Closed Wet Cooling Towers Research Articles

CALCULATION OF CLOSED WET COOLING TOWERS USING MODIFIED Ε-NTU APPROACHES

Two modified ε-NTU approaches developed for air-to-air indirect contact evaporative coolers have been adapted for calculation of heat/mass transfer in the closed wet cooling towers. These approaches use the analogy between the heat and mass transfer and the linearization of the direct or inverse correlations of moist air enthalpy on the temperature of the wet bulb. The results of calculation based on the modified ε-NTU approaches were compared with the data obtained by the traditional method, using the average logarithmic temperature and concentration differences. The comparison was carried out in terms of heat transfer efficiency according to the temperature of the wet bulb, this temperature is the theoretical limit of the outlet water temperature for this type of heat and mass exchange equipment of the indirect evaporative type. Calculations showed that both ε-NTU approaches demonstrate almost identical results, the possible inconsistency between them may be related to the inaccuracy of linearization. These data agree well with the data of the calculations, which are based on application of the average logarithmic difference of the flow parameters and the empirical dependencies of the heat and mass transfer coefficients. A small discrepancy (up to 7%) occurs with a small flow rate of working water. Bibl. 15, Fig. 5

Deciphering the optimal exergy field in closed-wet cooling towers using Bi-level reduced-order models

This paper introduces a bi-level reduced-order models (ROMs) approach for quickly deciphering the optimal exergy fields in closed wet cooling towers (CWCTs) with consideration of weather variations. First, an efficient sampling method based on stochastic reduced-order model is performed for the approximation of the multivariate probability distributions by generating a finite set of samples. The uncertainty associated with input variables is propagated via multi-sample CFD simulations of the CWCT model for each of the samples. The results of the state and output variables stored in the CFD solutions are used to construct the data-driven and physics-based ROMs by combining principal component analysis and artificial neural network methods. The constructed data-driven ROM is embedded in a sampling-based stochastic optimization model that seeks the maximization of the expected exergy efficiency ratio. The physics-based ROM is used to visualize the optimal field profiles of the thermal-, mechanical-, and chemical-exergy fluxes. Finally, the results of a case study demonstrate that the main strengths of the proposed approach is to simultaneously obtain the optimal exergy efficiency ratios and the exergy field profiles of the CWCT system in a computationally efficient manner.

Optimisation and Analysis Based on Bi-Level Reduced-Order Model for Coupled Heat and Mass Transfer Processes Under Uncertainty

This work proposes an integrated framework to build fast and high-fidelity bi-level reduced-order models (ROMs) for heat and mass transfer processes systems. In the proposed framework, the bi-level ROMs including the ROM for output variables (yROM) and the ROM for state variables (zROM) corresponding to its input variables of the systems are developed by integrating principal component analysis (PCA) with artificial neural networks (ANN), which can closely approximate the results of computational fluid dynamics (CFD). In particular, the CPU time is significantly reduced with the fast bi-level ROMs. A case study is used to demonstrate the benefits of using our proposed bi-level ROMs methodology for simulation and optimisation for heat and mass transfer processes systems, where a bi-level ROMs for the closed wet cooling towers (CWCTs) is built with the help of exergy analysis. The design points served for CFD simulations are randomly sampled by stochastic reduced-order modelling (SROM). The developed yROM is used for stochastic optimisation of exergy efficiency ratio and then the corresponding optimal exergy flux fields are then reconstructed and analysed based on zROM.

Model reductions for multiscale stochastic optimization of cooling water system equipped with closed wet cooling towers

Incorporation of closed wet cooling tower (CWCT) in the existing circulating water system has been recognized as a viable path to reduce water use in process industry. This paper introduces a specifically tailored framework based on model reductions for multiscale optimization of CWCT-based cooling water system considering environmental variations. An optimal design of experiment is performed for accurate approximation of the multivariate probability distributions by generating a finite set of samples over the entire input space. The probability distributions are propagated via multi-sample CFD simulations for constructing the physics-based and data-driven reduced models of CWCTs. Based on the developed reduced models, a multiscale optimization model is proposed for performing integrated design and management of CWCTs and cooling water system. It employs sampling-based stochastic programming and the heterogeneous integration of reduced models of CWCTs and other shortcut models. Finally, the performance of the proposed approach is illustrated through its comparison with a deterministic approach.

Experimental Study on Performance of a Closed Wet Cooling Tower for Air Wet-Bulb Temperature near 0°C

Closed wet cooling towers (CWCTs) are used widely because of their better water quality and smaller water consumption. However, the operating parameters shown in the technical documents are only for rated conditions in summer, not for any other conditions, especially in low air wet-bulb temperature areas such as those near 0°C. In addition, CWCTs often fail to achieve the designed cooling effect at low air wet-bulb temperatures. A experiment set of CWCT was built, and the performance of the CWCT at low air wet-bulb temperature near 0°C was investigated. The impact of the operating parameters (air flow rate, cooling water flow rate, and spray water flow rate) on the heat and mass transfer performance of the CWCT was measured and analyzed. The results show the cooling performance of the CWCT at an air wet-bulb temperature 0°C, 2°C, and 4°C is about 47%–63% of the rated operating condition; the optimal operating parameters of these conditions for the CWCT are just the same as those of the rated operating condition in summer. According to the experimental results, some operating advices are given.

Simulation and optimization of cooling water systems with closed wet cooling tower based on reduced order model

This study presents a CFD (computational fluid dynamics)-assistant multi-scale modeling and optimization strategy for synthesizing cooling water systems which incorporates multiple closed wet cooling towers (CWCTs) installed with plain tubes. The utilization of CFD is capable of providing accurate and visual results with respect to complex thermal and fluid flow phenomena that can be used to explore the synergistic mechanism of multi-parameter for CWCTs. Based on CFD results, a systematic reduced order model (ROM) for CWCTs is developed by closely approximating the high-fidelity CWCTs models through integrating principal component analysis (PCA) with Kriging-based surrogate model. This ROM is accessible to the desired result through input data without accounting for the complex geometries and detailed thermal-hydraulics property models. The proposed CFD-assistant ROM for the CWCTs is applied to enhance solution convergence and obtain optimal results of cooling water systems. An industrial case study is used to demonstrate the superiority of the proposed CFD-assistant strategy for the cooling water system synthesis and optimization.

Deciphering the thermal and hydraulic performances of closed wet cooling towers with plain, oval and longitudinal fin tubes

In this paper we numerically investigate the thermal-hydraulic performances of the closed wet cooling towers (CWCTs) installed with plain, oval, and longitudinal fin tubes in the heat exchanger. Important operational/structural parameters such as the flow rates of air and spray water are analysed by combining the discrete phase model with the species transport model of CFD. It is observed that the normal mesh, with a grid number of 43,000, is acceptable, while the realizable model is employed to describe the turbulent flows. The heat transfer coefficient of the fin tube case is the highest when the air flow rate exceeds a critical value, 3.2kgm−2s−1. However, overall, the oval tube case is recommended due to its high thermal-hydraulic performances and the modest pressure drop. The correlations of the heat and mass transfer coefficients as well as the pressure drop are developed. Furthermore, an uncertainty analysis finds that, the mean of the heat transfer coefficient increases from the plain tube case (1178.2), to the fin tube case (1898.4), to the oval tube case (1943.4). Statistically, the variances in the air flow rate, inlet air temperature, and spray water temperature contribute 52.8–70.6%, 20.4–33.8%, and 9.0–13.4% of the uncertainty, respectively.

Performance Analyses of Counter-Flow Closed Wet Cooling Towers Based on a Simplified Calculation Method

As one of the most widely used units in water cooling systems, the closed wet cooling towers (CWCTs) have two typical counter-flow constructions, in which the spray water flows from the top to the bottom, and the moist air and cooling water flow in the opposite direction vertically (parallel) or horizontally (cross), respectively. This study aims to present a simplified calculation method for conveniently and accurately analyzing the thermal performance of the two types of counter-flow CWCTs, viz. the parallel counter-flow CWCT (PCFCWCT) and the cross counter-flow CWCT (CCFCWCT). A simplified cooling capacity model that just includes two characteristic parameters is developed. The Levenberg–Marquardt method is employed to determine the model parameters by curve fitting of experimental data. Based on the proposed model, the predicted outlet temperatures of the process water are compared with the measurements of a PCFCWCT and a CCFCWCT, respectively, reported in the literature. The results indicate that the predicted values agree well with the experimental data in previous studies. The maximum absolute errors in predicting the process water outlet temperatures are 0.20 and 0.24 °C for the PCFCWCT and CCFCWCT, respectively. These results indicate that the simplified method is reliable for performance prediction of counter-flow CWCTs. Although the flow patterns of the two towers are different, the variation trends of thermal performance are similar to each other under various operating conditions. The inlet air wet-bulb temperature, inlet cooling water temperature, air flow rate, and cooling water flow rate are crucial for determining the cooling capacity of a counter-flow CWCT, while the cooling tower effectiveness is mainly determined by the flow rates of air and cooling water. Compared with the CCFCWCT, the PCFCWCT is much more applicable in a large-scale cooling water system, and the superiority would be amplified when the scale of water distribution system increases. Without multiple iterative calculations and extensive experimental data, the simplified method could be used to effectively analyze the thermal performance of counter-flow CWCTs in operation. It is useful for optimization operation of counter-flow CWCTs such that to improve the energy efficiency of the overall cooling water system.

Computational Analysis of Closed Wet Cooling Towers

The thermal and hydraulic performance of plain and oval tubes closed wet cooling tower (CWCT) are investigated numerically. Computational fluid dynamics (CFD), ANSYS Fluent 12.1, is implemented for the numerical solution. Species transport without reactions is adopted to simulate the mass transfer from the air-deluge water interface to the air. Different turbulence models and near-wall treatments are used to assess which model fits the data better. The mass transfer Colburn factor j m , and friction factor f are presented and compared with experimental data. The proposed CFD model is also applied to predict the mass transfer coefficient of another CWCT and compares well with the experimental data.

Thermal performance analysis of closed wet cooling towers under both unsaturated and supersaturated conditions

A closed wet cooling tower (CWCT) can be regarded as a cooling tower with the packing replaced by a bank of tubes. The heat and mass transfer in CWCT has been little investigated under supersaturated condition. The governing equations for heat and mass transfer in CWCT under both unsaturated and supersaturated conditions were derived based on Poppe method. Based on a sectional method, two analytical models were proposed and a program was coded to investigate the thermal performance by automatically selecting the corresponding governing equations whether the air was supersaturated or not. The results show that the air temperature, humidity ratio and enthalpy distributions under both unsaturated and supersaturated conditions could be accurately predicted with the analytical models. Then the effect of degree of saturation of inlet air on the heat transfer rate and process-water outlet temperature was presented, and little difference was found between the unsaturated and supersaturated conditions if the outlet air was supersaturated.

Experimental and computational analysis of thermal performance of the oval tube closed wet cooling tower

Abstract Closed wet cooling towers (CWCTs) characterize by smaller investment and exploitation cost as well as fewer materials and less space. It can be regarded as a cooling tower with the packing replaced by a bank of tubes. However, heat and mass transfer in an oval tube closed wet cooling tower (CWCT) have been little investigated under different operating conditions. The thermal performance characteristics of an oval tube CWCT were conducted. From the experimental results, correlations for the water-film heat transfer coefficient and air–water mass transfer coefficient are obtained. Experimental tests show the water-film heat transfer coefficient is a function of deluge-water temperature, deluge-water mass velocity and air mass velocity, which is different from that in the public literatures. The air–water mass transfer coefficient is a function of air mass velocity. In addition, a mathematical model with three ordinary differential functions is presented to predict the outlet process water temperature. The model is solved by a sectional method. The results show the analytical outlet temperature compares well with the experimental data. The method is suitable for evaluating the performance of an oval tube CWCT.

Simplification of analytical models and incorporation with CFD for the performance predication of closed-wet cooling towers

Simplified analytical models are developed for evaluating the thermal performance of closed-wet cooling towers (CWCTs) for use with chilled ceilings in cooling of buildings. Two methods of simplification are used with regard to the temperature of spray water inside the tower. The results obtained from these models for a prototype cooling tower are very close to experimental measurements. The thermal performance of the cooling tower is evaluated under nominal conditions. The results show that the maximum difference in the calculated cooling water heat or air sensible heat between the two simplified methods and a general computational model is less than 3%. The analytical model distribution of the sensible heat along the tower is then incorporated with computational fluid dynamics (CFD) to assess the thermal performance of the tower. It is found that CFD results agree well with the analytical results when the air flow is simulated with air supply from the bottom of the tower, which represents a uniform air flow. CFD shows the importance of the uniform distribution of air and spray water to achieve optimum design. Copyright © 2002 John Wiley & Sons, Ltd.

Theoretical and computational analysis of closed wet cooling towers and its applications in cooling of buildings

Theoretical analysis and computational modelling of closed wet cooling towers (CWCTs) are presented. Experimental measurements for performance of a prototype tower are used to define the tower transfer coefficients. Tower flow rates and number of tubes and rows are optimised for the required cooling load and to achieve a high coefficient of performance (COP). The performance of a cooling system used to cool office buildings is simulated using the transient system (TRNSYS) simulation environment. The cooling system consisted of a CWCT, chilled ceilings, pumps and a fan. A control strategy, including night cooling, is introduced in the simulation. The results indicate efficient performance and high COP values for the system.

Application of CFD to closed-wet cooling towers

Computational fluid dynamics (CFD) is applied to predicting the performance of closed-wet cooling towers (CWCTs) for chilled ceilings according to the cooling capacity and pressure loss. The prediction involves the two-phase flow of gas and water droplets. The predicted thermal performance is compared with experimental measurement for a large industrial CWCT and a small prototype cooling tower. CFD is then applied to the design of a new cooling tower for field testing. The accuracy of CFD modelling of the pressure loss for fluid flow over the heat exchanger is assessed for a range of flow velocities applied in CWCTs. The predicted pressure loss for single-phase flow of air over the heat exchanger is in good agreement with the empirical equation for tube bundles. CFD can be used to assess the effect of flow interference on the fluid distribution and pressure loss of single- and multi-phase flow over the heat exchanger.