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Brayton Cycle

Operationg Principle

Main parameters affecting gas turbine performance

Definition of Themodynamic

External Factors

Internal Factors

ISO Condition

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•TEMPERATURE 59° F (15°C)

•BAROMETRIC PRESSURE 14,7 psi (1013 mbar)

•RELATIVE HUMIDITY 60%

•INLET/EXHAUST PRESSURE DROP 0 mmOH 2

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• IT DRAWS IN AIR FROM THE SURROUNDING ENVIRONMENT• IT COMPRESS IT TO HIGHER PRESSURE

• IT INCREASES THE ENERGY LEVEL OF THE COMPRESSED AIR BY ADDING AND BURNING FUEL IN A COMBUSTION CHAMBER

• IT DIRECTS HIGH PRESSURE, HIGH TEMPERATURE AIR TO THE TURBINE SECTION, WHICH CONVERTS THERMAL ENERGY INTO MECHANICAL ENERGY THAT MAKES THE SHAFT REVOLVE; THIS SERVES, ON THE ONE HAND, TO SUPPLY USEFUL ENERGY TO THE DRIVEN MACHINE, COUPLED TO THE MACHINE BY MEANS OF A COUPLING AND, ON THE OTHER HAND, TO SUPPLY ENERGY NECESSARY FOR AIR COMPRESSION, WHICH TAKES PLACE IN A COMPRESSOR DIRECTLY WITH THE TURBINE SECTION• IT EXHAUST LOW PRESSURE, LOW TEMPERATURE GASES RESULTING FROM THE ABOVE-MENTIONED TRANSFORMATION INTO THE ATMOSPHERE.

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EXHAUST

COMBUSTION

INLET

COMPRESSOR

TURBINE

PR

ES

SU

RE

TEM

PER

ATU

RE

ATMOSPHRIC LEVEL

TEMPERATURE

PRESSURE

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The thermodiynamic cycle of a gas turbine is known as the BRAYTON CYCLE.

The following diagram is useful to understand the meaning of the thermodynamic cycle more easily.

FUEL

COMPRESSOR

COMBUSTION

EXHAUST

LOAD

AIR INLET

1

23

4

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FUEL

COMPRESSOR

COMBUSTION

EXHAUST

LOAD

AIR INLET

P

V

T

S

1

1

1

2

2

2

3

3

3

4

4

4

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ß=10,5

963°C

(1765°F)

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P

T

V

S1

1

2

2

3

3

4

4

FUEL

COMPRESSOR

COMBUSTION EXHAUST

LOAD

AIR INLET

1

2 34

Wc

Wt

Q1

Q2

HR

Nu

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P

T

V

S1

1

2

2

3

3

4

4

FUEL

COMPRESSOR

COMBUSTION EXHAUST

LOAD

AIR INLET

1

2 34

)( 12)( 12TTcWc TTpm

Mesured in

airinletkg

Kj

_

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P

T

V

S1

1

2

2

3

3

4

4

FUEL

COMPRESSOR

COMBUSTION EXHAUST

LOAD

AIR INLET

1

2 34

)( 43)( 43TTcWt TTpm

Mesured in

gaskg

Kj

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P

T

V

S1

1

2

2

3

3

4

4

FUEL

COMPRESSOR

COMBUSTION EXHAUST

LOAD

AIR INLET

1

2 34

Mesured in

gaskg

Kj

)( 23)(1 23TTcQ TTpm

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P

T

V

S1

1

2

2

3

3

4

4

FUEL

COMPRESSOR

COMBUSTION EXHAUST

LOAD

AIR INLET

1

2 34

Mesured in

gasehxaustkg

Kj

_

)( 14)(2 14TTcQ TTpm

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P

T

V

S1

1

2

2

3

3

4

4

FUEL

COMPRESSOR

COMBUSTION EXHAUST

LOAD

AIR INLET

1

2 34

This equation tell us that, by parity of heat Q1, introduced into the combustion chamber by fuel, efficiency will increase as heat Q2 “dissipated” into the atmosphere decreases

1

21 )(

Q

QQ

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P

T

V

S1

1

2

2

3

3

4

4

FUEL

COMPRESSOR

COMBUSTION EXHAUST

LOAD

AIR INLET

1

2 34

Mesured in

cairtinletgasturbineu WGWGN _

Kj

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P

T

V

S1

1

2

2

3

3

4

4

FUEL

COMPRESSOR

COMBUSTION EXHAUST

LOAD

AIR INLET

1

2 34

uN

QHR 1

Heat Rate is the inverse of efficiency, in that it indicates the ratio between thermal energy, resulting from the combustion process, and mechanical energy, obtained on the power shaft.

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P

T

V

S

1

1

2

2

3

3

4

4

In the Brayton Cycle are very important the following parameters:

THERMAL EFFICIENCY

SPECIFIC POWER )( skgKw

FIRING TEMPERATURE 3T

PRESSURE RATIO 1

2P

P

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P

T

V

S

1

1

2

2

3

3

4

4

Section A refers the so called “TURBINE INLET TEMPERATURE”, wich is the average temperature of gas at plane A.

Section C refers to the so-called “ISO FIRING TEMPERATURE”, wich is the average gas temperature at plane C, calculated as a function of the air and fuel flow rates via a thermal balance of combustion according to the ISO 2314 procedure.

The difference in the interpretation of temperatures in section A and B consists in the fact that the section B temperature takes account of mixing with 1st stage nozzle cooling air, wich was not involved in the combustion process, but mixes with burnt gases after cooling the surface of the nozzle.

According to the NUOVO PIGNONE-GENERAL ELECTRIC standard, the temperature that best repreesents point (3) is the one in section B

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