Heat transfer & heat exchangers - Lecture 6: Heat exchangers

Nội dung text: Heat transfer & heat exchangers - Lecture 6: Heat exchangers

1. Lecture 6 Heat exchangers
2. Classification Recuperative / Regenerative Transport: Direct / Indirect Geometry: Tube / Plate / Extended Phase: Single / Phase change Fluid arrangement: Parallel / Counter / Cross
3. Classification Counter flow Parallel flow Single pass Cross flow Split flow Divided flow Flow arrangement Cross counter flow Extended surface Cross parallel flow Plate Compound flow Multiple pass Parallel counter flow Shell & Tube Split flow Divided flow
4. Classification Spray & tray condenser Cooling tower
5. Classification Columns Evaporator
6. Classification Fired boiler
7. Classification Spiral plate Spiral tube
8. Classification Plate heat exchange (gasket)
9. Classification 1 tube pass Baffles Head for 2 tube pass Mini, 1 shell pass & 1 tube pass Shell and tube
10. Technical requirements 1 • Overall heat transfer coefficient 2 • Pressure drop 3 • Heat transfer area 4 • Operating under temperature and pressure design 5 • Structure and leakage
11. Fluid arrangment Gas flow Gas flow Mixed – Unmixed flow Unmixed – Unmixed flow
12. Shell and Tube
13. Configuration Rear header Fluid in shell Fluid in tube (1 pass) (2 passes) Shell Front header Tube bundle
14. Specifications Large surface area in a small volume For high pressure Well–established fabrication techniques A wide range of materials Easily cleaned Well–established design procedures
15. TEMA types TEMA: Tubular Exchanger Manufacturers Association • Size of heat exchanger is represented by the inside diameter of shell (or bundle diameter) and the tube length in inches • Type and name of a heat exchanger is designed by three letters (front header – shell – rear header) • Front header (stationary header) is where the fluid enters the tube side of the exchanger • Rear header is where the tube side fluid leaves the exchanger or is returned to the front header with multiple passes • Bundle comprises the tubes, tube sheets, baffles and tie rods to hold the bundle together • Shell contains the tube bundle
16. Front header types • Easy to repair and replace • Allow access to the tubes for cleaning or repair without having to disturb the pipeline • There are two seals (tube sheet–header and header–end plate), risk of leakage • Higher cost than B type
17. Front header types • For high pressure applications > 100 • Allow access to the tube without disturbing the pipeline • Difficult to repair and replace (the tube bundle is an integral part of the header)
18. Front header types • Allow access to the tubes without disturbing the pipeline • Difficult to maintain and replace (the header and tube sheet are an integral part of the shell) • Cheaper than an A type
19. Shell types • Pure countercurrent flow is required in a two tube side pass (two shells side passes by a longitudinal baffle) • Thermal and hydraulic leakage across the baffle
20. Shell types • Similar applications to G type but tends to be used when larger units are required
21. Shell types • For reboilers only to provide a large disengagement space in order to minimize shell side liquid carry over • To be used as a chiller, cool the tube side fluid by boiling a fluid on the shell side
22. Rear header types • For fixed tube sheets only (the tube sheet is welded to the shell), so it’s impossible to access to the outside of the tubes is not possible • Allow access to the inside of the tubes without having to remove any pipeline and the bundle to shell clearances are small • Small thermal expansions and this limits the operating temperature and pressure
23. Rear header types • Allow access the tubes without disturbing the pipeline • Difficult to maintain and replace (the header and tube sheet are an integral part of the shell) ‘N’
24. Rear header types • Allow the bundle to be removed • Unlimited thermal expansion • Smaller shell to bundle clearances than the other floating head types. Difficult to dismantle for bundle pulling and the shell diameter and bundle to shell clearances are larger than for fixed head type exchangers • Most expensive
25. Rear header types • The simplest design, unlimited thermal expansion, not pure counter flow unless an F type shell is used, limited to even numbers of tube passes • Allows the bundle to be removed to clean the outside of the tubes, the tightest bundle to shell clearances • Design pressure is up to 64 푠, temperature is 450℃ • Cheapest of all removable bundle designs, but slightly more expensive than a fixed tube sheet design at low pressures
26. Construction 1 Stationary Head–Channel 21 Floating Head Cover – External 2 Stationary Head–Bonnet 22 Floating Tubesheet Skirt 3 Stationary Head Flange–Channel or Bonnet 23 Packing Box Flange 4 Channel Cover 24 Packing 5 Stationary Head Nozzle 25 Packing Gland 6 Stationary Tube sheet 26 Lantern Ring 7 Tubes 27 Tie Rods and Spacers 8 Shell 28 Transverse Baffles or Support Plates 9 Shell cover 29 Impingement Plate 10 Shell Flange–Stationary Head End 30 Longitudinal Baffle 11 Shell Flange–Rear Head End 31 Pass Partition 12 Shell Nozzle 32 Vent Connection 13 Shell Cover Flange 33 Drain Connection 14 Expansion Joint 34 Instrument Connection 15 Floating Tubesheet 35 Support Saddle 16 Floating Head Cover 36 Lifting Lug 17 Floating Head Cover Flange 37 Support Bracket 18 Floating Head Backing Device 38 Weir 19 Split Shear Ring 39 Liquid Level Connection 20 Slip-on Backing Flange 40 Floating Head Support
27. Construction • Floating head backing (longitudinal baffle)
28. Construction • Externally sealed floating tubesheet (BEW)
29. Construction • Pull through floating head (BET)
30. Construction • Fixed tubesheet exchangers (BEM)
31. Construction • U–tube exchangers (CFU)
32. Construction U–tube exchangers Spec Advantages Disadvantages Because of U–bend, some tubes are omitted at the Allows differential thermal centre of the tube bundle, tubes can be cleaned only expansion between the shell by chemical methods (difficult for mechanical and the tube bundle as well cleaning), so tube side fluids should be clean as for individual tubes Due to U–nesting, individual tube is difficult to replace Normally use M, U Both the tube bundle and Mixed counter and parallel flow type the shell side can be inspected and cleaned Tube wall thickness at the U–bend is thinner than at mechanically straight portion of the tubes Less costly than floating head Draining of tube circuit is difficult when positioned or packed floating head with the vertical position with the head side upward designs
33. Construction • Kettle floating head reboiler (AKT)
34. Construction • Two exchangers in series
35. Construction Internal Outside– Pull– Packed lantern Fixed tube floating head packed through Type of design U–tube ring floating sheet (split backing floating floating head ring) head head Exterior tube cleaning mechanically: Triangular pitch No No No No No No Square pitch No Yes Yes Yes Yes Yes Hydraulic–jet cleaning: Tube interior Yes Special tools required Yes Yes Yes Yes Tube exterior No Yes Yes Yes Yes Yes Double tube sheet Yes Yes No No Yes No feasible Number of tube No practical Any even number Limited to one No practical No practical No practical passes limitations possible or two passes limitations limitations limitations Internal gaskets Yes Yes Yes No Yes No eliminated
36. Tube arrangement • Triangular = 1,25 • : bundle outside diameter 표 • 표: tube outside diameter 훾 = 0,75 − 36 • : number of tube 표 • : tube pitch 훾 = −24 ÷ 24 More tubes in a given space 1 푡 푒 푠푠: = 1298 + 74,86훾 + 1,283훾2 − 0,0078훾3 − 0,0006훾4 2 푡 푒 푠푠: = 1266 + 73,58훾 + 1,234훾2 − 0,0071훾3 − 0,0005훾4 4 푡 푒 푠푠: = 1196 + 70,79훾 + 1,180훾2 − 0,0059훾3 − 0,0004훾4 6 푡 푒 푠푠: = 1166 + 70,72훾 + 1,269훾2 − 0,0074훾3 − 0,0006훾4
37. Standard tube dimensions Weight, low carbon 풅 풅 Thickness Internal area External surface Internal surface 풐 풊 steel, 0,2836 lb/in3 푪 (in) (in) (in) (in2) (ft2/ft) (ft2/ft) (lb/ft) 0,194 0,028 0,0296 0,0654 0,0508 0,066 46 1 0,206 0,022 0,0333 0,0654 0,0539 0,054 52 4 0,214 0,018 0,0360 0,0654 0,0560 0,045 56 0,218 0,016 0,0373 0,0654 0,0571 0,040 58 0,277 0,049 0,0603 0,0982 0,0725 0,171 94 3 0,305 0,035 0,0731 0,0982 0,0798 0,127 114 8 0,319 0,028 0,0799 0,0982 0,0835 0,104 125 0,331 0,022 0,0860 0,0982 0,0867 0,083 134 0,370 0,065 0,1075 0,1309 0,0969 0,302 168 1 0,402 0,049 0,1269 0,1309 0,1052 0,236 198 2 0,430 0,035 0,1452 0,1309 0,1126 0,174 227 0,444 0,028 0,1548 0,1309 0,1162 0,141 241
38. Standard tube dimensions Weight, low carbon 풅 풅 Thickness Internal area External surface Internal surface 풐 풊 steel, 0,2836 lb/in3 푪 (in) (in) (in) (in2) (ft2/ft) (ft2/ft) (lb/ft) 0,482 0,134 0,1825 0,1963 0,1262 0,833 285 0,510 0,120 0,2043 0,1963 0,1335 0,808 319 0,532 0,109 0,2223 0,1963 0,1393 0,747 347 0,560 0,095 0,2463 0,1963 0,1466 0,665 384 3 0,584 0,083 0,2679 0,1963 0,1529 0,592 418 4 0,606 0,072 0,2884 0,1963 0,1587 0,522 450 0,620 0,065 0,3019 0,1963 0,1623 0,476 471 0,634 0,058 0,3157 0,1963 0,1660 0,429 492 0,652 0,049 0,3339 0,1963 0,1707 0,367 521 0,680 0,035 0,3632 0,1963 0,1780 0,268 567
39. Standard tube dimensions Weight, low carbon 풅 풅 Thickness Internal area External surface Internal surface 풐 풊 steel, 0,2836 lb/in3 푪 (in) (in) (in) (in2) (ft2/ft) (ft2/ft) (lb/ft) 0,670 0,165 0,3526 0,2618 0,1754 1,473 550 0,732 0,134 0,4208 0,2618 0,1916 1,241 656 0,760 0,120 0,4536 0,2618 0,1990 1,129 708 0,782 0,109 0,4803 0,2618 0,2047 1,038 749 0,810 0,095 0,5153 0,2618 0,2121 0,919 804 1 0,834 0,083 0,5463 0,2618 0,2183 0,814 852 0,856 0,072 0,5755 0,2618 0,2241 0,714 898 0,870 0,065 0,5945 0,2618 0,2278 0,650 927 0,902 0,049 0,6390 0,2618 0,2361 0,498 997 0,930 0,035 0,6793 0,2618 0,2435 0,361 1060
40. Standard tube dimensions Weight, low carbon 풅 풅 Thickness Internal area External surface Internal surface 풐 풊 steel, 0,2836 lb/in3 푪 (in) (in) (in) (in2) (ft2/ft) (ft2/ft) (lb/ft) 1,232 0,134 1,1921 0,3927 0,3225 1,957 1860 1,282 0,109 1,2908 0,3927 0,3356 1,621 2014 11 2 1,334 0,083 1,3977 0,3927 0,3492 1,257 2180 1,370 0,065 1,4741 0,3927 0,3587 0,997 2300 1,760 0,120 2,4328 0,5236 0,4608 2,412 3795 1,782 0,109 2,4941 0,5236 0,4665 2,204 3891 2 1,810 0,095 2,5730 0,5236 0,4739 1,935 4014 1,834 0,083 2,6417 0,5236 0,4801 1,701 4121
41. Tubesheet 풅풐 (mm) 16 20 25 38 57 Tubesheet thickness (mm) 21 26 32 48 70
42. Tube bundle
43. Shells • Shell diameter: 푖 = 6 ÷ 20 𝑖푛 (normally  24) Minimum thickness Shell diameter Material 1000 Carbon steel 5 6 6 6 Alloy 3 4 4 6
44. Nozzles & Impingements Limitation of fluid velocity in nozzle 풗 Τ풔 9000 Shell side nozzles < 휌퐿푖푞 2230 Tube side nozzles, noncorrosive and nonabrasive single phase fluids < 휌퐿푖푞 740 Tube side nozzles, other fluids < 휌퐿푖푞 • Impingement protection is required for gases, vapor, two phases mixtures, corrosive or abrasive
45. Expansion joint Toroidal Bellows Bellows with reinforcing rings & insulation cover Flanged & flued heads
46. Baffles Disc and doughnut Triple segmental
47. Baffles Tie rod and baffles
48. Tube passes 1 tube pass 2 tube passes
49. Tube passes
50. Shell passes 1 shell pass 2 shell passes 3 shell passes
51. Flow arrangement Cocurrent Counter current 1 − 2 2 − 4 1 − 1 Cross flow 1 − 2 Cross flow
52. Temperature for fluid properties For hot stream: 푖푛 ≥ 표 푡 ∆ = 푖푛 − 표 푡 For cold stream: 푡푖푛 ≤ 푡표 푡 ∆푡 = 푡표 푡 − 푡푖푛 ∆1−∆2 Log mean temperature difference: ∆ 퐿 = ∆ ln 1 ∆2 1 푡ҧ = 푡 + 푡 • ∆ > ∆푡 ቐ 2 푖푛 표 푡 ത = 푡ҧ + ∆ 퐿 푡ҧ = ത − ∆ 퐿 • ∆ < ∆푡 ቐ 1 ത = + 2 푖푛 표 푡 1 푡ҧ = 푡 + 푡 2 푖푛 표 푡 1 • ∆ = ∆푡 ത = + 2 푖푛 표 푡 ∆ 퐿 = ∆1= ∆2
53. Heat transfer coefficient of tube side • Condensation: 1Τ3 휌퐿 휌퐿 − 휌 ℎ푡 = 0.76 퐿 휇퐿훾 stratified flow 휌 1 + 퐿 휌 = 0.021푅푒0.8푃 0.43 푡 2 annular flow : gravitational acceleration 9.81 Τ푠2 퐿: condensate thermal conductivity, 푊Τ ℃ 3 휌퐿: condensate density, Τ 3 휌 : vapor density, Τ 2 휇퐿: condensate dynamic viscosity, 푠Τ 훾: tube loading (condensate flow per unit length of tube), Τ푠
54. Heat transfer coefficient • Boiling ℎ = ℎ + 푛ℎ푛 ℎ 퐿 0.8 0.4 = = 0.019푅푒퐿 푃 퐿 푣퐿퐿휌퐿 1 − 푅푒퐿 = 휇퐿 ℎ : heat transfer coefficient by convaction, 푊Τ ℃ 퐿: characteristic dimension, 푣퐿: liquid velocity, Τ푠 3 휌퐿: liquid density, Τ : mass fraction of vapor 2 휇퐿: liquid dynamic viscosity, 푠Τ 퐿: liquid thermal conductivity, 푊Τ ℃
55. Heat transfer coefficient • Boiling ℎ = ℎ + 푛ℎ푛 푞 = 표푛푠푡 푛푡: 0.17 1.2 10 0.69 0.7 푃 푃 푃 ℎ푛 = 0.104푃 푞 1.8 + 4 + 10 푃 푃 푃 ℎ: heat transfer coefficient, 푊Τ ℃ 푃: operating pressure, 푃 : liquid critical pressure, 2 푞 = ℎ 푤 − 푠 : heat flux, 푊Τ
56. Heat transfer coefficient 1.0 0.9 0.8 0.7 0.6 푛 0.5 0.4 0.3 0.2 0.1 1 104 2 3 4 5 6 7 8 9 105 2 3 4 5 6 7 8 9 106 1.25 푅푒퐿
57. Overall heat transfer coefficient • For dirty: 푄 푖 푡 1 푈 푖 푡 = = 표∆ 1 표 표 표 1 + 푅표 + ln + + 푅푡 ℎ푠 2 퐿 푖 푖 ℎ푡 1 푈 푖 푡 ≅ 1 표 1 + 푅표 + + 푅푡 ℎ푠 푖 ℎ푡
58. Pressure drop of tube side 2 2 0.14 퐿푛푡푣푡 휌푡 휇푡푤 Darcy equation ∆ 푡= 5 145 × 10 푖푠 휇푡 2 2푣푡 푛푡 “return loss” ∆ = ∆ = ∆ 푡 + ∆ : pressure drop of tube side 푠𝑖 ∆ 푡: pressure drop in tube side 푠𝑖 ∆ : return loss (lost due to direction change) 푠𝑖 : friction factor 푡2Τ𝑖푛2 푠: specific gravity 3 휌푡: density of fluid in tube 푙 Τ 푡 푣푡: tube velocity 푡Τ푠 : gravity acceleration 32.174 푡Τ푠2 퐿: tube length 푡 푛푡: number of tube passes 푖: tube inner diameter 푡
59. Pressure drop of shell side 2 2 0.14 + 1 푖푣푠 휌푠 휇푠푤 Darcy equation ∆ 푠= 5 145 × 10 푒푠 휇푠 푣 휌 푠 푠 푒 푠 푣 = 2 2 푅푒 = 푠 휌 12 − 표 휇푠 푠 푠 푒 = ; triangular 표 2 2 − 퐿 4 − 표 = 표 푖 = − 1 푒 = ; square 푠 표 ∆ 푠: pressure drop of shell side 푠𝑖 : number of baffles : friction factor 푡2Τ𝑖푛2 : baffle spacing 2 푠: specific gravity 푠: cross section area of shell 푡 3 휌푠: density of fluid in shell 푙 Τ 푡 푖: shell inner diameter 푡 푣푠: shell velocity 푡Τ푠 푒: equivalent diameter of shell 푡 표: tube outer diameter 푡 : tube pitch 푡
60. Example of heat exchanger design Summary Total area 표 = 푛푡; 푛푠; 푈표 Tube length 퐿 = 푛푡; 푛푠; 푈표; ; 푣푡 Shell diameter 푖 = 푛푡; 푛푠; ; 푣푡 Heat coefficient of tube ℎ푖 = ; 푣푡 Heat coefficient of shell ℎ푠 = 푛푡; 푛푠; ; ; ; 푣푡 Pressure drop of tube ∆ = 푛푡; 푛푠; 푈표; ; 푣푡 Pressure drop of shell ∆ = 푛푡; 푛푠; 푈표; ; ; ; 푣푡