The Process Heat Exchangers Engineering Essay

The Process light up Exchangers Engineering EssayIn this chapter, a full unit of warmth money changer will be designed including its chemical and mechanical design. A passion exchanger is a device built for efficient warmness transfer between two eloquents from one medium to another. The medium whitethorn be separated by a solid wall, so that the fluids never mix, or the fluids may never be in direct contact. Two fluids of assorted temperatures will fall down through the heat exchanger. genus Oestrus exchangers be widely used in space rut, refrigeration, air conditioning, power plants, chemical plants, petrochemical plants, petroleum refineries, and natural gas processing.3.1.1 Classification of Heat ExchangerHeat exchangers may be classified according to their flow arrangement. There ar two main flow arrangements which are parallel-flow and counter-current-flow. In parallel-flow heat exchangers, the two fluids enter the exchanger at the comparable end, and travel in pa rallel to one another to the other locating. In counter-flow heat exchangers the fluids enter the exchanger from opposite ends. Compared both flow arrangements, the counter current design is most efficient, in that it can transfer the most heat from the heat transfer medium.3.1.2 pillow slips of Heat ExchangerThere are many types of heat exchanger in industry. The types chosen based on the function of the heat exchanger itself. Choosing the right heat exchanger requires companionship of different type of heat exchanger as well as well as the environment in which the heat exchanger will operate. With sufficient knowledge of heat exchanger types and operating requirements, the best selection can be made in optimizing the process. Below, in Table 3.1 are list of types and functions of each heat exchanger.Table 3.1 Types and Functions of Heat Exchanger in IndustryNo.TypesFunctions1.Double pipe heat exchangerThe simplest type. Use for heating and cooling.2.Shell and tube heat exchange rUsed for all application.3.Plate exchangerUse for heating and cooling.4.Plate-fin exchangerUse for heating and cooling.5.Spiral heat exchangerUse for heating and cooling.6.Air cooledCooler and condenser.7.Direct contactCooling and quenching.8.Agitated vassUse for heating and cooling.9.Fired heatersUse for heating and cooling.Source Chemical Engineering radiation pattern, R.K.Sinnott.3.1.3 Selections of Heat ExchangerTypically in the manufacturing industry, several different types of heat exchangers are used for just the one process or system to derive the last product. In order to select an appropriate heat exchanger, one would showtimely consider the design limitations for each heat exchanger type. Although cost is often the first criterion evaluated, there are several other important selection criteria which includeHigh/ Low military press limitsThermal PerformanceTemperature rangesProduct conflate (liquid/liquid, leave-takingiculates or high-solids liquid)Pressure Drops a cross the exchangerFluid flow capacityClean-ability, maintenance and repairMaterials required for constructionAbility and ease of future thrashing out3.2 BASIC PRINCIPLES OF DESIGN3.2.1 Design Criteria for Process Heat ExchangersThere are some criteria that a process heat exchanger moldiness satisfy are easily enough stated if we confine ourselves to a certain process. The criteria includeThe heat exchanger must meet the process requirements. This meanspiriteds that it must personnel the desired change in thermal condition of the process stream within the allowable pressure drops. At the same time, it must continue doing this until the next scheduled shut down for maintenance.The heat exchanger must withstand the service conditions of the environment of the plant which includes the mechanical stresses of installation, startup, shutdown, normal operation, emergencies and maintenance. Besides, the heat exchanger must also resist corrosion by the environment, processes and stream s. This is mainly a matter of choosing materials of construction, but mechanical design does have some effect.The heat exchanger must be maintainable, which ordinarily implies choosing a configuration that permits cleaning and replacement. In order to do this, the limitations is the positioning the exchanger and providing clear space around it. Replacement usually involves tubes and other components that may be especially vulnerable to corrosion, erosion, or vibration.The cost of the heat exchanger should be consistent with requirements. Meaning of the cost here instrument to the cost of installation. Operation cost and cost of lost production due to exchanger malfunction or unavailable should be considered earlier in the design.The limitations of the heat exchanger. Limitations are on length, diameter, weight and tube specifications due to plant requirements and process flow.3.2.2 Structure of the Heat ExchangerThe basic structure of heat exchanger is the same whether using hand design method or computer design method. The logical structure of the heat exchanger design procedure is shown in frame of reference 2.15. From the figure, clearer view and steps of designing a heat exchanger can be obtained. insure 3.1 Basic Logical Structure of Heat Exchanger Design3.3 CHEMICAL DESIGN3.3.1 Problem IdentificationIn designing a heat exchanger in production of 100, 000 metric tonnes/year of Acrylonitrile, there is moreover one heat exchanger exists. The function of it is to exchange the temperature between the stream from Reactor with the temperature from 125C to 25C and the stream comes from Reboiler 5 from 90C to 120C.90.0 0C125.0 0C450.0 0C120.0 0CFigure 3.2 Diagram of shell and tube heat exchanger3.3.2 Determination of physical propertiesTable 3.2 Physical Properties of the tube side fluid (water)PropertiesInletMeanOutletTemperature (0C)90.0105120Pressure (kPa)70.139120.82198.52Specific heat (kJ/kg0C)4.2044.2244.249Thermal conductivity (W/m0C)0.11540.11980.112 7Density (kg/m3)0.4310.6230.721Viscosity (N sm-2)3.145 x 10-42.677 x 10-42.321 x 10-4Table 3.3 Physical Properties of shell fluid ( process fluid)PropertiesAverage Temperature, Tave = 287.5 0CPressure (kPa)150Specific heat (kJ/kg0C)1.1Thermal conductivity (W/m0C)0.1553Density (kg/m3)1.255Viscosity (N sm-2)4.529 x 10-4Only the thermal design will be carried out by using Kerns method. Since water is corrosive, so the tube-side is assign.Logarithmic mean temperature,Where, T1 = Inlet shell side fluid temperatureT2 = Outlet shell side fluid temperaturet1 = Inlet tube side fluid temperaturet2 = Outlet tube side fluid temperatureThus, Log mean temperature= 131.4477 0CThe true temperature difference is given by,Where, is the temperature correction instrumentFrom Figure 12.19, Chemical Engineering Design,Thus,0CFrom Table 12.1(Sinnott 2005), we assume value of overall coefficient, U = 500.0 W/m2.oC.Heat LoadHeat transfer force field,Where, Q = heat transferred per unit time (W)U = overal l heat transfer coefficient,(W/m2.oC)Tm = the mean temperature difference (oC)Thus,= 190.126 m23.3.3 Tube-side coefficientTable 3.4 proportionality of Heat-Exchanger tubesMaterialCarbon SteelOuter diameter, Dto (mm)50.8Length of tube Lt (m)5.0Inner diameter, Dti (mm)45.26BWG number12.0Source Transport Processes and Separation Process Principles, C. J. GeankoplisHeat transfer area of a tube, At = DoL= (50.8 x 10-3) 5= 0.798 m2Number of tube, Nt = A/At= 190.126 / 0.798= 238.25 = 239 tubesCross sectional area of a tube = (Di2) / 4= (45.26 x 10-3)24= 1.6089 x 10-3 m2By using two passesTotal tube area, AT = (239 / 2) (1.6089 x 10-3)= 0.1923 m2Mass velocity, Gs = flowrate / A= 29.96 / 0.1923= 155.798 kg/m2.sReynolds number, Re = Gsdi / = 155.798 x 0.04526 / 4.529 x 10-4= 1.557 x 10 4Prandtl number,= 3.1731 x 155.798 / 0.1553= 3183.275Nusselt number, NuD = 0.027 Rea Prb / wc= 0.027 (1.557 x 10 4)0.8 (3183.275)0.3 x 1= 685.578Stanton number, St = NuD / Re(Pr)= 685.578 / 1.557 x 10 4 x 3183.275 = 1.383 x 10-5Heat transmit factor, jh = St Pr0.67= 1.383 x 10-5 ( 3138.275 )0.67 x 1= 3.045 x 10-3Tube-side heat transfer coefficient, hi= 2329.599 W/ m2.0C3.3.4 Shell side coefficient1.25 triangular pitch was chosen to calculate the sheaf diameter. From table 12.4 (Sinnott 2005), constants value for 2 tube passes condition is K1 = 0.249 and n1 = 2.207Bundle diameter, Db = Dto (Nt / K1) 1/n1= 50.8 ( 239 / 0.249)1/2.207= 1122.575 mmPull-through floating head type was the best selection. From Figure 12.10 (Sinnott 2005), bundle diameter clearance is 95 mm.Shell diameter, Ds = 1122.575 + 95= 1217.575 mmFor selecting baffle spacing, the optimum spacing chosen is 0.2 times the shell diameters. scotch spacing, B = 0.2 Ds = 0.2 (1217.575) = 243.515mmTube pitch pt = 1.25 Do = 1.25 (50.8) = 63.5mmCross-flow area,= 0.0593 m2Mass velocity, Gs = Ws / As= 47.7672 / 0.0593= 805.518 kg/m2.sEquivalent diameter,= 36.07 mmShell-side heat transfer coefficient, hoReynolds number, Re = Gsdi / = 805.518 x 36.07 x 10-3 / 2.677 x 10-4= 1.0854 x 10 5Prandtl number,= 2.677 x 10-4 (2.4923 x 103) / 0.1553= 4.296Note that 45% baffle cut has been chosen, neglect the viscosity correction term. From Figure 12.29 (Sinnott, 2005), jh = 2.8 x 10-3= 1640.892 W/m2.0C3.3.5 Overall CoefficientTable 3.5 Dimensions in overall coefficientMaterialCarbon steelThermal conductivity of carbon steelKw = 45 W/m0CThe fouling factor for cooling waterhid 5000 W/m2.0CThe fouling factor for aqueous salt solutionsh0 =3000 W/m2.0CSource Chemical Engineering Design, R.K.Sinnott.The relationship between overall coefficient and individual coefficients is given byUO = 583.359 W/m2.0CWell approximately the initial estimate of 600 W/m2.0C, so design has adequate area for the obligation required.3.3.6 Tube-side Pressure DropReynolds number,= 14526.371From Figure 12.24 of Chemical Engineering. (Vol. 6) Friction factor, jf = 0.045Tube side pressure drop,Where, m = 0.25 for laminar flow, Re2100Np = nu mber of tube side passes= 23135.87 N/m2= 2.3135 kPa (Acceptable)3.3.7 Shell-side Pressure DropReynolds number, Re = 1.0854 x 10 5From the Figure 12.30 (Sinnott 2005), Friction factor, jf = 0.024Shell side pressure drop,= 64327.95 N/m2= 64.328 kPa (Acceptable)3.3.8 Summary of CalculationType of shell and tube is carbon steel with Kw of 45 W/m.0C. While, specification of inside diameter is 45.27mm, outside diameter is 50.8mm and length of 5m.Table 3.6Tube-side specificationParameterResultsTlm131.4477 oCR10.833S0.833FT0.93Tm122.246 oCArea, A190.126 m2Number of tubes, Nt239 tubesWater linear velocity, ut155.798 kg/m2.sHeat transfer coefficient, hi2329.599 W/m2.0CPressure drop, Pt2.3135 kPaTable 3.7 Shell-side specificationParameterResultsBundle diameter, Db1122.575 mmShell diameter, Ds1217.575 mmBaffle spacing, lB243.515mmShell area, As0.0593 m2Mass velocity, Gs805.518 kg/m2.sEquivalent diameter, de36.07 mmShell coefficient, ho1640.892 W/m2.0CPressure drop, Ps64.328 kPaOverall coefficie nts, U583.359 W/m2.0C3.4 MECHANICAL DESIGN OF heating EXCHANGER3.4.1 Design ParameterTable 3.8 Design ParameterParametreSI UnitEnglish UnitDesign temperature, TD460 OC860 OFOperating pressure, Po300 kPa43.51 pounds per square inchInternal diameter, Di1.217 m47.913 ftHemispherical length0.65 m2.13 ftShells length5.0 m16.40For this heat exchanger, the design pressure is 43.51 psi and above the aura pressure (15 psi). Based on study, if Po Patm (Pgage = Pabs Patm), the calculation for this heat exchanger is under internal pressure and the pressure that will used is,Po = Pabs Pgage = 43.51 psi 15 psi = 28.51 psiCalculation of design pressure for each part of heat exchanger by taking 10% safety factorP1 = PO + PH= 28.51 + 0.433 (2.13) = 29.431 psi x 1.1= 32.38 psiBecause this heat exchanger design is horizontal, so the value P1 = P2 = P3 = 32.38 psiThickness for each part of vesselthemispherical , t =tcylindrical Circumferentialt =Longitudinalt =For cylindrical, the highest ponde rousness value calculated will be chosen. So, from the calculation above the thickness for cylindrical part is 0.0446 inch. Now by adding corrosion hire, CA of 2 mm (0.07874 in.),themispherical = 0.0223in + 0.07874in = 0.101intcylindrical = 0.0446in + 0.07874in = 0.12334inThe material construction for this heat exchanger is carbon steel due to price and work in many applications. The highest value from these two types of wall thicknesses is 0.12334 inch, so the minimum wall thickness of this heat exchanger is 0.12334 inch (3.133mm). The nominal wall thickness for carbon steel at market is 0.1182 inch (3mm). Because of the nominal wall thickness is lower than the calculated we must take the calculated thickness t = 0.12334 inch (3.133 mm) as value of wall thickness.To calculate the maximum allowable working pressure for each part, MAWPpart , the thickness must subtract the corrosion allowancet = 0.12334in 0.07874in = 0.0446inMAWPpart (hemispherical)P =MAWPpart (cylindrical)Circumfe rentialP =LongitudinalP =The smallest value of pressure will be chosen. So, the internal pressure for cylindrical part is 32.383 psi.By subtracting the hydrostatic pressure, PH for each part,MAWPpart (hemispherical) = 64.812 psi (0.433)(2.13) = 63.889 psi =440.5 kPaMAWPpart (cylindrical) = 32.383 psi (0.433)(16.01) = 25.451 psi =175.478 kPaThe smallest value of pressure is taken as MAWPpart which is 25.451psi. This value is the maximum allowable pressure for the whole vessel.