Heat Exchanger

.
Introduction to
HEAT EXCHANGERS
Joy Mukherjee

What is a Heat Exchanger?
A heat exchanger is a device which transfers heat from one medium to another.

Classification of
Heat Exchangers

Types of HE
q Double-pipe exchanger
q Shell and tube exchangers
q Plate and frame exchangers
q Plate-fin exchangers.
q Spiral heat exchangers.
q Air cooled heat exchangers
q Agitated vessels.
q Fired heaters.

Based on transfer process
qIndirect Contact – Shell & Tube Heat Exchangers
qDirect Contact – Cooling Towers
qGas-Liquid exchangers
qLiquid-Liquid exchangers
qGas-Gas heat exchangers
Based on phase of fluids

Based on construction
q Tubular
§ Double pipe heat exchanger
§ Shell and tube heat exchangers
§ Spiral heat exchangers
q Plate-type
§ Plate and frame heat exchangers
§ Spiral plate heat exchangers
q Extended Surface
§ Plate-fin exchanger
§ Tube-fin exchanger

Based on flow arrangements
qParallel flow / Co-current flow
qCounter flow
qCross flow

Heat Transfer Coefficient
q Heat transfer rate, = ∆
U = overall heat transfer coefficient, W/(m2C)
A = heat transfer surface area, m2
∆ = mean temperature difference, oC
q 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)
q Mean temperature difference, ∆
∆ = ∆
qFt depends on R & S
=
−
−
=
−
−

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

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

Classification by Construction
Fixed-tube-sheet heat exchanger
qHas straight tubes secured at both ends to tube-sheets
welded to the shell
qLow cost, simplest construction.
qBundle is "fixed" to the shell so outside of the tubes
cannot be cleaned mechanically.
qApplication is limited to clean services on the shell side

Fixed-tube-sheet heat exchanger

U-tube heat exchanger
q Tubes are bent in the shape of a U
q Only one tube-sheet
q Bending of tubes adds to the cost
q Tube bundle is removable, outside of tubes can be cleaned.
q Because of the U-bend, inside of the tubes can’t be cleaned
mechanically
q Can’t be used for dirty fluids inside tubes.

Floating head exchanger
qMost versatile and costliest.
qOne tube-sheet is fixed relative to the shell, and the other
is free to “float” within the shell.
qCleaning of both the insides and outsides of the tubes
qCan be used for services where both the shell-side and
the tube-side fluids are dirty
Widely used in Petroleum Industry

Straight-Tube ( 2-Pass )
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4

Transportation of Heat Exchanger

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 Tube Side
— Viscous Fluids
— Lower Flow Rates
— Cleaner Fluids
— 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
Ø TUBE PITCH RATIO:
Min 1.25 times of tube OD
1.333 times of tube OD
1.5 times of tube OD
Ø 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
qShortest distance between two adjacent tubes.
qTEMA specifies a minimum tube pitch of 1.25*(OD)
qMinimum tube pitch leads to smallest shell diameter for a
given number of tubes.
qTo reduce shell-side pressure drop, the tube pitch may be
increased to a higher value.

Tube-sheet
q Barrier between shell-side and tube-side fluids.
q Mostly circular with uniform pattern of drilled holes.
q Tubes are attached to tube-sheet

Tie rods and spacers
Tie rods and spacers are used for:
q holding the baffle assembly together
q maintaining the selected baffle spacing
qhelp the bundle to slide out from the shell
qCan also be used as tie rods to hold the bundle in
position.
Sliding strips

Sealing strips and Seal rods
qSealing strips prevent shell side fluid from bypassing the
bundle.
qSealing strips block the resulting large open area at top or
bottom of the shell.
qSeal rods are also used to control the leakage streams.

TUBE PASS LAYOUT
Ribbon
Quadrant
H-Bend

TYPES OF BAFFLES
qSegmental type;
§ Single – horizontal & vertical
§ Double
§ Triple
§ No-Tubes in Window (NTIW)
qOrifice type
qDisc and doughnut type
qRod type
qImpingement type
qLongitudinal (pass partitions)

Ø ORIENTATION:
— Horizontal for heating or cooling with no phase change
— Vertical for shell side condensation
Ø CUT:
— 15 % to 45 % of shell ID for Single Segmental
— 25 % to 35 % of shell ID for Double Segmental
Baffle cut

Baffle cut
q Height of the segment that is cut in a baffle to permit the shell-
side fluid to flow across the baffle.
q 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
Ø CUT:
— 15 % to 45 % of shell ID for Single Segmental
— 25 % to 35 % of shell ID for Double Segmental

Flow Pattern depending on Baffle

Baffle/ Nozzle orientation
q The orientation of the baffle cut is important for heat exchanger
installed horizontally.
Ø When the shell side heat transfer is sensible heating or cooling with
no phase change, the baffle cut should be horizontal.
Ø For shell side condensation, the baffle cut for segmental baffles is
vertical.
Ø For shell side boiling, the baffle cut may be either vertical or
horizontal depending on the service.
q Positioning of inlet/ outlet nozzle is also important for the proper
functioning of exchangers.
Ø In cooling water services, the inlet nozzle should be at the bottom
and outlet nozzle should be at the top.
Ø 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

Shell Outlet
Channel Inlet
Channel
Outlet
Shell Outlet
SINGLE SEGMENTAL BAFFLES - Vertical

Shell Outlet
Shell Inlet
DOUBLE SEGMENTAL TRANSVERSE BAFFLES

DOUGHNUT AND DISC TYPE BAFFLES

Baffle Spacing
qBaffle spacing is the longitudinal or centreline-to-
centreline distance between adjacent baffles.
qAccording to TEMA, the minimum baffle spacing should
be one-fifth of the shell inside diameter or 2 in.,
whichever is greater.
qThe maximum baffle spacing is the shell inside diameter.

Impingement devices
q 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 tube-sheet
• Leakage through gasketted joint of floating head

Bypass & Leakage streams:
TINKER FLOW MODEL
Ø B stream: Main heat transfer stream, follows a path around baffles and
through tube bundle
Ø A stream: Leakage stream, flowing through clearance between tubes and
holes in baffles
Ø C stream: Tube bundle bypass stream in the gap between the tube bundle
and shell wall
Ø E stream: Leakage stream between baffle edge and shell wall
Ø 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 %
Allowable limits of leakage streams

Leakage prevention-Sealing Strip
Tubular-type heat exchangers contain both
baffle plates and sealing strips. The baffles
force the flow to pass normal to the axes of the
tubes and they serve to support the tubes. In
order to facilitate assembly of the heat
exchanger, a space exists between the tube
bundle and the retaining shell. This space
offers a hydraulic short circuit to the fluid,
thus reducing the effectiveness of the device to
exchange heat. Sealing strips, which are
metal strips mounted on the shell and running
parallel to the tubes, are introduced to
partially block this leakage flow, thereby
increasing the effectiveness of the device.

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:
Ø Baffle spacing and baffle cut
Ø Tube layout angle and tube pitch
Ø Clearance between the tube and the baffle hole
Ø Clearance between the shell I.D. and the baffle
Ø Location & no. of sealing strips and sealing rods

Temperature Cross (Co-current)
qOutlet temperature of cold stream
cannot be greater than the outlet
temperature of the hot stream.
qAn F shell has 2 shell passes, so if
there are 2 tube passes as well, it
represents a pure counter-current
flow

Plate and Frame heat exchanger

Spiral heat exchanger
Ideal for handling fouling fluids, viscous media,
slurries, sewage sludge, and other dirty process fluids
and effluents with suspended solids or fibres.

Examples of Fouling
Fouling is the accumulation of unwanted material on solid surfaces
to the detriment of function. The fouling materials can consist of
either living organisms (bio-fouling) or a non-living substance
(inorganic or organic). reduces thermal efficiency, decreases heat
flux, increases temperature on the hot side, decreases temperature
on the cold side, induces under-deposit corrosion, increases use of
cooling water;
Piping, flow channels – reduces flow, increases pressure drop,
increases upstream pressure, increases energy expenditure, may
cause flow oscillations, slugging in two-phase flow, cavitation; may
increase flow velocity elsewhere, may induce vibrations, may cause
flow blockage.

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

Steps in Design
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.

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
qHeat Duty, Q (single phase)
= ∆ (sensible heat)
here, m = flow rate of process fluid
Cp= specific heat of process fluid
∆ = temp diff for process fluid
qHeat Duty, Q (phase change)
= λ(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
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Transport Phenomenon (CH 306)6
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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
qUtility flow rate, mw
=
∆
Here, m = flow rate of utility
Cpw= specific heat of utility
∆ = temp diff for utility

LMTD & MTD Calculation
Difference (LMTD)

LMTD & MTD Calculation
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Difference (LMTD)

Provisional area required
qHeat transfer rate, Q = ∆
U = overall heat transfer coefficient, W/(m2C)
A = heat transfer surface area, m2
∆ = mean temperature difference, oC
=
∆

Number of tubes
=
Here, Nt= No. of tubes
do= tube o.d.
L= Tube length
qDecide the number of passes & tube layout, pitch

§ 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

§ 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 v/s % 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
§ 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 v/s Shell Side Pressure drop
§ 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 %
§ % 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
qBundle diameter, Db

Tube side coefficient
qMean utility temp, t
qTube c/s area, At
= ×
4
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
= /

jh = tube side heat transfer factor

Shell Side Coefficient
qChoose baffle spacing, lB and tube pitch, pt
q1/5th of the shell dia or 2 in., whichever is greater
qCross-flow area
Here, lB is baffle spacing, m.
qCalculate mean temperature
qDecide baffle cut % (start with 25%)

Ws = shell side flow-rate, kg/s

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

jh = shell side heat transfer factor

Overall Coefficient
qOverall 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
qTube pressure drop
qHere, jf = friction factor for tube side
qRange of tube pressure drop is
qGases = 14 kPa
qLiquids = 30 to 70 kPa
= ×
Fluid velocity
= /

jf = friction factor Tube side

Shell Side Pressure Drop
qShell Side Pressure Drop
qHere, jf = friction factor for shell side
qRange of shell pressure drop is
qLiquids 48 to 60 kPa
qGases 4 to 20 kPa

jf = friction factor shell side

To Reduce Tube Side Pressure Drop
qTube pressure drop
ØDecrease number of tube passes
ØIncrease tube diameter
ØDecrease tube length
ØIncrease number of tubes & hence increase shell diameter
= ×
Fluid velocity
= /

To Reduce Shell Side Pressure Drop
qShell Side Pressure Drop
ØIncrease the baffle cut
ØIncrease the baffle spacing
ØIncrease tube pitch
ØIncrease tube diameter
ØDecrease shell diameter

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