Planning the optimum cleaning schedule based on simulation of heat exchangers under fouling

An investigation of variations in outlet temperatures of heat exchangers under fouling was carried out. The simulation of heat exchangers was performed by employing a linear fouling deposit function. The formation of deposits reduces heat exchangers effectiveness. There is inherently a linear nature between outlet and inlet temperatures of heat exchangers. The outlet temperatures can also be affected by up‐stream exchangers serving the same streams, and the up‐stream influence can be transferred in the heat system. The mathematical model of the cleaning cycle was outlined, based on the objective function of minimizing cost in unit operation time. According to the results, some heat exchangers can be given cleaning priority when the system is shut down, in order to maximize economic benefit. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20384     

Modeling of plate heat exchangers with generalized configurations

A mathematical model is developed in algorithmic form for the steady-state simulation of gasketed plate heat exchangers with generalized configurations. The configuration is defined by the number of channels, number of passes at each side, fluid locations, feed connection locations and type of channel-flow. The main purposes of this model are to study the configuration influence on the exchanger performance and to further develop a method for configuration optimization. The main simulation results are: temperature profiles in all channels, thermal effectiveness, distribution of the overall heat transfer coefficient and pressure drops. Moreover, the assumption of constant overall heat transfer coefficient is analyzed.     

Optimal configuration design for plate heat exchangers

A screening method is presented for selecting optimal configurations for plate heat exchangers. The optimization problem is formulated as the minimization of the heat transfer area, subject to constraints on the number of channels, pressure drops, flow velocities and thermal effectiveness, as well as the exchanger thermal and hydraulic models. The configuration is defined by six parameters, which are as follows: number of channels, numbers of passes on each side, fluid locations, feed relative location and type of channel flow. The proposed method relies on a structured search procedure where the constraints are successively applied to eliminate infeasible and sub-optimal solutions. The method can be also used for enumerating the feasible region of the problem; thus any objective function can be used. Examples show that the screening method is able to successfully determine the set of optimal configurations with a very reduced number of exchanger evaluations. Approximately 5% of the pressure drop and velocity calculations and 1% of the thermal simulations are required when compared to an exhaustive enumeration procedure. An optimization example is presented with a detailed sensitivity analysis that illustrates the application and performance of the screening method.     

Optimising heat exchangers for air-to-air space-heating heat pumps in the United Kingdom

The paper deals with some of the major aspects of heat-exchanger design for electric heat pumps. After a discussion of heat-transfer theory, it describes a method that can be used in the design and sizing of air-to-refrigerant heat exchangers and in calculating temperature distributions. As an illustration, economically optimum sizes for exchanger coils are given for heat pumps of output 13.9 kW, 5.6 kW and 5 kW at 5°C outside the ambient temperature. At several stages, manufacturer's experimental data have been used, and the final results are compared with the design of heat exchangers used in commercially available models. Some temperature measurements made on a heat-pump installation in an experimental house are also reported. At least doubling the size of presently used indoor coils is shown to be economically justifiable, increasing the seasonal coefficient of performance from about 2.4 to 2.8-3.0. Reassessment of outdoor-fan size is also shown to be necessary. Throughout the work it is assumed that the heat pump is required for heating only, as would be the case in the United Kingdom.     

Experimental study on heat and mass transfer characteristics of louvered fin-tube heat exchangers under wet condition

An experimental study was conducted to investigate the effect of a tube row, a fin pitch and an inlet humidity on air-side heat and mass transfer performance of louvered fin-tube heat exchangers under wet condition. Experimental conditions were varied by three fin pitches, two rows, and two inlet relative humidities. From the experimental results, it was found that the heat transfer performance decreased and the friction increased with the decrease of a fin pitch, for 2 row heat exchanger. The effect of a fin pitch on heat transfer performance was negligible with 3 row heat exchanger. The change in a relative humidity was not affected heat transfer and friction. However, the mass transfer performance was slightly decreased with the increase of a relative humidity and with the decrease of a fin pitch. The mass transfer performance decreased with the decrease of a fin pitch. The mass transfer performance of the louvered fin-and-tube heat exchanger was different according to the number of a tube row.     

板式换热器黏液形成菌生物污垢特性的实验研究

为探讨黏液形成菌在板式换热器里的结垢规律,对不同流速、温度及体积分数下黏液形成菌在板式换热器内的污垢特性进行了实验研究。结果表明：随着流速的增加,黏液形成菌的污垢热阻渐近值逐渐减小;随着温度的升高,黏液形成菌结垢的诱导期缩短,并且达到稳定的时间增加,在实验温度范围内,污垢热阻渐近值在35℃时最大;而随着细菌体积分数的增加,污垢热阻值呈现明显幅度的增长。     

紧凑式两相换热器中管式换热元件沸腾传热实验研究

以烃类物质（丙烷和正戊烷）作为工质,进行了紧凑式换热器中带有加工配置表面的管式换热元件池沸腾实验研究。其中,单管实验温度工况为２５３Ｋ￣２９３Ｋ（饱和工质）。实验中所采用的换热元件为重入式结构加工配置表面的强化传热管和光管以及低助管。针对由４５根光管或带有加工配置表面的管子所构成的叉排管束进行了实验研究,实验工质为丙烷和正戊烷,实验温度分别为两种工质在２６３Ｋ和３０８Ｋ之间的饱和和温度。并将所得实     

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.     

Transient response of plate heat exchangers considering effect of flow maldistribution

Plate heat exchangers have been playing important role in the power and process industries in the recent past. Hence, it is important to develop simulation strategies for plate heat exchangers accurately. This analysis represents the dynamic behaviour of the single pass plate heat exchangers, considering flow maldistribution from port to channel. In addition to maldistribution the fluid axial dispersion is used to characterise the back mixing and other deviations from plug flow. Due to unequal distribution of the fluid, the velocity of the fluid varies from channel to channel and hence the heat transfer coefficient variation is also taken into consideration. Solutions to the governing equations have been obtained using the method of Laplace transform followed by numerical inversion from frequency domain. The results are presented on the effects of flow maldistribution and conventional heat exchanger parameters on the temperature transients of both U-type and Z-type configurations. It is found that the effect of flow maldistribution is significant and it deteriorates the thermal performance as well as the characteristic features of the dynamic response of the heat exchanger. In contrast to the previous studies, here the axial dispersion describes the inchannel back mixing alone, not maldistribution, which is physically more appropriate. Present method is an efficient and consistent way of describing maldistribution and back mixing effects on the transient response of plate heat exchangers using an analytical method without performing intensive computation by complete numerical simulation.     

Optimal design of heat exchangers using the progressive quadratic response surface model

The optimal values of the design variables which minimize the pressure loss under the required temperature rise are obtained numerically in a plate-fin heat sink. In thermal/fluid systems, three fundamental difficulties such as a high computational cost for function evaluations (i.e., pressure drop and thermal resistance), the absence of design sensitivity information, and the occurrence of numerical noise are commonly confronted. Thus, sequential approximate optimization (SAO) algorithms have been used to overcome the above mentioned problems. In the present work, the progressive quadratic response surface method (PQRSM), which is one of the SAO algorithms, is proposed for constrained nonlinear optimization problems and is coupled with the computational fluid dynamics (CFD) for the optimization of heat sink. The optimal solutions obtained from the PQRSM are also compared with those of the sequential quadratic programming (SQP) method, which is one of the gradient-based optimization algorithms, to validate the efficiency and fidelity of the PQRSM.     

Benefit of filtration in physical water treatment for the mitigation of mineral fouling in heat exchangers

The objective of the present study was to investigate the benefit of filtration in the physical water treatment used to mitigate mineral scale build-up in heat exchangers. Two types of filters were used: a 5-μm cartridge fabric filter and a sand filter with 20-μm pores, both of which were used at a side-stream loop in a laboratory cooling tower. Heat transfer fouling experiments were conducted using a double-pipe heat exchanger with cooling water at 5 and 8 cycles of concentration with a make-up water hardness of 200–240 ppm. The test results demonstrated that the filtration enhanced the performance of the PWT synergistically, resulting in a near initial peak heat transfer performance in the double-pipe heat exchanger.     

Experimental comparison and analysis on silica gel and polymer coated fin-tube heat exchangers

In this paper, two desiccant-coated heat exchangers, which are actually fin-tube heat exchanging devices coated with silica gel and polymer materials respectively, are investigated experimentally. Due to the hygroscopic properties of the desiccant materials, both the sensible heat and the latent heat of the process air can be handled by using this kind of heat exchanger. An experimental setup was designed and built to test the performance of this unit. It is found that this desiccant-coated fin-tube heat exchanger well overcomes the side effect of adsorption heat which occurs in desiccant dehumidification process, and achieves good dehumidification performance under given conditions. The silica gel coated heat exchanger behaves better than the polymer one. The influences of regeneration temperature, inlet air temperature and humidity on the system performance in terms of average moisture removal rate D_avg and thermal coefficient of performance COP_th were also analyzed.     

Theoretical and experimental examination of air conditioner heat exchangers

Heat and mass transfer characteristics of finned-tube heat exchangers under dehumidifying and dry conditions are examined both theoretically and experimentally in the present study. Six types of heat exchangers were tested on an experimental facility. It was determined that there is less than 1 °C of outlet air temperature difference between computer program results and literature for water and R22 as a refrigerant. The outlet air specific humidity difference between them is less than 6.44% for water and 9% for R22. According to the experimental results, louvers on fin cause an increase of airside heat transfer coefficient of about 50% or 100%.     

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.     

翅片间距对平直翅片管换热器传热与阻力特性影响试验研究

为了获得翅片间距Pf对平直翅片管换热器的传热与阻力特性的影响规律,根据相似模化原理对3种不同Pf的平直翅片管换热器进行了试验研究。结果表明:雷诺数Rea在4 000~8 000范围内,努赛尔数Nua数随Rea的增大而增加,欧拉数Eua随Rea的增大而降低;同一Rea下,Nua随Pf减小而增加,但增加不明显,Eua随Pf增大而降低;同一Rea下,Pf越大,综合流动传热性能越好,但实际换热面积会减小,需综合考虑。研究成果可为汽轮发电机平直翅片管换热器的结构和性能优化提供依据。     

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.     

静电水处理器在鼓冷工段螺旋板换热器中的应用分析

针对螺旋板换热器易结垢、礁清洗的生产难题,采用新型表龟水处理器,对循环冷却水进行处理,实现节能降耗,由此对静电水处理器的应用前景做出展望。提出了静电防垢除垢的机理,并在微观上对其做出合理解释。     

Impact of scale build‐up models on energy utilization in heat exchangers

In this paper an energy utilization index is presented. It is used to investigate the impact of scale build‐up in heat exchange equipment. The investigation is carried out using several scale build‐up models. The models employed in this paper are the linear, logarithmic, and the asymptotic one. It has been demonstrated by a realistic example that the type of scale build‐up model assumed greatly affects energy utilization and maintenance scheduling for heat exchange equipment. Copyright © 1999 John Wiley & Sons, Ltd.     

Design and multi-objective optimization of heat exchangers for refrigerators

In this study, a detailed thermodynamic model for a refrigerator based on an irreversible Carnot cycle is developed with the focus on forced-air heat-exchangers. A multi-objective optimization procedure is implemented to find optimal design values for design variables. Minimizations of energy consumption and material cost were the two objectives considered. Since these objectives are conflicting, no single design will satisfy both simultaneously. The result of this research is a set of multiple optimum solutions, which are called ‘Pareto optimal solutions’. Air and refrigerant side correlations were combined with an elemental approach to model the heat exchangers. This paper presents a detailed design model development. A limited validation is presented with experimental test-data obtained from a typical household refrigerator. Detailed simulation models are typically complex and computationally demanding. An optimization algorithm requires several evaluations of such models. Response surface based metamodels for objective functions were used to save computational effort. A genetic-algorithm based optimization tool is used for multi-criteria optimization.     

换热器结垢工况下换热系数变化的分析研究

介绍了换热器污垢热阻的数学模型，包括污垢沉积模型和剥蚀模型。分析了换热器结垢工况下的换热系数的变化，重点研究了时间、流体雷诺数Re和流体—污垢界面温度Ts对换熟系数K的影响，以及在结垢诱导期内换热系数K的变化。得到了冷却水流速与污垢热阻之间的关系式，界面温度Ts与污垢热阻和换热系数之间关系的示意图，并得出了诱导期内的换热系数K大于结垢过程的其他四个阶段的结论。最后，阐述了分析结果对工程的实际指导意义。