Predictive tool for estimation of convection heat transfer coefficients and efficiencies for finned tubular sections

Fins or extended surfaces are widely used to increase the air side heat transfer rate in various heat exchange applications. In this paper, firstly, an attempt has been made to formulate a novel and simple-to-use correlation for the prediction of efficiencies for uniform thickness finned tubular sections as well as fin tip temperature for wide range of conditions (covering finned pipe diameter to pipe diameter ratios of up to 3). Secondly, another simple correlation is developed to approximate external convection heat transfer coefficients for nominal pipe size (NPS) steel pipes of 75, 100, and 150 mm arranged in staggered rows surrounded by combustion gases for temperature up to 600 °C and gas mass flow rates of up to 3 kg/(m² s). A simple interpolation formula generalizes this correlation for wide range of steel pipes. Average absolute deviations between the reported data and the proposed correlations are found to be around 1% demonstrating the excellent performance of proposed correlation. The tool developed in this study can be of immense practical value for engineers and scientists to have a quick check on the efficiencies for uniform thickness finned tubular sections as well as external convection heat transfer coefficients for various steel pipes arranged in staggered rows and surrounded by combustion gases flowing externally to heat a liquid in a pipe at various conditions without opting for any experimental measurements. In particular, practice engineers would find the predictive tool to be user-friendly with transparent calculations involving no complex expressions.     

Energy efficiency and indoor thermal perception: a comparative study between radiant panel and portable convective heaters

This study investigates experimentally the thermal perception of indoor environment for evaluating the ability of radiant panel heaters to produce thermal comfort for space occupants as well as the energy consumption in comparison with conventional portable natural convective heaters. The thermal perception results show that, compared with conventional convection heater, a radiantly heated office room maintains a lower ambient air temperature while providing equal levels of thermal perception on the thermal dummy head as the convective heater and saves up to 39.1% of the energy consumption per day. However, for human subjects’ vote experiments, the results show that for an environmentally controlled test room at outdoor environment temperatures of 0°C and 5°C, using two radiant panel heaters with a total capacity of 580 W leads to a better comfort sensation than the conventional portable natural convective heater with a 670 W capacity, with an energy saving of about 13.4%. In addition, for an outdoor environment temperature of 10°C, using one radiant panel heater with a capacity of 290 W leads to a better comfort sensation than the conventional convection heater with a 670 W capacity, with an energy saving of about 56.7%. From the analytical results, it is found that distributing the radiant panel heater inside the office room, one on the wall facing the window and the other on the wall close to the window, provides the best operative temperature distribution within the room.     

Long time investigation of the effect of fouling on the super‐heaters in a circulating fluidized biomass boiler

The present investigation involves measurements and theories on the mechanisms of the forming of deposit layers on super‐heater tubes in a biomass‐fired CFD boiler. The deposit layer thickness and the soot‐blowing frequency effect on the super‐heaters heat transfer are the main subject of the study that has been conducted over a 3‐year period. The measurements show a deposit growth rate on the super‐heaters of approximately 4 g m^?2 h^?1. The distribution of the deposit material varies significantly between the windward and the leeward side of the tubes, with the thickest layers on the windward side. Further down stream of the first super‐heater, the fouling problem on the super‐heater and re‐heater tubes are not so severe. A theoretical model shows that a deposit layer of 20 mm will decrease the heat transfer rate of the first super‐heater by nearly 40%. The soot‐blowing system shows a strong positive effect on the heat transfer rate of the super‐heater a few hours after a soot‐blowing sequence has been completed. However in the long run, the varied soot‐blowing frequency does not have a significant influence on the deposit layer growth rate. Copyright © 2006 John Wiley & Sons, Ltd.     

加热炉炉管节能改造和防护技术探讨

某油田拥有200多台火(管)筒式加热炉,由于腐蚀老化、负荷增大以及热洗周期缩短等原因,加热炉损坏和高能耗问题日益突出,每年耗气近9500×104m3,且现场多次发生加热炉着火事故。据统计,损坏的加热炉中,炉管烧损鼓包占53%,烟炉管、封头、烟箱、烟囱腐蚀减薄或穿孔占32%,由此判断炉管是加热炉的易损部件。加热炉炉管鼓包是因为结垢导致炉管上部局部过热以及环境温度变化过大,炉管和弯头等部位的腐蚀主要是因为溶解氧腐蚀、垢下腐蚀及硫酸露点腐蚀。根据加热炉损坏的原因,提出了通过使用FHC-D型红外线新型节能涂料、物理防垢设备、直燃式燃烧器辐射管、炉管高温和泄漏监测装置等防护措施,有效控制炉管损坏问题,并预防因炉管烧穿而引起的火灾事故;此外,在加热炉运行管理中,建议通过推广低温常温集输、加热炉交替使用、合理调整天然气和空气配比等方式,达到减缓结垢速度、保护炉管的目的。     

Experimental study on titanium heat exchanger used in a gas fired water heater for latent heat recovery

In general, latent heat recovery is usually accompanied by the corrosion of the heat exchanger, which is caused by the strongly acidic condensate when the temperature of the flue gas is lowered below the acid dew point. The present study has been conducted to investigate the heat and mass transfer characteristics in a titanium heat exchanger with excellent corrosion resistance used for waste heat recovery with the condensation arranged in a gas fired water heater. In addition, the thermal efficiency of the gas fired water heater was evaluated based on the net calorific value at the maximum rated output during latent heat recovery from the exhaust flue gas. Parametric studies were conducted for the flue gas flow rate, inlet temperature and mass flow rate of the supplied water, respectively. Different arrangements of the tubes of the heat exchanger including in-line and staggered configurations were investigated. The experimental results indicate that the thermal efficiency of the gas fired water heater with a latent heat recovery (LHR) heat exchanger was enhanced by about 10% compared with conventional instantaneous water heaters, i.e., water heaters without heat recovery. In addition, in terms of the Nusselt number and the Sherwood number, the heat and mass transfer performance of the staggered tube bank type were approximately 50% and 10% higher than that on the in-line tube bank type when the Reynolds number of the flue gas was 10³.     

Performance of artificially roughened solar air heaters—A review

The conversion, utilization and recovery of energy invariably involve a heat exchange process, which makes it imperative to design more efficient heat exchanger. The use of artificial roughness in different forms, shapes and sizes is the most common and effective way to improve the performance of a solar air heater. Several studies have been carried out to determine the effect of different roughness element geometries on heat transfer and friction in solar air heaters. This study reviews various roughness element geometries employed in solar air heaters for performance enhancement. Based on the correlations of heat transfer and friction factor developed by various investigators, an attempt has been made to compare the thermohydraulic performance of roughened solar air heaters.     

A review on methodology of artificial roughness used in duct of solar air heaters

In order to enhance rate of heat transfer to flowing air in the duct of a solar air heater, artificially roughened surface of absorber plate is considered to be an effective technique. Investigators reported various roughness geometries in literature for studying heat transfer and friction characteristics of an artificially roughened duct of solar air heaters. In the present paper an attempt has been made to categorize and review the reported roughness geometries used for creating artificial roughness. Heat transfer coefficient and friction factor correlations developed by various investigators for roughened ducts of solar air heaters have also been reported in the present paper.     

Design and Development of a Test Method for Analyzing Protective Performance of Gloves Exposed to Radiant Heat Based on Computational Fluid Dynamics Analysis

Gloves are one of the most important items in the protective ensemble as they protect hand which is the most frequently burned body part. At present, no international standard or standardized test method is available for analyzing thermal protective performance of gloves exposed to purely radiant heat exposures. In the present work, a systematic approach for developing test setup for radiant heat exposure is proposed using computational fluid dynamics (CFD). A coupled CFD–radiation heat transfer model is first developed and validated with available experimental data. Effects of horizontal and vertical orientations of radiant heaters are then analyzed. Based on this study, an optimized configuration of the experimental setup which results in uniform heat flux distribution throughout the hand under lower level of radiant heat exposure is proposed. Furthermore, effects of different heater temperatures on heat flux distribution are studied. It is found that the proposed configuration works satisfactorily for other higher and lower heat flux levels as well.     

电站锅炉回转式空气预热器积灰监测模型的研究

基于回转式空气预热器受热面积灰程度与烟气侧流动阻力间的依赖关系及其相关影响因素，提出了折算压差的概念，推导了积灰监测的计算模型。根据发电机组计算机数据采集系统（DAS）烟气侧实时检测的烟气侧压差等参数，实现了300MW燃煤电站锅炉回转式空气预热器积灰状态的在线监测。     

Mixed Integer Nonlinear Programming model for the optimal design of fired heaters

In this paper, a mathematical optimization model for the optimal design of industrial furnaces/fired heaters is presented. Precisely, a detailed Mixed Integer Nonlinear Programming (MINLP) model including operational and geometric constraints is developed to get an efficient furnace design. Discrete decisions connected with the geometric design such as number of tubes in convection and radiation sections, number of shield tubes, number of passes and number of tubes per pass are modelled by using integer variables. Continuous variables are used for process conditions (temperatures, flow-rates, pressure, velocities, pressure drops among others).The mathematical model and the solution procedure are implemented in General Algebraic Modelling System (GAMS), Brooke [A. Brooke, D. Kendrick, A. Meeraus, A. A. GAMS – A User’s Guide (Release 2.25), The Scientific Press, San Francisco, CA, 1992]. Based on a typical furnace configuration, several applications are successfully solved by applying the proposed MINLP model. In this paper, three case studies with increasing complexity are presented. In the first case study, the accuracy of results from the proposed model is compared satisfactorily with literature. In the second case study, the MINLP model is applied to optimize the fire heater’s efficiency. Finally, the total annual cost of the fired heater is minimized in the Case Study III. Also, a sensitive analysis of the unitary cost of fuel and capital investment is investigated. The developed model is characterized by its robustness and flexibility.     

A review on roughness geometry used in solar air heaters

The use of an artificial roughness on a surface is an effective technique to enhance the rate of heat transfer to fluid flow in the duct of a solar air heater. Number of geometries of roughness elements has been investigated on the heat transfer and friction characteristics of solar air heater ducts. In this paper an attempt has been made to review on element geometries used as artificial roughness in solar air heaters in order to improve the heat transfer capability of solar air heater ducts. The correlations developed for heat transfer and friction factor in roughened ducts of solar air heaters by various investigators have been reviewed and presented.     

Effective efficiency of solar air heaters having different types of roughness elements on the absorber plate

The use of an artificial roughness on a surface is an effective technique to enhance the rate of heat transfer to fluid flow in the duct of a solar air heater. This paper presents a comparison of effective efficiency of solar air heaters having different types of geometry of roughness elements on the absorber plate. The effective efficiency has been computed by using the correlations for heat transfer and friction factor developed by various investigators within the investigated range of operating and system parameters.     

某国产300MW机组暖风器疏水系统改造热经济性分析

某国产300 MW机组暖风器疏水回收方案为"暖风器—疏水泵—除氧器",运行过程中经常发生暖风器开裂、泄漏;疏水泵汽蚀;疏水无法正常回收等故障。为了解决此问题,建议采用"暖风器—疏水器—凝汽器"方案。利用等效热降理论对两种方案进行了热经济性比较,并给出详细的计算过程。最终得出"暖风器—疏水器—凝汽器"方案系统简单,经济性好。     

Heat transfer characteristics of a rotary regenerative combustion system (RRX)

With a view to saving fuel, the use of regenerative burners as a heating source has been spreading in the field of industrial furnaces. By combining a burner with a regenerative air preheater, a second generation regenerative burner—the Rotary Regenerative Combustion System (RRX)—has been developed, which makes for lower emission of air pollutants and compactness, in addition to fuel savings. In this paper, heat transfer characteristics of RRX were deduced theoretically based on the heat transfer theory of a regenerative air preheater and investigated experimentally using two test rigs. A commercially operating fired heater was revamped in the summer of 1994 to install three sets of RRXs, and it has been successfully operated for one year. As a result, it was determined that the heat transfer rate in RRX can be predicted to within ±10%, by considering not only convective but also radiative heat transfer. Furthermore, it was confirmed both theoretically and experimentally that fuel efficiency exceeding 90% was stably attained in a commercial fired heater. Around 60 ppm of NOx emission (as dry, 6% O₂) was also measured, although the preheated air temperature was calculated to be as high as 930 K. © 1999 Scripta Technica, Heat Trans Jpn Res, 27(8): 584–596, 1998     

Local Convective Heat Transfer from Small Heaters to Impinging Submerged Axisymmetric Jets of Seven Coolants with Prandtl Number Ranging from 0.7 to 348

INTRODUCTIoNJetimpingementhasbeenextensivelyemployedintechnicalprocessestoproducerelativelyhighheat/massfluxes.Incomparisonwiththeheat/masstransferratesprovidedbyconventionaltechniqueswithfluidfiowsparalleltotheheat/masstransfersur-face,aremarkableincreaseintransfercoefficientscanbeobtainedinthisfashion.Inmostcasesairisusedastheworkingmedium.Examplesofairjetapp1icationsincludecoolingofturbinebladesandelectroniccom-ponents,annealingofmetallicandplasticsheets,dry-ingoftextilesandpaper,andtem…     

高温空气燃烧技术蓄热式燃气辐射管燃烧器的研制开发和应用

介绍了应用高温空气燃烧技术研制开发W型和U型蓄热式燃气辐射管燃烧器的工作原理、技术特点和性能指标。研究指出．蓄热式燃气辐射管燃烧器是一种高效、节能、低污染的工业炉用加热装置。可广泛应用于机械、冶金、石化、电子、兵器、轻工、电器、交通、建材、纺织、汽车、航空航天和食品工业等加热炉设备上,具有广阔的应用前景。     

Analysis of the influence of operating conditions on fouling rates in fired heaters

Fouling due to chemical reaction in preheat trains for the processing of crude oil plays a key role in the operation and maintenance costs and on greenhouse emissions to atmosphere in crude processing plants. A preheat train consists of a set of heat transfer units that provide the crude oil stream the required amount of thermal energy to reach its target temperature either by heat recovery or by direct firing. Fired heaters supply external high temperature heating through the burning of fuel which result in complex heat transfer processes due to the large temperature and pressure changes and vaporization that takes place inside the unit. In this work, a thermo-hydraulic analysis of the performance of fired heaters is carried out through the application of commercial software to solve the mathematical models using finite difference methods; the analysis is applied to the crude side of a vertical fired heater in order to evaluate the impact of process conditions such as throughput and crude inlet temperature (CIT) on the fouling that take place at the early stages of operation. Using a fouling rate model based on thermo-hydraulic parameters, fouling rates are predicted assuming steady state operation and clean conditions. Although variations in process conditions are known to influence fouling rates, little work has been done on the subject. In this work excess air and steam injection are studied as a means to mitigate fouling. Results show that throughput reduction brings about a marked increase in the fouling rates. A decrease in CIT affects only the convection zone and it is found that this effect is negligible. In terms of excess air, it is found that although it affects negatively the heater efficiency it can be used to balance heat transfer between the convection and radiation zone in a way that fouling rates are reduced; however this strategy should be considered right from the design stage. Finally it is observed that steam injection is an effective method to reduce fouling rates since it results in lower film temperatures and larger shear stress.     

Ambient condition effects on process heater efficiency

Process heaters are typically located outside and subject to the weather. Although heaters are typically tuned at a given set of conditions, actual operating conditions vary significantly from season to season and sometimes even within a given day. Unfortunately, most heaters are not properly adjusted for actual operating conditions. Ambient air temperature, pressure and humidity all significantly impact process heater efficiency. This paper shows how changing ambient conditions can reduce efficiency if proper adjustments are not made.Combustion efficiency is related to air:fuel ratio and to air–fuel mixing. A general industry rule-of-thumb is that operating at 2–3% excess O₂ (dry basis) results in the best combination of efficiency and flexibility. At higher O₂ levels, efficiency is reduced because the additional O₂ and N₂ absorb heat, much of which exits the exhaust stack. At lower O₂ levels, efficiency can be substantially reduced because some fuel is uncombusted. Low O₂ levels can also lead to soot and coke buildup on process tubes reducing heat transfer to the process fluid and reducing efficiency.Several examples demonstrate how ambient conditions affect heater efficiency. Calculations and graphs for a wide range of operating conditions demonstrate how efficiency can be affected by changes in ambient conditions for process heaters.     

Exergy based analysis of solar air heater having discrete V-down rib roughness on absorber plate

The artificially rib roughened solar air heaters perform thermally better than the conventional flat-plate solar air heater under same operating conditions. However, the artificial rib roughness leads to higher friction factor thereby increasing pumping power. The second law based exergy analysis is suitable for design of rib roughened solar air heaters as it incorporates quality of useful energy output and pumping power. The exergetic efficiency of a solar air heater having discrete V-down rib roughness is studied analytically and the results obtained are compared with that of a conventional flat-plate solar air heater. Flow Reynolds number and rib-roughness parameters, viz., relative roughness pitch, relative gap position, relative gap width, angle of attack and relative roughness height have combined effect on heat transfer as well as fluid friction. The exergy based criterion suggests use of the discrete V-down rib roughened solar air heater for the Reynolds number range normally used in solar air heaters. It was found that there exist optimum roughness parameters of the discrete V-down rib for a given Reynolds number (or temperature rise parameter) at which the exergetic efficiency is highest. Curves of optimum rib-roughness parameters are also plotted.     

Simple method for estimation of effectiveness in one tube pass and one shell pass counter-flow heat exchangers

In one tube pass and one shell pass counter-flow heat exchangers, when both streams change temperature by different amounts, the effectiveness is defined as the temperature change for the stream with lower capacity divided by the maximum possible change and the effectiveness depends on the number of transfer units and the thermal capacity ratio. In this paper, an attempt has been made to formulate a simple-to-use method which is easier than existing approaches, less complicated and with fewer computations for accurate and rapid estimation of effectiveness in one tube pass and one shell pass counter-flow heat exchangers as a function of number of transfer units and the thermal capacity ratio. The proposed method permits estimating the exit temperature for a one tube pass and one shell pass counter-flow heat exchanger without a trial-and-error calculation. The average absolute deviations between the reported data and the proposed correlations are found to be less than 2% demonstrating the excellent performance of proposed correlation. The tool developed in this study can be of immense practical value for engineers and scientists to have a quick check on the effectiveness in one tube pass and one shell pass counter-flow heat exchangers at various conditions without opting for any experimental measurements. In particular, practice engineers would find the predictive tool to be user-friendly with transparent calculations involving no complex expressions.