MER 439 - Design of Thermal Fluid Systems
Heat Exchanger Analysis
Professor Anderson
Spring Term 2012
1
Outline
(1) Heat Exchanger Types
(2) Heat Exchanger Analysis
Methods
Overall
Heat Transfer Coefficient
 fouling, enhanced surfaces
LMTD Method
 Effectiveness-NTU Method
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HX Classifications
3
HX Classifications
4
Heat Exchanger Types
Concentric tube (double piped)
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Heat Exchanger Types
Concentric tube (double piped)


One pipe is placed concentrically within
the diameter of a larger pipe
Parallel flow versus counter flow
Fluid B
Fluid A
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Heat Exchanger Types
Shell and Tube
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Heat Exchanger
Types
Compact Heat
Exchangers
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Heat Exchanger Types
Cross Flow
 finned
versus unfinned
 mixed versus unmixed
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Heat Exchanger Types
10
Heat Exchanger Types
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Heat Exchanger Analysis



Overall Heat Transfer Coefficient
LMTD
Effectiveness-NTU
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Overall Heat Transfer Coefficient
The overall coefficient is used to analyze heat exchangers. It contains the effect of hot and cold side
convection, conduction as well as fouling and fins.
Rf ,c
Rf ,h
1
1
1


 Rw 

UA (o hA)c (o A)c
(o A) h (o hA) h
Rf  foulingfactor
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Enhanced Surfaces
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Log-Mean Temperature Difference
To relate the total heat transfer rate to inlet and
outlet fluid temperatures. Apply energy balance:
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Log-Mean Temperature Difference
We can also relate the total heat transfer rate to the
temperature difference between the hot and cold
fluids.
let T  Th  Tc
Q  UATLM
.
16
The log mean temperature difference
depends on the heat exchanger
configuration
Th,in
Th,in
Th,out
Tc,out
Tc,in
Th,out
Tc,out
Tc,in
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LMTD Parallel-Flow HX
Q  UATLM
TLM
T2  T1

ln(T2 / T1)
Wherefor P arallelFlow :
T1  Th,1  Tc,1  Th,i  Tc,i
T2  Th, 2  Tc,2  Th,o  Tc,o
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LMTD Counter-Flow HX
Q  UATLM
T2  T1
TLM 
ln(T2 / T1)
Wherefor CounterFlow :
T1  Th,1  Tc,1  Th, i  Tc, o
T2  Th,2  Tc,2  Th, o  Tc, i
Tlm,CF > Tlm,PF FOR SAME U: ACF < APF
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LMTD- Multi-Pass and Cross-Flow
Apply a correction factor to obtain LMTD
Q  UATLM
TLM  FTLM ,CF
t: Tube Side
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LMTD Method
Sizing a Heat Exchanger:




Calculate Q and the unknown outlet
temperature.
Calculate DTlm and obtain the correction
factor (F) if necessary
Calculate the overall heat transfer
coefficient.
Determine A.
The LMTD method is not as easy to use for
performance analysis….
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The Effectiveness-NTU Method

Define Qmax
for Cc < Ch
for Ch < Cc
or
Qmax = Cc(Th,i - Tc,i)
Qmax = Ch(Th,i - Tc,i)
Qmax = Cmin(Th,i - Tc,i)
e
q
qmax

Ch (Th,i  Th,o )
Cmin (Th,i  Tc,i )

Cc (Tc,o  Tc,i )
Cmin (Th,i  Tc,i )
Q = eCmin(Th,i - Tc,i)
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The Effectiveness-NTU Method

For any heat exchanger:
e  f(NTU,Cmin/Cmax)

NTU (number of transfer units)
designates the nondimensional heat
transfer size of the heat exchanger:
UA
NTU 
Cmin
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The Effectiveness-NTU Method
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The Effectiveness-NTU Method
PERFORMANCE ANALYSIS




Calculate the capacity ratio Cr = Cmin/Cmax and
NTU = UA/Cmin from input data
Determine the effectiveness from the
appropriate charts or e-NTU equations for the
given heat exchanger and specified flow
arrangement.
When e is known, calculate the total heat
transfer rate
Calculate the outlet temperature.
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The Effectiveness-NTU Method
SIZING ANALYSIS

When the outlet and inlet temperatures are
known, calculate e.

Calculate the capacity ratio Cr = Cmin/Cmax

Calculate the overall heat transfer coefficient, U


When e and C and the flow arrangement are
known, determine NTU from the e-NTU
equations.
When NTU is known, calculate the total heat
transfer surface area.
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The Homework
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