inlet air velocity of 2.0 m/s; The ranges of friction fouling factor and heat transfer fouling factor are 19.8%–43.1% and −15.6%−13.1%, respectively; a small quantity of biofouled particles can enhance heat transfer at low Reynolds number, and the enhancement effect decreases with the increase of Reynolds number.     

Convective boiling and particulate fouling of stabilized CuO-ethylene glycol nanofluids inside the annular heat exchanger

Thermal performance of convective flow boiling heat transfer and particulate fouling of CuO/EG nanofluids has been experimentally studied inside the annular heat exchanger. CuO nanoparticles were well-dispersed and stabilized using a new combinational method (adding surfactant, stirring, pH control and sonication) in ethylene glycol (EG) as the base fluid in different weight fractions of nanoparticles (0.1–0.4%). Despite stabilizing the nanofluids, a considerable boiling heat transfer reduction due to the fouling resistance has been reported. Subsequently, scale formation and particulate fouling of nanofluids in term of fouling resistance has experimentally been investigated. Influences of operating parameters on the fouling resistance and heat transfer coefficient are investigated and briefly discussed.     

Air-side heat transfer and friction characteristics of biofouled evaporator under wet conditions

The effects of biofouling on air-side heat transfer and friction characteristics under wet conditions of three biofouled finned tube heat exchangers and one clean finned tube heat exchanger were investigated experimentally. Experimental results indicate that the biofouled fin efficiency of the evaporator decreases by 15.5% compared with the clean evaporator under the condition of the biofouled area ratio of 60% at the inlet air velocity of 2.0m/s; The ranges of friction fouling factor and heat transfer fouling factor are 19.8%―43.1% and ―15.6%―13.1%, respectively; a small quantity of biofouled particles can enhance heat transfer at low Reynolds number, and the enhancement effect decreases with the increase of Reynolds number.     

Heat transfer enhancement by flow-induced vibration in heat exchangers

The flow-induced vibration in heat exchanger is usually considered as a detrimental factor for causing the heat exchanger damage and is strictly prevented from its occurrence. Its positive role for the possible heat transfer enhancement has been neglected. In this article a novel approach is proposed to enhance the heat transfer by using the flow-induced vibration of a new designed heat transfer device. Thus the flow-induced vibration is effectively utilized instead of strictly avoiding it in the heat exchanger design. A heat exchanger is constructed with the new designed heat transfer devices. The vibration and the heat transfer of these devices are studied numerically and experimentally, and the correlation of the shell-side convective heat transfer coefficient is obtained. It is found that the new designed heat exchanger can significantly increase the convective heat transfer coefficient and decrease the fouling resistance. Therefore, a lasting heat transfer enhancement by the flow-induced vibration can be achieved.     

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

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

Experimental observation of surface morphology effect on crystallization fouling in plate heat exchangers

In this study, stainless steel test plates with different surface roughness and textures, which are used as the heat transfer surface of a plate heat exchanger, are tested individually in calcium carbonate fouling experiments. The present experimental results clearly indicate a strong correlation between the surface roughness and the amount of crystallization fouling deposit. Through detailed image analysis, four stages of the formation of crystallization fouling are identified, and the impact of the surface morphology on the extent of crystallization fouling is described qualitatively.     

Study on oil fouling in a double pipe heat exchanger with mitigation by a surfactant

Fouling of oils on heat exchanger surfaces and pipelines is a common problem in a variety of industrial applications. This is because the oil deposits on the heat transfer surface causes an increase in pressure drop and a decrease in heat exchanger efficiency. In the current work, oil fouling in double pipe heat exchanger was investigated and mitigated using a surface‐active agent for the flow of a dispersion fluid containing different dispersed oil fractions in water. The effect of the dispersed oil fraction (5%vol and 10%vol) and temperature (35°C‐55°C) on the oil fouling rate was studied and discussed under turbulent flow conditions for both hot and cold fluids. Different amounts of alkylbenzene sulfonate as a surfactant were added to reduce the fouling rate under turbulent flow. It was found that the fouling thermal resistance (R_f) increases when the fluid temperature decreases. The higher the dispersed oil fraction, the higher the R_f for all temperatures due to higher oil deposition. Addition of 0.2%vol to 0.5%vol of alkylbenzene sulfonate caused an appreciable reduction in R_f depending on oil fraction and Reynolds number. The mitigation percent was higher for a lower Reynolds number, reaching up to 96%.     

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.     

Application of a heat pump system using untreated urban sewage as a heat source

Untreated urban sewage contains large amounts of thermal energy; and its temperature is suitable as a heat source in heat pumps for the heating and cooling of buildings. However, it is not widely used in heat pump systems due to the problem of filth. This paper presents an untreated sewage source heat pump (USSHP) system in which auto-avoiding-clogging equipment is used to continuously capture suspended solids in the sewage. Thus, the block problems caused by filtration and fouling in the heat exchanger tubes can be efficiently resolved in this system. In an actual engineering application, the characteristic parameters of USSHP system are tested under typical operating conditions for heating status. Based on the test results, the performances of the USSHP system are examined. The results indicate that the thermal resistance of the convective heat transfer and fouling on the sewage side in the sewage exchanger is 80% of its total thermal resistance. The COP of the heat pump unit and the COP of the USSHP system are 4.3 and 3.6, respectively.     

Experimental study of thermo-hydraulic and fouling performance of enhanced heat exchangers

The present study investigated the effect of two tube inserts (wire coil and wire mesh) on the heat transfer enhancement, pressure drop and mineral salts fouling mitigation in tube of a heat exchanger. A 3/4-in. tube that is heated by band heaters, is used which simulated a tube of heat exchanger. Working fluid is water with certain quality. The heat transfer rate averagely increased by 22–28% for wire coil (p/d = 0.125, e/d = 0.00375) and 163–174% for wire mesh (medium density) over a plain tube value, depending on type of tube insert, density of wire torsion and flow velocity. However, the pressure drop also increased substantially by 46% for wire coil and 500% for wire mesh. Wire coil insert with vibration mitigate mineral salts fouling (scaling) about 34%, and wire mesh have no effect on scaling, however it sometimes increased deposit rate.     

某型柴油机换热器设计及仿真分析

设计独立的换热器,降低水下工作柴油机中高负荷工况的排气温度。运用SolidWorks建立换热器模型并进行仿真分析,研究圆柱形换热器4种换热管布置方式对换热器温降与压损的影响。分析结果表明:设计的换热器可将排气温度由550.00℃降低到161.94℃,废气在换热器中的压损为5.95 kPa,降温效果和压损均满足相关工程要求;换热管管径和管心距不变,正三角形和转角三角形布置方式的换热器换热管数量较多,换热面积较大,换热效果较好,其中正三角形布置方式换热器的换热效果更好,但压损更高。本研究可为水下动力装置排气降温系统设计提供参考。