Vapour power cycle a

05/03/2023 Jhangirabad institute of technology 1

By:Naphis ahmadAssistant professorJIT, Barabanki

UNIT IV


Operating principles Vapor power plants The ideal Rankine vapor power cycle Efficiency

◦ Improved efficiency - superheat

Rankine Cycle: The Ideal Cycle for Vapor Power Cycles


The simple ideal Rankine cycle.


Assume a Carnot cycle operating between two fixed temperatures as shown.

A hypothetical vapor power cycle

T

s

1

2 3

4


All processes are internally reversible.s

T

1

2

3

4

3*

4*

The ideal Rankine cycle


All processes are internally reversible.

s

T

1

2

3

4

3*

4*

Reversible constant pressure heat rejection (4 1)

Reversible constant pressure heat addition (2 3)

Isentropic compression (1 2)

Isentropic expansion to produce work (3 4) or (3* 4*)

The ideal Rankine cycle


The ideal Rankine cycle(h-s diagram)

h

s

4

3

2WOUT

QH

QC1

WIN


s

43

12

43

hhQhhWhhW

H

IN

OUT

H

NET

QW

(3)

h

s

4

3

2WOUT

QH

QC1

WIN

Rankine cycle efficiency


FIGURE 10-6The effect of lowering the condenser pressure on the ideal

Rankine cycle.


FIGURE 10-7The effect of superheating the steam to higher temperatures on the ideal Rankine cycle.


FIGURE 10-8The effect of increasing the boiler pressure on the ideal Rankine cycle.


FIGURE 10-10T-s diagrams of the three cycles discussed in Example 9–3.


A hypothetical vapor power cycle with superheat

Superheating the working fluid raises the average temperature of heat addition.

T

s1

23

4

TH,2

TH,1


A hypothetical vapor power cycle: A Rankine cycle with superheat

s

T

d

b

c

THT

HT

Superheating the working fluid raises the average temperature with a reservoir at a higher temperature.

a


The extra expansion via reheating to state “d” allows a greater enthalpy to be released between states “c” to “e”.

s

T

f

a

b

c

p1p2

d

e

HT

CT

The Rankine cycle with reheat


FIGURE 10-11The ideal reheat Rankine cycle.


The Rankine cycle with regeneration


The first part of the heat-addition process in the boiler takes place at relatively low temperatures.


The ideal regenerative Rankine cycle with an open feedwater heater.


FIGURE 10-16The ideal regenerative Rankine cycle with a closed feedwater heater.


FIGURE 10-17A steam power plant with one open and three closed feedwater heaters.


Reheat (multiple stages) Regeneration (multiple extractions) Nearly ideal heat addition

◦ Constant temperature boiling for water

Commercial steam-power plants


Heat transfer characteristics of steam and water permit external combustion systems

Compression of condensed liquid produces a favorable work ratio.



The Rankine cycle with reheat and regeneration is advantageous for large plants.

Small plants do not have economies of scale◦ Internal combustion for heat addition.◦ A different thermodynamic cycle



Cogeneration


A simple process-heating plant.


An ideal cogeneration plant.


FIGURE 10-22A cogeneration plant with adjustable loads.


10-9 Combined /gas-Vapor Power Cycles


Combined gas–steam power plant.


Mercury–water binary vapor cycle.

05/03/2023

A steam turbine is a prime mover in which potential energy is converted into kinetic energy and then to Mechanical energy.

Potential Energy

Kinetic energy

Mechanical Energy

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Steam passageBoiler-Super heater- Economiser-Air pre heater-Turbine- Condenser

Water flowCondenser-Feed water pump- Boiler


WORK IN A TURBINE VISUALIZED

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Description of common types of Turbines.

1. Impulse Turbine.2. Reaction Turbine.

The main difference between these two turbines lies in the way of expanding the steam while it moves through them.



05/03/2023 Jhangirabad institute of technology

In the impulse turbine, the steam expands in the nozzles and it's pressure does not alter as it moves over the blades.

In the reaction turbine the steam expanded continuously as it passes over the blades and thus there is gradually fall in the pressure during expansion below the atmospheric pressure.

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PRESSURE-VELOCITY DIAGRAM FOR A TURBINE NOZZLE

ENTRANCEHIGH THERMAL ENERGY

HIGH PRESSURELOW VELOCITYSTEAM INLET

EXITLOW THERMAL ENERGY

LOW PRESSUREHIGH VELOCITY

STEAM EXHAUST

PRESSURE

VELOCITY

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Simple impulse Turbine.

It the impulse turbine, the steam expanded within the nozzle and there is no any change in the steam pressure as it passes over the blades



IMPULSE TURBINE PRINCIPLE

NOZZLE

STEAMCHEST

ROTOR


PRESSURE-VELOCITY DIAGRAM FORA MOVING IMPULSE BLADE

VELOCITY

PRESSURE

TURBINESHAFT

DIRECTION OF SPIN

ENTRANCEHIGH VELOCITYSTEAM INLET

REPRESENTS MOVINGIMPULSE BLADES

EXITLOW VELOCITY

STEAM EXHAUST


Reaction Turbine

In this type of turbine, there is a gradual pressure drop and takes place continuously over the fixed and moving blades. The rotation of the shaft and drum, which carrying the blades is the result of both impulse and reactive force in the steam. The reaction turbine consist of a row of stationary blades and the following row of moving blades

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The fixed blades act as a nozzle which are attached inside the cylinder and the moving blades are fixed with the rotor as shown in figure

When the steam expands over the blades there is gradual increase in volume and decrease in pressure. But the velocity decrease in the moving blades and increases in fixed blades with change of direction.


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Because of the pressure drops in each stage, the number of stages required in a reaction turbine is much greater than in a impulse turbine of same capacity.

It also concluded that as the volume of steam increases at lower pressures therefore the diameter of the turbine must increase after each group of blade rings.



REACTION TURBINE PRINCIPLE

STEAM CHEST

ROTOR


PRESSURE-VELOCITY DIAGRAM FORA MOVING REACTION BLADE

TURBINESHAFT

DIRECTION OF SPIN

ENTRANCEHIGH PRESSUREHIGH VELOCITYSTEAM INLET

REPRESENTS MOVINGREACTION BLADES

EXITLOW PRESSURELOW VELOCITY

STEAM EXHAUST

PRESSURE

VELOCITY


http://en.wikipedia.org/wiki/File:Turbines_impulse_v_reaction.png


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Compounding in Steam Turbine.

The compounding is the way of reducing the wheel or rotor speed of the turbine to optimum value.

Different methods of compounding are:

1.Velocity Compounding 2.Pressure Compounding 3.Pressure Velocity Compounding.

In a Reaction turbine compounding can be achieved only by Pressure compounding.



Velocity Compounding: There are number of moving blades separated by rings of fixed blades as shown in the figure. All the moving blades are keyed on a common shaft. When the steam passed through the nozzles where it is expanded to condenser pressure. It's Velocity becomes very high. This high velocity steam then passes through a series of moving and fixed blades. When the steam passes over the moving blades it's velocity decreases. The function of the fixed blades is to re-direct the steam flow without altering it's velocity to the following next row moving blades where a work is done on them and steam leaves the turbine with allow velocity as shown in diagram.

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Velocity Compounding


Pressure Compounding:

These are the rings of moving blades which are keyed on a same shaft in series, are separated by the rings of fixed nozzles.

The steam at boiler pressure enters the first set of nozzles and expanded partially. The kinetic energy of the steam thus obtained is absorbed by moving blades. The steam is then expanded partially in second set of nozzles where it's pressure again falls and the velocity increase the kinetic energy so obtained is absorbed by second ring of moving blades.

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Pressure Compounding


Pressure velocity compounding:

This method of compounding is the combination of two previously discussed methods. The total drop in steam pressure is divided into stages and the velocity obtained in each stage is also compounded. The rings of nozzles are fixed at the beginning of each stage and pressure remains constant during each stage as shown in figure. The turbine employing this method of compounding may be said to combine many of the advantages of both pressure and velocity staging By allowing a bigger pressure drop in each stage, less number stages are necessary and hence a shorter turbine will be obtained for a given pressure drop.

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PRESSURE-VELOCITY COMPOUNDED


Reaction turbine pressure compounding

Engineering

Vapour power cycle a