International Journal of Engineering Trends and Technology (IJETT) - Volume4 Issue7- July 2013
A Cooling Unit to Increase the Efficiency of Gas Turbine Plant
Shirsendu Das
Department of ME, TIT Agartala
Narsingarh, West Tripura, India
R
Abstract - Gas Turbine power plant is one of the most
important power plant in current century. Brayton Cycle is
the standard cycle for gas turbine power plant. Day by day
many modifications have been introduced to improve the
efficiency of turbine plant like reheat, regeneration,
intercooling, combination of reheat-regeration-intercooling
etc. Here I have introduced a mechanism which can employ
in any type of gas turbine circuit (reheat, regeneration,
intercooling etc) to improve the efficiency of the plant.
Ex
C
H
C2
G
Keywords – Cooling system, effect of compressor inlet
temperature in efficiency.
I.
INTRODUCTION
T2
C1
N
It is observe that the efficiency of gas turbine plant
increase with the decreased value of compressor inlet
temperature. A cooling system has been employed at the
inlet of the compressor to cool the air before entering
compressor. The cooling system delivers the air slightly
below the atmospheric temperature at the inlet of
compressor which is corresponding to maximum efficiency.
II.
PLANT MOUNTINGS
a. Compressor: - The air is compressed at high
pressure isentropically inside the compressor.
b. Combustion Chamber: - The compressed air
enters inside the combustion chamber & mix
up with fuel and then ignites to produce a gas
of high pressure & temperature.
c. Turbine: - The combust gas is expanding
inside the turbine isentropically & produces
shaft power in turbine shaft. This shaft power
is utilised to run the compressor & electric
generator.
d. Reheater: - It is the part where the escape
gases from the turbine exchange their heat
energy with compressed gases.
ISSN: 2231-5381
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R
C.C
T1
Ge
(H = Heat Exchanger, C1= Compressor 1, R= Reheater,
C.C= Combustion Chamber, T1= turbine 1, Ge= Generator,
T2= Turbine 2, G=Gear box, C2=Compressor 2, C=
Condenser, Ex= Expansion device, R=Receiver, N=Nozzle)
(Figure-1)
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International Journal of Engineering Trends and Technology (IJETT) - Volume4 Issue7- July 2013
III.
PLANT ACCESSORIES
a.
b.
c.
d.
e.
f.
g.
h.
Nozzle: - The exhaust gas after reheating the
compressed air goes through a nozzle where
the pressure will drop & gases will achieve
some kinetic energy.
Turbine 2:- The turbine 2 is a small turbine
just use to produce available power to run the
small compressor of cooling unit. The exhaust
gases then escape into the atmosphere at the
outlet of turbine 2.
Compressor 2 :- It is also a small compressor
running by the power of turbine 2.The
compressor is a part of cooling system which
is use to compressed & circulate the coolants
Condenser :- Compressor consists of coil of
pipes in which coolant of high pressure &
temperature is cooled & condensed
Receiver: - The condensed liquid coolant is
stored in receiver.
Expansion Valve: - It allows liquid coolants of
high pressure & temperature to pass at a
controlled rate. Some of the liquid coolants
evaporate as they pass through expansion
valve.
Heat Exchanger (Evaporator):- It consists of a
coil of pipe in which liquid-vapour coolants at
low pressure & temperature is evaporated &
change into vapour coolant. During the
evaporation process the liquid-vapour
coolants absorb the latent heat of vaporisation
from the air entering at the inlet of the
compressor. So the temperature of the air
decreases.
Gear Box: - A gear box is provided in
between turbine 2 & compressor 2 to engage
& disengage the power transmission from the
turbine to compressor. This arrangement is
provide to maintain a constant temperature of
the air, entering the compressor 1. Due to
continuous running of compressor when the
temperature of the cooling unit will fall below
the required inlet temperature that time the
gear box will cut the power transmission from
turbine 2 to compressor 2.
ISSN: 2231-5381
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The design of the cooling unit will be like so that it can
check the unwanted heat loss or heat transfer to the
atmosphere.
IV.
CALCULATION FOR EFFICIENCY
6
R
3
4
C.C
5
2
CC
C
T
Ge
1
( Flow Diagram)
( C= Compressor, T= Turbine, Ge= Generator,
R= Reheater)
(Figure-2)
4
5’
T
3
5
2 2’
6
1
S
(1-2-3-4-5-6-1= Theoretical Cycle &
1-2’-3-4-5’-6-1= Actual Cycle)
(Figure-3)
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International Journal of Engineering Trends and Technology (IJETT) - Volume4 Issue7- July 2013
1-2 = Isentropic compression in compressor.
1-2’= Actual process in compressor.
2’-3= Heat addition in reheater.
3-4= Heat addition in combustion chamber.
4-5= Isentropic expansion in turbine.
4-5’= Actual process in turbine.
5’-6= Heat rejection by exhaust gases.
Ta
Tb
Tc
(
)
Effectiveness of heat exchanger =(
)
Td
Theoretically due to regeneration the heat supplied & the
heat rejection both will reduce by same amount. So there is
an increase of mean temperature of heat addition &
decrease of mean temperature of heat rejection due to
regeneration.
=
So, the supplied heat in C.C
(
Heat rejection
=
(
−
)
Turbine work
=
(
−
)
(
−
=
Compressor work
The efficiency of the cycle
(
=
)
In figure-4, Ta, Tb, Tc & Td are different value of
compressor inlet temperature where Ta<Tb<Tc<Td.
From the expression of efficiency & figure-4 it is clear that
the thermal efficiency will increase with the decrease value
of compressor inlet temperature. At higher compressor inlet
temperature efficiency first increases but fall rapidly. But at
lower value of inlet temperature curves are more flat at the
optimum value of pressure ratio.
)
= 1−
V.
)
=1−(
Theoretically,
−
)
=
&
(Figure-4)
EFFECT OF COMPRESSOR INLET
TEMPERATURE ON OTHER PARAMETERS
1.
(
)
=1−(
)
= 1−
= 1−
Since,
=( )
×
=
Air rate: - It is the mass of the air required to
produce 1 KW power in 1 hour. Variation of
air rate with compressor inlet temperature is
shown in figure 4. It is found that when the
compressor inlet temperature increases air rate
increases. When the air rate increase
compressor work increase so the net work of
the plant decrease. So a good gas turbine plant
requires lower value of compressor inlet
temperature.
=( )
Kg/KW-h
Where,
So,
= Pressure ratio.
= 1−
( )
Compressor inlet temperature (Figure-5)
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International Journal of Engineering Trends and Technology (IJETT) - Volume4 Issue7- July 2013
2.
Work ratio: - Compressor work decreases
with the decreased value of inlet temperature.
So for a fix value of turbine work it increases
the work ratio.
Consider an elementary area
of the heat exchanger. The
heat flow rate through the elementary area
=
(
)= .
−
.
Due to this heat transfer
through the elementary area
the temperature of the inlet air decrease by
& coolant
temperature increased by
.
Ti
So,
=−
.
.
=
.
.
= .
.
Here, & are the mass of inlet air & coolant and is negative (-ve) as it decreasing.
Work ratio
=−
=−
.
(Ti= Compressor inlet temperature)(Figure-6)
VI.
=
COOLING PROCES
Cooling system consist of compressor, condenser, receiver,
expansion device & a heat exchanger. Details working of
these components have been discussed earlier. We can use
vapour compression cycle for the cooling process. The
temperature in which maximum efficiency can be achieved
at the optimum value of pressure ratio, it has to maintain at
the compressor inlet. Now a day water is injected at the
inlet air of the compressor to decrease its temperature. But
this mechanism is more efficient than water injection
because here we can maintain a desirable inlet temperature.
The circulation also very first due to compressor work. No
external power source is required to drive the compressor.
Because it is run by a small turbine, which is driven by
exhaust gases.
Where,
=
.
=
.
= Heat capacity of inlet air &
= Heat capacity of the coolant.
−
1
=−
1
=−
=− .
(
+
−
=−
VII.
=
.
HEAT TRANSFER FROM THE COOLANT TO
INLET AIR
1
1
)
1
.
1
+
+
+
1
1
−
Consider,
Coolant
1
=− .
+
=
1
Inlet air
Air
Ta
Temperature
Tc
Tc2
1
log
=− .
.
1
+
1
Now, the overall heat transfer rate between coolant & inlet
)= ( − )
air
= (
−
dA
1
Area
ISSN: 2231-5381
+
dQ
Ta2
Coolant
Tc1
1
=−
Ta1
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=
−
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International Journal of Engineering Trends and Technology (IJETT) - Volume4 Issue7- July 2013
1
log
=
Where,
−
=
−
=− .
[(
−
−
=
[4]
+
)−(
−
)] =
&
−
=
=
. (
−
[5]
−
(
−
Heat Transfer & Aerodynamics Study of Multiple Cooling Holes by
Kamil Abdullah,Onodera Hitato, Funazaki Ken-ichi & Ideta
Takeomi.
‘Non-equilibrium condensing Flow Modelling in Nozzle & Turbine
Cascade’ from International Journal of Gas Turbine, propulsion &
power system.
)
)
log
= . .
Where,
=
= Logarithmic mean temperature
difference of inlet air & coolant.
VIII.
MERITS OF THE MECHANISM
1.
2.
3.
4.
5.
IX.
The mechanism can provide quick & efficient
output.
No external efforts or power is required to run the
mechanism.
It can give more efficient effect than water
injection system at compressor inlet.
It can maintain a desirable constant temperature at
the inlet of the compressor corresponding to
maximum efficiency at optimum pressure ratio.
Due to this cooling action compressor work
decreases to neat work increase.
ACKNOWLEDGEMENT
The author would like to thank Mr. Badal Debnath for his
support in this work.
REFERENCES
[1]
[2]
[3]
Analysis of gas turbine performance with inlet air cooling
Techniques Applied to Brazilian site by Ana Paula Santos &
Claudia R. Andrade.
Reference from American Society of Heating, Refrigerating & Air
Conditioning Engineers.
Intake system for industrial gas turbine by Zuniga M. O. V
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