Heat Exchanger
Heat exchangers can be described as:
they include all devices that are designed for exchanging
heat.
Only heat exchangers that are meant to exchange heat
between two fluids are taken into account.
These fluids can be gasses as well as liquids. It is still
difficult to have an overview, and a classification needs to
be made. It is possible to classify heat exchangers in a
number of ways.
Classification Heat Exchangers
1.Classification of heat exchangers depending on the basic
of the fluid paths through the heat exchanger.
Difference is made between :
• parallel flow,
• counter flow
• and cross flow.
Classification
cross flow air to liquid heat exchanger
This is a heat exchangers that is
found on the top buildings
and is for instance needed for
air conditioning inside. Here a
fluid is leaded between the
plates
at the top of the heat exchanger
and flows horizontally. Air is
blown vertically against the
plates to cool the fluid inside.
Classification
Classification
The second classification made, is depending on the state of the media
in the heat exchanger.
Liquid-to-liquid exchangers are those in which two liquids interact.
Also gas-to-gas heat exchangers like air preheaters in steam plants and
helium-cooled reactor gas turbine plants have to be mentioned. These
devices operate with heat transfer coefficients that are between ten and
one hundred times lower than the coefficients of liquid-to-liquid
exchangers. Gas to gas exchangers are general much larger and heavier
if a same amount of transferred heat is demanded.
The third type is the liquid-to-gas heat exchanger (or vice versa),
usually water and air are used, for instance in automotive radiators.
Because of the lower heat transfer coefficients on the gas-side there are
usually fines placed on the exchanging surfaces.
Classification
The third classification method is based the purpose of heat
exchanger.
The last classification is actually the most important choice
of the designer of a heat exchanging system. This is the
choice what kind of construction he is going to use.
Classification
series of U-type constructions
Shell-and-tube-heat exchanger with one shell pass and one tube pass; cross- counterflow operation
Regenerative Heat Exchangers
The regenerator represents class of heat exchangers in
which heat is alternately stored and removed from a
surface
This heat transfer surface is usually referred to as the
matrix of the regenerator.
For continuous operation, the matrix must be moved into
and out of the fixed hot and cold fluid streams. In this case,
the regenerator is called a rotary regenerator.
If, on the other hand, the hot and cold fluid streams are
switched into and out of the matrix, the the regenerator is
referred to as a fixed matrix regenerator.
Heat Transfer of Heat Exchanger
•
•
•
•
•
•
•
•
•
How is the heat transfer?
Mechanism of Convection
Applications .
Mean fluid Velocity and Boundary and their effect on the rate of heat
transfer.
Fundamental equation of heat transfer
Logarithmic-mean temperature difference.
Heat transfer Coefficients.
Heat flux and Nusselt correlation
Simulation program for Heat Exchanger
How is the heat transfer?
• Heat can transfer between the surface of a solid conductor
and the surrounding medium whenever temperature
gradient exists.
Conduction
Convection
Natural convection
Forced Convection
Natural and forced Convection
Natural convection occurs whenever heat flows
between a solid and fluid, or between fluid
layers.
As a result of heat exchange
Change in density of effective fluid layers taken
place, which causes upward flow of heated
fluid.
If this motion is associated with heat transfer mechanism
only, then it is called Natural Convection
Forced Convection
 If this motion is associated by mechanical means such as
pumps, gravity or fans, the movement of the fluid is
enforced.
 And in this case, we then speak of Forced convection.
Heat Exchangers
• A device whose primary purpose is the transfer of energy
between two fluids is named a Heat Exchanger.
Applications of Heat Exchangers
Heat Exchangers
prevent car engine
overheating and
increase efficiency
Heat exchangers are
used in Industry for
heat transfer
Heat
exchangers are
used in AC and
furnaces
• The closed-type exchanger is the most popular one.
• One example of this type is the Double pipe exchanger.
• In this type, the hot and cold fluid streams do not come
into direct contact with each other. They are separated by a
tube wall or flat plate.
Principle of Heat Exchanger
•
First Law of Thermodynamic: “Energy is conserved.”
0
0
0
0
dE


ˆ
ˆ
 .hin   m
 .hout   q
w
s  e
generated
 m
dt
out
 in

Qh  A.m h .C ph .Th
Qc  A.m c .C pc .Tc
•Control Volume
 m .hˆ   m .hˆ
in
out
COLD
HOT
Cross Section Area
Thermal Boundary Layer
THERMAL
Region III: Solid –
Cold Liquid
Convection
BOUNDARY LAYER
Energy moves from hot fluid
to a surface by convection,
through the wall by
conduction, and then by
convection from the surface to
the cold fluid.
NEWTON’S LAW OF
CCOLING
dqx  hc .Tow  Tc .dA
Th
Ti,wall

To,wall
Tc
Region I : Hot LiquidSolid Convection
Q hot
Q cold
NEWTON’S LAW OF
CCOLING
dqx  hh .Th  Tiw .dA
Region II : Conduction
Across Copper Wall
FOURIER’S LAW
dT
dqx  k.
dr
• Velocity distribution and boundary layer
When fluid flow through a circular tube of uniform crosssuction and fully developed,
The velocity distribution depend on the type of the flow.
In laminar flow the volumetric flowrate is a function of the
radius.
r D/2
V
 u2rdr
r 0
V = volumetric flowrate
u = average mean velocity
 In turbulent flow, there is no such distribution.
• The molecule of the flowing fluid which adjacent to the
surface have zero velocity because of mass-attractive
forces. Other fluid particles in the vicinity of this layer,
when attempting to slid over it, are slow down by viscous
forces.
Boundary
layer
r
• Accordingly the temperature gradient is larger at the wall
and through the viscous sub-layer, and small in the
turbulent core.
Tube wall
qx  hAT
qx  hA(Tw  T)
heating
Warm fluid
Metal
wall

h
Twh
cold fluid
Twc
qx 
cooling
Tc
k

A(Tw  T)
• The reason for this is
1) Heat must transfer through the boundary layer by
conduction.
 conductivity (k)
2) Most of the fluid have a low thermal
3) While in the turbulent core there are a rapid moving
eddies, which they are equalizing the temperature.
U = The Overall Heat Transfer Coefficient [W/m.K]
Region I : Hot Liquid –
Solid Convection
Region II : Conduction

Across Copper Wall
Region III : Solid –
Cold Liquid Convection

Th  Tc 
qx  hhot .Th  Tiw .A
qx 
qx
R1  R2  R3
1
A.R

qx  hc To,wall  Tc Ao

qx  U.A.Th  Tc 
U
kcopper .2L
r
ln o
ri
Th  Tiw 


qx
hh .Ai
ro 
qx .ln 
ri 
To,wall  Ti,wall 
kcopper .2L
qx
To,wall  Tc 
hc .Ao


ro 
ln 


r
1
1
 i 

Th  Tc  qx 


hh .Ai kcopper .2L hc .Ao 






 ro 
ro . ln 


r
r
1
 i 

U   o 
 hhot .ri
kcopper .ri
hcold 




1
r
r
i
o
+
Calculating U using Log Mean Temperature
Hot Stream :
 h .C ph .dTh
dqh  m
Cold Stream:
 c .C .dTc
dqc  m
d (T )  dTh  dTc
T  Th  Tc
c
p
dq  dqhot  dqcold
 dq  U .T .dA
 1
1 

d (T )  U .T .dA.

 m .C h m .C c 
c
p 
 h p

T2
T1

T2
T1
 dqh
dqc 

d (T ) 

 m .C h m .C c 
c
p 
 h p
 Th Tc  A2
d (T )
. dA
 U .

T
qc  A1
 qh
 1
d (T )
1
 U .

 m .C h m .C c
T
c
p
 h p
 A2
. dA
 A1

 T 
U . A.
Th  Tc    U .A Thin  Thout  Tcin  Tcout
ln 2   
q
q
 T1 

 
q  U .A
Log Mean Temperature

T2  T1
 T2 
ln
 T 

1


Log Mean Temperature evaluation
Ýh .CÝph .T3  T6  m
Ýc .CÝpc .T7  T10 
m
T2  T1
TLn 
U

 T2 
A.TLn
A.TLn

ln
 T1 
COUNTER CURRENT FLOW
1 CON CURRENT FLOW
2

1
2
T3
T4
T6
T1
∆ T1
∆ T2
T6
Wall
T7
T2
T8
T9
T10
∆A
A
A
T10
T1
T4
T5
T2
T10
T1
T6
T3
T4
T2
T5
T3
T6
T9
T8
T7
Parallel Flow
T1  T  T  T3  T7
in
h
in
c
T2  Thout  Tcout  T6  T10
T8
T7
T9
Counter - Current Flow
T1  T  Tcout  T3  T7
in
h
T2  Thout  Tcin  T6  T10
q  hh Ai Tlm
(T  T )  (T6  T2 )
Tlm  3 1
(T  T )
ln 3 1
(T6  T2 )
1
2
T3
T4
T1
T6
T6
Wall
T2
T7
T8
T9
T10
q  hc Ao Tlm
Tlm 
(T1  T7 )  (T2  T10 )
(T1  T7 )
ln
(T2  T10 )
A
DIMENSIONLESS ANALYSIS TO CHARACTERIZE A HEAT EXCHANGER
Nu  f (Re, Pr, L / D, b / o )
h.D
k
v.D.
C p .

k
•Further Simplification:
Nu  a.Re b .Pr c
Nu 
Can Be Obtained from 2 set of experiments
One set, run for constant Pr

And second set, run for constant Re
q
k

h
A(Tw  T)
D

•Empirical Correlation
•For laminar flow
Nu = 1.62 (Re*Pr*L/D)
•For turbulent flow
Nu Ln
 b 
 0.026. Re . P r . 
 o 
0.8
1/ 3
•Good To Predict within 20%
•Conditions:
L/D > 10
0.6 < Pr < 16,700
Re > 20,000
0.14
Experimental
Apparatus
Switch for concurrent
and countercurrent
flow
Temperature
Indicator
Hot Flow
Rotameters
Cold Flow
rotameter
Heat
Temperature
Controller Controller
• Two copper concentric pipes
•Inner pipe (ID = 7.9 mm, OD = 9.5 mm, L = 1.05 m)
•Outer pipe (ID = 11.1 mm, OD = 12.7 mm)
•Thermocouples placed at 10 locations along exchanger, T1 through T10
Theoretical trend
y = 0.8002x – 3.0841
Examples of Exp. Results
Theoretical trend
y = 0.026x
6
Experimental trend
y = 0.0175x – 4.049
ln (Nu)
5.5
5
4 .5
4
3 .5
Experimental trend
y = 0.7966x – 3.5415
3
2 .5
2
9 .8
10
10 .2
10 .4
10 .6
10 .8
250
11
200
Nus
ln (Re)
150
100
Theoretical trend
y = 0.3317x + 4.2533
50
0
150
ln (Nu)
4.8
2150
4150
6150
8150
10150
12150
Pr^X Re^Y
4.6
4.4
Experimental Nu = 0.0175Re0.7966Pr0.4622
4.2
Theoretical
4
0.6
0.8
1
ln (Pr)
1.2
1.4
Experimental trend
y = 0.4622x – 3.8097
Nu = 0.026Re0.8Pr0.33
Effect of core tube velocity on the local and
over all Heat Transfer coefficients
Heat Transfer Coefficient Wm -2K-
35000
30000
25000
hi (W/m2 K)
ho (W/m2 K)
U (W/m2 K)
20000
15000
10000
5000
0
0
1
2
3
4
-1
Velocity in t he core t ube (ms )