An Experimental Study on Latent Heat Recovery of Exhaust Wet Flue Gas

The experiment was conducted to investigate the heat transfer performance of wet flue gas in a vertical tube. The factors influencing the convective condensation of wet flue gas were experimentally investigated. The measured results indicate that the convective heat transfer of bulk flow and condensation heat transfer of vapor have significant contribution to the total heat transfer and the dominant transport mechanism is dependent upon the vapor fraction in mixture.     

Theoretical prediction of sulfuric acid condensation rates in boiler flue gas

Prediction of acid condensation has a critical role in designing heat exchangers to recover water vapor from power plant flue gas. Rates of mass transfer for condensation of sulfuric acid vapors onto heat exchanger tubes were theoretically investigated and a computer program for numerical simulations of sulfuric acid (H₂SO₄) condensation in a flue gas condensing heat exchanger was developed. Governing equations based on mass and energy balances for the system were derived to predict variables such as flue gas exit temperatures, cooling water outlet temperatures, and molar fractions and condensation rates of water and sulfuric acid vapors. The associated equations were solved using an iterative solution technique and a one dimensional finite difference method with forward differencing. The Controlled Condensation Method (EPA Method 8B) was applied to experimentally obtain concentration profiles of sulfuric acid vapor in flue gas along downstream in the system. Predicted results of sulfuric acid vapor condensation were compared with empirical data for model validation, and the discrepancy is analyzed in terms of measurement and computation uncertainties. It is found that from both modeling and test results sulfuric acids as well as water vapors are reduced and separated in condensing heat exchanger due to mass transfer with condensation in flue gas. The modeling methodology described here is applicable to theoretical prediction of sulfuric acid and water condensation in full scale flue gas condensing heat exchanger applications.     

低温烟气凝结换热及对天然气热效率的影响实验研究

为研究烟气露点附近及以下的低温烟气对流凝结换热规律与烟气换热对天然气利用热效率的影响,建立了烟气在翅片管换热器内对流凝结换热实验系统,研究了不同烟气温度、水蒸气含量对烟气凝结换热的影响,得出了烟气凝结换热实验准则关联式,分析对比了天然气利用热效率实验值与理论值.实验结果表明:当被加热水温度为23℃,烟气出口温度为73℃...     

含湿混合气体水平管外对流冷凝换热实验研究

回收烟气中的潜热和显热在提高锅炉效率和环境保护方面都具有重要意义。主要针对含湿混合气体在水平单管管外的对流冷凝换热进行了实验研究。通过对实验数据的分析,得到了烟气进口温度、冷却水进口温度、水蒸气的质量分数以及Re的变化对含湿混合气体在水平单管管外冷凝换热的影响。     

Modelling and analyses of heat exchangers in a biomass boiler plant

A boiler plant is presented, in which the fuel is dried before combustion in a silo with air. The drying air is heated in a recuperative heat exchanger by the heat of flue gases. Hot air is then blown through the bed of fuel in the drying silo, while the fuel dries and the air cools down and becomes humidified. Heat of the moist exhaust air of the silo is recovered for the drying air and combustion air by a recuperative heat exchanger. Modelling of the thermal behaviour of the plant helps in understanding complex interdependencies of the two heat exchangers, the boiler and the dryer. The models of the heat exchangers and applications in analysing the boiler system are described in this paper. Calculating the combinations of extreme operational conditions gives the input data needed in comparing different types of heat exchangers, dimensioning the heat transfer area, choosing the control strategy and selecting the operating parameters and set‐values of the control system. Results of verification measurements and practical operation at a 40 kW_th pilot plant and a 500 kW_th demonstration plant are also discussed. Using engineering correlation formulas for heat and mass transfer, an adequate accuracy between the model and the measurements was achieved. Fouling was detected to be a major problem with the flue gas heat exchanger. However, in absence of condensation, the increase of a fouling layer with respect to time was observed to be low. Fouling was also a problem with the drying exhaust gas heat exchanger, but after the installation of a simple dust collector, a reasonable cleaning period was achieved. A mixed‐flow configuration was found to be the most appropriate for the flue gas heat exchanger. In order to avoid condensation of the flue gas the drying exhaust gas heat exchanger is indispensable in Finnish climate in the considered system. In addition to this, it decreases the need of fuel. A parallel‐flow type was found the most appropriate as the drying exhaust gas heat exchanger. Copyright © 2004 John Wiley & Sons, Ltd.     

Analysis of two‐stage waste heat recovery based on natural gas‐fired boiler

Waste heat recovery (WHR) is crucial to the efficiency improvement of natural gas‐fired boiler systems. Two‐stage WHR systems based on the natural gas‐fired boiler were analyzed from the viewpoints of thermal efficiency and heat transfer irreversibility. An overall entransy dissipation‐based thermal resistance was derived to evaluate the irreversibility of WHR, including the entransy dissipations during condensation and in absorption heat pump (AHP). Compared with the basic WHR system, the two‐stage WHR systems have higher boiler efficiency and less irreversibility. The air‐humidified system recycles both the heat and vapor in flue gas, while the unutilized latent heat in the recovered vapor causes the boiler to be less efficient than the AHP system. Investigation on heat exchanger effectiveness of two‐stage WHR systems illustrated: in the two‐stage WHR system with air humidification, the increasing effectiveness of both heat exchangers could effectively increase boiler efficiency and reduce heat transfer irreversibility. In the two‐stage WHR system with AHP, boiler efficiency has a local optimum when the dew point occurs near the outlet of the first heat exchanger; increasing the second heat exchanger effectiveness is more efficient in improving boiler efficiency. The present work may provide available references and guidance for the design and optimization of the two‐stage WHR systems.     

Condensation heat transfer on tubes in actual flue gas

In order to improve boiler efficiency, latent heat recovery from flue gas is a very important concept. Condensation heat transfer on horizontal stainless‐steel tubes was investigated experimentally by using an actual flue gas from a natural gas boiler. The experiment was conducted at different air ratios of the flue gas and a wide range of tube wall temperatures. The condensation pattern was similar to a dropwise condensation near the dew point. By decreasing the wall temperature, the wall region covered with a thin liquid film increased. The heat and mass transfer behavior was well predicted with the analogy correlation at the high‐wall‐temperature region. At the low‐wall‐temperature region, the total heat transfer was higher than that predicted by the analogy correlation. © 2001 Scripta Technica, Heat Trans Asian Res, 30(2): 139–151, 2001     

Heat transfer research on vapor‐gas mixture with condensation in a vertical tube

The convection‐condensation heat transfer of vapor‐gas mixtures in a vertical tube was studied theoretically and experimentally. The effects of the condensation of a small amount of water vapor (8 to 20%) on heat transfer in a vertical tube were discussed. Comparisons show that theoretical solutions obtained through modified film model and experimental results are in good agreement. The results show that the condensation heat transfer of a small amount of water vapor and single‐phase convection heat transfer in the vapor‐gas mixtures are of the same order of magnitude, and these two modes of heat transfer could not be neglected. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(7): 531–539, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10055     

Condensation heat transfer on tubes in actual flue gas (Parametric study for condensation behavior)

In order to improve boiler efficiency, latent heat recovery from the flue gas is a very important concept. Condensation heat transfer on horizontal tubes was investigated experimentally by using flue gas from a natural gas boiler. The parametric studies concerning the effects of tube stages, gas velocities, SO₂ in the flue gas, and Teflon‐lined tubes were conducted at a wide range of tube surface temperature. The heat transfer was enhanced at the second stage due to the wake turbulence of the first stage. No significant decrease of the condensation in the second stage due to the condensate falling from the first stage could be observed. Even at the high gas velocity condition, the condensation pattern was similar to that observed in previous low gas velocity experiments. The effect of SO₂ in the flue gas on the condensation behavior could not be observed. The heat and mass transfer behaviors on stainless‐steel and Teflon‐lined surfaces were well predicted with the analogy correlation. The above results suggested that the heat and mass transfer were dominated by the convection and diffusion process on the gas side. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(2): 153–166, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10079     

Experimental Study of Condensation in a Shell and Tube Heat Exchanger in the Presence of a Noncondensable Gas

In this paper, an experimental study of the condensation of water vapor from a binary mixture of air and low‐grade steam has been depicted. The study is based upon diffusion heat transfer in the presence of high concentration of noncondensable gas. To simplify the study, experimental analysis is supported by empirical solutions. The experimental setup is custom designed for testing a new shell and tube type heat exchanger supplied by the manufacturer. Air–vapor mixture at 80 °C (max) and 20.2% relative humidity enters the heat exchanger at a mass flow rate of 480 kg/h and condenses 27 kg/h vapor using cooling water at an inlet temperature of 7 °C to 10 °C and mass flow rate of 3500 kg/h. By using the experimental data of constant inlet air mass fraction, mixture gas velocity, and different volumetric flow rate of the cold fluid, the local heat transfer coefficients are obtained. The main objective of this work is to establish an approximate value for surface area and overall heat transfer coefficient of a horizontal shell and tube condenser used in process space. Under designed working conditions, the condenser is found to work efficiently with 90% vapor condensation by mass.     

燃气锅炉的烟气凝结换热

燃气锅炉排放的烟气温度很高且含有一定数量的水蒸汽,在回收烟气余热时,水蒸汽的换热是影响余热回收效率的主要因素之一.利用多孔介质理论对各控制方程进行重新修正;采用标准的k-ε模型和两相界面上凝结换热边界条件,对换热过程进行数值计算.结果表明,有凝结换热发生时,换热能力的计算值与实验值的误差在允许范围内.     

Evaluation of retrofitting a conventional natural gas fired boiler into a condensing boiler

The exit flue gas temperature of a conventional gas fired boiler is usually high and a great amount of heat energy is lost to the environment. If both sensible heat and latent heat can be recovered by adding a condensing heat exchanger, the efficiency of the boiler can be increased by as much as 10%. In this paper, based on combustion and heat transfer calculations, the recoverable heat and the efficiency improvement potential of different heat recovery schemes at various exit flue gas temperatures are presented by performing design calculations. The payback period method has been used to analyze the feasibility of retrofitting a conventional gas fired boiler into a condensing boiler in a heating system in detail. The results show that the most economical exit flue gas temperature is 40–55 °C when a conventional natural gas fired boiler is retrofitted into a condensing boiler simply by adding a condensing heat exchanger. It is feasible to use the return water of a heating system as the cooling medium of the condensing heat exchanger because the return temperature varies with the ambient temperature and is lower than the dew point of the water vapor in the flue gas in most periods of a heating season in some regions, which has been verified by retrofitted case.     

Convection Condensation Heat Transfer of Steam-Air Mixture With Heat Pipe Heat Exchanger

A heat pipe heat exchanger (HPHE) was used to investigate the heat transfer performance of steam-air mixture condensation, analogous to the dew condensation of flue gas, when the steam volume fraction ranged from 0 to 20%, and the inlet temperature of steam-air mixture varied from 70 to 120°C. Self-assembled monolayers (SAMs) treatment with n-octadecyl mercaptan was adopted to modify the condensation surface of the heat pipe. The comparisons were conducted to examine the influence of SAMs on condensation heat transfer of steam-air mixture vapor. The results indicate that the convection condensation heat transfer coefficient increases with the increase of steam volume fraction and Re number of the steam-air mixture. As the inlet temperature increases, the heat transfer coefficient decreases accordingly. The heat transfer performance can be improved by the SAMs surface, with a heat transfer enhancement ratio up to 1.6 at a condition of 20% of the steam volume fraction and 1500 Re number.     

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.     

增压富氧煤燃烧烟气凝结换热的计算

针对含有少量水蒸气的增压富氧煤燃烧产生的烟气在竖直管内的对流凝结换热进行了分析研究.利用修正的膜模型与Nusselt凝结理论建立了换热数学模型,并对不同壁面温度、不同雷诺数和不同水蒸气份额下烟气的凝结换热进行了计算.结果表明:壁面温度升高时,烟气的凝结速率、换热流率和凝结液膜的厚度均减小;混合气体的雷诺数增大时,烟气的凝结速率和换热流率增大,凝结液膜的厚度减小;烟气中水蒸气的份额减小时,烟气的凝结速率和换热流率减小,凝结液膜的厚度减小不明显.     

新型复合防腐表面烟气对流凝结换热实验研究

烟气对流凝结换热强化和换热表面防腐是天然气热能动力设备烟气余热回收利用关键技术。不同防腐表面耐腐蚀性能不同,且换热性能也不同。采用CCD高速摄像仪,对烟气在新型复合防腐表面上的凝结形态和凝结过程进行了可视化观测和换热实验研究,采用对图像边缘提取法,获得凝结液的边缘曲线。研究表明,烟气在新型复合防腐表面上的凝结为珠状凝结,凝结液珠最大粒径为0.2～0.28 mm,与其他表面形成的膜状凝结相比,在实验范围内,珠状凝结换热可提高约7倍。为增强烟气对流凝结换热和开发烟气冷凝余热回收利用技术提供了参考和依据。     

Prediction and Measurement of Apparent Heat Transfer Coefficient by Condensation in Finned-Tube Heat Exchangers

The apparent heat transfer coefficient by condensation in finned-tube heat exchangers is determined experimentally and numerically in this paper. The film method is used to predict the partial or total condensation of the water vapor contained in the humid air over the smooth or finned tube-heat recuperators. Based on this method, a computer code is developed here. The mathematical formulation is validated by our experimental results, using tube bundles in staggered and aligned arrangements. The determination of the fin portion, which functions in wet regime, is carried out by the prediction of thermal field over a circular fin. The condensation of the water vapor contained in the humid air is done preferentially with the last rows of the heat exchanger. The heat flux is predicted in a range of 20% and 5% in wet and dry regimes, respectively. The apparent heat transfer coefficient by condensation can exceed 10 times the value of the air-side heat transfer coefficient in dry regime.     

石墨烯-PFA复合材料换热器与金属换热器的传热性能对比

氟塑料换热器以其耐腐蚀、耐磨损等优点而备受关注,但氟塑料热导率较低,换热能力差,限制了其广泛应用。石墨烯-PFA复合材料兼具石墨烯优异的导热性和可熔性聚四氟乙烯(PFA)良好的耐酸碱腐蚀性,是新一代的换热器材料。搭建了余热回收测试实验台,对石墨烯-PFA复合材料换热器和金属换热器的传热性能进行对比。研究了不同烟气流速、不同进口烟气温度以及不同石墨烯配比对复合材料传热性能的影响。结果表明:对于金属换热器和复合材料换热器,当烟气流速从2.0增加到4.0 m/s时,传热系数分别增加到原来的1.19和1.34倍;随着进口烟温的升高,两种材质的传热系数分别降低了15.6%和14.7%;随着石墨烯含量增加,复合材料的导热系数以及传热系数均增加。     

Theoretical research of convective condensation heat transfer for mixture gas on a horizontal tube

The mechanism of convective condensation heat transfer of moist mixed gas across a horizontal tube was studied in this paper. The models referring to how the liquid film flows and the heat transfers on the tube are set up by combining modified film model and Nusselt condensation theory. The effects of Re number, wall temperature, and water vapor concentration on condensation heat transfer are discussed. Results predict that the film thickness profile on the tube is influenced greatly by vapor shear force on liquid film. Local Nusselt number depends remarkably on gas phase heat resistance, which is different from pure vapor and very similar to single‐phase gas. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(6): 324–333, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20169     

Numerical analysis of the tubular heat exchanger designed for co-generating units on the basis of microturbines

The application of numerical simulations using the computational fluid dynamics (CFD) analysis when mapping processes in the course of which the heat transmission occurs has become an essential part of the heat transfer systems. The present contribution deals with the possibility to use the waste heat of the flue gas produced by small microturbines. The waste heat is mapped by means of both the numerical simulations applying the FLUENT software and the practical experiment. Utilizing a part of the waste heat for water heating and decreasing the outlet temperature of the flue gas into atmosphere when applying in co-generating units represents one of the partial benefits. The present paper brings information concerning the newly designed type of heat exchanger including the results of its numerical analysis.The analysed heat exchanger designed in the system with microturbine (MT) C30 reached generally the efficiency of 75%. Both the results of simulations and the carried out practical experiment confirmed the temperature of the flue gas to be sufficient behind the exchanger to prevent the condensation of water from the flue gas. On the contrary, except for heating water the exchanger under consideration offers – thanks to its design – also other possibilities to use of the flue gas. The practical experiment confirmed the results of the CFD prediction with rather small differences as the temperature of water obtained from the exchanger was 359 K and the designed shape of the exchanger did not result in substantial pressure losses in flue gas approximately 50 Pa. The mean logarithmic temperature difference of the mapped and verified exchanger was ～203 K.