Steam Power Cycle and Basics of Boiler

1. THERMAL POWER PLANT
by: Mulugeta T.
1

2. CHAPTER-2:
2. Analysis of Steam Cycles
2.1 Introduction
• A steam power plant continuously converts the energy
stored fossil fuels (coal, petroleum, and natural gas ) or
fissile fuels (uranium, thorium) OR other energy
resources in to shaft work and ultimately into
electricity.
• Steam power plants are commonly referred to as coal
plants, nuclear plants, or natural gas plants,
depending on the type of fuel used to supply heat to the
steam.
• The working fluid water which is some times liquid
phase and some times in vapor phase during its
performing thermodynamic cycle of operation. 2

3. …Important Definitions
• Saturation temperature: is the
temperature of a pure substance start
boiling at certain pressure, this pressure is
called saturation pressure.
• Saturated liquid: if a pure substance
exists as liquid at saturation temperature
and pressure, it is called saturated liquid.
• Wet mixture: is the mixture of liquid and
its vapor.
• Saturated vapor: if a pure substance
exists as vapor at saturated temperature
and pressure, it is called saturated vapor.
• Moisture content: is the ratio of liquid
mass to the total mass (mass of liquid and
mass of vapor). y=ml/(ml+mv)
• Dryness fracture (x): is a ratio of vapor
mass to the total mass. x=mv/(ml+mv)
y+x=1
3
s

4. …..
• Enthalpy of vaporization (hfg): or latent heat of vaporization:- It represent the amount
of energy needed to vaporize a unit mass of saturated liquid at a given temperature or
pressure. It decreases as the temperature or pressure increases, and it is becomes Zero at
the critical point.
• Super heated vapor: When the temperature of the vapor is higher than the saturated
temperature of this vapor is called super heated vapor.
• Degree of super heated: is the difference between the saturated temperature and super
heated temperature.
Degree = Tsup. - Tsat.
• Enthalpy of water (hf): is the enthalpy of heat absorbed by unit mass of water at
constant pressure until it reaches to the temperature of vapor forming from (0 oC).
hf = c (T - 0)
• T: temp. of vapor forming.
• c: specific heat of water (4.2 kJ/kg.k)
• Enthalpy of dry steam (hg): is the quantity of head which needed to change unit mass of
water at (0 oC) to dry steam.
hg = hf + (hfg)
• Enthalpy of wet steam (hx):
hx = (1 - x) hf + x hg
• This relation can also be expressed as
hx = hf + x hfg (WHERE : x is a dryness of vapor mixture)
4

5. …Basic Consideration in the Analysis of
Power Cycles
i. Actual Cycle
• The cycles encountered in actual devices are difficult
to analyze because of the presence of complicating
effects, such as friction and the absence of sufficient
time for establishment of the equilibrium conditions
during the cycle.
ii. Ideal Cycle
• When the actual cycle is stripped of all the internal
irreversibilities and complexities, we end up with a
cycle that resembles the actual cycle closely but is
made up totally of internally reversible processes.
Such a cycle is called an Ideal cycle.
5

6. …Steam Cycle
• Steam cycle thermal power plant are designed for
the purpose of converting primary energy
resources to work and their performance is
expressed as thermal efficiency.
ɳth=Wnet/Qin
• The Idealization and Simplification of steam cycle:
a) The cycle does not involve any friction.
b) All expansion and compression process take place in a
quasi-equilibrium manner.
c) The pipe connecting the various component of a
system are well insulated and heat transfer and
pressure drop through them are negligible.
6

7. 7
i. THE CARNOT VAPOR CYCLE
T-s diagram of two Carnot vapor cycles.
The Carnot cycle is the most efficient cycle operating between two specified temperature limits
but it is not a suitable model for power cycles. Because:
Process 1-2 Limiting the heat transfer processes to two-phase systems severely limits the
maximum temperature that can be used in the cycle (373.95°C and 22.06 Mpa for water)
Process 2-3 The turbine cannot handle steam with a high moisture content because of the
impingement of liquid droplets on the turbine blades causing erosion and wear.
Process 4-1 It is not practical to design a compressor that handles two phases.
The cycle in (b) is not suitable since it requires isentropic compression to extremely high
pressures and isothermal heat transfer at variable pressures.
1-2 isothermal heat addition in
a boiler (TH)
2-3 isentropic expansion in a
turbine
3-4 isothermal heat rejection in
a condenser (TL)
4-1 isentropic compression in a
compressor
Thermal efficiency of Carnot
cycle
ɳ= 1-TL/TH

8. 8
ii. RANKINE CYCLE: THE IDEAL CYCLE
FOR VAPOR POWER CYCLES
Many of the impracticalities associated with the Carnot cycle can be eliminated by
superheating the steam in the boiler and condensing it completely in the condenser.
The cycle that results is the Rankine cycle, which is the ideal cycle for vapor power
plants. The ideal Rankine cycle does not involve any internal irreversibilities.
The simple ideal Rankine cycle.

9. 9
Energy Analysis of the Ideal Rankine Cycle
Specific steam consumption (ssc)
is the steam flow in kg/h required
to develop 1 kW
ssc=3600/Wnet, Kg/Kjh
The thermal efficiency can be interpreted as the ratio
of the area enclosed by the cycle on a T-s diagram to
the area under the heat-addition process.
Steady-flow energy equation
Dryness fracture (x): is a ratio
of vapor mass to the total
mass. x=mv/(ml+mv)
x

10. 10
2.2 DEVIATION OF ACTUAL VAPOR POWER CYCLES
FROM IDEALIZED ONES
(a) Deviation of actual vapor power cycle from the ideal Rankine cycle.
(b) The effect of pump and turbine irreversibilities on the ideal Rankine cycle.
The actual vapor power cycle differs from the ideal Rankine cycle as a result of
irreversibilities in various components.
Fluid friction and heat loss to the surroundings are the two common sources of
irreversibilities. Isentropic efficiencies

11. Example-1: Ideal Rankine Cycle
Turbine
pump condenser
1
2 3
4
Qout
Qin
Wout
Win
boiler
Consider the Rankine power cycle as shown.
Steam enters the turbine as 100% saturated
vapor at 6 MPa and saturated liquid enters the
pump at a pressure of 0.01 MPa. If the net
power output of the cycle is 50 MW.
Determine:
(a) the thermal efficiency,
(b) the mass flow rate of the system,
(c) the rate of heat transfer into the boiler,
(d) the mass flow rate of the cooling water
from the condenser, in kg/s, if the cooling
water enters at 20°C and exits at 40°C.
T
s
1
2
3
4

12. Solution
• At the inlet of turbine, P3=6MPa, 100% saturated vapor x3=1, from saturated
table A-5, h3=hg=2784.3(kJ/kg), s3=sg=5.89(kJ/kg K)
• From 3-4, isentropic expansion: s3=s4=5.89 (kJ/kg K)
• From 4-1, isothermal process, T4=T1=45.8°C (why?)
From table A-5, when T=45.8°C, sf4=0.6491, sfg4=7.5019, hf4=191.8, hfg4=2392.8
x4 = (s4-sf4)/sfg4 = (5.89-0.6491)/7.5019 = 0.699
h4 = hf4+x4* hfg4 = 191.8+0.699(2392.8) = 1864.4 (kJ/kg)
• At the inlet of the pump: saturated liquid h1=hf1=191.8
qout = h4-h1=1672.6(kJ/kg)
• At the outlet of the pump: compressed liquid v2=v1=vf1=0.00101(m3/kg)
work input to pump Win = h2-h1 = v1 (P2-P1) = 0.00101(6000-10) = 6.05
h2 = h1 + v1 (P2-P1) =191.8 + 6.05 = 197.85 (kJ/kg)
• In the boiler, qin=h3-h2=2784.3-197.85=2586.5(kJ/kg)

13. Solution (cont.)
(a) The thermal efficiency h = 1-qout/qin= 1-1672.6/2586.5=0.353=35.3%
(b) Net work output (dW/dt) =50MW=(dm/dt)(Wout-Win)=(dm/dt)((h3-h4)-
(h2-h1))
mass flow rate (dm/dt)=50000/((2784.3- 1864.4 )-(197.85-
191.8))=54.7(kg/s)
( c) heat transfer into the boiler qin = (dm/dt)(h3-h2)=54.7(2586.5)
=141.5(MW)
(d) Inside the condenser, the cooling water is being heated from the heat
transfered from the condensing steam.
q cooling water = qout = (dm/dt)(h4-h1) = 54.7(1672.6) = 91.49 (MW)
(dm/dt)cooling water Cp (Tout - Tin) = q cooling water
C p, water = 4.177(kJ/kg K)
(dm/dt)cooling water = 91490/(4.177*(40-20)) = 1095.2 (kg/s)
Very large amount of cooling water is needed 

14. Example-2: Ideal Rankine Cycle
• A steam power plant operates between a boiler
pressure of 42 bar and a condenser pressure of
0.035 bar. Calculate for these limits the cycle
efficiency, the work ratio and the specific steam
consumption (ssc):
I. For Carnot cycle using wet steam
II. For Rankine cycle with dry saturated steam at entry to
the turbine.
III. For the Rankine cycle of (b) when the expansion
process has an isentropic efficiency of 80%.
14

15. 15

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19. 19

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21. 21
2.3 HOW CAN WE INCREASE THE EFFICIENCY OF THE RANKINE
CYCLE?
The effect of lowering the condenser pressure on the
ideal Rankine cycle.
The basic idea behind all the modifications to increase the thermal efficiency
of a power cycle is the same: Increase the average temperature at which heat is
transferred to the working fluid in the boiler, or decrease the average temperature at
which heat is rejected from the working fluid in the condenser.
i. Lowering the Condenser Pressure (Lowers Tlow,avg)
To take advantage of the increased
efficiencies at low pressures, the condensers
of steam power plants usually operate well
below the atmospheric pressure. There is a
lower limit to this pressure depending on
the temperature of the cooling medium
Side effect: Lowering the condenser
pressure increases the moisture content of
the steam at the final stages of the turbine.

22. 22
The effect of superheating the steam
to higher temperatures on the ideal
Rankine cycle.
ii. Superheating the Steam to High Temperatures (Increases
Thigh,avg)
Both the net work and heat input increase
as a result of superheating the steam to a
higher temperature. The overall effect is an
increase in thermal efficiency since the
average temperature at which heat is added
increases.
Superheating to higher temperatures
decreases the moisture content of the
steam at the turbine exit, which is desirable.
The temperature is limited by metallurgical
considerations.
Presently the highest steam temperature
allowed at the turbine inlet is about 620°C.

23. Example-3:Super heating steam
• Compare the Rankine cycle performance of Example-2 with
that obtained when the steam is superheated to 500 oC.
Neglect the feed pump work.
23

24. 24
• To calculate cooling load of water for condenser for
both examples by the law:
Ssc*(h2-h3)
i. Dry saturated steam
– Condenser heat load= 3.64(1808-112)=6175 (kJ/h)/kW
ii. with superheated steam
– Condenser heat load= 2.71(2113-112)= 5420 (kJ/h)/kW

25. 25
iii. Increasing the Boiler Pressure (Increases Thigh,avg)
The effect of increasing the boiler
pressure on the ideal Rankine cycle.
For a fixed turbine inlet temperature,
the cycle shifts to the left and the
moisture content of steam at the
turbine exit increases. This side effect
can be corrected by reheating the
steam.
A supercritical Rankine cycle.
Today many modern steam power
plants operate at supercritical
pressures (P > 22.06 MPa) and have
thermal efficiencies of about 40% for
fossil-fuel plants and 34% for nuclear
plants.

26. 26
iii. THE IDEAL REHEAT RANKINE CYCLE
How can we take advantage of the increased efficiencies at higher boiler pressures without
facing the problem of excessive moisture at the final stages of the turbine?
1. Superheat the steam to very high temperatures. It is limited metallurgically.
2. Expand the steam in the turbine in two stages, and reheat it in between (reheat)
The ideal reheat Rankine cycle.

27. Example-4: Ideal reheat Rankine cycle
• Calculate the ζR and S.S.C if reheat is included in the
plant of example-2: the steam conditions at inlet to the
turbine are 42 bar and 500 oC, and the condenser
pressure is 0.035 bar as before. Assuming that the
steam is just dry saturated on leaving the first turbine,
and is reheated to its initial temperature. Neglect the
feed pump term.
27

28. 28

29. Mollier Diagram (h-s diagram)
29

30. 30

31. Home Work:
1. Steam at a pressure of 15bar and 250 °C is expanded through a
turbine at first to a pressure of 4bar. It is then reheated at constant
pressure to the initial temperature of 250 °C and is finally
expanded to 0.1bar. Estimate the work done per kg of steam
flowing through the turbine and amount of heat supplied during
the process of reheat. Compare the work output when the
expansion is directed from 15bar to 0.1bar without any reheat.
Assume all expansion process to be isentropic.
2. Consider a steam power plant that operates on a reheat Rankine
cycle and has a net power output of 80 MW. Steam enters the high-
pressure turbine at 10 MPa and 500 °C and the low-pressure
turbine at 1 Mpa and 500°C. Steam leaves the condenser as a
saturated liquid at a pressure of 10 kPa. The isentropic efficiency
of the turbine is 80 percent, and that of the pump is 95 percent.
Show the cycle on a T-s diagram with respect to saturation lines,
and determine:
(a) the quality of the steam at the turbine exit,
(b) the thermal efficiency of the cycle, and
(c) the mass flow rate of the steam.
31

32. 32
The average temperature at which
heat is transferred during reheating
increases as the number of reheat
stages is increased.
The single reheat in a modern power plant
improves the cycle efficiency by 4 to 5% by
increasing the average temperature at which
heat is transferred to the steam.
The average temperature during the reheat
process can be increased by increasing the
number of expansion and reheat stages.
As the number of stages is increased, the
expansion and reheat processes approach
an isothermal process at the maximum
temperature. The use of more than two
reheat stages is not practical. The
theoretical improvement in efficiency from
the second reheat is about half of that
which results from a single reheat.
The reheat temperatures are very close or
equal to the turbine inlet temperature.
The optimum reheat pressure is about one-
fourth of the maximum cycle pressure.

33. 33
IV. THE IDEAL REGENERATIVE RANKINE CYCLE
The first part of the heat-addition
process in the boiler takes place at
relatively low temperatures.
Heat is transferred to the working fluid during
process 2-2 at a relatively low temperature.
This lowers the average heat-addition
temperature and thus the cycle efficiency.
In steam power plants, steam is extracted
from the turbine at various points. This steam,
which could have produced more work by
expanding further in the turbine, is used to
heat the feedwater instead. The device where
the feedwater is heated by regeneration is
called a regenerator, or a feedwater heater
(FWH).
A feedwater heater is basically a heat
exchanger where heat is transferred from the
steam to the feedwater either by mixing the
two fluid streams (open feedwater heaters) or
without mixing them (closed feedwater
heaters).

34. 34
a) Open Feedwater Heaters
The ideal regenerative Rankine cycle with an open feedwater heater.
An open (or direct-contact) feedwater
heater is basically a mixing chamber, where
the steam extracted from the turbine mixes
with the feedwater exiting the pump.
Ideally, the mixture leaves the heater as a
saturated liquid at the heater pressure.

35. If the Rankine cycle of Ex. 2 modified to include one feed
water heater, calculate the cycle efficiency and the s.s.c.
35
Example-4: Regenerative ideal Rankine cycle

36. 36

37. 37

38. 38
Closed Feedwater Heaters
The ideal regenerative Rankine cycle with a closed feedwater heater.
Another type of feedwater heater frequently used in steam power plants is
the closed feedwater heater, in which heat is transferred from the extracted
steam to the feedwater without any mixing taking place. The two streams now can
be at different pressures, since they do not mix.

39. 39
A steam power plant with one open
and three closed feedwater heaters.
The closed feedwater heaters are more complex because of the internal tubing network, and
thus they are more expensive. Heat transfer in closed feedwater heaters is less effective
since the two streams are not allowed to be in direct contact. However, closed feedwater
heaters do not require a separate pump for each heater since the extracted steam and the
feedwater can be at different pressures.
Open feedwater
heaters are simple
and inexpensive and
have good heat
transfer
characteristics. For
each heater,
however, a pump is
required to handle
the feedwater.
Most steam power
plants use a
combination of open
and closed
feedwater heaters.

40. Open vs. Closed Feedwater Heater
Open FWHs
• Open feedwater heaters are
simple and inexpensive.
• They have good heat
transfer characteristics.
• For each feedwater heater
used, additional feedwater
pump is required.
Closed FWHs
• The closed feedwater heaters are
more complex because of the
internal tubing network.
• Thus they are more expensive.
• Heat transfer in closed feedwater
heaters is less effective since the
two streams are not allowed to
be in direct contact.
• The closed feedwater heaters do
not require a separate pump for
each FWH since the extracted
steam and the feedwater can be
at different pressures.
40

41. 41
V. COMBINED GAS–VAPOR POWER CYCLES
• The continued quest for higher thermal efficiencies has resulted in rather innovative
modifications to conventional power plants.
• A popular modification involves a gas power cycle topping a vapor power cycle, which is
called the combined gas–vapor cycle, or just the combined cycle.
• The combined cycle of greatest interest is the gas-turbine (Brayton) cycle topping a
steam-turbine (Rankine) cycle, which has a higher thermal efficiency than either of the
cycles executed individually.
• It makes engineering sense to take advantage of the very desirable characteristics of the
gas-turbine cycle at high temperatures and to use the high-temperature exhaust gases
as the energy source for the bottoming cycle such as a steam power cycle. The result is a
combined gas–steam cycle.
• Recent developments in gas-turbine technology have made the combined gas–steam
cycle economically very attractive.
• The combined cycle increases the efficiency without increasing the initial cost greatly.
Consequently, many new power plants operate on combined cycles, and many more
existing steam- or gas-turbine plants are being converted to combined-cycle power
plants.
• Thermal efficiencies over 50% are reported.

42. 42
Combined gas–steam power plant.

43. 43
Summary
• The Carnot vapor cycle
• Rankine cycle: The ideal cycle for vapor power
cycles
– Energy analysis of the ideal Rankine cycle
• Deviation of actual vapor power cycles from
idealized ones
• How can we increase the efficiency of the Rankine
cycle?
– Lowering the condenser pressure (Lowers Tlow,avg)
– Superheating the steam to high temperatures (Increases Thigh,avg)
– Increasing the boiler pressure (Increases Thigh,avg)
• The ideal reheat Rankine cycle
• The ideal regenerative Rankine cycle
– Open feedwater heaters
– Closed feedwater heaters
• Second-law analysis of vapor power cycles
• Cogeneration
• Combined gas–vapor power cycles

44. Chapter-4
4. Steam Generator (Boilers)
Essentials of Steam Power Plant Equipment
• A steam power plant must have following
equipment :
(a) A furnace to burn the fuel.
(b) Steam generator or boiler containing water. Heat
generated in the furnace is utilized to convert water
into steam.
(c) Main power unit such as an engine or turbine to use
the heat energy of steam and perform mechanical
work.
(d) Piping system to convey steam and water.
44

45. Cont…
• The function of a steam generator or a boiler
is to convert water into steam at the desired
temperature and pressure to suit the turbine
which it serves.
• The basic components of steam generator are
furnace and fuel burning equipment, water
walls, boiler surface (drum and tubes), super
heater surface, air heater (air pre-heater)
surface, re-super heater surface, economizer
surface (feed water heating), and several
othe accessories. 45

46. A requirements boiler:
i. Safety : The boiler should be safe under operating
conditions.
ii. Accessibility : The various parts of the boiler should be
accessible for repair and maintenance.
iii. Capacity : The boiler should be capable of supplying steam
according to the requirements.
iv. Efficiency : To permit efficient operation, the boiler should
be able to absorb a maximum amount of heat produced
due to burning of fuel in the furnace.
v. It should be simple in construction and its maintenance
cost should be low.
vi. Its initial cost should be low.
vii. The boiler should have no joints exposed to flames.
viii. The boiler should be capable of quick starting and loading.46

47. 4.1 Types of Boilers
• Classification of boilers can be happen to according to several
methods.
A. According to fuel type:
• Coal
• Wood
• Waste material
• Oil fired
• Light or heavy fuel
• Gas fired
• Natural gas, LPG
B. According to fluid flow
• Natural circulation
• Forced circulation
• Once through
C. According to heat utilization
• Fire tube
• Water tube
47

48. • Water tube boiler: water circulates through tubes & hot flue gases
flow over them.
 Less liable to explosion, produce high pressure steam, high efficiency, heating surface is large
• Fire tube boiler: hot flue gases pass through the tubes which are
surrounded by water.
 Low cost, compact in size, heating surface is small, cannot produce high pressure steam, liable to
explode, low efficiency
a Water tube boiler b Fire tube boiler

49. Water tube boilers
• Water tube boilers are classified as follows :
• Horizontal Straight Tube Boilers
(a) Longitudinal drum
(b) Cross-drum.
• Bent Tube Boilers
(a) Two drum
(b) Three drum
(c) Low head three drum
(d) Four drum.
• Cyclone Fired Boilers
49

50. Cont…
• Various advantages of water tube boilers are as
follows:
(a) High pressure can be obtained.
(b) Heating surface is large. Therefore steam can be
generated easily.
(c) Large heating surface can be obtained by use of
large number of tubes.
(d) Because of high movement of water in the tubes
the rate of heat transfer becomes large resulting
into a greater efficiency.
50

51. Fire tube boilers
Fire tube boilers are classified as follows :
• External Furnace
(a) Horizontal return tubular
(b) Short fire box
(c) Compact.
• Internal Furnace
–Horizontal Tubular
(a) Short firebox
(b) Locomotive
(c) Compact
(d) Scotch. 51

52. Cont…
• Vertical Tubular
(a) Straight vertical shell, vertical tube
(b) Cochran (vertical shell) horizontal tube.
• Various advantages of fire tube boilers are as
follows :
(a) Low cost
(b) Fluctuations of steam demand can be met easily
(c) It is compact in size.
• According to position of furnace :
(a) Internally fired
(b) Externally fired
• In internally fired boilers the grate combustion chamber are
enclosed within the boiler shell whereas in case of extremely
fired boilers and furnace and grate are separated from the
boiler shell. 52

53. Cont…
According to the position of principle axis :
(a) Vertical
(b) (b) Horizontal
(c) Inclined.
• According to application :
(a) Stationary
(b) Mobile, (Marine, Locomotive).
• According to the circulating water :
(a) Natural circulation
(b) Forced circulation.
• According to steam pressure :
(a) Low pressure
(b) Medium pressure
(c) Higher pressure. 53

54. 4.2 Major Components and Their
Functions
i. Economizer
• The economizer is a feed water heater,
deriving heat from the flue gases.
• The justifiable cost of the economizer depends
on the total gain in efficiency.
• In turn this depends on the flue gas
temperature leaving the boiler and the feed
water inlet temperature.
54

55. 55
Coal-fired Power Plant Steam Generator Highlighting the Air Pre-heater
Location (Radiant Section Tubing is Not Shown)

56. ii. Air Pre-heater
• The flue gases coming out of the economizer is
used to preheat the air before supplying it to
the combustion chamber.
• An increase in air temperature of 20 degrees
can be achieved by this method.
• The pre heated air is used for combustion and
also to dry the crushed coal before pulverizing.
56

57. iii. Soot Blowers
• The fuel used in thermal power plants causes soot and this
is deposited on the boiler tubes, economizer tubes, air pre
heaters, etc.
• This drastically reduces the amount of heat transfer of the
heat exchangers.
• Soot blowers control the formation of soot and reduce its
corrosive effects.
• The types of soot blowers are fixed type, which may be
further classified into lane type and mass type depending
upon the type of spray and nozzle used.
• The other type of soot blower is the retractable soot
blower.
• The advantages are that they are placed far away from the
high temperature zone, they concentrate the cleaning
through a single large nozzle rather than many small
nozzles and there is no concern of nozzle arrangement with
respect to the boiler tubes. 57

58. iv. Superheater
• The superheater consists of a superheater
header and superheater elements.
• Steam from the main steam pipe arrives at the
saturated steam chamber of the superheater
header and is fed into the superheater
elements.
• Superheated steam arrives back at the
superheated steam chamber of the superheater
header and is fed into the steam pipe to the
cylinders.
• Superheated steam is more expansive.
58

59. v. Reheater
• The reheater functions similar to the
superheater in that it serves to elevate the
steam temperature.
• Primary steam is supplied to the high pressure
turbine.
• After passing through the high pressure
turbine, the steam is returned to the steam
generator for reheating (in a reheater) after
which it is sent to the low pressure turbine.
• A second reheat cycle may also be provided.
59

60. vi. Excess Air Control
• The steam outlet temperature
of a convection superheater
may be increased at partial
load by increasing the excess
air supply.
• The reduced gas temperature
decreases the furnace heat
absorption for the same steam
production.
• The increased gas mass flow
with its increased total heat
content serves to increase the
degree of superheat
60
Superheat control by increased
excess air

61. vii. Flue Gas Recirculation
• The recirculation of some
percentage of the combustion
gases serves to control steam
temperature in the same
manner as does an increase in
excess air.
• By introducing the hot gases
below the combustion zone,
relatively high efficiency may be
maintained.
61
Superheat Control by Flue Gas
Recirculation

62. viii. Gas By-pass Control
• The boiler convection banks
can be arranged in such a
manner that portion of the
gases can be by-passed around
the superheater elements.
• The superheater is oversized so
that it will produce the
required degree of superheat
at partial load conditions.
• As the load increases, some of
the flue gases are by-passed.
62
Superheat Control using Flue Gas By-
pass

63. ix. Control of Combination
Superheaters
• The control of combination radiant-convection
superheaters is relatively simple because of their
compensating characteristics.
• An increase in excess air reduces the radiant
heat transfer but increases the convection heat
transfer. The reduction in excess air has the
opposite effect.
• Thus the combination superheaters can be
operated over the entire control range without
additional equipment.
• Adjustable Burner Control
• With a multiple burner furnace it is possible to
distribute the burners over a considerable
burner wall height.
• This control is obtained by selective firing.
• Tiltable furnace may be adjusted to shift the
position of the combustion zone.
63
Superheat Control by Burner Tilt

64. x. Furnace
• Furnace should be designed so that in a given time, as
much of material as possible can be heated to a
uniform temperature as possible with the least
possible fuel and labour.
• To achieve this, the following parameters can be
considered.
– Determination of the quantity of heat to be imparted to
the material or charge.
– Liberation of sufficient heat within the furnace to heat the
stock and overcome all heat losses.
– Transfer of available part of that heat from the furnace
gases to the surface of the heating stock.
– Equalization of the temperature within the stock.
– Reduction of heat losses from the furnace to the minimum
possible extent.
64

65. Classification of Furnace
65

66. Pulverized Coal Systems .
• Pulverized coal firing is done by two systems :
(a) Unit System or Direct System.
(b) Bin or Central System
66
b) Bin or Central Systema) Unit or Direct System

67. 4.3 Draft System
• Draftt is defined as the difference between absolute gas
pressure at any point in a gas flow passage and the
ambient (same elevation) atmospheric pressure.
• What are the purpose of Draft.
(i) To supply required amount of air to the furnace for the combustion
of fuel. The amount of fuel can be burnt per square foot of grate
depends upon the quantity of air circulated through fuel bed.
(ii) To remove the gaseous products of combustion.
• Most boilers now depend on mechanical draft equipment
rather than natural draft.
• This is because natural draft is subject to outside air
conditions and temperature of flue gases leaving the
furnace, as well as the chimney height.
• All these factors make proper draft hard to attain and
therefore make mechanical draft equipment much more
economical.
• There are three types of mechanical draft : 67

68. Induced Draft
• This is obtained one of three ways, the first being the “stack
effect” of a heated chimney, in which the flue gas is less
dense than the ambient air surrounding the boiler.
• The denser column of ambient air forces combustion air
into and through the boiler.
• The second method is through use of a steam jet.
• The steam jet oriented in the direction of flue gas flow
induces flue gasses into the stack and allows for a greater
flue gas velocity increasing the overall draft in the furnace.
• This method was common on steam driven locomotives
which could not have tall chimneys.
• The third method is by simply using an induced draft fan
which removes flue gases from the furnace and forces the
exhaust gas up the stack.
• Almost all induced draft furnaces operate with a slightly
negative pressure.
68

69. Forced Draft
• Draft is obtained by forcing air into the furnace by
means of a fan and ductwork.
• Air is often passed through an air heater; which,
as the name suggests, heats the air going into the
furnace in order to increase the overall efficiency
of the boiler.
• Dampers are used to control the quantity of air
admitted to the furnace.
• Forced draft furnaces usually have a positive
pressure.
69

70. Balanced Draft
• Balanced draft is obtained through use of both
induced and forced draft. This is more
common with larger boilers where the flue
gases have to travel a long distance through
many boiler passes.
• The induced draft works in conjunction with
the forced draft fan allowing the furnace
pressure to be maintained slightly below
atmospheric.
70

71. 4.4 ENERGY PERFORMANCE ASSESSMENT
OF BOILERS
• Performance of the boiler, like efficiency and evaporation
ratio reduces with time, due to poor combustion, heat
transfer fouling and poor operation and maintenance.
• Deterioration of fuel quality and water quality also leads to
poor performance of boiler.
• Efficiency testing helps us to find out how far the boiler
efficiency drifts away from the best efficiency.
• Any observed abnormal deviations could therefore be
investigated to pinpoint the problem area for necessary
corrective action.
• Hence it is necessary to find out the current level of
efficiency for performance evaluation, which is a pre
requisite for energy conservation action in industry.
• Purpose of the Performance Test
• To find out the efficiency of the boiler
• To find out the Evaporation ratio 71

72. Efficiency
• In the boiler industry there are four common definitions of efficiency :
i. Combustion Efficiency
• Combustion efficiency is the effectiveness of the burner only and relates to its
ability to completely burn the fuel.
• The boiler has little bearing on combustion efficiency. A well-designed burner
will operate with as little as 15 to 20% excess air, while converting all
combustibles in the fuel to useful energy.
Ii. Thermal Efficiency
• Thermal efficiency is the effectiveness of the heat transfer in a boiler. It does not
take into account boiler radiation and convection losses.
iii. Boiler Efficiency
• The term boiler efficiency is often substituted for combustion or thermal
efficiency. True boiler efficiency is the measure of fuel to steam efficiency.
Efficiency= mw(h-hf)/C
Where C is the calorific value
iv. Fuel to Steam Efficiency
• Fuel to steam efficiency is calculated using either of the two methods as
prescribed by the ASME (American Society for Mechanical Engineers) power test
code, PTC 4.1. The first method is input output method. The second method is
heat loss method. 72

73. Performance Terms and Definitions
73

74. i. The Direct Method Testing
• This is also known as „input-output method‟ due to
the fact that it needs only the useful output (steam)
and the heat input (i.e. fuel) for evaluating the
efficiency.
• This efficiency can be evaluated using the formula :
74

75. 75
Direct Method Testing

76. Measurements Required for Direct Method Testing
Heat Input
• Both heat input and heat output must be measured.
• The measurement of heat input requires knowledge of the calorific
value of the fuel and its flow rate in terms of mass or volume, according
to the nature of the fuel.
For Gaseous Fuel
• A gas meter of the approved type can be used and the measured
volume should be corrected for temperature and pressure.
• A sample of gas can be collected for calorific value determination, but it
is usually acceptable to use the calorific value declared by the gas
suppliers.
For Liquid Fuel
• The meter, which is usually installed on the combustion appliance,
should be regarded as a rough indicator only and, for test purposes, a
meter calibrated for the particular oil is to be used and over a realistic
range of temperature should be installed.
• Even better is the use of an accurately calibrated day tank.
76

77. For Solid Fuel
• The accurate measurement of the flow of coal or
other solid fuel is very difficult.
• The measurement must be based on mass, which
means that bulky apparatus must be set up on the
boiler-house floor.
• Samples must be taken and bagged throughout
the test, the bags sealed and sent to a laboratory
for analysis and calorific value determination.
• In some more recent boiler houses, the problem
has been alleviated by mounting the hoppers over
the boilers on calibrated load cells, but these are
yet uncommon.
77

78. Heat Output
• There are several methods, which can be used for
measuring heat output.
• With steam boilers, an installed steam meter can be
used to measure flow rate, but this must be corrected
for temperature and pressure.
• In earlier years, this approach was not favored due to
the change in accuracy of orifice or venture meters with
flow rate. It is now more viable with modern flow
meters of the variable-orifice or vortex-shedding types.
• The alternative with small boilers is to measure feed
water, and this can be done by previously calibrating the
feed tank and noting down the levels of water during
the beginning and end of the trial.
• Care should be taken not to pump water during this
period. Heat addition for conversion 78

79. ii. The Indirect Method Testing
• The efficiency can be measured easily by measuring all the losses
occurring in the boilers using the principles to be described. The
disadvantages of the direct method can be overcome by this method,
which calculates the various heat losses associated with boiler.
• The efficiency can be arrived at, by subtracting the heat loss fractions
from 100.
• An important advantage of this method is that the errors in
measurement do not make significant change in efficiency.
• Thus if boiler efficiency is 90%, an error of 1% in direct method will
result in significant change in efficiency, i.e. 90+0.9 = 89.1 to 90.9.
• In indirect method, 1% error in measurement of losses will result in
• Efficiency = 100 – (10 +0.1) = 90+0.1 = 89.9 to 90.1
• The various heat losses occurring in the boiler are
• Efficiency = 100 – (1 + 2 + 3 + 4 + 5 + 6 + 7 + 8) (by indirect method)
79

80. Cont…
• The following losses are applicable to
liquid, gas and solid fired boiler :
– L1 – Loss due to dry flue gas (sensible heat)
– L2 – Loss due to hydrogen in fuel (H2)
– L3 – Loss due to moisture in fuel (H2O)
– L4 – Loss due to moisture in air (H2O)
– L5 – Loss due to carbon monoxide (CO)
– L6 – Loss due to surface radiation, convection
and other unaccounted*.
• Losses which are insignificant and are
difficult to measure.
• The following losses are applicable to solid
fuel fired boiler in addition to above :
– L7 – Unburnt losses in fly ash (Carbon)
– L8 – Unburnt losses in bottom ash (Carbon)
• Boiler Efficiency by indirect method
ɳb=100 – (L1 + L2 + L3 + L4 + L5 + L6 + L7 + L8)
80
Indirect Method Testing

81. Energy Balance
• Having established the
magnitude of all the
losses mentioned above,
a simple energy balance
would give the efficiency
of the boiler.
• The efficiency is the
difference between the
energy input to the boiler
and the heat losses
calculated.
81

82. 82