The document describes a project report submitted by four students for their Bachelor of Technology degree. The report details the design of a superheater for a 210 MW thermal power plant. It includes sections on the layout and components of the power plant such as the boiler, superheater, and turbine. The document also provides information on different types of superheaters and the advantages of using superheaters. The majority of the report focuses on measuring heat transfer coefficients and metal temperatures in the superheater and using this data to design the superheater for the power plant.

1. DESIGN OF SUPERHEATER FOR 210 MW THERMAL
POWERPLANT
A PROJECT REPORT
Submitted by
PULAK DASGUPTA
NIKHIL JAIN
MD SHAHID AMIN
KUNDAN CHAKRABORTY
in partial fulfillment for the award of the degree
of
BACHELOR OF TECHNOLOGY
IN
MECHANICAL ENGINEERING
MEGHNAD SAHA INSTITUTE OF TECHNOLOGY, KOLKATA
MAULANA ABUL KALAM AZAD UNIVERSITY OF TECHNOLOGY
JUNE 2016

2. MAULANA ABUL KALAM AZAD UNIVERSITY OF
TECHNOLOGY
BONAFIDE CERTIFICATE
Certified that this project report “DESIGN OF SUPERHEATER FOR 210MW
THERMAL POWER PLANT” is the bonafide work of “PULAK DASGUPTA,
NIKHIL JAIN, MD SHAHID AMIN, KUNDAN CHAKRABORTY” who
carried out the project work under my supervision.
SIGNATURE SIGNATURE
GOUTAM LAHA RABI SHANKAR SINGH
HEAD OF THE DEPARTMENT SUPERVISOR
PROFESSOR
MECHANICAL ENGINEERING MECHANICAL ENGINEERING
MEGHNAD SAHA INSTITUTE MEGHNAD SAHA INSTITUTE
OF TECHNOLOGY, KOLKATA OF TECHNOLOGY, KOLKATA

3. ABSTRACT
A small unit of improvement in the design of the tubes in the super heater gives a
large value of positive impact on the performance of super heater in the steam
generation system.
In order to avoid over-temperature, tube explosion of the superheater, the
measurements of metal temperatures and the heat transfer coefficients of the
superheater in a commercial 210 MW Circulating Fluidized Bed (CFB) boiler are
conducted in this work. The measured data is analyzed and the theoretical
calculation is made. On the basis, the reasonable surface area of tube and the value
range of heat transfer coefficient of the middle temperature superheater are applied
for design. Furthermore, based on operation experience from several 210 MW CFB
boilers, an arrangement of the superheater in the furnace is given.
[iii]

4. TABLE OF CONTENTS
CHAPTER NO. TITLE PAGE NO.
ABSTRACT iii
LIST OF TABLES v
LIST OF FIGURES vi
LIST OF SYMBOLS vii
1. INTRODUCTION viii
LAYOUT OF POWER PLANT ix
COMPONENTS & OPERATION x
2. SUPERHEATER xvii
TYPES OF SUPERHEATERS xix
APPLICATIONS xxi
ADVANTAGE & DISADVANTAGE xxi
OTHER DETAILS xxiii
3. DESIGN OF THE SUPERHEATER xxv
4. CONCLUSION xxxiii
5. FUTURE SCOPE OF ACTION xxxiv
6. REFERENCES xxxv

5. LIST OF TABLES
Table 1. Materials Used For Superheater and their allowable temperatures.
[v]

6. LIST OF FIGURES
1. LAYOUT OF THERMAL POWER PLANT
2. SIMPLIFIED LAYOUT OF A THERMAL POWER PLANT
3. SCHEMATIC DIAGRAM OF A BOILER FURNACE IN 210 MW THERMAL POWER
PLANT
4. MAIN PARTS OF A THERMAL POWER PLANT
5. IMAGE OF COOLING TOWER
6. SCHEMATIC DIAGRAM OF A SUPERHEATER
7. TYPES OF SUPERHEATERS
[vi]

7. LIST OF SYMBOLS, ABBREVIATIONS and
NOMENCLATURE
do, di = tube outer and inner diameters, mm
F = fraction of direct radiation absorbed
f = friction factor inside tubes
ffo, ffi = fouling factors outside and inside tubes, m² K/W
hg1, hg2 = gas enthalpy at inlet and exit of superheater, kJ/kg
hc, hn, ho = convective, non luminous and outside heat transfer coefficients, W/m²
hs1, hs2 = steam enthalpy at superheater inlet and exit, kJ/kg
K= tube thermal conductivity, W/m K
Le = tube effective length, m
M = a constant
Qc, Qn, Qr, Qs = energy due to convection, non luminous heat transfer, direct
radiation and that absorbed is by steam, W
S = surface area, m²
Tg, is = local gas and steam temperatures, °C
Wg, Ws = gas and steam flow, kg/s
w = steam flow per tube, kg/s
ΔT = log-mean temperature difference, °C
[vii]

8. INTRODUCTION
Superheater tubes are surfaces for heat exchange, with the object of increasing the
steam temperature, after it comes from the boiler drum, to a value higher than
saturation. This has two basic purposes: to increase the thermodynamic efficiency
of the turbine, in which the steam will be expanded; and to make the steam free of
humidity. In normal operation, the boiler analyzed in this paper, produces steam
that is superheated by approximately 200 °C at the inlet of the turbine. The steam
flow has to be intense to permit the heat absorption from the tube, avoiding
deformation because of high temperature.
The superheater can be divided in two sections, primary and secondary, as in the
boiler studied, where the superheater tubes are within the radiation and convection
zone.
[viii]

9. 1. LAYOUT OF THERMAL POWER PLANT
2. SIMPLIFIED LAYOUT OF A THERMAL POWER PLANT
[ix]

10. 3. SCHEMATIC DIAGRAM OF A BOILER FURNACE IN 210 MW THERMAL
POWER PLANT
[ix]

11. COMPONENTS & OPERATION
3. MAIN PARTS OF A THERMAL POWER PLANT
1. Coal conveyor 2. Stoker 3. Pulverizer 4. Boiler 5. Superheater, Reheater 6. Air
preheater 7. Electrostatic precipitator 8. Smoke stack 9. Turbine 10. Condenser
11.Transformers 12. Cooling towers 13. Generator 14. High - voltage power lines
Basic Operation: A thermal power plant basically works on Rankine cycle.
Coal conveyor: This is a belt type of arrangement. With this coal is transported
from coal storage place in power plant to the place nearby boiler.
Stoker: The coal which is brought nearby boiler has to put in boiler furnace for
combustion. This stoker is a mechanical device for feeding coal to a furnace.
Pulverizer: The coal is put in the boiler after pulverization. For this pulverizer is
used. A pulverizer is a device for grinding coal for combustion in a furnace in a
power plant.
[x]

12. Boiler: Now that pulverized coal is put in boiler furnace. Boiler is an enclosed
vessel in which water is heated and circulated until the water is turned in to steam
at the required pressure.
Coal is burned inside the combustion chamber of boiler. The products of
combustion are nothing but gases. These gases which are at high temperature
vaporize the water inside the boiler to steam. Sometimes this steam is further
heated in a superheater as higher the steam pressure and temperature the greater
efficiency the engine will have in converting the heat in steam in to mechanical
work. This steam at high pressure and temperature is used directly as a heating
medium, or as the working fluid in a prime mover to convert thermal energy to
mechanical work, which in turn may be converted to electrical energy. Although
other fluids are sometimes used for these purposes, water is by far the most
common because of its economy and suitable thermodynamic characteristics.
Classification of Boilers:
Fire tube boilers: In fire tube boilers hot gases are passed through the tubes and
water surrounds these tubes. These are simple, compact and rugged in
construction. Depending on whether the tubes are vertical or horizontal these are
further classified as vertical and horizontal tube boilers. In this since the water
volume is more, circulation will be poor. So they can't meet quickly the changes in
steam demand. High pressures of steam are not possible, maximum pressure that
can be attained is about 17.5kg/sq cm. Due to large quantity of water in the drain it
requires more time for steam raising. The steam attained is generally wet,
economical for low pressures. The output of the boiler is also limited.
Water tube boilers: In these boilers water is inside the tubes and hot gases are
outside the tubes. They consist of drums and tubes. They may contain any number
[xi]

13. of drums (you can see 2 drums in fig).Feed water enters the boiler to one drum
(here it is drum below the boiler).This water circulates through the tubes connected
external to drums. Hot gases which surround these tubes will convert the water in
tubes in to steam. This steam is passed through tubes and collected at the top of the
drum since it is of light weight. So the drums store steam and water (upper
drum).The entire steam is collected in one drum and it is taken out from there (see
in layout fig).As the movement of water in the water tubes is high, so rate of heat
transfer also becomes high resulting in greater efficiency .They produce high
pressure, easily accessible and can respond quickly to changes in steam demand.
These are also classified as vertical, horizontal and inclined tube depending on the
arrangement of the tubes. These are of less weight and less liable to explosion.
Large heating surfaces can be obtained by use of large number of tubes. We can
attain pressure as high as 125 kg/sq cm and temperatures from 315 to 575
centigrade.
Superheater: Most of the modern boilers are having superheater and reheater
arrangement. Superheater is a component of a steam-generating unit in which
steam, after it has left the boiler drum, is heated above its saturation temperature.
The amount of superheat added to the steam is influenced by the location,
arrangement, and amount of superheater surface installed, as well as the rating of
the boiler. The superheater may consist of one or more stages of tube banks
arranged to effectively transfer heat from the products of combustion. Superheaters
are classified as convection, radiant or combination of these.
Reheater: Some of the heat of superheated steam is used to rotate the turbine
where it loses some of its energy. Reheater is also steam boiler component in
which heat is added to this intermediate-pressure steam, which has given up some
of its energy in expansion through the high-pressure turbine. The steam after
[xii]

14. reheating is used to rotate the second steam turbine (see Layout fig) where the heat
is converted to mechanical energy. This mechanical energy is used to run the
alternator, which is coupled to turbine, there by generating electrical energy.
Condenser: Steam after rotating steam turbine comes to condenser. Condenser
refers here to the shell and tube heat exchanger (or surface condenser) installed at
the outlet of every steam turbine in Thermal power stations of utility companies
generally. These condensers are heat exchangers which convert steam from its
gaseous to its liquid state, also known as phase transition. In so doing, the latent
heat of steam is given out inside the condenser. Where water is in short supply an
air cooled condenser is often used. An air cooled condenser is however
significantly more expensive and cannot achieve as low a steam turbine
backpressure (and therefore less efficient) as a surface condenser.
The purpose is to condense the outlet (or exhaust) steam from steam turbine to
obtain maximum efficiency and also to get the condensed steam in the form of
pure water, otherwise known as condensate, back to steam generator or (boiler) as
boiler feed water.
Cooling Towers: The condensate (water) formed in the condenser after
condensation is initially at high temperature. This hot water is passed to cooling
towers. It is a tower- or building-like device in which atmospheric air (the heat
receiver) circulates in direct or indirect contact with warmer water (the heat source)
and the water is thereby cooled (see illustration). A cooling tower may serve as the
heat sink in a conventional thermodynamic process, such as refrigeration or steam
power generation, and when it is convenient or desirable to make final heat
[xiii]

15. rejection to atmospheric air. Water, acting as the heat-transfer fluid, gives up heat
to atmospheric air, and thus cooled, is recirculated through the system, affording
economical operation of the process.
Two basic types of cooling towers are commonly used. One transfers the heat from
warmer water to cooler air mainly by an evaporation heat-transfer process and is
known as the evaporative or wet cooling tower.
4. IMAGE OF COOLING TOWER
Economiser: Flue gases coming out of the boiler carry lot of heat. Function of
economiser is to recover some of the heat from the heat carried away in the flue
gases up the chimney and utilize for heating the feed water to the boiler. It is
placed in the passage of flue gases in between the exit from the boiler and the entry
to the chimney. The use of economiser results in saving in coal consumption,
increase in steaming rate and high boiler efficiency but needs extra investment and
increase in maintenance costs and floor area required for the plant. This is used in
all modern plants. In this a large number of small diameter thin walled tubes are
placed between two headers. Feed water enters the tube through one header and
leaves through the other. The flue gases flow outside the tubes usually in counter
flow.
[xiv]

16. Air preheater: The remaining heat of flue gases is utilised by air preheater. It is a
device used in steam boilers to transfer heat from the flue gases to the combustion
air before the air enters the furnace. Also known as air heater; air-heating system.
It is not shown in the lay out. But it is kept at a place nearby where the air enters in
to the boiler.
The purpose of the air preheater is to recover the heat from the flue gas from the
boiler to improve boiler efficiency by burning warm air which increases
combustion efficiency, and reducing useful heat lost from the flue. As a
consequence, the gases are also sent to the chimney or stack at a lower
temperature, allowing simplified design of the ducting and stack. It also allows
control over the temperature of gases leaving the stack (to meet emissions
regulations, for example).After extracting heat flue gases are passed to electrostatic
precipitator.
Electrostatic precipitator: It is a device which removes dust or other finely
divided particles from flue gases by charging the particles inductively with an
electric field, then attracting them to highly charged collector plates, also known as
precipitator. The process depends on two steps. In the first step the suspension
passes through an electric discharge (corona discharge) area where ionization of
the gas occurs. The ions produced collide with the suspended particles and confer
on them an electric charge. The charged particles drift toward an electrode of
opposite sign and are deposited on the electrode where their electric charge is
neutralized. The phenomenon would be more correctly designated as
electrodeposition from the gas phase.
The use of electrostatic precipitators has become common in numerous industrial
applications. Among the advantages of the electrostatic precipitator are its ability
to handle large volumes of gas, at elevated temperatures if necessary, with a
reasonably small pressure drop, and the removal of particles in the micrometer
[xv]

17. range. Some of the usual applications are: (1) removal of dirt from flue gases in
steam plants; (2) cleaning of air to remove fungi and bacteria in establishments
producing antibiotics and other drugs, and in operating rooms; (3) cleaning of air
in ventilation and air conditioning systems; (4) removal of oil mists in machine
shops and acid mists in chemical process plants; (5) cleaning of blast furnace
gases; (6) recovery of valuable materials such as oxides of copper, lead, and tin;
and (7) separation of rutile from zirconium sand.
Smoke stack: A chimney is a system for venting hot flue gaseous smoke from a
boiler, stove, furnace or fireplace to the outside atmosphere. They are typically
almost vertical to ensure that the hot gases flow smoothly, drawing air into the
combustion through the chimney effect (also known as the stack effect). The space
inside a chimney is called a flue. Chimneys may be found in buildings, steam
locomotives and ships. In the US, the term smokestack (colloquially, stack) is also
used when referring to locomotive chimneys. The term funnel is generally used for
ship chimneys and sometimes used to refer to locomotive chimneys. Chimneys are
tall to increase their draw of air for combustion and to disperse pollutants in the
flue gases over a greater area so as to reduce the pollutant concentrations in
compliance with regulatory or other limits.
Generator: An alternator is an electromechanical device that converts mechanical
energy to alternating current electrical energy. Most alternators use a rotating
magnetic field. Different geometries - such as a linear alternator for use with
stirling engines - are also occasionally used. In principle, any AC generator can be
called an alternator, but usually the word refers to small rotating machines driven
by automotive and other internal combustion engines.
[xvi]

18. Transformers: It is a device that transfers electric energy from one alternating-
current circuit to one or more other circuits, either increasing (stepping up) or
reducing (stepping down) the voltage. Uses for transformers include reducing the
line voltage to operate low-voltage devices (doorbells or toy electric trains) and
raising the voltage from electric generators so that electric power can be
transmitted over long distances. Transformers act through electromagnetic
induction; current in the primary coil induces current in the secondary coil.
[xvii]

19. SUPERHEATER (LITERATURE)
A superheater is a device used to convert saturated steam or wet steam
into superheated steam used in steam engines or in processes, such as steam
reforming.
It is integral part of boiler and is placed in the path of hot flue gases from the
furnace. The heat recovered from the flue gases is used in superheating the steam
before entering into the turbine (i.e. prime mover).Its main purpose is to increase
the temperature of saturated steam without raising its pressure.
Most of the modern boilers are having superheater and reheater arrangement.
Superheater is a component of a steam-generating unit in which steam, after it has
left the boiler drum, is heated above its saturation temperature. The amount of
superheat added to the steam is influenced by the location, arrangement, and
amount of superheater surface installed, as well as the rating of the boiler. The
superheater may consist of one or more stages of tube banks arranged to effectively
transfer heat from the products of combustion. Superheaters are classified as
convection, radiant or combination of these.
5. SCHEMATIC DIAGRAM OF A SUPERHEATER [xviii]

20. Types of Superheaters
There are three types of superheaters namely: radiant, convection, and separately
fired. A superheater can vary in size from a few tens of feet to several hundred feet
(a few metres to some hundred metres).
 A radiant superheater is placed directly in the combustion chamber.
 A convection superheater is located in the path of the hot gases.
 A separately fired superheater, as its name implies, is totally separated from the
boiler.
0
6. TYPES OF SUPERHEATERS
[xix]

21. Radiant Type Superheater:
The radiant type of superheater receives its heat by radiation in the furnace area of
the boiler. An increase in load on a boiler increases the rate of steam flow through
the superheater tubes.
To maintain a constant superheater temperature the heat input to the superheater
must also increase.
Since radiant heat is proportional to the furnace temperature, and the furnace
temperature remains fairly constant with an increase in the number of fires or firing
rate the amount of heat entering the superheater per pound of steam flow will
decrease.
Therefore, with an increase in load with a radiant type superheater, the outlet steam
temperature decreases.
Convection Type Superheater:
The convection type superheater is located in the path of the combustion gas flow
and receives its heat from the convective flow of these hot combustion gases past
the tubes. With an increase in the load the rate of steam flow through the
superheater increases.
To support the load increase more fuel is burned and more air is used, increasing
the amount of combustion gases, and increasing the convective flow of heat to the
superheater.
This increase in the convection air flow is greater than the increase in steam flow,
hence the amount of heat entering the superheater per pound of steam increases.
Therefore, with the convection type superheater, an increase in load causes the
outlet temperature of the superheater to increase.
[xx]

22. Applications of Superheater:
Superheaters are used for:
Power plants
Steam engines
Locomotive use
Damper and shifting valve
Front-end throttle
Advantage and Disadvantage:
The main advantages of using a superheater are reduced fuel and water
consumption but there is a price to pay in increased maintenance costs. In most
cases the benefits outweighed the costs and superheaters were widely used. An
exception was shunting locomotives (switchers). British shunting locomotives
were rarely fitted with superheaters. In locomotives used for mineral traffic the
advantages seem to have been marginal. For example, the North Eastern
Railway fitted superheaters to some of its NER Class P mineral locomotives but
later began to remove them.
Without careful maintenance superheaters are prone to a particular type of
hazardous failure in the tube bursting at the U-shaped turns in the superheater tube.
This is difficult to both manufacture, and test when installed, and a rupture will
cause the superheated high-pressure steam to escape immediately into the large
flues, and then back to the fire and into the cab, to the extreme danger of the
locomotive crew.
[xxi]

23. Superheated steam increases the plant’s capacity since each pound of steam
contains higher energy content (BTU) per pound than saturated steam
Superheated steam reduces condensation in steam lines
Superheated steam reduces the engines steam consumption
Superheated steam eliminates erosion of turbine blading by insuring that only dry
steam enters the turbine
Superheated steam minimizes the possibility of carryover since the steam leaving
the dry pipe must pass through the superheater before entering the engine.
Superheated steam reduces the size of the boiler, turbine and connecting piping for
a given output.
Table 1. Materials Used For Superheater and their allowable temperatures:
[xxii]

24. Other Details:
Causes of Change in Superheater Outlet Temperature:
► Excess Air
► Change in Feed water Temperature
► Soot Accumulation
► Waterside Deposits Carryover
The common methods used for controlling the superheat temperature of the
steam are discussed below:
1. Bypassing the furnace gas around the superheater. At lower loads on the
power plant, the part of the gases is bypassed with the help of damper. Until
recently, this method of control was used successfully. But the troubles with
satisfactory materials to withstand erosion and high temperatures in the gas
passages have limited the use of damper method of control.
2. Tilting burners in the furnace. The temp of the steam coming out of
superheater is controlled by titling burners up or down through a range of 30°C.By
tilling the burner downward in a furnace much of the heat is given to the water
walls by the gas and the gas entering the superheater region is relatively cool. If the
burner is turned upward, then the heat given to the boiler water wall is less and
hotter gas enters the superheater region to increase the steam temperature.
3. Auxiliary burners. The temperature of the steam can be controlled by turning
the auxiliary burners in addition to main burners. The effect of this is similar to
tilting burners.
[xxiii]

25. 4. Desuperheater using water spray. The temperature of the steam can be
controlled by injecting the water either before the superheater or between sections
of a superheater.
5. Pre-condensing control. The temperature of the steam can he controlled by
condensing the steam coming out of boiler with a small condenser with the help of
feed water. Automatic control regulates the amount of feed water by-passed.
6. Gas recirculation. The gas coming out of economiser is partly recirculated into
the furnace with the help of a fan the recirculated gas acts like excess air and
blankets the furnace wall. This reduces the heat absorption by water wall and
increases the heat absorption by superheater.
7. Twin furnace arrangement The twin furnace arrangement is an extension of
the separately fired superheater. Varying the firing rates between furnaces controls
the superheat temperature.
Superheater Protection:
► Desuperheater (Auxiliary and Control)
► Superheater Safety Valve
► Superheater Vent
► Superheater Protection Steam
[xxiv]

26. DESIGN OF THE SUPERHEATER FOR 210 MW THERMAL
POWER PLANT
In order to understand the performance of any superheater, its overall heat transfer
co-efficient must be evaluated, and then based on the gas temperature and surface
area, its energy transfer may be estimated.
The total energy transferred superheater is given by:
Q=UA (∆T)
(∆T = LMTD)
If A is base on external surface area, then U is also based on external surface area.
Similarly for the inner diameter:
UoAo= UiAi
Uo and Ui overall heat transfer co-efficients based, on external and internal areas of
tube.
If external radiation Qr from a cavity, flame or furnace is received by the
superheater, then the equation is modified as:
Q – Qr= QC+Qh=UA (∆T)
Qc, Qn are energy transferred by convection and non-luminous radiations,
respectively
The energy given by hot fluid, namely, the gases is absorbed by the colder fluid,
say, steam in superheater.
Wh∆hh (1 – hl)=Wc (∆hc)=Q
Q is energy transferred (KW).
Qr is neglected when direct radiation from furnace or cavity is absent.
A=surface area (m2
)
W is flue gas flow (kg/s)
(Subscripts c and h stand for cold and hot fluids respectively)
[xxv]

27. ∆h =Cp∗ (temperature change)
Cp is specific heat (KJ/kg K), ∆h is change in enthalpy (KJ/kg)
hl = heat loss ranging from 0.1% to 1%
The convective heat transfer co-efficient depends on insulation thickness, ambient
temperature and wind velocity conditions. For plain or base tubes,U0may be
obtained as follows:
1/U0=d/hidi +ffi∗(d/di)+(d/2Km)ln(d/di)+ff0+1/h0
do,di are outer and inner diameters, Km is thermal conductivity of tube wall.
f fi, f fo are fouling factors inside and outside the tubes, respectively (m2
K/W)
hi and h0 are the tube inside and outside heat transfer co-efficients (W/m2
K)
h0, outside heat transfer co-efficient, consists of a connective part and a non-
luminous part hn.
h0=hc+hn
The gas co-efficient h0 governs U in the superheaters, the other term can be
neglected.
1/U0≈1/h0
Heat transfer co-efficient outside Plain Tubes:
Fisherden & Gaunder Correlation for hc for cross-flow of gases over plain tube
banks takes the following form:
Nµ=0.35FhRc
0.6
Px
0.3
Fh depends on tube geometry whether staggered or inline.
A conservative correlation for inline and staggered arrangements is as follows:
Nu=0.33Re
0.6
Pr
0.33
--------------(B)
[xxvi]

28. Where, Re=GD/µ
G is gas mass velocity,kg/m2
s
G=Wg/[Nw∗L∗(ST-d)]
Wg is flow over tubes (kg/s)
d is tube outer diameter (m)
Nw is no of tuber per row as tubes wide L is effective length of tube (m)
µ is viscosity of gas (kg/m.s or Pa.s)
Nµ= (hcd) /k
hc is convective heat transfer co-efficient (W/m2
k)
d is in m,
k is thermal conductivity of gas (W/m2
k)
Pr=µcp/k,
cp is specific heat (J/kgK)
ST and SL are transverse & longitudinal pitch respectively.
Note that all the thermal properties for heat transfer co-efficient for plain tubes are
estimated at the gas film temperature.
Substituting Nµ1, Re and Pr and simplifying we have:
Hc=0.33F(G0.6
/d0.4
)
Where, F= (k0.67
CP
0.33
)/µ0.27
Grimson correlation is widely used in boiler design practice:
Nu=BRN
[xxvii]

29. We know for 210MW power plant the gas temperature is 7590
C and tube wall
temperature is 5200
C.
Now, the average film temperature is 6400
C.
Let the tube be 11 rows deep.
Tube OD=51 mm
Transverse and longitudinal pitch = 102 mm
Effective tube length is 3.5 m, and there are 12 tubes/ row,
Now,
For 6000
C, specific heat (Cp) =1.26KJ/KgK
Viscosity (µ) = 0.0000362kg/ms
Thermal conductivity (K) =0.0443W/mK
Taking the mass flow rate of flue gases as 145 tonnes/hr (=40.2 kg/s)
G=40.2/{12∗ 305∗ (0.102-0.051)}=18.76 kg/m2
s
Re=Gd/µ= {(18.76∗0.051)/0.0000362}=26.440
Grimson co-efficient for inline arrangement:
ST/d=SL/d = 2
B= 0.229
N=0.632
Nµ=0.229∗(26440)0.632
=142.7 W/m2
k
[xxviii]

30. Now,
Nµ=0.35Re
0.6
Pr
0.3
Nu=0.35∗(26440)0.6
∗(0.894)0.3
Nµ=0.35∗(26440)0.6
∗(0.894)0.3
or, hc∗(0.051/0.0440) = 158.93
or, hc=138 w/m2 0
C
h0=hc+hn
h0=138 W/m2 0
C
Now,
1/U0≈ 1/h0
U0=138 W/m2 0
C
Design Analysis:
Heat transferred to steam:
Q=mc(∆t)
m=mass flow rate of fluid in kg/s
c= Specific heat of steam in kJ/Kg⁰C
Q=147.96∗1.996∗71
=20.96 MW
[xxix]
Assumption:
1. The properties remain constant under state conditions.
2. Neglect the surrounding losses.
3. Neglecting the kinetic and potential energy.

31. Heat loosing fluid:
Qc =mc (∆t)
=40.2∗ 1.289∗ (723.468)
=12.7 MW
We know, superheaters used in the steam power plants are cross-flow tube heat
exchangers.
ΔTm= (ΔTb - ΔTa) /ln (ΔTb/ΔTa), for counter-flow
= F (ΔTm) , for cross-flow
F Correction Factor
Now for primary superheater,
R = (T1 – T2) / (t2 – t1)
= (723 – 468) / (426 – 355) = 3.59
P = (t2 – t1) / (T1 – t1)
= (468 – 355) / (723 – 355) = 0.19 ≈ 0.2
Hence from the graph, F = 0.9 (For one shell pass)
Now,
ΔTa= T2– t1 = 113 ⁰C
ΔTb= T1 – t2= 297 ⁰C
[xxx]

32. (ΔTm)cross = F(ΔTm)counter
We get, ΔTm = 190.4 ⁰C
From heat transfer equation, we get the area of the superheater as follows:
Q = UA(ΔTm)
or, A = Q / {U∗ (ΔTm)}
= (12.7 ∗ 106
)/ (138 ∗ 190.4)
= 483.3 m2
……(1)
From average velocity in the tube and discharge, we calculate the total flow area.
m = Mass flow rate of steam = 147.96 kg/s
A = tube flow area
u = velocity of flow = 20 m/s
Now, from the steam table we get the specific volume at 150 bar and 550o
C,
v=.0229 m3
/kg
Hence, volume flow rate,Q=m*v=3.39m3
/s
[xxxi]

33. From continuity equation,
Q=A*u
=0.169 m2
A =
𝜫
4
𝒅²
or, d = 0.46 m
=46 cm
Let the number of tubes be, n=10
From equation ①, the area is 483.3 m2
.
Then the total surface area in 4 tube pass is given below:
𝟒𝐧𝚷𝐝𝐋 = 483.3
or, 4 ∗ 10 ∗ 3.14 ∗ 0.046 ∗ 𝐋 = 483.3
or, 𝐋 =
483.3
(4∗10∗3.14∗0.046)
= 83.65 m
Number of passes = 4
Number of tubes = 10
Hence, length of each tube=2.1 m
[xxxii]

34. CONCLUSION
Superheaters are the tube bundles that attain the highest temperatures in a boiler
and consequently require the greatest care in the design and operation. The
complex superheater tube arrangements permit the economic trade-off between
material unit costs and surface area required to obtain the prescribed steam outlet
temperature. Very often, various alloy steels are used for each pass in modern
boilers. High temperature heat exchangers, like steam superheaters, are difficult to
model since the tubes receive energy from the flue gas by two heat transfer modes:
convection and radiation. The division of superheaters into two types: convection
and radiant superheaters is based on the mode of heat transfer that is predominant.
Correct determination of the heat flux absorbed through the boiler heating surfaces
is very difficult. This results, on the one hand, from the complexity of heat transfer
by radiation of flue gas with a high content of solid ash particles, and on the other
hand, from the fouling of heating surfaces by slag and ash. The degree of the slag
and ash deposition is hard to assess, both at the design stage and during the boiler
operation. In consequence, the proper size of superheaters can be adjusted after
taking the boiler into operation. In cases when the temperature of superheated
steam at the exit from the superheater stage under examination is higher than
design value, then the area of the surface of this stage has to be decreased.
However, if the exit temperature of the steam is below the desired value, then the
surface area is increased. A numerical model of multipass steam superheater with
eight passes was developed. The convection and radiation heat transfer was
accounted for on the flue gas side. In addition, the deposit layer was assumed to
cover the outer surface of the tubes. The calculation results were compared with
the experimental data. The computed steam temperature increase over the entire
superheater corresponds very well with the measured steam temperature rise.
[xxxiii]

35. The developed modeling technique can especially be used for modeling tube heat
exchangers when detail information on the tube wall temperature distribution is
needed.
FUTURE SCOPE OF ACTION
In this semester we have designed a simple superheater without considering many
of its possibilities. In future we will use fins on the superheater tubes to increase
the heat transfer rate and different way of arrangement of the tubes in furnace to
get the maximum heat transfer possible. We will also take the help of CFD design
software to design a superheater that gives the maximum plant efficiency.
[xxxiv]

36. REFERENCES
1. Ganapathy V, Steam Generators & Waste Heat Boilers For process and
Plant Engineers, CRC Press.
2. John H Lienhard IV/John H Lienhard V Heat Transfer Textbook III Edition.
3. Heat and Mass Transfer Data book by C P Kothandaraman, S Subramanyan.
4. Fundamentals of Heat and Mass Transfer by R C Sachdeva, IV Edition, 2010.
5. Fundamentals of Heat and Mass Transfer by P K NAG, Tata McGraw Hill,
III Edition, 2009.
6. Thermal engineering by RK Rajput, Lakshmi publications, VIII Edition 2010.
[xxxv]