Chapter 10

Vapor and Combined

Power Cycles
 RANKINE CYCLE: THE IDEAL CYCLE
FOR VAPOR POWER CYCLES

The Rankine cycle, which is the ideal cycle for vapor

power plants. The ideal Rankine cycle does not involve
any internal irreversibilities and consists of the following
four processes:
1-2 Isentropic compression in a pump
2-3 Constant pressure heat addition in a boiler
3-4 Isentropic expansion in a turbine
4-1 Constant pressure heat rejection in a condenser
RANKINE CYCLE: THE IDEAL CYCLE
FOR VAPOR POWER CYCLES
Steam Turbine and Electric Generator Couple
 RANKINE CYCLE: THE IDEAL CYCLE
FOR VAPOR POWER CYCLES
• Water enters the pump at state 1 as saturated liquid and is compressed
isentropically to the operating pressure of the boiler.
• Water enters the boiler as a compressed liquid at state 2 and leaves as a
superheated vapor at state 3.
• The superheated vapor at state 3 enters the turbine, where it expands
isentropically and produces work by rotating the shaft connected to an
electric generator.
• The pressure and the temperature of steam drop during this process to the
values at state 4, where steam enters the condenser. At this state, steam is
usually a saturated liquid–vapor mixture with a high quality.
• Steam is condensed at constant pressure in the condenser, which is
basically a large heat exchanger, by rejecting heat to a cooling medium
such as a lake, a river, or the atmosphere.
• Steam leaves the condenser as saturated liquid and enters the pump,
completing the cycle.
 RANKINE CYCLE: THE IDEAL CYCLE
FOR VAPOR POWER CYCLES
Energy Analysis of the Ideal Rankine Cycle
• All four components in the Rankine cycle (the pump, boiler, turbine, and
condenser) are steady-flow devices, and can be analyzed as steady-flow
processes.
• the steady-flow energy equation per unit mass of steam (neglecting ke and pe)

The boiler and the condenser do not involve any work, and the pump and the
turbine are assumed to be isentropic.
RANKINE CYCLE: THE IDEAL CYCLE
FOR VAPOR POWER CYCLES
 RANKINE CYCLE: THE IDEAL CYCLE
FOR VAPOR POWER CYCLES
Consider a steam power plant operating on the simple ideal Rankine cycle.
Steam enters the turbine at 3 MPa and 350°C and is condensed in the
condenser at a pressure of 75 kPa. Determine the thermal efficiency of this
cycle.
 DEVIATION OF ACTUAL VAPOR POWER
CYCLES FROM IDEALIZED ONES
The actual vapor power cycle differs from the ideal Rankine cycle as a result of
irreversibilities in various components. Fluid friction and heat loss to the
surroundings are the two common sources of irreversibilities.
 DEVIATION OF ACTUAL VAPOR POWER
CYCLES FROM IDEALIZED ONES

• A pump requires a greater work

input, and
• a turbine produces a smaller work
output as a result of irreversibilities.
 HOW CAN WE INCREASE THE EFFICIENCY
OF THE RANKINE CYCLE?

The basic idea behind all the modifications to

increase the thermal efficiency of a power cycle is
the same:
• Increase the average temperature at which heat
is transferred to the working fluid in the boiler,
• Decrease the average temperature at which heat
is rejected from the working fluid in the
condenser.
 HOW CAN WE INCREASE THE EFFICIENCY
OF THE RANKINE CYCLE?
• Lowering the Condenser Pressure (Lowers Tlow,avg)
Steam exists as a saturated mixture in the
condenser at the saturation pressure inside
the condenser. Therefore,
lowering the operating pressure of the
condenser automatically lowers the
temperature of the steam, and thus the
temperature at which heat is rejected.
• The colored area represents the increase in net work output.
• The heat input requirements also increase (curve 2-2’), but this increase is very
small.
• Thus lowering the condenser pressure increases in the thermal efficiency of the
cycle.
BUT
There is a lower limit on the condenser pressure that can be used. It cannot be lower
than the saturation pressure corresponding to the temperature of the cooling medium.
 HOW CAN WE INCREASE THE EFFICIENCY
OF THE RANKINE CYCLE?
Superheating the Steam to High Temperatures
(Increases Thigh,avg)
• The average temperature at the boiler exit can
be increased without increasing the boiler
pressure.
• Both the net work and heat input increase as a
result of superheating the steam to a higher
temperature.
• The overall effect is an increase in thermal
efficiency, however, since the average
temperature at which heat is added increases.
Superheating the steam decreases the moisture content of the steam at the turbine
exit, as can be seen from the T-s diagram.
The temperature to which steam can be superheated is limited, however, by
metallurgical considerations. Presently the highest steam temperature allowed at the
turbine inlet is about 620°C. Ceramics ??
 HOW CAN WE INCREASE THE EFFICIENCY
OF THE RANKINE CYCLE?
Increasing the Boiler Pressure (Increases Thigh,avg)

• Increasing the operating pressure of the

boiler, which automatically raises the
temperature at which boiling takes place.

• This raises the average temperature

and thus raises the thermal efficiency
of the cycle.

Notice that for a fixed turbine inlet temperature, the cycle shifts to the left
and the moisture content of steam at the turbine exit increases. Why
moisture content is undesired??

This undesirable side effect can be corrected, however, by reheating the steam.
 SUPERCRITICAL POWER PLANTS

• Today many modern steam power

plants operate at supercritical
pressures (P > 22.06 MPa) and have
thermal efficiencies of about 40 percent
for fossil-fuel plants and 34 percent for
nuclear plants.

• There are over 150 supercritical-

pressure steam power plants in
operation in the United States.

• Bekirli Thermal Power Plant in

Çanakkale in operation since 2011.
 THE IDEAL REHEAT RANKINE CYCLE

How can we take advantage of the increased

efficiencies at higher boiler pressures without
facing the problem of excessive moisture at the
final stages of the turbine?
1. Superheat the steam to very high temperatures before it enters the
turbine. This is not a viable solution since it requires raising the steam
temperature to metallurgically unsafe levels.
2. Expand the steam in the turbine in two stages, modify the simple
ideal Rankine cycle with a reheat process.

Reheating is a practical solution to the excessive moisture problem in

turbines, and it is commonly used in modern steam power plants.
THE IDEAL REHEAT RANKINE CYCLE
 THE IDEAL REHEAT RANKINE CYCLE
• The incorporation of the single reheat
in a modern power plant improves the
cycle efficiency by 4 to 5 percent by
increasing the average temperature at
which heat is transferred to the steam.
• As the number of stages is increased,
the expansion and reheat processes
approach an isothermal process at the
maximum temperature.

• The use of more than two reheat stages, however, is not practical.
The theoretical improvement in efficiency from the second reheat is
about half of that which results from a single reheat.
• Double reheat is used only on supercritical-pressure (P >22.06 MPa)
power plants
EXAMPLE: Consider a steam power plant operating on the ideal reheat Rankine
cycle. Steam enters the high-pressure turbine at 15 MPa and 600°C and is condensed
in the condenser at a pressure of 10 kPa. If the moisture content of the steam at the
exit of the low-pressure turbine is not to exceed 10.4 percent, determine (a) the
pressure at which the steam should be reheated and (b) the thermal efficiency of the
cycle. Assume the steam is reheated to the inlet temperature of the high-pressure
turbine.
 THE IDEAL REGENERATIVE RANKINE CYCLE

The temperature of the liquid leaving

the pump (called the feedwater) before
it enters the boiler can be raised using
regenerators.
A practical regeneration process in
steam power plants is accomplished by
extracting steam from the turbine at
various points.
• The steam, which could have produced more work by expanding
further in the turbine, is used to heat the feedwater instead. The
device where the feedwater is heated by regeneration is called a
regenerator, or a feedwater heater (FWH).
• A FWH is basically a heat exchanger where heat is transferred from
the steam to the feedwater either by mixing the two fluid streams
(open FWH) or without mixing them (closed FWH).
Open Feed Water Heater
 THE IDEAL REGENERATIVE RANKINE CYCLE
Open Feedwater Heaters

An open (or direct-contact) feedwater heater is basically a mixing

chamber, where the steam extracted from the turbine mixes with the
feedwater exiting the pump.
Ideally, the mixture leaves the heater as a saturated liquid at the
heater pressure.
 THE IDEAL REGENERATIVE RANKINE CYCLE
Open Feedwater Heaters
For each 1 kg of steam leaving the boiler, y kg
expands partially in the turbine and is extracted at
state 6. The remaining (1-y) kg expands completely
to the condenser pressure.
If the mass flow rate through the boiler is m it is
(1-y)m through the condenser.
 5
6 Turbine
3 WT
OFWH
2
6
7

m2*h2+m6*h6=m3*h3 m5*h5=m6*h6+m7*h7 +WT

y = m6/m3 h5 = y.h6 + (1-y).h7 +wT
(1-y).h2 + y.h6 =1.h3 wT= h5 + h6 –h6 - y.h6 - (1-y).h7
h3-h2 = y(h6-h2) wT = (h5-h6) + (1-y)(h6-h7)
y = (h3-h2)/(h6-h2)
h3 = hf@P3
 THE IDEAL REGENERATIVE RANKINE CYCLE
Open Feedwater Heaters

• The thermal efficiency of the Rankine cycle increases as a result of

regeneration.
• Regeneration raises the average temperature at which heat is
transferred to the steam in the boiler by raising the temperature of
the water before it enters the boiler.
• The cycle efficiency increases further as the number of feedwater
heaters is increased.
• Many large plants in operation today use as many as eight
feedwater heaters. The optimum number of feedwater heaters is
determined from economical considerations.
• The use of an additional feedwater heater cannot be justified
unless it saves more from the fuel costs than its own cost.
 THE IDEAL REGENERATIVE RANKINE CYCLE
Closed Feedwater Heaters

Another type of FWH frequently used in steam power plants is the

CFWH, in which heat is transferred from the extracted steam to the
feedwater without any mixing taking place.
Closed Feed Water Heater
Closed Feed Water Heater
 THE IDEAL REGENERATIVE RANKINE CYCLE
Closed Feedwater Heaters

Another type of FWH frequently used in steam power plants is the

CFWH, in which heat is transferred from the extracted steam to the
feedwater without any mixing taking place.
 THE IDEAL REGENERATIVE RANKINE CYCLE
Closed Feedwater Heaters
The two streams now can be at different
pressures, since they do not mix.
In an ideal CFWH, the feedwater is heated
to the exit temperature of the extracted
steam, which ideally leaves the heater as a
saturated liquid at the extraction pressure.

In actual power plants, the feedwater leaves the heater below the exit
temperature of the extracted steam because a temperature difference
of at least a few degrees is required for any effective heat transfer to
take place.
 THE IDEAL REGENERATIVE RANKINE CYCLE

The condensed steam is then either pumped to the feedwater line or routed to
another heater or to the condenser through a device called a trap.
A trap allows the liquid to be throttled to a lower pressure region but traps the
vapor. The enthalpy of steam remains constant during this throttling process.
 THE IDEAL REGENERATIVE RANKINE CYCLE
• OFWHs are simple and inexpensive and have good heat transfer
characteristics. They also bring the feedwater to the saturation state.
For each heater, however, a pump is required to handle the
feedwater.
• The CFWHs are more complex because of the internal tubing
network, and thus they are more expensive. Heat transfer in closed
feedwater heaters is also less effective since the two streams are not
allowed to be in direct contact. However, closed feedwater heaters
do not require a separate pump for each heater since the extracted
steam and the feedwater can be at different pressures.
• Most steam power plants use a combination of open and closed
feedwater heaters.
THE IDEAL REGENERATIVE RANKINE CYCLE
The Ideal Reheat–Regenerative Rankine Cycle
 COGENERATION

Many systems or devices, however,

require energy input in the form of
heat, called process heat.
Some industries that rely heavily on
process heat are chemical, pulp and
paper, oil production and refining, steel
making, food processing, and textile
industries.
Energy is usually transferred to the
steam by burning coal, oil, natural gas,
or another fuel in a furnace.
 COGENERATION

Industries that use large amounts of

process heat also consume a large amount
of electric power. Therefore, it makes
economical as well as engineering sense
to use the already-existing work potential
to produce power instead of letting it go to
waste. The result is a plant that produces
electricity while meeting the process-heat
requirements of certain industrial
processes.
Such a plant is called a cogeneration
plant. In general, cogeneration is the
production of more than one useful form
of energy (such as process heat and
electric power) from the same energy
source.
 COGENERATION

Probably the most striking feature of the

ideal steam-turbine cogeneration plant is
the absence of a condenser.
In other words, all the energy
transferred to the steam in the boiler is
utilized as either process heat or electric
power. Thus it is appropriate to define a
utilization factor εu for a cogeneration
plant as
 COGENERATION

Actual cogeneration plants have

utilization factors as high as 80
percent. Some recent cogeneration
plants have even higher utilization
factors.
 COGENERATION

The ideal steam-turbine cogeneration

plant described above is not practical
because it cannot adjust to the
variations in power and process-heat
loads.
Under normal operation, some steam is
extracted from the turbine at some
predetermined intermediate pressure P6.
The rest of the steam expands to the
condenser pressure P7 and is then
cooled at constant pressure. The heat
rejected from the condenser represents
the waste heat for the cycle.
 COGENERATION

At times of high demand for process heat, all the

steam is routed to the process-heating units and
none to the condenser (m7= 0). The waste heat is
zero in this mode.
If this is not sufficient, some steam leaving the
boiler is throttled by an expansion or pressure-
reducing valve (PRV) to the extraction pressure P6
and is directed to the process-heating unit.
Maximum process heating is realized when all the
steam leaving the boiler passes through the
PRV (m5= m4). No power is produced in this
mode.
When there is no demand for process heat, all
the steam passes through the turbine and the
condenser (m5= m6= 0), and the cogeneration
plant operates as an ordinary steam power plant.
 COGENERATION

The rates of heat input, heat rejected, and

process heat supply as well as the power
produced for this cogeneration plant can
be expressed as follows:

The use of cogeneration dates to the beginning of this century when power
plants were integrated to a community to provide district heating space, hot
water, and process heating for residential and commercial buildings. The
district heating systems lost their popularity in the 1940s owing to low fuel
prices. However, the rapid rise in fuel prices in the 1970s brought about
renewed interest in district heating.
COGENERATION
 COMBINED GAS–VAPOR POWER CYCLES

A 1350-MW combined-cycle power plant built in Ambarli, Turkey, in 1988 by Siemens

of Germany is the first commercially operating thermal plant in the world to attain
an efficiency level as high as 52.5 percent at design operating conditions. This plant
has six 150-MW gas turbines and three 173-MW steam turbines. Some recent
combined-cycle power plants have achieved efficiencies above 60 percent.