Lecture 6: Boiler, economizer, superheater, condenser, and Turbines

1. Introduction
Fossil-Fuel Steam Generators

2. • What is a Boiler?
• Difference between steam boiler and steam generator?
• Boilers are classified upto 16 different ways. e.g.
1) Tube Content 2) Type of Fuel 3) Tube shape & position
4) Manufacturer’s trade name 5) Mode of circulation of fluid
Boilers
• Focus on (1): Is fire-tube or water-tube type. Each has its own types.
• Classify the boiler in your steam lab.
• Boiler horse power (BHP): Was originally defined as 10 ft2 of boiler
heating surface. Now, it is heat required to evaporate 34.5 lbm/h of
saturated water to saturated steam at 1 atm. i.e. 33475.35 Btu/h.
Usually rounded off to 1 BHP = 33480 Btu/h.

3. Construction and Flow Diagram
Note: See diagram of
steam power plant in
J.B. Jones book.
Purpose of Economizer and air preheater?
Advantages of a high flue gas exit temperature?
Recall minimum stack temperature.
Feedwater from
feedwater heater
Flue gas to
stack
Induced-draft
fan
Forced-
draft
fan
Air preheater
Air

4. • Feedwater is fed from the economizer, saturated steam is separated from the
boiling water and remaining water is recirculated.
• Should be of sufficient volume.
• Most important function is separation of steam from boiling water.
• Simplest method is gravity separation but not good at high presssure.
• In high-pressure boilers, mechanical separation that assists gravity
separation takes place in 2 steps: primary & secondary (drying). In Primary
(P), most of the water is removed from steam & prevents carryunder. In
secondary (S) part, remaining fine droplets are removed. Accomplished by
baffles (P), screens (S), plates (P,S) and turbo separators (used at high
pressures).
The Steam Drum

5. Determine the boiler efficiency if the boiler generates 2000 kg/hr of
dry saturated steam at 2 MPa. The rate of coal used is 220 kg/hr and
the calorific value (CV) of coal is 30096 kJ/kg. The outlet temperature
of feedwater is 88 °C.
Solution:
Boiler Efficiency (Numerical)
kg
kJ
h
h
kg
kJ
h
h
C
f
fw
MPa
g
st
/
84
.
367
/
5
.
2799
88
@
2
@





%)
45
.
73
(
7345
.
0
30096
*
220
)
84
.
367
5
.
2799
(
2000
or





CV
m
h
h
m
Supplied
Heat
Load
Heat
fuel
fw
st
st
.
)
(

 




6. Introduction
Economizer

7. In order to utilize heat of waste flue gases before they are discharged, they are passed
through the heat recovery equipment such as economizer, which heats the feedwater.
Economizer
• Placement: Generally placed before air preheater. In some cases, a low-temperature
economizer (called a stack cooler) is placed after the air preheater.
• Types: There are 2 types: 1) Steaming 2) Non-Steaming
Steaming economizers raise the feedwater temperature to boiling point and
partially evaporate it (x < 0.25). On the other hand, in Non-Steaming
economizers, temperature of water is (20 – 30 °C) below the boiling point.
• Tubes: Cast iron or steel tubes. Plain or extended surface.
• Advantages: These are as follows:
(I) Reduces heat loss by flue gases and, thus, the consumption of fuel as well.
(II) Increases life of boiler.
(III) Improves the efficiency of the boiler installation. Boiler efficiency rises by
about 1% for each 10 °F rise in feedwater temperature.
• Efficiency: The ratio of the heat absorbed by the BFW in the economizer to the
heat supplied by the flue gases in the economizer; the temperature of the flue gases
being reckoned above the temperature of the air supplied to the boiler.

8. Efficiency in Economizer
In a steam generation unit, the inlet and outlet temperature of
feedwater is measured to be 18 °C and 88 °C, respectively.
Atmospheric temperature is 20 °C and the temperature of the entering
flue gas is 350 °C. Determine the economizer efficiency if the boiler
feedwater(fw) and flue gas(g) flow rate is 2,000 kg/hr and 4,150 kg/hr.
Solution:
%)
56
.
42
(
4256
.
0
)
20
350
(
*
005
.
1
*
4150
)
18
88
(
*
184
.
4
*
2000
or






Question: What is the combined efficiency of boiler and economizer?
)
(
.
)
(
.
,
,
,
,
,
atm
in
g
g
p
g
in
fw
out
fw
fw
p
fw
T
T
c
m
T
T
c
m






C
kg
kJ
c
C
kg
kJ
c o
fw
p
o
g
p .
/
184
.
4
;
.
/
005
.
1 ,
, 


9. Introduction
Superheater

10. • General: 1) Degree of superheat is the temperature imparted to steam above its
saturation temperature at a given pressure.
2) Final temperature of steam depends upon the flow rate & temperature of (flue) gas
leaving the super-heater section.
3) The (flue) gas temperature should be nearly 175 °C higher than the temperature of
super-heated steam.
4) In modern utility high pressure steam generators, more than 40% of the total heat
absorbed takes place in the superheater.
Superheater
• Tubes: Tubes range from 1-3 in. of diameter. Material used for tubes should have
high temperature strength & high resistance to oxidation. Special steel alloys such as
chromium molybdenum is used for the tubes. Extended surfaces may be added.
• Types: There are 3 types: 1) Convective 2) Radiant 3) Combined
• Advantages: ?

11. Condenser Efficiency
Condenser

12. Condenser Efficiency
Consider the condenser of steam power plant that is at 10 kPa. If the
inlet and outlet temperatures of cooling water to the condenser are 28
°C and 42 °C, respectively, calculate the condenser efficiency.
Solution:
%)
6
.
78
(
786
.
0
28
81
.
45
28
42
or






i
w
sat
C
i
w
o
w
T
T
T
T
,
,
,
,




C
T
C
T
C
T o
i
w
o
o
w
o
kPa
sat
C 28
;
42
;
81
.
45 ,
,
10
@
, 



 Rise in temperature of cooling water
Condenser saturation temperature – Inlet temperature of cooling water