This document outlines the Rankine cycle and methods to increase the efficiency of steam power plants. The ideal Rankine cycle consists of four processes: an isentropic compression in a pump, isobaric heat addition in a boiler, isentropic expansion in a turbine, and isobaric heat rejection in a condenser. The thermal efficiency of the cycle can be increased by lowering the condenser pressure, increasing the boiler pressure, and superheating the steam to raise average temperatures. These modifications aim to increase the average fluid temperature during heat addition and decrease it during heat rejection.

1. Topic:
Rankine Cycle
&
How To Increase The Efficiency
Presented by:
Dolat Ram ( 15ME14 )
Thermal Power Plants

2.  Outlines
 Ideal Rankine Cycle
 TS & PV Diagram
 Energy Analysis
 Thermal Efficiency
 Methods To Increase The Efficiency Of Rankine Cycle
 Lowering The Condenser Pressure
 Increasing The Boiler Pressure
 Superheating The Steam

3. Rankine Cycle: The Ideal Cycle for Vapor Power Cycles
Steam Power Plant
 Steam Engine and Steam Turbines in which steam is used as working medium follow Rankine cycle
 Ideal Rankine Cycle does not involve any Internal Irreversibilities and consists of the following 4 processes
1-2: Isentropic Compression in a pump (ΔQ=0 ΔS=0) Win
2-3: Isobaric Heat Addition in a boiler (ΔP=0) Qin
3-4: Isentropic Expansion in a turbine (ΔQ=0 ΔS=0) Wout
4-1: Isobaric Heat Rejection in a condenser (ΔP=0) Qout

4. Steam Power Plant
 Area under the process curve on a T-S Diagram represents the heat transfer for internally reversible
processes
o Area under process curve 2-3: heat transferred to the
water in the boiler
o Area under process curve 4-1: Heat rejected in the
Condenser
o Difference between these two (the area enclosed by
the cycle curve) is the Net Work produced during the
cycle
Rankine Cycle: The Ideal Cycle for Vapor Power Cycles

5. Steam Power Plant
Energy Analysis of the Ideal Rankine Cycle
o All four components are steady-flow devices & all four processes are steady-flow processes.
o Boiler and the Condenser do not involve any work, and the Pump and the Turbine are assumed to be Isentropic.
• Considering 1 kg of fluid :
o Applying Steady Flow Energy Equation (S.F.E.E.) to Boiler, Turbine, Condenser and Pump:

6. Thermal Efficiency of the Ideal Rankine Cycle
Steam Power Plant
o Thermal Efficiency of the Rankine cycle is
OR
 The ratio of the area enclosed by the cycle on a t-s diagram to the area under the heat-addition process.

7. Methods To Increase The Efficiency Of The Rankine Cycle
Steam Power Plant
 Basic Idea behind all the modifications to increase the thermal efficiency of a power cycle is:
o Av. Fluid Temperature should be as high as possible during Heat Addition and
o as low as possible during Heat Rejection
1- Lowering the Condenser Pressure (Lowers Tlow,avg)
 Colored area on this diagram represents increase in net
work output as a result of lowering the condenser pressure
from P4 to P4/ .
 Heat Input requirements also increase (represented by the
area under curve 2/-2), but this increase is very small.
 Overall Effect of lowering the Condenser Pressure (lowers
temperature at which heat is rejected) is an increase in η.

8. Steam Power Plant
Methods To Increase The Efficiency Of The Rankine Cycle
1- Lowering the Condenser Pressure (Lowers Tlow,avg)
 To take advantage of the increased η at low pressures, the
condensers of steam power plants usually operate well below the
Atmospheric Pressure
 Pcond cannot be lower than the saturation pressure corresponding to
the temperature of the cooling medium
 Lower Pcond creates the possibility of air leakage into the condenser
 Lower Pcond increases the moisture content of the steam at the final
stages of the turbine
 presence of large quantities of moisture is highly undesirable in
turbines because it decreases the turbine efficiency and erodes the
turbine blades

9. Steam Power Plant
2-Superheating the Steam to High Temperatures (Increases Thigh,av)
Methods To Increase The Efficiency Of The Rankine Cycle
 Av.Temp at which heat is transferred to steam can be increased without
increasing the boiler pressure by superheating the steam to high
temperatures
 Colored Area on TS-diagram represents increase in the network
 Total Area under the process curve 3-3’ represents the inc: in Heat Input
 Overall effect is an increase in Thermal Efficiency, due to increased Tm
 Superheating of steam decreases the moisture content of the steam at
the turbine exit (4 vs 4’)
 Temp. to which steam can be superheated is limited, by Metallurgical
Considerations
 Presently, Highest Steam Temperature allowed at the turbine inlet is
about 620°C (1150ᵓF).

10. 3- Increasing the Boiler Pressure (Increases Thigh,av)
Methods To Increase The Efficiency Of The Rankine Cycle
Steam Power Plant
 Increasing the operating pressure of the boiler automatically
raises the temperature at which boiling takes place
 It raises the average temperature at which heat is transferred
to the steam and thus raises ηcycle
 For a Fixed Turbine Inlet Temp., cycle shifts to the left and the
moisture content of steam at the turbine exit increases →
Undesirable (side effect can be corrected, by reheating).
Superheat → metallurgically unsafe.
 Max Moisture Content at Turbine Exhaust is not allowed to
exceed 12% or the quality of steam to fall below 88 %

11. Steam Power Plant
Methods To Increase The Efficiency Of The Rankine Cycle
3- Increasing the Boiler Pressure (Increases Thigh,av)
 Max Steam Temp. at Turbine inlet is fixed by the Materials used
 Min Temp. of Heat Rejection is fixed by the Ambient Conditions
 Min Quality of Steam at the Turbine Exhaust is fixed by Turbine
Blade Erosion
⇒Max Steam Pressure at the Turbine Inlet also gets fixed

12.  References:
 THERMODYNAMICS: AN ENGINEERING APPROACH, EIGHTH EDITION
By Yunus A. Çengel & Michael A. Boles
 G.R.Nagpal-power Plant Engineering
 Https://Www.Google.Com
 Https://Www.Slideshare.Net

13. •Interrogation…???

14. Thank you 