GAS TURBINES

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COURSE OBJECTIVES

At the end of the course, delegates will be able to:

1- Explain the function of a gas turbine.

2- List the most common types of gas turbines in use.

3- Understand the thermodynamic principles of gas turbine systems.

4- Understanding some of the criteria for selecting a gas turbine for a given set of

design conditions.

5- Explain the main aspects of gas turbine protection and control.

6- Appreciate how the design of an overall-plant control scheme relates to the gas

turbine system.

7- Explore the safe and efficient operation of a gas turbine system, using dynamic

simulation model.

8- Explore common operating problems.

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CONTENTS

1- INTRODUCTION

- Fluid mechanics properties and laws

- Thermodynamics principles

- Definition of a gas turbine

- Historical background about gas turbine

- Brief idea about types of gas turbine

- Brief idea about advantages of gas turbine

- Brief idea about applications of gas turbine

2- TECHNICAL PRINCIPLES OF GAS TURBINES

- Working principles of gas turbine

- Turbine operation

3- MAIN COMPONENTS OF GAS TURBINES

- Compressor

- Combustion

- Turbine

- Air intake system

- Exhaust System

4- AUXILIARY SYSTEMS FOR GAS TURBINES

- Lube oil system

- Fuel oil system

- Fuel gas system

- Hydraulic trip oil system

5- OPERATION AND MAINTENANCE OF GAS TURBINES

- Introduction

- Turbine operation

- Maintenance

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UNIT (1)

INTRODUCTION

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Definition of a gas turbine

1- Definitions:

Since long time, the theory and method by which gas turbine operates was

known even before knowing the material from which the gas turbine should be built

and before knowing the fluid mechanics theories and fluid flow. Engineers waited

the development of material science very long to obtain some materials that endure

high temperatures that happen in the turbine system.

The gas turbine is defined as a type of prime movers that can transform the fuel

energy to useful mechanical energy developed at its shaft at high rotational speeds.

The gas turbine consists, in its simplest form, of two main parts, they are:

1- Gas generator section.

2- Power conversion section.

The gas generator section by itself consists of:

One- The compressor.

Two- The combustion chamber,

Three- The turbine, the turns the compressor.

There are different types of gas turbines depending on the nature of inlet and

exit of the gases. Figures (1/1, 2/1, 3/1, 4/1) show the different types of these

turbines. Figure (1/1) is a section of a gas turbine that operates according to the basic

cycle, while Fig. (2/1) is a section in a turbo-jet that is equipped with afterburner. As

for Fig. (3/1) is a section in non-mixed turbo-fan engine.

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PRINCIPLES OF

FLUID MECHANICS

&

THERMODYNAMICS

FLUID MECHANICS

♣♣♣♣ PROPERTIES OF FLUIDS

The equations of fluid mechanics allow us to predict the behavior of fluids in

various flow situations. To use the equations, however, there must be

information regarding properties. The properties, which are discussed in this

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chapter, include viscosity, pressure, density, kinematic viscosity, surface

tension, specific heat, internal energy, enthalpy, and compressibility.

♥♥♥♥ Viscosity:

A fluid has many properties. One important property is viscosity,

which is a measure of the resistance the fluid has to an external applied

shear. Because this property arises from the definition of a fluid, it is

examined in that regard. Consider again a fluid-filled space formed by

two horizontal parallel plates shown in figure. The upper plate has an

area A in contact with the fluid and is pulled to the right with a force F/

at a velocity Vi. If the velocity at each point within the fluid could be

measured, a velocity distribution like that illustrated in the figure might

result. The fluid velocity at the moving plate is Vi because the fluid

adheres to that surface. At the bottom, the velocity is zero with respect to

the boundary, owing to the non-slip condition. The slope of the velocity

distribution is: dV1/dy.

If this experiment is repeated with F2 as the force, a different slope or

strain rate results: dV1/dy. In general, to each applied force there corresponds

only one shear stress and only one strain rate. If data from a series of these

experiments were plotted as T versus dV1/dy., the shown figure would result

for a fluid such as water. The points lie on a straight line that passes through

the origin. The slope of the resulting line in the figure is the viscosity of the

fluid because it is a measure of the fluid's resistance to shear. In other words,

viscosity indicates how a fluid will react (dV/ d y) under the action of an

external shear stress (τ).

The plot of that figure is a straight line that passes through the origin.

This result is characteristic of a Newtonian fluid, but there are other types of

fluids called non-Newtonian fluids. A graph of T versus dV/ dy, called a

rheological diagram, is shown in figure for several types of fluids.

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Newtonian fluids follow Newton's law of viscosity and are represented by

the equation

)/( dydVµτ =

Where:

τ = the applied shear stress in dimensions of F/L2{ Ibf/ft2 or N/m2)

µ = the absolute or dynamic viscosity of the fluid in dimensions of F. T/L2 (Ibf.s/ft

2 or

N.s/m2 )

dV/ dy = the strain rate in dimensions of 1/T (rad/s).

In the cgs system of units, the unit for viscosity is poise, corresponding

to 1 g/cm .s. The centipoise is 1/100 of a poise. The SI system, unit for

viscosity is 1 kg/m.s. It has no particular name. It is 10 times the size of the

poise, as it is clear from the basic units.

Examples of Newtonian fluids are water, oil, and air. If a fluid cannot be

described by Equation 1.3, it is called a non-Newtonian fluid. On the basis

of their behavior, these fluids are divided into three categories: time-

independent, time-dependent, and viscoelastic as in figure.

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♥♥♥♥ Density:

The density of a fluid is its mass per unit volume, represented by the

letter p. If the mass of 1 ft3 of water is 1.94 slug, its density is p = 1.94

slug/ft3. If the mass of 1 m3 of liquid is 820 kg, its density is p = 820 kg/m3.

Density has dimensions of M/L3. The density of various substances is given

in the property tables in the appendices of any Fluid Mechanics HandBook.

One quantity of importance related to density is specific weight.

Whereas density is mass per unit volume, Specific weight is weight per unit

volume. Specific weight is related to density by:

SW = ρg , with dimension F/L3 (Ibf/ft

3 or N/m

3)

Another useful quantity is specific gravity, which is also related to

density of a substance. The specific gravity of a substance is the ratio of its

density to the density of water at 4 °C:

SG= ρ /ρw (ρw density of water)

For usual ρw is taken to be 1.94 slug/ft3 or 1000 kg/m

3.

♥♥♥♥ Specific heat:

The specific heat of a substance is the heat required to raise a unit mass

of | the substance by 1° . The dimension of a specific heat is energy/ (mass.

temperature): F. L /(M. t). The process by which the heat is added also

makes a difference, particularly for gases. The specific heat for a gas that

undergoes a process occurring at constant pressure involves a different

specific heat than that for a constant volume process. For example, the

specific heat at constant pressure Cp for air is 0.24 Btu/(Ibm . °R), or 1005

J/(kg . K), and the specific heat at constant volume Cy is 0.17 Btu/(Ibm .

°R), or 717 J/(kg . K). Also of importance when dealing with these

properties is the ratio of specific heats, defined as

γ = Cp /Cv, (For air, the ratio of specific heats y is 1.4)

The Btu (British thermal unit) is the unit of energy measurement in the

English engineering System. One Btu is defined as the energy required to

raise the temperature of 1 Ibm of water by 1 °F. However, because we are

using the British gravitational system, the units encountered are Btu/(slug.

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°R). Both specific heats vary with temperature for real substances, but to

simplify calculations they are assumed to be constant.

♥♥♥♥ Internal Energy:

Internal energy is the energy associated with the motion of molecules of

a substance. Consider a quantity of gas. The gas can have three types of

energy: energy of position (potential energy), energy of translation (kinetic

energy), and energy of molecular motion (internal energy). Adding heat to a

quantity of gas at constant volume affects only the motion of the molecules

and does not increase the potential or kinetic energies of the gas. This effect

is manifested as an increase in temperature. In fact, for a perfect gas with

constant specific heats, it can be shown that:

∆ U = Cv ∆ T

where ∆ U is a change in the internal energy per unit mass of the gas with dimensions of energy /mass (F. L / M).

♥♥♥♥ Enthalpy:

A quantity that appears often in equations is (u + p/r), this quantity is

given the special name enthalpy, h. adding heat at constant pressure goes

into increasing the internal energy of the gas and raising the position. Again

for a perfect gas with constant specific heats, it can be shown that:

∆ h = Cp ∆ T

♥♥♥♥BERNOULLI'S THEOREM:

Bernoulli's Theorem is a special application of the first law of

thermodynamics for flowing fluids. The energy content of a flowing fluid

can be split into three components:

Elevation Head, Velocity Head, Pressure Head

Bernoulli's Theorem states that the sum of these three energy terms at one

set of conditions is equal to their sum at another set of conditions.

Accelerating fluid

Bernoulli's equation shows that we can convert the Kinetic energy of a gas

to pressure (potential) energy by decreasing it's velocity.

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Velocity Head + Pressure Head + Elevation Head = Constant

Decrease in velocity head causes an increase in pressure head.

Bernoulli's Equation

conszg

p

g

U=++

ρ2

2

U = fluid velocity, g = gravitational constant, p = pressure

ρ = density, z = elevation head

headvelocityg

U=

2

2

, headpressureg

p=

ρ , z = Elevation Head

NB: Each term has dimensions of length and can be regarded as

representing a contribution to the total fluid head. Incompressible,

frictionless flow is assumed.

THERMODYNAMICS

PRESSURE: The pressure is the normal force per unit area, and its

dimension is (N/m2). The pressure measured by any instrument, that reads

zero when it is open to the atmosphere, is called gauge pressure. The

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absolute pressure is the sum of gauge pressure plus the atmospheric

pressure. The standard atmospheric pressure, (at sea level and 15 oC), is:

1 bar (105 Pa or 10

5 N/m

2)

or 76 cm Hg

or 10.3 m of H2O

or 14.7 Psi

Then:

Absolute pressure = gauge pressure + atmospheric pressure

TEMPERATURE: Two types of temperature scales are found, namely

centigrade and Fahrenheit. The absolute temperature is the local

temperature plus 273 for centigrade scale or 460 for Fahrenheit scale. To

convert from one scale to another the following two relation are used:

C= (F-32) x (5/9)

F= C x (9/5) +32

First Law Of Thermodynamics (Conservation Of Energy): Energy

cannot be created nor destroyed during a process (e.g. compression),

although it may change from one form to another.

Potential Energy: Energy due to a body's elevation (pressure).

Kinetic Energy: Energy due to a body's motion (velocity).

From the first law of thermodynamics, it is possible to convert energy

from one form to another. To understand the principles of compression we

need to consider thermodynamics.

Gas Laws:

•••• Boyle's Law:

At constant temperature, the volume of an ideal gas is inversely

proportional to the pressure.

pV

1α

Therefore: PV = Constant, Where: V = Volume and P = Pressure

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•••• Charles' Law

At constant pressure, the volume of an ideal gas is proportional absolute

temperature.

TVα

Therefore: V/T = Constant, Where: V = Volume and T =

Absolute temperature

•••• Ideal Gas Equation

From Boyle's Law and Charles' Law: PV = RT Where: R = Universal

Gas Constant

This is true for all ideal gases, no gases are ideal and this equation is

corrected by using compressibility factors, which are determined

experimentally. Several forms of ideal gas equation are as follows:

Pv = RT

PV = mRT

PV = nMRT

PV = nℜT

(Where: P is the gas absolute pressure, v is the gas specific volume, R is

the gas constant, T is the gas absolute temperature, m is the mass of the

gas, n is number of moles of the gas, M is the gas molecular weight, is the

universal gas constant and ℜ=MR)

•••• Decreasing Volume

From Boyle's Law (PV = Constant), it can be seen that if you reduce

the volume of a gas then it's pressure will increase. PV = Constant, as V

reduces, P increases (assuming constant temperature).

•••• Compressibility

All gases deviate from the ideal gas law and these deviations are

accounted for by the compressibility factor (Z). The ideal gas equation

is modified to:

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PV = ZRT, Rearranging:

RT

pvZ = Z is experimentally derived from actual

gas data and is often generated from equations of state.

•••• The Second Law of Thermodynamics

This is more abstract and can be stated several ways.

♣ Heat cannot, of itself, pass from a colder to a hotter body. ♣ Heat can be made to go from a body at lower temperature to one at higher temperature only if external work is done.

♣The available energy of the isolated system decreases in all real processes.

♣ Heat or energy (or water), of itself, will flow only downhill.

Basically, these statements say that energy exists at various levels and is

available for use only if it can move from a higher to a lower level. In

Thermodynamics a measure of the unavailability of energy has been

devised and is known as entropy. It is defined by the differential equation:

T

dQdS = , Where: S = Entropy, Q = Heat, T = Temperature

Note that: entropy (as a measure of unavailability) increases as a system

loses heat, but remains constant when there is no gain or loss of heat (as in

an adiabatic process).

DEFINITIONS:

♥ISOTHERMAL PROCESS: constant temperature process

♥POLYTROPIC PROCESS: a reversible process

♥ADIABATIC PROCESS: no heat gain to or lost from the system

♥ISENTROPIC PROCESS:reversible and adiabatic process

•••• Compressor Cycles

Compressors are compared to theoretical compression cycles as a basis

for calculations and comparisons.

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♥♥♥♥Isothermal Compression: This occurs when the temperature is kept

constant as the pressure increases. This requires continuous heat removal.

constVpVp == 2211

It is not commercially possible to remove all heat although compressors

are usually designed for as much heat removal as possible.

♥♥♥♥Adiabatic Reversible (Isentropic) Compression: This occurs when no

heat is added or removed during compression.

constVpVp kk == 2211 , Where k = ratio of specific heat

Adiabatic compression is never obtained because there is always some

heat removed or added.

♥♥♥♥ Polytropic Compression: This is the cycle along which actual compression takes place.

constVpVp nn == 2211

The exponent n is determined experimentally for a given type of

machine and may be lower or higher than the adiabatic exponent k. In

positive displacement compressors n is usually less than k.

n or n

n 1− can be calculated from test data if the suction and discharge

n pressures and temperatures are known.

n

n

p

p

T

T1

1

2

1

2

−

= (can be derivation)

2- Historical background about gas turbines:

One- Early model of gas turbine:

Utilization of gas turbines, driven by combustion gases flowing from fire,

began since long time. It was during the era of the Hero of Alexandria at year

150 BC. In addition, at the same time Chinese used windmills (that may be

considered one type of gas turbines).

Two- Gas turbine models between 1791-1930:

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1- John Barbar turbine (Fig. 4/1):

It is to be sure that the real gas turbines era had begun in 1791 when John

Barbar get a patent for his pioneer invention concerning gas turbines. That

invention consists of reciprocating compressor and a combustion chamber

and an impulse turbine. He was able to recognize the need of cooling the

turbine blades and suggested the water injection.

2- Stolze turbine:

In year 1872, Stolze was able to get a patent for his invention concerning

gas turbines. The design of that invention consisted of the following parts:

• Multi-stage axial flow compressor, which perhaps was the first type at

all.

• Multi-stage reaction turbine whose shaft is connected to the shaft of the

compressor.

• Heat exchanger. • Combustion chamber. The turbine was ten stages, while the compressor was only nine stages.

3- Charles G. Curtiss turbine:

In June 1884, Charles G. Curtiss presented a complete gas turbine that

was the first gas turbine designed in USA.

4- Stolze second turbine:

The second gas turbine built by Stolze in France in year 1900, was the

first real turbine capable to produce work. However, the tests carried on it

were not encouraging because its very low unsatisfactorily efficiency.

5- Armanganed Brother’s turbine:

Armanganed Brother carried series of effective trials to build a large gas

turbine in Paris at the same time during which Stolze was building his

turbine. They began their trial on a 25-HP De Laval turbine using an air

compressor that work by the compressed air from the principal Paris

compressed air network. After this, they produced another turbine using

a centrifugal compressor of four bars designed by Rateau in year 1905. In

addition, they used Curtiss turbine wheel whose diameter was 37.4 inches.

The turbine was running at 4250 rpm, while the turbine exit gas

temperature was 1040 oF. That turbine was able to produce compressed

air instead of mechanical power and its thermal efficiency was 3%.

6- Holzwarth turbine:

The efforts of Holzwarth in the field of gas turbines were the most

important efforts appeared after this, and to him was the grace in building

the first practical and economical gas turbine. Holzwarth turbine was

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operating by explosion cycle without pre-compression. It was a rotating

engine of intermittent combustion. The first turbine of this type was built

and tested in Hanover. Who is interested of more details about Holzwarth

turbine should look to Stodola book.

7- Sanford Moss turbine:

Sanford Moss was able to operate the first turbine in USA in year 1919,

and at the same year, the first civil airplane could fly using a gas turbine.

Three- Developments between 1930-1940:

Many developments happened during the thirties especially Velox boilers

and the first gas turbine that worked successfully in power generation was

built by the efforts of Brown Boveri. The British and German governments

did great efforts to develop gas turbines which are used in airplanes

propulsion. Among the efforts exerted during the thirties are:

1- Brown Boveri efforts:

Thanks to Brown Boveri, that building of gas turbines to generate

electrical energy had increased either in power stations or in other

industrial applications. The first gas turbine built by Brown Boveri was

used to operate combustion air compressors used in Velox boilers. The

first turbine built to be used in Velox boilers in the year 1932. That

turbine used auxiliary power, as the power needed by the compressor

was higher than that developed by the turbine. After while it was able

to increase the developed power from the turbine to be higher than that

needed by the compressor. This was realized by using charging sets

added to the turbine.

In November 1936, new utilization of the charging sets appeared

when Sun Oil Company in Philadelphia in USA declared the discovery

of the supercharger. It was to burn the carbon residuals of petroleum

product distillation to produce the maximum power of the combustion

process. In year 1939, Brown Boveri had built the first power station

using large gas turbines to produce 4000 kW. That unit worked on a

simple cycle, as it was stand-by unit. In year 1940 that unit was used in

an underground-power station in Newchatel. The total hours use of this

unit did not increase more than 1200 hrs, till year 1953.

Brown Boveri invited Stodola to carry standard tests on that unit.

The results of these tests are shown in table (1/1). In year 1939, the first

gas turbine-using mazoot to produce 2 megawatt. It was used during

6000 hrs. since that year till 1977.

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Table (1/1) test results carried by Stodola on Brown Boveri gas turbine

Type of test results

1st test 2nd test 3rdtest

Load (kW) light 4021 3057

Fuel mazout mazout Mazout

Compressor pressure ratio 3.82 4.38 4.38

Compressor efficiency (%) 86.4 86.4 86.4

Compressor speed (rpm) 3020 3020 3030

Compressor air flow (Lb/hr) 499620 498176 498049

Turbine inlet temperature (oF) 705.2 1067 987.8

Turbine efficiency (%) 85.4 88.4 88.4

Fuel consumption (Lb/kW) - 1.078 1.193

Thermal efficiency (%) - 18.04 16.37

2- British efforts:

With the beginning of the thirties two separate groups began in

building and testing gas turbines utilized in power stations and

airplanes in England. One of these groups worked under the

supervision of Whittle in jet engines using centrifugal compressors.

The other group worked under the supervision of Griffith and Constant

in building and testing axial flow compressors.

In 1930 Whittle had registered his first patent in this field but he

failed to get financial support from British aviation ministry or special

association that is why he oriented his efforts to theoretical studies

since 1930 till 1936. Power Jet Ltd. Company was formed and made

the commitment to transform his theoretical studies to simple jet

turbine. It was a single-stage centrifugal compressor of bilateral intakes

and a single-stage turbine connected directly to the compressor having

single combustion chamber. The test of the first turbine of this type was

in 12 April 1937, during 11 days. The combustion chamber represented

the principal problem and the compressor performance was less than all

expectations.

Redesigning this turbine was in 16 April 1938 till 6 May when big

damage happened due to the failure of wheel blades. The third trial was

under test in October 1938.

In summer 1939 British aviation ministry signed a contract with

Power Jet Ltd. Company for building an airplane turbine knew by W1

which was tested in 1941.

The other group under the supervision of Griffith and Constant

began building and testing the axial flow compressors in the Royal

Aircraft Establishment.

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3- German efforts:

Hans Von Ohain paid attention to gas turbines used in propulsion

at the beginning of the year 1935. He was able to register a patent in

turbo-jet engine supplied with centrifugal compressor and Ernst

Heinkel appointed him as a general manager to Ernst Heinkel

company in 1936. In 1938, he was able to test the first turbine used for

airplanes and after redesigning this turbine to be the model He 5-36,

that had the following specifications:

Compressor Centrifugal

Weight of unit 795 Lb

Static thrust for unit 1100 Lb

Specific fuel consumption for thrust 1.6 Lb/(Lb-hr)

4- USA efforts:

The efforts were concentrated on raising the gas turbine efficiency.

The efficiency was raised to be between 60-65% for compressor and 65-

70 for the turbine

Four- Developments between 1940-1945:

1- British efforts:

Power Jet Ltd., under supervision of Whittle, continued developing

the turbine W1 and the turbine wheel was cooled by water. The first

Gloucester airplane trip powered by gas turbine was in May 1941. In

June 1945, Rolls-Royce produced a more powerful engine known as

Derwent (V) of repulsion 3500 Lb.

2- German efforts:

Sinc 1939 to 1942, German began developing the Von Ohain

engine using centrifugal compressor after this at the end of 1941 they

replaced it by Heinkel engine of axial-flow compressor. In November

1942, Junkers 004 (known as Jumo 004) was tested and installed in

airplane Me-262.

3- USA efforts:

• Turbo dyne (2500 HP) was suggested to the army in 1940 by Northup

Aircraft Inc.

• Lockhead Aircraft suggested engine L1000 • Westinghouse Electric Corporation built (The 19) as a turbo-jet using

axial flow compressor.

• Wright Aeronautical Corporation failed to produce Whittle engine.

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• General Electric began in 1941, after getting the design and drawings of W2B from Power Jet, Ltd. And produced engine I installed in

sirplane Bell P-59 A in 1942. In 1945 they began building engine G35.

4- Other efforts:

A side of using the gas turbine in airplanes, Swiss Federal Railway

installed an engine of 2200 HP in a train.

Five- Developments between 1945-1950:

During this period, old companies returned their efforts because the

governmental support was good in this field. Therefore, gas turbine

development continued in a fast rate to compete other prime mover either

diesel engines or steam turbines.

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BRIEF IDEA ABOUT TYPES OF GAS TURBINE

There are two types of gas turbines, they are:

1- Constant volume gas turbine.

2- Constant pressure gas turbine.

Constant volume gas turbine. The constant volume gas turbine, Fig. (5/1) consists of an air compressor that

sucks atmospheric air at point (1). It compresses the air and delivers it to the

combustion chamber through charging valve (a). At the combustion chamber a

quantity of fuel is injected through nozzle (c) by the fuel injection pump. Therefore

the fuel-air mixture burns by the electric spark (d), due to which the pressure

increases suddenly follows by opening the valve (b) to discharge the combustion

products to the turbine. There, the pressure energy is converted to mechanical

energy on the turbine shaft that may be used to drive an electric generator to

produce electric power. Finally, the exhaust gases escapes to the atmosphere.

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THE HEAT CYCLE

The constant volume gas turbine follows a cycle called constant volume cycle,

as the combustion of the fuel-air mixture takes place at constant volume. The only

effect due to heat addition in the combustion chamber is to increase the pressure

only.

Figure (6/1) shows the heat cycle curve, where air of volume (1-5) enters at

atmospheric pressure at point (1). Air is compressed to point (2) where its pressure

is increased and its volume becomes (2-6). At this point air enters the combustion

chamber and its temperature increases due to burning of the fuel injected. Therefore,

the pressure increases from point (2) to point (3) at which delivery valve (b) opens

to permit gases to pass and expand through the turbine. The gas pressure decreases

and its volume increases, which is represented by curve (3-4). The exhaust gases

leaves the turbine at point (4).

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In the constant volume gas turbine, usually multi-combustion chambers are used

that are charged by the compressed air one after another using the same compressor.

An automatic timer for the valves regulates the charging periods. That is, after

charging a combustion chamber by air the charging valve (a ) is closed, fuel is

injected inside this combustion chamber, the fuel-air mixture is burned that an

increase in pressure occurs after which delivery valve (b) opens to exit the exhaust

gases as a fast stream facing the turbine blades. As the pressure inside the

combustion chamber decreases the gas speed decreases. When the pressure reaches

to almost atmospheric pressure the delivery valve (b) closes and charging valve (a)

opens to recharge the combustion chamber by air.

The most important disadvantage of this type of turbines is the reduced of

thermal capability, that is why it is not widely used and direction to the other type

(constant pressure gas turbine) happened.

Constant pressure gas turbine. The constant pressure gas turbine, Fig. (7/1) consists of an air compressor that

sucks atmospheric air at point (1). It compresses the air and delivers it to the

combustion chamber, where it burns at constant pressure. The air volume and its

temperature increase. Then, the gases pass to the turbine, where it expands and its

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pressure decreases to atmospheric pressure. The pressure energy changes to

mechanical energy on the turbine shaft.

THE HEAT CYCLE

The constant-pressure gas turbine follows Brayton cycle called constant

pressure cycle, as the combustion of the fuel-air mixture takes place at constant

pressure. The only effect due to heat addition in the combustion chamber is to

increase the air volume only.

Figure (8/1) shows the heat cycle curve, where air of volume (1-5) enters at

atmospheric pressure at point (1). Air is compressed to point (2) where its pressure

is increased and its volume becomes (2-6). At this point air enters the combustion

chamber and its temperature increases due to burning of the fuel injected. Therefore,

the volume increases from point (2) to point (3) without no pressure increase. The

delivery valve (b) opens to permit gases to pass and expand through the turbine. The

gas pressure decreases and its volume increases, which is represented by curve (3-

4). The gas volume increases from (3-6) to (4-5) and the pressure decreases from

point (3) to point (4). The exhaust gases leaves the turbine at point (4).

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From the cycle, it is clear that exhaust and combustion take place at constant

pressure while compression and expansion take place continuously without

intermittence. Therefore, output power is generated at constant rate.

BRIEF IDEA ABOUT ADVANTAGES OF GAS TURBINE:

Gas turbines have the following advantages:

a- Supply and installation of a gas turbine can be done in short time not

exceeding few months.

b- It does not need water source for operation.

c- Operation and loading of the unit takes short time not exceeding few minutes.

d- It is possible to use either liquid or gaseous fuel or their mixture for gas

turbine operation.

e- It is possible to use exhaust gases in heating processes or to help in operation

of steam turbine in combined systems.

f- It can be installed in deserts and remote areas, as it does not need water for

cooling.

g- It needs few operators and can be remote-operated.

BRIEF IDEA ABOUT APPLICATIONS OF GAS TURBINE:

It is well known that airplanes use gas turbines but they are also used in surface

transportation and many other stationary applications. Scientifically, the field of

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stationary applications includes many purposes of high efficiency. This field is

always increasing while that of aviation has no innovation. Here, in a scientific view

the stationary applications are discussed as follows:

• In electricity generation • In oil and gas industry • In combined cycles or heating purposes • In chemical and process industry applications