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Raghv pandey tata steel project

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BAF Heat Exchanger

BAF Heat Exchanger

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Raghv pandey tata steel project Raghv pandey tata steel project Document Transcript

  • 8-Jun-13 PROJECT REPORT ON: Efficiency improvement and failure prevention of BAF heat exchanger. SUBMITTED BY: RAGHVENDRA KUMAR PANDEY VT20130074 UNDER GUIDANCE OF: Mr. P.KAISER Mr. PRAKASH Senior Manager CRM Dept. TATA Steel BAF/ECL/SPM
  • 8-Jun-13 May -June 2013 CERTIFICATE This is to certify that following students have successfully completed the project on “EFFICIENY IMPROVEMENT AND FAILURE PREVENTION OF BAF HEAT EXCHANGER” as a part of Vocational Training at CRM complex, TATA STEEL, Jamshedpur during May14, 2013 to June 12, 2013 under our guidance. They have successfully completed this project and worked very sincerely to our satisfaction. RAGHVENDRA KUMAR PANDEY
  • 8-Jun-13 Mr. P. KAISER Senior Manager BAF /ECL/SPM ACKNOWLEDGEMENT “Outstanding achievement is not possible in hydraulic. It needs lot of help and assistance besides a healthy environment, luckily I have.” First and foremost, I would like to express my hearty thanks and indebtedness to Mr. PERWAIZ KAISER for his enormous help and encouragement throughout the project. It would have been impossible for me to have a clear idea and way of approach without his support. I am thankful to all the employees of the department who stood by my side to help me out in the understanding of the concepts and techniques in the process. View slide
  • 8-Jun-13 I am also thankful to SNTI department for allowing me to register for the project. CONTENTS: 1. INTRODUCTION a. About Tata Group b. About Tata Steel c. Introduction to Steel Manufacturing and processes at Tata steel. 2. Safety induction 3. Cold Rolling Mill a. Introduction b. Basics c. Pickling & Tandem Cold Mill (PLTCM) d. Batch Annealing Furnace (BAF) e. Flow chart of B.A.F f. Skin Pass Mill (SPM) g. Galvanizing Lines (CGL) 4. About Heat exchanger 4.1 Type of Heat exchanger View slide
  • 8-Jun-13 4.2 Type of flow 5. Project outline 6. About BAF heat exchanger 6.1 Design analysis 6.2 Effectiveness improvement 7. Failure of heat exchanger 7.1 Failure statement of heat exchanger tube 7.2 Suspected reasons for failure of a heat exchanger 7.3 Methods of failure prevention 8. Conclusion & Reference INTRODUCTION: About Tata Group TATA group of companies has always believed strongly in the concept of collaborative growth, and this vision has seen it emerge as one of India’s and world’s most respected and successful conglomerates. The Tata Group has traced a route of growth that spans through six continents and embraces diverse cultures. The total revenue of Tata companies, taken together, was $83.3 billion (around Rs3, 796.75 billion) in 2010-11, with 58 per cent of this coming from business outside India. In the face of trying economic challenges in recent times, the Tata Group has steered India’s ascent in the global map through its unwavering focus on sustainable development. Tata companies employ over 425,000 people worldwide. It is the largest employer in India in the Private Sector and continues to lead with the same commitment towards social and community responsibilities that it has shown in the past.
  • 8-Jun-13 The Tata Group of Companies has business operations (114 companies and subsidiaries) in seven defined sectors – Materials, Engineering, Information Technology and Communications, Energy, Services, Consumer Products and Chemicals. Tata Steel with its acquisition of Corus has secured a place among the top ten steel manufacturers in the world and it is the Tata Group’s flagship Company. Other Group Companies in the different sectors are – Tata Motors, Tata Consultancy Services (TCS), Tata Communications, Tata Power, Indian Hotels, Tata Global Beverages and Tata Chemicals. About Tata Steel
  • 8-Jun-13 Tata Steel - Asia's first and India's largest integrated private sector steel company began with a modest capacity of two 200 - tons blast furnaces, four 4 -tons steam-driven blooming mills and a rail and structural mill. Today it stands as a state-of-the-art 3.5 million tons steel plant capable of making the most rigorous demands of its customers. Established in 1907, Tata Steel is among the top ten global steel companies with an annual crude steel capacity of over 28 million tonnes per annum (mtpa). It is now one of the world's most geographically-diversified steel producers, with operations in 26 countries and a commercial presence in over 50 countries. The Tata Steel Group, with a turnover of US$ 26.13 billion in FY 2011- 2012, has over 81,000 employees across five continents and is a Fortune 500 company. Tata Steel’s vision is to be the world’s steel industry benchmark through the excellence of its people, its innovative approach and overall conduct. Underpinning this vision is a performance culture committed to aspiration targets, safety and social responsibility, continuous improvement, openness and transparency. Tata Steel’s larger production facilities include those in India, the UK, the Netherlands, Thailand, Singapore, China and Australia. Operating companies within the Group include Tata Steel Limited (India), Tata Steel Europe Limited (formerly Corus), NatSteel, and Tata Steel Thailand (formerly Millennium Steel). TATA Steel is one of the lowest cost steel producer as well as it is supplier of high-end steel like supplied to auto OEM. Tata Steel is one of the bestintegrated steel plants and is having facilities right from raw material production to finishing stage rolling.
  • 8-Jun-13 INTRODUCTION TO STEEL MANUFACTURING: Steel making is basically done in three stages: 1. Iron making process 2. Conversion to steel 3. Rolling Process Steel is manufactured by injecting oxygen through liquid iron. This is done in huge vessels known as LD (Linz Donnawitz) vessel. In this process oxygen is passed at supersonic velocity. The oxygen used is 99.5 % pure. This is done to reduce the carbon content in iron to less than 2%. Flow dia. below describes the steel manufacturing process at TATA Steel.
  • 8-Jun-13 Sinter plant:Iron ore fines are not suitable for use in the Blast Furnace. Hence, the iron ore fines are agglomerated into larger porous lumps, which is suitable for use in the Blast Furnace. A green mix of carefully proportioned iron ore fines, fluxes and coke breeze is prepared in granular form in Mixers. Heat generated through combustion within the mass itself produces large lumps of hot Sinter. This Sinter is cooled, sized and stored for use in the blast furnace.
  • 8-Jun-13 Iron ore fines are recycled to make sinter, to help produce hot metal of predictable and standard quality in the Blast Furnace. Sinter plant layout Coke plant:-
  • 8-Jun-13 Naturally found coal contains fixed carbon (FC), Volatile matter (VM), Ash, Moisture and other impurities. Its poor crushing strength and the volatile matter content makes it unsuitable for use in Blast Furnace. Hence, naturally found coal is converted into coke in the coke oven for use in Blast Furnaces. Heating coal in the absence of air carbonising it to form a hard porous mass, devoid of volatile matter produces coke. Coal after carbonization, which gives blast furnace quality coke, is called metallurgical coal. Coal is graded as prime, medium and blend able, based on its coking properties. Blending of different grades of coal is necessary in order to conserve metallurgical coal, yet ensure uniform coking properties. Currently, there are 6 batteries of coke oven supplying coke to the blast furnaces. The coke plant blends coal from different sources, converts coal to coke and cuts to the correct size for use in the blast furnace. Coke plant layout
  • 8-Jun-13 Blast Furnace:The Blast Furnace is a ceramic refractory lined tall reactor, used for the production of liquid iron called Hot Metal. Iron oxide, present in the iron bearing raw materials, is reduced inside the reactor by coke and carbon monoxide. Coke is used for combustion to attain the high temperatures required for reduction. Coke on combustion generates carbon monoxide, which acts as the reducing agent and converts the iron oxides into molten iron. Fluxes are used to make low melting slag and control the quality of Hot Metal. Hot Metal and Slag are collected in the hearth and tapped periodically. Blast Furnaces A, B, C, D, E and F together produce 2.8 million tons of hot metal, annually. G-Blast Furnace produces 1.30 million tons of hot metal annually. Blast Furnace F has been rebuilt in 2002 to enlarge its capacity to 1 million tonnes. Blast Furnaces are used for producing Hot Metal.
  • 8-Jun-13 BLAST FURNACE LAYOUT LD#1:Hot Liquid Iron (commonly called Hot Metal in Tata Steel) is converted to Steel in the Steel Melting Shops. Hot Metal from the Blast Furnace is stored in Mixers in LD#1 shop. The Hot Metal is converted to Steel in the LD converters by removing its carbon, silicon, sulphur and phosphorous contents. The liquid steel from the converter is converted to billets using Continuous Casting machine. A small portion of steel is teemed into ingots through Bogie Bottom Poring process using cast iron moulds. The liquid steel is treated in on line purging, Ladle Refining Furnace or Argon Rinsing station before continuous casting. Special grades of steel, which are cast as ingots, are processed in On-line purging, followed by the Vacuum Arc degassing & refining unit.
  • 8-Jun-13 The Steel Melting Shop requires an Oxygen Plant to cater to the requirement of oxygen for steel making. The Lime Calcining Plant and the Tar Dolo Plant are auxiliary units required for the manufacture of Steel. Hot metal is converted to Steel and cast into Billets. LD#1 LD#2:- The LD#2 Shop has three Converters of 140 tons capacity each, producing 2.6 million tons of crude steel per annum. Hot Metal is brought from A, D, E, F and G Blast Furnaces in Torpedo ladles. The metal from the Torpedo ladle is taken into the Hot Metal for Desulphurisation. It is then charged into the vessel.
  • 8-Jun-13 Primary refining of steel is done in the Ladle Furnace (LF) and RH Degasser (RH) to make cleaner steel of different value added grades. LD 2 makes superior & cleaner grades of steel required to process Flat products of world-class standards. LD#2 Hot strip milling:-
  • 8-Jun-13 HSM.mp4
  • 8-Jun-13 Wire mill (WRM):- WRM
  • 8-Jun-13 WRM.mp4 Merchant mill (MM):-
  • 8-Jun-13 MM
  • 8-Jun-13 2. SAFETY INDUCTION:What is PPE? PPE is defined in the Regulations as ‘all equipment (including clothing affording protection against the weather) which is intended to be worn or held by a person at work and which protects him against one or more risks to his health or safety’, e.g. safety helmets, gloves, eye protection, high visibility clothing, safety footwear and safety harnesses. Hearing protection and respiratory protective equipment provided for most work situations are not covered by these Regulations because other regulations apply to them. However, these items need to be compatible with any other PPE provided. Cycle helmets or crash helmets worn by employees on the roads are not covered by the Regulations. Motorcycle helmets are legally required for motorcyclists under road traffic legislation. The hazards and types of PPE Eyes Hazards: chemical or metal splash, dust, projectiles, gas and vapor, radiation. Options: safety spectacles, goggles, face shields, visors.
  • 8-Jun-13 Head Hazards: impact from falling or flying objects, risk of head bumping, hair entanglement. Options: a range of helmets and bump caps. Breathing Hazards: dust, vapor, gas, oxygen deficient atmospheres. Options: disposable filtering face piece or respirator, half or full face respirators, aired helmets, and breathing apparatus. Protecting the body Hazards: temperature extremes, adverse weather, chemical or metal splash, spray from pressure leaks or spray guns, impact or penetration, contaminated dust, excessive wear or entanglement of own clothing. Options: conventional or disposable overalls, boiler suits, specialist protective clothing, e.g. chainmail aprons, high visibility clothing.  Hands and arms Hazards: abrasion, temperature extremes, cuts and punctures, impact, chemicals, electric shock, skin infection, disease or contamination. Options: gloves, gauntlets, mitts, wrist cuffs, armlets.  Feet and legs Hazards: wet, electrostatic buildup, slipping, cuts and punctures, falling objects, metal and chemical splash, abrasion.
  • 8-Jun-13 Options: safety boots and shoes with protective toe caps and penetration-resistant mid-sole, gaiters, leggings.
  • 8-Jun-13 3.COLD ROLLING MILL • Introduction CRM.mp4 The HR Coil coming from HSM is Cold Rolled because 1. Shape and Profile are better in CR as compared to HR. 2. Surface finish in HR Coil is not good. 3. Gauge accuracy of HR is not good. The Cold Rolling Mill (CRM) of TATA Steel is fully automated. The CRM uses Hot Rolled coil as its input and produces Cold Rolled galvanized and Cold Rolled Annealed coils. Reputed makers like Hitachi, Mitsubishi, IHI, LOI, CMI, ABB, SIEMENS etc have supplied equipments.
  • 8-Jun-13 Cold rolled steel sheets are produced by way of series of cold rolling processes, using hot rolled steel coils as the starting material. In the cold strip mill, the scale on the hot rolled coils is removed by the process called pickling (producing a product referred to as pickled coil). The pickled coil is then cold rolled at room temperature to a thickness of 0.15 to 3.2 mm. The cold rolling process is followed by the annealing process and temper rolling. Compared to hot rolled sheets, cold rolled sheets are thinner, Have superior thickness accuracy, and a smooth, attractive surface. Cold rolled sheets are also superior in mechanical properties, especially in workability. These features mean that cold rolled sheets are suitable for application which require small thickness of less than 1.2 mm, which is outside the conventional size range of hot rolled steels sheets, and require not only stricter thickness accuracy but also an attractive surface and excellent formability. Cold rolled steels sheets are also used as the substrate for various kinds coated steels sheets such as zinc galvanized steel sheets, tin mill black plates, and others
  • 8-Jun-13 CRM OVERALL FLOW CHART
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  • 8-Jun-13 A-BAsICs Of CRM 1. The hot strip mill sends hot rolled coil (thickness 2 to 6 mm) width (800-1500mm) to the CRM. 2. First the hot coil passes through the pickling section containing HCL acid in order to remove scales, making them ready for cold rolling. 3. Trimming section where the edges of the pickled hot rolled coils are trimmed, if needed. 4. The coils are then fed into the main mill, viz. Tandem Cold Mill (TCM) (6 Hi, 5 stand mill). The CR strip thickness is 0.25 to 3.2 mm. The strip thickness and shape is controlled within a very tight tolerance. 5. After cold rolling, the strip is either processed through annealing (BAF) + skin passing (SPM) or through any of the two galvanizing lines. 6. Part of coils which are sent through BAF + SPM, are first processed in ECL. 7. About 2/3rd of CR goes to the Batch Annealing Furnaces (BAF) where the coil are stacked, covered and heated in closed atmosphere in a 100% hydrogen environment. This process improves the mechanical properties of the strip and helps in stress relieving. These coils are then processed through Skin Pass Mill (SPM). Skin passing removes Luder bands, impart desired surface finish, and improve flatness. 8. About 1/3rd of the production from PL-TCM goes to the Galvanizing Lines (CGL#1 & CGL#2), where the zinc coating is applied on the strip by taking it through molten zinc bath. This zinc coating gives a sacrificial layer on the cold rolled strip for corrosion protection. 9. Most of the coils are then taken through recoiling & Inspection lines (RCL 1, 2 & 3) and are then packaged either manually or through coil packaging line.
  • 8-Jun-13 B-PICkLING & TANDeM COLD MILL (PLTCM) Pickling is an important stage in the steel processing sequence. After hot rolling prior to cold rolling of strip. The aim of this operation is to eliminate oxide formed on the surface during hot rolling. Pickling consist of treatment of the surface of the strip with inorganic acid either hydrochloric or sulphuric. In order to improve the efficiency of pickling it is preceded by mechanical de scaling (with help of tension leveller) to crack and partially eliminate the oxides. The pickling process then removes the remaining oxides and leaves a clean surface. Pickling rate is faster with Hydrochloric Acid (HCL) then sulphuric acid. Owing to enhancement of productivity along with other advantage we use HCL acid as the pickling medium. Various types of baths / tanks are used for pickling, for Tata steel CRM shallow bath HCL continuous pickling has been chosen as the pickling process.Major purposes of pickling  To eliminate scales from hot rolled strip.  Improve cold rolling operatibility.  Ensure the appearance of strip.  Improve the efficiency and quality of processes downstream, the size of coil increased by welding.  A strip is trimmed on both the edges to a specified width.
  • 8-Jun-13  Any portions of strip that are bound to be detrimental to downstream processes or that to turn defective are eliminated. PROCESS FLOW OF PL-TCM C-COLD ROLLING MILL After the hot rolled coil surface is free from scale; it is rolled at room temperature, referred to as cold rolling. For cold rolling mill complex it has been decided to go in for a coupled pickling line (with shallow bath pickling) and a 5-stand tandem mill (each 6-hi). This line has the capability to produce strips with thickness accuracy better than +- 1%.
  • 8-Jun-13 TANDEM COLD MILL D-BATCh ANNeALING fuRNACe (BAf) After cold rolling, annealing is done to soften the sheet for obtaining superior mechanical properties for deep drawing application. Annealing process restore ductility increases internal stresses and restores formability of the cold rolled strips. Two widely different annealing technologies are a) Batch or box annealing Furnace (BAF) b) Continuous annealing For aluminium killed low carbon steels, which form a major portion of the product-mix, Batch Annealing has a distinct advantage over continuous annealing.
  • 8-Jun-13 For the Tata steel’s cold rolling complex, 100% batch annealing has been chosen as the annealing route. The batch annealing has following advantages. a) b) c) d) e) BAF PLANT Development of 100% H2 batch annealing Flexibility in input raw material Process simplicity Economic lot size Flexibility in capacity addition.
  • 8-Jun-13 B.A.F
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  • 8-Jun-13 B.A.F FLOW CHART
  • 8-Jun-13 STEPS INVOLVED IN B.A.F:-  Rolled steels are placed on the base and each rolled steels are separated with the help of convector.  After that inner cover was placed on the rolled coils.  N2 gas is passed through the inner cover to make the atmosphere inert by the removal of air.  After that N2 gas was replaced by H2 gas.  When inner cover is filled with H2 gas then heating hood is placed over the inner cover.  After that the process of heating was done with the help of COG, compressed air & spark.  When the heating was completed then the heating hood was removed & apply cooling hood.  The process of cooling was performed with the help of blower.  When the cooling is completed then cooling hood was removed carefully with the help of crane.  10. Finally, inner cover was removed and the steel coils were passed to CCSU.
  • 8-Jun-13 REASONS FOR USING CO GAS AS BURNER AND H2 FOR FURNACE:-
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  • 8-Jun-13 e-skIN PAss MILL (sPM) It is necessary to impart a small amount of deformation (typically between 0.5 to 1.5%, depending on the grade) to annealed steel strips for primarily a) Preventing stretcher-strains mark during subsequent forming application b) Imparting the desired surface texture (roughness) on the strip. For CRM complex a skin pass mill of 1 mtpa capacity has been selected having following facility. a) 4 hi single stand mill with work roll bending b) Wet type skin passing (using temper agent) c) Work roll texturing using the Electrode Discharge texturing (EDT) method. Skin Pass Mill
  • 8-Jun-13 f- GALvANIzING LINes (CGL) Galvanizing the coils protects steel sheets from corrosion attack by acting as a barrier between steel and the environment. Zinc and Zinc alloys have the ability to react at scratches and other damages through an electrochemical (galvanic) action between steel and zinc. In the galvanic process zinc sacrifices itself to protect structural integrity of the steel. TWO LINES 1) One for construction( CGL # 1) 2) One for auto/ white goods (CGL #2). CGL#1
  • 8-Jun-13 CGL#1 FLOW CHART CGL#2FLOW CHART
  • 8-Jun-13 4 ABOuT heAT exChANGeR: The process of heat exchange between two fluids that are at two different temperatures and separated by solid wall occurs in many engineering applications. The device used to implement this exchange is termed as ‘Heat Exchanger’, a specific application may be found in space heating, refrigerator and air conditioners, automobile industry, power production and waste heat recovery. The classic example of a heat exchanger is found in an internal combustion engine in which a circulating fluid known as engine coolant flows through radiator coils and air flows past the coils, which cools the coolant and heats the incoming air. 4.1 heAT exChANGeR TyPes: Shell and tube heat exchanger:Shell and tube heat exchangers consist of a series of tubes. One set of these tubes contains the fluid that must be either heated or cooled. The second fluid runs over the tubes that are being heated or cooled so that it can either provide the heat or absorb the heat required. A set of tubes is called the tube bundle and can be made up of several types of tubes: plain, longitudinally finned, etc. Shell and tube heat exchangers are typically used for high-pressure applications (with pressures greater than 30 bar and temperatures greater than 260 °C). This is because the shell and tube heat exchangers are robust due to their shape. Several thermal design features must be considered when designing the tubes in the shell and tube heat exchangers: • Tube diameter: Using a small tube diameter makes the heat exchanger both economical and compact. However, it is more likely for the heat exchanger to foul up faster and the small size makes mechanical
  • 8-Jun-13 • cleaning of the fouling difficult. To prevail over the fouling and cleaning problems, larger tube diameters can be used. Thus to determine the tube diameter, the available space, cost and the fouling nature of the fluids must be considered. Tube thickness: The thickness of the wall of the tubes is usually determined to ensure: • There is enough room for corrosion • That flow-induced vibration has resistance • Axial strength • Availability of spare parts • Hoop strength (to withstand internal tube pressure) • Buckling strength (to withstand overpressure in the shell) • Tube length: heat exchangers are usually cheaper when they have a smaller shell diameter and a long tube length. Thus, typically there is an aim to make the heat exchanger as long as physically possible whilst not exceeding production capabilities. However, there are many limitations for this, including space available at the installation site and the need to ensure tubes are available in lengths that are twice the required length (so they can be withdrawn and replaced). Also, long, thin tubes are difficult to take out and replace. • Tube pitch: when designing the tubes, it is practical to ensure that the tube pitch (i.e., the centre-centre distance of adjoining tubes) is not less than 1.25 times the tubes' outside diameter. A larger tube pitch leads to a larger overall shell diameter, which leads to a more expensive heat exchanger. • Tube corrugation: this type of tubes, mainly used for the inner tubes, increases the turbulence of the fluids and the effect is very important in the heat transfer giving a better performance. • Tube Layout: refers to how tubes are positioned within the shell. There are four main types of tube layout, which are, triangular (30°), rotated
  • 8-Jun-13 triangular (60°), square (90°) and rotated square (45°). The triangular patterns are employed to give greater heat transfer as they force the fluid to flow in a more turbulent fashion around the piping. Square patterns are employed where high fouling is experienced and cleaning is more regular. • Baffle Design: - baffles are used in shell and tube heat exchangers to direct fluid across the tube bundle. They run perpendicularly to the shell and hold the bundle, preventing the tubes from sagging over a long length. They can also prevent the tubes from vibrating. The most common type of baffle is the segmental baffle. The semicircular segmental baffles are oriented at 180 degrees to the adjacent baffles forcing the fluid to flow upward and downwards between the tube bundles. Baffle spacing is of large thermodynamic concern when designing shell and tube heat exchangers. Baffles must be spaced with consideration for the conversion of pressure drop and heat transfer. For thermo economic optimization it is suggested that the baffles be spaced no closer than 20% of the shell’s inner diameter. Having baffles spaced too closely causes a greater pressure drop because of flow redirection. Consequently having the baffles spaced too far apart means that there may be cooler spots in the corners between baffles. It is also important to ensure the baffles are spaced close enough that the tubes do not sag. The other main type of baffle is the disc and donut baffle, which consists of two concentric baffles. An outer, wider baffle looks like a donut, whilst the inner baffle is shaped like a disk. This type of baffle forces the fluid to pass around each side of the disk then through the donut baffle generating a different type of fluid flow. Fixed tube liquid-cooled heat exchangers especially suitable for marine and harsh applications can be assembled with brass shells, copper tubes, brass baffles, and forged brass integral end hubs.
  • 8-Jun-13  Plate heat exchanger:-
  • 8-Jun-13  Plate and shell heat exchanger: Adiabatic wheel heat exchanger: Plate fin heat exchanger:This type of heat exchanger uses "sandwiched" passages containing fins to increase the effectiveness of the unit. The designs include cross flow and counter flow coupled with various fin configurations such as straight fins, offset fins and wavy fins. Plate and fin heat exchangers are usually made of aluminum alloys, which provide high heat transfer efficiency. The material enables the system to operate at a lower temperature and reduce the weight of the equipment. Plate and fin heat exchangers are mostly used for low temperature services such as natural gas, helium and oxygen liquefaction plants, air separation plants and transport industries such as motor and aircraft engines. Advantages of plate and fin heat exchangers: • High heat transfer efficiency especially in gas treatment • Larger heat transfer area • Approximately 5 times lighter in weight than that of shell and tube heat exchanger. • Able to withstand high pressure Disadvantages of plate and fin heat exchangers: • Might cause clogging as the pathways are very narrow • Difficult to clean the pathways • Aluminum alloys Embrittlement Failure are susceptible to Mercury Liquid
  • 8-Jun-13  Pillow plate heat exchanger: Phase-change heat exchangers:In addition to heating up or cooling down fluids in just a single phase, heat exchangers can be used either to heat a liquid to evaporate (or boil) it or used as condensers to cool a vapor and condense it to a liquid. In chemical plants and refineries, re boilers used to heat incoming feed for distillation towers are often heat exchangers. Distillation set-ups typically use condensers to condense distillate vapors back into liquid. Power plants that use steam-driven turbines commonly use heat exchangers to boil water into steam. Heat exchangers or similar units for producing steam from water are often called boilers or steam generators. In the nuclear power plants called pressurized water reactors, special large heat exchangers pass heat from the primary (reactor plant) system to the secondary (steam plant) system, producing steam from water in the process. These are called steam generators. All fossil-fueled and nuclear power plants using steam-driven turbines have surface condensers to convert the exhaust steam from the turbines into condensate (water) for re-use. To conserve energy and cooling capacity in chemical and other plants, regenerative heat exchangers can transfer heat from a stream that must be cooled to another stream that must be heated, such as distillate cooling and re boiler feed pre-heating. This term can also refer to heat exchangers that contain a material within their structure that has a change of phase. This is usually a solid to liquid phase due to the small volume difference between these states. This change of phase effectively acts as a buffer because it occurs at a constant temperature but still allows for the heat exchanger to accept additional heat. One example where this has been investigated is for use in high power aircraft electronics.
  • 8-Jun-13  Direct contact heat exchanger:- 4.2:- Types of flow:- A) Parallel and Counter flow
  • 8-Jun-13 B) Cross Flow:- 5:- pRoJeCT oUTlINe
  • 8-Jun-13 The main aim of the project was to increase the efficiency of the heat exchanger at BAF. This aim could have been fulfilled in many different ways like changing the entire design of the heat exchanger, modifying existing dimensions, and material or flow rates of the fluids of the heat exchanger or by reducing the failure rate of the heat exchangers. The entire design or even the existing dimensions cannot be changed as that would require constructional changes to the plant which was not feasible. The flow rates also cannot be changed because that would require a change in the pumping system which was also not feasible. We were told that the plant was having major problems regarding the failure rate of the heat exchanger. Therefore, we decided to increase its efficiency by analyzing two problems – one through the design point of view and another through the failure point of view. The analysis through the design point of view was done by measuring the current efficiency of the heat exchanger and suggesting possible methods to increase its efficiency remaining within the constraints. The analysis through the failure point of view was done by investigating the possible causes of failure and suggesting possible remedies to eradicate the practices causing them. 6-AboUT bAf heAT exChANgeR:The cold rolled coils coming out of tandem mill are strain hardened and having very less formability, thus unsuitable for automobile industries and other industries which ask for high formable steel. Hence the CR coil is sent to batch annealing furnace where coils are annealed for a period of ~50 hours in the hydrogen atmosphere. In BAF, coils undergo ramp of heating, control heating and soaking for better grain structure. After the heating coils are cooled, firstly slow cooling, and then cooling using cooling hood and then a rapid cooling is required for time saving. Cooling hood cools down the wall of furnace while a heat exchanger cools the hot hydrogen inside the furnace and it is re circulated using a blower.
  • 8-Jun-13 BILL OF MATERIALS:Component Round bar Angles Square bar Rivets Name plate bracket Name plate Angles Plate (LHS Channel) Plate (LHS Channel) Cover plate RHS tube sheet LHS tube sheet Tubes Stay bar LHS partition plate RHS partition plate Sealing angle ᶲ CS Bolts and nuts Gasket Cover plate Channel flange Channel plate Channel plate Quantity 2 2 1 4 1 1 4 1 1 1 1 1 207 14 2 3 18 Standard IS-2062 IS-2062 IS-2062 SS IS-2062 IS-2062 IS-2062 IS-2062 IS-2062 IS-2062 IS-2062 IS-2062 IS-2062 IS-2062 IS-2062 IS-2062 IS-2062 42 1 1 1 2 2 IS-2062 IS-2062 IS-2062 IS-2062 IS-2062 IS-2062 Selection of tube material To preamp to transfer heat well, the tube material should have good thermal conductivity. Because heat is transferred from a hot to a cold side through the tubes, there is a temperature difference through the width of the tubes. Because of the tendency of the tube material to thermally expand differently at various temperatures, thermal stresses occur during operation. This is in addition to any stress from high pressures from the fluids themselves. The tube material also should be compatible with both the shell and tube side fluids for
  • 8-Jun-13 long periods under the operating conditions (temperatures, pressures, pH, etc.) to minimize deterioration such as corrosion. All of these requirements call for careful selection of strong, thermally-conductive, corrosion-resistant, high quality tube materials, typically metals, including copper alloy, stainless steel, carbon steel, non-ferrous copper alloy, Inconel, nickel, Hastelloy and titanium. Poor choice of tube material could result in a leak through a tube between the shell and tube sides causing fluid cross-contamination and possibly loss of pressure. For increasing the effectiveness of the heat exchanger, tubes are finned :Details of fin tubes:Type of fin  fully crimped Material of construction cast steel 8fins/inch Fin details49 OD x 26 G.THK 6.1:- Design Analysis : Given: Di = 18 mm Tube Material = Mild Steel Do = 22 mm Fin Material = Mild Steel L tot = 1210 mm Fin Pitch = 8 fins/inch L fin = 1140 mm Number of Tubes = 207 t = 26G = 0.405 mm Number of Tube Passes = 6 Pin, water = 3.5 bar Tube Configuration = Staggered Tin, water = 30° C Number of Shell passes = 1 Tout, water = 58° C Height of Tube Bundle, H = 605 mm
  • 8-Jun-13 Tin, H2 = 550° C Width of Tube Bundle, W = 810 mm V̇ water = 35 m3/h Transverse Pitch of the Tubes ST = 50 mm V̇H2 = 15000 m3/h Longitudinal Pitch of the Tubes SL = 44 mm ρ = 0.072 Kg/m3 Average no. of tubes per pass, n = 34.5 H2 Properties: Mild Steel: k = 55 W/m-K Hydrogen (T̅ = 590 K): µ = 1.377 x 10-5 Ns/m2 C p = 14.54 KJ/Kg K k = 0.32 W/m K Pr = 0.68 Water (T̅ = 315 K): µ = 6.31 x 10-4 Ns/m2 Cp = 4.179 KJ/Kg K K = 0.634 W/m K Pr = 4.16 ρ = 991.08 Kg/m3 To calculate effectiveness of the heat exchanger we need to calculate the overall heat transfer coefficient. 1/UA = 1/(η0hA)c + R f ,c”/(η0A)c + R w + R f, h”/(η0A)h + 1/(η0hA)h Neglecting fouling & η0,c = 0 (since inside of tube is not finned), 1/UA = 1/h c A c + R w + 1/η0,h hhAh To calculate water side convection coefficient h c u = V̇/An= 35×4/(3600×π×34.5×0.0182) =1.107m/s
  • 8-Jun-13 Re = ρ u D/μ = 991.08×1.107×0.018/6.31×10-4 = 31296.767 Therefore, flow is turbulent. For turbulent flow in circular tubes, Dittus – Boelter equation is used, N u D = 0.023 Re4/5Prn Where n = 0.4 for heating & n = 0.3 for cooling N u D = 0.023 (31296.767)4/5(4.16)0.4 = 160.767 H c = N u D k/D = 160.607*0.634/0.018 = 5656.94 W/m2K To calculate gas side convection coefficient hh , we will consider two different models, one the flow across banks of tubes & other that of compact heat exchanger. 1. Flow Across bank of Tubes For staggered configuration, the maximum velocity may occur at either the transverse plane A1 or diagonal plane A2.
  • 8-Jun-13 It will Occur at A2, if 2(SD – D) < (ST – D) The factor 2 results from the bifurcation experienced by the fluid moving from A1 to A2 planes. SD = {SL2 + (ST/2)2}1/2 = (442 + 252)1/2 = 50.606 mm SD – D = 28.606 mm ST – D = 28 mm 2(SD – D) > (ST – D) V max will occur at A1, which is given by V max = {ST/(ST – D)}.V V= V̇H2/A fr = 15000/3600×0.605×1.21 = 5.69 m/s V max = {50/(50 – 22)}*5.69 = 10.16 m/s
  • 8-Jun-13 Re D, max = ρV max D/μ = 0.072×10.16×0.022/1.377×10-5 = 1168.73 For this range of Re, it is appropriate to use the results of Zukauskas, Nu D = C2C Re m D, max Pr 0.36(Pr/Pr s)1/4 Where P r s = Prandtl no. at the surface temperature Assuming Ts = T̅ water = 315 K Pr s = 0.701 C2, C and m are noted from the relevant table C2 = 0.98 For C, ST/SL = 1.136 & the configuration is staggered
  • 8-Jun-13 C = 0.35(ST/SL)1/5 = 0.359 & m = 0.6 N u D = 0.98*0.359*(1168.73)0.6(0.68)0.36(0.68/0.701)1/4 = 21.05 H h=NuDk/D = 21.05×0.32/0.022 = 306.25 W/m2K 2. Compact Heat Exchanger Tube banks with the product a*b <1.252 are referred to as compact, while those with a*b> 4 are considered widely-spaced, where a = ST/D and b = SL/D In this case, a = 50/22 = 2.27 and b = 44/22 = 2 Therefore, a*b = 4.54 Although this value suggests the heat exchanger is not compact, we can approximate it as compact as the deviation is not much. The overall heat transfer coefficient based on the gas-side surface area is given by: 1/Uh = 1/h c(Ac/Ah) + A h R w + 1/η0,hhh If fin thickness is assumed negligible, Ac/Ah ≈ Di/Do*{1-(A f, h/Ah)} (Approximation Valid to within 10%) 2 2 A f, h = fin area = π/2(Df – Di ) + π D f t = π/2(0.0492 - 0.0222) + π×0.049×0.405×10-3 = 3.074×10-3 m2 Ah = Total hot side surface area = A f, h + π Ds Where s = distance between two fins Ah = 3.074×10-3 + π×0.022×0.00278 = 3.266×10-3 m2 A f ,h /Ah = 3.074×10-3/3.266×10-3 = 0.94 Ac/Ah ≈ 18/22*(1 - 0.94) = 0.049 A h R w = ln (Do/Di)/ (2πLk/Ah) =Di ln (Do/Di)/{2k(Ac/Ah)}
  • 8-Jun-13 = 0.018*ln (22/18)/ (2*55*0.049) = 6.7×10-4 m 2K/W Mass Velocity, G=ṁ/σ A f r σ=A ff/A f r A ff = Free Flow area A f r = Frontal Area ṁ = (15000/3600) ×0.072 = 0.3 kg/s A ff = πDh2/4 D h = 4A/P = 4Weff He ff/2(W eff + H eff) H eff = H = 0.605 m W eff = W – (N fin t fin) = 1210 – (359×0.405) = 1064.605 mm Dh = 2 × 1064.605 × 605/(1064.605 + 605) = 771.54 mm A ff = π × 0.77152/4 = 0.4675 m2 A fr = 1.21 x 0.605 = 0.73205 m2 σ=A ff/A fr = 0.4675/0.73205 = 0.6386 G = 0.3/(0.6386×0.73205) = 0.6417 Re = G Dh/μ = 0.6417 × 0.7715/1.377×10-5 = 35932.91 The following correlation for j factors is recommended by Briggs and Young [see Webb (1994)], for individually-finned tubes on staggered tube banks. J H = 0.134 Red-0.319(s/lf)0.2(s/δ f)0.11 Where lf is the radial height of the fin, δ f is the fin thickness, p f is the fin pitch and s = p f – δ f is the distance between adjacent fins. This equation is valid for the following ranges: 1100 ≤ Red ≤ 18,000, 0.13 ≤ s/lf ≤ 0.63, 1.01 ≤ s/δ f ≤ 6.62, 0.09 ≤ lf/do ≤ 0.69, 0.011 ≤ δ/do ≤ 0.15, 1.54 ≤ X t/do ≤ 8.23; fin root diameter do between 11.1 and 40.9 mm; and fin density N (= 1/p f)
  • 8-Jun-13 between 246 and 768 fins per meter. The standard deviation of this equation from experimental results has been computed at 5.1%. Checking for validity, Re = 35932.91 (not within range) s/ lf = 2.78/13.5 = 0.206 (within range) s/ δf = 2.78/0.405 = 6.86 (almost within range) lf/ d0 = 13.5/22 = 0.61 (within range) δf/ d0 = 0.405/22 = 0.018 (within range) Xt/d0 = ST/Do = 50/22 = 2.27 (within range) d0 = Do = 22 mm (within range) Nf = 315 fins/meter (within range) Considering only Reynolds number is not within range, we can safely say that this equation will yield satisfactory results. Therefore j H = 0.134 (35932.91)-0.319(0.206)0.2(6.86)0.11 = 0.00425 H h ≈ j H G c p/Pr2/3 = 0.00425 × 0.6417 ×14540/0.682/3 = 51.28 Now to calculate η0,h, the hot side overall surface efficiency, we need the fin efficiency η f, which is defined as η f ≡ q f/q max = qf/h A f θ b Where A f is the surface area of the fin. For convective heat transfer tip condition, qf = (hPkAc)1/2θb {sinh(mL) + (h/mk) cosh(mL)}/{cosh(mL) + (h/mk) sinh(mL)} Where m = (hP/kAc)1/2 In lieu of the somewhat cumbersome expression for heat transfer from a straight rectangular fin with an active tip, it has been shown that approximate, yet accurate, predictions may be obtained by using the adiabatic tip result, with a corrected fin length L c = L + (t/2) for a rectangular fin. The correction is based on assuming equivalence between
  • 8-Jun-13 heat transfer from the actual fin with tip convection and heat transfer from a longer, hypothetical fin with an adiabatic tip. Hence, with tip convection, qf = M tanh(mLc) where M = (hPkAc)1/2θb η f = tanh(mLc)/mLc Errors associated with the approximation are negligible if (ht/k) ≤ 0.0625 ht/ k=0.000378 Hence error associated is negligible. If the width of a rectangular fin is much larger than its thickness, w>>t, the perimeter may be approximated as P = 2w, and mLc = (hP/kAc)1/2Lc = (2h/kt)1/2Lc Multiplying and dividing by Lc1/2 & introducing a corrected fin profile area, A p = L ct mLc = (2h/kAp)1/2Lc3/2 A graph is plotted between Lc3/2(h/kAp)1/2 & ηf for different geometries.
  • 8-Jun-13 The efficiency can also be calculated from the following formula for an adiabatic tip ηf={2r1/(m(r22-r12))}*{K1(mr1)I1(mr2)I1(mr1)K1(mr2)/K0(mr1)I1(mr2)+I0(mr1)K1(mr2)} Where, I0 and K0: modified zero order Bessel functions of the first and second Kinds, respectively. I1 and K1: modified first order Bessel functions of the first and second Kinds, respectively. This result may be applied for an active (convecting) tip, if the tip radius r 2 is replaced by a corrected radius of the form r2c = r2 + t/2. The Bessel functions are tabulated at the end. As the same result is shown graphically as a plot, we will calculate the efficiency directly from the plot. For the first model of flow over tube banks, Lc3/2(h/kAp)1/2 = (13.7025x10-3)3/2(h)1/2/(55x5.5495x10-6)1/2 Where Lc =L + t/2 = (r2 – r1) + t/2 = 13.5 + 0.405/2 = 13.7025 mm And Ap = Lct = 13.7025 x 0.405 = 5.5495 mm2 Therefore, Lc3/2(h/kAp)1/2 = 0.0918(h)1/2 = 0.0918(306.25)1/2 = 1.607 To get the efficiency from the plot, we also require r 2c/r1 r2c = r2 + t/2 = 24.5 + 0.405/2 = 24.7025 mm r1 = 11 mm Therefore, r2c/r1 = 2.2457 ≈ 2.25 Hence, ηf ≈ 0.35
  • 8-Jun-13 For the second model of compact heat exchanger, Lc3/2(h/kAp)1/2 = 0.0918(51.28)1/2 = 0.657 Hence, ηf ≈ 0.73 From the efficiency values of both models, we can safely say that the compact heat exchanger model is more realistic as the value of 0.35 seems very low to have been manufactured. Now, η0,h = 1 – {Af(1 – ηf)/A} = 1 – {0.94(1 – 0.73)} = 0.746 Therefore, 1/Uh = {1/(5656.94 x 0.049)} + 6.7 x 10-4 + {1/(0.746 x 51.28)} = 3.6 x 10-3 + 6.7 x 10-4 + 26.14 x 10-3 = 30.14 x 10-3 m2K/W Uh = 32.88 W/m2K Now, Cc = ṁccp,c = V̇waterρwcp,c = (35/3600) x 991.08 x 4179 = 40266.75 W/K Ch = ṁhcp,h = V̇H2ρH2cp,h = (15000/3600) x 0.072 x 14540 = 4362 W/K Therefore, Cmin = Ch = 4362 W/K q = Cc(Tc,o – Tc,i) = Cc(Tout,water – Tin,water) = 40266.75(58° - 30°) = 1127.469 kW qmax = Cmin(Th,I – Tc,i) = Cmin(Tin,H2 – Tin,water) = 4362(550° - 30°) = 2268.24 kW ε = q/qmax =1127.469/2268.24 = 0.497
  • 8-Jun-13 The hydrogen with oxygen makes a highly explosive mixture and nitrogen can be used at place of hydrogen but one major disadvantage behind using nitrogen in BAF is that it will take approximately 150 hours for the same amount of annealing for which hydrogen only takes 50 hours, hence more production at CRM.
  • 8-Jun-13 6.2:- Methods of iMproving effectiveness:1] BY Using corrUgAted fins:-Use Of corrugated fins upon the tubes significantly increases the heat transfer rate and if carefully designed then it may reach2-3 times of heat transfer rate that of non-finned heat exchangers. Thickness of fins 0.405 mm Outer diameter of fins 49 mm Inner diameter on fins=outer diameter of pipes22 mm Efficiency of fins calculated = 53% One of the major contributors to the efficiency of heat exchanger is the fin efficiency. This can easily be improved by using a material having a better thermal conductivity than the present material which is Mild Steel. Initially, we considered both Aluminium and Copper as the substitutes but the temperature range of the hot side (≈550) prevented us from using Aluminium as it is very near to its melting temperature. So, we have considered copper as a substitute. The overall heat transfer coefficeint consists of three resistances; one between the cold fluid and the tube wall, one through the tube wall and one between the tube wall and the hot side. Using copper basically reduces the thermal resistance of the hot side, which enhances the heat transfer between the fluids. This can be shown numerically by considering the overall heat transfer coefficient. In the calculations above we can see that the largest contribution was of the resistance between the tube wall and the hot fluid. So, decreasing the resistance there would be a big factor towards enhancing heat transfer. 2]Using inserts :- If some king of inserts which take the form of complex wire shape or flat strips made of non corrosive materials like fibber or
  • 8-Jun-13 thermosetting plastic is inserted inside the tubes and fitted in various styles in such a way that the reduce in boundary layer resistance can exceed the rate of pressure loss in the tube then this continuous disturbance of boundary layer of tube side fluid increase the amount o turbulence within the fluid and this will increase the values of NUSSELT NUMBER , and providing the tube side fluid have higher resistance to heat flow. Will increase the overall rate at which heat is transferred. A movable insert which is fixed on a wire made of thermosetting plastic fitted inside the tube will easily do this work and due to wavy motion of insert inside the tube will sufficiently create the turbulence and also help in deposition of fouling and negligibly cost the assembly and fabrication. Due to width of this insert is along the flow direction of fluid hence it will not drop the pressure much more and hence reduce in boundary layer resistance would be higher than the pressure loss of fluid inside the pipe. 3] Using corrrUgAted tUBes:Use of corrugated pipes with the corrugated fins in BAF heat exchanger is definitely a better option than the using a straight tube with corrugated fins. This is because of:Corrugated tube will consist more fins the straight tube and effective area of heat exchanger is increased increasing the overall effectiveness of heat exchanger Manufacturing is very easy like the straight one. straight pipes show more affinity toward the thermal stress and fatigue failure chances are higher than the corrugated tubes because a corrugated tube will turn to straight because of thermal stress and do not fail easily like the straight one.
  • 8-Jun-13 7:-fAilUre of heAt exchAnger:- General corrosion Cogging due to calcium and magnesium ions
  • 8-Jun-13 Erosion corrosion Steam water hammering 7.1 fAilUre stAteMent of heAt exchAnger tUBe:Problem:The tubes of BAF bypass heat exchanger at CRM have failed in service. The tubes are getting punctured at random locations along their length resulting in leakage. ICW which flow through the tubes is used to cool the hot hydrogen gas flowing over the tubes:Sample details:-
  • 8-Jun-13 Department Cold rolling mill Component Heat exchanger tubes Equipment BAF bypass heat exchanger Life expected 5 years Life obtained 2-3 years Material IS:2062 Grade B UMC 0566A2378 Heat exchanger type Shell and exchanger tube type heat Industrial cooling water is flowing through the tubes and cools the hot hydrogen gas hydrogen gases passing over the finned tubes. The heat exchange is comprised of a casing and several tubes which are covered with the fins. The fins were found ruptured in multiple locations. Most are the tubes were fouled and ions of magnesium and calcium along with the other impurities were deposited (sticking in inner surface) in the tubes causing the blockage of flow and, poor convective heat transfer between cold water and metallic tube hence reducing the overall effectiveness of the heat exchanger. Once it has been reported that within 5 span of months 14 such heat exchanger assemblies has been affected by same issues of serious fouling.
  • 8-Jun-13 Corrosion of pipe due to use of ICW Tube Failure of Finned shell and tube type heat exchanger • Effect of Cleaning Agent on Corrosion and Failure • The cleaning agent used for the tube bundle was a solution under the trade name of ‘Alkali’, a combination of compounds, including Na2CO3 , Silicates and mostly(>80%) NaOH.
  • 8-Jun-13 • The solution is primarily used in ECL (Electrolytic Cleaning), but was found to be suitable for cleaning tube bundles in BAF Heat Exchanger. • Effect of NaOH on metals • Sodium hydroxide is the largest strongly basic commodity produced by the chemical industry today. It is the eighth largest tonnage product of the chemical industry and is found, either directly or indirectly, in a variety of products and processes. • At any concentration, sodium hydroxide should not be stored in aluminium or zinc (galvanized) containers or allowed to come in contact with magnesium. The most common materials of construction for the storage of sodium hydroxide are carbon steel and polyethylene. • This is because of the amphoteric property of certain elements like Al, Zn, Mg, Cr etc. Amphoteric property suggests that elements react similarly with acids and bases. • 2Al(s) + 6H+ (aq)  2Al3+(aq) + 3H2(g) Zn(s) + 2H+ (aq)  Zn2+(aq) + H2(g) • 2Al(s) + 2OH- (aq) + 6H2O(l)  2[Al(OH)4] -(aq) + 3H2(g) Zn(s) + 2OH- (aq) + 2H2O(l)  [Zn(OH)4]2-(aq) + H2(g) • Sodium Hydroxide being one of the strongest bases commercially available will react with these metals. Zn + 2NaOH + 2H2O  Na2 [Zn (OH)4] + H2(g) • However, Iron (or Carbon Steel) being a non-amphoteric metal, does not react with bases such as Sodium Hydroxide in such a way. • • Why care about amphoteric metals if the tube material is carbon steel? • It is because the tubes are made of Galvanized steel. Galvanized steel is made by putting a thin layer of zinc on the steel. This
  • 8-Jun-13 layer protects the steel from rusting when wet. Zinc reacts with Sodium hydroxide to form Sodium Zincate ( which dissolves in the water. Eventually all the zinc will dissolve and the iron will then lose the protection of the zinc and begin to rust. ), • Evidence from Lab Test of failed samples • The lab test of eroded and punctured tubes presented us with a few facts • Iron oxides were the most common material found in the residue apart from compounds of Calcium and Magnesium, although water samples didn’t indicate high iron content in the inlet. This indicates that iron was rusted. • Zinc was completely absent from the residue, although galvanized steel is expected to have zinc at the surface and hence as part of the residue. This indicates that zinc was completely worn out. • • • • • Conclusion: •  Both the points support the theory that zinc was dissolved and iron in carbon steel began to rust. Suggested alternate cleaning agents/methods: Ethylene diamine tetra acetic acid (EDTA)
  • 8-Jun-13 • EDTA is a versatile chelating agent. It can form four or six bonds with a metal ion, and it forms chelates with both transition-metal ions and main-group ions. EDTA is frequently used in soaps and detergents, because it forms complexes with calcium and magnesium ions. • In other applications, EDTA dissolves the CaCO3 scale deposited from hard water without the use of corrosive acid. • It is a better option, compared to other chemicals such as NaOH or HCl, as it is unlikely to react with the tube surface, but will remove the scale formed due to Calcium and Magnesium 7.2 sUspected reAsons for fAilUre of A heAt exchAnger tUBe:- There might be several reasons for the failure of heat exchanger like:• Pipe and tubing imperfections • Welding Faults • Fabrication Issues • Improper design Specifications • Improper materials • Improper operating conditions
  • 8-Jun-13 • Pitting • Stress-corrosion cracking (SCC) • Corrosion fatigue • General corrosion • Crevice corrosion • Design errors • Selective leaching, or de alloying • Erosion corrosion • Thermal fatigue therMAl fAtigUe:Tubes, predominantly in the U-bend sections, can fail as a result of fatigue from accumulated stresses related to constant thermal cycling. This problem is significantly aggravated as the temperature difference across the U-bends increase Temperature differences caused tube flexing, which subsequently produced a stress load that, until the materials tensile strength was exceeded and therefore cracked. The resulting crack most commonly runs in radials around the tube, and may result in a complete failure.
  • 8-Jun-13 Thermal fatigue foUling:If there is a gradual decline in heat transfer, fouling may be the culprit. Heat exchanger software can give the available fouling as compared to design fouling. Sometimes fouling is so severe that tubes can be plugged inside or the shell side ligaments between the tubes can be filled. This is sometimes seen when bundles from a refinery are sent to be repaired. Actual fouling can be much higher than the TEMA (Tubular Exchanger Manufacturer's Association) specification. If you suspect that fouling is a problem, check the exchanger's operating history. Are there deviations from design conditions? Are there periods of operation where flows are lower than design? Heat exchangers will foul faster at low velocities. If water fouling is a problem, have the water flows been cut back in the winter? If you determine that fouling is a problem, make a chemical analysis of the fouling material. If the fouling cannot be controlled, a tube electro polishing process can slow scale and other buildup. It eliminates small ridges and pits that contribute to fouling What is scale? Scale is a hard deposit of predominantly inorganic material on heating transfer surfaces caused by the precipitation of mineral particles in water. As water evaporates in a cooling tower or an evaporative condenser, pure vapour is lost and the dissolved solids concentrate in the remaining water.
  • 8-Jun-13 If this concentration cycle is allowed to continue, the solubility of various solids will eventually be exceeded. The solids will then settle in pipelines or on heat exchange surfaces, where it frequently solidifies into a relatively soft, amorphous scale. Problems Scale, in addition to causing physical blockage of piping, equipment, and the cooling tower, also reduces heat transfer and increases the energy use. For example, the thermal conductivity BTU/ [hr (ft 2) (F/in)] of copper is 2674, while the common cooling water scale calcium carbonate has a thermal conductivity of 6.4 BTU/ [hr (ft 2) (F/in)]. A calcium carbonate scale of just 1.5 mil thickness is estimated to decrease thermal efficiency by 12.5 %. In compression refrigeration systems, scale translates into higher head pressures, hence an increase in power requirements and costs. For example, 1/8" of scale in a 100 ton refrigeration unit represents an increase of 22% in electrical energy compared to the same size unit free of scale. Fouling Literature show that the fouling of heat exchangers can occur by any of the following ways Fouling of heat exchanger (which is basically clogging of tubes) can occur by any of five mechanisms or combination of all five:-
  • 8-Jun-13 A) Sedimentation fouling B) Inverse solubility fouling C) Chemical reaction of fluid and tube material D) Corrosion product fouling E) Biological fouling  In the case of sedimentation fouling the suspended solid in the cooling water settle down upon the heat transfer surface. But in this case the amount of suspended particle is within range in the ICW of plant.  For inverse solubility fouling commonly found salt in natural water notably calcium salts (predominantly calcium carbonate) crystallizes upon the heat transfer surface and forms strong and adherent scale in due course.  Chemical reaction due to exposure to hot heat transfer surface n degradation of one of the process stream resulting in carbonaceous deposits, resulting in chemical reaction fouling.  Many cooling water sources and a few process streams contain organisms, which attach to the heat transfer surface and causes biological fouling.  Due to mixing of oil in the water, biological fouling takes place and organism generate and deposit at a faster rate, if and tube is cracked at PL-TCM section heat exchanger then from there oil mixes to the ICW which is carried to the water tank of ICW and circulated to BAF heat exchanger. This is also a major reason for the biological fouling.
  • 8-Jun-13 design And fABricAtion proBleM:Not removing enough tubes under the shell nozzles can cause pressure drop and vibration problems. Refer to figure, where the lowest edge of the nozzle is below the top tube row. The entering shell fluid cannot disburse in all directions. It can only go parallel to the tubes and down between the tubes. The opposite is true at the exit. Tubes too close to the nozzles will cause a high pressure drop and possibly bundle vibration. This problem is normally found when older exchangers are used in a new service. The problem can occur if nozzles are enlarged to handle more flow for the new service, but the bundle layout is not changed. One cure for exchangers with high bundle entrance/exit pressure drops is to add
  • 8-Jun-13 another nozzle so two parallel streams enters or leave the exchanger. This cure is also good if there is a vibration problem in the bundle end zones Excess ‘environmental’ vibration from equipment including air compressors, refrigeration machines or other motors can cause tube failures that form as a result of fatigue stress cracks and or erosion where the tubes make contact with the baffles. Ideally, heat exchangers should be isolated from all forms of vibration. Fluid velocities that exceed 4 FPS could cause vibration induced damage in the tubes, often causing baffle supports to cut into the tubes (Figure C). Velocity-induced vibrations may also cause fatigue failures by hardening the tubes at the contact points between baffles or in U-Bend segments, eventually leading to cracks and splits. Fluid velocity in excess of the manufacturer recommendation on either the shell or tube side of the heat exchanger will likely cause erosion damage as metal wears from the tubing surfaces. If any corrosion is already present on the exchanger, the erosion is accelerated, exposing the underlying metal to further attack without a protective coating. A metal erosion problem most often occurs inside the tubes, along the U bend and near the tube entrances. Figure (A) is an example of metal loss in a section of U bend caused by extremely high-temperature water flashing over to steam.
  • 8-Jun-13 Metal erosion fAilUre dUe to poor welding of tUBes At flAnges:Welding defects can greatly affect weld performance and longevity. Having an understanding of the various defects, their causes and remedies can help to ensure higher-quality and longer lasting welds. This article details some of the more common welding defects, their causes and possible preventative and corrective measures. Chocked tube due to ICW Fouling
  • 8-Jun-13 7.3:-Methods of fAilUre prevention:-  Analysing the microstructure of tube samples near the puncture, it can be observed that these are the locations where tubes have undergone corrosive attack by the deposits resulting the thinning of tube and eventually leading to failure (puncture). The etched surface of tube structure may be due to ferrite matrix with very small amount of pearlite (Bainite formation). Hence, if tube grains structure is carefully studied and set then failure of tube can be prevented. Specifications as given in IS: 4503  Material of construction Allowable fluid temperature (deg. Centigrade) carbon steel 540 C –Mo steel 590 Cr –Mo steel 650 Low alloy steel Cr <6% 590 Alloy steel Cr <17 % 590 Austenitic Cr-Ni steel 650 Cast iron 200 Brass 200
  • 8-Jun-13  By the EDS analysis and element mapping of the fouling one can get the reasons of fouling and ions present in the water used in heat exchanger. Generally major reasons of fouling in a heat exchanger tube are deposition of layers of oxide of iron, calcium and magnesium ion. Hence, use of anti-corrosive material as the material of pipe can significantly resist the formation of oxide layer and hence less fouling and increased overall effectiveness of heat exchanger.  If some moisture content is present in the coating of electrode used for welding then the water is dissociated during the welding operation into hydrogen and oxygen and hydrogen is get trapped in the weld pool lattice which makes the weld more brittle. This is called hydrogen embrittlement and corrosion starts from the weld junction. Above defects can be eliminated if the electrode is kept in furnace at approx 600 o C for some time.  Due to improper heat treatment, there are chances that the less ferrite matrix with very small amount of pearlite (bainatic structure) formed in the abundance at the weld section. Bainite structure is an acicular microstructure that forms in steel at250600 o C. A fine non-lamellar structure, bainite commonly consists of cementite and dislocation-rich ferrite. The high concentration of dislocations in the ferrite present in bainite makes this ferrite harder than it normally would be. Hence due to high hardness and abundant bainite structure, weld section becomes brittle and corrosion starts from welding joint. This can be prevented using high pearlite matrix better heat treatment of weld sections.  Optimisation is must while choosing the pipe material, cold fluid, methods of manufacturing of heat exchanger (cost) and life span of a heat exchanger, since production of company must not stop.
  • 8-Jun-13 8-conclUsion:I conclude based upon the forgoing that the all the analysis made by me is the correct in best of my knowledge and margin of error is within the tolerance limit. Safety at TATA STEEL is first and foremost thing to keep in mind and at any level compromise with the safety is strictly prohibited. If a corrugated tube with corrugated fins is used with the galvanised tubes and a proper anti fouling agent then life of a heat exchanger can be increased significantly. Due to larger effective surface area overall effectiveness of a heat exchanger is increased. It is always undesirable that the tube fail within the designed life of heat exchanger or corrodes, hence reducing overall effectiveness of heat exchanger. If precautions are taken during welding operation of tubes to the flanges and a controlled heat treatment have been done after the welding operation then suitable amount of perlite-ferrite matrix will lower down the brittle character of joint and prevent the corrosion resulting joints will have higher life. reference: WWW.GOOGLE.COM WWW.TATA.COM WWW.WIKIPEDIA.COM WWW.TATASTEEL.COM www.sciencedirect .com BOOKS:  FUNDAMENTAL OF HEAT AND MASS TRANSFER BY- INCROPERA/DEWITT/BERGMAN/LAVINE Heat and mass transfer by J.P. Hollman ISO STANDARDS: -
  • 8-Jun-13 IS: 2062 TEMA- Associations