Performance characteristics of a trickle fill in cross- and counter-flow configuration in a wet-cooling tower

In cooling towers packed with trickle or splash fills, which have almost isotropic or anisotropic flow resistance, the air flow through the fill is oblique or in cross-counterflow to the water flow, particularly at the cooling tower inlet when the fill loss coefficient is small or when the fill hangs down into the air inlet region. This results that the fill Merkel number or transfer characteristic for cross-counterflow is between that of purely counter- and cross-flow fills.When using CFD to model natural draught wet-cooling tower performance for isotropic or anisotropic fill resistance, two- or three-dimensional models and fill characteristics are therefore required to determine overall fill performance.In this paper, the test facility, measurement techniques and methods of analysis used to determine fill performance characteristics in counter- and cross-flow configuration are presented and discussed. Results obtained for a specific fill are presented and discussed which can be used for the evaluation of cross-counterflow fill performance.     

A CRITICAL INVESTIGATION INTO THE HEAT AND MASS TRANSFER ANALYSIS OF CROSSFLOW WET-COOLING TOWERS

The heat and mass transfer process of evaporative cooling in crossflow wet-cooling tower fills is investigated. The governing equations of the crossflow evaporative process are derived from first principles. A detailed account is given of how to solve these equations. The governing equations, according to the Poppe, Merkel, and e-NTU methods of analysis, are considered. The equations of the Poppe method of analysis are extended to give a more detailed representation of the transfer characteristic or Merkel number. The results of a crossflow wet-cooling tower fill analysis according to the Merket, Poppe, and e-NTU methods of analysis are presented. The differences between the results of these methods are evaluated.     

Drop size distribution below different wet-cooling tower fills

Drop size distribution is measured photographically below three different counter-flow wet-cooling tower fills (cross-fluted film, trickle and fibre cement) and a number of splash grid configurations installed below a trickle fill. The air and water flow rates are varied to investigate their influence on drop size. The data for each test case is presented as a cumulative mass distribution curve, Sauter mean diameter, Rosin–Rammler distribution curve and Rosin–Rammler function. The main objectives are to describe the implemented experimental apparatus and measurement techniques and to determine the drop size distribution beneath different fills and ultimately the drop size reduction capability of grids installed below the fill. An important conclusion is that the Rosin–Rammler distribution curve generally does not fit the measured data adequately and thus should be avoided.     

Contrastive analysis of cooling performance between a high-level water collecting cooling tower and a typical cooling tower

A three-dimensional (3D) numerical model is established and validated for cooling performance optimization between a high-level water collecting natural draft wet cooling tower (HNDWCT) and a usual natural draft wet cooling tower (UNDWCT) under the actual operation condition at Wanzhou power plant, Chongqing, China. User defined functions (UDFs) of source terms are composed and loaded into the spray, fill and rain zones. Considering the conditions of impact on three kinds of corrugated fills (Double-oblique wave, Two-way wave and S wave) and four kinds of fill height (1.25 m, 1.5 m, 1.75 m and 2 m), numerical simulation of cooling performance are analysed. The results demonstrate that the S wave has the highest cooling efficiency in three fills for both towers, indicating that fill characteristics are crucial to cooling performance. Moreover, the cooling performance of the HNDWCT is far superior to that of the UNDWCT with fill height increases of 1.75 m and above, because the air mass flow rate in the fill zone of the HNDWCT improves more than that in the UNDWCT, as a result of the rain zone resistance declining sharply for the HNDWCT. In addition, the mass and heat transfer capacity of the HNDWCT is better in the tower centre zone than in the outer zone near the tower wall under a uniform fill layout. This behaviour is inverted for the UNDWCT, perhaps because the high-level collection devices play the role of flow guiding in the inner zone. Therefore, when non-uniform fill layout optimization is applied to the HNDWCT, the inner zone increases in height from 1.75 m to 2 m, the outer zone reduces in height from 1.75 m to 1.5 m, and the outlet water temperature declines approximately 0.4 K compared to that of the uniform layout.     

基于散堆填料的空气湿化塔性能实验研究

搭建了内径约600mm的空气湿化塔性能实验台,以塑料阶梯环散装填料塔为研究对象,实验并分析了气速、水气比、填料总高度对湿化塔的压力损失、加湿效果和传质单元高度的影响,以及进水温度对后两者的影响,同时获得了空气湿化填料塔精细设计数据,并对结果进行了拟合,得到填料的传质单元高度经验关联式。     

Thermodynamic study of wet cooling tower performance

An analytical model was developed to describe thermodynamically the water evaporation process inside a counter‐flow wet cooling tower, where the air stream is in direct contact with the falling water, based on the implementation of the energy and mass balance between air and water stream describing thus, the rate of change of air temperature, humidity ratio, water temperature and evaporated water mass along tower height. The reliability of model predictions was ensured by comparisons made with pertinent experimental data, which were obtained from the literature. The paper elaborated the effect of atmospheric conditions, water mass flow rate and water inlet temperature on the variation of the thermodynamic properties of moist air inside the cooling tower and on its thermal performance characteristics. The analysis of the theoretical results revealed that the thermal performance of the cooling tower is sensitive to the degree of saturation of inlet air. Hence, the cooling capacity of the cooling tower increases with decreasing inlet air wet bulb temperature whereas the overall water temperature fall is curtailed with increasing water to air mass ratio. The change of inlet water temperature does not affect seriously the thermal behaviour of the cooling tower. Copyright © 2005 John Wiley & Sons, Ltd.     

自然通风湿式冷却塔防冻数值研究

由于我国北方地区冬季气温较低,冷却塔普遍存在结冰问题,必须采取一定的防冻措施。通过FLUENT软件模拟分析得出进塔水温对塔内不同特征面水温的影响规律,并进一步模拟加装不同层数挡风板后冷却塔内流场的变化。计算结果表明:①当运行工况其它条件不变时,随着进塔水温的升高,塔内不同特征面最低水温升高;②加装一定层数挡风板能使填料下面空气温度场和进风口上沿面空气速度场分布逐渐趋于均匀,有利于防止塔内结冰;③选取进塔水温分别为17.80℃、21.95℃、26.77℃、30.04℃时,分别加装4、3、2、1层挡风板可有效防止塔内结冰。     

New Developments in Reboilers and Evaporators

Abstract

A prototype cooling tower was used to explore the potential of using cooling towers compared with radiator cooling systems with 3 MW diesel engines. The working parameters were the water mass flow rate, water inlet temperature, air mass flow rate, and humidity ratio. The water mass flow rate was relatively the most effective. Three methods of calculation were used to evaluate performance—namely, heat and mass balance, psychrometric chart, and the heat and mass transfer method. The first was the best in comparison with experiments. The economic analysis of both the cooling tower and radiator systems showed that it would be more economical in the long run to use cooling towers for diesel engines.     

Loss coefficient correlation for wet-cooling tower fills

Loss coefficient correlations given in the literature for wet-cooling tower fills are relatively simple and generally do not represent the pressure drop accurately over a wide range of operational conditions. A new form of empirical equation is proposed that correlates fill loss coefficient data more effectively when compared to other forms of empirical equations commonly found in the literature.     

Modeling of Thermal Performance of a Cooling Tower Using an Artificial Neural Network

Abstract

In the present study, the ability of an artificial neural network model to evaluate the thermal performance of a cooling tower, which used in the heating, ventilating, and air conditioning industries to reject heat to the atmosphere, is examined. The network is trained with the following experimental values: the ratio of the water mass flow rate to air mass flow rate, the inlet water temperature, and the outlet water temperature, and the inlet air wet-bulb temperature are selected as input variables, while the output is the coefficient of performance. It is concluded that a well-trained neural network provides fast, accurate, and consistent results, making it an easy-to use tool for preliminary engineering studies.     

Comparative analysis of two different types of fills used in wet cooling tower for higher‐scale water with conventional film type fill

In the present study, an induced draft wet cooling tower has been experimentally investigated using film, glass, and ball fills. Film fills are used only in industry where scaling is less or low‐scale process water is used because of clogging of fills. Whereas, glass fills and ball fills are used for higher‐scale water. The main objective of this study is to investigate the performance and compare the result of film fills with glass fills and ball fills and also predict the loss as compared to film fills when glass and ball fills were used for high‐scaling water under such environmental conditions and different ratio of water to air flow and water inlet temperatures. Various performance parameters are analyzed for each of the fills and it was found that tower characteristics of film fills are 18.56% and 15.59% more than that of glass and ball fills, respectively, at water inlet temperature of 43°C and water‐to‐air flow ratio of 0.62. Moreover, exergy destruction was also calculated for each of the fills and it was observed that film fills destruct less exergy as compared with glass and ball fills.     

Three-dimensional numerical study on thermal performance of a super large natural draft cooling tower of 220m height

Based on the heat and mass transfer theory and the characteristics of general-purpose software FLUENT, a three-dimensional numerical simulation platform, composed of lots of user defined functions（UDF）, has been developed to simulate the thermal performance of natural draft wet cooling towers（NDWCTs）. After validation, this platform is used to analyse thermal performances of a 220m high super large cooling tower designed for inland nuclear plant under different operational conditions. Variations of outlet temperature of the cooling tower caused by changes of water flow rates, inlet water temperatures are investigated. Effects of optimization through non-uniform water distributions on outlet water temperature are discussed, and the influences on the flow field inside the cooling tower are analyzed in detail. It is found that the outlet water temperature will increase as the water flow rate increases, but the air flow rate will decrease. The outlet water temperature will decrease 0.095K and 0.205K, respectively, if two non-uniform water distribution approaches are applied.     

Study on the heat transfer characteristics of an evaporative cooling tower

In the present study, both experimental and theoretical results of the heat transfer characteristics of the cooling tower are investigated. A column packing unit is fabricated from the laminated plastic plates consists of eight layers. Air and water are used as working fluids and the test runs are done at the air and water mass flow rates ranging between 0.01 and 0.07 kg/s, and between 0.04 and 0.08 kg/s, respectively. The inlet air and inlet water temperatures are 23 °C, and between 30 and 40 °C, respectively. A mathematical model based on the conservation equations of mass and energy is developed and solved by an iterative method to determine the heat transfer characteristics of the cooling tower. There is reasonable agreement from the comparison between the measured data and predicted results.     

AP1000核电机组巨型冷却塔型体优化数值计算

根据气水两相间热质传递原理,利用Fluent数值计算软件建立了APl000核电机组用20000m。淋水面积巨型冷却塔的三维计算模型,对双曲线塔筒的母线方程中的特征值及喉部半径的最优取值进行了计算验证,综合分析了特征值及喉部半径对塔内流场、进塔风量、蒸发水量和平均出塔水温的影响,得到了210m高巨型冷却塔的最优型体结构参数．计算结果表明：喉部半径与塔底半径的比值r0／r2=0．6--0．7、塔筒母线方程中的曲率特征值a=0．17～0．18时,冷却塔运行最优．     

Experimental study on the performance of a mechanical cooling tower fitted with different types of water distribution systems and drift eliminators

Water drift emitted from cooling towers is objectionable for several reasons, mainly due to human health hazards. Generation and control of drift depends mostly on the couple of elements water distribution system and drift eliminator. The configuration of these two components not only affects drift but also the cooling tower thermal performance. However, no references regarding the effect of the water distribution system on the cooling tower characteristic have been found in the reviewed bibliography. This paper presents an experimental investigation of the thermal performances of a forced draft counter-flow wet cooling tower fitted with a gravity type water distribution system (GWDS) for six drift eliminators and when no drift eliminator was fitted. The interaction between distribution system and drift eliminators is analyzed. Heat and mass transfer processes taken place in the cooling tower have found to be affected by the mass transfer coefficient and the exchange mass-heat area per unit of cooling tower volume. The comparison between the obtained results and those found in the literature indicates that the pressure water distribution systems type (PWDS) achieve better performances than the GWDS. Maximum averaged differences of 38.66% in terms of cooling tower performance have been obtained between the two water systems. The data registered in the experimental set-up were employed to obtain correlations of the tower characteristic. The outlet water temperature predicted by these correlations was compared with the experimentally registered values, obtaining a maximum averaged difference of ±1.61% for the water-to-air mass flow ratio correlation and ±0.95% for the water and air mass flow ratios.     

膜加湿器热质交换过程的理论分析

膜加湿器是保证质子交换膜燃料电池(PEMFC)正常高效运行的重要组成部分.以燃料电池的板式膜加湿器为研究对象,根据热质交换原理对膜加湿器的传热传质过程进行了理论计算,分析了空气质量流量、膜内加湿侧进口温度和膜内加湿侧进口湿度对传热传质过程的影响.在传热方面:当空气质量流量不同时,随着膜内加湿侧进口温度的变化,膜内的热流量变化趋势不一致;当膜内加湿侧进口相对湿度为95%时,随着空气质量流量的变化,膜内热流量变化不大.在传质方面:当加湿侧进口相对湿度不变时,膜中水传输速率随着空气质量流量的增大而减小;当空气质量流量不变时,膜中水传输速率随着加湿侧进口相对湿度的增大而增大.     

Numerical study of the thermal–hydraulic performance of evaporative natural draft cooling towers

In this paper, the mathematical and physical models governing the flow, mass and heat energy of moist have been set up for an evaporative natural draft cooling tower. The models consider the effect of non-spherical shape of water drops on the flow, heat and mass transfer. Experimental data has been adopted to validate the numerical scheme. Average difference between the measured and the predicted outlet water temperature is 0.26°C. Distributions of the velocity components of the moist air, density, pressure, enthalpy and moisture content, the water temperature and its mass flux have been predicted. The simulation shows that some recirculation exits under the lower edge of the shell, where the air enthalpy, temperature, humidity and moisture content are higher, but the density is lower. The simulation also proves that the main transfer processes take place in the fill region where the percentage of latent heat transfer is predicted as 83%. However, about 90% of the heat energy is transferred via evaporation in the rain region although the total heat transfer rate there is very small compared to the fill region. Hourly performance of a natural draft cooling tower under the meteorological condition of Singapore has also been predicted.     

Solution of heat and mass transfer in counterflow wet-cooling tower fills

Model of heat and mass transfer in wet cooling tower fills is presented. The model consist of a set of four 1D ODEs describing the mass and energy conservation and kinetics with boundary conditions prescribed on opposite sides of the computational domain. Shooting technique with self adaptive Runge–Kutta step control is applied to solve the resulting model equations. The developed model is designed to be included in a large scale CFD calculations of a natural draft cooling tower where the fill is treated as a porous medium with prescribed distributions of mass and heat sources. Thus, the technique yields the spatial distributions of all flow parameters, specifically the heat and mass sources. Such distributions are not directly available in standard techniques such as Merkel, Poppe and e-NTU models of the fill where the temperature of the water is used as an independent variable. The method is validated against benchmark data available in the literature.     

The effect of inlet parameters on the performance of packed tower-regenerator

An investigation on the operation of packed tower for the regeneration of liquid desiccant is presented. A theoretical model demonstrating the effect of the system parameters is described. The experimental results are plotted to illustrate the effect of air and liquid parameters on the output variables. The regeneration process is shown to be highly dependent on the air inlet conditions, namely, temperature, humidity and flow rate. Also, the effects of the liquid temperature, concentration and flow rate is discussed. Data obtained were correlated to estimate the rate of water evaporation (regeneration rate) from values of variables that influence the rate of mass transfer in the column.     

Theoretical Relation between the Number of Diffusion Units and Pressure Drop in Direct Contact Heat Transfer

Abstract

A simple theoretical expression has been developed to relate the number of diffusion units involved in thermal processes controlled by mass transfer with the pressure loss in associated convection or forced air flows. The expression allows the analytical evaluation of a previously defined thermo-fluid dynamic parameter as a function of the liquid/gas ratio in cooling towers. It has been found that experimental data, obtained from different commercial cooling tower fills, behave in a similar way to the calculated theoretical values. A thermo-fluid dynamic efficiency has also been derived that is useful for the quantitative qualification of cooling towers, cooling tower fills, and heat transfer processes in which the mass transfer prevails.