We were discussing Brayton cycle, an ideal cycle for
gas turbine engine in our recent post. We have also seen the effect of regeneration on Brayton cycle efficiency in our previous post.
Today we will see here the effect of intercooling on
Brayton cycle with the help of this post.
Brayton cycle with intercooling
As we have already discussed during study of open cycle gas turbine engine and also during study of closed cycle gas turbine engine
that resultant work from a gas turbine engine will be the difference between
the work output from the turbine due to the expansion of working fluid and work
energy required by the compressor for compressing the working fluid.
Net work from the gas turbine engine = Work output
from the turbine – Work input to the compressor
Wnet = WT-WC
We can conclude from above equation that net work
from the gas turbine engine could be increased by decreasing the compressor
input work or by increasing the turbine work or both. Hence, efficiency of
Brayton cycle could also be increased in this way by increasing the net work
from the cycle.
Work energy required for compressing the gas between
two specific pressure levels could be reduced by carrying out the process of
compression in stages and cooling the gas between two successive stages. This
method of increasing the net work from the gas turbine engine is termed as
multi-stage compression with intercooling.
Let us see the following figure, we will have basic
arrangements of various components, PV diagram and TS diagram here to show the
effect of intercooling on Brayton cycle
Process 1-2: Adiabatic compression of the working
fluid
Process 2-3: Working fluid is cooled to the initial
temperature in a heat exchanger which is termed as intercooler.
Process 3-4: Further adiabatic compression of
working fluid to the specified pressure level
Process 4-5: Heat energy addition to the working
fluid at constant pressure in heating chamber
Process 5-6: Adiabatic expansion of the working
fluid through turbine or also termed as power stroke
Process 6-1: Rejection of heat energy at constant
pressure in cooling chamber
Process 1-2’: Adiabatic compression of the working
fluid from one specified pressure level to another specified pressure level and
we have not considered the effect of intercooling here.
Therefore, cycle 1-2-3-4-5-6-1 indicates the gas
turbine cycle with intercooling having two stage compression and cycle 1-2’-5-6-1
indicates the gas turbine cycle without intercooling i.e. ideal closed gas
turbine cycle or ideal closed Brayton cycle.
Suppose a compressor has to compress the working
fluid from pressure P1 to pressure P2, there will be
three methods for compressing the working fluid from pressure P1 to
pressure P2.
Isentropic compression
Isothermal compression
Multi-stage compression with intercooling
If we plot these three processes on PV diagram to
know the work energy required for compressing the working fluid from pressure P1
to pressure P2 for each process, we will come to know that work will
be required maximum during the process of isentropic compression and work will
be required minimum during the process of isothermal compression.
PV diagram for three processes for compressing gas
from P1 to P2
Whereas during the process of multi-stage
compression with intercooling, work will be required less than as for
isentropic compression process and will be more than as for isothermal compression
process.
As we know that when compressor will compress the
working fluid, temperature of working fluid will be increased and hence
isothermal compression process will not be possible and therefore work input
for compressor for compressing the working fluid between two specified pressure
levels could be decreased by adopting the multi stage compression with
intercooling.
We have shown above two stage compression of working
fluid with one intercooler between two compressors. There might be more stages
of compression process but usually more than two intercooling will not be
preferred because it will be quite complex.
We have shown above in PV diagram one intermediate
pressure PX, compressor input work will be dependent over the value
of intermediate pressure PX. Compressor input work will be minimum
if it will follow the following equation.
Do you have any suggestions? Please write in comment
box.
We will see another topic i.e. Brayton cycle with reheating in
our next post in the category of thermal engineering.
Reference:
Engineering thermodynamics by P. K. Nag
Engineering thermodynamics by Prof S. K. Som
Image courtesy: Google
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