Heat exchangers are devices that transfer thermal energy between two or more fluids at different temperatures. There are several types of heat exchangers classified by design and flow arrangement. The document discusses types including shell and tube, concentric tube, and compact heat exchangers. It also covers key heat exchanger concepts such as the overall heat transfer coefficient, log mean temperature difference, fouling factors, and the differences between parallel and counterflow heat exchangers.

1. HEAT EXCHANGERS
BY Sharwan Kumar Sharma

2. Syllabus
• Heat exchanger:
• Types of heat exchangers,
• arithmetic and logarithmic mean temperature differences
• heat transfer coefficient for parallel, counter and cross flow
type heat exchanger;
• effectiveness of heat exchanger
• N.T.U. method,
• fouling factor.
• Constructional and manufacturing aspects of Heat
Exchangers.

3. Heat Exchanger
• a heat exchanger is a device which transfers heat from one medium to another or
A heat exchanger is a device that is used to transfer thermal energy (enthalpy)
between two or more fluids, between a solid surface and a fluid, or between solid
particulates and a fluid, at different temperatures and in thermal contact.
 a Hydraulic Oil Cooler for example will remove heat from hot oil by using cold water or air.
 Alternatively a Swimming Pool Heat Exchanger uses hot water from a boiler or solar heated water
circuit to heat the pool water.
• In heat exchangers, there are usually no external heat and work interactions.
Typical applications involve heating or cooling of a fluid stream of concern and
evaporation or condensation of single- or multicomponent fluid streams
• Heat is transferred by conduction through the exchanger materials which
separate the mediums being used.
 A shell and tube heat exchanger passes fluids through and over tubes, where as an air cooled heat
exchanger passes cool air through a core of fins to cool a liquid.

4. Classification of Heat exchangers
• On the basis of operating Principle or nature of heat exchange process
 Direct Contact
 Regenerators
 Recuperators
• On the basis of Arrangement of flow path or direction of flow of fluids
 Co-current or parallel flow
 Counter- current or counter flow
 Cross flow
• On the basis of the Design
 Concentric tubes
 Shell and tube
 Multiple shell and tube passes
• Physical state of the heat exchanging fluids
 Condenser
 Evaporator

5. Continued………..

6. Classification Continued…….
Direct contact or open heat exchangers
 The energy transfer between the hot and cold fluid is brought about by their complete physical mixing.
 There is simultaneous transfer of heat and mass.
 Use of this units restricted to the situations where mixing between the fluids is either harmless or desirable.
 Examples are water cooling towers and jet condensers in steam power plants
Regenerator
 Hot fluid is passed through a certain medium called matrix.
 the heat is transferred to solid matrix and accumulate there, this operation called heating period.
 The heat stored in the matrix transferred to the cold fluid by allowing it to pass over the heated matrix
 The operation of the regenerator is intermittent.
 Examples are regenerator of open hearth and glass meting furnaces, air heaters of blast furnaces.
Recuperators
 The fluid flow simultaneously on either side of the separating wall.
 The heat transfer occurs between fluid streams without mixing or physical contact with each other.
 The heat transfer occurs through convection between hot fluid and the wall, conduction through the wall,
convection between wall and cold fluid
 Examples are boilers, superheaters, condensers, economisers and air pre heaters in steam power plants
automobile radiators, condenser and evaporators in refrigeration units.

7. Classification Continued…….
• Co-current or parallel flow
− fluid enter the unit from the same side and flow in the same
direction and subsequently leave from the same side.
− Flow is unidirectional
− Parallel to each other
• Counter current or counter flow
− fluid enter the unit from the opposite ends and flow in the
opposite direction and subsequently leave from the opposite
ends.
− Flow is opposite in direction
• Counter current or counter flow
− Two fluids are directed at right angle to each other
Flow is opposite in direction

8. Classification Continued…….
• Concentric tubes
 Two concentric pipes are used
 Each carrying one of the fluids
 The direction of the flow may correspond to
unidirectional or counter flow arrangement
• Shell and tube
 In this type of heat exchanger, one of the fluid flows through
 A bundle of tubes enclosed in shell.
 The other fluid is forced through the shell and flows over the
outside surface of the tubes.
 The direction of the flow for either or both fluids may change
during its passage through the heat exchanger
• Multiple shell and tube passes
 The two fluids may flow through the exchanger only
once, one or both fluids may traverse the exchanger
 more than once

9. Log Mean Temperature Difference
Log Mean Temperature Difference
If Th or Tc varies with position in the heat exchanger, the use of the relation
q = UAΔT
may Not be appropriate. In this case, it is necessary to work with a rate equation of the form:
q = UAΔTm
where ΔTm is an appropriate mean temperature difference.
The Parallel – Flow Heat Exchanger
The temperature difference ΔT is initially large but decays
rapidly with increasing x, approaching zero asymptotically.
Th,i = Th,l, Th,0 = Th,2, Tc,i =Tc,1, and Tc,0 = Tc,2

10. Log Mean Temperature Difference
Applying an energy balance to each of the differential elements of Figure 11.7, it follows that
mhcp,hTh = dq + mhcp,h (Th + dTh) or dq = -mhcp,hdTh = -ChdTh
Similarly, dq = mccp,cdTc = CcdTc
where Ch and Cc are the hot and cold fluid heat capacity rates, respectively.
The heat transfer across the surface area dA may also be expressed as:
dq = UΔTdA
where ΔT = Th – Tc is the local temperature difference between the hot and cold fluids. The differential form of
the equation is d(ΔT) = dTh - dTc
(11.10)
(11.11)
Substituting Eqs. (11.10) and (11.11) into it to obtain
dA
C
C
U
T
T
d
or
C
C
TdA
U
C
C
dq
T
d
c
h
c
h
c
h
)
1
1
(
)
(
),
1
1
(
)
1
1
(
)
(














11. Chapter 11: Heat Exchangers
• Heat Exchange between two fluids that are at different temperatures and
separated by a solid wall.
•Specific applications include: space heating and air-conditioning, power
production, waste heat recovery, and chemical processing.
Heat Exchanger Types
• Concentric tube heat exchangers (the simplest heat exchanger, parallel or
counter-flow arrangement, Fig. 11.1).
• Cross-flow heat exchangers (Fig. 11.2).
• Shell-and-tube heat exchanger with one shell pass and one tube pass, cross-
counterblow mode of operation, Fig. 11.3).
• Shell-and-tube heat exchanger (Fig. 11-4). (a) One shell pass and two tube
passes, (b) Two shell passes and four tube passes.
• Compact heat exchanger cores (Fig. 11.5).

12. Heat Exchanger Types

13. Heat Exchanger Types

14. Heat Exchanger Types

15. THE OVERALL HEAT TRANSFER COEFFICIENT
hot fluid
cold fluid
δ
Ah
Ac
wall
q
T∞,h, hh
T∞,c, hc
T∞,h
Ts,h Ts,c T∞,c
Rw
or
A
kw
  
L
k
r
r
w

2
ln 1
2
tot
c
w
h
c
h
R
T
hA
R
hA
T
T
q





 

)
(
1
)
(
1
,
,

16. THE OVERALL HEAT TRANSFER COEFFICIENT
An overall heat transfer coefficient, analogous to Newton’s law of
cooling, is introduced,
tot
c
c
h
h
R
T
T
A
U
T
A
U
q






A
A
c
w
h
tot
c
c
c
w
h
tot
A
A
tot
h
A
A
U
hA
R
hA
R
A
U
or
hA
R
hA
R
A
U
or
R
A
U
1
)
(
1
)
(
1
1
)
(
1
)
(
1
1
1











For a heat exchanger, fins are often added to
surfaces exposed to either or both fluids.
Surfaces are also subject to fouling by fluid
impurities, rust formation, or other reactions
between the fluid and the wall material. The
subsequent deposition of a film or scale on the
surface can greatly increase the resistance to
heat transfer between the fluids.

17. THE OVERALL HEAT TRANSFER COEFFICIENT
For a plat surface without fins: )
( 

 T
T
h
A
q b
b
For a surface with fins: )
( 

 T
T
h
A
q b
t
o

So , in the expressions for a flat surface, replacing
b
A by t
o A

to obtain the expression for a surface with fins:
h
h
c
o
t
w
h
o
t
tot
c
c
c
o
t
w
h
o
t
tot
h
h
tot
h
h
A
U
hA
R
hA
R
A
U
or
hA
R
hA
R
A
U
or
R
A
U
1
)
(
1
)
(
1
1
)
(
1
)
(
1
1
1















For fouling , introduce an additional thermal resistance, termed
the fouling factor,
f
R 

c
o
t
c
o
t
c
f
w
h
o
t
h
f
h
o
t
tot
h
h
c
c hA
A
R
R
A
R
hA
R
A
U
A
U )
(
1
)
(
)
(
)
(
1
1
1 ,
,
















18. TTHE OVERALL HEAT TRANSFER COEFFICIENT
The overall surface efficiency can be expressed as:
)
1
(
1 f
t
f
o
A
A

 


If a straight or pin fin of length L is used and an adiabatic tip is
assumed:
mL
mL
f
)
tanh(



19. Log Mean Temperature Difference
If Th or Tc varies with position in the heat exchanger, the use of the
relation
q = UAΔT
may Not be appropriate. In this case, it is necessary to work with a
rate equation of the form:
q = UAΔTm
where ΔTm is an appropriate mean
temperature difference.
The Parallel – Flow Heat Exchanger
The temperature difference ΔT
is initially large but decays
rapidly with increasing x,
approaching zero
asymptotically.
Th,i = Th,l, Th,0 = Th,2, Tc,i =Tc,1,
and Tc,0 = Tc,2

20. Log Mean Temperature Difference
Applying an energy balance to each of the differential
elements of Figure 11.7, it follows that
mhcp,hTh = dq + mhcp,h (Th + dTh) or dq = -mhcp,hdTh = -ChdTh
Similarly, dq = mccp,cdTc = CcdTc
where Ch and Cc are the hot and cold fluid heat capacity rates, respectively.
The heat transfer across the surface area dA may also be
expressed as:
dq = UΔTdA
where ΔT = Th – Tc is the local temperature difference between the hot and
cold fluids. The differential form of the equation is d(ΔT) = dTh - dTc
(11.10)
(11.11)
Substituting Eqs. (11.10) and (11.11) into it to obtain
dA
C
C
U
T
T
d
or
C
C
TdA
U
C
C
dq
T
d
c
h
c
h
c
h
)
1
1
(
)
(
),
1
1
(
)
1
1
(
)
(














21. Log Mean Temperature Difference
Integrating across the heat exchanger, we obtain

 



 2
1
2
1
)
1
1
(
)
(
dA
C
C
U
T
T
d
c
h
)
1
1
(
)
/
ln( 1
2
c
h C
C
UA
T
T 




Substituting for Ch and Cc from:
q = mhcp,h(Th,i - Th,0) = Ch (Th,i - Th,0)
q = mccp,c(Tc,o – Tc,i) = Cc (Tc,o – Tc,i)
Respectively, it follows that
  )
)(
/
(
)
(
)
(
)
)
(
)
/
ln(
2
1
,
,
,
,
,
,
,
,
1
2
T
T
q
UA
T
T
T
T
q
UA
q
T
T
q
T
T
UA
T
T
o
c
o
h
i
c
i
h
i
c
o
c
o
h
i
h

















)
ln(
1
2
1
2
T
T
T
T
UA
q






2
1
2
1
1
2
1
2
ln
ln
T
T
T
T
T
T
T
T
Tlm














22. Log Mean Temperature Difference
q = UAΔTlm
For the parallel-flow exchanger,
ΔT1 = Th,1- Tc,1 = Th,i- Tc,i
ΔT2 = Th,2- Tc,2 = Th,0- Tc,0
The Counter flow Heat Exchanger
ΔT1 = Th,1- Tc,1 = Th,i- Tc,0
ΔT2 = Th,2- Tc,2 = Th,0- Tc,i
For the same inlet and outlet
temperatures,
ΔTlm,CF > ΔTlm,PF
Also Tc,o can exceed Th,o for counter flow
but not for parallel flow.

23. Log Mean Temperature Difference
Special Operating Conditions -- Evaporators and condensers
When one of the fluids flowing through a heat exchanger changes phase, that
fluid will remain at a constant temperature, provided that its pressure does not
change and there is no superheating or subcooling.
q = UA ΔTlm = UA = UA
= mcp(T0 – Ti)
)]
/(
)
ln[(
)]
/(
)
ln[(
/ i
c
o
c
o
c
i
c
p T
T
T
T
T
T
T
T
c
m
UA 







Take antilog gives
)
1
)(
(
)
/(
)
(
)
/(
)
(
/
/
p
p
c
m
UA
i
c
i
o
i
c
i
i
o
c
i
c
o
c
c
m
UA
e
T
T
T
T
T
T
T
T
T
T
T
T
T
T
e
















24. Log Mean Temperature Difference
With a circular tube for the cold fluid flowing inside the tube,
A =πDx
)
1
)(
(
)
(
/ p
c
m
Dx
U
i
c
i e
T
T
T
x
T







If U = h , Tc = Ts, the above solution is the solution for constant wall
temperature in Chapter 8.
Multipass and Cross-flow Heat Exchangers
ΔTlm = F ΔTlm,CF

25. Heat Exchanger Analysis: The effectiveness – NTU Method
When the fluid inlet temperatures are known and the outlet temperatures are specified or readily
determined from the energy balance expressions, it is a simple matter to use the log mean temperature
difference (LMTD) method. If only the inlet temperatures are known, use of the LMTD method requires
an iterative procedure. The effectiveness – NTU method is more convenient.

28. Effectiveness – NTU Relations

29. NTU Relations

30. Effectiveness Relations

31. Effectiveness Relations

32. Effectiveness Relations

33. Effectiveness-NTU Relations