Presentation on theme: "Subject: M.E. 108 , 143 Fundamentals of Heat Exchanger Design "— Presentation transcript:
1 Subject: M.E. 108 , 143 Fundamentals of Heat Exchanger Design Wednesday ,July 27, 2011Lecturer: Prof. Gennaro J. Maffia
2 ME-108 Principles of Heat Exchanger Design, Part I This seminar provides an overview of the equipment used for the transfer of heat in the process industries, including that used in chemical plants, refineries, and cogeneration power plants. The transformation of the basic heat balance into a T-Q curve and theresulting zone analysis to produce an effective design is covered. Selection of the appropriate TEMA designation, as well as economic and environmental considerations will be addressed. The seminar will conclude with a discussion of the techniques, such as Pinch Technology, in developing a heat exchanger network.ME-143 Principles of Heat Exchanger Design, Part IIThis seminar provides a continuation of the discussion of the equipment used for thetransfer of heat. Specific examples and case studies will highlight analysis techniquesand step-wise procedures. Emphasis will be placed on the calculation of the film coefficientfor a variety of different geometries and how to compose an overall heat transfer coefficient.Transfer line exchangers, plate-fin exchangers and Joule-Thompson exchangers are amongthe examples covered.
3 ME-108 Principles of Heat Exchanger Design, Part I overview of the equipment used for the transfer of heat in the process industries,including that used in chemical plants, refineries, and cogeneration power plants.basic heat balance into a T-Q curve and the resulting zone analysis to produce an effective designTEMA designation, as well as economic and environmental considerations will be addressed.Pinch Technology
4 Fundamentals of Heat Exchanger Design This seminar provides an overview of the used for the transfer of heat in the process industries, including that used in chemical plants, refineries, and cogeneration power plants.
5 The transformation of the basic heat balance into a T-Q curve and the resulting zone analysis to produce an effective design is covered. Selection of the appropriate TEMA designation, as well as economic and environmental considerations will be addressed.
6 The seminar will conclude with a discussion of the techniques, such as Pinch Technology, in developing a heat exchanger network.
7 Dr. Gennaro Maffia, D.E., MBA After twenty years of industrial experience with large multinational companies, Dr. Maffia served as the Chairman of the Chemical Engineering Department at Widener University until He is currently Prof. of Chem. Eng at Manhattan College, Prof Emeritus at Widener University and Adjunct Prof. of Chem. and Biol. Engr. at Drexel University. Prof. Maffia is a member of many professional and honor societies as well as being an associate of several consulting companies. His research interests include the development of environmental remediation and biomedical technologies bases on water retention properties of unraveled bovine hide collagen. He holds a doctorate from Dartmouth College and a MBA from NYU.
11 Founded in 1821, Widener University is a multi-campus, independent, metropolitan university. It offers a student-centered learning environment where course work connects to societal issues. Dynamic teaching, active scholarship, personal attention, and experiential learning are key components of the Widener experience.The university provides a unique combination of liberal arts and professional education through its eight schools and colleges.
12 Collagen Research Lineage Started at Dartmouth ‘85cell cultureVerax - FBBRWidenerEnvironmental AngleflocculationfiltrationfractionationTailor Made SubstratesOther Applications1
13 Collagen Research Group Department of Chemical Engineering Widener UniversityTime Capsule1985 – 1988 Dartmouth College - Initial Research1985 – 1995 Verax Corp., Lebanon, NH1994 – 2006 Collagen Research Group, Widener U150+ students, professors, and colleagues have worked on protein research at Widener since 1994HS summer interns and summer campers –50-75 per year since 1996
14 Collagen Research Group Department of Chemical Engineering Major Collagen Projectsmanufacture and applications of the main productscollagen dispersionswater treatmentenvironmental remediationintermediatelyophilized collagen matricescontrolled releaselost protein technologysterile, crosslinked collagen matricesbiotech/biomedicaltissue engineering
15 Some Chemical Engineering Students and Professors Donna Stauffer and Prof. MaffiaBrian Coll, Megan Winkelman,and Prof. MaffiaJennifer HubbellFreshman and Prof. MaffiaProf. McNeil, Charlene Emlet,Eric Shaw and Donna StaufferWidener UniversitySome Chemical EngineeringStudents and Professorscontact:Audra RizzettoMonica Beistline
16 C B T M A P C R G Biotechnology Miscellaneous milling and dispersion (smart material)BiotechnologyCell Immobilizationreactor designbiosensingtissue engineeringBiomolecule DeliveryControlled ReleaseMiscellaneousDispersing AgentInfrastructureInsulationMicro-encapsulationCoating of Medical DevicesAdditive for Photocatalyst CoatingLost Protein& Catalyst ManufactureEnvironmentalExtractionFiltration AidFlocculationand SettlingCRG
17 AFTER MILLING AND WASHING nm diameterAFTER MILLING AND WASHING
21 not viable in most situations Indirect streams do not mix Heat TransferDirectstream mixingcheapnot viable in most situationsIndirectstreams do not mix
22 Heat Transfer Mechanisms ConductionConvectionRadiation
23 Conduction is the transfer of thermal energy from a region of higher temperature to a region of lower temperature through direct molecular communication within a medium or between mediums in direct physical contact without a flow of the material medium. Metals (eg. copper) are usually the best conductors of thermal energy. Fluids (liquids (except liquid metals) and gasses) are not typically good conductors.
24 Convection is a combination of conduction and the transfer of thermal energy by fluid circulation or movement of the hot particles in bulk to cooler areas in a material medium. Unlike the case of pure conduction, now currents in fluids are additionally involved in convection. This movement occurs into a fluid or within a fluid, and cannot happen in solids. In solids, molecules keep their relative position to such an extent that bulk movement or flow is prohibited, and therefore convection does not occur.
25 Radiation is transfer of heat through electromagnetic radiation Radiation is transfer of heat through electromagnetic radiation. Hot or cold, all objects radiate energy at a rate equal to their emissivity times the rate at which energy would radiate from them if they were a black body.No medium is necessary for radiation to occur; radiation works even in and through a perfect vacuum.Both reflectivity and emissivity of all bodies is wavelength dependent. The temperature determines the wavelength distribution of the electromagnetic radiation as limited in intensity by Plank’s law of black-body radiation.
26 Heat Transfer Mechanisms ConductionConvectionRadiation
27 Shell and tube heat exchanger indirect heat transfermost common type of heat exchangerin oil refineries and other large chemical processessuited for higher-pressure applications
28 Shell and tube heat exchanger consists of a shell (a large pressure vessel) with a bundle of tubes inside it.One fluid runs through the tubes, and another fluid flows over the tubes (through the shell) to transfer heat between the two fluids.The set of tubes is called a tube bundle, and may be composed by several types of tubes: plain, longitudinally finned, etc.
29 BasicsTwo fluids, of different starting temperatures, flow through the heat exchanger. One flows through the tubes (the tube side) and the other flows outside the tubes but inside the shell (the shell side).Heat is transferred from one fluid to the other through the tube walls, either from tube side to shell side or vice versa.This process is best represented by a T-Q diagram
30 Bottom LineAt every point in the HX, the hot stream must be hotter than the cold stream by a distance called the driving force
54 The fluids can be either liquids or gases on either the shell or the tube side. In order to transfer heat efficiently, a large heat transfer area should be used, so there are many tubes.In this way, waste heat can be put to use. This is a great way to recuperate energy.
55 Heat exchangers with only one phase (liquid or gas) on each side can be called one-phase or single-phase heat exchangers. Two-phase heat exchangers can be used to heat a liquid to boil it into a gas (vapor), sometimes called boilers, or cool a vapor to condense it into a liquid (called condensers), with the phase change usually occurring on the shell side.
56 In large power plants with steam-driven turbines, shell-and-tube surface condensers are used to condense the exhaust steam exiting the turbine into condensate water which can be recycled back to be turned into steam, possibly into a shell-and-tube type boiler.
57 Shell and tube heat exchanger design There can be many variations on the shell and tube design. Typically, the ends of each tube are connected to plenums (sometimes called water boxes) through holes in tubesheets. The tubes may be straight or bent in the shape of a U, called U-tubes.
62 In nuclear power plants called pressurized water reactors, large heat exchangers called steam generators are two-phase, shell-and-tube heat exchangers which typically have U-tubes.They are used to boil water recycled from a surface condenser into steam to drive the turbine to produce power.
63 Most shell-and-tube heat exchangers are either 1, 2, or 4 pass designs on the tube side. This refers to the number of times the fluid in the tubes passes through the fluid in the shell.In a single pass heat exchanger, the fluid goes in one end of each tube and out the other.
64 Surface condensers in power plants are often 1-pass straight-tube heat exchangers. In other facilities such as refineries, two and four pass designs are common because the fluid can enter and exit on the same side. This makes construction much simpler.
65 There are often baffles directing flow through the shell side so the fluid does not take a short cut through the shell side leaving ineffective low flow volumes.Countercurrent heat exchangers are most efficient because they allow the highest log mean temperature difference between the hot and cold streams.
66 Many companies however do not use single pass heat exchangers because they can break easily in addition to being more expensive to build.Often multiple heat exchangers can be used to simulate the countercurrent flow of a single large exchanger.
67 Selection of tube material To be able 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.
68 Tube materialThe tube material also should be compatible with both the shell and tube side fluids for 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. 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.
69 The Tubular Exchanger Manufacturers Association, Inc. (TEMA) is trade association of leading manufacturers of shell and tube heat exchangers, who have pioneered the research and development of heat exchangers for over sixty years.The TEMA Standards and software have achieved worldwide acceptance as the authority on shell and tube heat exchanger mechanical design.
70 TEMA members are market-aware and actively involved, meeting several times a year to discuss current trends in design and manufacturing.The internal organization includes various subdivisions committed to solving technical problems and improving equipment performance.
71 TEMAChoosing the right supplier is an important and difficult decision. Users need to be absolutely sure that their manufacturer is reliable, producing heat transfer equipment that is safe, effective, and economical. Considerable time and money are spent having a heat exchanger designed and built to exact specifications, design codes and requirements.TEMA, was founded in 1939 and has grown to include a select group of member companies. Members adhere to strict specifications.TEMA members develop and update today's standards. TEMA Standards and Software have achieved worldwide acceptance as the authority on shell and tube heat exchanger mechanical design.
72 TEMATEMA has also developed engineering software that complements the TEMA Standards in the areas of flexible shell elements (expansion joints) analysis, flow induced vibration analysis and fixed tubesheet design and analysis.This state-of-the-art software features a materials data-bank of 38 materials. The programs handle many complex calculations, so users can focus on the final results.
73 TEMABefore a company can even become a member of TEMA, it must have a minimum of 5 years of continuous service in the manufacture, design and marketing of shell and tube heat exchangers. All TEMA companies must have in-house thermal and mechanical design capabilities, and thoroughly understand current code requirements and initiate strict quality control procedures.Additionally, all welding must be done by the company's own personnel, and the company must have its own quality control inspectors.
74 Tubular Exchanger Manufacturers Association, Inc. 8th Edition of the Standards of the Tubular Exchanger Manufacturers Associationsections have been reviewed to incorporate new data which were not available at the time of the 1988 printing, including suggestions which resulted from theextensive use of the Standards by both manufacturers and users of shell and tube heat exchangers.cooperation of the American Petroleum Institute (API) and the American Society of Mechanical Engineers (ASME)
75 TEMA’s Eighth Edition: Metrification has been included where feasible and appropriate. Methods for calculating several types of floating head backing rings have been added. A method for incorporating pass partition rib area into flange design has been incorporated. The vibration section has been expanded and vibration amplitude for vortex shedding and acoustic resonance have been added. Nozzle flange pressure/temperature rating tables from ASME Standard B w/ 1998 addenda are included.
76 TEMA’s Eighth Edition: New materials have been included in coefficient of thermal expansion, modulus of elasticity, and thermal conductivity tables. Design equations for double tubesheets have been added. A method for calculating the mean metal temperature for tubesheets has been added. Stress multipliers have been added to account for the stiffness of knuckles on flanged and flued expansion joints. Suggested calculation methods have been incorporated for both vertical and horizontal supports. Design methods have been added for lifting lugs.
77 More on T-Q Ticket to HX design and rating Basis for Heat Integration Used for process design
80 Overview and ContentBackgroundDefinitionsKey AreasSteam reformer
81 fuel + air oxidation products Fired Heaters are used widely in chemical processing and petroleum refining plantssupply heat to various process fluids by burning fuel in a combustion chamberfuel is usually oil or gas or a combinationburners are placed on the floor of the heater or on the wall
82 Heat Transfer Mechanisms Radiation (T4)Conduction (DTsolid)Convection (DTconvective film)
83 Radiation tubes are installed along the walls and roof combustion chamber called radiant sectionheat is transferred to the tube wall primarily by radiationflue gases are then passed through the convection section of the heater
84 Convection Section Characterized by extended surfaces fins or studs improve heat transferincrease effective arearequired area governed by LMTD and UoEconomical heat recoverysteam generationboiler feed water preheatcombustion air preheatFlue gas passes through a stack to the atmosphere
85 Convection Section outside film outside fouling tube wall inside film Typical Indirect heat transfer mechanismT bulk outsideoutside filmoutside foulingtube wallinside filminside foulingT bulk inside
86 most of the heat is transferred in the radiant section Radiant Efficiency => Radiant Duty/Fired DutyOverall Efficiency => Absorbed Duty/Fired DutyIn large heatersmost of the heat is transferred in the radiant sectionConvection sectionincreases the overall efficiency of the heaterSome fired heaters, for very low heat duty services, have no convection sectionthermal efficiencylowest capital investment.Most fired heaters have both a radiant and convection sectionNew-fired heatersair preheating systemNOx reduction system
87 Process Fluid:Fluid characteristics of the process fluids should be considered before designing a heater.for example,Very high viscosity fluids have tendency to attain very high film temperature, as the fluid in the film does not readily mix with the bulk fluid.This results in uneven distribution of heat in the fluid and develops hot spots, where vaporization and degradation occurs.
88 Heat Duty:Total furnace heat dutysum of heat transferred to all process streams, including auxiliary services such as steam superheaters.Amount of heat duty affects the selection of type and configuration of heater.
91 What Flue Gas Temperature Tells Us Flue gas temperature is an indication of how effectively combustion heat is being transferred to the boiler water. In general, lower flue gas temperature indicates better heat transfer and higher overall efficiency. It means that less energy is going up the stack and, everything else being equal, more is going into the water. If you find flue gas temperature gradually increasing over time, this indicates gradually deteriorating heat transfer within the boiler. This could be caused by soot buildup on the fire side of the heat transfer surfaces, or scale buildup on the water side. A flue gas temperature increase of 100°C indicates a boiler efficiency decrease of 4 to 5 per cent.
92 Average Radiant Heat Flux: Average radiant heat flux rate is very important parameter for design of a fired heater.Higher the design radiant flux, less the heat transfer surface, smaller the heater and lower the cost.Unduly high radiant rates, however, result in higher maintenance cost due to shortened life of components and coke deposition.Allowable average radiant heat flux rate is a function of various factors such as heater type, feedstock, service, coil outlet temperature etc. and, therefore, established by experience.
93 Mass Flow Velocity:Cracking and polymerization occur in the film of the fluid inside the tube wall surface.To minimize coking and fouling in coils, fired heaters should be designed with high enough mass velocities.However, too high a mass velocity will cause a high coil pressure drop, resulting in high pumping or compressor costs, increased design pressure of the coils and upstream equipment and possible erosion at the heater return bends.Design mass velocity is usually kept in the range of 250 to 350 lb/sec-ft2. Under turndown conditions, mass velocity should be kept above 150 lb/sec-ft2 in order to prevent excessive coking and fouling of the coils.
94 Vaporization in Process Fired Heaters: It is desirable to avoid a situation when the liquid or partially vaporized stream reaches a point within the heater in which it is 100% vaporized.Foreign material or polymer formed in tankage, which does not vaporize, may deposit on the tube and cause coking.Therefore, limit the maximum vaporization to about 80%.
95 Tube size, Number of Passes and Fluid Pressure Drop: A combination of the tube size and number of passes is selected to satisfy the mass flow velocity, throughput and fluid pressure drop requirements.Tube diameters are normally selected from standard tube sizes, in the range of 3 to 8 inches.Non-standard sizes can also be used when design parameters cannot be met with standard sizes.
96 Turndown:Turndown requirements are set by process considerations.In general, turndown rates of 60% can be used without falling below mass velocity rates needed to prevent excessive coking rates.
97 Burner turndown is a function of burner design and the type of fuel. However, burner turndown does not normally affect furnace turndown, but burners can be turned off or excess air increased when furnace is operated at greatly reduced firing rates.
98 Stack Temperature and Optimum Heater Efficiency: The economic stack temperature or the optimum efficiency of the heater is a function of fuel value, inlet oil temperature, investment cost of the incremental convection section and the required rate of return from incremental investment.Stack temperature usually ranges from 350°F to 700°F, however, a temperature of 250°F can be achieved for low sulfur fuel using air preheater.Stack temperature must be high enough to prevent acid condensation on the convection section inlet tubes and air preheater.Most of the new fired heaters have convection sections and air preheater. As a result, heater efficiencies have increased to more than 90%.
99 Tube/Coil Materials:Heater tubes are usually made from carbon steel, alloy steel or stainless steel pipes.Tubing material is selected based on service life, corrosion resistance and cost.Allowable stresses in the tube material decrease with increasing temperatures, therefore, higher tube temperatures require thicker tubewalls or higher alloy-content.Carbon steel is the most widely used material for heater tubing where corrosion resistance is relatively mild.
104 Fired Heaters or Furnaces, are used for various purposes in the Refining & Petrochemical Industry, Power Generation Facilities and in Homes and Apartments, etc.In refineries applications include:Air Heater, Boilers, Reboilers, Vacuum Charge Heater, Crude Furnace, Hydrocracker, Pyrolysis Furnace, Reforming Furnace, Visbreaker Furnace, etc.
111 The first step in the refining process is the separation of crude oil into various fractions or straight-run cuts by distillation in atmospheric and vacuum towers.The main fractions or "cuts" obtained have specific boiling-point ranges and can be classified in order of decreasing volatility into gases, light distillates, middle distillates, gas oils, and residuumAt the refinery, the desalted crude feedstock is preheated using recovered process heat. The feedstock then flows to a direct-fired crude charge heater where it is fed into the vertical distillation column just above the bottom, at pressures slightly above atmospheric and at temperatures ranging from 650° to 700° FAll but the heaviest fractions flash into vapor. As the hot vapor rises in the tower, its temperature is reduced.
112 VACUUM DISTILLATION PROCESS In order to lower the temperatures in further distilation of residual crude from the atmospheric tower, reduced pressure.Vacuum towers may produce gas oils, lubricating-oil base stocks, and heavy residual.Vacuum towers are typically used to separate catalytic cracking feedstock from surplus residuum.
113 Hydrocracking Process Hydrocracking Process. In the first stage, preheated feedstock is mixed with recycled hydrogen and sent to the first-stage reactor, where catalysts convert sulfur and nitrogen compounds to hydrogen sulfide and ammonia.Limited hydrocracking also occurs. After the hydrocarbon leaves the first stage, it is cooled and liquefied and run through a hydrocarbon separator. The hydrogen is recycled to the feedstock. The liquid is charged to a fractionator. Depending on the products desired (gasoline components, jet fuel, and gas oil), the fractionator is run to cut out some portion of the first stage reactor out-turn. Kerosene-range material can be taken as a separate side-draw product or included in the fractionator bottoms with the gas oil.
114 VISBREAKING PROCESSVisbreaking, a mild form of thermal cracking, significantly lowers the viscosity of heavy crude-oil residue without affecting the boiling point range. Residual from the atmospheric distillation tower is heated ( degrees F) at atmospheric pressure and mildly cracked in a heater.It is then quenched with cool gas oil to control overcracking, and flashed in a distillation tower. Visbreaking is used to reduce the pour point of waxy residues and reduce the viscosity of residues used for blending with lighter fuel oils. Middle distillates may also be produced, depending on product demand. The thermally cracked residue tar, which accumulates in the bottom of the fractionation tower, is vacuum flashed in a stripper and the distillate recycled.
120 Importance of DraftFired heaters are major consumers of energy in the chemical process industries (CPI), especially at petroleum refineries and petrochemical plants--accounting for as much as 70% of total plant consumption in some instances. While most plant engineers and operators are aware of the importance of controlling excess oxygen in fired heaters, they often overlook a key determinant of efficient heater operation: the control of their draft, namely, the negative pressure inside the vessel with respect to the atmosphere.
121 In most fired heaters, the draft is maintained at almost four times the value recommended. At the other end of the spectrum, some heaters run with no draft--in fact, with positive pressure at the radiant arch (the transition zone between the radiant and convection sections).Neither situation is desirable; they can cause considerable loss of energy, and can even be hazardous.
123 ZONE ANALYSISdefined as regimes of phase changes where the overall heat transfer coefficient (Uo) will varyUsing T-Q (Temperature-Heat) diagrams are the best way to pinpoint zones. Chemical #1 enters the shell at 200 0C as a superheated vapor. In Zone 1, it releases heat to the tubeside chemical (Chemical #2). Zone 1 ends just a Chemical #1 begins to condense. The tubeside (Chemical #2) enters as a liquid or gas and does not change phase throughout the exchanger.
124 ZONE ANALYSISChemical #1 leaves Zone 1 and enters Zone 2 at its boiling temperature, Tb1. T* marks the temperature of Chemical #2 when Chemical #1 begins to condense. In Zone 2, Chemical #1 condenses to completion while Chemical #2 continues to increase in temperature. The temperature of Chemical #2 when Chemical #1 is fully condensed is denoted at T**. Finally, in Zone 3, both chemicals are liquids. Chemical #1 is simply liberating heat to Chemical #2 as it becomes a subcooled liquid and exits the shell at 100 0C.
125 ZONE ANALYSISDefining zones is one of the most important aspects of heat exchanger design. It is also important to remember that if a process simulator does not support zoned analysis you should model each zone with a separate heat exchanger. Thus, the previous illustration would require 3 heat exchangers in the simulation and also may require three separate bundles in the real plant.
126 GJM Procedure, part 1 Assemble Physical Properties Assemble Service RequirementsConstruct T-Q DiagramEstablish Zones and Calculate LMTDGuess a UoCalculate Required Surface Area
127 GJM Procedure, part 2 Pick Tube Dimensions Calculate Number of Tubes size, gauge, lengthCalculate Number of TubesCalculate Flow Characteristics on Shell and Tube SidesVelocity, Re, Pr, NuCalculate hi, ho
128 GJM Procedure, part 3 Calculate Uo Check versus Assumed Uo Iterate or Goal Seek (Excel) Until the Uo MatchesCalculate Pressure Drop
129 If using a process simulator, obtaining the physical properties of the streams are easy. Get the physical properties for each zone separately to ensure accuracy, but in some cases it is acceptable to use an average value. This would be true for sensible heat transfer since the material is not changing phase or undergoing a truly significant temperature change (over 1000C). Physical properties will include: heat capacity, viscosity, thermal conductivity, density, and latent heat (for phase changes). These are in addition to the boiling points of the streams at their respective pressures.
130 Guessing UoEstimates for the heat transfer coefficients can be found in most textbooksMain equation Q=UoADTlmThe above equation must be used to get an area for each zone, then add them together.
132 The determination of U is often tedious and needs data not yet available in preliminary stages of the design. Therefore, typical values of U are useful for quickly estimating the required surface area. The literature has many tabulations of such typical coefficients for commercial heat transfer services. Lower values are for unfavorable conditions such as lower flow velocities, higher viscosities, and additional fouling resistances. Higher values are for more favorable conditions. Coefficients of actual equipment may be smaller or larger than the values listed. Note that the values should not be used as a replacement of rigorous methods for the final design of heat exchangers, although they may serve as a useful check on the results obtained by these methods.
133 What geometric configuration is right for the exchanger? Once you've selected a shell diameter, tubesheet layout, baffle and tube spacing, etc., it's time to check your velocity and pressure drop requirements to see if they're being met. Experienced designers will usually combine these steps and actually obtain a tube size that meets the velocity and pressure drop requirements and then proceed. Some guidelines may be as follows: 3/4 in. and 1.0 in. diameter tubes are the most popular and smaller sizes should only be used for exchangers needing less than 30 m2 of area. If your pressure drop requirements are low, avoid using four or more tube passes as this will drastically increase your pressure drop.
134 This is where you'll spend much of your time in designing a heat exchanger. Although many textbooks showNu=0.027(NRE)0.8(NPR)0.33as the "fundamental equation for turbulent flow heat transfer", what they sometimes fail to tell you is that the exponents can vary widely for different situations. For example, condensation in the shell has different exponents than condensation in the tubes. Use this fundamental equation if you must, but you should consult a good resource for accurate equations.
136 Lytron's heat exchanger technologies include tube-fin, oil cooler, plate-fin, and liquid-to-liquid brazed plate heat exchangers. With almost five decades of experience in heat exchanger design and manufacturing, Lytron's expertise is second-to-none. Our heat exchangers are used in many industries including military and aerospace, medical and industrial lasers, medical imaging, analytical instrumentation, power electronics, semiconductor equipment, machine tools and telecommunications.Any of our heat exchanger technologies can be customized to your exact requirements. We also supply heat exchanger subassemblies, adding fittings, hoses, fans, sensors and other instrumentation to your specification. And our seven product ranges with over 130 standard parts ensure that when you need an off-the-shelf part we are likely to have something that meets your requirements.Lytron’s broad range of heat exchanger technologies combined with almost five decades of thermal transfer experience enables us to offer tailored solutions for a wide range of applications. Our tube-fin, oil cooler, plate-fin, and liquid-to-liquid brazed plate heat exchangers can all be customized and integrated into subassemblies.Performance comparisonThis chart compares the performance of different heat exchanger technologies, including plate-fin, oil cooler, and tube-fin heat exchangers. Performance is shown as Q/ITD, the heat load divided by the difference in incoming temperature of the liquid and air. Units are not shown so that technologies can be compared regardless of size.As many heat exchangers are customized, a range of typical values is shown for each technology. All performances are compared using water as the cooling fluid.Fluid compatibilityCoolant compatibility with wetted surfaces must be considered when selecting a heat exchanger technology. A copper fluid path is compatible with water and most common coolants. A stainless steel fluid path is necessary when using deionized water and other corrosive fluids. Aluminum offers excellent performance with ethylene glycol/water mixture (EGW), oils and other fluids, but is not compatible with untreated water. The table below shows fluid/heat exchanger compatibility.
137 ME-143 Principles of Heat Exchanger Design, Part II This seminar provides a continuation of the discussion of the equipment used for the transfer of heat. Specific examples and case studies will highlight analysis techniques and step-wise procedures. Emphasis will be placed on the calculation of the film coefficient for a variety of different geometries and how to compose an overall heat transfer coefficient. Transfer line exchangers, plate-fin exchangers and Joule-Thompson exchangers are among the examples covered.
138 Energy Balance Mechanical design and process arrangement OutlineHeating and cooling requirementsIndirect or direct heat transferVarious types of heat exchangersMechanical design and process arrangementKey vendors and producersTubular Exchanger Manufacturers Association
139 What is Heat ? Universe: matter and energy Energy causes the atoms and molecules to always be in motionMotion of atoms and molecules creates heat or thermal energyIn space, matter still has a very small amount of heat energy
141 Force, Pressure, Work, Energy, Power Force: Newton, (kg)(m)(s-2)Pressure: Pascal, (N)(m-2)Work: Joule, (N)(m)Energy: Joule, (N)(m)Power: Watt, (J)(s-1)Example: a 60 W light bulb expends 216 kJ every hour of use and costs the consumer about 1 cent (US)
142 Energy in Colloquial Terms 1 BTU = 1055 J1 BTU = kJ1 kWh = 3600 kJEnergy in Colloquial TermskWh = 1 kJ/s of power expended for 1 hCost (based on PECO): $0.165/kWh
143 Current Energy Production Picture – US and World-Wide Language of Energy Production and UsageMagnitudeEnergy NeedsEnvironmental Impactas a energy and environmental primer:in combustion: 1 kg C yields ~ 3.7 kg CO2 and ~ 33 MJ
144 Unit Levels – SI Prefixes 1 EW exa103 PW peta106 TW tera109 GW giga1012 MW mega1015 kW kilo1018 W -----1021 mW milli1024 mW micro
145 World Energy Consumption Fuel typePower in TWEnergy in EJOil5.6180Gas3.5110Coal3.8120Hydroelectric0.930NuclearGeothermal, wind, solar, wood0.134Total15471as a reference: 174,000 TW (174 PW) incoming solar power
146 Value of Energy - Energy Information Administration Price SummaryYear 2007 2008 2009 2010 WTI Crude ($/barrel)72.3299.5760.1272.42Gasoline ($/gal) 2.813.262.342.7Diesel ($/gal)2.883.82.47Heating Oil ($/gal)2.723.382.512.78Natural Gas ($/mcf)13.0313.6711.9211.56Electricity (cents/kwh)10.6511.3611.6411.4
147 Basic Heat Balance The First Law of Thermodynamics: expression of the principle of conservation of energystates that energy can be transformed but is not created or destroyed
148 not viable in most situations Indirect streams do not mix Heat TransferDirectstream mixingcheapnot viable in most situationsIndirectstreams do not mix
149 Heat Transfer Mechanisms ConductionConvectionRadiation
150 Conduction Transfer of thermal energy (heat) from a region of higher temperature to a region of lower temperature by direct contact (Second Law of Thermodynamics)
151 Conduction Direct molecular communication without a flow of material Metals (eg. copper) are usually the best conductors of thermal energy. Fluids are not typically good conductors. HX tubes are usually made of copper, CS, or SS
152 Convection combination of conduction and the transfer of thermal energy by fluid circulation or movement of the hot particles in bulk to cooler areas in a material medium.T bulkconvective film coefficientT surface
153 This movement occurs into a fluid or within a fluid, and cannot happen in solids In solids, molecules keep their relative position to such an extent that bulk movement or flow is impossible
154 RadiationTransfer of heat through electromagnetic radiationObjects radiate energy at a rate equal to their emissivity times the rate at which energy would radiate from them if they were perfectradiators
155 RadiationNo medium is necessary for radiation to occur; radiation works even in and through a perfect vacuum
156 RadiationReflectivity and emissivity of all bodies is wavelength dependentTemperature determines the wavelength distribution of the electromagnetic radiation
160 Heat Transfer Equations Beginnings of a T-Q Curve Q, kJ/h heat transferredl, latent heatcp, specific heat
161 Shell and tube heat exchanger Indirect heat transferMost common type of heat exchangerin oil refineries and other large chemical processesSuited for higher-pressure applications
162 Shell and tube heat exchanger Consists of a shell (a large pressure vessel) with a bundle of tubes inside it.One fluid runs through the tubes, and another fluid flows over the tubes (through the shell) to transfer heat between the two fluids.The set of tubes is called a tube bundle, and may be composed by several types of tubes: plain, longitudinally finned, etc.
163 Shell and tube heat exchanger design There can be many variations on the shell and tube design. Typically, the ends of each tube are connected to plenums (sometimes called water boxes) through holes in tubesheets.The tubes may be straight or bent in the shape of a U, called U-tubes.
166 The Tubular Exchanger Manufacturers Association, Inc. TEMAThe Tubular Exchanger Manufacturers Association, Inc.(TEMA) is trade association of leading manufacturers of shell and tube heat exchangers, who have pioneered the research and development of heat exchangers for over sixty years.The TEMA Standards and software have achieved worldwide acceptance as the authority on shell and tube heat exchanger mechanical design.
167 TEMA MissionUsers need to be absolutely sure that their manufacturer is reliable, producing heat transfer equipment that is safe, effective, and economical.Considerable time and money are spent having a heat exchanger designed and built to exact specifications, design codes and requirements.
168 TEMA HistoryTEMA, was founded in 1939 and has grown to include a select group of member companies. Members adhere to strict specificationsTEMA members develop and update today's standards. TEMA Standards and Software have achieved worldwide acceptance as the authority on shell and tube heat exchanger mechanical design
169 TEMA Outreach and Standards TEMA members are market-aware and actively involved, meeting several times a year to discuss current trends in design and manufacturingThe internal organization includes various subdivisions committed to solving technical problems and improving equipment performance
174 Overview of HX Design – Part 1 Equipment used for the transfer of heat energyBasic heat balance as a T-Q curveAppropriate TEMA designationEconomic and environmental considerationsPinch Technology for heat exchanger networks
175 Fluid Flow Arrangements The fluids can be either liquids or gases on either the shell or the tube side.In order to transfer heat efficiently, a large heat transfer area should be used, so there are many tubes.In this way, waste heat can be put to use. This is a great way to recuperate energy.
176 Sensible Heat Transfer vs Phase ChangeHeat exchangers with only one phase (liquid or gas) on each side can be called one-phase or single-phase heat exchangersTwo-phase heat exchangers can be used to heat a liquid to boil it into a gas (vapor), sometimes called boilers, or cool a vapor to condense it into a liquid (called condensers), with the phase change usually occurring on the shell side.
177 Amongst the Largest HXs are….. In large power plants with steam-driven turbines, shell-and-tube surface condensers are used to condense the exhaust steam exiting the turbine into condensate water which can be recycled back to be turned into steam, possibly into a shell-and-tube type boiler
184 CFU Designation (C). Channel Integral with. Fixed Tube Sheet (F) CFU Designation (C) Channel Integral with Fixed Tube Sheet (F) Two Pass Shell with Longitudinal Baffle (U) U-tube (two pass tubes)
185 AKT Designation (A). Channel and Removable. Cover (K). Kettle (T) AKT Designation (A) Channel and Removable Cover (K) Kettle (T) Pull Through Floating Head
186 Heat Transfer Equations - sign convention Exothermic – gives off heatUsually written as negativeEndothermic – absorbs heatUsually written as positiveGJM – “think positively”Using words like heat removed and heat absorbed and plot on the same axis
190 BasicsTwo fluids, of different starting temperatures, flow through the heat exchanger. One flows through the tubes (the tube side) and the other flows outside the tubes but inside the shell (the shell side).Heat is transferred from one fluid to the other through the tube walls, either from tube side to shell side or vice versa.This process is best represented by a T-Q diagram
191 Indirect Heat Transfer Example Plate and FrameHeat Exchanger
192 2nd Law of Thermodynamics - Bottom Line At every point in the HX, the hot stream must be hotter than the cold stream by a distance called the driving force
193 J-T Heat Exchanger System With Brazed Aluminum Type plate Exchanger
194 Composite Curves in the Plate Exchanger composite coldcurve
195 Overview of HX Design – Part 2 Step 1: Gather duty requirements and physical propertiesStep 2: Construct a T-Q CurveStep 3: Guess an Overall Heat Transfer CoefficientStep 4: Calculate the LMTDStep 5: Calculate the F (correction factor)Step 6: Calculate the Heat Transfer Area
196 Overview of HX Design – Part 2 Step 7: Assign the tube specifications – diameter, BWG and lengthStep 8: Calculate the outside surface area per tubeStep 9: Calculate the number of tubes to provide the required areaStep 10: Calculate the velocity - shell side and tube sideStep 11: Calculate the tube and shell side coefficientsStep 12: Calculate the Overall Heat Transfer CoefficientStep 13: Check with Step 3, adjust and recalculate
200 OutlineAssessing the performance of a heat exchangerValue of energy transferredCapital cost of the heat exchangerEconomic performanceEconomic sensitivity
201 Assessing the Performance of a Heat Exchanger OutlineLeveraging issues, such as the level of energy transferredFouling, cleaning and sparing philosophyFuture of heat exchanger designNew technologyBreakthrough design options
202 Assessing the performance of a heat exchanger Calculate the required heat transfer area and theassociated costCalculate the cost of any utility or utility savingsPerform an economic analysis based on deltacosts and delta returns – that is will additionalsurface area of a heat exchanger provide a highenough reduction in utility costs
203 Assessing the performance of a heat exchanger Energy CostHeat Exchanger Area
207 Basics of Pinch Technology for Heat Integration Pinch technology presents a simple methodologyfor systematically analyzing chemical processesand the surrounding utility systems with the help ofthe First and Second Laws of Thermodynamics.The temperature approach is the minimumallowable temperature difference (DTmin) in thestream temperature profiles, for the heat exchangerunit.
208 Basics of Pinch Technology for Heat Integration, continued…. The temperature level at which DTmin isobserved in the process is referred to as"pinch point" or "pinch condition"The pinch defines the minimum driving forceallowed in the exchanger unit.
209 Objectives of Pinch Technology for Heat Integration Pinch Analysis is used to identify energy cost andheat exchanger network (HEN) capital cost targetsfor a process and recognizing the pinch pointThe procedure first predicts, ahead of design, theminimum requirements of external energy, networkarea, and the number of units for a given process atthe pinch point
226 Questions Give an example of direct heat transfer Give an example of indirect heat transferEnergy is transferred from hot to cold (T/F ?)What is the main S&T HX design equation?What is a T-Q diagram?Does the T-Q for the cold material intersect the T-Q curve for the hot material?Discontinuities on the T-Q diagram represent phase change (T/F?)What is the main trade organization for S&T HXs?What does LMTD stand for?Provide a reasonable Uo for a gas-gas HX?