Mr. E.
MANOJ KUMAR
Associate Professor
Department of Mechanical Engineering
Vardhaman College of Engineering
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SYLLABUS
UNIT – II
BOILERS: Classification, Working principles with sketches
including H.P. Boilers, Mountings and Accessories, Working
principles, efficiency and heat balance , Draught, classification,
artificial draught, induced and forced draught.
STEAM CONDENSERS: Requirements of steam condensing
plant, Classification of condensers, working principle, vacuum
efficiency and condenser efficiency, air leakage, sources and its
affects, air pump, cooling water requirement.
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Steam condenser is the one in which steam condenses by
transferring heat from steam to water.
Condenser is a type of heat exchanger which is used to
transfer the heat from steam to cooling water. The steam
condenses (steam converts into water) by transfering heat
from steam to cooling water.( Pr below the atm Pr)
Condensation is effected due to removal of latent heat from
exhaust steam by using circulating cooling water.
Latent heat:temperature constant, phase changes
Sensible heat:temp increases,no phase change
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Adventages of condenser
1) It will increase the work developed by turbine i.e., the
efficiency of steam power plant increases with the help of
the condenser.
2) The amount of pump work is decreased due to specific volume
of feed water is small compared with specific volume of steam.
3)The condensed steam is used as feed water for the boiler.
4) Hight pressure ratio provides larger enthalpy drop.
5)Work output per kg of steam increases.Thus specific steam
consumption decreases
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6)Condensate can be reused as hot feed water to the boiler.
As the feed water is hot, it reduces the time of evaporation
and hence fuel economy.
7) No feed water treatment is required. This reduces the
cost of the plant.
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Components of condensing plant.
1) condenser
2) condensate pump
3)hot well
4)cooling water pump
5)cooling tower
6)air pump
7)feed pump
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Absolute pressure = gauge pressure + atmospheric pressure
Absolute pressure = atmospheric pressure-vacuum pressure
1 atm pressure = 1.01325 bar=760mm of Hg
1.01325
1 mm of Hg = 0.001333bar
760
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Vacuum in the condenser
The vacuum in a condenser is produced by removing air and
condensate from the condenser.
The vacuum in a condenser is measured by means of vacuum gauge.
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Absolute pressure = Barometer reading (atm pres)
- vacuum gauge reading
or
vacuum in the condenser = barometer reading
- absolute pressure
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Corrected vacuum
Vacuum is referred to a standard barometer reading (760 mm Hg).
If the barometer reading not coincides with standard value, the
corrected vacuum with respect to standard barometer may be
calculated by using the relation.
corrected vacuum in mm of Hg =
standard barometer reading - absolute actual pressure in the
condensor in mm of Hg
=760 - (actual barometer reading - actual vacuum reading)
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Vacuum efficieny
It is defined as the ratio of actual vaccum in the condenser to
the ideal vacuum (max obtainable vacuum)
actual vacuum
vacuum
max vacuum
maximum vacuum in the condenser is obtained when there is
no air in the condenser
Barometer reading - Abs pressure in condenser
vacuum
Barometer reading - at condensate temp abs pressure of steam
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Absolute pressure at condensate temp is obtained by
using steam tables
p= pS pa
ps partial pres of steam
p a partial pres of air
p = Total pressure in the condenser
p bar p
vacuum
p bar ps
if there is no air, then ps p and vacuum is 100%
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Dalton's law
According to dalton's law, the total pressure of mixture is
sum of partial pressure of individual components.
In a mixture of gases, which do not interact chemically,
each gas behave as if the other were not present.
consider air and vapour contained in a condenser. Then
the total pressure exerted on the walls of condenser is
equal to the sum of the partial pressure of the air and
steam at the common temperature.
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p = p a + ps 1
where p= total pressure in the condenser
pa = partial pressure of air
ps = partial pressure of steam
air and steam occupies whole volume (space) of the condenser
in which they are contained.
V = Vs = Va
(i.e., specific volume of air = specific volume of steam)
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The mass of air present in the condenser is calculated from
characteristic gas equation
PV=mRT
i.e., Pa Va =ma RTa
Pa Va
mass of air ma 2 3
(Va in m )
RTa
Pa Va
mass of air/kg of steam ma 3 (Va in m3 / kg)
RTa
ma RTa
similarly volume of air Va 4
Pa
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Condenser efficiency
It is defined as the ratio of the difference between the outlet
and inlet temperatures of cooling water to the difference between
the temperature corresponding to the vacuum in the condenser
and inlet temperature of the cooling water.
rise in temperature of cooling water
condenser efficiency =
temp corresponding
to vacuum in the inlet temp of
cooling water
condenser
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rise in temperature of cooling water
temp corresponding to
the absolute pressure inlet temp of
cooling water
in the condenser
t 2 t1
cond
t s t1
where t s saturation temp of steam at condenser pressure
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P) The outlet and inlet temp of cooling water to a condenser are
37.50C and 300C respectively. If the vacuum in the condenser
is 706 mm of Hg with barometer reading 760mm of Hg.
Determine condenser efficiency.
Inlet temp of cooling water t1 300C
Outlet temp of feed water t 2 37.50C
vacuum pressure = 706mm of Hg
Barometer pressure (Atm pr)=760mm of Hg
t s is saturation temperature of steam at condenser pressure
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Absolute pressure in condenser = Atmospheric pressure
- Vacuum pressure
=760-706 = 54 mm of Hg
760 mm of Hg 1.013 bar
54 mm of Hg ?
54
Absolute pressure = 1.013 0.07 bar
760
from steam tables t s 39.03 C
0
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t 2 t1 actual rise in temp of cooling water
cond
t s t1 maximum rise in temp of cooling water
37.5 30
0.8306 83.6%
39.03 30
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P) In a surface condenser, the vacuum maintained is 700 mm
of Hg. The barometer reads 754 mm of Hg. If the temp of
condensate is 180C. Determine 1) Vacuum efficiency
2) Air present in the condenser per kg of steam.
vacuum pressure = 700 mm of Hg
Barometer pressure (Atm pr)=754mm of Hg
temp of condensate 180C
from steam tables at condensate temp 180C
ps is the absolute pressure at condensate temp =0.02 bar
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760 mm of Hg 1.013 bar
? 0.02 bar
0.02 760
= 15.004 mm of Hg
1.013
actual vacuum actual vacuum
1)vac
max vacuum barometer reading -
absolute pre at condensate temp
700
94.723%
754-15.004
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2)mass of air/kg of steam
pa va
ma
R a Ta
we know that p = pa + ps
pa = p - ps
ps =absolute pressure = 0.02 bar
p = absolute pressure = barometric pressure - vacuum pressure
= 754 - 700
=54 mm of Hg
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760 mm of Hg 1.013 bar
54 mm of Hg ?
54
Absolute pressure = 1.013 0.072 bar
760
Now pa = p - ps
=0.072 - 0.02
=0.052 bar
from steam tables at 180C va vg 65.112 m3 / kg
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pa va
ma
R a Ta
0.052 65.112 105
0.287 18 273 10 3
4.198 kg/kg of steam
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cooling water requirement
The amount of cooling water is calculated by using energy
balance equation
heat lost by steam = heat gained by cooling water 1
heat lost by steam = Latent heat of steam
+ sensible heat of water
= ms x.h fg ms Cp t s t c 2
heat gained by cooling water = mw Cp t 2 t1 3
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from 1, 2 & 3
ms x.h fg ms Cp t s t c mw Cp t 2 t1 4
equation 4 is used to calculate the required amount of
cooling water into the condenser and dryness fraction of
steam after expansion in turbine i,e., quality of steam at
inlet to condenser.
ms is mass of steam
x is dryness fraction of steam
h fg is latent heat of evaporation
C p is specific heat of vapour
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t s is saturation temp of steam
t c is condensate temp(feed water temp)
t 2 is outlet temp of cooling water
t1 is inlet temp of cooling water
m w is mass flow rate of cooling water
In jet condenser the temp of outlet of cooling water is equal to
temp of condenser. i.e., t 2 t c
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P) The following observations are recorded during test
on a steam condenser.
condenser vacuum=710 mm of Hg,
barometer reading = 765 mm of Hg,
mean condenser temperature = 340C,
temperature of hot well = 28.50 C,
condensate collected =1800 kg/hr,
mass flow rate of cooling water = 57,500 kg/hr,
inlet temp of cooling water = 8.50C,
outlet temperature of cooling water = 200 C,
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Calculate
a) vacuum corrected to the standard barometer reading
b) under cooling of condenser
c) quality of steam entering the condenser
d) mass of air per kg of steam
e) mass of air per m3 of condenser vacuum
f) vacuum efficiency
g) condenser efficiency
a) corrected vacuum in mm of Hg =
standard barometer reading - absolute actual pressure in the
condensor in mm of Hg
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=760 - (actual barometer reading - actual vacuum reading)
=760-(765-710) =705 mm of Hg
b) under cooling of condenser (degree of cooling of condensate)
= temp of condensate - Hot well temp
0
=34 - 28.5 = 5.5 C
c) quality of steam
energy balance eqn
ms x.h fg ms Cp t s t c mw Cp t 2 t1
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ms
x.h fg C p t s t c mw Cp t 2 t1
ts tc
ms x.h fg mw Cp t 2 t1
1800 x.h fg 57500 4.2 20 8.5 1
absolute pressure = barometric pressure - vacuum pressure
=765 - 710
=55 mm of Hg
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760 mm of Hg 1.013 bar
55 mm of Hg ?
55
Absolute pressure = 1.013 0.073 bar
760
at 0.07 bar hfg 2409.2 kJ/kg
from eqn 1
1800 x 2409.2 57500 4.2 20 8.5
x = 0.64 x<1 the steam is in wet condition
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d) mass of air per kg of steam
characteristic gas equation
PV=mRT
i.e., Pa Va =ma RTa
from Dalton's law
p = p a + ps
pa =p- ps
p= 0.073 bar
ps pressure at condensate temp i.e at 340C = 0.05319 bar
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pa = 0.073 - 0.05319 = 0.02 bar
Pa Va
mass of air/kg of steam ma
RTa
at 340C va vg 26.613 m3 / kg
0.02 26.613 10 5
ma
0.287 34 273 103
0.604 kg/kg of steam
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e) mass of air per m3 of condenser vacuum
Pa
ma ; Va 1 m3
RTa
0.02 10 5
0.287 34 273 103
0.023kg/m3
f) vacuum efficiency
actual vacuum
vacc
barometric reading-absolute pressure
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absolute pre at condensate temo 340 C
from steam tables 0.0532 bar
760 mm of Hg 1.013 bar
? mm of Hg 0.0532 bar
710
= =97.91%
765-39.91
t 2 -t1
g) condenser efficiency cond
t s -t1
20 8.5
45.09%
34 8.5
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Classification of Condensers
condensers are classified into two types
1) Jet condenser 2) Surface condenser
In jet condensers, the exhaust steam and water come in direct
contact with each other and temperature of the condensate is
the same as that of cooling water leaving the condenser. The
cooling water is usually sprayed into the exhaust steam to cause,
rapid condensation.
Jet condensers are may be classified as
1) Parallel flow
a) low level
b) high level
2) Counter flow
3) Ejector
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In surface condensers, the exhaust steam and water do not come
into direct contact. The steam passes over the outer surface of
the tubes through which a supply of cooling water is maintained.
There may be single pass or double pass. In single pass condensers,
the water flows in one direction only through all the tubes, while
in two pass condenser the water flows in one direction through the
tubes and returns through the remainder.
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Jet condensers
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Surface condensers
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Sources of air leakage into condenser
The main sources through which air enters into the
condensers are given below
1) Air leaks through various joints into the condenser
2) The air is supplied to the condenser along with the steam
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Effects of air leakage into the condenser
1) The work developed by turbine is decreased
2) The performance of the condenser decreases
3) The efficiency of plant decreases
4) Heat transfer from steam to cooling water is decreases
due to the formation of air filled over the surface of tubes.
5)It requires air pump to maintain desired vacuum in the
condenser
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Air Pump
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