2. 17-09-2015Transport Phenomenon (CH 306)2
Transfer of thermal energy between
two or more fluids
between a solid surface and a fluid
between solid particulates and a fluid
4. Types of HE
17-09-2015Transport Phenomenon (CH 306)4
Double-pipe exchanger
Shell and tube exchangers
Plate and frame exchangers
Plate-fin exchangers.
Spiral heat exchangers.
Air cooled heat exchangers
Agitated vessels.
Fired heaters.
5. Based on transfer process
17-09-2015Transport Phenomenon (CH 306)5
Indirect Contact – Shell & Tube Heat Exchangers
Direct Contact – Cooling Towers
Gas-Liquid exchangers
Liquid-Liquid exchangers
Gas-Gas heat exchangers
Based on phase of fluids
6. Based on construction
17-09-2015Transport Phenomenon (CH 306)6
Tubular
Double pipe heat exchanger
Shell and tube heat exchangers
Spiral heat exchangers
Plate-type
Plate and frame heat exchangers
Spiral plate heat exchangers
Extended Surface
Plate-fin exchanger
Tube-fin exchanger
7. Based on flow arrangements
17-09-2015Transport Phenomenon (CH 306)7
Parallel flow / Co-current flow
Counter flow
Cross flow
8. Heat Transfer Coefficient
17-09-2015Transport Phenomenon (CH 306)8
Heat transfer rate, 𝑞 = 𝑈𝐴∆𝑇 𝑚
U = overall heat transfer coefficient, W/(m2C)
A = heat transfer surface area, m2
∆𝑇 𝑚 = mean temperature difference, oC
Overall Heat Transfer Coefficient, Uo
ho= outside fluid film coefficient, W/(m2.oC)
hi= inside fluid film coefficient, W/(m2.oC)
hod= outside dirt coefficient (fouling factor), W/(m2.oC)
hid= inside dirt coefficient, W/(m2.oC)
kw= thermal conductivity of the tube wall material, W/(m2.oC)
di= tube inside diameter, m
do= tube outside diameter, m
16. Classification by Construction
17-09-2015Transport Phenomenon (CH 306)16
Fixed-tubesheet heat exchanger
Has straight tubes secured at both ends to tubesheets
welded to the shell
Low cost, simplest construction.
Bundle is "fixed" to the shell so outside of the tubes
cannot be cleaned mechanically.
Application is limited to clean services on the shell side
18. 17-09-2015Transport Phenomenon (CH 306)18
U-tube heat exchanger
Tubes are bent in the shape of a U
Only one tubesheet
Bending of tubes adds to the cost
Tube bundle is removable, outside of tubes can be
cleaned.
Because of the U-bend, inside of the tubes can’t be
cleaned mechanically
Can’t be used for dirty fluids inside tubes.
20. 17-09-2015Transport Phenomenon (CH 306)20
Floating head exchanger
Most versatile and costliest.
One tubesheet is fixed relative to the shell, and the other
is free to “float” within the shell.
Cleaning of both the insides and outsides of the tubes
Can be used for services where both the shell-side and
the tube-side fluids are dirty
Widely used in Petroleum Industry
27. For dirty
tube side
For clean
tube side
For
Hazardous
fluid
For horizontal
thermosyphon
reboilers
For No
Temp
Cross
Large temp
difference
between shell
& tube fluids
Allowable
pressure drop
on shell side is
very low
Fixed tube-
sheet on the
rear side of the
shell
TEMA Types
17-09-2015Transport Phenomenon (CH 306)27
28. FLUID ALLOCATION
Shell Side
Viscous Fluids
Lower Flow Rates
Cleaner Fluids
Tube Side
Fluids which are prone to
fouling
Corrosive fluids
Toxic fluids to increase
containment
High pressure streams, since
tubes are less expensive to
build strong
Streams with low allowable
pressure drop
Cooling water to be put on
tube side only
17-09-2015Transport Phenomenon (CH 306)28
29. Tubes
17-09-2015Transport Phenomenon (CH 306)29
Tubes should be able to withstand:
Operating temperature and pressure on both sides
Thermal stresses due to the differential thermal expansion
between the shell and the tube bundle
Corrosive nature of both the shell-side and the tube-side
fluids
TUBE PITCH RATIO:
Min 1.25 times of tube OD
1.333 times of tube OD
1.5 times of tube OD
TUBE PASS: Based on pressure drop & velocity limit on tube
side
30. TUBE LAYOUT ANGLE
17-09-2015Transport Phenomenon (CH 306)30
30o
FLOW
90o
FLOW
45o
FLOW
60o
FLOW
30o Triangular
60o Rotated
Triangular
45o Rotated
Square
90o Square
Triangular layouts give more
tubes in a given shell Square layouts give cleaning
lanes with close pitch
31. 17-09-2015Transport Phenomenon (CH 306)31
Feature Tube Layout Pattern
Lower ΔP on shell-side Square (effective only at low
Re number)
Shell-side fouling Square - easier cleaning
Horizontal shell-side
Boiling
Square
Smaller shell size Fit 15% more tubes if
triangular pitch used
32. Tube pitch
17-09-2015Transport Phenomenon (CH 306)32
Shortest distance between two adjacent tubes
TEMA specifies a minimum tube pitch of 1.25*(OD)
Minimum tube pitch leads to smallest shell diameter for a
given number of tubes.
To reduce shell-side pressure drop, the tube pitch may be
increased to a higher value.
33. Tubesheet
17-09-2015Transport Phenomenon (CH 306)33
Barrier between shell-side and tube-side fluids.
Mostly circular with uniform pattern of drilled holes.
Tubes are attached to tubesheet
34. Tie rods and spacers
17-09-2015Transport Phenomenon (CH 306)34
Tie rods and spacers are used for:
holding the baffle assembly together
maintaining the selected baffle spacing
help the bundle to slide out from the shell
Can also be used as tie rods to hold the bundle in
position.
Sliding strips
35. Sealing strips and Seal rods
17-09-2015Transport Phenomenon (CH 306)35
Sealing strips prevent shell side fluid from bypassing the
bundle.
Sealing strips block the resulting large open area at top or
bottom of the shell.
Seal rods are also used to control the leakage streams.
37. TYPES OF BAFFLES
17-09-2015Transport Phenomenon (CH 306)37
Segmental type;
Single – horizontal & vertical
Double
Triple
No-Tubes in Window (NTIW)
Orifice type
Disc and doughnut type
Rod type
Impingement type
Longitudinal (pass partitions)
39. 17-09-2015Transport Phenomenon (CH 306)39
ORIENTATION:
Horizontal for heating or cooling with no phase change
Vertical for shell side condensation
CUT:
15 % to 45 % of shell ID for Single Segmental
25 % to 35 % of shell ID for Double Segmental
40. Baffle cut
17-09-2015Transport Phenomenon (CH 306)40
Height of the segment that is cut in a baffle to permit the shell-
side fluid to flow across the baffle.
Baffle cut should be set carefully because a baffle cut that is
either too large or too small can increase the possibility of
fouling in the shell, and moreover it would also lead in
inefficient shell-side heat transfer
CUT:
15 % to 45 % of shell ID for Single Segmental
25 % to 35 % of shell ID for Double Segmental
42. Baffle/ Nozzle orientation
17-09-2015Design of HC Process Equipments42
The orientation of the baffle cut is important for heat exchanger
installed horizontally.
When the shell side heat transfer is sensible heating or cooling with
no phase change, the baffle cut should be horizontal.
For shell side condensation, the baffle cut for segmental baffles is
vertical.
For shell side boiling, the baffle cut may be either vertical or
horizontal depending on the service.
Positioning of inlet/ outlet nozzle is also important for the proper
functioning of exchangers.
In cooling water services, the inlet nozzle should be at the bottom
and outlet nozzle should be at the top.
For condensing services exit should be from the bottom nozzle.
46. DOUGHNUT AND DISC TYPE BAFFLES 17-09-2015Transport Phenomenon (CH 306)46
47. Baffle Spacing
17-09-2015Transport Phenomenon (CH 306)47
Baffle spacing is the longitudinal or centreline-to-
centreline distance between adjacent baffles.
According to TEMA, the minimum baffle spacing should
be one-fifth of the shell inside diameter or 2 in.,
whichever is greater.
The maximum baffle spacing is the shell inside diameter.
49. Impingement devices
17-09-2015Design of HC Process Equipments49
Impingement rod, Impingement plate, Nozzle Impingement baffle
are the various devices used in heat exchangers to trim down the
effects of high velocity at entry nozzles over tube bundle.
50. TUBE PROBLEMS
17-09-2015Transport Phenomenon (CH 306)50
• Scaling of inside/outside of the tube surface
• Blockage of tube passage
• By passing across the baffle
• Puncture in the tube
• Leakage through the tube to tubesheet
• Leakage through gasketted joint of floating head
51. Bypass & Leakage streams:
TINKER FLOW MODEL
17-09-2015Transport Phenomenon (CH 306)51
B stream: Main heat transfer stream, follows a path around baffles and
through tube bundle
A stream: Leakage stream, flowing through clearance between tubes and
holes in baffles
C stream: Tube bundle bypass stream in the gap between the tube bundle
and shell wall
E stream: Leakage stream between baffle edge and shell wall
F stream: Bypass stream in flow channel partitions due to omissions of
tubes in tube pass partitions.
52. 17-09-2015Transport Phenomenon (CH 306)52
FLOW FRACTIONS ALLOWABLE LIMITS
A Stream < 10 %
B Stream > 40 %
C Stream < 10 %
E Stream < 15 %
F Stream < 10 %
54. Bypass & Leakage Streams
17-09-2015Design of HC Process Equipments54
Since the flow fractions depend strongly upon the path resistances, varying any
of the following construction parameters will affect stream analysis and
thereby the shell side performance of an exchanger:
Baffle spacing and baffle cut
Tube layout angle and tube pitch
Clearance between the tube and the baffle hole
Clearance between the shell I.D. and the baffle
Location & no. of sealing strips and sealing rods
55. Temperature Cross (Co-current)
17-09-2015Transport Phenomenon (CH 306)55
Outlet temperature of cold stream
cannot be greater than the outlet
temperature of the hot stream.
An F shell has 2 shell passes, so if
there are 2 tube passes as well, it
represents a pure counter-current
flow
56. Air cooled heat exchanger
17-09-2015Transport Phenomenon (CH 306)56
57. Plate and Frame heat exchanger
17-09-2015Transport Phenomenon (CH 306)57
59. Aims of Thermal Design
17-09-2015Transport Phenomenon (CH 306)59
1. Achieve the specified duty at minimum overall cost
2. To achieve high heat transfer coefficient within allowable pressure
drops.
3. 10-20 % Overdesign margin (design safety)
4. Pressure drops should be in limits
5. To keep fluid velocities in limit
6. To keep shell side flow fractions in limit
61. 17-09-2015Transport Phenomenon (CH 306)61
1. Calculate the heat duty.
2. Select cooling/heating medium
3. Calculate utility flow-rate.
4. Collect the fluid physical properties : density, viscosity,
thermal conductivity.
5. Allocate the fluids on shell side and tube side.
6. Decide the exchanger type
7. Determine LMTD and MTD ΔTm
8. Select a trial value for the overall coefficient, U
9. Estimate the provisional area required.
Steps in Design
62. 17-09-2015Transport Phenomenon (CH 306)62
10. Tube geometry : Number of tubes & number of tube
passes etc.
11. Calculate the shell diameter.
12. Determine the shell side and tube side heat transfer
coefficients.
13. Calculate the overall coefficient and compare with the
trial value.
14. Find the area provided based on U value and then
calculate % excess area.
15. Calculate the shell side and tube side pressure drop.
16. Optimize the design
63. Heat Duty
17-09-2015Transport Phenomenon (CH 306)63
Heat Duty, Q (single phase)
𝑄 = 𝑚𝐶 𝑝∆𝑇(sensible heat)
here, m = flow rate of process fluid
Cp= specific heat of process fluid
∆𝑇 = temp diff for process fluid
Heat Duty, Q (phase change)
𝑄 = 𝑚λ(latent heat)
here, m = flow rate of process fluid
λ= latent heat of process fluid
64. Selection of cooling/heating medium
17-09-2015Transport Phenomenon (CH 306)64
Based on cost and availability
Cooling medium
• Cooling water (35-100
oC)
• Chilled water (< 35 oC)
• Air (> 60 oC)
• Brine (< 8 oC)
Heating medium
• Steam (100-180 oC)
• Oil (180-300 oC)
• Dowtherm oils (180-400 oC)
• Molten Salt (400-590 oC)
• Na – K alloys (500-750 oC)
• Flue gas or Hot air (750-1100 oC)
65. Utility Flow rate
17-09-2015Transport Phenomenon (CH 306)65
Utility flow rate, mw
𝑚 𝑤 =
𝑄
𝐶 𝑝𝑤∆𝑇 𝑤
Here, m = flow rate of utility
Cpw= specific heat of utility
∆𝑇 = temp diff for utility
66. LMTD & MTD Calculation
17-09-2015Transport Phenomenon (CH 306)66
Log Mean Temperature
Difference (LMTD)
71. Provisional area required
17-09-2015Transport Phenomenon (CH 306)71
Heat transfer rate, Q = 𝑈𝐴∆𝑇 𝑚
U = overall heat transfer coefficient, W/(m2C)
A = heat transfer surface area, m2
∆𝑇 𝑚 = mean temperature difference, oC
𝐴 𝑝𝑟𝑜 =
𝑄
𝑈∆𝑇 𝑚
72. Number of tubes
17-09-2015Transport Phenomenon (CH 306)72
𝐴 𝑝𝑟𝑜 = 𝑁𝑡 𝜋𝑑 𝑜 𝐿
Here, Nt= No. of tubes
do= tube o.d.
L= Tube length
Decide the number of passes & tube layout, pitch
73. Parametric study for 2, 4, 6 and 8 passes
To avoid fouling, tube velocity is kept between 1-2 m/s
At 2 and 4 passes, tube velocity is less than 1 m/s
At 8 passes, tube velocity is more than 2 m/s
Number of passes = 6
0
0.5
1
1.5
2
2.5
2 4 6 8
TubeSideVelocity,m/s
No. of Passes
Tube Passes vs Tube Side Velocity
1.35 m/s
17-09-201573 Transport Phenomenon (CH 306)
74. Parametric study for 2, 4, 6 and 8 passes
Allowable value of Tube side pressure drop is around 70 kPa
At 6 passes, optimum value of pressure drop is obtained, i.e. 45.06 kPa
Number of passes = 6
0
20
40
60
80
100
120
140
2 4 6 8
TubeSidePressuredrop,kPa
No. of Passes
Tube Passes vs Tube Side Pressure drop
45.06 kPa
Allowable pressure drop = 70 kPa
17-09-201574 Transport Phenomenon (CH 306)
75. 0
5
10
15
20
25
2 4 6 8
%Overdesign
No. of Passes
Tube Passes vs % Overdesign
16.03 %
Parametric study for 2, 4, 6 and 8 passes
% Overdesign should be between 10 to 20 %
At 6 passes, optimum value of % overdesign is obtained, i.e. 16.03 %
Number of passes = 6
17-09-201575 Transport Phenomenon (CH 306)
76. Optimization of Baffle Spacing
Parametric study for 200, 250, 300 and 350 mm spacing
At 250 mm, B-stream flow fraction is optimum i.e. 87 %
Baffle spacing = 250mm
0.86
0.862
0.864
0.866
0.868
0.87
0.872
0.874
0.876
0.878
0.88
200 250 300 350
Bstreamflowfraction
Baffle Spacing, mm
Baffle Spacing vs B stream flow fraction
0.87
17-09-201576 Transport Phenomenon (CH 306)
77. 1
1.25
1.5
1.75
2
2.25
2.5
200 250 300 350
ShellSidePressuredrop,kPa
Baffle Spacing, mm
Baffle Spacing vs Shell Side Pressure drop
Parametric study for 200, 250, 300 and 350 mm spacing
At 250 mm, , optimum value of shell side pressure drop is obtained, i.e. 1.391 kPa
Baffle spacing = 250 mm
17-09-201577 Transport Phenomenon (CH 306)
78. 10
12
14
16
18
20
22
200 250 300 350
%Overdesign
Baffle Spacing, mm
Baffle Spacing vs % Overdesign
16.03 %
Parametric study for 200, 250, 300, 350 mm spacing
% Overdesign should be between 10 to 20 %
At 250 mm, optimum value of % overdesign is obtained, i.e. 16.03 %
Baffle spacing = 250 mm
17-09-201578 Transport Phenomenon (CH 306)
79. 760
780
800
820
840
860
880
30 45 60 75 90
No.ofTubes
Tube Layout
Tube Layout vs No. of Tubes
Selection of Tube Layout
10
15
20
25
30
35
40
30 45 60 75 90
%Overdesign
Tube Layout
Tube Layout vs % Overdesign
Parametric study for 30o, 45o,60o and 90o layout
If service requires continuous cleaning lanes choose square layout only
45o gives optimum number of tubes and optimum value of % overdesign
Tube Layout = 45o Square
17-09-201579 Transport Phenomenon (CH 306)
82. Tube side coefficient
17-09-2015Transport Phenomenon (CH 306)82
Mean utility temp, t
Tube c/s area, At
𝐴 𝑡 =
𝑁𝑡
𝑁𝑝
×
𝜋
4
𝑑𝑖
2
Here, Np = No. of tube passes
di = tube i.d.
83. 17-09-2015Transport Phenomenon (CH 306)83
Sieder-Tate Equation
Re < 2000
Dittus-Bolter Equation
Re > 4000
Generalized Equation
Re = 10 to 106
For Water Service
C = 0.021 for gases,
C = 0.023 for non-viscous liquids,
C = 0.027 for viscous liquids.
86. Shell Side Coefficient
17-09-2015Transport Phenomenon (CH 306)86
Choose baffle spacing, lB and tube pitch, pt
1/5th of the shell dia or 2 in., whichever is greater
Cross-flow area
Here, lB is baffle spacing, m.
Calculate mean temperature
Decide baffle cut % (start with 25%)
95. Shell Side Pressure Drop
17-09-2015Transport Phenomenon (CH 306)95
Shell Side Pressure Drop
Here, jf = friction factor for shell side
Range of shell pressure drop is
Liquids 48 to 60 kPa
Gases 4 to 20 kPa
97. To Reduce Tube Side Pressure Drop
17-09-2015Design of HC Process Equipments97
Tube pressure drop
Decrease number of tube passes
Increase tube diameter
Decrease tube length
Increase number of tubes & hence increase shell diameter
𝑨 𝒕 =
𝑵 𝒕
𝑵 𝒑
×
𝝅
𝟒
𝒅𝒊
𝟐 Fluid velocity
𝒖 𝒕 = 𝒎 𝒘/𝑨 𝒕
98. To Reduce Shell Side Pressure Drop
17-09-2015Design of HC Process Equipments98
Shell Side Pressure Drop
Increase the baffle cut
Increase the baffle spacing
Increase tube pitch
Increase tube diameter
Decrease shell diameter
ADVANTAGES OF FIXED TUBESHEET HE:
Low cost, simple construction
Tubes can be cleaned mechanically after removal of channel cover.
Leakage of shell side fluid is minimized since there are no flange joints.
DISADVANTAGES OF FIXED TUBESHEET HE:
Outside of tubes cant be cleaned mechanically as they cant be removed.
It can only be used for clean services.
For high differential temp. expansion bellow on shell side is required.
ADVANTAGES OF U-TUBE HE:
As one end is free, the bundle can expand or contract in response to stress differentials.
The outside of the tubes can be cleaned, as tube bundle is removable.
DISADVANTAGES OF U-TUBE HE:
The inside of the tubes cant be cleaned mechanically as U-bend requires flexible end drill shafts for cleaning.
Cant be used for dirty fluids inside tubes.
Costlier than fixed tube bcoz of u-bend radius requirement.
ADVANTAGES OF FLOATING HEAD HE:
Free expansion of the tube bundle & cleaning of tubes in & out.
Closer spacing results in poor bundle penetration by shell side fluid & difficulty in mechanical cleaning of tubes.
Low baffle spacing results in a poor stream distribution.
Horizontal cut for single phase fluid minimizes accumulation of deposits at shell bottom & prevents stratification.
In cooling water services, the inlet nozzle should be at the bottom (as the water get heated up it becomes less dense and give a convective upside flow) and outlet nozzle should be at the top.
For condensing services exit should be from the bottom nozzle (as condensed liquid will come down because of gravity).
Closer spacing results in poor bundle penetration by shell side fluid & difficulty in mechanical cleaning of tubes.
Low baffle spacing results in a poor stream distribution.