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Heat Exchangers
Chapter 18
ChEN 4253
Terry A. Ring
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Flow Patterns
• Parallel Flow
• Counter Current Flow
• Shell and Tube with baffles
• Cross Flow
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Temperature Profiles
∆T = Approach Temperature
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Heat
Exchanger
Temperature
Profiles
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Flow Structure
Q=U A F ∆Tlm-counter
( )
( ) ( )( )
incoldinhot
incoldoutcold
incoldoutcold
outhotinhot
TT
TTS
TT
TTR
RRS
RRSR
RS
SR
F
−
−=
−
−=
+++−
+−+−−
−−
+=
112
112ln1
1
1ln1
2
2
2
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Overall Heat Transfer Coefficient
• Series of Resistances
• Basis
– Inside
– Outside
( ) ( )[ ] ( )1
,, 1)ln(21)1(
−
++++= ofoioowiio
i
oifo RhDDDkhDDA
ARU
Inside
Tubes
Rf=fouling factors, inside and outside
See table 18.5 for range of U values for different cases.
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Heat Transfer inside a tube
Turbulent000,10Re
700,16Pr7.0
esmooth tub,60/
PrRe027.0
14.0
3/18.0
>
<=<
>
==
f
p
w
b
f
k
C
DL
k
hDNu
µ
µµ
Also other correlations valid over wider ranges
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Heat Transfer outside of Tube
000,100Re40
300Pr67.0
2.525.0
Pr)Re06.0Re4.0(
25.0
4.03/25.0
<<
<=<
<<
+==
f
p
w
b
w
b
f
k
C
k
hDNu
µ
µµ
µµ
Also other correlations valid over wider ranges
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Thermal Conductivity
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What Temperature Approach
• Heuristic 26.
– Near-optimal minimum temperature approaches
in heat exchangers depend on the temperature
level as follows: – 10°F or less for temperatures below ambient,
– 20°F for temperatures at or above ambient up to 300°F,
– 50°F for high temperatures,
– 250 to 350°F in a furnace for flue gas temperature above
inlet process fluid temperature.
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Where are the Heat Exchangers?
What is happening in each
Octane Reaction
2C2H4 + C4H10 � C8H18
P= 20 psia, T=93C,
X=98% Conversion
TBP
C2H4 −103.7 °C
C4H10 +0.5 °C
C8H18 +125.52 °C
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Where are the Heat Exchangers?
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Heat Transfer With Phase
Change
• Tricky Problems
– Examples
• Reboiler on Distillation Unit
• Condenser on Distillation Unit
• Flash Units
• Boilers
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A Word About Steam
• Simulator Assumptions
– Inlet – Saturated Vapor– Pressure
– 100% Vapor
– Outlet – Saturated Liquid
• Liquid Only Leaves via steam trap
– Pressure = Pin- ∆P (1.5 psi, Heuristic-31)
– 100% Liquid
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Where are the Tricky
Heat Exchangers?
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Condensation Heat Transfer
• Drop Wise Condensation
– Special Case
• Very High Heat Transfer
• 5 to 10 x Film Condensation
• Film Condensation
– Laminar4/13
)(4
)(
−
∆−==
xTT
kHg
k
xhNu
wvl
lvapvll
l
xx µ
ρρρ
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Laminar to Turbulent Condensate
Flow
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Boiling Heat Transfer Coefficient
Various correlations depending upon boiling mechanism
Highest Heat
Transfer Coef.
But hard to
control HX
operating here
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Heuristic 28
• Boil a pure liquid or close-boiling liquid
mixture in a separate heat exchanger, using
a maximum overall temperature driving
force of 45 F to ensure nucleate boiling and
avoid undesirable (low h) film boiling.
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Effective
Flow
Conditions
with
Boiling in
Thermo
siphon
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Kettle (Re)Boiler Design
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Aspen - Zone AnalysisProMax – Heat Release Increments
• Heuristic 29.
– When cooling and condensing a stream in a heat exchanger, a zone
analysis, described in Section 18.1, should be made to make sure
that the temperature difference between the hot stream and the cold
stream is equal to or greater than the minimum approach
temperature at all locations in the heat exchanger. The zone
analysis is performed by dividing the heat exchanger into a number
of segments and applying an energy balance to each segment to
determine corresponding stream inlet and outlet temperatures for
the segment, taking into account any phase change. A process
simulation program conveniently accomplishes the zone analysis.
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Pressure Drop
& Flow Rate
• Laminar vs. Turbulent
– Heuristic 31.
• Estimate heat-exchanger pressure drops as follows:
– 1.5 psi for boiling and condensing,
– 3 psi for a gas,
– 5 psi for a low-viscosity liquid,
– 7-9 psi for a high-viscosity liquid,
– 20 psi for a process fluid passing through a furnace.
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Controlling ∆P in Simulator
• Shell side– Nozzle diameter
• Inlet and Outlet
– Number of Baffles
– Tubes• Number, diameter, pitch, No. passes
• Tube side– Nozzle diameter
• Inlet and Outlet
– Tubes• Number, diameter, pitch, No. passes
Note interactions!
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Shell Heads, Shell Type
• See ProMax Help/index “Shell, types”
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HX Cost
• Size Factor HX Area
• CBase(6-2000)=exp[11.0545-0.9228*ln(A)+0.09861*ln(A)2]
• Purchase Price– CP-fob=FP(P)*FMaterial(A)*FL(L)*CBase*(CPI/394)
– CBM=FBM*CP-fob
– CBM=3.17*CP-fob
• Cost depends on HX Area
• Pumping Cost– Work = Q*∆P
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Controlling A in Simulator
• A = Ntubes π Dtubes Ltubes
• Shell
– Shell Diameter and pitch determines Ntubes
• Tubes
– Dtubes
– Ltubes
– Tube pitch-The transverse pitch is the shortest distance from the center lines of two adjacent tubes.
– Tube pitch ratio 1.25 to 1.5 typically
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Controlling U in a Simulator
• For a given heat duty and geometry - U determines the HX area
• Steps– Identify the controlling heat transfer resistance– ho-Manipulate the shell side Reynolds number
• Shell diameter
• Tube pitch
• Number of baffles
– hi-Manipulate the tube side Reynolds number• Tube diameter
• Number of tubes (shell diameter and tube pitch)
• Number of passes
– If odd things happen check to see that you have the same controlling heat transfer resistance
Note interactions!
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Other Issues
• Materials of Construction
– Strength at temperature, life time, heat conduction,
fouling
• Design layout
– Tube pitch, baffles, tube and shell diameters
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Heat Exchanger
Problems
• Temperatures Cross Each Other
– Non-functioning Exchanger
– To solve increase approach ∆T
• Condensation/Evaporation
– Heat transfer with multiple heat transfer coefficients in a single apparatus
• Various regimes of boiling
• Various regimes of condensation