Dry Cooling Towers Research Articles

Spray Effect on the Thermal Performances of a Dry Cooling Tower

Pre-cooling the inlet air of a dry cooling tower by means of a spray can improve the tower performance during periods of high temperature. To study the spray effect on the thermal performance of natural draft dry cooling towers (NDDCTs), in this study 3-D numerical simulations of such a process have been conducted using Fluent 16.2 (a two-way coupled Eulerian-Lagrangian approach). The considered NDDCT is 120 m high and only half system is simulated due to its structural symmetry. Three different spray strategies have been investigated at a typical crosswind speed of 4 m/s, which is the most frequent wind speed. The results have shown that: (1) The three implemented spray strategies can improve the thermal performance of the studied NDDCT with a varying degree of success. In one case, the heat rejection rate can be increased by 35.2%, and the tower outlet water temperature can be decreased by 2.1°C when compared with the no spray case; (2) To improve the thermal performance of the NDDCT using a small amount of water, the design of the spray pre-cooling system must include more nozzles on the windward and fewer or even no nozzles on the leeward sides of the NDDCT.

Study on Pollutant Emission in a Natural Draft Dry Cooling tower with Flue Gas Injection based on LES

This study investigated the Computational Fluid Dynamics (CFD) simulation of pollutant emission in a natural draft dry cooling tower (NDDCT) with flue gas injection. In order to predict the diffusion and distribution characteristics of the pollutant more accurately, Large Eddy Simulation (LES) was applied to predict the flow field and pollutant concentration field and compared with Reynolds Average Navier-Stokes (RANS). The relationship between pollutant concentration pulsation and velocity pulsation is emphatically analyzed. The results show that the maximum value of LES is about 43 times that of RANS for the prediction of pollutant concentration in the inner shell of cooling tower. Compared with RANS, LES can simulate flow field pulsation with a smaller scale and higher frequency.

Aluminum compound forms in the Al–H2O system as applied to operating conditions of Dry Cooling Towers

One of important indicators of reliable operation of the steam-water cycle at power plants is water chemistry. Special chemicals are used to adjust properties of working fluids. At designing a new unit the chemicals are selected on the basis of parameters of water, steam and materials, used in different parts of the cycle.Another indicator is minimizing the cooling water loss. Typical loss of cooling water is about 5% of total water amount. This loss is associated with evaporation, droplet carry-over, and blowdown in cooling towers.For power plants, it is typically recommended to use dry cooling towers for minimizing the cooling water loss. In dry cooling systems, cooling water with low amount of impurities goes to mixing condenser, where turbine steam condensation occurs. Then one portion of mixed water flow at the condenser outlet is supplied to the boiler or steam generator feed water loop. Another portion is returned back to the cooling tower. Inside the cooling tower, water is cooled down by passing through air-cooled heat exchangers manufactured of aluminium or steel alloys.On the basis of Pourbaix diagram it is proposed to evaluate the influence of water chemistry on the forms of existence of aluminium compounds in dry cooling towers. The results of calculations are compared with the experimental data.

Investigation into Operating Conditions of a Dry Mechanical Draft Cooling Tower at the TETs-12 Cogeneration Power Station of PAO Mosenergo

The results of the comprehensive investigation into operation of a dry mechanical draft cooling tower (DMDCT) of a PGU-220 power unit at the TETs-12 cogeneration power station of PAO Mosenergo are presented. Thermal performance of the main and auxiliary DMDCTs and their individual sections is examined. To determine the cooling-air flowrate, a rake consisting of 11 velocity transducers and thermocouples was installed at the fan outlet above the fan diffuser of the examined section. The DMDCT heat balances calculated on the basis of water- and cooling-air conditions agree with each other with a maximum error of 8%. The overall thermal performance indices of the main and auxiliary DMDCTs were lower than those specified by the supplier. Performance of DMDCT fans was investigated at different blade angles α. According to our calculations, the effect of better fan performance in terms of an increase in the power of a steam turbine in a combined-cycle unit at α > 15° does not compensate for an increase in the power consumption of fan motors and even reduces the amount of delivered power. Therefore, the range of α = 10°–15° for the fans of the main DMDCT may be considered optimal. As to the auxiliary cooling tower, an increase in the air flow at α = 17° can offer a wider operation range for the fans at high ambient air temperatures, which is important considering limitations on the supply temperature (below 35°С) of the auxiliary equipment cooling water. The investigation of DMDCT operation during the cold season yielded the dependence of the minimum heat load of DMDCT, required for its safe operation, on the ambient air temperature. The study into the performance of the humidification system revealed that its efficiency is inadequate. The best solution to this problem is to replace the normal humidification nozzles with nozzles with smaller diameter holes to improve water dispersion that will prevent the water drops from falling onto the ground and improve the efficiency of the humidification system while decreasing the water flow to the humidification nozzles.

Study of flue gas emission and improvement measure in a natural draft dry-cooling tower with flue gas injection under unfavorable working conditions

In this paper, we observed that flue gas from a natural draft dry-cooling tower (NDDCT) with flue gas injection could not be discharged smoothly under unfavorable working conditions (e.g., when the louvers are closed in winter), which may have caused severe corrosion on the inner shell of the cooling tower. In light of this issue, we proposed an improvement measure. On the basis of the computational fluid dynamics (CFD) method, we established a numerical model for NDDCT with flue gas injection to predict the flow field and pollutant diffusion. We simulated flue gas emissions for a NDDCT with flue gas injection under normal and unfavorable working conditions. The results showed that the air mass flow rate and heat rejection of the cooling tower decreased significantly, and the air flow in the cooling tower became disorganized when the louvers were closed under unfavorable working condition. Compared with normal working conditions, the pollutant concentration on the inner shell of the cooling tower increased by 1.5 times under unfavorable working conditions. After increasing the height of flue gas outlet, the pollutant concentration on the inner shell of the cooling tower decreased obviously.

Internal flow reconstruction strategies to improve both thermo-flow performance and flue gas diffusion characteristic of the integrated dry-cooling tower and stack system

The interaction between thermo-flow performance and flue gas injection in an integrated dry-cooling tower and stack system was revealed. Flue gas injection slightly increases the ventilation and heat rejection of the tower, and rear radiators are protected due to the blocking effect of the desulfurizer and stack. Both the swirl and downwind deflection of internal rising airflows contribute to the chaotic diffusion of flue gas. Two internal flow reconstruction strategies were proposed. One works in the stage of inflow air mixing, and the other works in the following stage of airflow rising. Numerical results show that the former strategy performs better on tower performance improvement, since the latter one causes a decline of tower ventilation at low wind speed. Both the two strategies have evident superiority in controlling gas diffusion inside the tower, especially at the medium wind speed of 10 m/s, the chaos of internal rising airflows reduces remarkably, and the contact area between flue gas and inner tower shell reduces over 85%. Thanks to the mitigation of plume downwash after flow reconstruction, diffusion passage of flue gas in the atmosphere is uplifted, and the reduction of contact area between flue gas and outer tower shell reaches 40%.

CFD Simulation of Pollutant Emission in a Natural Draft Dry Cooling Tower with Flue Gas Injection: Comparison between LES and RANS

Accurate prediction of pollutant dispersion is vital to the energy industry. This study investigated the Computational Fluid Dynamics (CFD) simulation of pollutant emission in a natural draft dry cooling tower (NDDCT) with flue gas injection. In order to predict the diffusion and distribution characteristics of the pollutant more accurately, Large Eddy Simulation (LES) was applied to predict the flow field and pollutant concentration field and compared with Reynolds Average Navier-Stokes (RANS) and Unsteady Reynolds Average Navier-Stokes (URANS). The relationship between pollutant concentration pulsation and velocity pulsation is emphatically analyzed. The results show that the flow field and concentration field simulated by RANS and URANS are very close, and the maximum value of LES is about 43 times that of RANS and URANS for the prediction of pollutant concentration in the inner shell of cooling tower. Pollutant concentration is closely related to local flow field velocity. RANS and URANS differ greatly from LES in flow field prediction, especially at the outlet and downwind of cooling tower. Compared with URANS, LES can simulate flow field pulsation with a smaller scale and higher frequency.

Thermodynamic Analysis of the Air-Cooled Transcritical Rankine Cycle Using CO2/R161 Mixture Based on Natural Draft Dry Cooling Towers

Heat rejection in the hot-arid area is of concern to power cycles, especially for the transcritical Rankine cycle using CO2 as the working fluid in harvesting the low-grade energy. Usually, water is employed as the cooling substance in Rankine cycles. In this paper, the transcritical Rankine cycle with CO2/R161 mixture and dry air cooling systems had been proposed to be used in arid areas with water shortage. A design and rating model for mixture-air cooling process were developed based on small-scale natural draft dry cooling towers. The influence of key parameters on the system’s thermodynamic performance was tested. The results suggested that the thermal efficiency of the proposed system was decreased with the increases in the turbine inlet pressure and the ambient temperature, with the given thermal power as the heat source. Additionally, the cooling performance of natural draft dry cooling tower was found to be affected by the ambient temperature and the turbine exhaust temperature.

Combined air-cooled condenser layout with in line configured finned tube bundles to improve cooling performance

Air-cooled condensers (ACCs) are commonly arranged in the A-frame form to extend the heat transfer surface with some well-recognized design flaws that inhibit the cooling performance. For overcoming this geometric defect, the combined natural draft ACC is proposed with the horizontally in line arranged finned tube bundles inside the dry-cooling tower and vertically in line configured bundles outside the tower. A three dimensional CFD method with the realizable k-ε model, radiator model and porous media model is developed and experimentally validated. The thermo-flow characteristics of the proposed ACC are analyzed and compared with two types of traditional ACCs. Besides, the effects of the platform heights from 5 m to 50 m are also investigated for the novel ACC layout. The results show that the new proposed ACC can endow with splendid cooling performance at the wind speeds lower than 9 m/s, especially in the absence of winds with the heat transfer rate increased by nearly 20% in comparison with two conventional ones. While at high wind speeds, the traditional ACC with vertically arranged heat exchanger bundles shows the highest cooling efficiency, thus is applicable to the regions with strong prevailing winds. Moreover, for the novel ACC, there exist the optimal platform heights of 15 m and 40 m at the low wind speeds and high wind speeds respectively, which can contribute to the optimal design of natural draft air-cooled condenser in power plants.

The Effects of Windbreak Walls with Various Widths on Thermal Performance Enhancements of the Dry Cooling Tower under Crosswind

A numerical model of the dry cooling tower is established, considering the detailed structures of the delta-type radiators. Heat transfer characteristics of the radiators, along with cooling water temperature drop of the whole tower are obtained, and the impacts of windbreak walls with various widths on thermal performance enhancement of the tower are analyzed and compared. It’s found that the effects caused by windbreak walls are not always positive, walls with small width would lead to performance deterioration of the tower. As wall width increases, the walls would enhance thermal performance of the tower gradually, and the optimal benefit brought by windbreak walls appears at the width between radiator height and 5/4 radiator height. Moreover, at large wall width, the positive effect brought by the walls is decreasing, but thermal performance of the tower in this condition is still superior to that at no-wall condition. This study provides guidance for the design, manufacture and practical application of windbreak walls in the dry cooling towers.

Wind Effects and the Challenge to Enhance Thermal Performance of Three Aligned Natural Draft Dry Cooling Towers

The performance of natural draft dry cooling towers (NDDCTs) depends on the environmental conditions. Crosswind is known to have the most destructive effect, whereas the ambient temperature is marked as the next. A 3-D numerical model was developed to investigate the thermal performance of a single as well as three aligned units of NDDCTs under crosswind conditions. The computed results showed major discrepancies between computed velocity patterns and pressure fields around three aligned towers and those of the single tower due to windshield effect of neighboring cooling towers. The thermal performance losses for the single tower under crosswind showed higher values compared with those of towers in the aligned arrangement. The best thermal performance of three aligned NDDCTs was achieved when the crosswind was directed along the towers’ connection line.

Heat Transfer in Saline Water Evaporative Cooling

ABSTRACTThe present paper provides an overview and review on heat and mass transfer involved in evaporation of single saline water droplets as well as pure and saline water sprays for cooling purposes. The aim is to demonstrate the advances that have been made toward the deployment of Natural Draft Dry Cooling Tower operating with saline water. A new theoretical model for evaporation of single solid containing droplet is reviewed, and the corresponding advantages are discussed. Moreover, a comparison between pure and saline water sprays is reported and performance correlations are compared. The new approaches to implement saline water injections in numerical simulation and nozzle arrangement in a cooling tower reported in this work provides valuable tools for designing an efficient cooling tower. This work shows that in addition to fresh water preservation and cost savings due to water purification, using saline water can also lead to enhance cooling efficiency by up to 8% and build more compact cooling towers. Overall it has been shown that the use of saline water is a viable and promising concept that requires further exploration.

Preliminary Analysis of Direct and Indirect Heat Rejection Systems for a Small sCO2 Brayton Cycle Using an Existing Natural Draft Dry Cooling Tower

AbstractThe supercritical carbon dioxide (sCO2) Brayton cycle has been the focus of much research in recent years because of its high efficiency and compactness. One of the key issues is the heat r...

Performance Recovery of Natural Draft Dry Cooling Systems by Combined Air Leading Strategies

The cooling efficiency of natural draft dry cooling system (NDDCS) are vulnerable to ambient winds, so the implementation of measures against the wind effects is of great importance. This work presents the combined air leading strategies to recover the flow and heat transfer performances of NDDCS. Following the energy balance among the exhaust steam, circulating water, and cooling air, numerical models of natural draft dry cooling systems with the combined air leading strategies are developed. The cooling air streamlines, volume effectiveness, thermal efficiency and outlet water temperature for each cooling delta of the large-scale heat exchanger are obtained. The overall volume effectiveness, average outlet water temperature of NDDCS and steam turbine back pressure are calculated. The results show that with the air leading strategies inside or outside the dry-cooling tower, the thermo-flow performances of natural draft dry cooling system are improved under all wind conditions. The combined inner and outer air leading strategies are superior to other single strategy in the performance recovery, thus can be recommended for NDDCS in power generating units.

Annularly arranged air-cooled condenser to improve cooling efficiency of natural draft direct dry cooling system

For natural draft direct dry cooling system, the triangularly arranged air-cooled condenser may lead to the unexpected airflow deviations and unbalanced flow distributions although with an additional buoyancy force from the dry-cooling tower. In this work, an annularly arranged air-cooled condenser is proposed for natural draft direct dry cooling system. By numerical simulations, the flow and heat transfer performances of these two dry cooling systems are analyzed and compared at various wind speeds. The results show that for the annularly arranged air-cooled condenser, the air flow interactions between the neighboring cooling columns are totally avoided at small wind speeds, and moreover, the vortices at the inlets of the cooling columns may vanish for the middle sector at high wind speeds. The air inflow and outflow deviations through the cooling columns are clearly restrained, which improve the local thermo-flow performances, thus increase the heat rejection conspicuously and recover the cooling efficiency of natural draft direct dry cooling system compared with the triangularly arranged air-cooled condenser, especially at small wind speeds. The annular configuration of air-cooled condenser could be recommended for the potential engineering applications thanks to its more energy efficient performance.

Solar-Enhanced Air-Cooled Heat Exchangers for Geothermal Power Plants

This paper focuses on the optimization of a Solar-Enhanced Natural-Draft Dry-Cooling Tower (SENDDCT), originally designed by the Queensland Geothermal Energy Centre of Excellence (QGECE), as the air-cooled condenser of a geothermal power plant. The conventional method of heat transfer augmentation through fin-assisted area extension is compared with a metal foam-wrapped tube bundle. Both lead to heat-transfer enhancement, albeit at the expense of a higher pressure drop when compared to the bare tube bundle as our reference case. An optimal design is obtained through the use of a simplified analytical model and existing correlations by maximizing the heat transfer rate with a minimum pressure drop goal as the constraint. Sensitivity analysis was conducted to investigate the effect of sunroof diameter, as well as tube bundle layouts and tube spacing, on the overall performance of the system. Aiming to minimize the flow and thermal resistances for a SENDDCT, an optimum design is presented for an existing tower to be equipped with solar panels to afterheat the air leaving the heat exchanger bundles, which are arranged vertically around the tower skirt. Finally, correlations are proposed to predict the total pressure drop and heat transfer of the extended surfaces considered here.

Thermo-flow performances of natural draft direct dry cooling system at ambient winds

Natural draft direct dry cooling system (NDDDCS) uses the natural draft dry-cooling tower instead of axial flow fans, so can effectively reduce the power consumption, operating costs and also the noise caused by fans. In this work, two types of NDDDCS with horizontally arranged heat exchanger bundles (HAHEBs) and vertically arranged heat exchanger bundles (VAHEBs) are proposed respectively. Furthermore, the cooling delta apex angles from 60° to 180° are further investigated. The air-side velocity, pressure and temperature fields are presented and the heat rejections for various configurations of NDDDCS are calculated and compared. The results show that the cooling performance of NDDDCS with VAHEBs is superior to that with HAHEBs in any case, especially at high wind speeds. The VAHEBs with the apex angle of 150° have the best cooling performance, so can be recommended in the potential practical engineering of natural draft direct dry cooling system in power plants.

Comparative Analysis on Thermal Performance of Different Natural-draft Dry Cooling Towers under Crosswind Condition

This article aims to study the thermal performances of four different natural draft coolingtowers under crosswind condition. The windbreakers and the oblique exit plane have been simultaneouslyincluded in the structure of the new cooling tower. A finite volume method using SIMPLE algorithm wasused to simulate the flow field around each cooling tower. The thermal performance of the new geometryhas been compared with those of others for the generally investigated wind velocity profile for 10 m/s,and also two uniform wind velocities for 3 and 7 m/s. The cooling capacity of the cooling tower utilizingwindbreakers and the oblique exit plane was predicted as 98.3% of the design value in the presence ofgenerally studied wind velocity profile of 10 m/s, while that of the cooling tower utilizing windbreakerswas predicted as 93.5%. Of course, the percentage of the thermal improvements of the different restoringstrategies are sensitive to the profile of an approaching wind. The uniform wind velocity decreases thethermal efficiency of the cooling tower more than the distributed one, while the restoring strategies usingwindbreakers provide a higher percentage of thermal improvements in the presence of uniform windvelocity.

Flue gas diffusion for integrated dry-cooling tower and stack system in power plants

For the integrated dry-cooling tower and stack, the flue gas diffusion characteristics are of great concerns for the pollutant control and tower anticorrosion. In this work, the computational models of flue gas diffusion for the integrated dry-cooling tower and stack system are developed and experimentally validated. By means of numerical simulations, the flue gas diffusions inside and outside the dry-cooling tower are investigated and the flue gas concentrations at different heights of the inner tower shell and on the ground level are obtained. The rising height of flue gas is studied and compared with the separate stack. The total mass flow rate of cooling air and heat rejection of air-cooled heat exchanger for both the dry cooling system with and without the stack inside the dry-cooling tower are also obtained. The results show that not only the thermo-flow performances of the natural draft dry cooling system with the integrated stack can be improved, but also the air pollution alleviation can be guaranteed. At high wind speeds, the inner tower shell faces corrosion risks due to the flue gas deposition, so the anticorrosion measures for the tower should be taken.

Economic Analyses of Natural Draft Dry Cooling Towers Pre-cooled Using Wetted Media

Evaporative pre-cooling with wetted-medium can improve heat exchange of natural draft dry cooling towers (NDDCTs) in hot days (usually in summer). However, the media introduce extra pressure drop which reduces the air flow of a NDDCT, and as a result, impairs the tower performance. Scholars studied the effect of evaporative pre-cooling on performance of cooling tower through experiments or simulations by taking into account both the benefit of evaporative pre-cooling and the disadvantage of extra pressure loss. They found that the performance of NDDCTs can be improved by wetted-medium evaporative pre-cooling when the ambient air is hot and dry. Nevertheless, whether the pre-cooling proposal will bring potential increase in the revenue for the owner of the power plants employing NDDCTs requires further study. To this end, the current study investigates the benefit of NDDCT with wetted-medium pre-cooling through calculating the electricity production and water consumption.