Despite efforts to develop alternative energy
converters, electricity from steam will continue, for
many years, to provide the power that energizes the
Steam cycles used in electrical power plants and in
the production of shaft power in industry are based
on the familiar Rankine cycle.
Rankine cycle is an ideal cycle for comparing the performance of
steam plants. It is a modified form of Carnot cycle, in which the
condensation process is continued until the steam is condensed into water.
The Rankine cycle is a thermodynamic cycle which converts heat into
work. The heat is supplied externally to a closed loop, which usually uses
water as the working fluid.
This cycle generates about 80% of all electric power used throughout the
world, including virtually all solar thermal, biomass, coal and nuclear power
In the simple Rankine cycle, steam flows to a turbine, where part of its
energy is converted to mechanical energy that is transmitted by rotating
shaft to drive an electrical generator.
The reduced-energy steam flowing out of the turbine condenses to liquid
water in the condenser.
A feed water pump returns the condensed liquid (condensate) to the steam
The heat rejected from the steam entering the condenser is transferred to a
separate cooling water loop that in turn delivers the rejected energy to a
neighbouring lake or river or to the atmosphere.
The efficiency of a Rankine cycle is usually limited by the working fluid.
Without the pressure going super critical the temperature range the cycle can
operate over is quite small, turbine entry temperatures are typically 565 C
(the creep limit of stainless steel) and condenser temperatures are around
This gives a theoretical Carnot efficiency of around 63% compared with an
actual efficiency of 42% for a modern coal-fired power station.
This low turbine entry temperature (compared with a gas turbine) is why the
Rankine cycle is often used as a bottoming cycle in combined cycle gas
turbine power stations.
The working fluid in a Rankine cycle follows a closed loop and is re-used
The water vapor and entrained droplets often seen billowing from power
stations is generated by the cooling systems (not from the closed loop
Rankine power cycle) and represents the waste heat that could not be
converted to useful work.
Note that cooling towers operate using the latent heat of vaporization of the
While many substances could be used in the Rankine cycle, water is
usually the fluid of choice due to its favorable properties, such as nontoxic
and unreactive chemistry, abundance, and low cost, as well as its
Process of rankine cycle
Process 1-2: The working fluid is pumped from low to high
pressure, as the fluid is a liquid at this stage the pump requires little
Process 2-3: The high pressure liquid enters a boiler where it is heated
at constant pressure by an external heat source to become a dry
Process 3-4: The dry saturated vapor expands through a
turbine, generating power. This decreases the temperature and
pressure of the vapor, and some condensation may occur.
Process 4-1: The wet vapor then enters a condenser where it is
condensed at a constant pressure and temperature to become a
saturated liquid. The pressure and temperature of the condenser is
fixed by the temperature of the cooling coils as the fluid is undergoing
Heat absorbed during the complete cycle = Heat absorbed during
isothermal operation 1- 2 + Heat absorbed during warming operation 4 – 1
Work done during the cycle = Heat absorbed – Heat rejected
The work done by the extraction and boiler feed pumps in increasing the
Heat absorbed during warming operation 4 1(P3h= P4)hto4 the h f 2 h f 3
pressure of water from the condenser pressure
(P1 = P2) is very small. Hence neglected
x 3 h f g 3 ( h f 3
hf 4 )
Increasing the Efficiency of Rankine Cycle
We know that the efficiency is proportional to: 1-TL/TH
That is, to increase the efficiency one should increase the average
temperature at which heat is transferred to the working fluid in the boiler,
and/or decrease the average temperature at which heat is rejected from the
working fluid in the condenser.
Decreasing the of Condenser Pressure (Lower TL)
Superheating the Steam to High Temperatures (Increase TH)
Increasing the Boiler Pressure (Increase TH)
Decreasing the of Condenser Pressure
Lowering the condenser pressure will increase the area
enclosed by the cycle on a T-s diagram which indicates that the
net work will increase. Thus, the thermal efficiency of the cycle
will be increased.
The condenser pressure cannot be lowered than the saturated
pressure corresponding to the temperature of the cooling
medium. We are generally limited by the thermal reservoir
temperature such as lake, river, etc.
Steam exits as a saturated mixture in the condenser at the
saturation temperature corresponding to the pressure in the
So lower the pressure in the condenser, lower the temperature
of the steam, which is the heat rejection temperature.
Superheating the Steam to High Temperatures
Superheating the steam will increase the net work output and the
efficiency of the cycle.
It also decreases the moisture contents of the steam at the turbine exit.
The temperature to which steam can be superheated is limited by
metallurgical considerations (~ 620 C).
By superheating the stream to a high temperature (from state 3 to state
3'), the average steam temperature during heat addition can be
Increasing the Boiler Pressure (Increase TH)
Increasing the operating pressure of the boiler leads to an increase in
the temperature at which heat is transferred to the steam and thus
raises the efficiency of the cycle.
If the operating pressure of the boiler is increased, (process 2-3 to
process 2'-3'), then the boiling temperature of the steam raises
For a fixed inlet turbine temperature, the blue area is the net work
increased and the gray area is the net work decreased. Also, the
moisture content of the steam increases from state 4 to state 4', which
is an undesirable side effect.
This side effect can be corrected by reheating the steam, and results in
the reheat Rankine cycle.
The Rankine cycle has been modified to produce more output work by
introducing two stage steam turbines, using intermediate heating.
Basically, in this modified Rankine cycle, the full expansion of steam is
interrupted in the high-pressure turbine and steam is discharged after
This exhaust steam is passed through a cold reheat line to the steam
generator where it gains heat while passing through hot tubes.
This reheated steam is supplied to a low pressure turbine for full expansion
and reaches the condenser pressure.
This results in low pressure turbine expansion work. Thus, reheat increases
work output because of low pressure turbine expansion work.
The use of reheat also tends to increase the average temperature at which
heat is added.
If the steam from the low-pressure turbine is superheated, the use of reheat
may also increase the average temperature at which heat is rejected.
It may reduce or increase the thermal efficiency depending on the specific
cycle conditions viz. thermal (heat addition and heat reduction temperature) or
mechanical (condenser vibrations and air leakage, pump vibrations, turbine
blade vane deflection etc).
Indeed the the net work of the reheat cycle is the algebraic sum of work of the
two turbines and the pump work and the total heat addition is the sum of the
heat added in the feed-water and reheat passes through the steam generator.
The presence of more than about 10% moisture in the turbine exhaust can
cause erosion of blades near the turbine exit and reduce energy conversion
Determination of a suitable reheat pressure level is a significant design
problem that entails a number of considerations.
The cycle efficiency, the net work, and other parameters will vary with the
reheat pressure level for a given throttle and condenser conditions.
One of these may be numerically optimized by varying the reheat pressure
level while holding all other design conditions constant.
The efficiency of a simple Rankine cycle is improved by using intermediate
reheat cycle, enabling improved thermal conditions of the working fluid.
However, it cannot reach the thermal conditions as in the case of the
Carnot cycle where heat addition and heat rejection occurs at a specified
The regeneration is vital to improve the efficiency as it uses the sensible
heat of exhaust steam for the preheating of feed water. Inclusion of a FWH
also introduces an additional pressure level into the Rankine cycle as seen
in the T-s diagram.
Hence, the extraction pressure level is another parameter under the
control of the designer.
The control of steam condenser pressure i.e. condenser vacuum and
supply of condenser tube cooling water is another parameter which affects
the steam thermal power plant efficiency.