1 HEAT TRANSFER HEAT EXCHANGERS CONDENSORS AND REBOILERS

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

Contents § HEAT TRANSFER LAW APPLIED TO HEAT EXCHANGERS § HEAT TRANSFER BY CONDUCTION § The Heat Conduction Equation § HEAT TRANSFER BY CONVECTION § Forced Convection § Natural Convection § HEAT TRANSFER BY RADIATION § OVERALL HEAT TRANSFER COEFFICIENT § PROBLEMS 2 3 9 12 12 14 15 18 22 DESIGN STANDARDS FOR TUBULAR HEAT EXCHANGERS SIZE NUMBERING AND NAMING SIZING AND DIMENSION TUBE-SIDE DESIGN SHELL-SIDE DESIGN § Baffle type and spacing § GENERAL DESIGN CONSIDERATION § THERMAL AND HYDRAULIC HEAT EXCHANGER DESIGN § DESIGN OF SINGLE PHASE HEAT EXCHANGER § Kern’s Method § Bell’s method § Pressure drop inside the shell and tube heat exchanger § DESIGN OF CONDENSERS § DESIGN OF REBOILER AND VAPORIZERS § DESIGN OF AIR COOLERS 9 § MECHANICAL DESIGN FOR HEAT EXCHANGERS 10 § DESIGN LOADINGS § TUBE-SHEET DESIGN AS PER TEMA STANDARDS § DESIGN OF CYLINDRICAL SHELL, END CLOSURES AND FORCED HEAD § REFERENCES 23 23 27 32 33 33 35 37 37 45 49 57 65 72 85 88 88 90 91 95 § § § 2

HEAT TRANSFER LAW APPLIED TO HEAT EXCHANGERS 3

Heat Transfer by Conduction W/m 2 W/m. K 4

Thermal Conductivity of solids 5

Thermal Conductivity of liquids 6

Thermal conductivity of gases 7

Example 8 Calculate the heat flux within a copper rod that heated in one of its ends to a temperature of 100 o. C while the other end is kept at 25 o. C. The rode length is 10 m and diameter is 1 cm.

Example 9 An industrial freezer is designed to operate with an internal air temperature of -20 o. C when external air temperature is 25 o. C. 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 thickness of this insulation material that allows a heat transfer of 15 W/m 2 to pass through the three layers, assuming the area normal to heat flow is 1 m 2?

The Heat Conduction Equation Rate of heat conduction into control volume + Rate of heat generation inside control volume = Rate of heat conduction out of control volume + 10 Rate of energy storage inside control volume

The Heat Conduction Equation 11

Heat Transfer by Convection 12

Reynolds and Prandtl Numbers Re < 2100 Laminar flow Re > 2100 Turbulent flow Values of Prandtl number for different liquids and gases 13

Flow through a single smooth cylinder This correlation is valid over the ranges 10 < Rel < 107 and 0. 6 < Pr < 1000 where 14

Flow over a Flat Plate Re < 5000 Laminar flow Re > 5000 Turbulent flow 15

Natural Convection 16

Heat Transfer by Radiation q = ε σ (Th 4 - Tc 4) Ac Th = hot body absolute temperature (K) Tc = cold surroundings absolute temperature (K) Ac = area of the object (m 2) σ = 5. 6703 10 -8 (W/m 2 K 4) The Stefan-Boltzmann Constant 17

Emissivity coefficient for several selected material 18 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 Zink Tarnished 0. 95 - 0. 963 0. 25

Overall heat transfer coefficient For a wall For cylindrical geometry 19

Typical value for overall heat transfer coefficient Shell and Tube Heat Exchangers Coolers Hot Fluid Cold Fluid U [W/m 2 C] Water 800 - 1500 Organic solvents Organic Solvents 100 - 300 Light oils 100 - 400 Heavy oils 50 - 300 Reduced crude Flashed crude 35 - 150 Regenerated DEA Foul DEA 450 - 650 Gases (p = atm) 5 - 35 Gases (p = 200 bar) 100 - 300 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 Gases Brine 15 - 250 20

Heat Exchangers Hot Fluid Cold Fluid U [W/m 2 C] 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 Aqueous vapors Water 1000 - 1500 Organic vapors Water 700 - 1000 Refinery hydrocarbons Water 400 - 550 Water 500 - 700 Vacuum condensers Water 200 - 500 Steam Aqueous solutions 1000 - 1500 Steam Light organics 900 - 1200 Steam Heavy organics 600 - 900 Heat Transfer (hot) oil Refinery hydrocarbons 250 - 550 Condensers Vapors with some non condensable Vaporizers 21

DESIGN STANDARDS FOR TUBULAR HEAT EXCHANGERS 22 • 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.

Heat exchanger nomenclatures 23

The standard nomenclature for shell and tube heat exchanger 1. Stationary Head-Channel 20. Slip-on Backing Flange 30. Longitudinal Baffle 2. Stationary Head-Bonnet 21. Floating Head Cover-External 31. Pass Partition 24 3. Stationary Head Flange-Channel or 22. Floating Tube sheet Skirt 32. Vent Connection Bonnet 23. Packing Box 33. Drain Connection 4. Channel Cover 24. Packing 34. Instrument Connection 5. Stationary Head Nozzle 25. Packing Gland 35. Support Saddle 6. Stationary Tube sheet 26. Lantern Ring 36. Lifting Lug 7. Tubes 27. Tie-rods and Spacers 37. Support Bracket 8. Shell 28. Support Plates 38. Weir 9. Shell Cover 29. Impingement Plate 39. Liquid Level Connection 10. Shell Flange-Stationary Head End 40. Floating Head Support 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

25 Removable cover, one pass, and floating head heat exchanger Removable cover, one pass, and outside packed floating head heat exchanger

26 Channel integral removable cover, one pass, and outside packed floating head heat exchanger

27 Removable kettle type reboiler with pull through floating head

Tube sizing: Birmingham Wire Gage Gauge 00000 (5/0) 0000 (4/0) 000 (3/0) 00 (2/0) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 (B. W. G. ) (inches) 0. 500 0. 454 0. 425 0. 380 0. 340 0. 300 0. 284 0. 259 0. 238 0. 220 0. 203 0. 180 0. 165 0. 148 0. 134 0. 120 0. 109 0. 095 0. 083 0. 072 0. 065 0. 058 0. 049 0. 042 0. 035 0. 032 0. 028 (B. W. G. ) (mm) Gauge 12. 7 11. 5 10. 8 9. 7 8. 6 7. 2 6. 6 6. 0 5. 6 5. 2 4. 6 4. 2 3. 8 3. 4 3. 0 2. 8 2. 4 2. 1 1. 8 1. 7 1. 5 1. 2 1. 1 0. 9 0. 8 0. 7 23 24 25 26 27 28 29 30 31 32 33 34 35 36 28 (B. W. G. ) (inches) 0. 025 0. 022 0. 020 0. 018 0. 016 0. 014 0. 013 0. 012 0. 010 0. 009 0. 008 0. 007 0. 005 0. 004 (B. W. G. ) (mm) 0. 6 0. 5 0. 4 0. 3 0. 2 0. 1

Tube sizing: Birmingham Wire Gage 29

Tube-side design Arrangement of tubes inside the heat exchanger 30

Shell-side design 31 (a) one-pass shell for E-type, types of shell passes (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.

Shell-side design Shell thickness for different diameters and material of constructions 32

Baffle type and spacing 33

General design consideration 34 Factor Tube-side Shell-side Corrosion More corrosive fluid Less corrosive fluids Fouling Fluids with high fouling Low fouling and scaling Fluid temperature High temperature Low temperature Operating pressure Fluids with low pressure Fluids with high pressure drop Viscosity Less viscous fluid More viscous fluid Stream flow rate High flow rate Low flow rate

THERMAL AND HYDRAULIC HEAT EXCHANGER DESIGN Design of Single phase heat exchanger Design of Condensers Design of Reboiler and Vaporizers Design of Air Coolers 35

Design of Single phase heat exchanger 36

Typical values for fouling factor coefficients 37

Temperature profile for different types of heat exchangers 38

39 For counter current For co-current

40 one shell pass; two or more even tube 'passes

41 two shell passes; four or multiples of four tube passes divided-flow shell; two or more even-tube passes

42 split flow shell, 2 tube pass cross flow heat exchanger

Shell-side heat transfer coefficient 43

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Shell diameter 45

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47 Bundle diameter clearance

Tube-side heat transfer coefficient 48

49 Tube-side heat transfer factor

Shell and Tube design procedure 50 • Kern’s Method 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. • 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

Kern’s Method 51

Bell’s method 52

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56 Figure 34 Baffle cut geometry

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Pressure drop inside the shell 59

Pressure drop inside the tubes 60

Design of Condensers • • • 61 For reactor off-gas quenching Vacuum condenser De-superheating Humidification Cooling towers Direct contact cooler

Condensation outside horizontal tubes For Laminar flow For turbulent flow, 62

Condensation inside horizontal tubes 63 stratified flow annular flow

Design of Reboiler and Vaporizers 64 • Suitable to carry viscous and heavy fluids. • Pumping cost is high Forced-circulation reboiler • 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 Thermosyphon reboiler • 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 Kettle reboiler

Boiling heat transfer and pool boiling Nucleate pool boiling Critical heat flux Film boiling 65

Nucleate boiling heat transfer coefficient 66

Critical flux heat transfer coefficient Film boiling heat transfer coefficient 67

Convection boiling Effective heat transfer coefficient encounter the effect of both convective and nucleate boiling 68

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Design of air cooler 71

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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 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 73

Design loading 74

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