Heat Transfer Lecture

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Heat Transfer FundamentalsHeat Exchange Equipments1Temperature, Heat, and Expansion What is temperature? A measure of how warm or cold an object is with respect to some standard Related to the random thermal motion of the molecules in a substance Measure of avg. translational kinetic energy of molecules What is heat? The energy transferred between objects due to a temperature difference Energy in transit (similar to work) How are the two concepts related? Heat always flows from hotter to colder objects2Heat and Internal Energy Internal energy Total energy contained in a substance translation, rotational, vibrational kinetic energies interparticle potential energies When an object absorbs (gives off) heat, its internal energy increases (decreases)Which has more internal energy?3Specific Heat Capacity The quantity of heat needed to raise the temperature of one gram of a substance by 1 Celsius Measures the resistance of a substance to temp. changes Thermal inertia Works both ways Substances that take longer to heat up also take longer to coolWhich has a higher specific heat,the filling or the crust?4How is the heat transfer? Heat can transfer between the surface of a solid conductor and the surrounding mediumwhenever temperature gradient exists.ConductionConvection Natural convection Forced ConvectionThermal Radiation5Modes of heat transfer Conduction: diffusion of heat due to temperature gradients. A measure of the amount of conduction for a given gradient is the heat conductivity. Convection: when heat is carried away by moving fluid. The flow can either be caused by external influences, forced convection; or by buoyancy forces, natural convection. Convective heat transfer is tightly coupled to the fluid flow solution. Radiation: transfer of energy by electromagnetic waves between surfaces with different temperatures, separated by a medium that is at least partially transparent to the (infrared) radiation. Radiation is especially important at high temperatures, e.g. during combustion processes, but can also have a measurable effect at room temperatures.6Conduction Heat transfer by successive atomic collisions Ex.A metal rod held in a fire Conductors Solids built from atoms that have free outer electrons Readily transport energy via collisions Ex. Metals Insulators Tightly bound outer electrons Wood, cork, styrofoam, air Air vs. rack in an ovenDoes cold flow into your finger,or does heat flow out? 7Convection Heat transfer due to the actual motion of a fluid Operates in liquids and gases What causes convection currents? Convection currents Heated fluid expands and rises due to buoyant force Cooler, denser fluid descends and takes its place Rising fluid cools as it expands8Natural and forced ConvectionNatural convection occurs whenever heat flows between a solid and fluid, or between fluid layers.As a result of heat exchangeChange in density of effective fluid layers taken place, which causes upward flow of heated fluid.If this motion is associated with heat transfer mechanism only, then it is called Natural Convection9Forced ConvectionIf this motion is associatedby mechanical means such as pumps, gravity or fans, the movement of the fluid is enforced.And in this case, we then speak of Forced convection.10Radiation Energy transmitted in the form of electromagnetic waves Only mechanism that can transmit energy across the vacuum of space Conduction requires collisions in a material object Convection requires a fluid Ex.Solar energy received by the Earth11Overview dimensionless numbers Nusselt number:Ratio between total heat transfer in a convection dominated system and the estimated conductive heat transfer. Grashof number: Ratio between buoyancy forces and viscous forces. Prandtl number:Ratio between momentum diffusivity and thermal diffusivity. Typical values are Pr= 0.01 for liquid metals; Pr= 0.7 for most gases; Pr= 6 for water at room temperature. Rayleigh number:The Rayleigh number governs natural convection phenomena. Reynolds number:Ratio between inertial and viscous forces.. /fk hL Nu !wT g L Gr V R V F2 3/ A !E Q F V Q F V / /3 2 3T g L k T c g L Pr Gr RapA ! A ! !. / k c PrpQ !. / Q VUL Re !12Heat Transfer Equipment Heat Transfer EquipmentHeat Exchangers Heat ExchangersEvaporators EvaporatorsBoilers BoilersProcess Fired Heaters Process Fired Heaters13Heat Exchangers Heat Exchangers14Applications of Heat ExchangersHeat Exchangers preventcar engine overheating and increase efficiencyHeat exchangers are used in Industry for heat transferHeat exchangers are used in AC and furnaces15What is a Heat Exchanger? A device built for efficient flow of thermal energy from one fluid to another at a different temperature, whether the fluids are separated by a solid wall so that they never mix, or the fluids are directly contacted. Heat exchangers are used to transfer heat from one stream to another.They are used to heat streams and to cool streams.The streams can be heated or cooled by other process streams, or they can be heated by steam and cooled by cooling water. Widely used in refrigeration, air conditioning, electronics, space heating, power production, and chemical processing. 16Heat ExchangersThe driving force for the operation of a heat exchanger is the temperature difference between the fluids.The higher the temperature difference, the smaller the heat exchanger.However, the temperature difference is limited by boiling points of the liquids, scaling, materials of construction, etc.The proper design of a heat exchanger is a trade-off of price and performance.171819Heat Exchangers - TypesExchange heat between fluidsLatent heat and sensible heat transferHeat exchangers come in many different types:1. Co-current.2. Counter-current3. Cross flow4. Double pipe5. Spiral6. Finned7. Compact8. Shell and tube (most common in chemical process industries)20Heat Exchangers - TypesCo- and Counter- current:The exchanger is constructed of concentric tubes.One fluid occupies the outer pipe, the other the inner.Co-current: fluids flow in same directionCounter-current:fluids flow in opposite directionCo-currentCounter-current21CounterflowHeat ExchangerParallel Heat ExchangerTYPES OF HEAT EXCHANGERS22Crossflow Heat ExchangerTYPES OF HEAT EXCHANGERS23Some important concepts:ApproachRangeLog mean Temperature Difference (LMTD)Heat fluxOverall heat transfer coefficientIndividual heat transfer coefficient24Q hot Q coldThTi,wallTo,wallTcRegion I : Hot Liquid-Solid ConvectionNEWTONS LAW OF CCOLINGdqx! hh. Th Tiw ).dARegion II : Conduction Across Copper WallFOURIERS LAWdqx! k.dTdrRegion III: Solid Cold Liquid ConvectionNEWTONS LAW OF CCOLING dqx! hc. Tow Tc ).dATHERMALBOUNDARY LAYEREnergy moves from hot fluid to a surface by convection, through the wall by conduction, and then by convection from the surface to the cold fluid. 25BulkFluidTemperatureTemperatureOutsideFluidTube WallOutside FilmOutside Fouling (Scale)Inside Fouling (Scale)Inside FilmBulk FluidTemperatureInside FluidRrordormdiriResistance to Heat Transfer Resistance to Heat Transfer26Principle of Heat Exchanger First Law of Thermodynamic: Energy is conserved.generated sin outout ine w q h m h mdtdE + + + '+

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! .. !out inh m h m..hhp h hT C m A Q A ! . . . ccp c cT C m A Q A ! . . . 00 0 0Control VolumeCross Section AreaHOTCOLDThermal Boundary Layer27Region I : Hot Liquid Solid Convection Th Tiw!qxhh.Ai qx! hhot. Th Tiw ).ARegion II : Conduction Across Copper Wall qx!kcopper.2xLlnrori To,wall Ti,wall!qx.lnrori ' + ' kcopper.2xLRegion III : Solid Cold Liquid Convection To,wall Tc!qxhc.Ao qx! hcTo,wall Tc )Ao+ Th Tc! qx1hh.Ai+lnrori ' + ' kcopper.2xL+1hc.Ao | qx!U.A. Th Tc )11.ln ..

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+ !cold i copperiooi hotoh r krrrr hrUU = The Overall Heat Transfer Coefficient [W/m2.K]] 3 2 1 [ R R R qx T Tc h+ + ! U !1A.LRrori28Q Q Q ! !give getmQ KAt! AmQq KtA! ! Aheat transfer Eq.Basic Eq. of heat transferthermodynamicswhere Q heat transfer rateWA transport aream2 q heat transfer fluxW/m2tmmean temperature difference between hot and cold fluids, K K heat transfer coefficientW/m2.K heat balanceor29Heat Exchangers - TypesDouble-Pipe:A heat exchanger built from concentric tubes.These are simple to construct, but may require a lot of physical space to achieve the desired heat transfer area. Can be used for gases and liquids.3031Simplified Process Flow DiagramThiThoTciTcoQout, TSQin, TSQin, SSQout, SS327.2.1task of heat transfer1 no phase transformation W1Cp1(T1 - T2) = W2Cp2(t2 - t1)single pass exchangerQrelease=QabsorbW2,Cp2,t2W2,Cp2,t1W1,Cp1,T1 W1,Cp1,T22hot fluid condense, cold fluid heating W11= W2Cp2(t2 - t1) 3 cold fluid evaporation, hot fluid cooling W1Cp1(T1 - T2)= W224two fluids phase transformation W11= W22W2,Cp2,t2W2,Cp2,t1W1,r1,TW1, r1,T W1,Cp1,T1 W1,Cp1,T2W2,r2,tW2,r2,tW1,r1,TW1, r1,TW2,r2,tW2,r2,t33Shell&Tube Heat Exchangers Shell and Tube:Most common in chemical process industries.This type provides a large heat transfer surface in a small space, can operate at high pressures, are easy to clean and can be made of a wide variety of materials.1-1 Counter Current Exchanger (1-shell, 1 tube pass)343536Heat Exchangers - Types1-2 Heat Exchanger:1 shell pass, 2 tube passes.37Heat Exchangers - Types2-4 Heat Exchanger:2 shell passes, 4 tube passes.38cold fluidt1cold fluidt2hot fluid T1hot fluidT2Hot and cold fluids do not contactdirectly. Heat is exchanged throughsolid wall conduction .Heat exchangerspace inside tubetube passspace outside tubeshell passfluid flows mtimes in tubesmtube passfluid flows mtimes in shellmshell passQQtubular exchangersingel pass shell-tube exchangerdouble pass shell-tube exchanger39Heat Exchangers - Temperatures40Shell and Tube Heat Exchanger Shell and Tube Heat Exchanger41Parts of S&T Exchangers Tubes & Tube sheet Shell Inlet& outlet nozzles Baffles Guide rods Longitudinal baffles Head Stuffing box42Important things related to S&T Exchanger Tube pitch (triangular or square)CleaningShell side pressure dropHeat transfer area Tube length (BWG8,12,16,20) Shell diameter (up to 23) Baffle spacing (should not be less than one fifth of shell dia or more than the inside diameter of the shell) Clearance (center to center distance between tubes)4344Shellside Shellside FlowFlow Out OutTubeside Tubeside FlowFlow In InTubeside Tubeside FlowFlow Out OutShell ShellTube Bundle Tube BundleShellside Shellside FlowFlow In InShell and Tube Heat ExchangerShell and Tube Heat Exchanger Details Details45Shellinlet ShellinletChannel Inlet Channel InletChannel ChannelOutlet OutletShell Outlet Shell OutletSINGLE SEGMENTAL TRANVERSE BAFFLES SINGLE SEGMENTAL TRANVERSE BAFFLESShell and Tube Heat Exchanger Shell and Tube Heat Exchanger46Shell and Tube Heat Exchanger Shell and Tube Heat Exchanger47Doughnut and Disc Type BafflesShell and Tube Heat Exchanger Shell and Tube Heat Exchanger48Heat Exchangers Problems with CalculationsTemperature of fluid changes (possibly on shell and tube side).Baffles and leakage around baffles.Easy part is tube side use existing correlations to determine heat transfer coefficient.Difficult side is shell side as fluid flows across the tubes.49FoulingFouling is when a fluid goes through the heat exchanger and the impurities precipitate onto the surface of the tubes. Precipitation of these impurities are cause by not cleaning the heat exchanger on a regular basis, using the heat exchanger often, reducing the velocity of the fluids that are moving through the heat exchanger, over sizing the heat exchanger. The effects of fouling are more obvious in the cold tubes than the hot tubes because the impurities are not likely to be dissolved in cold fluid. This is because the solubility increases as the temperature increases. Fouling has the effect of reducing the cross sectional area for the heat to be transferred and causes an increase in the resistance to heat transfer across the heat exchanger. This leads to less efficiency in the heat exchanger which causes an increase in pumping and maintenance costs.50Tube Bending Shell and Tube Heat Exchanger Tube Bending Shell and Tube Heat Exchanger51Tube Failure Shell and Tube Heat Exchanger Tube Failure Shell and Tube Heat Exchanger52FACTORS FOR U THE BOUNDARY LAYER SCALE/CHEMICAL DEPOSITS SOOT/DIRT BUILDUP53Fouling in Shell and Tube Heat Exchanger Fouling in Shell and Tube Heat Exchanger54common heating agents and their applied temperatureheating agent hot watersaturatedsteammineral oilbiphenylmixture*melt salts*flue gasT / 40 100 100 180180 250255 380 142 530500 1000*melt saltsKNO353%NaNO240% NaNO37%common cooling agents and their applied temperaturecoolingagentwater & airaqueous salt solution*liquid ammonia liquid ethane liquid etheneT/ 20 30 -4-20 -33.4 -88.6 -103.7*salt ofNaCl and CaCl3, etc.55Heat Exchangers - TypesSpiral:Constructed from sheets of metal wound in a circular fashion.The fluids flow in adjacent chambers between the sheets of metal.This design is based on the ease of fabrication.5657Plate Heat Exchangers consist of a number of very thin corrugated stainless steel heat transfer plates clamped together in a frame. Every second channel is open to the same fluid. Between each pair of plates there is a rubber gasket, which prevents the fluids from mixing and from leaking to the surroundings. Heat is thus transferred from the warm fluid to the colder fluid via the thin stainless steel plate. The corrugations support the plates against differential pressure and create a turbulent flow in the channels. In turn, the turbulent flow provides high heat transfer efficiency, making the plate heat exchanger very compact compared with the traditional shell-and-tube heat exchanger. In most cases the plate type heat exchanger is the most efficient heat exchanger.58 Plate flow offers superior heat transfer coefficients compared to shell and tube heat exchangers. Plate flow offers true countercurrent flow. This maximizes the mean temperature difference between the fluids.Disadvantagesof plate heat exchangers are their initial expense, they don't work well under high pressure rates and they are not well suited for processing pulpy products or product with particulates. The main weakness of the plate and frame heat exchanger is the necessity for the long gaskets which holds the plates together5960The plates are clamped together, separated by spacing gaskets, and the heating and cooling fluids are arranged so that they flow between alternate plates. Suitable gaskets and channels control the flow and allow parallel or counter current flow in any desired number of passes.A substantial advantage of this type of heat exchanger is that it offers a large transfer surface that is readily accessible for cleaning. The banks of plates are arranged so that they may be taken apart easily. Overall heat transfer coefficients are of the order of 2400-6000 J m-2s-1 C-1.61Plate Type Heat ExchangerA popular heat exchanger for fluids of low viscosity, such as milk, is the plate heat exchanger, where heating and cooling fluids flow through alternate tortuous passages between vertical plates as illustrated in Figure621095843261 71Plate and Frame Heat Exchanger Plate and Frame Heat Exchanger63Heat Exchangers - TypesFinned:contain fins on one heat exchange surface to increase the heat exchange surface.64Heat Exchangers - TypesCompact heat exchangers:constructed from round or square channels.Car radiator is an example of this type.65SerratedFinsSolid FinsStudsExtended Surface Area Extended Surface Area6667PLAINA sheet of metal with corrugated fins at right angles to the plates PERFORATEDA plain fin constructed from perforated material HERRINGBONEMade by displacing the fins sideways at regular intervals to produce a zig-zag effect. SERRATEDMade by simultaneously folding and cutting alternative sections of fins. These fins are also known as the lanced or multi-entry pattern. Fin Types in Plate-Fin Exchangers6869Finned Tubes70Scraped Surface Heat Exchanger One type of heat exchanger, that finds considerable use in the food processing industry particularly for products of higher viscosity. consists of a jacketed cylinder with an internal cylinder concentric to the first and fitted with scraper blades, as illustrated in Figure. The blades rotate, causing the fluid to flow through the annular space between the cylinders with the outer heat transfer surface constantly scraped. Coefficients of heat transfer vary with speeds of rotation but they are of the order of 900-4000 J m-2s-1C-1. These machines are used in the freezing of ice cream and in the cooling of fats during margarine manufacture. 71Scraped Surface Heat Exchanger72JacketedPan In a jacketed pan, the liquid to be heated is contained ina vessel, which may also be provided with an agitator to keep the liquid on the move across the heat-transfer surface. The source of heat is commonly steam condensing in the vessel jacket. Practical considerations of importance are: 1. There is the minimum of air with the steam in the jacket.2. The steam is not superheated .3. Steam trapping to remove condensate and air is adequate. The action of the agitator and its ability to keep the fluid moved across the heat transfer surface are important. Some overall heat transfer coefficients are shown in Table . 7374Condensing fluidHeated fluid Pan materialHeat transfer coefficientsJ m-2s-1C-1Steam Thin liquidCast-iron 1800Steam Thick liquid Cast-iron 900Steam Paste Stainless steel 300Steam Water, boiling Copper160075Heating Coils Immersed in Liquids76Problem: Water flows at 50C0inside a 2.5cm inside diameter tube such that hi= 3500/C0m2.the tube has a wall thickness of 0.8mm with a thermal conductivity of 16 W/mC0.the outside of the tube looses heat by free convection with h0=7.6/m2C0.calculate the overall heat transfer coefficient and heat loss per unit length to surrounding air at 20C0.77