International Journal of Emerging Trends in Engineering and Development Issue 3, Vol.
2 (March 2013)
Available online on http://www.rspublication.com/ijeted/ijeted_index.htm ISSN 2249-6149
Page 1
A new Optimization Method for Evaluating Thermal Parameters in a Single segmental Shell and
Tube Heat Exchanger
Sandeep K. Patel
LDRP-ITR, Gandhinagar.
PG student, Mechanical Engineering Department,
Alkesh M. Mavani
LDRP-ITR, Gandhinagar.
Asso. Professor, Mechanical Engineering Department,
_____________________________________________________________________________________________
Abstract
This paper presents a simple but accurate method to calculate thermal parameters in a single segmental shell and
tube heat exchanger. In this paper attempt is made to overcome some theoretical assumptions and serve practical
approach as much as possible for shell and tube heat exchanger design and optimization. Numbers of iteration and
their comparisons as well as analysis is performed in HTRI software. The case of minor error is theoretical
assumption made in manual calculations. The final results are helpful to run shell and tube heat exchanger water
cooler at optimal mass flow rate and baffle spacing.
Keywords
Heat exchangers, Pressure drop, Baffle spaces, Baffle cut, Heat transfer co-efficient, Mass flow rate.
_____________________________________________________________________________________________
1. Introduction
Heat Exchanger is a device which provides a flow of thermal energy between two or more fluids at different
temperatures. Heat exchangers are used in a wide variety of engineering applications like power generation, waste
heat recovery, manufacturing industry, air-conditioning, refrigeration, space applications, petrochemical industries
etc.
In recent years, new softwares for design of heat exchangers has been focusing in adapting the equipment to the
required process and new solutions have been found that makes the design time shorter.
2. Pressure drop in STHE
2.1Preliminary Calculation
A selected shell and tube heat exchanger must satisfy the process requirements with the allowable pressure drops
until the next scheduled cleaning of plant. The methodology to evaluate thermal parameters is explained with
suitable assumptions. The following are the major assumptions made for the pressure drop analysis;
1. Flow is steady and isothermal, and fluid properties are independents of time.
2. Fluid density is dependent on the local temperature only or is treated as constant.
3. The pressure at a point in the fluid is independent of direction.
4. Body force is caused only by gravity.
5. There are no energy sink or sources along streamline; flow stream mechanical energy dissipation is idealized as
zero.
6. The friction factor is considered as constant with passage flow length.
Heat transfer or the size of heat transfer exchanger can be obtained from equation,
Q = U
o
A
o
T
m
(1)
The overall heat transfer coefficient U
o
based on the O.D. of tubes can be estimated from the estimated values of
individual heat transfer coefficients, the wall and fouling resistance and the overall surface efficiency using equation
(2)
International Journal of Emerging Trends in Engineering and Development Issue 3, Vol.2 (March 2013)
Available online on http://www.rspublication.com/ijeted/ijeted_index.htm ISSN 2249-6149
Page 649
(2)
For the single tube pass, purely countercurrent heat exchanger, F= 1.00. For preliminary design shell with any even
number of tube side passes, F may be estimated as 0.9
Heat load can be estimated from the heat balance as:
Q = (mC
p
)
c
(T
c2
T
c1
) = (mC
p
)
h
(T
h2
T
h1
) (3)
If one stream changer phase:
Q = mh
fg
(4)
LMTD (Log Mean Temperature Difference Method) calculation:
If three temperatures are known, the fourth one can be found from the heat balance,
(5)
Heat transfer area can be calculated from equation (1). Number of tubes of diameter (d
o
), shell diameter (D
s
) to
accommodate the number of tubes (N
t
), with given tube length (L) can be estimated,
(6)
One can find the shell diameter (D
s
), which would contain the right number of tubes (N
t
), of diameter (d
t
).
Figure 1: Square and Triangular Pitch Tube Layout
[1]
The total number of tubes can be predicted in fair approximation as function of the shell diameter by taking the shell
circle and dividing it by the projected area of the tube layout (Figure 1) pertaining to a single tube A
1
.
(7)
Where CTP is the tube count calculation constant that accounts for the incomplete coverage of the shell diameter by
the tubes.
Based on fixed tube sheet the following values are suggested:
One tube pass CTP = 0.93
Two tube pass: CTP = 0.90
Three tube pass: CTP = 0.85
A
1
= (CL) (P
T
)
2
(8)
Where CL is the tube layout constant:
CL = 1.0 for 90 and 45
CL = 0.87 for 30 and 60
Equation (7) can be written as:
(9)
Where P
R
is the Tube Pitch Ratio (P
R
= P
T
/d
o
).
The shell diameter in terms of main construction diameter can be obtained as from equations (6) and (9),
(10)
2.2 Tube side Pressure Drop
[B1][B2]B3]
1 1 1
fi fo
o
o w
o i i i i o o o
R R
A
A R
U A h h q q q q
(
= + + + +
(
1 2 2 1
1 2
2 1
( ) ( )
( )
ln
( )
h c h c
lm
h c
h c
T T T T
T
T T
T T
A =
o e t
A N L t =
2
( )
4
s
t
i
D
N CTP
A
t
=
2
2 2
r
0.875
(P )
s
t
o
D CTP
N
CL d
| |
=
|
\ .
1/ 2
2
r
(P )
0.637
o o
A d CL
Ds
CTP L
| |
=
|
\ .
International Journal of Emerging Trends in Engineering and Development Issue 3, Vol.2 (March 2013)
Available online on http://www.rspublication.com/ijeted/ijeted_index.htm ISSN 2249-6149
Page 650
The tube side pressure drop can be calculated by knowing the number of tube passes (N
p
) and length (L) oh
heat exchanger,
The pressure drop for the tube side fluid is given by equation
(11)
(12)
The change of direction in the passes introduction in the passes introduction an additional pressure drop due to
sudden expansions and contractions that the tube fluid experiences during a return that is accounted for allowing
four velocity head per pass
(13)
The total pressure drop of the side becomes:
(14)
2.3 Shell side Pressure Drop
[B1][B2]B3]
The shell side pressure drop depends on the number of tubes, the number of times the fluid passes the tube bundle
between the baffles and the length of each crossing.
The pressure drop on the shell side is calculated by the following expression:
(15)
Where,
s
= (
b
+
s
)
0.14
N
b
= Number of baffles
(N
b
+ 1) = Number of times fluid passes to the tube bundle
Friction factor (f) calculated from:
(16)
Where,
(17)
The correlation has been tested based on data obtained on actual exchangers. The friction coefficient also takes
entrance and exit losses into account.
2.4 Pumping power and pressure drop
[B1][B3]
The fluid pumping power is proportional to the pressure drop in the fluid across a heat exchanger, the equation can
give by:
(18)
Where is the pump or fan efficiency. ( = 0.80 to 0.85)
The cost in terms of increased fluid friction requires an input of pumping work greater than the realized benefit of
increased heat transfer. For gases and low density fluids and also for very high viscosity fluids, pressure drop is
always of equal importance to the heat transfer rate and it has a strong influence on the design of heat exchangers.
2
4
2
p
m
t
i
LN
P f
d
A =
2
4
2
p
i
t
i
LN
G
P f
d
A =
2
4
2
m
t p
P N
A =
2
4 4
2
p
m
t p
i
LN
P f N
d
| |
A = +
|
\ .
2
( 1)
2
s b s
s
s s
G N D
P f
D |
+
A =
exp( .576 0.19ln Re )
s
f o =
6
400 Re 1 10
s s
s
D o
< = s
p
m P
P
q
A
=
p
q
p
q
International Journal of Emerging Trends in Engineering and Development Issue 3, Vol.2 (March 2013)
Available online on http://www.rspublication.com/ijeted/ijeted_index.htm ISSN 2249-6149
Page 651
3. Problem definition
Feed water cooler specification
Data for feed water cooler is shown taken from ABB LTD. (Makarpura, Vadodara). Both the fluids are in liquid to
liquid face .It is liquid to liquid heat transfer process and there is counter flow in the heat exchanger. It is assumed
that shell and tube are made of carbon steel.
TEMA : AES
DUTY : 670KW
SHELL SIDE
Fluid : Sour water
Internal Diameter (m) : 0.387
Inlet Temperature (C) : 125.50
Fouling Factor (m
2
K/W) : 0.000334
Baffle Spacing : Single Segmental
TUBE SIDE
Fluid : Cooling water
Inlet Temperature (C) : 37.00
Outlet Temperature (C) : 80.20
Tube Length(m) : 0.6
Tube Pitch(mm) : 32
Tube Layout : 90
Tube Count : 64
This problem is solved by MATLAB as well as by HTRI Xchanger.
4. Result and discussion
4.1 Output 3D CAD Model of Feed Water Cooler
Three dimensional model of shell and tube type heat exchanger made in HTRI Xchanger is shown in figure 2
Figure 2 3D Model of Shell and Tube Heat Exchanger
International Journal of Emerging Trends in Engineering and Development Issue 3, Vol.2 (March 2013)
Available online on http://www.rspublication.com/ijeted/ijeted_index.htm ISSN 2249-6149
Page 652
Figure: 3 3D Models Internal Construction of Shell and Tube Heat Exchanger
Figure: 3 shows the internal constructions of shell and tube heat exchanger. The baffle and tubes arrangements are
shown.
Shell and tube heat exchanger analyzed at different mass flow rate and baffle spacing with help of HTRI Xchanger
6.0 software. The conclusions are made as below:
4.2 CONCLUSIONS BASED ON MASS FLOW ITERATIONS AT DIFFERENT MASS FLOW RATE
These conclusions are based on iterations made at various mass flow rates at fixed fouling condition of water.
- As the mass flow rate increase there is relative increase in pressure drop at constant fouling factor for shell
and tube both side.
- While increasing mass flow rate there is gradual drop in over design.
- As the mass floe rate increase at constant fouling factor there is no effect on EMTD parameters.
- On the condition of heat exchanged is optimized for such a mass floe rate and pressure drop in allowable
condition which satisfies required duty of 670 KW.
- The fouled heat transfer coefficient has significant role in heat transfer process, while increasing mass flow
rate there is gradual increase in fouled and clean heat transfer coefficient will increase the heat transfer rate
of heat exchanger
- As the mass floe rate increase there is increase in heat transfer .Heat transfer is strongly influenced by shell
side mass flow rate. Small change at shell side mass floe rate will give significant heat transfer. As the tube
and shell side mass floe rate increased there is a relative increase in pressure drop.
4.3 CONCLUSIONS BASED ON BAFFLE SPACING ITERATIONS AT OPTIMIZED MASS FLOW RATE
These conclusions are based on iterations made at various mass flow rates at fixed fouling condition of water.
- As the baffle spacing increase at constant fouling there is gradual drop in Cross pass.
International Journal of Emerging Trends in Engineering and Development Issue 3, Vol.2 (March 2013)
Available online on http://www.rspublication.com/ijeted/ijeted_index.htm ISSN 2249-6149
Page 653
- As the baffle spacing at constant fouling there is gradual drop in Over Design.
- As the baffle spacing at constant fouling there is minor drop in EMTD.
- As the baffle spacing increase at constant fouling there is gradual drop in Overall heat transfer coefficient.
- As the baffle spacing increase at constant fouling there is partially drop and rise condition in Shell side heat
transfer coefficient.
- As the baffle spacing increase at constant fouling there is no effect in tube side heat transfer coefficient.
- As the baffle spacing increase at constant fouling there is normal drop in Shell side velocity, but velocity
remains constant in tube side.
- As the baffle spacing increase at constant fouling there is normal pressure drop in shell side, but pressure
remains constant in tube side.
Initial heat exchanger data is available in ABB LTD.(VADODARA). After thermal analysis, I have achieved
following result for 200 mm and 160 mm baffle spacing with help of HTRI- 6.0 software. Table 6 indicates
optimized condition of thermal parameters, which is directly effect on heat exchanger performance. After
conclusion, 160 mm baffle spacing is more suitable for heat exchanger design.
Table:1 HTRI Calculation for 200(Original Design) mm baffle spacing and 160(Optimized Design) mm baffle
spacing
SR.
NO.
THERMAL
PARAMETERS
200 MM
BAFFLE
SPACING
160 MM
BAFFLE
SPACING
1 Over Design (%) 21.24 22.09
2 hs(w/m-k) 3582.79 3776.05
3 ht(w/m-k) 5958.17 5958.17
4 U(w/m-k) 786.92 795.85
5 Vs(m/s) 0.16 0.2
6 Vt(m/s) 1.04 1.04
7 Ps(bar) 0.051 0.061
8 Pt(bar) 0.324 0.324
5. SCOPE OF WORK
The literature survey revealed that the very limited studied has been done baffle spacing ratio was found to be
important design parameters which has a direct effect on pressure drop and causes a conflict between the
effectiveness and total cost.
Continuous pressure drop which will increase the pumping power. The high pumping power increase the operating
cost as well as reduce the overall efficiency which is the major problem in industries. To achieve optimal operating
International Journal of Emerging Trends in Engineering and Development Issue 3, Vol.2 (March 2013)
Available online on http://www.rspublication.com/ijeted/ijeted_index.htm ISSN 2249-6149
Page 654
condition trial and error or scientific optimization method has been used. These needs time and also may not be
economical. By using proper optimization software optimal operating condition can be evaluated. That is why in
present work, the effort has been made to provide methodology for optimization of mass flow rate and baffle
spacing in shell and tube heat exchanger. On the basis of above problem objective of present work has been framed.
i. Detail study of shell & tube heat exchangers construction, operation and designing for an optimal
condition for industrial applications.
ii. Optimization of Shell and Tube Heat Exchanger by controlling fluid flow rate with permissible pressure
drop to gain optimal thermal efficiency.
iii. To gain knowledge of optimization & designing software (Xchanger-HTRI) for achieve optimal thermal
efficiency for industrial application.
iv. Iteration based on baffle spacing for various parameters.
Acknowledgement
I take this opportunity to express my sincere gratitude to my advisor, Prof. Alkesh M. Mavani, Professor in
Mechanical Engineering Department, LDRP-ITR,Gandhinagar through his valuable guidance, motivation, co-
operation with encouraging attitude at all stages of my work. His hard work, personal values and excellent work
ethics have been a source of inspiration for me.
I am also grateful to Mr. Narendra K. Roy Executive Director at CHARISMA CAREERS PVT. LTD.,
VADODARA, for providing with all the facilities of the computer laboratory and workshop as well as all other help
that they granted me to pursue my research leading to the dissertation work.
It gives me pleasure to express my deep sense of gratitude to Prof. A.R. Patel and head of the Mechanical
Engineering Department to provide great opportunity to carry out this Literature Review work as a part of the
curriculum. I also extend my sincere thanks to all other faculty members of the department.
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International Journal of Emerging Trends in Engineering and Development Issue 3, Vol.2 (March 2013)
Available online on http://www.rspublication.com/ijeted/ijeted_index.htm ISSN 2249-6149
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