HEAT TRANSFER, HEAT EXCHANGERS, CONDENSORS AND REBOILERS, AIR COOLERS

Reyad Awwad Shawabkeh

Associate Professor of Chemical EngineeringKing Fahd University of Petroleum & Minerals

Dhahran, 31261

Kingdom of Saudi Arabia

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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

2

HEAT TRANSFER LAW APPLIED TO HEAT EXCHANGERS

3

Heat Transfer by Conduction

W/m2 W/m.K

4

Thermal Conductivity of solids

5

Thermal Conductivity of liquids

6

Thermal conductivity of gases

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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.

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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?

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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

+

10

The Heat Conduction Equation

11

Heat Transfer by Convection

12

Reynolds and Prandtl Numbers

Values of Prandtl number for different liquids and gases

Re < 2100 Laminar flow

Re > 2100 Turbulent flow

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Flow through a single smooth cylinder

This correlation is valid over the ranges 10 < Rel < 107 and 0.6 < Pr < 1000 where

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Flow over a Flat Plate

Re < 5000 Laminar flow

Re > 5000 Turbulent flow

15

Natural Convection

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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

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Emissivity coefficient for several selected materialSurface 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

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Overall heat transfer coefficient

For a wall

For cylindrical geometry

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Typical value for overall heat transfer coefficientShell 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

Gases Brine 15 - 250

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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

condensableWater 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

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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.

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Heat exchanger nomenclatures

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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

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Removable cover, one pass, and floating head heat exchanger

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

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

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Removable kettle type reboiler with pull through floating head

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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 0.60000 (4/0) 0.454 11.5 24 0.022 0.6000 (3/0) 0.425 10.8 25 0.020 0.500 (2/0) 0.380 9.7 26 0.018 0.5

0 0.340 8.6 27 0.016 0.41 0.300 7.6 28 0.014 0.42 0.284 7.2 29 0.013 0.33 0.259 6.6 30 0.012 0.34 0.238 6.0 31 0.010 0.35 0.220 5.6 32 0.009 0.26 0.203 5.2 33 0.008 0.27 0.180 4.6 34 0.007 0.28 0.165 4.2 35 0.005 0.19 0.148 3.8 36 0.004 0.1

10 0.134 3.4 25 0.020 0.511 0.120 3.0 26 0.018 0.512 0.109 2.8 27 0.016 0.413 0.095 2.4 28 0.014 0.414 0.083 2.1 29 0.013 0.315 0.072 1.8 30 0.012 0.316 0.065 1.7 31 0.010 0.317 0.058 1.5 32 0.009 0.218 0.049 1.2 33 0.008 0.219 0.042 1.1 34 0.007 0.220 0.035 0.9 35 0.005 0.121 0.032 0.8 36 0.004 0.122 0.028 0.7

Tube sizing: Birmingham Wire Gage

28

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Tube sizing: Birmingham Wire Gage

Tube-side design

Arrangement of tubes inside the heat exchanger

30

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.

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Shell-side design

Shell thickness for different diameters and material of constructions

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Baffle type and spacing

33

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

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THERMAL AND HYDRAULIC HEAT EXCHANGER DESIGN

Design of Single phase heat exchanger

Design of Condensers

Design of Reboiler and Vaporizers

Design of Air Coolers

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Design of Single phase heat exchanger

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Typical values for fouling factor coefficients

37

Temperature profile for different types of heat exchangers

38

For counter current

For co-current

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one shell pass; two or more even tube 'passes

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two shell passes; four or multiples of four tube passes

divided-flow shell; two or more even-tube passes

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split flow shell, 2 tube pass

cross flow heat exchanger

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Shell-side heat transfer coefficient

43

44

Shell diameter

45

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Bundle diameter clearance

47

Tube-side heat transfer coefficient

48

Tube-side heat transfer factor

49

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.

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Kern’s Method

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Bell’s method

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Figure 34 Baffle cut geometry

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Pressure drop inside the shell

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Pressure drop inside the tubes

60

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Design of Condensers

Direct contact cooler

• For reactor off-gas quenching• Vacuum condenser• De-superheating• Humidification• Cooling towers

62

Condensation outside horizontal tubes

For turbulent flow,

For Laminar flow

63

Condensation inside horizontal tubes

stratified flow

annular flow

64

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

65

Boiling heat transfer and pool boiling

Nucleate pool boiling

Critical heat flux

Film boiling

66

Nucleate boiling heat transfer coefficient

67

Critical flux heat transfer coefficient

Film boiling heat transfer coefficient

Convection boiling

68

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

69

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Design of air cooler

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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 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

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Design loading

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