Evaluation of laboratory crude oil threshold fouling data for application to refinery pre-heat trains

`Threshold' models for crude oil fouling present a logical framework for analysing chronic fouling problems in refinery pre-heat trains. Few complete data sets of fouling thresholds in crude oil exist: this paper presents a modelling (correlation) study of the data set obtained by Knudsen et al. [Understanding Heat Exchanger Fouling and its Mitigation, Begell House, New York, 1999, p. 265] for threshold fouling experiments performed in a pilot plant. The threshold fouling equation proposed previously by Ebert and Panchal, and extended by Panchal et al. [Understanding Heat Exchanger Fouling and its Mitigation, Begell House, New York, 1999, p. 273] has been modified following a critique of some of its underlying assumptions. These previous efforts in developing threshold fouling equations were based on observed fouling rates, extrapolated to yield operating conditions that presumably exhibit zero fouling. This study represents an attempt to model measured physical parameters that actually resulted in no observable fouling. The validity of such a model to describe threshold fouling conditions in refinery pre-heat exchangers is discussed.     

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.     

Neuro-based formulation to predict fouling threshold in crude preheaters

For preheat exchangers of a crude distillation unit (CDU), operating in conditions such that fouling minimized is crucial. A number of semi-empirical models called “threshold fouling models” were developed by various researchers to predict crude oil fouling behavior in crude preheaters of CDUs. In this study, an artificial neural network (ANN) model has been employed to develop a set of mathematical formulations to identify regions where there is less/no fouling. The comparisons between results of the developed neuro-based formulation and three threshold fouling models showed the use of neuro-based model resulted in significant improvements in terms of predicting crude oil fouling behavior of various laboratory and plant data. The approach of developing neuro-based models to predict fouling behavior can be readily applied in CDUs to identify more accurate fouling/no-fouling operating zones leading to an enhancement in the operation of crude preheaters.     

夹点分析在原油常减压蒸馏换热网络的应用

对常减压蒸馏过程的换热网络进行了夹点分析。得到了各热流体和冷流体在各自温度间隔的热负荷分布情况,绘制了组合曲线和总组合曲线。对于热流体,热端提供的热负荷较大,冷端提供的的热负荷较小,其中热物流减黏渣油(JNZY)对整个热负荷的贡献最大。对于冷流体,需要热负荷的冷流体主要是流体拔头油(BTY)和原油(YY),且集中在中低温度间隔内。组合曲线和总组合曲线说明换热网络热回收接近温差(ΔtHRAT)越小,回收的热量越大,需要的热公用工程和冷公用工程也越少,公用工程投资和冷热公用工程费用将减少,但是由于换热网络接近温差变小,整个换热网络的换热器面积将增大,从而增加了换热器投资,这表明在实际过程中要兼顾换热器等设备的投资成本。不同ΔtHRAT的换热网络,其夹点是变化的。     

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%.     

Effect of heat exchanger material and fouling on thermoelectric exhaust heat recovery

Thermoelectric devices are being investigated as a means of improving fuel economy for diesel and gasoline vehicles through the conversion of wasted fuel energy, in the form of heat, to useable electricity. By capturing a small portion of the energy that is available with thermoelectric devices can reduce engine loads thus decreasing pollutant emissions, fuel consumption, and CO₂ to further reduce green house gas emissions. This study is conducted in an effort to better understand and improve the performance of thermoelectric heat recovery systems for automotive use. For this purpose an experimental investigation of thermoelectrics in contact with clean and fouled heat exchangers of different materials is performed. The thermoelectric devices are tested on a bench-scale thermoelectric heat recovery apparatus that simulates automotive exhaust. It is observed that for higher exhaust gas flowrates, thermoelectric power output increases from 2 to 3.8 W while overall system efficiency decreases from 0.95% to 0.6%. Degradation of the effectiveness of the EGR-type heat exchangers over a period of driving is also simulated by exposing the heat exchangers to diesel engine exhaust under thermophoretic conditions to form a deposit layer. For the fouled EGR-type heat exchangers, power output and system efficiency is observed to be 5-10% lower for all conditions tested.     

Improved milk fouling simulation in a helical triple tube heat exchanger

Performance of a dairy heat exchanger declines as milk fouling deposits on the heating surface. It causes an increased resistance to heat flow thereby the milk outlet temperature decreases with increasing fouling thicknesses. Various models have been suggested for the prediction of fouling thickness and milk outlet temperature in a heat exchanger. The present paper describes an improved simulation model for the accurate estimation of fouling thickness and milk outlet temperature. Local fouling factor in terms of the Biot number is used in the work undertaken. Fouling thickness and milk outlet temperature are predicted as a function of time and over the entire length of the heat exchanger. Right from the beginning fouling occurred to a greater extent towards the outlet and with the progress of time the rate of increase of the fouling thickness decreased. The milk outlet temperature decreased with the time as the fouling increased.     

Influences of bubble formation on different types of heat exchanger fouling

In this paper, a systematic comparison is performed to investigate fouling of suspended particles under forced convective and subcooled flow boiling heat transfer. For this purpose, two different types of fouling are separately considered: crystallization fouling of dissolved CaSO₄ particles in water and particulate fouling of suspended Al₂O₃ particles in n–heptane. The effect of hydraulic parameters such as fluid velocity and also bubble generation under subcooled flow boiling are studied. Results of the experiments demonstrate that creation of boiling condition in the heat exchanger has opposite influence in these two types of fouling. It means that bubble generation on the heat transfer surface promotes scale formation under crystallization fouling. This is due to the fact that increased bubble generation creates higher supersaturation beneath the vapor bubble, therefore, increasing the crystal concentration in the boundary layer. On the other hand, boiling condition inhibits the scale formation under particulate fouling because the suspended particles are repelled from the boundary layer by the strong turbulences created by the swarm of bubbles.     

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.     

Conjugate fluid flow and kinetics modeling for heat exchanger fouling simulation

Thermal treatment of fluid foods represents a major unit operation in the food industry, to ensure the product's safety and quality features. But during the thermal treatments of such sensible fluids in common plate heat exchangers, food constituents such as proteins can be thermally damaged and precipitated to form fouling that greatly affect the treatment efficiency and alter the product's desired features.Computational Fluid Dynamics simulations can then be successfully exploited, bringing forth temperature and velocity information that yield for deposit distributions when coupled to biochemical notations for thermal denaturation of fluid constituents.The present work exploits such modeling for a single-channel heat exchanger during pasteurization of milk. The model enforces a conjugate system of differential equations to a heat exchanger's corrugated plate to combine flow, heat transfer and local transport of β-lactoglobulin. A preliminary computation has been performed that could be applied to geometry optimization (different corrugation shape and orientation) and for a variety of biochemically evolutive products.     

城市污水自冲洗除污换热器试验研究

城市原生污水作为热泵冷热源为建筑物空调供热,换热面的污染导致流动阻力增大,换热系数降低,是污水源热泵发展的主要技术障碍之一,基于此,开发了在线自清洗除污换热器。文章介绍了在线自清洗除污换热器的结构原理和在线自清洗除污换热器样机,构建了城市污水流动换热实验台。文章分别对不启动冲污注水头HRF换热器的工作状况和启动冲污注水头HRF换热器的除污效果进行了试验研究,结果表明,换热器机械性能良好,当除污周期为10 h,冲洗流速为5 m/s,单次冲洗时间为1 min时,一个采暖周期换热器污垢阻抗降低率为70%以上。     

Calculation of heat exchanger networks for limiting fouling effects in the petrochemical industry

The paper presents a method for designing of heat exchanger networks (HENs), which reduces the effects of thermal fouling resistance. The method is based on pinch technology, extended by two transformations. These are based on the criterion of minimum sensitivity to the fouling effects by a single heat exchanger and the HEN. The proposed method has been applied in the petrochemical industry where the two heat recovery systems, designed by the method described here, have been working successfully for some years.     

Entropy parameters for heat exchanger design

A general expression for entropy generation in counter-current heat exchangers is developed. It is applicable to incompressible liquids and perfect gases. Two new entropy generation numbers are defined, N_M and N_Q. The analysis is applied to an air-air counter-current heat exchanger. The three entropy generation numbers, N_S, N_M and N_Q, have a different variation with NTU at the various values of the capacity flow rate ratio employed in the calculations.     

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.     

Design of a novel, intensified heat exchanger for reduced fouling rates

This paper describes an integrated approach into the design and evaluation of a novel tube bundle heat exchanger that achieves higher heat transfer levels at lower levels of pressure drop, while remaining less susceptible to gas-side fouling. The approach combines laboratory scale experiments with industrial observations and numerical simulations of full-scale heat exchangers to study the thermal, hydraulic and fouling characteristics of tube bundle heat exchangers. Three arrangements are compared and the advantages of the proposed novel arrangement are demonstrated. Enhanced heat transfer rates are combined with reduced pressure drop and gas-side fouling rates through careful design of the shape of the tube cross-section and reduced transverse spacing.     

螺旋翅片管换热器的优化设计

本文以试验为基础,对螺旋翅片管换热器的管束结构进行了优化设计,建立了数学模型。对建模 过程中有关目标函数确定、变量分析、约束条件等进行了讨论。得到了一定限制条件下的最佳结构参 数,可为电站锅炉的省煤器、空气预热器等设备的优化设计提供参考。     

Optimum design of double pipe heat exchanger

Heat exchangers are used in industrial processes to recover heat between two process fluids. Although the necessary equations for heat transfer and the pressure drop in a double pipe heat exchanger are available, using these equations the optimization of the system cost is laborious. In this paper the optimal design of the exchanger has been formulated as a geometric programming with a single degree of difficulty. The solution of the problem yields the optimum values of inner pipe diameter, outer pipe diameter and utility flow rate to be used for a double pipe heat exchanger of a given length, when a specified flow rate of process stream is to be treated for a given inlet to outlet temperature.     

Optimum design parameters of a heat exchanger

In this study, the effects of the longitudinal and lateral seperations of consecutively enlarged-contracted arranged fin pairs, widths of the fins, angle of attack, heights of fins and flow velocity on the heat and pressure drop characteristics were investigated using the Taguchi experimental-design method. Nusselt number and friction factor were considered as performance parameters. An L₁₈(2¹13⁷) orthogonal array was selected as an experimental plan for the eight parameters mentioned above. First of all, each goal was optimized, separately. Then, all the goals were optimized together, considering the priority of the goals, and the optimum results were found to be fin width of 15 mm, angle of attack of 15°, fin height of 100 mm, span-wise distance between fins of 20 mm, stream-wise distance between fins of 10 mm, span-wise distance between slices of 20 mm, stream-wise distances between slices of 20 mm at a flow velocity of 4 m/s.     

Velocity effect on electronic-antifouling technology to mitigate mineral fouling in enhanced-tube heat exchanger

The purpose of the present study was to investigate the effect of flow velocity on an electronic-antifouling (EAF) treatment, which was considered to mitigate mineral fouling in a heat exchanger with cooling-tower water. Nine different tests were conducted with no-blowdown: six runs with an in-line EAF treatment and three runs with the EAF treatment at a side-stream loop. The flow velocities tested were 0.6, 1.2, 1.5 and 2.0 m/s. As the flow velocity was increased, the benefit of the in-line EAF treatment decreased. At a flow velocity of 0.6 m/s, the fouling resistance decreased by 80% in the case with the EAF treatment compared with that in the no-treatment case for both in-line and side-stream EAF treatments.     

Experimental measurement of fouling resistance in the heat exchanger of a geothermal heating system

Experimental measurements of fouling resistance were carried out in a double-pipe heat exchanger (HE), with geothermal water flowing in the inner tube and tap water in the annular space. Tests were performed using waters from a geothermal well in central Serbia whose production temperature remained practically constant (52 °C) throughout the 90-day experiment. Analysis of the data showed that the change in fouling resistance was a linear function of time.