LECTURE 19
19.1 Vapor Power Plants

 
Working Fluid
For High Temperature Applications
Sodium, Potassium, Mercury
For Low Temperature Applications
Benzene, Freon (Organic Fluids)
Low Cost, Availability, High Enthalpy of Vaporization
Steam

Fuel Type
Coal, nuclear, geothermal or natural gas.

19.2 Carnot Vapor Cycle

Process 2-3
Isentropic Expansion
The turbine will have to handle steam with low quality, i.e., steam with a high moisture content. The impingement of liquid droplets on the turbine blades causes erosion and is a major source of wear. Thus, steam with qualities less than 90% cannot be tolerated in the operation of power plants.
Process 4-1
Isentropic Compression

a. It is not easy to control the condensation process so precisely as to end up with the desired quality at state 4.

b. It is not practical to design a compressor that will handle two phases.

19.3 Rankine Cycle


Rankine cycle is the ideal cycle for vapor power plants. It does not involve any internal irreversibilities and consists of the following four processes:

 
Process 1-2
 Isentropic Compression in a Pump
 
Process 2-3
 Constant-Pressure Heat Addition in a Boiler
 
Process 3-4
 Isentropic Expansion in a Turbine
 
Process 4-1
 Constant-Pressure Heat Rejection in a Condenser


 
 
 
 
 


 
State 1
 Saturated Liquid
 
State 2
 Compressed Liquid
 
State 3
 Superheated/Saturated Vapor
 
State 4
 Saturated Liquid-Vapor Mixture with a High Quality


 
Steady-Flow Process
 
Energy
Equation
OR


 
Pump
 q=0,
 
Boiler
 w=0,
 
Turbine
 q=0,
 
Condenser
 w=0,


 
Thermal Efficiency
 
 
Back Work Ratio (BWR)
 


19.4 Deviation of Actual Cycle from Ideal Rankine Cycle

 
Two Most Common Sources of Irreversibilities
1. Fluid friction
2. Undesired heat loss to the surroundings

Fluid friction causes pressure drops in the boiler, the condenser, and the piping between various components. To compensate for these pressure drops, the water must be pumped to a sufficiently higher pressure than the ideal cycle calls for.

Of particular importance are the irreversibilities occurring within the pump and the turbine. The deviation of actual pumps and turbines from the isentropic ones can be accurately accounted for by utilizing adiabatic efficiency:

Adiabatic Efficiency
Pump
Turbine

 
 

 
19.5 To Increase the Efficiency of the Rankine Cycle


Basic idea to increase the thermal efficiency of a power cycle:

Increase the average temperature at which heat is transferred to the working fluid in the boiler, and decrease the average temperature at which heat is rejected from the working fluid in the condenser.

Three Ways to Increase the Efficiency of the Rankine Cycle
Method 1
Method 2
Method 3

Lowering the condenser pressure :
Lowers

Superheating the steam to high temperature :
Increases

Increasing the boiler pressure:
Increases

Operating Pressure of Boilers:

2.7 MPa (400 psia) in 1922 to over 30 MPa (4500 psia) today

Supercritical Rankine Cycle : Supercritical Pressure

 
Plants
Efficiency ()
 
Fossil-Fuel Plants
40%
 
Nuclear Plants
34%