The Use of a Thermo-Fluid Dynamic Efficiency in Cooling Towers

A thermo-fluid dynamic efficiency is defined for a mechanical draft water-cooling tower. It allows evaluating the heat transfer related to the pressure drop. It could also be used in isolated fills in different types of water-cooling towers. Thermo-fluid dynamic efficiency values obtained with available experimental results acquired from commercial fills are shown, and it can be concluded that this efficiency is not a function of the height of the fill.     

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.     

Optimization of Thermal-Flow Processes in a System of Conjugate Cooling Towers

Abstract

The paper presents the thermal-flow study of a closed cooling system with special emphasis on the working parameters of natural draft wet cooling towers. The authors analyze the possibility of the improvement of the overall cooling efficiency of a closed cooling system consisting of several cooling towers by the proper redistribution of cooling water between individual units. The problem of the optimal redistribution of circulating water between cooling towers is formulated as a mathematical issue involving finding the extrema of the multivariate function with constraints fixing the total mass flow rate of cooling water circulated in the hydraulic installation and the ranges of the hydraulic loads of individual cooling towers. The optimization process requires information about the individual characteristics of each cooling tower, which is achieved by experimental measurements done on real objects. The research done inside the cooling towers enables the identification of the heat and mass transfer processes across its radius. Next, these characteristics are used to calculate the optimal cooling water flow rates to the cooling towers, giving the highest possible mean cooling water temperature drop in the system.     

Investigation of Natural Draft Cooling Tower in China

ABSTRACT

The natural draft cooling tower has been widely used in thermal power plants because of its stable operation, lower maintenance cost, and smaller environmental impact. Hundreds of natural draft cooling towers have been built in China, including different types and various capacities of conventional cooling towers, seawater cooling towers, flue gas injection cooling towers, cooling towers with water collecting device, dry cooling towers, and others. A wide range of investigation for natural draft cooling towers was carried out for the first time by the Electric Power Planning & Engineering Institute in 2013 in China. Based on the collected data and summarized information, the statistical results and conclusive opinions are analyzed. The development trend of natural draft cooling tower technology is also explored in this paper. The investigation results are expected to be a reference for designing and operating natural draft cooling towers in thermal power plants.     

Refinement of the Transfer Characteristic Correlation of Wet-Cooling Tower Fills

Transfer characteristic correlations given in the literature for wet-cooling tower fills are generally only a function of the air and water mass flow rates. This is a gross simplification of a very complex heat and mass transfer (evaporative cooling) process. In addition to the effects of the air and water mass flow rates, effects of the inlet water temperature, air drybulb temperature, wetbulb temperature, and fill height on the transfer characteristic, or Merkel number, are investigated in the present study. The accuracy of two different empirical equations is also evaluated. It is found that the transfer characteristic correlations for wet-cooling tower fills are functions of the inlet water temperature and fill height but not of the air drybulb and wetbulb temperatures.     

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.     

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.     

Numerical Simulation of Influence of Arrangement of Flue Gas Discharge Devices on Performances of Indirect Dry Air Cooling Tower

ABSTRACT

For this paper, thre-dimensional (3D) numerical simulations have been carried out by means of one 3D model established using FLUENT software according to the indirect dry air cooling towers of one 2 × 660-MW power plant. The arrangement types of the flue gas discharge devices include the horizontal and the vertical arrangement types. The flow fields in the cooling towers have been studied. Meanwhile, the thermal and resistance performances have been analyzed. The study results include: (a) When the ambient wind is not considered, the arrangement types have little influence on the thermal and resistance performances of the indirect dry air cooling tower; and (b) when the wind is considered, the horizontal arrangement type plays a role of the cross-wall, which induces a mass flow increase compared with the case without the flue gas discharge devices.     

Theoretical model to estimate the thermal performance of an evaporative wind tower placed in an open space

A theoretical model to evaluate the thermal performance of an evaporative wind tower installed in open spaces with hot and dry climates has been developed. It was based on the laws of conservation of mass and energy and used TRNSYS as a simulation tool. Evaporative wind towers produce an adiabatic cooling which has been modelled taking into account all the heat and mass exchanges between the airflow and the injected water, and also considering the processes of radiation, convection and conduction. The system analyzed has a special design based on an existing installation placed in Madrid, which is composed of sixteen evaporative wind towers with one fan and six nozzles on the top of each one. A first validation of this theoretical model was done by comparing calculated results obtained through numerical simulation with experimental data. These last data were previously registered in a campaign carried out during the summer 2008 to evaluate the thermal behaviour of the system. To contrast both results, the same initial assumptions in fan and water operation as well as environmental conditions were considered. The comparison between them during the period of 18th to 20th July 2008, show an average temperature drop of 6.5 °C and an average increase of relative humidity of 27%. These values present a high correlation, up to 0.79, between experimental and calculated wet bulb depression. The average cooling power achieved by this system varies from 13 to 16 kW, with maximum peaks around 20 kW. So this theoretical model could be used for future energy estimations of wind towers design with similar constructive characteristics.     

Reduction in Performance Due to Recirculation in Mechanical Draft Cooling Towers

Abstract

The influence of recirculating warm plume air on the performance of mechanical-draft cooling towers is investigated analytically, numerically, and experimentally. It is shown that the amount of recirculation that occurs is a function of the flow and the thermal and geometric characteristics of the tower. The presence of a wind wall tends to reduce the amount of recirculation. An equation is presented with which the performance effectiveness due to recirculation can be evaluated approximately for a mechanical-draft cooling tower.     

Heat and Mass Transfer in an Indirect Contact Cooling Tower: CFD Simulation and Experiment

The present work is focused on the computational analysis of heat and mass transfer in an indirect contact cooling tower. The main objectives of the study are to contribute to the understanding of heat and mass transfer mechanisms involved in the problem and to check the possibility of making use of a commercial computational fluid dynamics (CFD) code for simulating mass and heat transfer phenomena occurring in indirect cooling towers. The CFD model uses as boundary conditions the temperatures of the tubes obtained by a correlation model developed by Mizushina. The available mass transfer correlations for indirect cooling towers are presented and compared with a correlation obtained from CFD simulations. The assumption of analogy between heat and mass transfer is also discussed.     

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.     

Numerical model of evaporative cooling processes in a new type of cooling tower

A numerical model for studying the evaporative cooling processes that take place in a new type of cooling tower has been developed. In contrast to conventional cooling towers, this new device called Hydrosolar Roof presents lower droplet fall and uses renewable energy instead of fans to generate the air mass flow within the tower. The numerical model developed to analyse its performance is based on computational flow dynamics for the two-phase flow of humid air and water droplets. The Eulerian approach is used for the gas flow phase and the Lagrangian approach for the water droplet flow phase, with two-way coupling between both phases. Experimental results from a full-scale prototype in real conditions have been used for validation. The main results of this study show the strong influence of the average water drop size on efficiency of the system and reveal the effect of other variables like wet bulb temperature, water mass flow to air mass flow ratio and temperature gap between water inlet temperature and wet bulb temperature. Nondimensional numerical correlation of efficiency as a function of these significant parameters has been calculated.     

超大型湿冷塔进风流场及导流装置数模研究及应用

基于Fluent软件,采用标准k-ε湍流模型进行应力封闭,对某工程塔内传热传质过程进行三维数值计算。计算分析了塔内外空气的速度场、温度场,建立了相关方程及气水两相间传热传质理论模型。结合工程实际情况,对冷却塔进风流场进行深入分析:1)导风板的存在降低了塔侧空气绕流流速,增大了冷却塔进风口流场的对称性,使塔内空气动力场的均匀程度增加;导风板安装高度和长度对冷却塔进风流场产生较大影响,以高11 m、长8 m导风板对# 1、# 2冷却塔性能的改善作用最大;2)导风板安装角度和块数对冷却塔进风流场产生一定影响,在导风板安装角度由0°至20°变化、在导风板安装块数由60块至90块变化时,两塔冷却性能变化影响较小。     

Performance Evaluation and Design of a New Cooling Tower Spray System for Uniform Water Distribution

ABSTRACT

The performance of wet cooling towers can be improved by installing sprayers that distribute the cooling water uniformly onto the fill while operating at a low pressure head. This paper presents the methodology that was followed to design a new cooling tower spray nozzle. The fluid dynamics of an orifice nozzle, such as the effect of a change in pressure head, spray angle, spray height, orifice diameter, and wall thickness on drop diameter and spray distance, is experimentally investigated, and ultimately a model with which a spray nozzle can be designed is presented. The manufacture and testing of a prototype spray nozzle show that it is possible to enhance the performance of sprayers and thus wet cooling towers by means of the methods presented.     

横流式冷却塔换热模型与影响因素研究

根据Merkel的冷却塔传热传质理论,推导了适应于横流式冷却塔的换热模型,通过理论模型正交试验和实测数据因子相关性分析,研究了横流式冷却塔换热性能的影响因素。实测数据因子相关性分析结果表明,风量对横流式冷却塔换热性能影响程度最小,与理论模型正交实验结果存在一定的差异。运行时应保证横流式冷却塔进水流量的分布均匀,才能更有效的利用换热面积,提高横流式冷却塔换热效率。     

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.     

Performance analysis and improvement of the use of wind tower in hot dry climate

Wind towers for passive evaporative cooling offer real opportunity for improving the ambient comfort conditions in building whilst reducing the energy consumption of air-conditioning systems.This study aims at assessing the thermal performance of a bioclimatic housing using wind towers realized in a hot dry region of Algeria. Performance monitoring and site measurement of the system provide data which assist model validation. The analysis and site measurement are encouraging, and they confirm the advantage of the application of this passive cooling strategies in hot dry climate.A mathematical model is developed using heat and mass transfer balances. For a more effective evaporative cooling, a number of improvements on wind tower configurations are proposed.     

Integrated analysis of cooling water systems: Modeling and experimental validation

Cooling towers are widely used in many industrial and utility plants as a cooling medium, whose thermal performance is of vital importance. Despite the wide interest in cooling tower design, rating and its importance in energy conservation, there are few investigations concerning the integrated analysis of cooling systems. This work presents an approach for the systemic performance analysis of a cooling water system. The approach combines experimental design with mathematical modeling. An experimental investigation was carried out to characterize the mass transfer in the packing of the cooling tower as a function of the liquid and gas flow rates, whose results were within the range of the measurement accuracy. Then, an integrated model was developed that relies on the mass and heat transfer of the cooling tower, as well as on the hydraulic and thermal interactions with a heat exchanger network. The integrated model for the cooling water system was simulated and the temperature results agree with the experimental data of the real operation of the pilot plant. A case study illustrates the interaction in the system and the need for a systemic analysis of cooling water system. The proposed mathematical and experimental analysis should be useful for performance analysis of real-world cooling water systems.     

Toward to the control system of mechanical draft cooling tower of film type

For changing atmospheric conditions the mathematical model of a control system of a mechanical draft cooling tower has been developed. The model includes a heat and mass transfer processes between water films and turbulent damp air flow at quasi-state approximation. Various regimes of cooling tower performance are compared and the optimization method is proposed.