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Transcript of Steam power plant complete
Mechanical Engineering Dept. HITEC 1
Steam Power Plant
References:
Thermodynamics by Yunus A Cengel
Power Plant Engineering by PK Nag
Mechanical Engineering Dept. HITEC 2
Steam Power Plant
Electric Power Generation
Faraday Law of Electromagnetic Induction
Voltage is induced in a circuit whenever relative motion exists between a Conductor and a
Magnetic Field and that the Magnitude of this Voltage is proportional to the rate of change of the
flux
Emf induced is called Induced Emf and if the conductor circuit is closed, the current will also
circulate through the circuit and this current is called induced Current
Mechanical Engineering Dept. HITEC 3
Steam Power Plant
Electric Power Generation
Faraday Law of Electromagnetic Induction—contd--
Amount of voltage (emf) induced in the coil using just magnetism can be increased by:
o Increasing the number of turns of wire in the coil
o Increasing the speed of the relative motion between the coil and the magnet
o Increasing magnetic field strength surrounding the coil
Magnitude of the electromagnetic induction is
directly proportional to:o Flux Density, β
o Number of loops giving a total length of the
conductor l in meters
o rate ν at which the magnetic field changes within
the conductor in m/s
Mechanical Engineering Dept. HITEC 4
Steam Power Plant
Power Plants & Types of Power Plant
Power Plant or Power Generating Station: an industrial location that is utilized for the
Generation and Distribution of Electric Power in mass scale, in the order of several 1000 Watts
All Power Generating Stations has an A.C. generator or an Alternator, which is basically a rotating
machine that is equipped to convert energy from the Mechanical Domain (Rotating Turbine) into
Electrical Domain by creating Relative Motion between a Magnetic Field and the Conductors
Depending on the type of fuel used, Power Generating Stations are broadly classified as:
1. Steam Power Plant
2. Diesel Power Plant
3. Gas Turbine Power Plant
4. Nuclear Power Plant
5. Hydro Electric Power Plant
THERMAL POWER PLANT: Converts Heat into Electric Energy
Mechanical Engineering Dept. HITEC 5
Steam Power Plant
Power Plants & Types of Power PlantThermal Power Generation
Mechanical Engineering Dept. HITEC 6
Steam Power Plant
Mechanical Engineering Dept. HITEC 7
Steam Power Plant
Mechanical Engineering Dept. HITEC 8
Steam Power Plant
Mechanical Engineering Dept. HITEC 9
Steam Power Plant
Power Plants & Types of Power Plant
Hydel Power Generation
Mechanical Engineering Dept. HITEC 10
Steam Power Plant
Power Plants & Types of Power Plant
Nuclear Power Generation
Mechanical Engineering Dept. HITEC 11
Steam Power Plant
Introduction to Steam Power Plant
Today, most of the electricity produced throughout the world is from Steam Power Plants
Steam Power Plant continuously converts the energy stored in fossil fuels (Coal, Oil, Natural
Gas) into shaft work and ultimately into electricity
Steam has the advantage that,
o it can be raised from water which is available in abundance
o it does not react much with the materials of the equipment of power plant
o is stable at the temperature required in the plant
Mechanical Engineering Dept. HITEC 12
Steam Power Plant
Introduction
Energy released by burning of fuel Q1 is transferred to water in Boiler (B)
Steam is generated (H2O(g)) at high pressure and Temperature
Steam expands in the Turbine (T) to a low pressure to produce shaft work WT
Steam leaving the Turbine (T) is condensed into water in the condenser (C)
Mechanical Engineering Dept. HITEC 13
Steam Power Plant
Introduction
In Condenser (C), Cooling water from a river or sea circulates carrying away the heat released
during condensation Q2
Water (Condensate) is fed back to the boiler by the pump (P) requiring power WP and cycle
repeats
Working substance (Water)is undergoing a Cyclic Process → No change in its Internal Energy over the
cycle: ∫ dE =0
Mechanical Engineering Dept. HITEC 14
Steam Power Plant
Introduction
Net Energy transferred to the unit mass of the fluid as Heat during the cycle must equal the net energy
transferred as Work from the fluid:
⇒
Efficiency of the Vapor Power Cycle:
Mechanical Engineering Dept. HITEC Univ. 15
Introduction to Refrigeration
Temperature and Pressure Relationship
Temperature at which a liquid boils is not
constant, but varies with the pressure
Mechanical Engineering Dept. HITEC Univ. 16
Boiling Point of water can be changed and controlled by controlling the
vapor pressure above the water⇒
Introduction to Refrigeration
Temperature and Pressure Relationship
When the pressure in the jar reaches the pressure that corresponds to the
boiling point of water at 70°F (21 oC), the water will start to boil and vaporize
Mechanical Engineering Dept. HITEC 17
Steam Engine and Steam Turbines in which steam is used as working medium follow Rankine cycle
Steam Power Plant
Rankine Cycle: The Ideal Cycle for Vapor Power Cycles
Ideal Rankine Cycle does not involve any Internal Irreversibilities and consists of the following 4
processes:1-2: Isentropic Compression in a pump 2-3: Isobaric Heat Addition in a boiler
3-4: Isentropic Expansion in a turbine 4-1: Isobaric Heat Rejection in a condenser
T
Mechanical Engineering Dept. HITEC 18
Steam Power Plant
Rankine Cycle: The Ideal Cycle for Vapor Power Cycles
Area under the process curve on a T-S Diagram represents the heat transfer for internally
reversible processes
Area under process curve 2-3:
heat transferred to the water in
the boiler
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
T
Mechanical Engineering Dept. HITEC 19
Steam Power Plant
Energy Analysis of the Ideal Rankine Cycle
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 :
Applying Steady Flow Energy Equation (S.F.E.E.) to Boiler, Turbine, Condenser and Pump:
(i) For Boiler (as control volume)
(ii) For Turbine (as control volume)
(iii) For Condenser (as control volume)
(iii) For Feed Pump(as control volume)
T
Mechanical Engineering Dept. HITEC 20
Steam Power Plant
Energy Analysis of the Ideal Rankine Cycle
Thermal Efficiency of the Rankine cycle is:
T
Where;
Example 5.1
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.
Mechanical Engineering Dept. HITEC 21
Steam Power Plant
Energy Analysis of the Ideal Rankine Cycle
Mechanical Engineering Dept. HITEC 22
Steam Power Plant
Deviation of Actual Vapor Power Cycles from Idealized Ones
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
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
⇒ requires a larger pump and larger work
input to the pump
Mechanical Engineering Dept. HITEC 23
Steam Power Plant
Other major source of irreversibility is the heat loss from the steam to the surroundings as the
steam flows through various components
To maintain the same level of net work output,
more heat needs to be transferred to the steam
in the boiler to compensate for these
undesired heat losses
⇒ Cycle Efficiency Decreases
Deviation of Actual Vapor Power Cycles from Idealized Ones
Mechanical Engineering Dept. HITEC 24
Steam Power Plant
Deviation of Actual Vapor Power Cycles from Idealized Ones
Deviation of actual pumps and turbines from the isentropic ones can be accounted for by utilizing
Isentropic Efficiencies
states 2a and 4a are Actual Exit States of the
pump and turbine
States 2s and 4s are corresponding states for
Isentropic Case
⇒ As a result of irreversibilities:
Pump requires a greater work input
Turbine produces a smaller work output
Mechanical Engineering Dept. HITEC 25
Steam Power Plant
Deviation of Actual Vapor Power Cycles from Idealized Ones
Example
A steam power plant operates on the cycle shown in Fig. If the isentropic efficiency of the turbine is 87
percent and the isentropic efficiency of the pump is 85 percent, determine:
(a) the thermal efficiency of the cycle and
(b) the net power output of the plant for a mass flow rate
of 15 kg/s.
Mechanical Engineering Dept. HITEC 26
Steam Power Plant
Deviation of Actual Vapor Power Cycles from Idealized Ones
Example
Mechanical Engineering Dept. HITEC 27
Steam Power Plant
Economizer, Evaporator and Superheater
Heat transfer in Steam Generator normally takes place in 3 steps
o Economiser (4-5): Sensible heating in liquid Phase till it becomes saturated Liquid
o Evaporator (5-6): Phase change by absorbing Latent Heat of Vaporization
o Superheater (6-1): Sensible heating of vapor to become Super Heated Vapor
Mechanical Engineering Dept. HITEC 28
Steam Power Plant
Economizer, Evaporator and Superheater
ORpv
hsTs
Mechanical Engineering Dept. HITEC 29
Steam Power Plant
Economizer, Evaporator and Superheater
Fractions of total heat absorbed in Economizer, Evaporator and Super Heater:
Mechanical Engineering Dept. HITEC 30
Steam Power Plant
Mean Temperature of Heat Addition
If Tm1 is the Mean Temperature of Heat Addition so that area under 4-1 is equal to area under 5-6, then heat
added is:
⇒
T2 Temperature of Heat Rejection
o Lower is T2 for a given Tm1, i.e., lower is the condenser
pressure, higher will be ηRankine
o Lowest practicable temperature of heat rejection T2 is
temperature of surroundings To
⇒
⇒ Higher is Tm1, higher will be ηRankine
Mechanical Engineering Dept. HITEC 31
Steam Power Plant
Methods to Increase the Efficiency of The Rankine Cycle
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) T
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 is
an increase in η
Mechanical Engineering Dept. HITEC 32
Steam Power Plant
Methods to Increase the Efficiency of The Rankine Cycle
1- Lowering the Condenser Pressure (Lowers Tlow,avg) – contd--
To take advantage of the increased η at low pressures, the condensers of steam power plants usually
operate well below the Atmospheric Pressure
T
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
o presence of large quantities of moisture is highly
undesirable in turbines because it decreases the
turbine efficiency and erodes the turbine blades
Mechanical Engineering Dept. HITEC 33
Steam Power Plant
Methods to Increase the Efficiency of The Rankine Cycle
2- Superheating the Steam to High Temperatures (Increases Thigh,av)
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
net work
Total Area under the process curve 3-3’ represents the
increase 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
Mechanical Engineering Dept. HITEC 34
Steam Power Plant
Methods to Increase the Efficiency of The Rankine Cycle
3- Increasing the Boiler Pressure (Increases Thigh,av)
Increasing the operating pressure of the boiler automatically raises the temperature at which boiling takes
place
o 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
Max Moisture Content at Turbine Exhaust is not allowed
to exceed 12% or the quality of steam to fall below 88 %
Mechanical Engineering Dept. HITEC 35
Steam Power Plant
Methods to Increase the Efficiency of The Rankine Cycle
3- Increasing the Boiler Pressure (Increases Thigh,av) –contd--
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
Mechanical Engineering Dept. HITEC 36
Steam Power Plant
Methods to Increase the Efficiency of The Rankine Cycle
4- Increasing the Boiler Pressure (Increases Thigh,av) –contd-
Increasing the operating pressure of the boiler automatically raises the temperature at which boiling takes
place
Operating pressures of boilers have gradually increased over
the years from about 2.7 MPa (400 psia) in 1922 to over 30 MPa
(4500 psia) today, i.e. above Tcritical of Water
Most modern fossil fuel plants employ the Supercritical Rankine
Steam Cycle which pushes the Thermal Efficiency of the plant
into the low to mid 40% range
Drawback: boiler and turbine must be built to withstand high
pressure and high temperatures → can be quite expensive
Majority of the additional heat input, relative to the Rankine
Cycle is converted into work
Mechanical Engineering Dept. HITEC 37
Steam Power Plant
Boiler (Steam Generator)
Key Boiler Components
Key Boiler Components involved in the process of heat transfer in a boiler are:
o Burner: mixes fuel and oxygen together and, with the assistance of an ignition device, provides a platform
for combustion
o Combustion Chamber: where Combustion takes place
o Heat Exchanger: heat generated in Combustion
Chamber is transferred to the water through the
heat exchanger
o Controls: to regulate:
• ignition
• burner firing rate
• fuel supply
• air supply
• exhaust draft
• water temperature
• steam and boiler pressure
Mechanical Engineering Dept. HITEC 38
Steam Power Plant
Boiler (Steam Generator)
1- Water Tube Boilers
Water flows in the inside of the tubes and hot gases from combustion flow around the outside of the tubes
Combustion Gases heat the water into a steam-water mixture which, because it becomes less dense than
liquid water inside the Feed-water Drum, rises
Mixture ascends in tubes called Risers to the Steam Drum
Mechanical Engineering Dept. HITEC 39
Steam Power Plant
Boiler (Steam Generator)
1- Water Tube Boilers – contd--
Steam from the water-vapor mixture is removed and released into the system
Water remaining in the steam drum returns to the feedwater drum through pipes called Downcomers
Mechanical Engineering Dept. HITEC 40
Steam Power Plant
Boiler (Steam Generator)
2- Fire Tube Boilers
Hot gases of combustion flow through a series of tubes surrounded by water
Heat Energy of the gasses is transferred to the water surrounds them
Steam is generated in the water and naturally comes up and is stored upon the water in
the same vessel of fire tube boiler
Mechanical Engineering Dept. HITEC 41
Steam Power Plant
Cooling Tower
A device that passes outside air over the water to remove the system heat from the water
26 oC
35 oC
35 oC
29 oC
Cooling Tower is limited in capacity to the amount of evaporation that occurs
Evaporation Rate is linked to the Wet-bulb Temperature of the outside air (humidity)
Towers can be either:
(1) Natural Draft
(2) Forced Draft
Evaporation takes heat from the remaining water and adds to the capacity of the tower
Mechanical Engineering Dept. HITEC 42
Dry Bulb Temperature (DB): Temperature of the air, as sensed by a thermometer, freely
exposed to the air but shielded from radiation and moisture
Wet Bulb Temperature (WB): Temperature sensed by a thermometer
whose bulb is wrapped with a water-soaked wick, in rapidly moving air
o If the surrounding air is very dry, the moisture will evaporate quickly,
causing the WB to drop lower
o If surrounding air is very wet (high relative humidity), rate of
evaporation will be very low and the WB reading will be closer to the
DB reading
o WB can never be higher than the DB
Steam Power Plant
Thermodynamics of the Compact Steam Power Plant
Dry Bulb and Wet Bulb Temperature
Mechanical Engineering Dept. HITEC 43
Steam Power Plant
Cooling Tower
1- Natural-Draft Towers
Natural-Draft Tower does not have a blower to move air through the tower
Water is sprayed into the top of the tower through spray heads, and some of the water evaporates as it falls
to the bottom of the tower
Must be located in the path of prevailing winds
Mechanical Engineering Dept. HITEC 44
Steam Power Plant
Cooling Tower
2- Forced or Induced-Draft Towers
They have a fan to move air over a wetted surface
Presence of fans provides a means of regulating air flow, to compensate for changing atmospheric and load
conditions, by fan capacity manipulation
Mechanical Engineering Dept. HITEC 45
Air Flow is directed
perpendicular to the water flow
Air Flow is directly opposite of
the water flow
Mechanical Engineering Dept. HITEC 46
Steam Power Plant
Dynamometer
A device that can measure Force, Power, or Speed
E.g., power produced by an engine motor or other rotating prime mover can be calculated by simultaneously
measuring Torque and Rotational Speed (RPM)
Torque produced in the housing is measured
by the strain gauge and speed by speed
sensor
Mechanical Engineering Dept. HITEC 47
Steam Power Plant
Thermodynamics of the Compact Steam Power Plant
Mechanical Engineering Dept. HITEC 48
Steam Power Plant
Thermodynamics of the Compact Steam Power Plant
Feed Pump for Boiler
From Condenser Tank
h1 = h0 + (p1 – pa)v0
h1 = Enthalpy of water after feed Pump (kj/kg)
h0 = Enthalpy of the Feed Water at feed water Temperature T1 (kj/kg)
pa = Atmospheric Pressure (kPa)
p1 = Boiler Pressure (kPa)
vo = Specific Volume of saturated feed water at T1
Mechanical Engineering Dept. HITEC 49
Steam Power Plant
Thermodynamics of the Compact Steam Power Plant
Boiler Efficiency
h2 = Enthalpy of saturated steam leaving the boiler (kj/kg)
h1 = Enthalpy of water after feed Pump (kj/kg)
mf = Mass flow rate of the fuel (kg/s)
mc = Mass flow rate of the condensate
= feed water flow rate (kg/s)
Qf = Lower Heating Value of Fuel (kJ/kg)
Mechanical Engineering Dept. HITEC 50
Steam Power Plant
Thermodynamics of the Compact Steam Power Plant
Amount of heat released when a fuel is burned completely in a steady-flow process
Heating Value of Fuel (Calorific Value)
Heating Value depends on the phase of H2O in the products
Measured as a unit of energy per unit mass or volume of substance (e.g., kcal/kg, kJ/kg, J/mol or Btu/m³)
1- Lower Heating Value (LHV, Net Calorific Value)
Amount of heat released by combusting a specified quantity
(initially at 25°C) and returning the temperature of the
combustion products to 150°C
It assumes the Latent Heat of Vaporization of water in the
reaction products is not recovered
⇒ water formed during combustion remains as steam
Mechanical Engineering Dept. HITEC 51
Steam Power Plant
Thermodynamics of the Compact Steam Power Plant
E.g., LHV and HHV of Gasoline are 44,000 kJ/kg and
47,300 kJ/kg, respectively
2- Higher Heating Value (HHV, Gross Calorific Value)
Amount of heat released by a specified quantity (initially at 25°C) once it is combusted and the products
have returned to a temperature of 25°C
It takes into account the Latent Heat of Vaporization of water in the
combustion products
Mechanical Engineering Dept. HITEC 52
Steam Power Plant
Thermodynamics of the Compact Steam Power Plant
Super Heat Efficiency
h2 = Enthalpy of saturated steam leaving the boiler (kj/kg)
h3 = Enthalpy of steam entering the steam turbine (kj/kg)
mf = Mass flow rate of the fuel (kg/s)
mc = Mass flow rate of the condensate
= feed water flow rate (kg/s)
Qf = Lower Heating Value of Fuel (kJ/kg)
Mechanical Engineering Dept. HITEC 53
Steam Power PlantSteam Power Plant
Thermodynamics of the Compact Steam Power Plant
Steam Turbine or Engine Mechanical Power Output and Efficiency
Efficiency of the Steam Engine or Steam Turbine:
Pom = Mechanical Power Output (W) Nd = Dynamometer speed (rpm) Tq = Dynamometer Torque (Nm)
h3 = Enthalpy of steam entering the steam turbine (kj/kg)
h4 = Enthalpy of steam leaving the steam turbine (kj/kg)
mc = Mass flow rate of the condensate
= feed water flow rate (kg/s)
Mechanical Engineering Dept. HITEC 54
Steam Power Plant
Thermodynamics of the Compact Steam Power Plant
Overall Power Plant Efficiency
Pom = Mechanical Power Output (W)
mf = Mass flow rate of the fuel (kg/s)
Qf = Lower Heating Value of Fuel (kJ/kg)
Mechanical (with a Dynamo):
Electrical (with a Generator):
Pom = Electrical Power Output (W)
Mechanical Engineering Dept. HITEC 55
Steam Power Plant
Thermodynamics of the Compact Steam Power Plant
Condenser Heat Transfer Efficiency
mc = Mass Flow Rate of the Condensate (kg/s)
mw = Mass Flow Rate of the Cooling Water (kg/s)
h4 = Enthalpy of steam leaving the steam
turbine (kj/kg)
h5 = Enthalpy of the Condensate (kj/kg)
T8 = Temperature of Cooling Water Outlet
from Condenser (oC)
T7 = Temperature of Cooling Water Inlet to
Condenser (oC)
Mechanical Engineering Dept. HITEC 56
Steam Power Plant
Thermodynamics of the Compact Steam Power Plant
Cooling Efficiency of Cooling Tower
T8 = Temperature of Cooling Water Outlet from Condenser (oC)
T7 = Temperature of Cooling Water Inlet to Condenser (oC)
Tw = Wet Bulb Temperature of air (oC)
Mechanical Engineering Dept. HITEC 57
Steam Power Plant
Thermodynamics of the Compact Steam Power Plant
Cooling Efficiency of Cooling Tower
Tw = 18 oC =
T9 = 21 oC =
T8 = 29 oC =