Steam Power Plant and Rankine Cycles

HOW TO INCREASE
EFFICIENCY OF RANKINE
CYCLE

Lowering the condenser pressure (Lower Tlow,avg)
Steam in saturated mixture
during condensation
rejectedisheattheofretemperatuthus
,mean  TP
Increase in net work output
Additional heat input requirements (2 to
2’), but small compared to Wnet
Condenser operating pressure is limited by
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 (4´).
Could cause air leakage to condenser and moisture content of the steam in the
turbine due to low pressure

•Superheating the steam to high temperatures (Increases Thigh,avg)
Steam is superheated at constant pressure (3
to 3’) which increase the net work output.
Decreases the moisture content of the steam
at the turbine exit.
Steam superheated temperature is
limited by metallurgical
considerations and material limitation
(may create damage to turbine
blades)
Possible solution: Expand the steam
in the turbine in two stages, and
reheat it in between. (Decrease in
Steam remains same as before)

•Superheating the steam to high temperatures with Reheat

Increasing the boiler pressure (Increases T high, avg)
Pboiler increase which will automatically
increase the boiling temperature
Increases the net work output, but at
the same time increases the moisture
content in the turbine
High moisture content (at 4’ than 4)
causes:
• erosion in turbine blades and
•Viscous effect i.e. resistance to flow

Ideal Regenerative Rankine Cycle
A feed water heater is basically a heat exchanger where
heat is transferred from the steam to the feed water .
An open (or direct-contact) feed water heater is
basically a mixing chamber, where the steam
extracted from the turbine mixes with the feed
water leaving the pump. Ideally, the mixture leaves
the heater as a saturated liquid at the heater
pressure.

Ideal Regenerative Rankine Cycle
•This improves the average heat-addition
and thus the cycle efficiency.
•In steam power plants, steam is extracted
from the turbine at various points to heat
the feed water to boiler and thus improves
cycle efficiency
•. The device where the feed water is heated
by regeneration is called a regenerator, or a
feed water heater (FWH).

Ideal Regenerative Rankine Cycle with Open FWH
1-2 pump-I.
2-3 Mixing of Feed water
6-3 Steam from turbine.
3-4 pump II.
4-5 heat addition in boiler.
5-6 Turbine Work out (‘y’ portion of
steam to FWH).
5-7 Turbine Work out (1-y portion of
steam).
7-1 heat rejection in condenser.

Ideal Regenerative Rankine Cycle with Open FWH
y x h6 + (1-y) x h7 = 1 x h3
y x h6
(1-y) x h71 x h3

Closed Feed-Water Heaters
Another type of feed water heater frequently used in
steam power plants is the closed feed water heater, in
which heat is transferred from the extracted steam to
the feed water without any mixing taking place. The
two streams now can be at different pressures, since
they do not mix.

Closed Feed-Water Heaters
m
1-m

Cogeneration
• In production of electricity some energy is rejected
as waste heat, but in cogeneration this thermal energy is
put to good use. Concept is to utilize waste heat from the
power plant
•Heat is extracted from at least two heat sources
• the first one being the flow rate of waste heat
obtained from the exhaust gases from the power
plant.
•Heat from the condensing steam taken from the low
pressure turbine extraction points
•The resulting low-temperature waste heat extracted is
then used for water or space heating.

Steam Power Plant and Rankine Cycles

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