MODAL ANALYSIS OF COOLING TOWER

INTERNATIONAL JOURNAL OF PROFESSIONAL ENGINEERING STUDIES Volume 10 /Issue 2 / SEP 2019

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MODAL ANALYSIS OF COOLING TOWER

1 Yaseen Shaik (M.Tech) 2 Dr. Mahesh Mallampati Professor 3 Dr. C. Govinda Rajulu, Head of the Department 1,2,3 Guntur Engineering College, Yanamadala, Andhra Pradesh, INDIA

1shaikyaseen 9999 @Gmail.Com [email protected] [email protected]

ABSTRACT

Natural Draught hyperbolic cooling towers are the characterizing land marks of power station. They contribute both to an efficient energy output & to a careful balance with our environment. These structures are most efficient measures for cooling thermal power plants by minimizing the need of water & avoiding thermal pollution of water bodies. This paper deals with the study of static and dynamic (model) analysis of hyperbolic cooling towers (i.e. self weight, static loads). The boundary conditions considered are Top end free and Bottom end fixed. The material used for cooling tower is concrete.

Three different cooling towers will modeled by using SOLIDWORKS 2016 software i.e:CT1, CT2, CT3, in which CT1 & CT3 are existing Cooling towers and CT2 is newly design intermediate Cooling tower obtain between two existing cooling towers.Static and model analysis Analysis will perform by using ANSYS 16.0 work bench software on self load condition due to gravity , stresses and deformation will find out on cooling tower due to load as static analysis, and frequencies on different deformation mode shapes will find out as model analysis.

1. INTRODUCTION

A cooling tower is a heat rejection device which rejects waste heat to the atmosphere through the cooling of a water stream to a lower temperature. Cooling towers may either use the evaporation of water to remove process heat and cool the working fluid to near the wet-bulb air temperature or, in the case of closed circuit dry cooling towers, rely solely on air to cool the working fluid to near the dry-bulb air temperature. Common applications include cooling the circulating water used in oil refineries, petrochemical and other chemical plants, thermal power stations and HVAC systems

for cooling buildings. The classification is based on the type of air induction into the tower: the main types of cooling towers are natural draft and induced draft cooling towers.

Cooling towers vary in size from small roof-top units to very large hyperboloid structures (as in the adjacent image) that can be up to 200 metres (660 ft) tall and 100 metres (330 ft) in diameter, or rectangular structures that can be over 40 meters (130 ft) tall and 80 meters (260 ft) long. The hyperboloid cooling towers are often associated with nuclear power plants, although they are also used to some extent in some large chemical and other industrial plants. Although these large towers are very prominent, the vast majority of cooling towers are much smaller, including many units installed on or near buildings to discharge heat from air conditioning.

Cooling tower

Hyperbolic cooling towers are large, thin shell reinforced concrete structures which Contribute to power generation efficiency, reliability and to environmental protection. Natural draft cooling tower is one of the most widely used cooling between the air inside the tower and outside the tower. Hyperbolic shape of cooling tower is usually preferred due to its strength and stability and larger available area at the base. Hyperbolic reinforced concrete cooling towers

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are effectively used for cooling large quantities of water in thermal power stations, refineries, atomic power plants, steel plants, air conditioning and other industrial plants. Natural draughts cooling towers (NDCT) is the characterizing landmarks of power stations and are used as heat exchangers in nuclear power plants. They contribute both to an efficient energy output and to a careful balance with our environment. These shell structures are subjected to environmental loads such as Seismic and thermal gradients that is stochastic in nature. A series of a hyperbolic cooling tower

2.CLASSIFICATIONS

Heating, ventilation and air conditioning (HVAC)

Two HVAC cooling towers on the rooftop of a shopping center (Germany)

An HVAC (heating, ventilating, and air conditioning) cooling tower is used to dispose of ("reject") unwanted heat from a chiller. Water-cooled chillers are normally more energy efficient than air-cooled chillers due to heat rejection to tower water at or near wet-bulb temperatures. Air-cooled chillers must reject heat at the higher dry-bulb temperature, and thus have a lower average reverse-cycle effectiveness. In areas with a hot climate, large office buildings, hospitals, and schools typically use one or more cooling towers as part of their air conditioning systems. Generally, industrial cooling towers are much larger than HVAC towers.

HVAC use of a cooling tower pairs the cooling tower with a water-cooled chiller or water-cooled condenser. A ton of air-conditioning is defined as the removal of 12,000 BTU/hour (3500 W). The equivalent ton on the cooling tower side actually rejects about 15,000 BTU/hour (4400 W) due to the additional waste heat-

equivalent of the energy needed to drive the chiller's compressor. This equivalent tonis defined as the heat rejection in cooling 3 US gallons/minute (1,500 pound/hour) of water 10 °F (6 °C), which amounts to 15,000 BTU/hour, assuming a chiller coefficient of performance (COP) of 4.0. This COP is equivalent to an energy efficiency ratio (EER) of 14.

Cooling towers are also used in HVAC systems that have multiple water source heat pumps that share a common piping water loop. In this type of system, the water circulating inside the water loop removes heat from the condenser of the heat pumps whenever the heat pumps are working in the cooling mode, then the externally mounted cooling tower is used to remove heat from the water loop and reject it to the atmosphere. By contrast, when the heat pumps are working in heating mode, the condensers draw heat out of the loop water and reject it into the space to be heated. When the water loop is being used primarily to supply heat to the building, the cooling tower is normally shut down (and may be drained or winterized to prevent freeze damage), and heat is supplied by other means, usually from separate boilers.

3.WET COOLING TOWER MATERIAL BALANCE

Quantitatively, the material balance around a wet, evaporative cooling tower system is governed by the operational variables of make-up flow rate, evaporation and windage losses, draw-off rate, and the concentration cycles.

In the adjacent diagram, water pumped from the tower basin is the cooling water routed through the process coolers and condensers in an industrial facility. The cool water absorbs heat from the hot process streams which need to be cooled or condensed, and the absorbed heat warms the circulating water (C). The warm water returns to the top of the cooling tower and trickles downward over the fill material inside the tower. As it trickles down, it contacts ambient air rising up through the tower either by natural draft or by forced draft using large fans in the tower. That contact causes a small amount of the water to be lost as windage/drift (W) and some of the water (E) to evaporate. The heat required to evaporate the water is derived from the water itself, which cools the water back to the original basin water

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temperature and the water is then ready to recirculate. The evaporated water leaves its dissolved salts behind in the bulk of the water which has not been evaporated, thus raising the salt concentration in the circulating cooling water. To prevent the salt concentration of the water from becoming too high, a portion of the water is drawn off/blown down (D) for disposal. Fresh water make-up (M) is supplied to the tower basin to compensate for the loss of evaporated water, the windage loss water and the draw-off water.

4.CYCLES OF CONCENTRATION

Cycle of concentration represents the accumulation of dissolved minerals in the recirculating cooling water. Discharge of draw-off (or blow down) is used principally to control the buildup of these minerals.

The chemistry of the make-up water, including the amount of dissolved minerals, can vary widely. Make-up waters low in dissolved minerals such as those from surface water supplies (lakes, rivers etc.) tend to be aggressive to metals (corrosive). Make-up waters from ground water supplies (such as wells) are usually higher in minerals, and tend to bescaling (deposit minerals). Increasing the amount of minerals present in the water by cycling can make water less aggressive to piping; however, excessive levels of minerals can cause scaling problems.

Relationship between cycles of concentration and flow rates in a cooling tower

As the cycles of concentration increase, the water may not be able to hold the minerals in solution. When the solubility of these minerals have been exceeded they can precipitate out as mineral solids and cause fouling and heat exchange problems in the cooling tower or the heat exchangers. The

temperatures of the recirculating water, piping and heat exchange surfaces determine if and where minerals will precipitate from the recirculating water. Often a professional water treatment consultant will evaluate the make-up water and the operating conditions of the cooling tower and recommend an appropriate range for the cycles of concentration. The use of water treatment chemicals, pretreatment such as water softening, pH adjustment, and other techniques can affect the acceptable range of cycles of concentration.

Concentration cycles in the majority of cooling towers usually range from 3 to 7. In the United States, many water supplies use well water which has significant levels of dissolved solids. On the other hand, one of the largest water supplies, for New York City, has a surface rainwater source quite low in minerals; thus cooling towers in that city are often allowed to concentrate to 7 or more cycles of concentration.

Since higher cycles of concentration represent less make-up water, water conservation efforts may focus on increasing cycles of concentration. Highly treated recycled water may be an effective means of reducing cooling tower consumption of potable water, in regions where potable water is scarce.

5.WATER TREATMENT

Besides treating the circulating cooling water in large industrial cooling tower systems to minimize scaling and fouling, the water should be filtered to remove particulates, and also be dosed with biocides and algaecides to prevent growths that could interfere with the continuous flow of the water. Under certain conditions, a biofilm of micro-organisms such as bacteria, fungi and algae can grow very rapidly in the cooling water, and can reduce the heat transfer efficiency of the cooling tower. Biofilm can be reduced or prevented by using chlorine or other chemicals.

Legionnaires' disease

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Legionella pneumophila (5000x magnification

Another very important reason for using biocides in cooling towers is to prevent the growth of Legionella, including species that causelegionellosis or Legionnaires' disease, most notably L. pneumophila,[19] or Mycobacterium avium. The various Legionella species are the cause of Legionnaires' disease in humans and transmission is via exposure to aerosols—the inhalation of mist droplets containing the bacteria. Common sources of Legionella include cooling towers used in open recirculating evaporative cooling water systems, domestic hot water systems, fountains, and similar disseminators that tap into a public water supply. Natural sources include freshwater ponds and creeks.

French researchers found that Legionella bacteria travelled up to 6 kilometres (3.7 mi) through the air from a large contaminated cooling tower at a petrochemical plant in Pas-de-Calais, France. That outbreak killed 21 of the 86 people who had a laboratory-confirmed infection.

Drift (or windage) is the term for water droplets of the process flow allowed to escape in the cooling tower discharge. Drift eliminators are used in order to hold drift rates typically to 0.001–0.005% of the circulating flow rate. A typical drift eliminator provides multiple directional changes of airflow to prevent the escape of water droplets. A well-designed and well-fitted drift eliminator can greatly reduce water loss and potential for Legionella or water treatment chemical exposure.

Many governmental agencies, cooling tower manufacturers and industrial trade organizations have developed design and maintenance guidelines for preventing or controlling the growth of Legionella in

cooling towers. Below is a list of sources for such guidelines:

Centers for Disease Control and Prevention (CDC) PDF (4.99 MB) - Procedure for Cleaning Cooling Towers and Related Equipment (pages 225 and 226)

Cooling Technology Institute PDF (240 KB) - Best Practices for Control of Legionella, July, 2006

Association of Water Technologies PDF (964 KB) - Legionella 2003: An Update and Statement

California Energy Commission PDF (194 KB) - Cooling Water Management Program Guidelines For Wet and Hybrid Cooling Towers at Power Plants

SPX Cooling Technologies PDF (119 KB) - Cooling Towers Maintenance Procedures

SPX Cooling Technologies PDF (789 KB) - ASHRAE Guideline 12-2000 - Minimizing the Risk of Legionellosis

SPX Cooling Technologies PDF (83.1 KB) - Cooling Tower Inspection Tips {especially page 3 of 7}

Tower Tech Modular Cooling Towers PDF (109 KB) - Legionella Control

GE Infrastructure Water & Process Technologies Betz Dearborn PDF (195 KB) - Chemical Water Treatment Recommendations For Reduction of Risks Associated with Legionella in Open Recirculating Cooling Water Systems

6.INTRODUCTION TO DYNAMIC ANALYSIS

Earthquakes are caused by faulting, a sudden lateral orvertical movement of rock along a rupture (break) surface.The surface of the Earth is continuous slow motion. This isplate tectonics--the motion of immense rigid plates at thesurface of the Earth in response to flow of rock within theEarth. The plates cover the entire surface of the globe. Sincethey are all moving they rub against each other in someplaces, sink beneath each other in others, or spread apart fromeach other. At such places the motion isn't

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smooth the platesare stuck together at the edges but the rest of each plate iscontinuing to move, so the rocks along the edges are distorted(what we call "strain"). As the motion continues, the strainbuilds up to the point where the rock can’t withstand any morebending. With a lurch, the rock breaks and the two sidesmove. An earthquake is the shaking that radiates out from thebreaking rock. Unfortunately, timing of this naturalphenomenon cannot be predicted scientifically. Historicalrecords reveal the tendency of earthquakes to revisit regionsafter an interval of time. This random time interval iscalled RETURN PERIOD. This is the basis of the seismicconation. There are four zones in the country and they aredenoted as II, III, IV and V. Zone I which existed in theearlier versions of the code, has been upgraded to Zone IIor higher. The higher the zone, the more vulnerable is thatregion to a major earthquake. The size of an earthquake ismeasured by the strain energy released along the fault. It isexpressed as MAGNITUDE. The commonly used scale forexpressing the magnitude is the Richter scale. Every unitincrease in magnitude implies an increase of about 31times the energy. Dynamic analysis may be performedeither by the Time History Method or by the ResponseSpectrum Method. For cases where a more refined designanalysis is desired, response spectra are used as the meansfor determining lateral forces. A Response spectrum for aparticular earthquake shows in a relatively simple way thedynamic characteristics of a given earthquake.

Generation of Response Spectra

For the design of RC structures for seismic loading adesign spectrum is obtained as per the recommendations ofIS 1893 (Part1): 2002 titled “Criteria for EarthquakeResistant Design of Structures”. The parameters consideredare type of soil, type of construction, the dynamic behaviorof the prototype structure and the appropriate seismic zone.The earthquake spectrum is an average smoothened plot ofmaximum acceleration as function of frequency or timeperiod of vibration for a specified damping and for a sitespecific condition. According to the code, India isclassified into four seismic zones i.e. Zone II, Zone III,Zone IV and Zone V. The code specifies forces foranalytical

design of structures standing on rocks or soil forabove four zones and different value of damping of thestructure. For the purpose of design acceleration spectrumhas been prepared for zone III assuming damping as 5%and the soft soil condition. 7.SOLID WORKS

Solid Works is mechanical design automation software that takes advantage of the familiar Microsoft Windows graphical user interface.

It is an easy-to-learn tool which makes it possible for mechanical designers to quickly sketch ideas, experiment with features and dimensions, and produce models and detailed drawings. A Solid Works model consists of parts, assemblies, and drawings.

Typically, we begin with a sketch, create a base feature, and then add more features to the model. (One can also begin with an imported surface or solid geometry).

We are free to refine our design by adding, changing, or reordering features.

Associatively between parts, assemblies, and drawings assures that changes made to one view are automatically made to all other views.

We can generate drawings or assemblies at any time in the design process.

The Solid works software lets us customize functionality to suit our needs. 8.MODELING OF COOLING TOWER:

Bellary thermal power station (BTPS) is a power generating unit near Kudatini village inBellary district, Karnataka state. India. Two existing cooling towers are considered as case study as Shownin Fig 1 & 2. BTPS is geographically located at 15º11’58” N latitude and 76º43’23” E longitude. Details of existing cooling towers 1) The total height of the tower is 143.5 m. The tower has a base, throat and top radii of 55 m, 30.5 mand 31.85 m respectively, with the throat located 107.75 m above the base. (Unit No- 2 Coolingtower in BTPS) 2) The total height of the tower is 175.5 m. The tower has a base, throat and top radii of 61 m,

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34.375m and 41.00m respectively, with the throat located 131.60 m above the base (Unit No- 3 coolingtower in BTPS).

Modeling of CT2:

Modeling of CT3:

9.INTRODUCTION TO SIMULATION

Simulation is a design analysis system. Simulation provides simulation solutions for linear and nonlinear static, frequency, buckling, thermal, fatigue, pressure vessel, drop test, linear and nonlinear dynamic, and optimization analyses.

Powered by fast and accurate solvers, simulation enables you to solve large problems intuitively while you design. Simulation comes in two bundles: simulation professional and simulation premium to satisfy your analysis needs. Simulation shortens time to market by saving time and effort in searching for the optimum design.

simulation example

10.ANALYSIS ON COOLING TOWER:

Material properties: Concrete

Cooling tower - 1 analysis

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Model

Material

Meshing

Fixed support

Self load condition (due to gravity)

Stress

Strain

Deformation

Modal analysis

Model analysis is done with ansys work bench with six mode shape. Natural frequencies and deformation values are evaluated through modal analysis.

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Modal analysis for existing cooling tower and obtained frequencies and deformation values are listed.

Mode1

Mode 2

Mode 3

Mode 4

Mode 5

Mode 6

Graph


Model

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Material

Meshing

Fixed support


Stress

Strain

Deformation

Modal analysis

Mode1

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Mode 2

Mode 3

Mode 4

Mode 5

Mode 6

Graph


Model

Material

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Meshing

Fixed support


Stress

Strain

Deformation

Mode1

Mode 2

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Mode 3

Mode 4

Mode 5

Mode 6

Graph

11.RESULTS TABLES

TABLE : STATIC RESULT

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CONCLUSION:

Design and analysis of cooling tower is done. Brief study about cooling tower , its applications

and types is done in this project Modeling of cooling tower is done in solid works

2016 design software by using different commands and tools.

Three different geometrical cooling towers (CT1, CT2, and CT3) are modeled of different heights and thicknesses.

The models are saved as IGES (neutral) files to import in ansys.

Concrete is selected as the material for cooling towers.

Static structural analysis is performed by applying self weight of the cooling tower i.e.: due to gravity, stress, strain and deformation due to load is obtained for each cooling tower.

MODAL analysis is performed on cooling tower by fixing it with ground , 6 different deformation modes shapes with respective frequencies are obtained as the result for each cooling tower.

Stresses, strain, deformation values and frequencies are noted and tabulated.

From the static result table we can conclude that as the height increases the stress are also increases.

From the modal analysis table we can conclude that as the height of the cooling tower increase the natural frequency will decrease.

0References [1] G. Murali, C. M. Vivek Vardhan and B. V. Prasanth Kumar Reddy “RESPONSE OF COOLING TOWERS TO WIND LOADS”, ARPN Journal of Engineering and Applied Sciences, VOL. 7, NO. 1, JANUARY 2012 ISSN 1819-6608 [2] A. M. El Ansary, A. A. El Damatty, and A. O. Nassef, “Optimum Shape and Design of Cooling Towers”, World Academy of Science, Engineering and Technology 60 2011. [3] Shailesh S. Angalekar, Dr. A. B. Kulkarni, “Analysis of natural draught hyperbolic cooling tower by finite element method using equivalent plate method”. International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 1, Issue 2, pp.144-148 [4] Prashanth N, Sayeed sulaiman, “To study the effect of seismic loads and wind load on hyperbolic cooling tower of varying dimensions and RCC shell thickness” International Journal of Emerging Trends in Engineering and Development Issue 3, Vol.4 (June-July 2013) ISSN 2249-6149. [5] N Prabhakar (Technical Manager), “Structural aspects of hyperbolic cooling tower”, National seminar on Cooling tower, jan1990, Technical session IV, paper no 9 [6] IS: 11504:1985, Criteria for structural design of reinforced concrete natural draught cooling tower, New Delhi, India: Bureau of Indian standards. [7] IS: 875 (Part3):1987, Code of practice for design loads (other than earthquake loads) for buildings and structures. New Delhi, India: Bureau of Indian Standards. [8] IS 1893 (part 1): 2002 Criteria for earthquake resistant design structure. [9] IS 1893 (part 4): 2005 Criteria for earthquake resistant design Part-4 Industrial structures including stack-like structures.

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MODAL ANALYSIS OF COOLING TOWER