Ch8-Heat Transfer Equipment –Design and cost UTAS Fall22.pdf

Heat Transfer Equipment

1. ENCH4119- PLANT
DESIGN AND ECONOMICS
Instructor:
Name : Dr. Saravana Kumar
Room No : B112
Email: [email protected]

2. Heat Transfer Equipment
–
Design and Cost
2

3. THE OVERALL HEAT TRANSFER COEFFICIENT
•A heat exchanger typically involves two flowing fluids
separated by a solid wall.
•Heat is first transferred from the hot fluid to the wall by
convection, through the wall by conduction, and from the wall
to the cold fluid again by convection.
•Any radiation effects are usually included in the convection
heat transfer coefficients.
Thermal resistance network associated
with heat transfer in a double-pipe heat
exchanger.
3
Where R= Thermal Resistance

4. U the overall heat transfer coefficient,
W/m
2
C
When
The overall heat transfer coefficient U is dominated by the smaller convection coefficient. When one of the
convection coefficients is much smaller than the other (say, h
i << h
o), we have 1/h
i >> 1/h
o, and thus U  h
i. This
situation arises frequently when one of the fluids is a gas and the other is a liquid. In such cases, fins are
commonly used on the gas side to enhance the product UA and thus the heat transfer on that side.
4

5. The overall heat transfer coefficient ranges from about
10 W/m
2
C for gas-to-gas heat exchangers to about
10,000 W/m
2
C for heat exchangers that involve
phase changes.
For short fins of high thermal
conductivity, we can use this total
area in the convection resistance
relation R
conv = 1/hA
s
To account for fin efficiency
When the tube is finned on one side to
enhance heat transfer, the total heat transfer
surface area on the finned side is
5

6. Fouling Factor
The performance of heat exchangers usually deteriorates with time as a result of accumulation of deposits on
heat transfer surfaces. The layer of deposits represents additional resistance to heat transfer. This is represented
by a fouling factor R
f.
The fouling factor increases with the operating temperature and the length of service and decreases with the
velocity of the fluids.
6

7. ANALYSIS OF HEAT EXCHANGERS
An engineer often finds himself or herself in a position
1.to select a heat exchanger that will achieve a specified temperature change in a fluid stream
of known mass flow rate - the log mean temperature difference (or LMTD) method.
2.to predict the outlet temperatures of the hot and cold fluid streams in a specified heat
exchanger - the effectiveness–NTU method.
The rate of heat transfer in heat
exchanger (HE is insulated):
heat capacity rate
Two fluid streams that have
the same capacity rates
experience the same
temperature change in a
well-insulated heat
exchanger.
7

8. Variation of fluid
temperatures in a
heat exchanger
when one of the
fluids condenses or
boils.
is the rate of evaporation or condensation of the fluid
h
fg is the enthalpy of vaporization of the fluid at the specified temperature or pressure.
The heat capacity rate of a fluid during a phase-change process must approach infinity since the temperature
change is practically zero.
T
m an appropriate mean (average) temperature difference
between the two fluids
8

9. THE LOG MEAN TEMPERATURE DIFFERENCE METHOD
Variation of the fluid temperatures
in a parallel-flow double-pipe heat
exchanger. log mean
temperature
difference
9

10. The arithmetic mean temperature difference
The logarithmic mean temperature difference T
lm is an
exact representation of the average temperature
difference between the hot and cold fluids.
Note that T
lm is always less than T
am. Therefore, using
T
am in calculations instead of T
lm will overestimate the
rate of heat transfer in a heat exchanger between the two
fluids.
When T
1 differs from T
2 by no more than 40 percent,
the error in using the arithmetic mean temperature
difference is less than 1 percent. But the error increases
to undesirable levels when T
1 differs from T
2 by greater
amounts.
10

11. Counter-Flow Heat Exchangers
In the limiting case, the cold fluid will be heated to the inlet
temperature of the hot fluid.
However, the outlet temperature of the cold fluid can
never exceed the inlet temperature of the hot fluid.
For specified inlet and outlet temperatures, T
lm a
counter-flow heat exchanger is always greater than that
for a parallel-flow heat exchanger.
That is, T
lm, CF > T
lm, PF, and thus a smaller surface area
(and thus a smaller heat exchanger) is needed to achieve
a specified heat transfer rate in a counter-flow heat
exchanger.
When the heat capacity rates of the two
fluids are equal
11

12. Multipass and Cross-Flow Heat Exchangers:
Use of a Correction Factor
F correction factor depends on the geometry of the
heat exchanger and the inlet and outlet temperatures
of the hot and cold fluid streams.
F for common cross-flow and shell-and-tube heat
exchanger configurations is given in the figure versus
two temperature ratios P and R defined as
1 and 2 inlet and outlet
T and t shell- and tube-side temperatures
F = 1 for a condenser or boiler
12

13. Correction factor F
charts for common
shell-and-tube heat
exchangers.
13

14. Correction factor F
charts for common
cross-flow heat
exchangers.
14

15. The LMTD method is very suitable for determining the size of a heat exchanger to realize
prescribed outlet temperatures when the mass flow rates and the inlet and outlet
temperatures of the hot and cold fluids are specified.
With the LMTD method, the task is to select a heat exchanger that will meet the prescribed
heat transfer requirements. The procedure to be followed by the selection process is:
1.Select the type of heat exchanger suitable for the application.
2.Determine any unknown inlet or outlet temperature and the heat transfer rate using an
energy balance.
3.Calculate the log mean temperature difference T
lm and the correction factor F, if
necessary.
4.Obtain (select or calculate) the value of the overall heat transfer coefficient U.
5.Calculate the heat transfer surface area A
s .
The task is completed by selecting a heat exchanger that has a heat transfer surface area
equal to or larger than A
s.
15

16. THE EFFECTIVENESS–NTU METHOD
A second kind of problem encountered in heat exchanger analysis is the determination of the heat transfer rate
and the outlet temperatures of the hot and cold fluids for prescribed fluid mass flow rates and inlet temperatures
when the type and size of the heat exchanger are specified.
Heat transfer effectiveness
the maximum possible heat transfer rate
C
min is the smaller of C
h and C
c
16

17. Actual heat transfer rate
17

18. The effectiveness of a heat
exchanger depends on the
geometry of the heat exchanger
as well as the flow arrangement.
Therefore, different types of heat
exchangers have different
effectiveness relations.
We illustrate the development of
the effectiveness e relation for the
double-pipe parallel-flow heat
exchanger.
18

19. Effectiveness relations of the heat exchangers typically involve the dimensionless group UA
s
/C
min.
This quantity is called the number of transfer units NTU.
For specified values of U and C
min, the value of NTU is a
measure of the surface area A
s. Thus, the larger the NTU, the
larger the heat exchanger.
capacity
ratio
The effectiveness of a heat exchanger is a function of the number of transfer units
NTU and the capacity ratio c.
19

20. 20

21. Effectiveness for
heat exchangers.
21

22. 22

23. When all the inlet and outlet temperatures are specified, the size of the heat exchanger can
easily be determined using the LMTD method. Alternatively, it can be determined from the
effectiveness–NTU method by first evaluating the effectiveness from its definition and then
the NTU from the appropriate NTU relation.
23

24. (e.g., boiler, condenser)
24

25. Observations from the effectiveness relations and charts
•The value of the effectiveness ranges from 0 to 1. It increases rapidly with NTU for small
values (up to about NTU = 1.5) but rather slowly for larger values. Therefore, the use of
a heat exchanger with a large NTU (usually larger than 3) and thus a large size cannot
be justified economically, since a large increase in NTU in this case corresponds to a
small increase in effectiveness.
•For a given NTU and capacity ratio c = C
min /C
max, the counter-flow heat exchanger has
the highest effectiveness, followed closely by the cross-flow heat exchangers with both
fluids unmixed. The lowest effectiveness values are encountered in parallel-flow heat
exchangers.
•The effectiveness of a heat exchanger is independent of the capacity ratio c for NTU
values of less than about 0.3.
•The value of the capacity ratio c ranges between 0 and 1. For a given NTU, the
effectiveness becomes a maximum for c = 0 (e.g., boiler, condenser) and a minimum for
c = 1 (when the heat capacity rates of the two fluids are equal).
25

26. Design of Single phase heat exchanger
26

27. Shell-side heat transfer coefficient
27

28. 28
bundle
diameter clearance, BDC
Where=Bundle diameter clearance, BDC

29. Shell diameter
29

30. 30

31. Bundle diameter clearance
31

32. Tube-side heat transfer coefficient
32

33. Tube-side heat transfer factor
33

34. Determination of Pressure drop in heat exchanger
34
The major source of pressure prop in the heat exchanger is friction encountered
By the fluid as it flows through either the shell or the tube of the exchanger.
Other Pressure drops occur because of friction due to the sudden expansion,
sudden contraction or reversal in the direction of flow of the fluids.

Changes in the vertical head and kinetic energy can influence the pressure drop,
but these two effects are relatively small and can be neglected in the most
design calculations

35. Pressure drop inside the shell
35

36. Pressure drop inside the tubes
36

37. SELECTION OF HEAT EXCHANGERS
The uncertainty in the predicted value of U can exceed 30 percent. Thus, it is natural to tend to overdesign
the heat exchangers.
Heat transfer enhancement in heat exchangers is usually accompanied by increased pressure drop, and
thus higher pumping power.
Therefore, any gain from the enhancement in heat transfer should be weighed against the cost of the
accompanying pressure drop.
Usually, the more viscous fluid is more suitable for the shell side (larger passage area and thus lower
pressure drop) and the fluid with the higher pressure for the tube side.
The proper selection of
a heat exchanger depends
on several factors:
•Heat Transfer Rate
•Cost
•Pumping Power
•Size and Weight
•Type
•Materials
The annual cost of electricity associated with the operation of
the pumps and fans
The rate of heat transfer in the prospective
heat exchanger
37

38. 38
Key Heat exchange types
Double pipe and multiple double pipe exchangers
Shell and tube heat exchangers
Scraped surface exchangers
Gasketed and welded plate exchangers
Spiral plate and tube exchangers
Compact exchangers
Air-cooled exchangers
Condensers
Evaporators

39. 39
Gasketed and
welded plate
exchangers
Shell and tube
Exchanger

40. 40
Spiral plate and tube exchangers
Compact exchangers

41. 41
Air-cooled exchangers
Condenser Evaporator

42. 42
Heat exchanger classifications.
A Recuperative Heat Exchanger has separate flow paths for each fluid and fluids flow simultaneously through the
exchanger exchanging heat across the wall separating the flow paths. A Regenerative Heat Exchanger has a single flow
path, which the hot and cold fluids alternately pass through.
In a regenerative heat exchanger, the flow path normally consists of a matrix, which is heated when the hot fluid passes
through it (this is known as the "hot blow"). This heat is then released to the cold fluid when this flows through the matrix
(the "cold blow"). Regenerative Heat Exchangers are sometimes known as Capacitive Heat Exchangers

43. 43
For a typical Heat exchanger design, the following
parameters and constraints are usually given:
•Fluids used and their properties

•Inlet and exit fluid temperatures

•Fluid flow rates

•Operating pressure

•Allowable pressure drop

•Fouling resistances
Constraints: •Maximum and minimum fluid Velocity
•Maximum and minimum temperatures
•Corrosion allowances
•Materials of construction
•Properties of tube vibration
•Special codes involved

44. Heat exchanger nomenclatures
44

45. 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

45

46. Removable cover, one pass, and floating head heat exchanger

Removable cover, one pass, and outside packed floating head heat exchanger 46

47. Channel integral removable cover, one pass, and outside packed floating head heat
exchanger
47

48. Removable kettle type reboiler with pull through floating head
48

49. TYPES OF HEAT EXCHANGERS
49

50. Compact heat exchanger: It has a large heat transfer surface area
per unit volume (e.g., car radiator, human lung). A heat exchanger
with the area density  > 700 m
2
/m
3
is classified as being compact.
Cross-flow: In compact heat exchangers, the two fluids usually move
perpendicular to each other. The cross-flow is further classified as unmixed
and mixed flow.
50

51. Shell-and-tube heat exchanger: The most common type of heat exchanger in industrial
applications.
They contain a large number of tubes (sometimes several hundred) packed in a shell with
their axes parallel to that of the shell. Heat transfer takes place as one fluid flows inside the
tubes while the other fluid flows outside the tubes through the shell.
Shell-and-tube heat exchangers are further classified according to the number of shell and
tube passes involved.
51

52. Regenerative heat exchanger: Involves the alternate
passage of the hot and cold fluid streams through
the same flow area.
Dynamic-type regenerator: Involves a rotating drum
and continuous flow of the hot and cold fluid through
different portions of the drum so that any portion of
the drum passes periodically through the hot stream,
storing heat, and then through the cold stream,
rejecting this stored heat.
Condenser: One of the fluids is cooled and
condenses as it flows through the heat exchanger.
Boiler: One of the fluids absorbs heat and vaporizes.
52

53. Plate and frame (or just plate) heat exchanger: Consists of a series of plates with corrugated flat flow
passages. The hot and cold fluids flow in alternate passages, and thus each cold fluid stream is surrounded
by two hot fluid streams, resulting in very effective heat transfer. Well suited for liquid-to-liquid applications.
A plate-and-frame liquid-
to-liquid heat exchanger.
53

54. Cost of the heat exchangers
54
Fig 14-17 Purchase cost of U tube heat exchanger

55. Shell and Tube design procedure
• Kern’s Method
• Bell – Delaware 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.
55

56. Tube-side design

Arrangement of tubes inside the heat exchanger
56

57. 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.
57

58. Shell-side design

Shell thickness for different diameters and material of constructions
58

59. Baffle type and spacing
59

60. 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
60

61. Kern’s Method
61

62. Bell’s method
62

63. 63

64. 64

65. 65

66. Baffle cut geometry
66

67. 67

68. 68

69. Other types of heat exchanger
Plate Heat Exchanger
69

70. Spiral Exchanger – often used as
condensers
70

71. Process Intensification – minimising
exchanger size
71

72. Compact Heat Exchangers
Widely used to achieve large heat rates per unit volume, particularly when one
or both fluids is a gas.
Characterized by large heat transfer surface areas per unit volume (>700
m
2
/m
3
), small flow passages, and laminar flow.
72

73. Design of Condensers
Direct contact cooler
• For reactor off-gas quenching
• Vacuum condenser
• De-superheating
• Humidification
• Cooling towers
73

74. Condensation outside horizontal tubes
For turbulent flow,

For Laminar flow
74

75. Condensation inside horizontal tubes
stratified flow
annular flow 75

76. TYPES OF EVAPORATION EQUIPMENT
1. Open kettle or pan
2.Horizontal-tube natural circulation evaporator
3.Vertical-type natural circulation evaporator
4.Long-tube vertical-type evaporator
5.Falling-film-type evaporator
6.Forced-circulation-type evaporator
7.Agitated-film evaporator
8.Open-pan solar evaporator

76

77. 77

78. 78

79. 79

80. Different types of evaporators: (a) horizontal-tube type, (b) vertical-tube type,
80

81. Different types of evaporators: (c) long-tube vertical type,
(d) forced-circulation type.
81

82. 1.Single-effect evaporators
2.Forward-feed multiple-effect evaporators
3.Backward-feed multiple-effect evaporators
4.Parallel-feed multiple-effect evaporators


Methods of Operation of Evaporators
82

83. 1. Single-effect evaporators

Simplified diagram of single-effect evaporator
83

84. Simplified diagram of forward -feed triple-effect evaporator.
84

85. 85
Reference text books
1)Plant Design and Economics for Chemical Engineers, 5th edition, Peters, Timmerhaus


2) Heat and Mass Transfer: Fundamentals & Applications, Fourth
Edition ,Yunus A. Cengel, Afshin J. Ghajar, McGraw-Hill, 2011


3) Process Heat Transfer - Donald Quentin Kern

86. 86