Applied Heat Transfer Part Two Heat Excangers Dr. Amad RAMAZANI S.A. Associate Professor Sarif University of Tecnology انتقال حرارت کاربردی احمد رمضانی سعادت ا بادی Autumn, 1385 (2006) Ramazani, Heat Excangers 1 Topics of Tis capter Introduction of Heat Excangers (HEXs) Te Overall Heat Transfer Coefficient Fouling Factor Types of Heat Excangers Te Log Mean Temperature Difference (LMTD) Metod Effectiveness-number eat transfer unit (NTU) Metod Effectiveness and Heat Transfer Rate Compact Heat Excangers Analysis for Variable Properties Heat Excanger design Consideration Ramazani, Heat Excangers 2 Heat Excanger Types Wat is a eat excanger? An equipment tat permits to transfer eat from a ot fluid to a cold one witout any direct contact of fluids Heat excangers, can be seen in quotidian life, as well as different industries. Heat Excanger Types Heat Excanger can be categorized according to Flow arrangement and Type of Construction Parallel flow (fig. a) and Counter-flow (fig. b) in concentric tubes (double-pipe) Almost all Cemical And Petrocemical Plants, Air Conditioning Systems, Power production, Waste Heat recovery, Automobile Radiator, Central Heating System Radiator, Electronic Parts,. Ramazani, Heat Excangers 3 Concentric Tubes: a) parallel flow; b) Counter flow Ramazani, Heat Excangers 4
Heat Excanger Types (Con.) Finned and unfinned tubular eat excanger wit cross flow Heat Excanger Types (Con.) Sell-and-tube eat excanger wit one sell pass and one tube pass (Cross counter flow mode of operation) Cross flow eat excangers a) finned; b) unfinned Ramazani, Heat Excangers 5 Ramazani, Heat Excangers 6 Heat Excanger Types (Con.) Heat Excanger Types (Con.) Oter types of Sell-and-tube eat excanger One sell pass and two tube passes Two sell passes and four tube passes Ramazani, Heat Excangers 7 Ramazani, Heat Excangers 8
Heat Excanger Types (Con.) Heat Excanger Types (Con.) Ramazani, Heat Excangers 9 Ramazani, Heat Excangers 10 Heat Excanger Types (Con.) Compact eat excanger cores Heat Excanger Types (Con.) Core of a plate Compact eat excanger wit counter flow from Aluminum, AKG America Corp. Ramazani, Heat Excangers 11 Ramazani, Heat Excangers 12
Heat Excanger Types (Con.) Heat Excanger wit fins on Surface, General Motors Corp., Lockport, NY Heat Excanger Types (Con.) Heat excanger wit fins Ramazani, Heat Excangers 13 Ramazani, Heat Excangers 14 Heat Excanger Types (Con.) Fins deposited on tubes internal surface for increasing eat transfer Heat Excanger Types (Con.) Heat transfer area density (m 2 /m3) for different types of eat excangers Ramazani, Heat Excangers 15 Ramazani, Heat Excangers 16
Te Overall Heat Transfer Coefficient Te Overall eat transfer coefficient for walls Te Overall Heat Transfer Coefficient (con.) Te Overall eat transfer coefficient for Duble-pipe (HEXs) (U can be determined from Total Termal resistance to eat transfer between two fluid) Ramazani, Heat Excangers 17 Ramazani, Heat Excangers 18 Te Overall Heat Transfer Coefficient (Con.) Table 10.1. Approximate Values of Overall Heat-transfer Coefficient Ramazani, Heat Excangers 19 Te Overall Heat Transfer Coefficient, Example 1. EXAMPLE 10.1. OVERALL HEAT.TRANSFER COEFFICIENT FOR PIPE IN AIR. Hot water at 98 C flows troug a 2-in scedule 40 orizontal steel pipe [k = 54 W/m. o C] and is exposed to atmosperic air at 20 C. Te water velocity is 25 cm/s. Calculate te overall eat-transfer coefficient (U) for tis situation, based on te outer area of pipe. Solution. From Appendix A (P. 653) te dimensions of 2-in scedule 40 pipe are ID = 2.067 in = 0.0525 m OD = 2.375 in = 0.06033 m Te eat-transfer coefficient for te water flow on te inside of te pipe is determined from te flow conditions wit properties evaluated at te bulk temperature. Te freeconvection eat-transfer coefficient on te outside of te pipe depends on te temperature difference between te surface and ambient air. Tis temperature difference depends on te overall energy balance. First, we evaluate i and ten formulate an iterative procedure to determine o. Te properties of water at 98 o C are p = 960 kg/m3 µ = 2.82*10^-4 kg/m.s k = 0.68 W/m. o C Pr = 1.76 Ramazani, Heat Excangers 20
Te Overall Heat Transfer Coefficient, Example 1. (Con.) Te Overall Heat Transfer Coefficient, Example 1. (Con.) Te Reynolds number is ρud (960)(0.25)(0.025) Re = = = 44680 4 µ 2.82 10 and since turbulent flow is encountered, we may use Eq. (64) For unit lengt of te pipe te termal resistance of te steel is Ramazani, Heat Excangers 21 Ramazani, Heat Excangers 22 Te Overall Heat Transfer Coefficient, Example 1. (Con.) Te Overall Heat Transfer Coefficient, Example 2. EXAMPLE 10-2. OVERALL HEAT-TRANSFER COEFFICIENT FOR PIPE IN STEAM. Te pipe and ot-water system of Example 10-1 is exposed to steam at 1 atm and 100 o C. Calculate te overall eat-transfer coefficient for tis situation based on te outer area of pipe. Solution. We ave already determined te inside convection eat-transfer coefficient in Example 10.1 as i = 1961 W/m 2. o C Te convection coefficient for condensation on te outside of te pipe is obtained by using Eq. (9-12), were T o is te outside pipe-surface temperature. Te water film properties are, Ramazani, Heat Excangers 23 Ramazani, Heat Excangers 24
Te Overall Heat Transfer Coefficient, Example 2. Te Overall Heat Transfer Coefficient, Example 2 (Con). Ramazani, Heat Excangers 25 Ramazani, Heat Excangers 26 Te Overall Heat Transfer Coefficient, Example 2 (Con). Ramazani, Heat Excangers 27 Te Overall Heat Transfer Coefficient (Fouling Factor) After a period of operation, eat transfer surfaces of HEXs may become Coated wit various deposits present in te flow systems Corroded Or, in general, deteriorated because of use Resulting in decreased performance because of additional resistance(s) to eat flow Te overall effect of tis deterioration is represented by a Fouling Factor R f R f 1 = U dirty 1 U clean Ramazani, Heat Excangers 28
Te Overall Heat Transfer Coefficient (Fouling Factor Con.) Te Overall Heat Transfer Coefficient Example 3. (Fouling Factor Con.) From previous example clean = 1961 W/m 2. o C and so from above equation we can obtain i Ramazani, Heat Excangers 29 Ramazani, Heat Excangers 30 Liquid Temperature Profile in Counterflow HEXs Liquid Temperature profile in a Counter-flow double pipe HEXs (Oil is ot fluid in tube and water is cold one in sell) Liquid Temperature Profile in Cross-flow HEXs Fluid Temperature profile in a Cross-flow Heat Excangers (Steam is ot fluid and water is te cold one, Steam condense on te tube at a constant temperature) Ramazani, Heat Excangers 31 Ramazani, Heat Excangers 32
Te Log Mean Temperature Difference (LMTD) Metod Fluid Temperature Profile in Double Pipe Heat Excangers Te Log Mean Temperature Difference (LMTD) Calculation Parallel Flow Counter Flow Heat balance on an element of HEX Q = UA T m T = Suitable mean temperature difference across HEX m Ramazani, Heat Excangers 33 Mixing two above relations Ramazani, Heat Excangers 34 Te Log Mean Temperature Difference (LMTD) Calculation (Con.) Te Log Mean Temperature Difference (LMTD) Calculation (Con.) Heat excanges at lengt an element of HEXs Putting values of m C and m c C c in relation obtained for ln of temperatures differences Ramazani, Heat Excangers 35 Ramazani, Heat Excangers 36
Correction Factors for LMTD Metod Correction Factors for LMTD Metod (Con.) Fig. 10.8. Correction Factor for HEX wit one Sell and two, four, or any multiple of tube passes Ramazani, Heat Excangers 37 Fig. 10.9. Correction Factor for HEX wit Two Sell and four, eigt, or any multiple of tube passes Ramazani, Heat Excangers 38 Correction Factors for LMTD Metod (Con.) Correction Factors for LMTD Metod (Con.) Fig. 10.10. Correction Factor for Single-pass Cross-flow Ramazani, Heat Excangers 39 HEXs, bot fluid unmixed Fig. 10.11. Correction Factor for Single -pass Cross-flow Ramazani, Heat Excangers 40 HEXs, one fluid mixed, te oter unmixed
Use of te LMTD for calculation excanger performance. EXAMPLE. 10-4. CALCULATION OF HEAT EXCHANGER SIZE FROM KNOWN TEMPERATURE. Water at te rate of 68 kg/min is eated from 35 to 75 O C by an oil aving a specific eat of 1.9 kj/kg. o C. Te fluids are used in counterflow double pipe eat excanger, and te oil enters te excanger at 110 o C and leaves at 75 o C. Te overall eat-transfer coefficient is 320 W/m 2. o C. Calculate te eat excanger area. Solution. Te total eat transfer is determined from te energy absorbed by te water Use of te LMTD metod for calculation excanger performance. (Con.) Since all te fluid temperatures are known, te LMTD can be calculated by using te temperature sceme in Fig. 10-7b: Ramazani, Heat Excangers 41 Ramazani, Heat Excangers 42 Use of te LMTD for calculation HEXs performance. (Con.) Use of te LMTD for calculation excanger performance. (Con.) So, Using Fig. 10.8. Correction factor is F=0.81 Ramazani, Heat Excangers 43 Ramazani, Heat Excangers 44
Use of te LMTD for calculation excanger performance. Example 10.6. Design of Sell and tube HEXs: Water at te rate of 30000lbm/ [3.783 kg/s] is eated from 100 to 130 o F [37.78 to 54.44 0 C] in a sell-and tube eat excanger. On te sell side one pass is used wit water as te eating fluid, 15,000 1b. m [1.892 kg/s], entering te excanger at 200 o F [93.33 o C]. Te overall eat-transfer coefficient is 250 Btu/. ft 2. o F [1419 W/m 2. o C], and te average water velocity in te 3/4in [1.905-cm] diameter tubes is 1.2 ft/s[ 0.366 m/s]. Because of space limitations te tube lengt must not be longer tan 8 ft [2.438 m]. Calculate te number of tubes per passes, and te lengt of te tubes. consistent wit tis restriction. Solution: We first assume one tube pass and ceck to see if it satisfies te conditions of tis problem. Te exit temperature of te ot water is calculated from q = m& C T T c c c = m& C T T (30000 )(1)(130 100 ) = (15000 )(1) = 60 m& ccc T = m& C o O Ramazani, Heat Excangers 45 F = 33.33 C Use of te LMTD for calculation excanger performance. (Con.) Ramazani, Heat Excangers 46 Use of te LMTD for calculation excanger performance. (Con.) Use of te LMTD for calculation excanger performance. (Con.) Ramazani, Heat Excangers 47 Ramazani, Heat Excangers 48
Effectiveness-NTU Metod Effectiveness-NTU Metod (Con.) Energy balance for a) parallel flow b) Counterflow Sell-tube HEXs Ramazani, Heat Excangers 49 Ramazani, Heat Excangers 50 Effectiveness-NTU Metod (Con.) Effectiveness-NTU Metod (Con.) Ramazani, Heat Excangers 51 Ramazani, Heat Excangers 52
Effectiveness-NTU Metod (Con.) Effectiveness-NTU Metod (Con.) Ramazani, Heat Excangers 53 Ramazani, Heat Excangers 54 Effectiveness-NTU Metod (Con.) Effectiveness-NTU Metod (Con.) Ramazani, Heat Excangers 55 Ramazani, Heat Excangers 56
Effectiveness-NTU Metod (Con.) Effectiveness-NTU Metod (Con.) Ramazani, Heat Excangers 57 Ramazani, Heat Excangers 58 Effectiveness-NTU Metod (Con.) Effectiveness-NTU Metod (Con.) Ramazani, Heat Excangers 59 Ramazani, Heat Excangers 60
Effectiveness-NTU Metod (Con.) Example: Application of Effectiveness-NTU Metod P. 578 P. 573 Ramazani, Heat Excangers 61 Ramazani, Heat Excangers 62 Example: Application of Effectiveness-NTU Metod (Con.) Example: Application of Effectiveness-NTU Metod (Con.) Effectiveness can be calculated using Eq. 10.21, P.573, to be Ramazani, Heat Excangers 63 Ramazani, Heat Excangers 64
Example: Application of Effectiveness-NTU Metod Example: Application of Effectiveness-NTU Metod (Con.) Fig. 10.15 Te eat transfer is ten q = m& C T = (2.887 )(1006 )(29.44 c c c = 40.34 kw [1.38 105Btu/] 15.55) Ramazani, Heat Excangers 65 Ramazani, Heat Excangers 66 Example: Application of Effectiveness-NTU Metod (Con.) Example: Application of Effectiveness-NTU Metod (Con.) We sould assume values for te water flow rate until we could matc te performance of HEX according to Fig. 10. 15 or table 10. 3. Te selected iterations to approac to correct values are as follow: Fig. 10.15 ; and q = m& C T Ramazani, Heat Excangers 67 q= We tus estimate te water o m& C = 645 W/ C T and Te w, exit 645 m& = = 0.154 kg/s [1221 lb /] m 4180 exit water temperatu re is accordingl y = 82.22 4.034 10 645 4 - flow = 19.68 rate O Ramazani, Heat Excangers 68 C as about
Boilers and Condenser (HEXs) Compact Heat Excangers Ramazani, Heat Excangers 69 Tese eat excangers are best for gases and low cases Ramazani, Heat Excangers 70 Compact Heat Excangers Compact Heat Excangers For tese types of eat excangers driving matematical relation can be difficult, owever some correlation are presented to able us to calculate eat transfer and Pressure drop values for tem. Tese correlations are based on Stanton and Reynolds dimensionless numbers Wic are written based on te mass velocities in te minimum flow crosssectional area and ydraulic diameter stated in Mass Velocity f is friction inside te tubes and v 1 and v 2 are specific volume at entrance and exit and v m is its average at HEXs Ratio of te free-flow = area to frontal area Ramazani, Heat Excangers 71 Ramazani, Heat Excangers 72
Compact Heat Excangers Fig. 10.20. Heat transfer and friction factor for finned circular-tube HEX Example: Compact Heat Excangers St Pr 2/3 or f Ramazani, Heat Excangers 73 Ramazani, Heat Excangers 74 Analysis for Variable Properties Analysis for Variable Properties (Con.) (10-33) Were, (10-34) Ramazani, Heat Excangers 75 Ramazani, Heat Excangers 76
Analysis for Variable Properties (Con.) Analysis for Variable Properties (Example) Solution Metod: Te numerical-analysis procedure is clear wen te inlet temperatures and flows given: 1) Coose a convenient value of A j for te analysis. 2) Calculate te value of U for te inlet conditions and troug te initial A increment. 3) Calculate te value of Q for tis increment From Eq. (10-32). 4) Calculate te values of T, T c, and T - T c, for te next increment, using Eqs 10.31 and 10.34 5)Repeat te foregoing steps until all increment in A are employed Ramazani, Heat Excangers 77 Ramazani, Heat Excangers 78 Analysis for Variable Properties (Example) Analysis for Variable Properties (Example) Fig. 10.16. A rock-bed termal-energy storage unit scematic Ramazani, Heat Excangers 79 Ramazani, Heat Excangers 80
Analysis for Variable Properties (Example) Analysis for Variable Properties (Example) Ramazani, Heat Excangers 81 Ramazani, Heat Excangers 82 Analysis for Variable Properties (Example) Heat-Excanger Design Considerations Stored energy wit time for te rock-bed termal-energy storage unit of figure 10.16 Ramazani, Heat Excangers 83 Ramazani, Heat Excangers 84
Heat-Excanger Design Considerations Ramazani, Heat Excangers 85