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Steam Condenser

  • Al Falah University India


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Steam Condenser
Dr Mohd Parvez
Al Falah University Faridabad
Steam condensers are devices in which the exhaust steam from the steam turbine is condensed by
means of cooling water. Condensation can be done by removing heat from exhaust steam using
circulating cooling water. During condensation, the working substance (steam) changes its phase
from vapour to liquid and rejects latent heat as shown in Figure 1.
The primary object of a condenser is to maintain a low pressure on the exhaust side of the rotor
of steam turbine. This enables the steam to expand to a greater extent which results in an increase
in available energy for conversation into mechanical work. The secondary object of condenser is
to supply to the boiler pure and hot feed water, as the condensed steam which is discharged from
the condenser and collect in a hot well can be used over again as feed water for the boiler.
The use of a condenser in a power plant is to improve the efficiency of the power plant by
decreasing the exhaust pressure of the steam below atmospheric pressure. Another advantage of
the condenser is that the steam condensed may be recovered to provide a source of pure feed
water to the boiler and reduce the water softening capacity to a considerable extent.
Advantages of a condenser in a steam power plant
The main advantages of incorporating a steam condenser in a steam power plant are as follows:
It increases the efficiency of the power plant due to increased enthalpy drop.
It reduces back pressure of the steam which results in more work output.
It reduces temperature of the exhaust steam which also results in more work output.
The condensed steam can be reused as feed water for boiler which reduces the cost of
power generation.
The temperature of the condensate is higher than that of the fresh water which reduces
the heat supplied per Kg of steam produced.
Function of condenser
The main function of condenser is to convert gaseous form of exhaust steam into liquid form at a
pressure of below atmosphere. Cooling medium is used water to convert steam into water.
Others important functions of condensers:
Function of the condenser is to create a vacuum by condensing steam
Remove dissolved non - condensable gases from the condensate.
Providing a leak tight barrier between the high grade condensate contained within the
shell and the untreated cooling water.
Providing leak tight barrier against air ingress, preventing excess back pressure on the
Elements of a steam condensing plant
The main elements of a steam condensing plants are:
A condenser in which the exhaust steam is condensed
Supply of cooling water for condensing exhaust steam
A pump to circulate the cooling water in case of a surface condenser
A pump called the wet air pump to remove the condensed steam (condensate) the air, and
uncondensed water vapour and gases from the condenser (separate pump may be used to
remove air and condensed steam)
A hot well where the condensed steam can be discharged and from which the boiler feed
water is taken
An arrangement (cooling pond or cooling tower) for cooling the circulation water when
a surface condenser is used and the supply of water is limited
Types of condensers
The steam condensers are classified as follows:
1. Jet condensers (mixing type condensers)
a. Parallel flow jet condenser
b. Counter flow jet condenser (low level)
c. Barometric or high level jet condenser
d. Ejector condenser
2. Surface condensers (non mixing type condensers)
a. Down flow surface condenser
b. Central flow surface condenser
c. Regenerative surface condenser
d. Evaporative condenser
Parallel flow jet condenser
In parallel flow jet condenser both the steam and the water enters from the top and flows in the
same direction as shown in Figure 1. The exhaust steam is condensed when it mixes up with
water. The condensate and the cooling water are delivered to the hot well from where surplus
water flows to the cooling pond through an overflow pipe. Sometimes a single pump know as
wet air pump is used to remove both air and the condensate but generally separate air pump is
used to remove air as it gives a great vacuum.
Counter flow or low level jet condenser
In counter flow or low level jet condenser, the exhaust steam enters from bottom and mixes with
the down coming cooling water as shown in Figure 2. The air pump mounted at the top of the
condenser shell creates vacuum as it suck air. This draws the supply of cooling water which falls
from a large number of jets through perforated conical plate.
The water then falls in the trays and flows through second series of jets and mixes with the
exhaust steam entering at the bottom. This cause rapid condensation after which the condensate
and the cooling water are delivered to the hot well by the condensate extraction pump.
Barometric or high level jet condenser
This type of condenser is provided at a high level as shown in Figure 3 having a long tail pipe.
The exhaust steam enters from the bottom and flows upwards. This steam then mixes with
cooling water which falls from the top through various baffles. The vacuum is created by the air
pump placed at the top of the condenser shell. The condensate and the cooling water flows
downwards through a vertical tail pipe to the hot well without the aid of any pump. The surplus
water from the hot well flows to the cooling pond through an overflow pipe.
Ejector condenser
In ejector condenser, the steam and water mix-up while passing through a series of metal cones
as shown in Figure 4. Water enters from the top through a number of guide cones. The exhaust
steam enters the condenser through a non return valve. The steam and air then pass through the
hollow truncated cones. After that it passes through the diverging cone where its kinetic energy
is partly transformed into pressure energy. The condensate and the cooling water are then
discharged to the hot well. The high exit pressure in the diverging cone allow discharged of
water automatically into the hot well at atmospheric pressure.
Surface condenser
In surface condenser, the condensate does not mix up with the cooling water. So the whole
condensate can be reused in the boiler. This type of condenser is used where is only limited
quantity of fresh water is available like ships. A sectional view of a two pass surface condenser is
shown in Figure 5. It consists of a horizontal cylindrical vessel made of cast iron packed with
tubes for cooling water. The cooling water flows in one direction through the lower half of the
tubes and in opposite direction through the upper half. The water tubes are fixed into vertical
perforated type plates at the ends so that leakage of water should not occur into the central
condensing space. The steam enters from top end. The extraction pump at the bottom sucks the
condensate resulting in the downwards flow of steam over the water tubes.
Down flow surface condenser
In down flow surface condenser, the steam enters from the top as shown in Figure 6. The exhaust
steam is forced to flow downwards over the water tubes due to suction of the extraction pump at
the bottom. The suction pipe of the dry air pump is provided near the bottom and is covered by a
baffle so that the condensed steam does not enter into it. As the steam flow perpendicular to the
direction of flow of cooling water, it is also called cross flow surface condenser.
Central flow surface condenser
In this type of surface condenser the suction pipe of the air extraction pump is placed in the
center of the tube nest as shown in Figure 7. The exhaust steam from turbine enters from the top
and flows radially inwards over the tubes. The condensate is collected at the bottom.
The advantage of central flow type surface condenser over the down flow type is that the steam
flows over the whole periphery of the water tubes as the steam flows radially inwards.
Evaporative condenser
In evaporative condenser the steam flows enters the gilled pipes and flows backwards and
forwards in a vertical plane as shown in Figure 8. The water pump sprays water on the pipes
which condenses the steam. The main advantage of this type of condenser is that the quantity of
cooling water needed to condense the steam can be reduced by causing the circulating water to
evaporate which decrease the temperature. The remaining water is collected in the cooling pond.
Regenerative surface condenser
In this type of condenser the condensate after passing the tube nest is heated using regenerator
which is located inside the exhaust steam which raises the temperature of the condensate. This
increases the efficiency of the plant as water in the boiler requires less heat input.
Jet condenser Surface condenser
1. Cooling water and steam are mixed up Cooling water and steam do not mix
2. It requires less quantity of circulating water
It requires large quantity of cooling water
3. It is economical and simple It is costly and complicated
4. The condensate cannot be used as feed
water in the boiler unless the cooling water
is free from impurities
Condensate can be reused as feed water as it
does not mix with the cooling water
5. More power is required for air pump Less power is required for air pump
6. Condensate is wasted Condensate is reused
7. Its maintenance cost is low Its maintenance cost is high
8. Manufacturing cost is low Manufacturing cost is high
9. Less suitable for high capacity plants due
to low vacuum efficiency
Suitable for high capacity plants as vacuum
efficiency is higher
Condenser vacuum and air leakage
The pressure inside the condenser is less than the atmospheric as condensation of steam reduces
its volume. The pressure below the atmospheric is called vacuum. In order to obtain the
maximum work from unit mass of steam, the pressure should be as low as possible. Because of
low pressure inside the condenser, the air infiltrates into the system so the condenser is always
filled with a mixture of water, steam and air.
The main sources of air leakage in condenser:
Leakage through joints and packing’s
Air with steam which is dissolved in feed water
In jet condenser, the air enters with injection water
Effects of air leakage in condenser:
The presence of air in the condenser
Decreases the thermal efficiency of steam power plant
Increases the requirement of cooling water
Reduces the heat transfer as it has poor thermal conductivity
Increases the possibility of corrosion
Due to these problems, the life of condenser is reduced
Vacuum efficiency
The vacuum efficiency is a measure of the degree of perfection in achieving the desired vacuum
in the condenser.
The vacuum efficiency may be defined as the ratio of actual vacuum as recorded by the vacuum
gauge to the ideal vacuum i.e. the vacuum obtained when there is no leakage of air.
 = 
 =
 − 
Condenser efficiency
The ideal condenser should remove only the latent heat so that the temperature of condensate is
equal to the saturation temperature corresponding to the condenser pressure. It means that these
should be no understanding of the condensate.
The maximum temperature of the outgoing cooling water is the condensate temperature ideally
but it is less than practically. So the condenser efficiency is defined as the ratio of actual rise in
temperature of cooling water to the maximum rise in temperature.
 = − 
− 
t1 = inlet temperature of cooling water
t2 = outlet temperature of cooling water
t3 = saturation temperature corresponding to condenser pressure
Latent heat: The heat required to convert a solid, into a liquid or vapour, without change of
Saturation temperature means boiling point of water
Cooling water supply
For the condensation of steam into condensate to be used as feed water to the boiler, cooling
water has to be supplied to the condenser. If there is abundant source of cooling water like river
or pond, there is no problem. But if the supply of water is limited with high cons, it is necessary
to use cooling tower for water cooled condensers. The method commonly used to overcome a
condition of limited cooling water supply is to repeatedly cool and circulate it through as shown
in Figure 9.
Figure 9
Cooling pond
The simplest system of removing heat from the cooling water consists of cooling it in an open
pond. The effectiveness of this method depends upon a very long surface area of the pond and
hence it is used mostly for small condenser only. In this system sufficient amount of water is lost
by evaporation and windage. The factors which affect the rate of heat dissipation from cooling
pond are area and depth of pond, temperature of water entering the pond and wind velocity,
atmospheric temperature, shape and size of water spray nozzle and relative humidity. Cooling
ponds are of two types namely:
1. Non directed flow type
2. Direct flow type
Open or once
Water cooled Air cooled
Direct dry
Spray pond
For a given cooling capacity, the size of pond in this case is much less than that of open pond.
Hot water coming out from power plant is sprayed into the atmosphere, through many nozzles.
Tiny particles of sprayed water loose the sensible heat to air and get cooled and are finally
collected into a reservoir from which cold water is supplied to the power plant for reuse. In this
case cooling achieved is more effective.
Cooling towers
The place where acquisition of land is very expensive, we may use cooling tower for cooling
purposes. A cooling tower requires smaller area than a spray pond. It is an artificial device used
to cool the hot cooling water coming out of condenser effectively. The cooling tower is a semi
enclosed device made of steel or concrete structure and corrugated surfaces or trough or baffles
are provided inside the tower for uniform distribution and better atomization of water in the
tower. The hot water coming out from the condenser falls down in radial sprays from height and
the atmospheric air enters from the base of the tower. The partial evaporation of water takes
place which reduces the temperature of circulating water.
This cooled water is collected in the pond at the base of the tower and pumped into the
condenser. Draft eliminators are provided at the top of the tower to prevent the escaping of water
particles with air.
According to the method of air circulation, cooling towers are classified as:
1. Natural draught type cooling tower
2. Mechanical draught type cooling tower
a. Forced draught type
b. Induced draught type
3. Hyperbolic draught cooling tower
Natural draught type cooling tower
In this hot water from condenser is pumped at the top where water sprays through a series of
spray nozzles. Then waterfalls over decks, the decks also increase the amount of wetted surface
in the tower and breaks up the water into droplets.
The air flowing across in transverse direction cools the falling water. These towers are used for
small capacity power plants such as diesel power plants.
Forced draught cooling tower
In this tower draught air fan is installed at the bottom of tower. The hot water from the condenser
enters the nozzles. The water is sprayed over the tower filling slats and the rising air cools the
water. The entrained water is removed by eliminators located at the top.
Induced draught cooling tower
The difference here is in the supply of air. The drought fans installed at the top of tower draw air
thought the tower. The hot water is allowed to pass thought the tower bellow the eliminators. The
air moving in the upward direction cools the down coming hot water particles issued from spray
nozzles some percentage (1%) of total water goes into air in the form of water vapour.
Hyperbolic cooling tower
It is usually made of steel reinforced cement concrete to withstand high wind pressure. First type
of this type was installed in US at big sandy station of kenvcky power Co. It is capable of
handling 120x103 gpm and cools the water from 43oc to 30o C. It has minimum diameter of 39.5
m and maximum diameter of 74.5 m and 400 meters high.
The hot water from the condenser is supplied to the ring troughs. Which are placed at 8-10 m
above the ground level? The nozzles are provided on the bottom side of troughs to break up
water into sprays. The air enters the cooling tower just above the pond located at the bottom,
from the air openings provided, rises upward and absorbs heat from the falling water spray. The
cooled water is collected into the pond. Some makeup water is supplied to overcome the losses
into the atmosphere along with air due to evaporation. It needs about 35% of makeup water for
compensating. This type of cooling tower is generally used since it is very efficient.
(1) No air fans are required so power cost and auxiliary equipments are totally eliminated.
(2) Hyperbolic structure creates its own draught assuring efficient operation even when there is
no wind.
(3) Operation and maintenance cost are reduced.
(4) Ground fogging is avoided in hyperbolic towers.
Major drawbacks
(1) Its initial cost inconsiderably very high.
(2) Its performance varies with the personal changes in DBT and RH of air.
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