Heat transfer

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Heat transfer

  1. 1. HEAT TRANSFER, HEAT EXCHANGERS, CONDENSORS AND REBOILERS, AIR COOLERS Reyad Awwad Shawabkeh Associate Professor of Chemical Engineering King Fahd University of Petroleum & Minerals Dhahran, 31261 Kingdom of Saudi Arabia
  2. 2. Contents  HEAT TRANSFER LAW APPLIED TO HEAT EXCHANGERS 2  Heat Transfer by Conduction 3  The Heat Conduction Equation 9  Heat Transfer by Convection 12  Forced Convection 12  Natural Convection 14  Heat Transfer by Radiation 15  Overall heat transfer coefficient 18  Problems 22  DESIGN STANDARDS FOR TUBULAR HEAT EXCHANGERS 23  Size numbering and naming 23  Sizing and dimension 27  Tube-side design 32  Shell-side design 33  Baffle type and spacing 33  General design consideration 35  THERMAL AND HYDRAULIC HEAT EXCHANGER DESIGN 37  Design of Single phase heat exchanger 37  Kern’s Method 45  Bell’s method 49  Pressure drop inside the shell and tube heat exchanger 57  Design of Condensers 65  Design of Reboiler and Vaporizers 72  Design of Air Coolers9 85  MECHANICAL DESIGN FOR HEAT EXCHANGERS10 88  Design Loadings 88  Tube-Sheet Design as Per TEMA Standards 90  Design of Cylindrical shell, end closures and forced head 91  References 95
  3. 3. HEAT TRANSFER LAW APPLIED TO HEAT EXCHANGERS
  4. 4. Heat Transfer by Conduction W/m2 W/m.K
  5. 5. Thermal Conductivity of solids
  6. 6. Thermal Conductivity of liquids
  7. 7. Thermal conductivity of gases
  8. 8. Example Calculate the heat flux within a copper rod that heated in one of its ends to a temperature of 100 oC while the other end is kept at 25 oC. The rode length is 10 m and diameter is 1 cm.
  9. 9. Example An industrial freezer is designed to operate with an internal air temperature of -20 oC when external air temperature is 25 oC. The walls of the freezer are composite construction, comprising of an inner layer of plastic with thickness of 3 mm and has a thermal conductivity of 1 W/m.K. The outer layer of the freezer is stainless steel with 1 mm thickness and has a thermal conductivity of 16 W/m.K. An insulation layer is placed between the inner and outer layer with a thermal conductivity of 15 W/m.K. what will be the thickness of this insulation material that allows a heat transfer of 15 W/m2 to pass through the three layers, assuming the area normal to heat flow is 1 m2?
  10. 10. The Heat Conduction Equation Rate of heat generation inside control volume Rate of energy storage inside control volume Rate of heat conduction into control volume + = Rate of heat conduction out of control volume +
  11. 11. The Heat Conduction Equation
  12. 12. Heat Transfer by Convection
  13. 13. Reynolds and Prandtl Numbers Values of Prandtl number for different liquids and gases Re < 2100 Laminar flow Re > 2100 Turbulent flow
  14. 14. Flow through a single smooth cylinder This correlation is valid over the ranges 10 < Rel < 107 and 0.6 < Pr < 1000 where
  15. 15. Flow over a Flat Plate Re < 5000 Laminar flow Re > 5000 Turbulent flow
  16. 16. Natural Convection
  17. 17. Heat Transfer by Radiation q = ε σ (Th4 - Tc4) Ac Th = hot body absolute temperature (K) Tc = cold surroundings absolute temperature (K) Ac = area of the object (m2) σ = 5.6703 10-8 (W/m2K4) The Stefan-Boltzmann Constant
  18. 18. Emissivity coefficient for several selected material Surface Material Emissivity Coefficient - ε - Aluminum Commercial sheet 0.09 Aluminum Foil 0.04 Aluminum Commercial Sheet 0.09 Brass Dull Plate 0.22 Brass Rolled Plate Natural Surface 0.06 Cadmium 0.02 Carbon, not oxidized 0.81 Carbon filament 0.77 Concrete, rough 0.94 Granite 0.45 Iron polished 0.14 - 0.38 Porcelain glazed 0.93 Quartz glass 0.93 Water 0.95 - 0.963 Zink Tarnished 0.25
  19. 19. Overall heat transfer coefficient For a wall For cylindrical geometry
  20. 20. Typical value for overall heat transfer coefficient Shell and Tube Heat Exchangers Hot Fluid Cold Fluid U [W/m2C] Heat Exchangers Water Water 800 - 1500 Organic solvents Organic Solvents 100 - 300 Light oils Light oils 100 - 400 Heavy oils Heavy oils 50 - 300 Reduced crude Flashed crude 35 - 150 Regenerated DEA Foul DEA 450 - 650 Gases (p = atm) Gases (p = atm) 5 - 35 Gases (p = 200 bar) Gases (p = 200 bar) 100 - 300 Coolers Organic solvents Water 250 - 750 Light oils Water 350 - 700 Heavy oils Water 60 - 300 Reduced crude Water 75 - 200 Gases (p = 200 bar) Water 150 - 400 Organic solvents Brine 150 - 500 Water Brine 600 - 1200
  21. 21. Heat Exchangers Hot Fluid Cold Fluid U [W/m2C] Heaters Steam Water 1500 - 4000 Steam Organic solvents 500 - 1000 Steam Light oils 300 - 900 Steam Heavy oils 60 - 450 Steam Gases 30 - 300 Heat Transfer (hot) Oil Heavy oils 50 - 300 Flue gases Steam 30 - 100 Flue gases Hydrocarbon vapors 30 -100 Condensers Aqueous vapors Water 1000 - 1500 Organic vapors Water 700 - 1000 Refinery hydrocarbons Water 400 - 550 Vapors with some non condensable Water 500 - 700 Vacuum condensers Water 200 - 500 Vaporizers Steam Aqueous solutions 1000 - 1500 Steam Light organics 900 - 1200 Steam Heavy organics 600 - 900 Heat Transfer (hot) oil Refinery hydrocarbons 250 - 550
  22. 22. DESIGN STANDARDS FOR TUBULAR HEAT EXCHANGERS • Size of heat exchanger is represented by the shell inside diameter or bundle diameter and the tube length • Type and naming of the heat exchanger is designed by three letters single pass shell The first one describes the stationary head type The second one refers to the shell type The third letter shows the rear head type TYPE AES refers to Split-ring floating head exchanger with removable channel and cover.
  23. 23. Heat exchanger nomenclatures
  24. 24. The standard nomenclature for shell and tube heat exchanger 1. Stationary Head-Channel 2. Stationary Head-Bonnet 3. Stationary Head Flange-Channel or Bonnet 4. Channel Cover 5. Stationary Head Nozzle 6. Stationary Tube sheet 7. Tubes 8. Shell 9. Shell Cover 10. Shell Flange-Stationary Head End 11. Shell Flange-Rear Head End 12. Shell Node 13. Shell Cover Flange 14. Expansion Joint 15. Floating Tube sheet 16. Floating Head Cover 17. Floating Head Cover Flange 18. Floating Head Backing Device 19. Split Shear Ring 20. Slip-on Backing Flange 21. Floating Head Cover-External 22. Floating Tube sheet Skirt 23. Packing Box 24. Packing 25. Packing Gland 26. Lantern Ring 27. Tie-rods and Spacers 28. Support Plates 29. Impingement Plate 30. Longitudinal Baffle 31. Pass Partition 32. Vent Connection 33. Drain Connection 34. Instrument Connection 35. Support Saddle 36. Lifting Lug 37. Support Bracket 38. Weir 39. Liquid Level Connection 40. Floating Head Support
  25. 25. Removable cover, one pass, and floating head heat exchanger Removable cover, one pass, and outside packed floating head heat exchanger
  26. 26. Channel integral removable cover, one pass, and outside packed floating head heat exchanger
  27. 27. Removable kettle type reboiler with pull through floating head
  28. 28. Gauge (B.W.G.) (inches) (B.W.G.) (mm) Gauge (B.W.G.) (inches) (B.W.G.) (mm) 00000 (5/0) 0.500 12.7 23 0.025 0.6 0000 (4/0) 0.454 11.5 24 0.022 0.6 000 (3/0) 0.425 10.8 25 0.020 0.5 00 (2/0) 0.380 9.7 26 0.018 0.5 0 0.340 8.6 27 0.016 0.4 1 0.300 7.6 28 0.014 0.4 2 0.284 7.2 29 0.013 0.3 3 0.259 6.6 30 0.012 0.3 4 0.238 6.0 31 0.010 0.3 5 0.220 5.6 32 0.009 0.2 6 0.203 5.2 33 0.008 0.2 7 0.180 4.6 34 0.007 0.2 8 0.165 4.2 35 0.005 0.1 9 0.148 3.8 36 0.004 0.1 10 0.134 3.4 25 0.020 0.5 11 0.120 3.0 26 0.018 0.5 12 0.109 2.8 27 0.016 0.4 13 0.095 2.4 28 0.014 0.4 14 0.083 2.1 29 0.013 0.3 15 0.072 1.8 30 0.012 0.3 16 0.065 1.7 31 0.010 0.3 17 0.058 1.5 32 0.009 0.2 18 0.049 1.2 33 0.008 0.2 19 0.042 1.1 34 0.007 0.2 20 0.035 0.9 35 0.005 0.1 21 0.032 0.8 36 0.004 0.1 22 0.028 0.7 Tube sizing: Birmingham Wire Gage
  29. 29. Tube sizing: Birmingham Wire Gage
  30. 30. Tube-side design Arrangement of tubes inside the heat exchanger
  31. 31. Shell-side design types of shell passes(a) one-pass shell for E-type, (b) split flow of G-type, (c) divided flow of J-type, (d) two-pass shell with longitudinal baffle of F-type (e) double split flow of H-type.
  32. 32. Shell-side design Shell thickness for different diameters and material of constructions
  33. 33. Baffle type and spacing
  34. 34. General design consideration Factor Tube-side Shell-side Corrosion More corrosive fluid Less corrosive fluids Fouling Fluids with high fouling and scaling Low fouling and scaling Fluid temperature High temperature Low temperature Operating pressure Fluids with low pressure drop Fluids with high pressure drop Viscosity Less viscous fluid More viscous fluid Stream flow rate High flow rate Low flow rate
  35. 35. THERMAL AND HYDRAULIC HEAT EXCHANGER DESIGN Design of Single phase heat exchanger Design of Condensers Design of Reboiler and Vaporizers Design of Air Coolers
  36. 36. Design of Single phase heat exchanger
  37. 37. Typical values for fouling factor coefficients
  38. 38. Temperature profile for different types of heat exchangers
  39. 39. For counter current For co-current
  40. 40. one shell pass; two or more even tube 'passes
  41. 41. two shell passes; four or multiples of four tube passes divided-flow shell; two or more even-tube passes
  42. 42. split flow shell, 2 tube pass cross flow heat exchanger
  43. 43. Shell-side heat transfer coefficient
  44. 44. Shell diameter
  45. 45. Bundle diameter clearance
  46. 46. Tube-side heat transfer coefficient
  47. 47. Tube-side heat transfer factor
  48. 48. Shell and Tube design procedure • Kern’s Method • Bell’s method This method is designed to predict the local heat transfer coefficient and pressure drop by incorporating the effect of leak and by-passing inside the shell and also can be used to investigate the effect of constructional tolerance and the use of seal strip This method was based on experimental work on commercial exchangers with standard tolerances and will give a reasonably satisfactory prediction of the heat-transfer coefficient for standard designs.
  49. 49. Kern’s Method
  50. 50. Bell’s method
  51. 51. Figure 34 Baffle cut geometry
  52. 52. Pressure drop inside the shell
  53. 53. Pressure drop inside the tubes
  54. 54. Design of Condensers Direct contact cooler • For reactor off-gas quenching • Vacuum condenser • De-superheating • Humidification • Cooling towers
  55. 55. Condensation outside horizontal tubes For turbulent flow, For Laminar flow
  56. 56. Condensation inside horizontal tubes stratified flow annular flow
  57. 57. Design of Reboiler and Vaporizers Forced-circulation reboiler Thermosyphon reboiler Kettle reboiler • Suitable to carry viscous and heavy fluids. • Pumping cost is high • The most economical type where there is no need for pumping of the fluid • It is not suitable for viscous fluid or high vacuum operation • Need to have a hydrostatic head of the fluid • It has the lower heat transfer coefficient than the other types for not having liquid circulation • Used for fouling materials and vacuum operation with a rate of vaporization up to 80% of the feed
  58. 58. Boiling heat transfer and pool boiling Nucleate pool boiling Critical heat flux Film boiling
  59. 59. Nucleate boiling heat transfer coefficient
  60. 60. Critical flux heat transfer coefficient Film boiling heat transfer coefficient
  61. 61. Convection boiling Effective heat transfer coefficient encounter the effect of both convective and nucleate boiling
  62. 62. Design of air cooler
  63. 63. Mechanical Design for HE A typical sequence of mechanical design procedures is summarized by the flowing steps • Identify applied loadings. • Determine applicable codes and standards. • Select materials of construction (except for tube material, which is selected during the thermal design stage). • Compute pressure part thickness and reinforcements. • Select appropriate welding details. • Establish that no thermohydraulic conditions are violated. • Design nonpressure parts. • Design supports. • Select appropriate inspection procedure
  64. 64. Design loading

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