Technological Structures for Air Cooled Condensers

This paper summarizes the knowledge and results obtained in the field of designing technological platforms for energy industry. Optimal solution of the layout of elements and material of a number of technological platforms with a specific number of modules was searched. Technological platforms are the main supporting structure of the air-cooled condensers (ACC), which ensure cooling of plants. The fundament of the solution is based on the schema that the platform is composed from the at least one standardized bed containing the supporting surface equipped with the supporting columns and at least one horizontal segment for the condenser exchanger support. The platform structure must ensure sufficient spatial rigidity and stability and ACC functionality. Design requirements are defined both by size and weight of each single module of condenser and the total number of modules in assembly.


I. INTRODUCTION
Every power source using a turbine set and Rankine cycle requires a cooling source in addition to a heat and steam source.This is where the heat is taken away from steam and the steam is condensed after passing through the turbine.Cooling is usually ensured by a water-cooled condenser.This means that the steam condensation process is provided by cooling water supplied either from an artificial reservoir or from a natural source such as a river.However, this cooling system cannot be used in all areas.The problem arises especially in situations where only a small amount of water is available at a given location, as is the case in dry areas where water is a rare commodity or in countries where strict environmental legislation does not allow water use or when a wet solution is very costly.Further problem arises if there is no suitable wastewater discharge location nearby, and there is a threat of system integrity in winter.In this case, it is ideal to use an air-cooled condenser, which avoids the above complications caused by the use of water [1]- [4].
Dry cooling technologies, which use air as a cooling medium, are more environmentally friendly.The application and use of air-cooled condensers logically save water, which allows us to achieve a greener option to improve care for the environment.These low-emission systems that do not discharge hot water into the sea or rivers do not affect the ecosystem.In addition, dry cooling systems have very low maintenance requirements, which make it possible to Published on November 30, 2019 Z. Říhová is with the Faculty of Civil Engineering, Czech Technical University in Prague, Thákurova 6, 166 29 Prague, Czech Republic (e-mail: zdenka.rihova.1@fsv.cvut.cz).
M. Kočová is with the Faculty of Civil Engineering, Czech Technical University in Prague, Thákurova 6, 166 29 Prague, Czech Republic (e-mail: marketa.kocova@fsv.cvut.cz).minimize overall costs.Individual fans of air condensers that provide air intake are placed in actual operation on a supporting structure which ensures their position in space and allows them to function.Such a structure must be able to support its own weight, the weight of the ACC and the supplementary equipment required for correct operation, the load caused by the operation device and an external load acting on the structure.

II. HISTORY
The first direct air-cooled power plant in North America was the Neil Simpson coal power plant with a 21.7 MW unit in Gillette, Wyoming.It started commercial operation in 1969.Currently, the plant has a capacity of 101.7 MW.Use of air cooling for power plants began to grow only in the late 1990s, when emission control, water control and favorable gas prices strongly influenced the choice of more efficient combined cooling cycles for coal-fired power plants [5].
A large group of power plants installed during the years 2000 -2004 is equipped with air-cooled condensers (ACC).Initial experience with these units has shown technical and operational shortcomings in the design, which have been gradually eliminated.Shortcomings have emerged especially in the control of water and steam chemistry, which is crucial for the reliable operation of the plant [6].
For reliable operation it was necessary to solve the recurrent problems.This was soon realized by owners of ACC-equipped facilities.Sharing technical information and addressing the recurring problems in a timely manner has been key, ensuring a big step forward and saving a lot of costs.Thus, an informal list of ``interested parties'' was created, followed one year later, in 2007, an unofficial interest group was set up as an unofficial subcommittee of the ASME Power Plant and Environmental Chemistry (PPEC) Research Committee [6].This goal was continued in 2009 by NV Energy and the publishers of COMBINED CYCLE Journal, who worked together to create an ACC user list.The expanded exchange of information was mainly related to problematic mechanical components, correction of performance deficiencies, control algorithms to reduce energy consumption, etc.
Another important objective in the development of this list was to strengthen industry's knowledge of the appropriate chemistry to ensure reliable ACC operation and to report on best practices down to the plants staffs with responsibility for the performance of this equipment [6].
Meanwhile, in 1979 the German company GEA, dealing with air-cooled systems, joined the study for the Electricity Supply Commission (Eskom) -A new generation of large dry-cooling power plants.This research led to the design, manufacture, supply, installation and commissioning of six air-cooled steam condensers for the Matimba Eskom power plant (6 x 665 MWe) in South Africa in 1982.Matimba was gradually put into operation from 1988 to 1993 [7].
This application represented the largest direct air-cooled steam condenser in the world by that time.The turnkey contract was awarded for the success of the Matimba power plant in another project.In 1984, Eskom ordered air-cooled condensers for the Majuba power plant (6 x 660 MWe).It was then put into operation in 1996 and of the six units in the power plant, only three units were finally equipped with air-cooled condensers [8].
Another interesting project from the Czech perspective is the construction of two other power plants in South Africa.These are the Medupi power plant (operated in 2015) with an output of 4 764 MW and Kusile (still under construction) with an output of 4,800 MW.The power plant in Medupi and Kusile, once completed, should be the world's largest dry-cooling plants.
Medupi is a coal-fired power plant where the first of the 6 planned units was completed in 2014.An improved design of Matimba and Majuba was made here.During the preparation of the project, numerous modeling of cooling with the influence of different temperatures and wind flow was performed.Hutní montáže Vítkovice was involved into the project of the power plant in Medupi as subcontracted works in the field of the installation of the pressure part of boilers for company Mitsubishi Hitachi Power Systems Africa.
Kusile is also a coal-fired power plant, like Medupi, it is only partially functioning so far.The original planned completion was scheduled for 2018, now the new estimates show 2021.Once completed, its output of 4,800 MW should be the largest coal-fired power plant in the world.The cooling system should be built at a height of 50 meters.
The representative of direct dry cooling in the Czech Republic is for example the Prague waste incineration plant ZEVO Malešice.Construction of the incinerator began in 1988 and ten years later, in 1998, was put into operation.During operation was gradually upgraded its technology to reduce emissions (which now is around 10 percent of the permitted emission limits) and flue gas cleaning.In 2010, the cogeneration unit has been added, allowing the incineration of waste and began using to generate electricity with a maximum power of 17.5 MW.Air cooled condenser is in the form of 6 modules with fans with a diameter of 10 m, which is located at a height of 20 m.During the colder weather are only 4 fans in operation [11].

III. PRINCIPLE OF AIR COOLED CONDENSER
Air condensation of steam is used as all kinds of cooling to remove excess heat generated by technology in all industrial plants.Thus, the air-cooled condenser dissipates low-potential heat when condensing water vapor.
In principle, it is cooling by atmospheric air with forced air flow.The waste steam flows from the turbine through the large diameter pipe to the ACC itself.The surface of the finned tubes is used for heat transfer.Inside them condensing medium -steam -flows and the cooling air flows outside the tubes.The steam condenses into a system of parallel tubes and flows back to the system.Axial fans located in each module ensure the passage of cooling air over the heat exchanger tubes [13].See Fig. 2.

IV. MATERIAL AND CONSTRUCTION
Material choice of structure for technological platform, on which condensers are arranged, depends on several different factors.The most significant of these are, for example, the geographical conditions, the position of the condensers in relation to the building and the total heat output of the plant.The power output defines the number of fans needed for cooling final amount of steam.This then results in the height of the supporting structure.As the number of fans increases, so does the height of the supporting structure, because the fans need sufficient space for the correct function of the so-called suction height.
The following material solutions are based on the above factors and the external location of the building.

A. Steel structures
Steel is the highest quality of commonly used building materials.It has high strength and therefore steel elements are subtle and light.This advantage is also evident during transport and assembly.With steel structures, large spans and heights can be achieved while carrying heavy loads.Steel elements are manufactured in an industrial way, which guarantees high standard of quality.Assembling of steel structures proceeds very quickly and is not limited by climatic conditions.Reconstruction of steel structures is easy and usually there is no need to interrupt the operation and the material is 90 \% recycled.In this sense, steel is a highly environmental-friendly material.The disadvantage o steel structures may be the need to protect elements against corrosion and fire.The protection of steel structures is technically solved, but very expensive.
Thanks to their high strength, steel structures are able to provide a subtle, light and economical structure that, while providing corrosion and fire protection, meets the technical requirements of the ACC technology platform [5].

B. Concrete structure
Concrete is a building material resulting from the hardening of a mixture of cement, aggregate and water.Its properties are influenced by the ratio of components.The special properties of the concrete mixture are achieved by the addition of admixtures and additives.This affects the workability of the concrete mixture as well as the final properties of the concrete.The degree of workability is consistency, which can objectively evaluate the properties of fresh concrete.Concrete is firm and durable, generally used in combination with reinforcement of various types, thereby improving the possibilities of use in the compressive strength and tensile strength; non-reinforced concrete, the so-called "plain concrete", resists well compressive stress, and "reinforced concrete" even in tension.The advantage of concrete is its good formability, corresponding to the formwork used and the possibility of recycling, therefore it is suitable for various applications of structures in civil engineering, also like for part structures made of completely different building materials as foundation.Concrete has become a term of durability during use.However, in order to really meet the demands placed on it; fresh concrete must be produced in the same quality.This is best achieved by production in specialized fresh concrete mass production plants, professional transport of fresh concrete and its professional placement [5].

C. Composite structure
The principle of these structures consists in the rigid connection of steel or timber beams and reinforced concrete slab in horizontal structures.For example, vertical constructions are concreted steel pipes or concreted rolled sections.The combination of steel and concrete allows for optimal use of the advantageous properties of both materials and the elimination of their disadvantages.By suitably arranging these materials in the structure, each material is given a kind of stress for which its specific properties are best applied.This means that the concrete elements are placed in the compressed area of the structure because the concrete is able to carry only minimal tensile stresses.In contrast, the steel elements are placed in the tensile region of the structure.Although steel has almost the same tensile and compressive strength, but todays rolled and welded profiles are characterized by high slenderness, leading to stability problems in the compression area [5].

V. DESIGN OF PLATFORMS
The supporting structures of the ACC were solved in two type series, namely A and B. The module size and the number of needed ACC fans are chosen according to the required capacity of the operation.A units are used for smaller capacities type, where the fan diameter is 5.5 m and the module dimensions of one condenser field are 8 x 8 m.Type B units have a fan diameter of 9.75 m with 12 m x 12 m module dimensions are considered for bigger performance.See Table I and Table II.
During this research was looked for the optimal solution of design of platform, which consisted in a suitable arrangement and size of the individual components so as to achieve maximum efficiency of construction and its elements, either in terms of a minimum of material consumption, and minimal labor requirements or combinations thereof.
As was already mentioned, technology platforms are the main load-bearing structure of air-cooled condensers (ACC) that provide real-life cooling.The platform must therefore ensure sufficient load-bearing capacity, spatial rigidity and stability, as well as ACC functionality.Design requirements are defined both by the size and weight of each single ACC module, and by the total number of modules in the condenser unit and the external environment in which the structure is implemented -climatic loads and seismicity.
The platform structure has been solved in three material variants, namely in steel, concrete and combinations thereof, a composite structure.See Fig. 3.In the steel structure, variants differing in the arrangement of the horizontal bracing were considered.The basic difference in the arrangement of braces is their concentration in one module with adjacent modules without bracing, or vice versa, placing braces in several modules evenly throughout the structure.The concrete structure was designed as a thin slab placed on girders.The solution was considered as fully prefabricated and partially prefabricated variant.The horizontal stiffness is ensured here by bending rigidity and by the columns rigidly fixed into the foundation structure.Composite structure differs from concrete structure only in design of columns.Columns for smaller dimensions are designed as concrete with rolled steel profile (HEA).For larger dimensions, concrete columns are designed as reinforced by steel pipe.For the largest models of composite structure, lateral bracing has been added.
Based on the platform design requirements defined primarily by location, ambient conditions and ACC operation, primary load cases were specified.Thus, the effects of wind, snow, seismicity and loads from the ACC structure were included.In addition, other load cases had to be introduced into the design, such as overpressure loads generated by air intake into the ACC structure.Dynamic load of axial and radial force caused by rotating fan (It was considered eccentric rotating mass according to the supplier's specifications).Exceptional load condition caused by possible destruction of the fan blade or the load of the drive assembly located on service bridge.
As with other climatic loads, the location of the structure in the Czech Republic was considered for seismic loads, which corresponds to an acceleration of 0.1 g.This assumption is in accordance with ČSN EN 1998-1.The seismic load was included in the calculation in simplified equivalent quasi-static load acting at the fan level.II.A variant of the material in which the modification was made is labeled o -steel, b -concrete.The steel variant was considered with the design of steel S235, the profiles used for columns and braces were mainly HEA (for series A) and HEB (for series B) and for less stressed elements IPE.The static scheme of structure contains mostly hinged or semirigid connections and spatial stiffness is ensured by lattice braces.In the concrete variant, the design envisaged concrete of strength class C 40/50 and reinforcement B 500b.Two production options were considered, either monolithic concrete or prefabricated concrete.In monolithic concrete, the connection between the columns and the beams is rigid.In the case of prefabricated structures, it is necessary to ensure the spatial stability of the system what can be done by composite of steel and concrete.

Summary of modeled dispositions units with different variants is shown in Table I and Table
The composite structure of columns can be achieved in several ways.In the case of this construction, a rigid connection at the point of connection of the beam to the column has been considered so far.After the addition of additional load cases, the addition of oblique bracings was considered.It also included a comparison of material solutions of a number of technological platforms -steel, concrete and composite variant, see graphs.
The result graph (see Fig. 4.) shows that the steel variant has the best weight results compared to other material solutions.Although the steel structure is the best in terms of weight, the several times heavier concrete structure is the most advantageous from a cost point of view, see the graph on the Fig. 5.This graph also shows that the steel structure is the most expensive for smaller systems (up to 50 units).At this point we can also notice the emerging trend in the graph when the number of units increases then the steel structure is slightly approaching to the concrete solution in price while the composite variant increases and jumps significantly in price.
These options can be chosen in dependence on the preference of the application: Subtler and lighter steel structure at a higher price, or cheaper but the more massive concrete structure.Composite designs proved to be the most expensive and not very effective.

VII. RESULTS OF THE PROJECT
The result of the project is the utility model application No. D17115253 "Technological platform for condensers" and the industrial design application No. D17115295 "Performance series of direct air condensers".

Zdeňka
Říhová was born in 1989 in Prague, Czech Republic.She received her Bc.degree in 2013 from the Czech Technical University in Prague, specializing in Civil engineering and Ing.degree in 2015 Physical and Material Engineering.At the present she is PhD.student dealing with design optimization of series of technological platforms for energy industry developing concrete and composite variants.Markéta Kočová was born in 1990 in Pribram, Czech Republic.She completed her barchelor degree (2013 -Civil Engineering) and master's degree (2015 -Physical and Material Engineering) at the Faculty of Civil engineering, Czech Technical University in Prague.Currently, she is studying as a PhD.student at Czech Technical University in Prague focusing on analysis of steel platform of energy industry with regard to their lifetime.Author's formal photo Author's formal photo

TABLE I :
MODULAR SERIES OF TYPE A UNITS