Natural Draft Dry Cooling Tower Research Articles

Thermal performance improvement of the three aligned natural draft dry cooling towers by wind breaking walls and flue gas injection under different crosswind conditions

The substantial objective of using circulating cooling water in a typical power plant is to remove the heat effectively from the vapor cycle and to expel it to the environment. One of the most popular cooling systems for this purpose is the natural draft dry cooling tower (NDDCT) which is widely used in regions with the shortage of water, such as Iran. Environmental conditions affect the Heller cooling tower's performance strictly. Temperature, density, and the mass flow rate of sucked air into the cooling towers will all influence their operation. To improve the cooling tower's performance and air suction capability, it is proposed to inject the steam generators' flue gas into the tower. In this study, a single natural draft dry cooling tower incorporating the flue gas duct is modeled to study the effect of flue gas injection on the sucked air flow rate and the radiator heat transfer. Considering the buoyancy effect term, for a 3-D incompressible flow, the Navier Stokes' equations, as well as the energy equation, are solved by computational fluid dynamic method (CFD) to obtain the airflow pattern inside and outside the tower as well as the heat transfer from the radiators. The tower is assumed to be equipped with windbreak walls in one case and without it in another case so that the effect of crosswind could be investigated. Having compared the thermal performance of the towers, the results show that the thermal deficiency is more pronounced for the towers in crosswind without any flue gas injection. It is also shown that the injection of the hot flue gas (with temperature of 130 °C) will increase air intake and the thermal performance of the towers which, in turn, results in the reduction of the outlet water temperature. The modeling is then extended to three aligned NDDCTs to determine the effect of flue gas injection in the presence of windbreak walls under crosswind condition. Consequently, a notable promotion had been observed in all three towers thermal performance but with different ratios.

Spray cooling system design and optimization for cooling performance enhancement of natural draft dry cooling tower in concentrated solar power plants

In concentrated solar power (CSP) plants built in dry and arid areas, natural draft dry cooling tower (NDDCT) are commonly employed to dissipate waste heat into the atmosphere. The cooling performance of NDDCT mainly depends on the induced air flow caused by the buoyancy effect. However, the high ambient temperature in summers reduce the cooling efficiency of dry cooling towers and cause significant power loss for CSP plants. To address this problem, spray cooling system utilizing water evaporation was developed to pre-cool the inlet hot air. Different designs of spray cooling systems were proposed and tested on a 20 m high experimental tower. Experimental data were collected to evaluate the performance of the spray cooling system. To our knowledge, this is the world's first attempt to practice spray cooling on a full-scale natural draft dry cooling tower. This study confirms the feasibility and effectiveness of employing spray cooling for cooling performance enhancement of NDDCT. With the goal of maximal cooling effect with least water consumption, the optimal design was proposed, which consists of 3 upward injections at the low level (Height = 2 m), 2 counterflow injections at the middle level (H = 3 m) and 3 counterflow injections at the high level (H = 4 m). The cooling capacity of NDDCT increases from 789 kW to 841.73 kW, as the result of an intensified natural convection. Moreover, in the spray zone, the presence of a low-temperature area is featured by high relative humidity (70%–80%). The intensified natural convection caused by pre-cooled air and the presence of high vapour concentration are attributed to spray evaporation, which confirms the necessity to introduce the spray cooling system.

Entransy analysis optimization of cooling water flow distribution in a dry cooling tower of power plant under summer crosswinds

The performance of natural draft dry cooling tower (NDDCT) of a coal-fired power generating unit is susceptible to ambient conditions, such as natural wind and temperature. An approach was proposed to relieve the adverse effects of ambient wind, which was the optimization of waterside flow re-distribution among heat exchangers of NDDCT under natural wind. With the help of entransy balance equations that described the heat transfer processes in each heat exchanger sector and condenser, all 20 influencing factors were considered to establish the optimization mathematical model of whole system, including both of the steam turbine subsystem and dry cooling system. A practical 660 MW dry cooling supercritical power generating unit was selected as object. The flow and heat transfer of NDDCT by natural wind effect were obtained numerically with verification. The results indicated that the performance of NDDCT could be improved by optimal distribution of waterside flow rate among the heat exchangers at any wind speed, especially at high wind speed. The comprehensive saving of power output coal consumption could be acquired although the pump power would raise by optimal water flow distribution. The decrease of coal consumption could reach 0.74 g/kWh at the natural wind speed of 12 m/s.

The adoption of windbreak wall partially rotating to improve thermo-flow performance of natural draft dry cooling tower under crosswind

To reduce the adverse effect of crosswind on thermo-flow performance of the natural draft dry cooling tower (NDDCT), a novel type of external flat windbreak walls is introduced. Taking the prevailing wind direction as the symmetry axis, windbreak wall arranged at each flank of the tower is divided into three rotatable columns according to the size of the researched NDDCT, with the rotating angle equal to air inflow deviation angle of nearest cooling delta. Subsequently, using the steady model and half tower simulations after comparison, effects of various combinations of the rotatable columns were revealed based on the analysis of velocity and temperature fields in and around the tower. At the general wind speeds, wall form of only middle column rotating contributes the largest performance improvement to the tower, which is over 17% at wind speed of 8 m/s, nearly 2.5% larger than that brought by the traditional radial walls. Meanwhile, it was found that outlet water temperatures of the next three cooling deltas at the back of inner column get increased under the optimal wall form. To overcome this problem, porous walls were tested to be applicable, but without overwhelming superiority, compared to the solid walls. In addition, when crosswind gradually deviates from the specified prevailing wind direction, wall form of only middle column rotating is still superior to the traditional radial walls, especially at a small crosswind deviation angle, but the difference between the benefits brought by the two wall forms to the tower is decreasing.

Design and performance study of dry cooling system for 25 MW solar power plant operated with supercritical CO2 cycle

Dry natural draft cooling towers are mostly employed in thermal power plants in arid areas with no energy consumption and maintenance compared with the mechanical draft cooling system. Different types and shapes of natural draft dry cooling towers (NDDCT) are available in many thermal and nuclear power plant industries. The main purpose of NDDCT is to generate air flow through the finned tube heat exchanger bundles by the influence of buoyancy due to the density difference between the ambient air and the warm air inside the tower. In the present chapter, a one-dimensional MATLAB code is developed to design a NDDCT with detailed specification of finned tube heat exchanger bundles for a 25 MW solar plant operated with supercritical CO2 (sCO2) Brayton cycle. The tower height, the tower inlet and outlet diameters and the number of heat exchanger bundles are evaluated after accomplishing the design requirements. The performance study of an air-cooled heat exchanger is performed for a range of sCO2 inlet temperature from 71 °C to 91 °C, operating pressure from 7.5 MPa to 9 MPa and ambient air temperature from 20 °C to 50 °C. The sCO2 inlet temperature and operating pressure to the heat exchanger significantly affect the cooling system performance. During high ambient temperature period, the cooling potential of NDDCT is significantly reduced.

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.

Positive impact of a tower inlet cover on natural draft dry cooling towers under crosswind conditions

This study proposes a tower inlet cover to improve the performance of the small natural draft dry cooling tower (NDDCT) under crosswind conditions. CFD analyses are performed on a small NDDCT with tower inlet covers of different lengths, and the CFD model is validated against experimental results. The air temperature, air pressure, air flow and heat flux fields are presented, and the thermal performance for each heat exchanger and the NDDCT are obtained using CFD simulations. The CFD simulation results show that the high-pressure zone around the tower side wall, formed by the crosswind, causes the decrease in air flow through the tower and the deterioration in tower performance with a crosswind. The tower inlet cover can improve the tower performance in crosswinds by increasing the air flow of the heat exchangers. Tower inlet covers with lengths of 1.5 m, 3 m and 4.5 m improve the tower heat load by 40–65%, 70–130% and 85–230%, respectively, when the crosswind increases from 4 m/s to 12 m/s.

Ventilation enhancement for a natural draft dry cooling tower in crosswind via windbox installation

A natural draft dry cooling tower (NDDCT) is demanded to save water for power generation in arid area, but its performance could be degraded greatly by the crosswind. To overcome the degradation, this paper proposes the installation of a windbox, which consists of windbreaks, an enclosure, a back barricade and a wind top to improve the pressure distribution outside the heat exchanger bundle of a NDDCT, so as to increase the ventilation rate under crosswind condition. Full dimensional computational fluid dynamics (CFD) modelling was conducted for the windbox installed around the NDDCT of a large scale coal-fired power plant. Different configurations of the windbox were studied. The flow characteristics along the streamline in the NDDCT were analyzed and quantified by adopting the concept of flow loss factor (FLF). Based on simulation and experiments, the installation of the windbox is shown to be effective. The windbox with a 120 m radius enclosure and a full size louver-type top could improve the ventilation rate of a NDDCT by ∼60% in gale crosswind condition, and keep high performance in breeze crosswind condition. Consequently, an annual reduction of coal consumption of 30,000–45,000 tons could be achieved on a 1000 MW unit, which is ∼3000,000–4500,000 $/a.

The transient start-up process of natural draft dry cooling towers in dispatchable thermal power plants

A one-dimensional (1-D) transient cooling tower model is presented that can be used to simulate the start-up process of natural draft dry cooling towers (NDDCTs). The model simulates the behaviour of a NDDCT following a step increase in the heat exchanger temperature. The start-up process is analysed in two successive stages. In the first stage, the dominant mechanism is natural convection operating through generation and propagation of hot plumes rising from the heat exchanger surface. An understanding of different phases of plume development based on scaled analysis helps to predict the air flow development in this first stage. In the second stage, the air flow is driven by the draft caused by the difference in the inside and outside densities. The cooling tower system air flow development in the second stage is simulated through a quasi-steady state solution of the well-known draft equation. The simulation is repeated for three different input temperatures. The results show that the higher the input temperature, the higher is the inlet air velocity and shorter the start-up process. The results are validated against data from the commissioning tests of the University of Queensland natural draft cooling tower Gatton test rig. This study aims to help fill the knowledge gap in understanding the NDDCT start-up process. This understanding is important to the employment of NDDCTs in future thermal power plants operating in dispatchable mode.

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.

A performance enhancement of a natural draft dry cooling tower in crosswind via inlet flow field reconstruction

With the wide utilization of a natural draft dry cooling tower (NDDCT) in power generation in arid areas, the degradation of its performance under crosswind conditions is increasingly concerned. Based on the influencing mechanisms of the crosswind, the paper reconstructs the destructed inlet flow field with a labyrinth structure. The effect of the labyrinth structure is firstly assessed by means of a verified computational fluid dynamics (CFD) model. Then, on the basis of CFD results, the labyrinth structure is further optimised by adopting a quantification method using a flow loss factor (FLF). Numerical results revealed that the proposed flow field reconstruction approach could increase the ventilation rate of a NDDCT by ∼62% under high speed crosswind condition, correspondently reducing the overall coal consumption by 23,100–33,000 t annually for a 660 MW coal-fired unit. Moreover, the negative effect of the crosswind on the performance of a NDDCT could be reversed to a positive one. The numerical results are well validated by the modelling experiments conducted in a wind tunnel.

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...

A quantitative approach identifies the critical flow characteristics in a natural draft dry cooling tower

The natural draft dry cooling tower (NDDCT) is a critical facility for an indirect dry cooling power plant in arid area for its merit of excellent water-saving. While crosswind degrades the performance of a NDDCT by changing the flow field inside and outside. In order to quantitatively study the influence of different flow characteristics on the performance of a NDDCT, hence to grasp the affecting mechanism of crosswind, a half-cylindrical computational fluid dynamics (CFD) model of a Heller type 660 MW NDDCT is developed and validated by a hot state modelling test rig. A flow loss factor (FLF) is derived and verified to linearly describe the effect of local flow field changing on the overall performance of a NDDCT. Based on the conjoint studies of the local FLF variation trends and the changing processes of correspondent flow characteristics in each specific flow segments, the critical factors influencing the performance of a NDDCT are identified under different crosswind conditions.

Fresh breeze cuts down one-third ventilation rate of a natural draft dry cooling tower: A hot state modelling

The natural draft dry cooling tower (NDDCT) has been increasingly used in power generation for its merits of excellent water-saving, high energy saving, simple maintenance and long life service. To study the performance of a newly installed 660 MW NDDCT under crosswind condition, a model with scale of 1:200 was built according to the scaling law of geometric similarity. The experiments were set up in self-similar region with high Reynolds to meet momentum similarity, while meeting the scaling law of Froude and Euler numbers. A first order law radiator resistance model is also proposed and verified by a systematic test. The exponent law profile of wind velocity above the ground was built and verified by experimental data. On the ground of a constant heating rate bases, the flow field inside the NDDCT and the ventilation rate were investigated at the crosswind range of 0–20 m/s.

Investigations on the influence of nozzle arrangement on the pre-cooling effect for the natural draft dry cooling tower

Natural draft dry cooling tower (NDDCT), with little water usage, is a primary choice for power plants in dried regions. However, the increased ambient temperature during summer days decreases the cooling performance of NDDCT. Inlet air pre-cooling is used to alleviate the tower deterioration by making use of water evaporation to remove excess heat from inlet air. To achieve the maximal cooling effect, the injection heights, radial distances and injection directions of employed nozzle LNN1.5 were studied based on the CFD results. The study shows that lower nozzle placement can cool the central part of the radiator while the higher one cools the middle part. Additionally, the increasing extended length can boost the evaporation process of generated spray. Moreover, the upward and co-flow injections have poorer performance than the downward and counter-flow injections. Furthermore, an introduction of wall cover changes the flow field and drives the pre-cooled air flow through the edge of radiator. Since the wall cover reduces the blockage caused by the near-wall vortex the resultant low-temperature region move outwardly.

Comparison of direct and indirect natural draft dry cooling tower cooling of the sCO2 Brayton cycle for concentrated solar power plants

This study investigates the performance of direct and indirect cooling in conjunction with natural draft dry cooling towers (NDDCT) for the heat rejection of a supercritical carbon dioxide (sCO2) recompression Brayton cycle. One dimensional models were developed for direct and indirect cooling NDDCTs. The inlet conditions to the heat rejection system that correspond to the peak cycle efficiency for the scenario considered were identified and used to determine the minimum NDDCT size for direct and indirect cooling. The direct cooling configuration requires a smaller tower than indirect cooling, due to the much higher temperature difference between the two fluid streams. When analysed in isolation the direct cooling NDDCT is found to over cool at both high and low inlet temperatures. The direct and indirect dry cooled recompression cycles were analysed at a range of ambient temperatures, using a coupled NDDCT and cycle model approach, in order to represent daily and annual variances. In both direct and indirect configurations, the cycle is overcooled at low ambient temperature, reducing cycle efficiency. A novel bypass arrangement was proposed that allows control of the compressor inlet temperature by reducing heat rejection from the tower, and allows both cycles to maintain peak efficiency at low ambient temperatures. At the design point ambient temperature of 20 °C the cycle efficiency is 49.6%. This is maintained down to 0 °C for both configurations, and drops to 44.7% and 43.8% at 50 °C ambient for direct and indirect respectively.

Effects of pressure loss coefficients of heat exchanger on thermal performance of the dry cooling tower

Crosswind deteriorates thermal performance of the natural draft dry cooling tower (NDDCT) mainly due to the unfavorable pressure distribution at tower entrance. Thus, effects of air cooled heat exchanger with various pressure loss coefficients are investigated with numerical simulation. The vortex inside the tower would cut down air inflow of heat exchanger of sideward and leeward parts. A quite large exponent of pressure loss coefficient expression of heat exchanger contributes to reduce heat transfer unevenness among cooling deltas, and would generate a small enhancement for the overall thermal performance of the tower.

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.

Entransy dissipation based optimization of a large-scale dry cooling system

The natural draft dry cooling tower (NDDCT) with heat exchangers accounts for about 10 percent of the total investment of a power plant, so it is meaningful to optimize the geometric parameters to explore the potential that matching the cooling load of a power generating unit in the most cost effective way. The entransy dissipation equations of two typical irreversible heat transfer processes, including that the ambient air heated by heat exchangers and circulating water heated by the exhausted steam of turbine, are derived in this paper. Combined with the force balance equation of NDDCT, all parameters that may affect the performance and annual cost of a cooling system are took into consideration. Such constituted constraints are applied to the Lagrange multiplier optimization of such a system. Based on the mathematical relation and conditional extremum method, an optimization equation group aiming to obtain the minimum of total annual cost is constructed. Finally, a practical 600MW power generating unit is taken as an example to illustrate the applications of entransy dissipation based optimization principle. The result indicates that total annual cost of the dry cooling system can drop from $6 420 643 to $5 665 660.

A study on multi-nozzle arrangement for spray cooling system in natural draft dry cooling tower

Natural draft dry cooling tower (NDDCT) technology is especially attractive to power plants built in arid regions with limited water resource. However, high ambient temperature in summer deteriorates the performance of built NDDCT. To address this problem, evaporative pre-cooling technology has been developed by using nozzles to disintegrate water into fine droplets to achieve quick evaporation. The pre-cooled air flowing through radiator, has an enhanced heat exchange with the hot working fluid in the tube side. This paper reports a spray cooling system for the experimental tower built in UQ by combining several nozzle LNN1.5 to cool the inlet air and consequently improve the cooling efficiency of the NDDCT. To minimize water usage, a careful arrangement of spray nozzles should be investigated to achieve the maximum cooling outcome. With five nozzles installment, the inlet air is cooled by 6.3°C, corresponding to 51.2% cooling efficiency. A dimensionless analysis is presented to correlate cooling efficiency with influencing factors. The advantage of this pre-cooling system lies in the efficient water usage: more than 96% of the injected water extracts substantial heat from hot air and evaporates into vapor, leading to a pre-cooled airflow.