Shell and tube heat exchanger equipment use in chemical plant

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Shell-and-Tube Heat
Exchangers

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What’s a shell and tube heat exchanger?
• Consist of two main
things as it’s name
implies Shell & Tubes
• The shell is a large
vassel with a number of
tubes inside it .
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Common types of shell and
heat exchanger

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There are many types of shell-tube
heat exchanger but the most
common types in use are :-
• U-Tube Heat Exchanger
• Straight-Tube ( 1-Pass )
• Straight-Tube ( 2-Pass )
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U-Tube Heat Exchanger
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U-Tube
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Simple Shell & Tube Heat Exchanger

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Straight-Tube ( 2-Pass )
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How dose it work ?
• The principle of operation is simple enough:
Two fluids of different temperatures are
brought into close contact but they are not
mixing with each other.
• One fluid runs through the tubes, and another
fluid flows over the tubes (through the shell)
to transfer heat between the two fluids.
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Construction
• Bundle of tubes in large cylindrical shell
• Baffles used both to support the tubes and to
direct into multiple cross flow
• Gaps or clearances must be left between the
baffle and the shell and between the tubes and
the baffle to enable assembly Shell
Tubes
Baffle
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Inner Details of S&T HX

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Tube Sheet lay Out
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Tube layouts
• Typically, 1 in tubes on a 1.25 in pitch or 0.75 in
tubes on a 1 in pitch
• Triangular layouts give more tubes in a given shell
• Square layouts give cleaning lanes with close pitch
pitch
Triangular
30o
Rotated
triangula
r
60o
Squar
e
90o
Rotate
d
square
45o
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Components of STHEs
• It is essential for the designer to have a good working
knowledge of the mechanical features of STHEs and how they
influence thermal design.
• The principal components of an STHE are:
• shell; shell cover;
• tubes; tubesheet;
• baffles; and nozzles.
• Other components include tie-rods and spacers, pass partition
plates, impingement plate, longitudinal baffle, sealing strips,
supports, and foundation.

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U-Tube STHE

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Fixed tube sheet

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Types of Shells

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Shell-side flow
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Types of Baffle Plates : Segmental Cut Baffles
• The single and double segmental baffles are most frequently
used.
•They divert the flow most effectively across the tubes.
•The baffle spacing must be chosen with care.
•Optimal baffle spacing is somewhere between 40% - 60% of the
shell diameter.
•Baffle cut of 25%-35% is usually recommended.

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Types of Baffle Plates
Disc and ring baffles are composed of alternating outer rings and
inner discs, which direct the flow radially across the tube field.
 The potential bundle-to-shell bypass stream is eliminated
 This baffle type is very effective in pressure drop to heat transfer
conversion
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Cross Baffles
• Baffles serve two purposes:
• Divert (direct) the flow across the bundle to obtain a higher heat
transfer coefficient.
• Support the tubes for structural rigidity, preventing tube vibration
and sagging.
• When the tube bundle employs baffles,
• the heat transfer coefficient is higher than the coefficient for
undisturbed flow around tubes without baffles.
• For a baffled heat exchanger the higher heat transfer coefficients
result from the increased turbulence.
• the velocity of fluid fluctuates because of the constricted area
between adjacent tubes across the bundle.

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Why shell-and-tube?
S&T accounted for 85% of new exchangers supplied to
oil-refining, chemical, petrochemical and power
companies in leading European countries. Why?
• Can be designed for almost any duty with a very wide range of
temperatures and pressures
• Can be built in many materials
• Many suppliers
• Repair can be by non-specialists
• Design methods and mechanical codes have been established
from many years of experience
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Scope of shell-and-tube
• Maximum pressure
– Shell 300 bar (4500 psia)
– Tube 1400 bar (20000 psia)
• Temperature range
– Maximum 600oC (1100oF) or even 650oC
– Minimum -100oC (-150oF)
• Fluids
– Subject to materials
– Available in a wide range of materials
• Size per unit 100 - 10000 ft2 (10 - 1000 m2)
Can be extended with special designs/materials
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Fluid Allocation : Tube Side
Tube side is preferred under these circumstances:
• Fluids which are prone to foul
• The higher velocities will reduce buildup
• Mechanical cleaning is also much more practical for tubes than for shells.
• Corrosive fluids are usually best in tubes
• Tubes are cheaper to fabricate from exotic materials
• This is also true for very high temperature fluids requiring alloy construction
• Toxic fluids to increase containment
• Streams with low flow rates to obtain increased velocities and turbulence
• High pressure streams since tubes are less expensive to build strong.
• Streams with a low allowable pressure drop

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Fluid Allocation : Shell Side
Shell side is preferred under these circumstances:
• Viscous fluids go on the shell side, since this will usually
improve the rate of heat transfer.
• On the other hand, placing them on the tube side will usually
lead to lower pressure drops. Judgment is needed.
• Low heat transfer coefficient:
• Stream which has an inherently low heat transfer coefficient
(such as low pressure gases or viscous liquids), this stream is
preferentially put on the shell-side so that extended surface may
be used to reduce the total cost of the heat exchanger.

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Heat Exchanger Calculations
• Measured Variables
– Condensate flow/hot fluid flow rate
– Condensate temperature/hot fluid inlet & outlet
temp
– Cooling water flow
– Cooling water inlet temperature
– Cooling water outlet temperature
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Simplified Process Flow Diagram
Thi
Tho
Tci Tco
Qout, TS
Qin, TS
Qin, SS
Qout, SS
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Heat Exchanger Calculations
• Heat transfer rate
 QTS = mCpDT
 QSS = mDH + mCpDT
• Overall heat transfer coefficient
 QSS = Uo(Ao*DTLM)
 Log mean temperature
 DTLM = ((Thi-Tco) – (Tho – Tci)) / ln[(Thi – Tco) – (Tho – Tci)]
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LMTD Correction Factor
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Example 1
Debutaniser overhead condenser
Hot side Cold side
Fluid Light hydrocarbon Cooling water
Corrosive No No
Pressure(bar) 4.9 5.0
Temp. In/Out (oC) 46 / 42 20 / 30
Vap. fract. In/Out 1 / 0 0 / 0
Fouling res. (m2K/W) 0.00009 0.00018
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Example 2
Crude tank outlet heater
Cold side Hot side
Fluid Crude oil Steam
Corrosive No No
Pressure(bar) 2.0 10
Temp. In/Out (oC) 10 / 75 180 / 180
Vap. fract. In/Out 0 / 0 1 / 0
Fouling res. (m2K/W) 0.0005 0.0001
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Rule of thumb on costing
• Price increases strongly with shell diameter/number of
tubes because of shell thickness and tube/tube-sheet
fixing
• Price increases little with tube length
• Hence, long thin exchangers are usually best
• Consider two exchangers with the same area: fixed
tubesheet, 30 bar both side, carbon steel, area 6060 ft2
(564 m2), 3/4 in (19 mm) tubes
Length Diameter Tubes Cost
10ft 60 in 3139 $112k (£70k)
60ft 25 in 523 $54k (£34k)
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Typical maximum exchanger sizes
Floating Head Fixed head & U tube
Diameter 60 in (1524 mm) 80 in (2000 mm)
Length 30 ft (9 m) 40 ft (12 m)
Area 13 650 ft2 (1270 m2) 46 400 ft2 (4310 m2)
Note that, to remove bundle, you need to allow at least
as much length as the length of the bundle
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Problems of Conventional S & T
Zigzag path on shell side leads to
• Poor use of shell-side pressure drop
• Possible vibration from cross flow
• Dead spots
– Poor heat transfer
– Allows fouling
• Recirculation zones
– Poor thermal effectiveness, 
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Fouling
Shell and tubes can handle fouling but it can be reduced by
• keeping velocities sufficiently high to avoid deposits
• avoiding stagnant regions where dirt will collect
• avoiding hot spots where coking or scaling might occur
• avoiding cold spots where liquids might freeze or where
corrosive products may condense for gases
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