30120140503007

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 5, Issue 3, March (2014), pp. 66-76, © IAEME
66
CFD ANALYSIS OF ECONOMIZER TO OPTIMIZE HEAT TRANSFER
K. Obual Reddy1
, M. Srikesh2
, M. Kranthi Kumar3
, V. Santhosh Kumar4
1
Asst Prof, Department of Mechanical Engineering,
GITAM UNIVERSITY HYDERABAD INDIA
2, 3, 4
UG Student, Department of Mechanical Engineering,
GITAM UNIVERSITY HYDERABAD INDIA
ABSTRACT
Economizer is the best mechanical component which is used for trapping the heat of flue
gasses. Economizers can be best applied in electricity generating power plants. Best way of
utilisation of flue gas heat is by transferring it to the boiler feed water. Hence economizers are
designed in such a way that feed water is allowed to pass the economizer through pipes. Economizer
shell is completely filled with the flue gasses. There is a cross flow heat exchange process takes
place between boiler feed water and flue gas. Boiler feed water is heated up to its boiling temperature
by consuming the heat from flue gas. Hence as the boiler feed water is already preheated the amount
of fuel burnt to generate steam is reduced in this way in boilers. This paper explains about one of the
ways to increase the heat transfer rate of economiser. Heat transfer is always enhanced by increasing
the surface area of heat transfer between heat exchanging bodies. Hence by providing addition area
between the flue gas and boiler feed water we can increase the heat transfer. Additional area is
provided with the help of fins which are considered as extended surfaces. A CFD (Computational
Fluid Dynamics) analysis is carried with two cases with and without fins. Analysis of two cases is
carried out to determine, how much value of heat transfer has enhanced with fins.
Keywords: CFD (Computational Fluid Dynamics), Economiser, K- Ԑ Model.
NOMENCLATURE
ρ = density of fluid
u = initial velocity
p = pressure
f = body force
Q = vector variable
INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING
AND TECHNOLOGY (IJMET)
ISSN 0976 – 6340 (Print)
ISSN 0976 – 6359 (Online)
Volume 5, Issue 3, March (2014), pp. 66-76
© IAEME: www.iaeme.com/ijmet.asp
Journal Impact Factor (2014): 7.5377 (Calculated by GISI)
www.jifactor.com
IJMET
© I A E M E

67
F = vector fluxes
V = volume of control volume
A = area of control volume
Ri = equation residual
Wi = weight vector
Ve
= volume of element.
k = turbulence kinetic energy
Ԑ = dissipation rate
ω = specific dissipation rate
Gk = generation of turbulence kinetic energy due to mean velocity gradients
uj = jth Cartesian component of the instantaneous velocity
τij = viscous stress tensor
fi = the body force
h = static enthalpy
Gb = generation of turbulent kinetic energy that arises due to buoyancy
YM = represents the fluctuating dilation
Sε, Sk = source terms defined by the user.
C1ε, C2ε, Cµ = constants that have been determined experimentally
σk, σε = turbulent Prandtl numbers for the turbulent kinetic energy, its dissipation rate.
S = mean rate-of-strain tensor.
1. INTRODUCTION
Economizer is a mechanical component which helps in pre heating of water which is feed
into boiler. It is a part of steam generating unit which intended to reduce energy consumption of
boilers in particular. Generally in power plants water which is introduced into boilers is called as
boiler feed water. This water is converted into steam after absorbing the heat which is produced from
combustion of fossil fuel. Combustion of fossil fuels gives flue gases as by products. These gases
have lower level of thermal energy within them which is trapped. Economizer carries a vital role in
trapping this heat and transferring it to the boiler feed water. Hence economizer acts as an external
heat transfer device where heat transfer takes place from flue gases to the boiler feed water. This
action preheats the boiler feed water; thereby reducing the amount of fuel required to convert the
boiler feed water into steam. Economizer is a tubular structure with main objective of obtaining
lower cost efficient and maximum heat transfer rate with the boiler feed water. This action of
economizer helps in economic production of power in power plants. It reduces the operating costs of
power plant also.
Heat is considered to be a form of energy. So as the saying goes energy is neither created nor
destroyed, we should not allow the heat to go out as waste in the flue gases. One of the ways to
conserve the heat energy which is going out as waste in the exhaust gasses is to extract the heat and
transfer it to a boiler feed water. One of the important components of heat transfer system in power
plants is the economizer. Economizer is widely used equipment in various industries such as power
generation, petroleum refining, chemicals paper, refrigeration and also in HVAC (Heating
Ventilation and Air Conditioning). According to a market survey conducted in Europe, economizers
are used in about 42% of the power plants. It also used for the heat exchange process in Nuclear
industry as well. Hence there arises a need for analysis of Economizer.
1.1. Fins extended surface
In order to optimize the heat transfer rate and increase it there are several ways in achieving
it. One of the easiest and simple ways of doing it is to increase the area of heat transfer between two

68
heat exchanging bodies. Hence this paper explains about the analysis which is carried out on an
economizer which has fins attached to its tubes. As these fins are considered as extended surface
areas, hence there is increase in heat transfer between the boiler feed water and flue gas. Orientation
of the fins surface with respect to the direction of flue gas, type of fins shapes, dimensions of the fins
and the material used as fin all play a significant role in the heat transfer between two bodies.
2. LITERATURE SURVEY
Krunal P. Mudafale & Hemant S. Farkade[1] work on “CFD analysis of economizer in a
tengential fired boiler”. In this a simulation of the economizer zone, which allows for the condition
of the shell-side flow and tube-side and tube-wall, thermal fields, and of the shell-tube heat-
exchange. As per their observation maximum number of cause of failure in economizer unit is due to
flue gas erosion. The past failure details revels that erosion is more in U-bend areas of Economizer
Unit because of increase in flue gas velocity near these bends. But it is observed that the velocity of
flue gases surprisingly increases near the lower bends as compared to upper ones. The model is
solved using conventional CFD techniques by STAR- CCM+ software. The Computational Fluid
Dynamics (CFD) approach is utilised for the creation of a three-dimensional model of the
economizer coil. With equilibrium assumption applied for description of the system chemistry. The
flue gas temperature, pressure and velocity field of fluid flow within an economizer tube using the
actual boundary conditions have been analyzed using CFD tool. Such as the ability to quickly
analyse a variety of design options without modifying the object and the availability of significantly
more data to interpret the results.
A.D.Patil, P.R.Baviskar, M.J.Sable, S.B.Barve[2] work on “To optimize economizer design
for better performance” This paper focuses on optimisation of economiser design with finned & bare
tube economiser. The aim of this work is to develop methodology which finds optimisation of
economiser design.CFD analysis is used to compare the new economiser design with traditional
strategies. A brief historical overview on economiser design & optimisation is given along with the
main advantage of the authors proposed method is also discussed.
A.D.Patil, P.R.Baviskar, M.J.Sable, S.B.Barve [3] work on “Optimization of Economizer
Design for the Enhancement of Heat Transfer Coefficient”. This paper presents an approach for the
optimization of economiser design. The aim of this work is to develop methodology which finds
optimization of economiser design. CFD analysis is used to compare the new economizer design
with traditional strategies. The most economical solution of this problem seems to distribute gas flow
uniformly at inlet of economiser by using vanes. In the present work commercial software Fluent is
used for the 3D simulation using its inbuilt K- Ԑ Reliable model. Optimization of economiser is done
for effective heat transfer with reducing number of tubes required. In the present work commercial
software Fluent is used for the 3D simulation using its inbuilt K- Ԑ Reliable model. Optimization of
economiser is done for effective heat transfer with reducing number of tubes required.
TSUNG-FENG WU[4] work on “failure analysis for economizer tube of the waste heat”.
This paper is about failure analysis of the leakage of the economizer tube of the waste heat boiler in
the energy factory. The results show that although the material and mechanical properties of the
failed tube, were inferior to those of the new one, most of them were still satisfactory to the criterion
requirement it is clear that the crack initiated in the outer surface and propagated toward the inner
surface of the tube and the crack was identified to be rectangular in shape.
Deendayal Yadav, Dr. G. V. Parishwad, P. R. Dhamangaonkar*, Dr. S. R. Kajale, Dr. M. R.
Nandgaonkar, Dr. S. N. Sapali.[5] work on “effect of arreasters on erosion in economizer zone and
its analysis”. The authors in this paper have attempted to suggest a probable solution for reduction of
erosion in economiser zone and its analysis using CFD tool. In this paper the authors have submitted
the findings of analysis of finned tube economizer with Arresters at different inclinations. A steady

69
3D CFD tool is used for analysis and flow of the flue gases over the coils has been observed. The
effect of provision of arresters on the surface temperature, the flue gas temperature, pressure and
velocity field of fluid flow within an economizer tube using the actual boundary conditions have
been analyzed using CFD tool. The optimum dimensions of arrester and feasible inclination is
recommended as a result of the study. The authors have analyzed the performance and tried to
comment on this issue too.
A.N. Aziz, P. Siregar, Y.Y. Nazaruddin, and Y. Bindar [6] work on “Improving the
Performance of Temperature Model of Economizer Using Bond Graph and Genetic Algorithm”. The
mathematical model of economizer, based on physical laws is derived using bond graph
methodology. Pseudo multi-energy bond graph, which employs mass flow-rate and enthalpy flow-
rate as flow variables, as well as pressure and temperature as effort variables, is used in achieving
that. Overall heat transfer coefficient of economizer is obtained by using logarithmic temperature
mean difference between flue gas and boiler feed water. A modification of overall heat transfer
coefficient in the form of parameterized polynomial is also done by using the help of genetic
algorithm technique. A step simulation of the model at maximum, continuous, and minimum boiler
operating condition demonstrates, that model’s performance has been improved.
3. COMPUTATIONAL FLUID DYNAMICS
Computational Fluid Dynamics CFD is one of the best suited software for the analysis of
flow patterns which involve the dynamic parameters in the flow. It is very stable robust and accurate
in providing the required outputs. Generally experiments on moving fluid particles are not feasible.
Dynamics study on moving particles involves many complex calculations and large variables. But
their analysis can be easily carried with the help of CFD.
3.1. Working of CFD
CFD is based on Finite Volume Method. The area in the flow pattern for the analysis is first
modelled in any of the modelling software. In terms of CFD this is called as Computational domain.
This computational domain is discretized into many small elements of finite volume. This process of
dividing the domain is termed as meshing. Hence here each volume is considered to be a control
volume. The control volume have certain properties like mass, momentum, energy, turbulence
quantities, and mixture fractions, species concentrations and material properties. Based on the flow
problem we pick up the control volume properties to be analysed and solve the flow problem. The
theoretical flow in control volume is represented physically with the help of numerically solvable
partial differential equations. These equations govern the flow of fluid in the computational domain.
These equations also undergo the discretization process for the flow analysis.
There are three equations which are applicable commonly to all fluid dynamics problems are
the conservation of mass, momentum and the energy equations. Equations when represented in
differential form:
Continuity equation:
డఘ
డ௧
+ ‫׏‬ (ߩ ‫)ݑ‬ = 0 (1)
Momentum equation
ߩሺ
ௗ௩
ௗ௧
+ ‫.ݒ‬ ߘv) = - ‫׏‬p ൅ ‫׏‬ T ൅ f (2)

70
Energy equation:
ࣔ
࢚ࣔ
(ρh) +
ࣔ
ࣔ࢞࢐
(ρujh) = -
ࣔࢗ࢐
ࣔ࢞࢐
+
ࣔρ
࢚ࣔ
+ uj
ࣔࡼ
ࣔ࢞࢐
+ τij
࢛ࣔ࢏
ࣔ࢞࢏࢐
(3)
Above equations along with many other equations depending on the type and problem of
flow are considered as PDE’s Partial Differential Equations. These equations when subjected to
discretization give out a set of algebraic equations. These algebraic equations contain terms that have
been used for specifications of that particular flow and also these equations contain our required
output parameters. After solving these equations, the solution of these algebraic equations is found
with the CFD program called as FLUENT. This process of solving the flow problem involves
generation of output at each element nodes for accurate and precise results.
Analysis in CFD involves repeated and sequential solving of algebraic equations. The main
purpose for this repeated iterations is to improve the quality of solution constantly after each and
every iterations. The process of iterations is continued until convincing values of the global residuals
are obtained. Global residuals are difference between the values of output parameters obtained in the
current and previous solution which are averaged over the entire computational domain. After
continuous iterations a state of “convergence” is obtained where the residuals have decrease by 4-5
orders of magnitude. One of the most important points to be considered when working with CFD
software is that the quality of its output is completely dependent on the quality of its input. Hence
when best quality inputs are given to the solver best quality outputs are obtained for the analysis.
3.2. K- Ԑ turbulence model
This model is most reliable and robust very frequently used CFD platform. This model of
analysis has some special from others models like K- omega, RANS, Large Eddy simulations etc. In
the K- Ԑ model K represents the kinetic energy and Ԑ represents the turbulence. Both of the
parameters are determined with the help of certain following equations
ࣔ
࢚ࣔ
(ρk) +
ࣔ
ࣔ࢞࢏
(ρkui) =
ࣔ
ࣔ࢞࢐
{[µ+ ೟
ఙೖ
]
ࣔ࢑
ࣔ࢞࢐
} + Gk + Gb – ρε - Ym + Sk (4)
ࣔ
࢚ࣔ
(ρε) +
ࣔ
ࣔ࢞࢏
(ρεui) =
ࣔ
ࣔ࢞࢐
{[µ+ ౪
ఙε
]
ࣔε
ࣔ࢞࢐
} + C1ε
ε
௄
(Gk+C3εGb) – C2ερ
Єଶ
௄
+ St (5)
µt = ρCµ
௄ଶ
ε
(6)
Gk = -ρui’uj’
࢛ࣔ࢐
ࣔ࢞࢏
(7)
S = √2SijSij (8)
Gb = βgi
୲
௉௥௧
ࣔࢀ
ࣔ࢞࢏
(9)
Ym = 2 ρЄMt
2
(10)
All the above terms have been defined in the nomenclature which is specified before the
starting of paper.

71
4. ANALYSIS OF ECONOMIZER
Analysis of heat exchanger to optimize the heat transfer takes place in two cases. First case is
plain tube economizer and second case is finned tube economizer. In order to provide accurate
results, the input boundary condition parameters are obtained from a real time coal fired power plant.
The table below represents the values which are given to fluent solver
Table 1: (Temperature constrains)
Flue gas inlet temperature into
economizer
508°C
Flue gas outlet temperature into
economizer
330°C
Feed water inlet temperature
into economizer
254°C
Feed water outlet temperature
into economizer
312°C
The geometric specifications of the computational domain are not in accordance with the real
time economizer design because of lack of proper computer capacity, but the domain is scaled as per
the power plant dimensions. And the table represents them
Table 2: (Geometrical specifications)
Specifications Dimension
Length (Shell) 60 centimetre
Width (Shell) 30 centimetres
Height (Shell) 30 centimetre
Diameter of tubes 0.2 centimetre
Material Steel and carbon
Number of tube assemblies 1
Some other specifications considered in the analysis are
Table 3: (Flue gas specifications)
Mass flow rate 17.6 Kg/s
Specific heat 1.12 kj/kg k
Thermal conductivity 0.00046 kw/mk
Density 1.337 kg/m3
viscosity 0.101 kg/mh
Table 4: (Feed Water Specifications)
Mass flow rate 6.5 Kg/s
Specific heat 0.42 kj/kg k
Thermal conductivity 1 kcal/m-hr-°c
density 913 kg/m3

72
4.1. Geometric model and meshing
Modelling of the computational domain is carried out on GAMBIT (Geometry and mesh
building intelligent kit). In order to get best accurate results the domain is discretized into small
elements. The shape of the elements is Tet/Hybrid which mainly includes Tetrahedron along with
some other shaped elements.
Case 1 meshed model
Figure 1: Plain tube economizer with mesh
In the modeling of case 2, only one finned tube economizer pipe is considered. From the
analysis of case 1 it is seen that temperature profile of all the water tubes are same and uniform.
Hence analysis of one finned tube gives information about all the remaining tubes in the economizer
shell.
Case 2 meshed economizer tube
Figure 2: Finned tube economizer model

73
4.2. Conduction and convection in finned tube economizer
Convection in the analysis is the heat transfer between the flue gas and the surface area of
economizer tube. Conduction is the heat transfer within the thickness of the water tube.
Convection in the case 2 increases because of increase the heat transfer surface area between
the flue gas and water tube. This increase is due to external fins which are provided on the water
tube.
Conduction in the case 2 decreases because of the increase in thickness due to fins.
Figure 3: Economizer water tube with and without fins
In the case 2 conduction, there is increase in the thickness of tube because of fins. When we
are considering the conduction formula the thickness of tube material is inversely proportional to the
thermal conductivity. So as the thickness increases when the fins are considered conduction in case 2
decreases.
5. RESULTS
These results are obtained after detailed analysis of economizer with two cases.
Case 1
Figure 4: Heat transfer along the economizer water tube (Static Temperature)

74
Figure 5: Heat transfer along the economizer water tube (Velocity Vector)
Figure 6: Heat transfer along the economizer shell tube (Velocity Vector)
Figure 7: three dimensional graph showing temperature profile of shell and water tube

75
Case 2
Figure 8: temperature profile of water tube in case 2
6. CONCLUSION
Hence from the analysis it is found that heat transfer rate has been enhanced by providing the
fins in case 2. As they are considered as extended heat transfer surface areas there is significant
increase in heat transfer rate. Other technical explanation for the increase in heat transfer is, by
creating fins in the case 2, uniform flow of flue gas in the economizer is disturbed. Where as in case 1
there is no obstruction in the motion of flue gas. Because of fins there is disturbance in its flow, which
creates turbulence into the motion of flue gas. This turbulence energy is transferred to the fins in the
water tube. Due to this more amount of heat is transferred to the fins. Hence more amount of heat is
transferred in the second case than the first case. And this heat energy is transferred to water flowing in
the tube. So in this way we experience increased heat transfer because of the external fins which act as
increased heat transfer area and also due to generation of turbulence.

76
REFERENCES
[1] Krunal P. Mudafale & Hemant S. Farkade CFD analysis of economizer in a tengential fired
boiler, International Journal of Mechanical and Industrial Engineering (IJMIE) ISSN No.
2231 –6477, Vol-2, Iss-4, 2012.
[2] A.D.Patil, P.R.Baviskar, M.J.Sable, S.B.Barve to optimize economizer design for better
performance, New aspects of fluid mechanics, heat transfer and environment isbn: 978-960-
474-215-8.
[3] A.D.Patil, P.R.Baviskar, M.J.Sable, S.B.Barve Optimization of Economiser Design for the
Enhancement of Heat Transfer Coefficient, International Journal of Applied Research In
Mechanical Engineering (IJARME), ISSN: 2231 –5950 Volume-1, Issue-2, 2011.
[4] TSUNG-FENG WU Failure Analysis for the Economizer Tube of the Waste Heat Boiler.
[5] Deendayal Yadav, Dr. G. V. Parishwad, P. R. Dhamangaonkar*, Dr. S. R. Kajale, Dr. M. R.
Nandgaonkar, Dr. S. N. Sapali Effect of Arresters on Erosion in Economizer Zone and its
Analysis, AMAE Int. J. on Production and Industrial Engineering, Vol. 01, No. 01, Dec 2010.
[6] A.N. Aziz, P. Siregar, Y.Y. Nazaruddin, and Y. Bindar Improving the Performance of
Temperature Model of Economizer Using Bond Graph and Genetic Algorithm, International
Journal of Engineering & Technology IJET-IJENS Vol: 12 No: 01.
[7] N.S. Venkatesh Kumar and Prof. K. Hema Chandra Reddy, “CFD Analysis of Wind Driven
Natural Cross Ventilation for a Generic Isolated Building”, International Journal of
Mechanical Engineering & Technology (IJMET), Volume 4, Issue 5, 2013, pp. 200 - 207,
ISSN Print: 0976 – 6340, ISSN Online: 0976 – 6359.
[8] Sanjay Kumar Patel and Dr. A.C. Tiwari, “Performance Analysis of Supercritical Boiler”,
International Journal of Mechanical Engineering & Technology (IJMET), Volume 3, Issue 2,
2012, pp. 422 - 430, ISSN Print: 0976 – 6340, ISSN Online: 0976 – 6359.
[9] Sunil Jamra, Pravin Kumar Singh and Pankaj Dubey, “Experimental Analysis of Heat
Transfer Enhancement in Circular Double Tube Heat Exchanger using Inserts”, International
Journal of Mechanical Engineering & Technology (IJMET), Volume 3, Issue 3, 2012,
pp. 306 - 314, ISSN Print: 0976 – 6340, ISSN Online: 0976 – 6359.

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