Cooling Towers: Structural Components and Operational Methodology

INTRODUCTION

Rooppur Nuclear Power Plant is being developed on the bank of the river Padma. The cooling towers of Rooppur NPP will use the water of this river for cooling purposes. To ensure sufficient amounts of water, cooling towers will adopt re-circulatory condenser water cooling systems to convert the used steam of turbines into water. Rooppur NPP has four natural draught cooling towers (11, 12, 21, 22 URA) which are designed to circulate water from power units 1 and 2. These towers are reinforced concrete shell structures which are supported by circular RCC racker columns. Inner components of the cooling tower are supported on 276 grillage columns having a haunch at its top to allow connection of beams with it. This parabolic tower of 175m in height can handle 170,000 cum/hr of condenser water. The quantity of condenser water flow for a tower of RNPP will be around 97,200 cum/hr. This water will additionally be used for the fire safety system. This paper aims at presenting an overview of Rooppur NPP cooling towers in terms of their structural components and water cooling methodology during its operation. 

STRUCTURE DESCRIPTION

A number of numerical and experimental studies have been carried out on cooling towers [1]. In experimental studies, parameters such as temperature and pressure values at different points of the tower, wind speed, and temperature are measured. The results obtained from the measured points are analyzed to find out the performance and heat transfer enhancement in the cooling tower. In this paper, the structural systems and operational methodology of RNPP cooling towers have been described based on design documents [2]. A schematic 3D view has been developed for better visualization of its components as shown in figure 1. 

FACILITY LAYOUT

Natural draught cooling towers are located in the southern part of the NPP site. Reference elevation of 0.000 corresponds to the top of the water storage basin wall. The location site for each cooling tower has been arranged down to the level of minus 0.150. These cooling towers are hyperbolic in shape. The hyperboloid shape is a very strong design for a large hollow structure. It has a broader base that allows for a greater area to encourage evaporation, then narrows to increase airflow velocity. It then widens slightly which could aid in mixing the warm moisture-laden air into the atmosphere.

Overall dimensions of natural draught cooling towers (11, 12, 21, 22 URA) are shown below:

- diameter along the external water storage basin wall surface – 135.72 m;

- diameter of the laying out axis for inclined colonnade struts at an elevation of minus 0.350 – 128.168m;

- cooling towers height – 175 m;

- shell diameter in the throat at an elevation of +125.000 m – 77.0 m;

- water storage basin bottom level - minus 2.350;

- circular footing level - minus 3.850 m;

- elevation of air inlet openings top - plus 10.300 m.

RNPP cooling tower is basically an RCC structure having a thick concrete shell of variable thickness supported on 100 racker columns and a water distribution pipeline system which is supported by precast grillage columns and beams. The basic elements of the RNPP cooling tower have been shown in figure 1.

 Figure 1:Basic Elements of NPP cooling Tower

The internal arrangement of the cooling tower is supported by a concrete Grillage column-beam arrangement system. These precast grillage columns have haunch at different elevations and directions to allow connection of beams at elevations +10.100m and +14. 600m.The first layer of the precast beam will be placed at elevation +10.800 as shown in figure 2. For visualization purposes, beams at different elevations have been presented in distinguished colors.

Figure 2: Beams at different elevations

WATER DISTRIBUTION METHODOLOGY OF COOLING TOWER

Water to each cooling tower from the turbine building (UMA) is supplied using two pipelines of 2600 mm with a further transition to 2200 mm. Pipelines are installed on supports which, in turn, are installed on the foundations made of monolithic reinforced concrete. The axis of the feed pipelines is located at an elevation of +1. 500. Feed pipelines supply water to four steel standpipes 2200 mm in diameter. The standpipes feed main pipelines from where water is distributed to operating pipelines within the irrigation area of the cooling tower. The diameters of the main pipelines are 1620 mm, 1220 mm, 1020 mm, 920 mm, 820 mm, and 630 mm. At an elevation of +14.050 operating pipelines are located for РАB system cooling where spray nozzles are installed. Elevation for operating pipeline’s location was specified with due account for needed sizes development of spray plumes under the condition of the most uniform water distribution over the irrigation area of the cooling tower. Operating pipelines shall be made of glass fiber plastic. The diameters of operating pipelines are 0.25 m, 0.20 m, and 0.15 m. Operating pipelines shall be hung to reinforced concrete beams. Separate sections of pipelines shall be connected by unions with rubber gaskets; at that water flow rate shall be maintained constant by pipeline diameter decrease. Water spraying shall be envisaged by plastic sprayers. Spraying direction shall be accepted to be downwards. Hydraulic resistance of feed and water distribution pipelines within the cooling tower -1.5 m. Head before spraying devices -1,05 m. At an elevation of +0.500 in the axes of 5-6 pipeline 1600 mm in diameter enters the cooling towers 11, 21URA supplying heated water from the power unit auxiliary equipment (РСВ) directly to the water storage basin of the cooling tower. At an elevation of +0.500 in the area of axes, 1-2 pipelines 800 mm in diameter enter the isolated water storage basin area from the cooling system for unessential consumers of refrigerators building. The pipe is opened only when the cooling tower is out of operation and delivers heated water into a special spraying system located over the water level in the isolated water storage basin area at an elevation of +0.500, which is a pipe of variable cross section 800-600-400 mm from where operating pipelines 200 mm and 150 mm in diameter branch out in two directions with splay nozzles installed in them. Studies showed relative humidity is higher there [3]. The cooled water is supplied to the pump station from the isolated area by a pipeline 1200 mm in diameter. In order to purge the circulation system as well as empty the water storage basin, a purge and overflow chamber is placed.

(a) Pipeline network embedded in concrete frame

 (b) Isolated pipe network

Figure 3: Water distribution system of the cooling tower

COOLING FILL

At an elevation of +11.100 a cooling fill is located where the main cooling of water occurs as shown in figure 4. The cooling fill provides for the following:

-                sufficient area of the cooled surface with optimum aerodynamic resistance;

-                installation with the full absence of visible crevices and leaks between cooling fill blocks and structures along the area of the cooling tower;

-                mechanical treatment of blocks (cutting) to change their geometrical dimensions while placed on the installation point;

-                keeping geometry and dimensions of cooling fill blocks with the account of the impact of cooling water flow, own weight, and possible sediments;

-                nonsusceptibility to deformation within the temperature range up to +60 °С;

-                nonsusceptibility to fouling;

-                ease of cleaning by water jets.

In terms of fire safety cooling fill, material parameters shall not be lower than those of combustibility group Г2 (hardly combustible) (as per ГОСТ 30244-94), flammability group В2 (medium flammable) (as per ГОСТ 304002-96). Cooling fill constitutes blocks laid directly on support mesh reinforcements which, in turn, are installed on reinforced concrete beams. Cooling fill blocks cover all the area of a cooling tower. Water is distributed over the cooling fill surface as a thin film, and the heat is transferred into the air which passes through the tower, mainly due to evaporation from the film surface. Height of the cooling fill – 1.8 m.

Figure 4: Cooling feel and Drift eliminator

 DRIFT ELIMINATOR

The drift eliminator is located at an elevation of +14.600 and is intended to a reduction of drop entrainment from cooling towers. Drift eliminator design provides for the following:

-                Maximum efficiency of drops entrainment at nominal aerodynamic resistance (drop entrainment from cooling tower shall not exceed 0.002 % of circulation water flow);

-                installation with total lack of through gaps and leaks between drift eliminator blocks and their interface with stack jacket;

-                mechanical treatment of blocks (cutting) to change their geometrical dimensions while placed at the designed elevation during installation;

-                unchangeability of elements and blocks geometry;

-                nonsusceptibility to deformation within temperature range up to +60 °С;

-                nonsusceptibility to fouling.

The drift eliminator consists of channeled panels assembled in blocks with the use of polypropylene fixtures. Panels are installed at the level of main concrete beams and thus are easily dismantled to provide access to water distribution pipelines. Availability of drift eliminator allows practical elimination of the unwanted impact of cooling towers on the environment.

COOLING ABILITY INDICES

Table 1 shows cooling water temperatures provided by the cooling tower for monthly average meteorological factors in terms of a year (50% probability for a year). As seen from the table, cooling water temperature for monthly average conditions of warm period changes from 21.8 °С up to 31.6 °С, and the average annual temperature of cooled water is 28.5 °С.

Water losses

In the cooling tower system located in the mass transfer lab, warm water is brought into contact with unsaturated air over the surface of packing plates. During this process,  part of the water evaporates, lowering the water temperature [4]. Irretrievable losses of water in a natural draught cooling tower include as follows:

-       evaporation losses;

-       losses due to drops entrainment by airflow through the outlet section of the tower.

Quantity of evaporation losses is defined by the following empirical formula, obtained as a result of field observations on a wide range of existing cooling towers:

q _ev =(0.1+0.002Θ) Δt, where

q _ev - quantity of evaporated water losses in percent of water consumption per cooling tower;

Θ – temperature of outside air by dry thermometer, ^оС;

Δt – temperature drop in cooling tower, оС.

Drop entrainment over the cooling tower top when using high-performance polymeric drift eliminators is accepted based on the results of simulation studies, and equals 0.002% of water consumption per cooling tower.

Irretrievable losses of water from the cooling tower for a single power unit are given in Table 2.

IMPACT ON ENVIRONMENT

The evaporating tiny water droplets carrying bacteria are airborne to the nearby localities and spread pneumonia and acute respiratory diseases. It is a known negative impact of cooling towers on the environment and public health when due attention is not given to tower maintenance during plant operation. In the case of Rooppur NPP, it will be prevented by the application of biocide chemicals and regular cleaning of towers.

CURRENT STATUS: ON OUR JOURNEY TO 175 METERS

The Joint Stock Company ‘Atomstroyexport’, of the Russian Federation, has been awarded the prestigious contract for the performance of construction and erection works of. Natural Draught Cooling Towers along with other associated facilities at Rooppur NPP. There are two Natural Draft Cooling towers in each unit. The total relative height of each cooling tower is 175 m.  For concrete works self-climbing formwork “DOKA SK-175” is being used which consists of 120 numbers of the segment (the inner side 60 pcs. And the outer side 60 pcs). This jump formwork climbs a total of 1500 mm for each lift in 5 stages.

Till 21/09/22 finished concreting height of cooling towers are the following:

11URA- 105.26 m     ( 60% of shell height)

12URA- 96.3 m         ( 55% of shell height)

21URA- 50.75 m       ( 29% of shell height)

     22URA-39.1 m          ( 22% of shell height)

Figure 4: Current View of Cooling Towers at RNPP

CONCLUSION

This paper briefly describes the design parameters and structural systems that were taken into consideration for RNPP cooling towers. The operating parameters, the fill depth, tower inlet height, water flow rate, ambient air temperature, and humidity, and the initial water droplet diameter and distribution in the rain zone, all of these meet the requirements suggested by several researchers for the successful operation of cooling towers [5, 6]. Considering the tropical weather of the country, four cooling towers have been suggested instead of two which will allow better maintenance and consistent operation of the plant. Skilled manpower will be in charge of the operation and maintenance of these cooling towers after they have completed their training from abroad to ensure the safe operation of the Rooppur NPP.

Mohammad Mahmudul Hasan Engineer, CRNPP, Email:mahmudul.hasan143@rooppurnpp.gov.bd; Sujan Kumar DasEngineer, CRNPP, Email:sujan.kumar149@rooppurnpp.gov.bd; Mohammad Shariful Islam, Principal Engineer, CRNPP, Email:shariful.islam222@rooppurnpp.gov.bd & Irtiaz Mahmud, Principal Engineer, CRNPP, Email: irtiaz.mahmud542@rooppurnpp.gov.bd

REFERENCES

 [1] Afshari, F., & Dehghanpour, H. (2019). A review study on cooling towers; types, performance and application. ALKÜ Fen Bilimleri Dergisi, 1-10.

[2] RPR-P0507050101-BAA0003, Design Documentation, Section 5, Data on engineering equipment, service networks, list of engineering and technical measures, contents of process solutions,5.7.5 Service water supply and hydraulic engineering solutions.5.7.5.1 Narrative. Volume 1 General information, Book 3 Natural draught cooling tower, (11URA, 12URA, 21URA, 22URA), Revision B02.

[3] Al-Dulaimi, M. J., Kareem, F. A., & Hamad, F. A. (2019). Evaluation of thermal performance for natural and forced draft wet cooling tower. Journal of Mechanical Engineering and Sciences, 13(4), 6007-6021.

[4] Shublaq, M., & Sleiti, A. K. (2020). Experimental analysis of water evaporation losses in cooling towers using filters. Applied Thermal Engineering, 175, 115418.

[5] Pradhan, P. P. (2022). Design of Cooling Tower. IJNRD-International Journal of Novel Research and Development (IJNRD), 7(5), 262-268.

[6] Hasan, M. R., Rahman, K. R., Shohag, M. R., & Uddin, M. M. (2020). Efficient water management and selection of cooling system for future NPP in Bangladesh. Int. J. Mat. Math. Sci, 2(6), 93-98.