3. RANKINE CYCLE
The Rankine cycle used in modern power plant has many more
components, but the four components are common to all power plants.
In this cycle, water is heated in the steam generator to produce high
temperature and pressure steam.
This steam is then expanded in a turbine to produce electricity from a
generator that is connected to the turbine.
The steam from turbine is then condensed back into water in the
The pump then returns the water to the steam generator.
5. INTRODUCTION TO CONDENSER
In systems involving heat transfer, a condenser is a device or unit used
to condense a substance from its gaseous to its liquid state, by cooling
it. In so doing, the latent heat is given up by the substance, and will
transfer to the condenser coolant.
Condensers are typically heat exchangers which have various designs
and come in many sizes ranging from rather small (hand-held) to very
large industrial scale units used in plant processes.
6. A surface condenser is an example of such a heat-exchange system. It
is a shell and tube heat exchanger installed at the outlet of every
steam turbine in thermal power stations.
Commonly, the cooling water flows through the tube side and the
steam enters the shell side where the condensation occurs on the
outside of the heat transfer tubes. The condensate drips down and
collects at the bottom, often in a built-in pan called a hotwell.
The shell side often operates at vacuum or partial vacuum.
7. SOURCES OF AIR IN CONDENSER
Following are the chief sources of air found in condensers :
Air leaks in condenser from atmosphere at the joints of the parts
which are internally under a pressure less than that of atmosphere- The
amount of air leaking in, mainly depends upon the accurate
workmanship and can, with care in the design and making of the-
vacuum joints, be reduced to a very small quantity.
Air also comes in with the steam from the boiler into which it enters
dissolved in feed water. The amount of air coming in depends upon the
treatment the feed water receives before it enters the boiler. The air
entering through this source is relatively small.
In case of jet condensers, some air comes in with the injection water
(cooling water) in which it is dissolved.
8. In the surface condensers of well designed and properly maintained
steam turbine plants, the amount of air entering condensers is about 5
kg per 10,000 kg of steam. With reciprocating steam engines, the air
entering is about 15 kg per 10,000 kg of steam.
In case of jet condensers the amount of air dissolved in injection water
is about 0-5 kg per 10,000 kg of water.
10. How to check the air leakage in condenser?
In order to check the following procedure is adopted:
1.Keep the plant running until the temperature and pressure
condition are steady in the condenser.
2.The steam condenser be isolated by shutting off steam and
simultaneously closing the condensate and air extraction pumps.
In case there is a leakage the readings of vacuum gauge and
thermometer will record a fall.
11. Following method is used to check the source
of air leakage:
1. Put the effect of steam condenser under the air pressure
and note its effect on soap water at the points where
infiltration is likely occur.
2.Put the pepprament oil on the suspected joints (when the
condenser is operating) and make a check on the
pepprament odour in the discharge of air ejector.
3.Large leakage in steam condenser under the vacuum can be
detected by passing candle flame over possible openings.
12. Effect of air leakage:
The effect of air leakage in a condenser are given below:
(1)It increase the back pressure on the turbine with the effect of
steam at with the effect that there is less enthalpy drop and low
thermal efficiency of the plant.
(2)The pressure of air in the condenser lowers the partial
pressure of steam will condense at a lower temperature and that
will require greater amount of cooling water.
(3) It reduces the rate of condensation of steam, because air
having poor thermal conductivity reduces the overall heat
transfer from the steam air mixture.
(4)The pressure of air in the condenser increase the corrosive
14. What is ‘PARTIAL PRESSURE’ ?
Partial Pressure is define as
“ It is the pressure which each
constituent of a gas mixture would exert if it alone
occupied the volume of the mixture at the same
15. Statement of Dalton’s partial pressure
In Thermodynamic Dalton’s partial pressure law is
“ The mixture of non-reacting gases,
the total pressure exerted is equal to the sum of
partial pressure of individual gases”
16. This empirical law was observed by ‘John Dalton’ in 1801.
This law is related for ideal gas laws.
As shown in figure the gases ‘a’ and ‘b’, originally
occupying volume V at temperature T are mixed in the third
vessel which is of the same volume and same temperature.
18. IN POWER PLANT ENGINEERING
Condensers contains air and steam mixture.
So, The statement is
“The pressure of the air and steam is equal to
the sum of the pressures which constituent would exert, if it
occupied the same space by self”
19. It means,
pc = pressure in condenser,
pa = partial pressure of air,
ps = partial pressure of steam.
20. Measurement of vacuum
•The vacuum in a condenser is usually expressed in millimeters of
mercury and it is the difference between the barometric pressure (or
barometric height) and absolute pressure in condenser. In order to know
the absolute pressure in the condenser, both the vacuum gauge and
barometer must be read. The difference between the barometer
and vacuum gauge readings will give the absolute pressure in the
• Barometric pressure is a variable quantity and varies from place
to place. Hence, it is more convenient for the purpose of comparison to
refer vacuum gauge readings to a standard barometer of 760 mm of
mercury (or 1.01325bar).
21. Standard or corrected vacuum in mm of Hg.= 760 mm of mercury -
absolute pressure in condenser in mm of Hg.
= 760 mm of Hg –
[Barometer reading in mm of Hg- Vacuum gauge reading in mm of
Since one standard atmosphere = 760 mm of Hg
=1.01325 bar (101.325 kPa),
1 bar = 760 / 1.01325 = 750 mm of Hg
So, Pressure equivalent of 1 mm of Hg = 1.01325/760
= 0.001333 bar or = 0.1333kPa.
22. Vacuum Efficiency
•In a steam condenser we have a mixture of steam and air, and the total
pressure which exists in the condenser is the sum of the partial pressures
exerted by the steam and air. With no air present in the condenser, the
total absolute pressure in the condenser would be equal to partial
pressure of steam corresponding to the temperature of condenser,
and maximum vacuum would be obtained in the condenser. The ratio of
the actual vacuum obtained at the steam inlet to the condenser, to this
maximum vacuum (or Ideal vacuum) which could be obtained in a
perfect condensing plant (with no air present) is called the
23. •Vacuum efficiency = Actual vacuum at the steam inlet to the condenser
[Barometric pressure -Absolute pressure
correspondending to the temperature of Condensation]
•[Barometric pressure] - Absolute pressure in the condenser
[Barometric pressure] - Absolute pressure corresponding to temperature
24. •If the absolute pressure of steam corresponding to the temperature of
condensation were equal to the absolute pressure in the condenser, the
vacuum efficiency would be 100%. In fact, there will always be some air
present in the condenser due to leakage and dissolved air present in the
steam entering the condenser. The value of vacuum efficiency, therefore,
depends upon the quantity of air removed from the condenser by
the air pump.
25. example : Steam enters a condenser at 32.88°C and with barometer
standing at 760 mm of Hg, a vacuum of 685 mm of Hg was
produced. Determine the vacuum efficiency.
From steam (Pressure) tables, at 32.88°C, partial pressure of steam
= 0.05 bar = 0.05 x 750 = 37.5 mm of Hg.
Vacuum efficiency =Actual vacuum/[Barometric Pressure-Absolute
pressure corresponding to temperature of Condensation]
=685/ [760-(0.05 X 750)]
= 685/722.5= 0.9481 or 94.81 %
or Vacuum efficiency =Actual vacuum/ vacuum corresponding to
saturation temperature of condensate
=Actual vacuum/ Ideal vacuum
=685/760 – 37.5
= 0.9481 or 94.81 %(same as before).
26. Condenser Efficiency:
The condenser efficiency is defined as the ratio of actual rise in tempe
to the maximum possible rise in temperature of cooling water.
Actual rise in temperature of cooling water
Inlet temperature of steam −Inlet temperature of cooling water
To − Ti
Ts − Ti
where , To = outlet temperature of cooling water,
Ti = Inlet temperature of cooling water ,
Ts = saturation temperature of steam corresponding to actual abso