Faculty of Engineering
Alexandria University
Mechanical Engineering Department
Heat Transfer Branch
Lecture (6)
Reference, thermodynamics 5th edition (Yunus A. Cengel)
Dr. Mohamed A. Qassem
Spring 2016
Steam Power Cycle
• RANKINE CYCLE:
The Ideal Cycle For Steam Power Plants
The ideal Rankine cycle 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
Steam Power Cycle
• RANKINE CYCLE:
Water enters the pump at state 1 as saturated liquid and is compressed isentropically
to the operating pressure of the boiler. The water temperature increases somewhat
during this isentropic compression process due to a slight decrease in the specific
volume of water. The vertical distance between states 1 and 2 on the T-s diagram
Water enters the boiler as a compressed liquid at state 2 and leaves as a superheated
vapor at state 3. The boiler is basically a large heat exchanger where the heat
originating from combustion gases, nuclear reactors, or other sources is transferred to
the water essentially at constant pressure. The boiler, together with the section where
the steam is superheated (the super-heater), is often called the steam generator.
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 (state 4), 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.
Steam Power Cycle
• Energy Analysis of the Ideal Rankine Cycle:
All four components associated with the Rankine cycle (the pump, boiler,
turbine, and condenser) are steady-flow devices, and thus all four processes
that make up the Rankine cycle can be analyzed as steady-flow processes.
The kinetic and potential energy changes of the steam are usually small
relative to the work and heat transfer terms and are therefore usually
neglected.
Then the steady-flow energy equation per unit mass of steam reduces to;
The boiler and the condenser do not involve any work, and the pump and the
turbine are assumed to be isentropic.
Steam Power Cycle
• Energy Analysis of the Ideal Rankine Cycle:
Then the conservation of energy relation for each device can be expressed as
follows:
The thermal efficiency of the Rankine cycle is determined from;
Steam Power Cycle
• Deviation Of Actual Steam Power cycle From Idealized One
The actual vapor power cycle differs from the ideal Rankine cycle, as a result
of irreversibilities in various components.
• Fluid friction causes pressure drops in the boiler, the condenser, and the
piping between various components.
• The other major source of irreversibility is the heat loss from the steam to
the surroundings as the steam flows through various components.
Deviation of actual vapor power The effect of pump and turbine
cycle from the ideal Rankine cycle. irreversibilities on the ideal Rankine cycle.
Steam Power Cycle
• Deviation Of Actual Steam Power cycle From Idealized One
The deviation of actual pumps and turbines from the isentropic ones can be
accounted for by utilizing isentropic efficiencies, defined as;
where states 2a and 4a are the actual exit states of the pump and the turbine,
respectively, and 2s and 4s are the corresponding states for the isentropic
case.
Steam Power Cycle
• How Can We Increase The Efficiency of The Rankine Cycle?
I.
Steam Power Cycle
• How Can We Increase The Efficiency of The Rankine Cycle?
II.
Steam Power Cycle
• How Can We Increase The Efficiency of The Rankine Cycle?
III.
Thank you
