Thermal Design of Shell and tube heat Exchanger

Transcription

1 King Abdulaziz University Mechanical Engineering Department MEP 460 Heat Exchanger Design Thermal Design of Shell and tube heat Exchanger March

2 Contents 1-Introduction 2-Basic components Shell types Tube bundle types Tube layouts baffle types 3-Basic design procedure Preliminary estimate of unit size Rating of preliminary design 4 Shell side heat transfer and pressure drop 5-Bell-Delaware method for rating a shell and tube heat exchangers 2

3 1-Introduction Most used heat exchangers Can accommodate high temperature high pressure fluids In some designs tubes can be replaced Many options to choose from Different designs to choose from for shell, tube layout and tube bundle Require more space when compared to plate gasketed heat exchangers 3

4 1-Introduction Main components of a shell & tube HX A-Shell types B- Tube bundle types C- Tube layouts D-Baffle types 4

5 A- Shell types TEMA Tubular Exchanger Manufacturers Association Standards Most common shell type are: E, F, G 5

6 A- Shell types V stands for vent 6

7 A- Shell types Kettle re-boiler shell 7

8 B- Tube bundle types U tube bundle Tube can expand Replacement of tubes is not possible except maybe the outer row Tubes can have fins 8

9 B-Tube bundle types Fixed tube sheet Design for ease cleaning of the inside of the tubes Not possible to access the outer surface of the tubes Has limited expansion Individual tubes can be replaced 9

10 Floating head B-Tube bundle types pull-through floating head Bundle can be removed and cleaned Good for fouled fluids 10

11 C-Tube layouts Only an E-shell with one tube pass and an F-shell with two tube passes result in nominal counterflow. All other multiple tube passes require a temperature correction (factor F), Tube metal is usually: Low carbon steel Low alloy steel Stainless steel Copper Admiralty Cupronickel Inconel Aluminum (in the form of alloys), or titanium. The wall thickness of heat exchanger tubes is standardized in terms of the Birmingham Wire Gage (BWG) of the tube. Tables 9.1 and 9.2 give data on heat exchanger tubes 11

12 C-Tubes & Tube layouts Tube diameters (8 15 mm) are preferred for greater area/volume density but are limited, for purposes of in-tube cleaning Larger tube diameters are often required for condensers and boilers. The tubes may be either bare or have low fins on the outside. Low fin tubes are used when the fluid on the outside of the tubes has a substantially lower heat transfer coefficient than the fluid on the inside of the tubes. As the tube length is increased the heat transfer area increased and the number of tubes decreased. Tube length is dictated by space available and transportation requirement. shell-diameter-to-tube-length ratio is typically between 1/5 to 1/15 [ D s /L =1/5 to 1/15] 12

13 C-Tubes & Tube layouts Commercial tube data Birmingham Wire Gage (BWG) 13

14 C-Tubes & Tube layouts Commercial tube data 14

15 C-Tubes & Tube layouts 15

16 C-Tube & tube layout Pitch angle p = = = = 16

17 C-Tube & tube layout P T is the tube pitch d o is the outside diameter PT/do, is between 1.25 and

18 C-Tube & tube layout Tube counts for different shell diameters and tube layout 18

19 Tube counts for different shell diameter and tube layout 19

20 Table 9.3 Tube count 20

21 D-Baffle types Baffle function: Baffles serve two functions: * Support the tubes for structural rigidity, preventing tube vibration and sagging, and * To divert the flow across the bundle to obtain a higher heat transfer coefficient baffle types Transverse and longitudinal Rod and plate 21

22 D-Baffle types transverse and longitudinal baffles Transverse baffles Longitudinal baffle 22

23 D-Baffle types Rod and plate baffles Rod baffles Plate baffles 23

24 D-Baffle types Types of plate baffles Single segmental Double segmental Triple segmental No tubes in the window Disk and doughnut 24

25 D-Baffle types Types of plate baffles 25

26 D-Baffle types Types of plate baffles 26

27 D-Baffle types Types of plate baffles 27

28 D-Baffle types Types of plate baffles 28

29 Orifice baffle D-Baffle types Types of plate baffles 29

30 D-Baffle types Rod and ring baffle 30

31 Baffle spacing and baffle cut Optimum baffle spacing is somewhere between 0.4 and 0.6 of the shell diameter and a baffle cut of 25% to 35% is usually recommended. The B 31

32 Baffle cut 32

33 33

34 34

35 Procedure for designing heat exchangers Select type of shell & tube HX Preliminary sizing of key parameters Kern method Bell Delaware method 35

36 Preliminary procedure to size a unit 1-Calculate LMTD and estimate the correction factor F 2-Estimate the overall heat transfer coefficient U (use table 9.4 and table 9.5 for individual h) 3-Calculate q from the known mass flow rates and the temperatures 4-Calculate approximately the heat transfer area A o using A o = q U o LMTD cf F 5-From the calculate A o one can estimate the number of tubes 36

37 Preliminary calculation for sizing shell and tube heat exchangers The size of a heat exchanger can be found using A o = q U o LMTD cf F Provided all temperatures are known and an approximate value of U o is available An estimate for U o can be found based on individual thermal resistances 1 U o = 1 h i (A i A o ) + R fi (A i A o ) + A or w + R fo + 1 h o 37

38 Preliminary procedure to size Shell & tube HX Estimating the individual heat transfer coefficient h 38

39 Preliminary procedure to size a unit Typical U value for some heat exchangers 39

40 L D e d o d i D s N t Length of the tube [m] Equivalent diameter [m] used in calculating Re s by Kern method Outside diameter of the tube [m] Inside diameter of the tube [m] Shell inside diameter[m] No. of tubes A 1 Area taken by single tube [m 2 ] CL CTP N p B B c P T Symbols and their meanings Tube layout constant. CL=1 for 90 and 45 layout, CL=0.87 for others Tube count calculation constant. One tube pass=0.93, two tube passes=0.9, Three tube passes=0.85 No of tube passes Baffle spacing [m] Baffle cut [m] Tube pitch [m] P T /d o Pitch outside diameter ratio [-] A s Min. flow area at the shell center line [m 2 ] G s Mass velocity m s A s [kg/(m 2.s)] 40

41 Preliminary procedure to size a unit q = m c Cp c (T co T ci ) q = m h Cp h (T hi T ho ) q = U o A o LMTD F (1) (2) (3) N t = CTP CL D s 2 PR 2 d o 2 (8) A o = q U o LMTD F A o = πd o N t L (4) (5) D s = CL CTP A o PR 2 d o L 1 2 (9) N t = CTP πd s 2 4A 1 2 A 1 = CL P T CL =1 CL=0.87 One tube pass Two tube pass Three tube passes For 90 and 45 For 30 and 60 CTP=0.93 CTP=0.9 CTP=0.85 (6) (7) PR=tube pitch ratio=p T /d o (10) 1-From Eq. (4) A o can be estimated 2-Assume a typical commonly used shell and tube layout estimate CTP, CL, PT and d o 3-Use Eq. (9) to estimate shell inside diameter D s 4-Use Eq. (8) to get the number of tubes N t 41

42 Example 9.1 on preliminary sizing of a shell and tube heat exchangers 42

43 Example 9.1 continue R f = m 2.K/W Baffle spacing B=0.6 D s Tubes: d o =19 mm, d i =16 mm m h =5000 kg/hr Water condensate T hi = 67 C 40 C heat exchanger length L<5 m T ho Baffle cut= 25% ΔP s < 5 psi 17 C city Water m c =30,000 kg/hr k t =60 W/(m.K) Required: Preliminary sizing of shell and tube heat exchanger ¾ tube P T /d o =1.25 d i =16 mm, d o =19mm 43

44 Example 9.1 continue h i =5000 W/(m^2.K) h o =4000 W/m^2.K 44

45 Example 9.1 continue 45

46 Example 9.1 continue 46

47 Example 9.1 continue 47

48 Example 9.1 continued 48

49 Rating a heat exchanger Kern and Bell-Delaware methods for rating shell and tube heat exchangers 49

50 Shell-Side Heat Transfer and Pressure Drop (Kern) McAdams expression for finding the shell side heat transfer coefficient h o D e k = 0.36 D eg s μ 0.55 cp μ k 1 3 μ b μ w < R es = G sd e μ < h o shell side heat transfer coefficient D e equivalent diameter G s shell side mass velocity D e = 4free flow area Wetted perimeter = 4A c Wetted perimeter 50

51 Shell-Side Heat Transfer and Pressure Drop (Kern) For square pitch D e = 4 P T 2 πd o 2 4 πd o For triangular pitch D e = 4 P T πd o 2 8 πd o 2 51

52 Shell-Side Heat Transfer and Pressure Drop (Kern) The bundle cross flow area A s at the centerline of the shell depends on the shell inside diameter D s, the tube layout pitch and the clearance between the tubes A s = D scb P T The shell side mass velocity is given by G s = m s A s 52

53 Shell side pressure drop The length is taken as the shell inside diameter and the flow make crosses over the bundle (N b +1) times Δp s = f G s 2 N b + 1 D s 2ρD e φ s f = exp ( ln Re s ) G s = m s A s 400 < Re s = G sd e μ φ s = μ b μ w 0.14 Where N b is the number of baffles N b = L B 1 B is the baffle spacing 53

54 Due to friction Tube side pressure drop N p is the number of tube passes Pressure drop due to change of direction of the flow Δp f = 4f L N p d i Which is taken as four velocity heads per pass ρ u 2 m 2 = 4f LN p d i Δp r = 4N p ρu m G t 2 ρ Therefore the total tube side pressure drop Δp tot = 4f LN p ρu2 m + 4N d p i 2 54

55 Example 9.2 Rating of a preliminary design It is required to rate the heat exchanger of example 9.1 output results from example 9.1 are: m h =5000 kg/hr Water condensate T hi = 67 C 40 C N t =117 tube T ho city Water m c =30,000 kg/hr 55

56 Correct count of tubes in a shell according to TEMA standards 56

57 Example 9.2 Rating of a preliminary design Input data for rating the heat exchanger are: No parameter value 1 D s =0.39 m 2 N t 124 tubes 3 d i 16 [mm] 4 d o 19 [mm] 5 k 60 [W/(m.K)] 6 B 0.2 and baffle cut =25% 7 P T [m] 8 N p 2 tube passes 57

58 Example 9.2 continue 58

59 Example 9.2 continue 59

60 Example 9.2 continue 60

61 Example 9.2 continue 61

62 Example 9.2 continue 62

63 Example 9.2 continue Calculating heat exchanger length 63

64 Example 9.2 continue 64

65 Example 9.2 continued 65