Heat exchangers

Introduction to
HEAT EXCHANGERS
Transport Phenomenon (CH 306)

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

Classification of
Heat Exchangers

Types of HE
 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.

Based on transfer process
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

Based on construction
 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

Based on flow arrangements
Parallel flow / Co-current flow
Counter flow
Cross flow

Heat Transfer Coefficient
 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

Temperature Profile: Co-current flow
Log Mean Temperature
Difference (LMTD)

Temperature Profile: Counter current flow
Difference (LMTD)

Temperature Correction Factor (Ft)
 Mean temperature difference, ∆𝑇 𝑚
∆𝑇 𝑚 = 𝐹𝑡∆𝑇𝑙𝑚
Ft depends on R & S
𝑅 =
𝑇ℎ𝑖 − 𝑇ℎ𝑜
𝑇𝑐𝑜 − 𝑇𝑐𝑖
𝑆 =
𝑇ℎ𝑖 − 𝑇𝑐𝑖

Temperature correction factor: one shell pass; two or more even tube passes

Components of a STHE
 Shell
 Shell cover
 Tubes
 Channel
 Channel Cover
 Tubesheet
 Baffles
 Floating-Head Cover
 Nozzles
 Tie-Rods & Spacers
 Pass Partition Plates
 Impingement Plates
 Sealing Strips & Sealing Rods

Classification by Construction
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

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.

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

U-Tube Heat Exchanger

Straight-Tube ( 1-Pass )

Straight-Tube ( 2-Pass )

TEMA Types

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

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

Tubes
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

TUBE LAYOUT ANGLE
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

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

Tube pitch
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.

Tubesheet
 Barrier between shell-side and tube-side fluids.
 Mostly circular with uniform pattern of drilled holes.
 Tubes are attached to tubesheet

Tie rods and spacers
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

Sealing strips and Seal rods
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.

TUBE PASS LAYOUT
Ribbon
Quadrant
H-Bend

TYPES OF BAFFLES
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)

 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

Baffle cut
 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

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.

Shellside Flow
Out
Tubeside Flow
In
Tubeside Flow
Out
Shell
Tube Bundle
Shellside Flow
In
SINGLE SEGMENTAL BAFFLES - Horizontal 17-09-2015Transport Phenomenon (CH 306)43

Shell Outlet
Channel Inlet
Channel
Outlet
Shell Outlet
SINGLE SEGMENTAL BAFFLES - Vertical 17-09-2015Transport Phenomenon (CH 306)44

Shell Outlet
Shell Inlet
DOUBLE SEGMENTAL TRANSVERSE BAFFLES

DOUGHNUT AND DISC TYPE BAFFLES 17-09-2015Transport Phenomenon (CH 306)46

Baffle Spacing
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.

Impingement devices
 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.

TUBE PROBLEMS
• 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

Bypass & Leakage streams:
TINKER FLOW MODEL
 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.

FLOW FRACTIONS ALLOWABLE LIMITS
A Stream < 10 %
B Stream > 40 %
C Stream < 10 %
E Stream < 15 %
F Stream < 10 %

Bypass & Leakage Streams
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

Temperature Cross (Co-current)
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

Air cooled heat exchanger

Plate and Frame heat exchanger

Spiral heat exchanger

Aims of Thermal Design
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

THERMAL DESIGN
Shell & Tube Heat Exchangers

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

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

Heat Duty
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

Selection of cooling/heating medium
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)

Utility Flow rate
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

LMTD & MTD Calculation
Difference (LMTD)

Temperature Correction Factor (Ft)
 Mean temperature difference, ∆𝑇 𝑚
∆𝑇 𝑚 = 𝐹𝑡∆𝑇𝑙𝑚
Ft depends on R & S
𝑅 =
𝑇ℎ𝑖 − 𝑇ℎ𝑜
𝑆 =
𝑇ℎ𝑖 − 𝑇𝑐𝑖

Temperature correction factor: one shell pass; two or more even tube passes

Overall
Coefficient

Provisional area required
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

Number of tubes
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

 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)

 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
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

0
5
10
15
20
25
2 4 6 8
%Overdesign
No. of Passes
Tube Passes vs % Overdesign
16.03 %
 % Overdesign should be between 10 to 20 %
 At 6 passes, optimum value of % overdesign is obtained, i.e. 16.03 %

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

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

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

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

Shell Diameter
Bundle diameter, Db

𝑫 𝒔 = 𝑫 𝒃 + 𝑪𝒍𝒆𝒂𝒓𝒂𝒏𝒄𝒆
Shell Diameter, Ds

Tube side coefficient
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.

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.

Here, de = Equivalent dia
Fluid velocity
𝒖 𝒕 = 𝒎 𝒘/𝑨 𝒕

jh = tube side heat transfer factor

Shell Side Coefficient
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%)

Ws = shell side flow-rate, kg/s

Shell side heat transfer coefficient, hs
jh = heat transfer factor

jh = shell side heat transfer factor

Overall Coefficient
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

Dirt factor / Dirt coefficient /Fouling factor

Tube Side Pressure Drop
Tube pressure drop
Here, jf = friction factor for tube side
Range of tube pressure drop is
Gases = 14 kPa
Liquids = 30 to 70 kPa
𝑨 𝒕 =
𝑵 𝒕
𝑵 𝒑
×
𝝅
𝟒
𝒅𝒊
𝟐 Fluid velocity

jf = friction factor Tube side

Shell Side Pressure Drop
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

jf = friction factor shell side

To Reduce Tube Side Pressure Drop
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

To Reduce Shell Side Pressure Drop
Shell Side Pressure Drop
Increase the baffle cut
Increase the baffle spacing
Increase tube pitch
Increase tube diameter
Decrease shell diameter

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