Steam power plant complete

Mechanical Engineering Dept. HITEC 1

Steam Power Plant

References:

Thermodynamics by Yunus A Cengel

Power Plant Engineering by PK Nag


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


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


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


Steam Power Plant

Power Plants & Types of Power PlantThermal Power Generation


Steam Power Plant


Steam Power Plant


Steam Power Plant


Steam Power Plant


Hydel Power Generation


Steam Power Plant


Nuclear Power Generation


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


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)


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


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


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


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


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


Steam Power Plant


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.


Steam Power Plant



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


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



Steam Power Plant


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


Steam Power Plant


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.


Steam Power Plant


Example


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


Steam Power Plant


ORpv

hsTs


Steam Power Plant


Fractions of total heat absorbed in Economizer, Evaporator and Super Heater:


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


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


Steam Power Plant


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


Steam Power Plant


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


Steam Power Plant


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 %


Steam Power Plant


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


Steam Power Plant


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


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


Steam Power Plant


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


Steam Power Plant


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


Steam Power Plant


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


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


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


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


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


Air Flow is directed

perpendicular to the water flow

Air Flow is directly opposite of

the water flow


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


Steam Power Plant



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


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)


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


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


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)






Steam Power PlantSteam 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)




Steam Power Plant


Overall Power Plant Efficiency

Pom = Mechanical Power Output (W)



Mechanical (with a Dynamo):

Electrical (with a Generator):

Pom = Electrical Power Output (W)


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)


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)


Steam Power Plant


Cooling Efficiency of Cooling Tower

Tw = 18 oC =

T9 = 21 oC =

T8 = 29 oC =

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