Shell and tube heat exchanger design - Coursework Example

? Shell and tube heat exchanger design Shell and tube heat exchanger design Introduction Heat exchangers operations are guided by the fact that heat transfer is a product of temperature variation between cold process stream and hot process stream. A thin solid layer is used in separation of the two streams. The wall has to be conducive to allow heat exchange and still be sufficiently strong to withstand fluid/gas pressures. In shell and tube heat exchangers, two closed process streams move across the unit; one move inside the tube and the other moves on the shell side. Convection and conduction allows heat to pass from hot stream to cold stream from the side of the tube side or from shell side. As temperature variation between the process streams rise, heat exchange rate for every surface area unit also rises. Conversely, heat exchangers per surface are unit drops non-linearly as temperature difference between the two process streams drops. Increasing the effective surface area of the entire system helps in maintenance of the total transfer of heat between two streams although eventually the system reaches a point where extra surface area has no effect on extra heat transfer. The other variable which affects heat exchange in shell and tube exchanger is each process stream’s velocity. This velocity directly contributes to a rise in convection cold process and hot process streams. Raising the velocity also raises heat exchange, more especially, in countercurrent design. Finally, velocity increments are limited by maximum permitted for a specific metallurgy constituting shell or tube. For carbon steel, for instance, velocity cannot exceed 6 ft. /sec. whilst for the case of stainless and high-alloy steel; rate is 12 ft. /sec. for liquids. The three conventional types of shell and tube heat exchangers are parallel, cross flow and countercurrent flow types. The names are derived from the process stream directions in relation to each other. In countercurrent heat exchanger type, average temperature variation between the process streams is optimized over the exchanger’s length, showing the highest heat transfer rate efficiency over a surface area unit. With respect to existing temperature variations observed during operation, parallel heat exchangers exhibit the lowest heat transfer rates, and then cross flow heat exchangers, and finally, countercurrent heat exchangers. Counterflow and parallel heat exchangers are illustrated below, Figure 1: Counterflow and parallel heat exchangers The design of shell and tube heat exchanger depends on flow pattern through the respective heat exchanger. It is however the most widely used heat exchanger in industries and can adopt counter-flow, parallel flow or cross-flow pattern. However, heat transfer area is a major factor in design calculation. Theoretically though, shell and tube heat exchanger flow patter is conventionally not specifically counter-flow, or parallel. Rather, it incorporates a mixture of counter-flow, parallel flow and cross-flow. Log mean temperature variation, used for design of shell and tube heat exchanger, works best for varied flow patterns occurring in this kind of heat exchanger. Shell and tube heat exchangers Shell and tube heat exchangers in their various construction modifications are probably the most widespread and commonly used basic heat exchanger configuration in the process industries. The reasons for this general acceptance are several. The shell and tube heat exchanger provides a comparatively large ratio of heat transfer area to volume and weight. It provides this surface in a form which is relatively easy to construction in a wide range of sizes and which is mechanically rugged enough to withstand normal shop fabrication stresses, shipping and field erection stresses, and normal operating conditions. There are many modifications of the basic configuration, which can be used to solve special problems. The shell and tube exchanger can be reasonably easily cleaned, and those components most subject to failure - gaskets and tubes – can be easily replaced. Finally, good design methods exist, and the expertise and shop facilities for the successful design and construction of shell and tube exchangers are available throughout the world. Flow patterns in shell and tube heat exchangers Rather than simply a single pipe inside another pipe, as is the case with the double pipe heat exchanger, shell and tube heat exchanger makes use of multiple tubes bundled inside a 'shell'. This provides a closely and firmly united heat exchanger for the respective heat transfer area, although the patterns of flow are more sophisticated for shell and tube heat exchanger. A general heat exchanger configuration is provided in the diagram below, Figure 2: generalized shell and tube heat exchanger Shell and Tube Heat Exchanger Design The basic heat exchange equation, to establish the shell and tube heat exchanger’s transfer surface area whereby, Q is the heat transfer rate between the two fluids (Btu/hr.), U is the overall heat transfer coefficient (BTU/hr-ft2-oF), A is the heat transfer surface area (ft2), and ?Tlm is the log mean temperature difference (F), calculated from inlet and outlet temperatures of the two heat exchanger fluids. When the equation is used as the design equation in calculation of the required surface are of heat transfer surface area, it is rearranged to produce the following equation, It should however be noted that heat exchangers design is an iterative process. Heat exchanger design equation is used in estimation of the required heat transfer area; on basis of the estimate of the overall coefficient of heat transfer, defined heat load, as well as the log means temperature difference, T in and T out. Design parameters and corresponding calculations In designing the heat exchanger, a number of items were identified including, 1. The fluids involved, 2. Heat capacity for each of the identified fluids were identified, 3. Required design initial and final temperatures for one of the fluids, 4. Initial temperature of the other fluid as per the design requirements, and 5. Initial Overall Heat Transfer Coefficient, U estimate The initial values were identified and tabulated as follows, Fluid1 mass flow rate, m1 = 25,000 lb/hr Fluid1 temp. in, T1in = 190 oF Fluid1 temp. out, T1out = 140 oF Fluid1 sp. heat, Cp1 = 0.74 Btu/lb-oF Fluid2 temp. in, T2in = 50 oF Fluid2 temp. out, T2out = 120 oF Fluid2 sp. heat, Cp2 = 1.0 Btu/lb-oF Overall heat transf. coeff. estim., U = 120 Btu/lhr-ft2-oF Equations used for calculations: Q = + (m1)(Cp1)(T1in - T1out) Q = + (m2)(Cp2)(T2in - T2out) DTlm = [(T1in - T2out) - (T1out - T2in)]/ln[(T1in - T2out)/(T1out - T2in)] Q = U A ?Tlm The heat transfer rate is calculated as follows, The log mean temperature difference is calculated as follows, Based on this, the preliminary area estimate is calculated as follows, The required mass flow rate of fluid two is calculated using the equation Q = m Cp ?T. Rearranging: For a shell and tube heat exchanger and desired tubes of 3 inch diameter and 10 ft. length. The numbers of tubes to be used are obtained as follows, The surface area per tube is given as follows, The numbers of tubes required are obtained as follows, n = 96.86 ft2/7.854 ft2/tube = 12.33 tubes (round up to 13 tubes). In general, heat exchanger design is a multi-step, iterative process and comprises the following steps: 1. Calculation of heat transfer rate, Q, in Btu/hr., on basis of specified information about fluid flow rates and temperatures. 2. Determination of the estimated value for overall heat transfer coefficient, U, on basis of the involved fluids. 3. Calculations of the log mean temperature difference, ?Tlm, using inlet and outlet temperatures of the two fluids. 4. Making an initial estimate of heat transfer area required, using: A = Q / (U ?Tlm). 5. Selection of a preliminary configuration for heat exchanger and making required calculations such as the number and size of tubes in the shell and tube heat exchanger or pipe diameters and length for double pipe heat exchanger 6. Estimation of the pressure drop across heat exchanger. Where pressure drop is too high or too low, the heat exchanger configuration is revised until an acceptable pressure drop is attained. 7. Creation of a detailed estimate of overall heat transfer coefficient, U, on basis of the existing heat exchanger configuration. 8. Where the latest U estimate significantly varies from previous estimates, steps 4 through 7 are repeated for as long necessary until the two estimates are similar to the desired degree of accuracy. Pressure Drop Calculation for Pipe Flow The conventionally used equation for either frictional head loss or frictional pressure drop in pipe flow, the following equations are used in calculation, Or alternatively in terms of pressure drop, Where, hL is frictional head loss, ft-lb/lb. L = pipe length, ft. D is pipe diameter, ft. V is average flow velocity of fluid (is Q/A), ft./sec g is acceleration due to gravity is 32.2 ft./sec2 f is friction factor, a dimensionless empirical factor that is a function of Reynolds Number (Re is DV?/?) and/or ?/D, where ? is an empirical pipe roughness, ft. ?Pf is frictional pressure drop, lb./ft2 ? is fluid density, slugs/ft3 Excel is used to provide the heat exchanger’s design parameters in SI units and the results provided as follows, Inputs Calculations Fluid1 mass flow Overall heat transf. coeff. estim., U = 2448 kJ/hr-m2-K rate, m1 = 100,000 kg/hr Fluid1 temp. in, T1in = 95 oC Heat Transfer Rate, Q = 16,119,180 kJ/hr Fluid1 temp. out, T1out = 40 oC Log Mean Temp Diff, DTlm = 30.8 oC Fluid1 sp. heat, Cp1 = 2.93076 kJ/kg-oC Heat Transfer Area, A = 213.88 m2 Fluid2 temp. in, T2in = 25 oC Fluid2 mass flow Fluid2 temp. out, T2out = 40 oC rate, m2 = 256471 kg/hr Fluid2 sp. heat, Cp2 = 4.2 kJ/kg-oC Overall heat transf. coeff. estim., U = 680 J/sec-m2-K Pipe dimensions Inputs Calculations Heat Transfer Area, A = 213.88 m2 Pipe Diam. in m, D = 0.075 m (from calculations above) Pipe length needed, L = 908 m pipe Diameter, Dmm = 75 mm (in mm) 1. Determ. Frict. Factor, f, assuming completely turbulent flow { f = [1.14 + 2 log10(D/e)]-2 } Inputs Calculations Pipe Diameter, D 75 mm Pipe Diameter, D 0.075 m Pipe Roughness, e 0.15 mm Friction Factor, f 0.02339 Pipe Length, L 908 m Cross-Sect. Area, A = 0.004418 m2 Pipe Flow Rate, Q 0.00345 m3/s Ave. Velocity, V 0.781 m/s Fluid Density, r 910 kg/m3 Reynolds number, Re 29,819 (tubeside fluid) Fluid Viscosity, m 0.001787 N-s/m2 (tubeside fluid) 2. Check on whether the given flow is "completely turbulent flow" (Calculate f with the transition region equation and see if it differs from the one calculated above.) f = {-2*log10[((e/D)/3.7)+(2.51/(Re*(f1/2))]}-2 Transistion Region Friction Factor, f: f = 0.0285 Repeat calc of f using new value of f: f = 0.0281 Repeat again if necessary: f = 0.0281 3. Calculate hL and DPf, for straight pipe flow, using the final value for f calculated in step 2 (hL = f(L/D)(V2/2g) and DPf = rghL) Frictional Head Loss, hL 10.56 m Frictional Pressure Drop, DPf 94299 N/m2 Frictional Pressure Drop, DPf 94.30 kN/m2 4. Calculate hL and DPf, for the 180o bends Inputs Calculations Pipe length between No. of 180o bends, bends, Lsect = 4 m NB = L/Lsect = 227 Minor Loss Coefficient Head loss due to bends, for 180o bends, K = 1.5 hB = NBK(V2/2g) = 10.58 m ( K = 1.5 for threaded pipe or 0.2 for flanged pipe. ) Pressure Drop due to bends, DPB = 94417 N/m2 Pressure Drop due to bends in psi = 94.42 kN/m2 5. Add the results from part 3 and part 4 to get total hL and DPf Total Frictional Head Loss, hL = 21.140 m Total Frictional Pressure Drop, DPf = 188716 N/m2 Total Frictional Pressure Drop in kN/m2 = 188.72 kN/m2 Design               SHELL-AND-TUBE HEAT EXCHANGER               CLIENT   EQUIP. NO PAGE                           REV PREPARED BY DATE APPROVAL W.O.   REQUISITION NO. SPECIFICATION NO. 0                       1             UNIT AREA PROCURED BY INSTALLED BY 2                                                   1 Size     TEMA Type       Connected in (series/parallel)         2 Surface per Unit 1557.84511   Shells per Unit 1 Surface per Shell   1,557.8 m? 3 Performance of One Unit 4 Fluid Allocation         Shell Side   Tube Side   6 Fluid Name         Distilled Water   Raw Water   7 Flow Total   kg/h     100,000   1,188,000   8 Vapor   kg/h (in/out)               9 Liquid   kg/h (in/out)   100,000     1,188,000     10 Steam   kg/h (in/out)               11 Water   kg/h (in/out)               12 Noncondensable kg/h (in/out)               13 Temperature (In/Out) °C (in/out)   95.0 40.0   25.0 40.0   14 Density   kg/m3     962.83 988.02   994.90 988.02   15 Viscosity   cP     0.30 0.66   0.90 0.66   16 Molecular Weight, vapor                 17 Specific Heat   kJ/kg-°C     4.21 4.20   4.20 4.20   18 Thermal Conductivity W/m-°C     0.7 0.6   0.6 0.6   19 Latent Heat   kJ/kg                 20 Inlet Pressure   kPa(g)g (inlet)   29.0   60.0   21 Velocity   m/s     105.03   0.80   22 Press Drop Allow/Calc kPa(g)     1.5 7,768,128.1   10.0 2.5   23 Fouling Factor   m?-°C/W     0.0005   0.0005   24 Heat Exchanged W     1,605,875 LMTD (corrected) °C 30.79   25 Service Coeff.   W/m?-°C   Dirty   #REF! Clean       26 Construction Data for One Shell 27         Shell Side Tube Side Sketch       28 Design/Test Press kPa(g)g                 29 Design Temperature °C                 30 No. Passes per Shell   1   1           31 Corrosion Allowance mm                 32 Connections Size & Rating In DN 80.00 300.00         33 Out   80.00 300.00         34 Intermediate                 35 Tubes No. 44078 OD, mm 0.75 Gauge 14 Length, m 15 Pitch layout, deg.   90 36   Type         Material Cr-alloy (0.5% Cr) Pitch ratio   1.33 37 Shell     OD, mm   ID, mm 21.25 Material         38 Channel or Bonnet OD, mm   Thick   Channel Cover         39 Tubesheet Type                   40 Floating Heat Cover         Impingement Protection       41 Baffles Cross (number) 1498   % Cut (d) 25%   Spacing C/C, mm 10.00 42 Baffles Long       Seal Type No           43 Supports Tube       U-Bend     Type     44 Bypass Seal Arrangement NA     Tube-Tubesheet Joint       45 Expansion Joint No.         Type         46 Rho-V2-Inlet Nozzle 18,090   Bundle Entrance 1,727   Bundle Exit 1,683   47 Gaskets - Shell Side         Tube Side         48 Floating Heat Cover         Supports         49 Code Requirements         TEMA Class         50 Weight per shell kg     Filled w/water     Bundle (48,900)   51                         52 Notes                       53                         54                         Read More