1

SEMINAR REPORT

ON

BRAYTON CYCLE

SUBMITTED BY: GUIDED BY:

NAME : GANDHI MISAL C. Mr. VINIT PATEL

ROLL NO: 110863119011 (Lect.)

MECHANICAL ENGINEERING DEPARTMENT

LAXMI VIDYAPEETH

LAXMI INSTITUTE OF TECHNOLOGY (086)

SARIGAM

APRIL-2012

2

CERTIFICATE

This is to certify that Mr./ Ms. GANDHI MISAL C. (Enroll. No.

110863119011) completed his/her Seminar (IV Semester) on

“BRAYTON CYCLE” as a part of the requirement of B.e

(Mechanical Engineering) of Laxmi Institute of Technology,

Sarigam (086). The work presented is satisfactory.

Date : 17/04/2012 Mr. AMRAT PATEL

Place : SARIGAM (Asst. Prof.)

Name of Guide : VINIT PATEL Head of the Department

Mechanical Engineering Department

Laxmi Institute of Technology

Laxmi Vidyapeeth

Sarigam.

3

INDEX

Content Page No.

Certificate i

Preface (Abstract) ii

List of Figures iii

Index iv

4

ABSTRACT

Power generation is an important issue today, especially on the West Coast. Demand is outweighing

supply because of lack of incentives for the utilities industry to build additional power plants over the past 10-

20 years. Electrical innovations (such as the personal computer) were not accounted for in earlier predictions

of power utilization and, now, the country is in dire need of streamlining the current power plants while

pushing through as many applications as possible for new power plants. In response to this situation, power

generation engineers will be in high demand. These engineers must have a thorough understanding of

thermodynamics and, in particular, the Brayton cycle. It is the backbone of power generation. In order to

deepen knowledge of how the Brayton cycle is applied at power generation plants, an interview was

conducted via e-mail with Brian Lawson, who has obtained the P.E. designation and is the Senior Mechanical

Engineer for Sierra Pacific Power Company’s Tracy Power Generating Station. This station provides a total

electrical power output of 454 MW and supplies the majority of the population in northern Nevada. The

italicized questions and answers asked and obtained are integrated throughout the various topics to provide

further insight and understanding for the beginning engineer entering the power generation field. Further,

bolded words are defined in detail at the end of each paragraph.

5

LIST OF FIGURES

FIG. NO. FIG. NAME PAGE NO.

2.1 OPEN CYCLE GAS TURBIN 10

2.2 CLOSED CYCLE GAS TURBINE 10

2.3 AXIAL FLOW COMPRESSOR 12

2.4 COMBUSTION CHAMBER CANE 13

2.5 TURBINE 14

2.6 P-V & T-S DIA. OF BRAYTON CYCLE 16

5.1 BRAYTON CYCLE WITH REGENERATION 24

5.2 T-S DIA. OF BRAYTON CYCLE WITH

REGENERATION

25

6.1 BRAYTON CYCLE WITH INTERCOOLING, REHEAT,

& REGENERATION

27

6.2 T-S DIA. BRAYTON CYCLE WITH INTERCOOLING,

REHEAT, & REGENERATION

27

7.1 NTPC - KAWAS POWER PLANT 30

6

INDEX

CH. NO. TOPIC PAGE NO.

1 INTRODUCTION 7

2 BRAYTON CYCLE 9

3 BRAYTON CYCLE ANALYSIS 17

4 DEVELOPMENT OF GAS TURBINE 21

5 BRAYTON CYCLE WITH REGENERATION 23

6 BRAYTON CYCLE WITH INTERCOOLING, REHEAT,

& REGENERATION

26

7 BRAYTON CYCLE USED IN INDUSTRY 29

7

8

INTRODUCTION

The basic gas turbine cycle is named for the Boston engineer, George Brayton, who first proposed the Brayton cycle around 1870.Now, the Brayton cycle is used for gas turbines only where both the compression and expansion processes take place in rotating machinery.

John Barber patented

the basic gas turbine in 1791. The two major application areas of gas-turbine engines are aircraft

propulsion and electric power generation. Gas turbines are used as stationary power plants to generate electricity as stand-alone units or in conjunction with steam power plants on the high-temperature side. In these plants, the exhaust gases serve as a heat source for the steam. Steam power plants are considered external-combustion engines, in which the combustion takes place outside the engine. The thermal energy released during this process is then transferred to the steam as heat.(2) The gas turbine first successfully ran in 1939 at the Swiss National Exhibition at Zurich. (3) The early gas turbines built in the 1940s and even 1950s had simple-cycle efficiencies of about 17 percent. This was because of low compressor and turbine efficiencies and low turbine inlet temperature due to metallurgical limitations at the time. The first gas turbine for an electric utility was installed in 1949 in Oklahoma as part of a combined-cycle power plant. It was built by General Electric and produced 3.5 MW of power. (2)

In the past, large coal and nuclear power plants dominated the base-load electric power

generation. However, natural gas-fired turbines now dominate the field because of their black start capabilities, higher efficiencies, lower capital costs, shorter installation times, better emission characteristics, and abundance of natural gas supplies. The construction cost for gas-turbine power plants are roughly half that of comparable conventional fossil-fuel steam power plants, which were the primary base-load power plants until the early 1980s. More than half of all power plants to be installed in the foreseeable future are forecast to be gas-turbine or combined gas-steam turbine types.

In the early 1990s, General Electric offered a gas turbine that featured a pressure ratio of 13.5

and generated 135.7 MW of net power at a thermal efficiency of 33 percent in simple-cycle operation. A

more recent gas turbine manufactured by General Electric uses a turbine inlet temperature of 1425°C

(2600°F) and produces up to 282 MW while achieving a thermal efficiency of 39.5 percent in the

simple-cycle mode. (2) Current low prices for crude oil make fuels such as diesel, kerosene, jet-engine

fuel, and clean gaseous fuels (such as natural gas) the most desirable for gas turbines. However, these

fuels will become much more expensive and will eventually run out. Provisions must therefore be made

to burn alternative fuels.

9

10

Brayton Cycle

FIG. 2.1 Open Cycle Gas Turbine

FIG. 2.2 Closed Cycle Gas Turbine

11

Gas turbines usually operate on an open cycle, as shown in Figure 1. Fresh air at ambient conditions is

drawn into the compressor, where its temperature and pressure are raised. The high-pressure air proceeds into

the combustion chamber, where the fuel is burned at constant pressure. The resulting high-temperature gases

then enter the turbine, where they expand to the atmospheric pressure through a row of nozzle vanes.

This

expansion causes the turbine blade to spin, which then turns a shaft inside a magnetic coil. When the shaft is

rotating inside the magnetic coil, electrical current is produced. The exhaust gases leaving the turbine in the

open cycle are not re-circulated.

The open gas-turbine cycle can be modeled as a closed cycle as shown in Figure 2 by utilizing the air-

standard assumptions. Here the compression and expansion process remain the same, but a constant-pressure

heat-rejection process to the ambient air replaces the combustion process. The ideal cycle that the working

fluid undergoes in this closed loop is the Brayton cycle, which is made up of four internally reversible

processes:

1-2 Isentropic compression (in a compressor)

2-3 Constant pressure heat addition

3-4 Isentropic expansion (in a turbine)

4-1 Constant pressure heat rejection

12

Brayton Cycle Components

1. Compressor

2. Combustion Chamber

3. Turbine

Compressor

FIG. 2.3 Axial Flow Compressor

Efficient compression of large volumes of air is essential for a successful gas turbine engine. This has

been achieved in two types of compressors, the axial-flow compressor and the centrifugal – or radial-flow

compressor. Most power plant compressors are axial-flow compressors. The object of a good compressor

design is to obtain the most air through a given diameter compressor with a minimum number of stages while

retaining relatively high efficiencies and aerodynamic stability over the operating range. Compressors contain

a row of rotating blades followed by a row of stationary (stator) blades. A stage consists of a row of rotor and

a row of stator blades. All work done on the working fluid is done by the rotating rows, the stators converting

the fluid kinetic energy to pressure and directing the fluid into the next rotor. The fluid enters with an initial

velocity relative to the blade and leaves with a final relative velocity at a different angle.

13

Combustion Chamber

FIG. 2.4 Combustion Chamber Cane

Combustion is the chemical combination of a substance with certain elements, usually oxygen,

accompanied by the production of a high temperature or transfer of heat. The function of the combustion

chamber is to accept the air from the compressor and to deliver it to the turbine at the required temperature,

ideally with no loss of pressure. Essentially, it is a direct-fired air heater in which fuel is burned with less than

one-third of the air after which the combustion products are then mixed with the remaining air. For the

common open-cycle gas turbine, this requires the internal combustion of fuel. This means the problem of fuel

operation, mixing and burning, must be addressed. Fuel is commonly gaseous or liquid. Solid fuel has not yet

advanced beyond the experimental stage. Gaseous or liquid fuels are usually hydrocarbons. Gases usually

being natural gas, mostly methane, and butane. Liquids may range from highly refined gasoline through

kerosene and light diesel oil to a heavy residual oil (Bunker C or No. 6 fuel oil). Combustion itself is seldom

difficult. The difficulty arises in the combination of combustion with low-pressure loss in a size of combustor

compatible with the high power-weight, high specific output potentialities, or the rotating elements. Almost

any fuel can be burnt successfully if sufficient pressure drop is available to provide the necessary turbulence

for mixing of air and fuel and if sufficient volume is available to give the necessary time for combustion to be

completed.

14

Turbine

FIG. 2.5 TURBINE

Gas turbines move relatively large quantities of air through the cycle at very high velocities. Among

the mechanical characteristics of gas turbine engines are very smooth operation and absence of vibration due

to reciprocating action. The high rotational speeds utilized require very accurate rotor balancing to avoid

damaging vibration. Rotor parts are highly stressed with low factors of safety. Blades are very finely tuned to

avoid resonant vibration. Gas turbines have relatively few moving (and no sliding) parts and are not subjected

to vibratory forces. As a result, they are highly reliable when properly designed and developed. The gas

turbine in its most common form is a heat engine operating through a series of processes. These processes

consist of compression of air taken from the atmosphere, increasing of gas temperature by the constant-

pressure combustion of fuel in the air, expansion of the hot gases, and finally, discharging of the gases to

atmosphere, in a continuous flow process. It is similar to the gasoline and Diesel engines in its working

medium and internal combustion, but is like the steam turbine in the steady flow of the working medium. The

compression and expansion processes are both carried out by means of rotating elements in which the energy

transfer between fluid and rotor is affected by means of kinetic action, rather than by positive displacement as

in reciprocating machinery.

15

Air-standard assumptions

Assumptions that the compression and expansion processes are adiabatic (insulated) and reversible

(isentropic), that there is no pressure drop during the heat addition process, and that the pressure leaving the

turbine is equal to the pressure entering the compressor.

Internally reversible processes

Thermodynamics states that, for given temperature limits, a completely reversible cycle has the

highest possible efficiency and specific work output, reversibility being both mechanical and thermal.

Mechanical reversibility is a succession of states in mechanical equilibrium, i.e. fluid motion without friction,

turbulence, or free expansion. Thermal reversibility is a consequence of the Second Law of thermodynamics,

which states that heat must be added only at the maximum temperature of the cycle and rejected at the

minimum temperature.

16

Brayton Cycle p-v & T-S Diagram

FIG 2.6 P-V & T-S DIAGRAM OF BRAYTON CYCLE

1-2 Isentropic compression (in a compressor)

2-3 Constant-pressure heat addition

3-4 Isentropic expansion (in a turbine)

4-1 Constant-pressure heat rejection

17

18

Brayton cycle analysis

As with any cycle, we’re going to concern ourselves with the efficiency and net work output:

Efficiency:

Net work:

1 to 2 (isentropic compression in compressor), apply first law:

When analyzing the cycle, we know that the compressor work is in (negative). It is standard convention to

just drop the negative sign and deal with it later:

2 to 3 (constant pressure heat addition - treated as a heat exchanger)

3 to 4 (isentropic expansion in turbine)

4 to 1 (constant pressure heat rejection)

We know this is heat transfer out of the system and therefore negative. In book, they’ll give it a positive sign

and then subtract it when necessary.

Let’s get the net work:

in

net

q

w

compturbnet www

comp 2 1w h h

12 hhwcomp

2323in hhqq

or,hhw 34turb

43turb hhw

,hhq 41out

14out hhq

compturbnet www

19

Substituting:

Let’s get the efficiency:

Let’s assume cold air conditions and manipulate the efficiency expression:

Using the isentropic relationships

Let’s define:

)h(h)h(hw 1243net

in

net

q

w

)h(h

)h(h)h(h

23

1243

)h(h

)h(h1

23

14

)(T

)(T1

23

14

Tc

Tc

p

p

1

11

23

14

2

1

TT

TT

T

T

;

1

1

2

1

2k

k

p

p

T

T

k

k

k

k

p

p

p

p

T

T1

2

1

1

3

4

3

4

4

3

1

2

P

P

P

Pratiopressurerp

20

Let’s assume cold air conditions and manipulate the efficiency expression:

Then we can relate the temperature ratios to the pressure ratio:

Plug back into the efficiency expression and simplify:

)(T

)(T1

23

14

Tc

Tc

p

p

1

11

23

14

2

1

TT

TT

T

T

4

31

1

2

T

Tr

T

T kk

p

kk

pr 1

11

21

22

Development of Gas Turbines

The gas turbine has experienced phenomenal progress and growth since its first successful

development in the 1930s. The early gas turbines built in the 1940s and even 1950s had simple cycle

efficiencies of about 17 percent because of the low compressor and turbine efficiencies and low turbine inlet

temperatures due to metallurgical limitations of those times. Therefore, gas turbines found only limited use

despite their versatility and their ability to burn a variety of fuels. The efforts to improve the cycle efficiency

concentrated in three areas:

1. Increasing the turbine inlet (or firing) temperatures:

This has been the primary approach taken to improve gas-turbine efficiency. The turbine inlet

temperatures have increased steadily from about 540°C in the 1940s to 1425°C and even higher today. These

increases were made possible by the development of new materials and the innovative cooling techniques for

the critical components such as coating the turbine blades with ceramic layers and cooling the blades with the

discharge air from the compressor. Maintaining high turbine inlet temperatures with an air-cooling technique

requires the combustion temperature to be higher to compensate for the cooling effect of the cooling air.

However, higher combustion temperatures increase the amount of nitrogen oxides (NOx), which are

responsible for the formation of ozone at ground level and smog. Using steam as the coolant allowed an

increase in the turbine inlet temperatures by 200°F without an increase in the combustion temperature. Steam

is also a much more effective heat transfer medium than air.

2. Increasing the efficiencies of turbo machinery components

The performance of early turbines suffered greatly from the inefficiencies of turbines and

compressors. However, the advent of computers and advanced techniques for computer-aided design made it

possible to design these components aerodynamically with minimal losses. The increased efficiencies of the

turbines and compressors resulted in a significant increase in the cycle efficiency.

23

24

THE BRAYTON CYCLE WITH REGENERATION

In gas-turbine engines, the temperature of the exhaust gas leaving the turbine is often considerably higher than

the temperature of the air leaving the compressor. Therefore, the high pressure air leaving the compressor can

be heated by transferring heat to it from the hot exhaust gases in a counter-flow heat exchanger, which is also

known as a regenerator or a recuperator.

A sketch of the gas-turbine engine utilizing a regenerator and the T-s diagram of the new cycle are shown in

Figs. The thermal efficiency of the Brayton cycle increases as a result of regeneration since the portion of

energy of the exhaust gases that is normally rejected to the surroundings is now used to preheat the air

entering the combustion chamber. This, in turn, decreases the heat input (thus fuel) requirements for the same

net work output. Note, however, that the use of a regenerator is recommended only when the turbine exhaust

temperature is higher than the compressor exit temperature. Otherwise, heat will flow in the reverse direction

(to the exhaust gases), decreasing the efficiency. This situation is encountered in gas-turbine engines

operating at very high pressure ratios.

FIG 5.1 BRAYTON CYCLE WITH REGENERATION

25

FIG. T-S DIAGRAM

The highest temperature occurring within the regenerator is T4, the temperature of the exhaust gases

leaving the turbine and entering the regenerator. Under no conditions can the air be preheated in the

regenerator to a temperature above this value. Air normally leaves the regenerator at a lower temperature, T5.

In the limiting (ideal) case, the air exits the regenerator at the inlet temperature of the exhaust gases T4.

26

27

THE BRAYTON CYCLE WITH INTERCOOLING, REHEATING AND REGENERATION

FIG 6.1 BRAYTON CYCLE WITH INTERCOOLING, REHEATING AND REGENERATION

FIG. 6.2 T-S DIAGRAM

28

The net work of a gas-turbine cycle is the difference between the turbine work output and the

compressor work input, and it can be increased by either decreasing the compressor work or increasing the

turbine work, or both. As mentioned before that the work required to compress a gas between two specified

pressures can be decreased by carrying out the compression process in stages and cooling the gas in between

—that is, using multistage compression with inter cooling. As the number of stages is increased, the

compression process becomes nearly isothermal at the compressor inlet temperature, and the compression

work decreases. Likewise, the work output of a turbine operating between two pressure levels can be

increased by expanding the gas in stages and reheating it in between—that is, utilizing multistage expansion

with reheating. This is accomplished without raising the maximum temperature in the cycle. As the number of

stages is increased, the expansion process becomes nearly isothermal. The foregoing argument is based on a

simple principle: The steady-flow compression or expansion work is proportional to the specific volume of the

fluid. Therefore, the specific volume of the working fluid should be as low as possible during a compression

process and as high as possible during an expansion process.

Combustion in gas turbines typically occurs at four times the amount of air needed for complete

combustion to avoid excessive temperatures. Therefore, the exhaust gases are rich in oxygen, and reheating

can be accomplished by simply spraying additional fuel into the exhaust gases between two expansion states.

The working fluid leaves the compressor at a lower temperature and the turbine at a higher

temperature, when intercooling and reheating are utilized. This makes regeneration more attractive since a

greater potential for regeneration exists. Also, the gases leaving the compressor can be heated to a higher

temperature before they enter the combustion chamber because of the higher temperature of the turbine

exhaust.

A schematic of the physical arrangement and the T-s diagram of an ideal two-stage gas-turbine cycle

with intercooling, reheating, and regeneration are shown in Figs.The gas enters the first stage of the

compressor at state 1, is compressed isentropically to an intermediate pressure P2, is cooled at constant

pressure to state 3 (T3 = T1), and is compressed in the second stage isentropically to the final pressure P4. At

state 4 the gas enters the regenerator, where it is heated to T5 at constant pressure. In an ideal regenerator, the

gas leaves the regenerator at the temperature of the turbine exhaust, that is, T5 = T9. The primary heat

addition (or combustion) process takes place between states 5 and 6. The gas enters the first stage of the

turbine at state 6and expands isentropically to state 7, where it enters the reheater. It is reheated at constant

pressure to state 8 (T8 = T6), where it enters the second stage of the turbine. The gas exits the turbine at state 9

and enters the regenerator, where it is cooled to state 10 at constant pressure. The cycle is completed by

cooling the gas to the initial state (or purging the exhaust gases).

29

30

BRAYTON CYCLE USED IN INDUSTRY

FIG. 7.1 NTPC – KAWAS

Kawas combined cycle Gas Power Project situated on the western seacoast around 15kms. From ―Silk

City‖ Surat in the state of Gujarat, is one of the prestigious combined cycle gas power station of its kind both

within NTPC and in the country. The unique features of the power station are multi fuel firing facility in the

Gas Turbine, a black start facility of 2.85 MW operational on HSD for start-up of the plant and maintaining

the plant mandatory systems in case of grid failure and a simulator complex for the combined cycle plant to

train power engineers.

31

MANUFACTURING PROCESS

STEP-1: STORAGE AND HANDLING OF HSD/ARN/NGL

HSD that is coming by tankers to the LFHS is manually measured and unloaded through pipeline by a

set of pumps and stored in HSD storage tank ARN/NGL that is transported from ONGC/HPCL depot through

pipeline. HPCL is received directly in the Naptha storage tanks. Measurement is done by fliw meters installed

in the line. In the case of emergency, ARN/NGL through road tankers is unloaded in LFHS unloading bay

using unloading pumps.

STEP-2: SUPPLY OF HSD/ARN.NGL TO GAS TURBINES

HSD and ARN/NGL are stored in storage tanks in the tank farm area. HSD is supplied to the units by

2 nos. forwarding pumps provided in LFHS pump house. Whenever there is a demand in HSD the forwarding

pump will start and the filtered and metered HSD is sent to the on-base system through a 3-way valve. If any

of the unit is running on the HSD/ARN/NGL the forwarding pump will be running. Naptha /NGL are supplied

to the units by 3 nos. Naptha /NGL/HSD is received and stored in liquid fuel handling station and is pumped

to the unit through a pipeline.

STEP- 3: SUPPLY OF RAW WATER TO RESERVOIR AND PT PLANT

2 Kiroskar make pumps are provided at the various pumping station of NTPC, KAWAS, for the

supply of raw water from the Tapti River through pipeline to NTPC plant site. Raw water can also be taken

from Gujarat state irrigation canal in case of emergency or availability. Raw water from the dead reservoir

after setting goes to live reservoir from where the 3 raw water pumps take section. The discharge from raw

water pumps is given to PT plant.

STEP- 4: GENERATION OF ELECTRICITY BY THE OPERATION AND

CONTROL OF GAS TURBINE WITH GAS/ LIQUID FUEL

There are 4 gas turbines (GT’s) that can be operated either on gas or on liquid fuel for the purpose of

generation of electricity. Each GT has a compressor, combustors, gas turbine, a generator and some auxiliaries

coupled to it. Filtered air form atmosphere is compressed in the compressor and sent to combustors.

STEP- 5: MONITORING AND MEASUREMENT OF COMBUSTION OF FUEL

IN GT

There are 4 gas turbines (GT’s) that can be operated either on gas or on liquid fuel. . Each GT has a

compressor, combustors, gas turbine, a generator and some auxiliaries coupled to it. The fuel gas from the gas

turbine outlet is delivered through the duct into the bypass stack during open cycle generation, which is let out

to the atmosphere at a height of 55mtr. In the case of combined cycle generation the fuel gas instead of

passing through the bypass stack is diverted through a diverter assembly into waste heat recovery boiler.

32

STEP- 6: GENERATION OF STEAM IN WHRB AND SUPPLY OF STEAM TO

STEAM TURBINE

The exhaust of gas turbine is diverted to the Waste Heat Recovery Boiler (WHRB) to recover the heat

through steam generation. The fuel gas passes through the vertical type boiler thereby transferring its heat

content to DM water and transforming it into superheated steam.

STEP- 7: MONITORING AND MEASUREMENT OF STEAM AND WATER

CYCLE

The DM water from DM plant is taken in the condenser. The DM water required for the boiler is

supplied from DM plant which is taken in condenser. The condensate is pumped through the pre heated circuit

into the de-aerator wherein the dissolved oxygen is removed to the desire level.

The de-aerated water is sent through the HP and LP economizer into the respective drums of a boiler by two

different sets of pumps taking from deaerator. The HP and LP drum water are circulated through the

evaporators by HP and LP circulation pumps whereby water is converted into steam and is collected in the

respective drums.

The saturated steam from the drums are passed through the super heaters and then sent into the steam

turbine to generate motive power for a generator. HP steam enters the HP turbine, after expansion mixes up

with the LP steam, and enters the LP turbine. The steam turbine is rated to generate 116.1 MW. Since total

heat cannot be converted into mechanical power, large quantity of water is needed to cool the residual steam.

This cooling water, after becoming hot, is

cooled in cooling towers.

STEP- 8: SUPPLY OF GENERATED ELECTRICITY TO WRLDC

Power generated in all the six units is supplied to western region grid through 220 KV transmission

lines. Name of the transmission lines are given below

a) Kawas- Navsari line 1

b) Kawas- Navsari line 2

c) Kawas- Valthan single line

d) Kawas- Ichhapore single line

e) Kawas- Haldarva line 1

f) Kawas- Haldarva line 2