Aircraft Refrigeration Cycles-2
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Aircraft Refrigeration Cycles-2

September 28, 2019


Hello I welcome you all in this course on
refrigeration and air conditioning. Today we will continue our discussions on
aircraft refrigeration cycles. In this lecture we will cover the simple air
refrigeration cycle with evaporative cooling. Bootstrap air refrigeration cycle with evaporative
cooling. Air refrigeration cycle with regeneration. Reduced ambient air refrigeration cycle, DART
irreversibility in air refrigeration cycle, where we start with simple air refrigeration
cycle with evaporative cooler. In previous lecture we have already discussed
the simple air refrigeration cycle that is 0, 1 then compression 2 then cooling constant
pressure cooling this is P1 this is P2 constant pressure cooling and then expansion in expansion
turbine. Now in this evaporative cooler in this cycle
evaporative cooler is introduced just before the expansion turbine. This evaporative cooler further pulls down
the temperature of air before the expansion and expansion turbine by providing evaporative
cooler the gas is further pulled up to the state 5 and then expansion takes place and
we get state 6 this is state 4. So you can see from here the temperature at
state 6 is less than the temperature at state 4 we get in this case we get a cooler air
entering the passenger cabin. Or we can say the same amount of circulated
air can take more refrigerating load, same arrangement is made in the bootstrap of a
cycle. In bootstrap cycle you know there is a two
time compression first, then second compression three, and then cooling four, and four to
five expansion takes place, but in this case the gas will get further cool to six, and
expansion will take place up to seven. The evaporative cooler they provide additional
cooling to the gas which is entering the expander by virtue of which we get cooler air at the
exit of the expander or the turbine. Now evaporative cooling is attained either
with the help of some refrigerant or liquid nitrogen and this evaporative cooling type
of arrangement is very useful when the aircraft is moving with a very high velocity or supersonic
velocity. Now we will discuss air refrigeration cycle
with regeneration. Now air refrigeration cycle with regeneration
initially the processes are same due to running action the pressure and the temperature of
the air is increased starting from state zero to state one, after the state one, the state
two is attained in a compressor state two, the air coming out of the compressor is cool
at a constant pressure this is the constant pressure line this is temperature on ordinate
on abscissa there is entropy so at constant pressure the cooling of air takes place and
state three is attained. After the strain three there is a heat exchanger
this heat exchanger as shown in the figure takes air after the expansion of turbine so
part of the air after the expansion of turbine it is re-circulated through this heat exchanger
so that the temperature of air is further cool to state four. So due to this heat exchanger instead of containing
this temperature three sorry 1, 2, 3 the gas is further or air is further cooled up to
temperature 4 and further expansion takes place in this case also we get cooler air
at the exit of the turbine now here the point is suppose 1 kg/sec is the gas circulation
rate part of this air maybe 5% or 10% will be tapped at state 5 and this tribe air will
be sent to the heat exchanger. And in the heat exchanger this low temperature
air will take heat from the air which is entering the turbine and it will further cool it so
this type of arrangement is also possible in air refrigeration cycles now the last one
is a reduced ambient air refrigeration cycle in a reduced ambient air refrigeration cycle
before entering the compressor the air is trapped and it is passed through a turbine
which produces work and the pressure of air is reduced because it is an adiabatic process
reversible adiabatic process. So if you look at this state 5 we say at state
1 the air may be I will never let us say – 20 degree centigrade but after the extension
the temperature of air will further reduce so this low temperature air will now be used
for cooling the gas coming from the compressor so definitely this will also improve the performance
of the cycle and whatever work is produced in this auxiliary turbine during process 1
to 5 this work is added to the frame work because in this cycle if an run is run by
the auxiliary turbine this is expansion turbine the fan is run by the expansion turbine and
the air circulated by this fan is used to take away heat in that cooling heat exchanger. Now these are the 4 cycle 6 type of cycles
which are used for air refrigeration systems now if we compare these cycles if you have
compare these cycles with each other I have taken on the x-axis Mac number on y-axis. It is the dart is dry air rated temperature,
dry air aerated temperature is the temperature of air at the exit of expansion domain or
temperature at state here in the state five or State six so dart if you look at the dart
and with the help of tried if we try to compare the performance of all these cycles if you
look at the simple air refrigeration cycle dart keep on increasing if we increase the
velocity of the aircraft. The dart keep on increasing so this cycle
cannot be used for very high velocity aircraft and if we look at the bootstrap, bootstrap
is much better than the simple refrigeration cycle and we can go for the bootstrap even
the supersonic aircraft it is because if let us say Mac number is 1.2 the value of dart
is -40 in bootstrap cycle and further we can go up to 1.2 1.4 upto 1.6 but for further
increasing the Mac number the regeneration type of regeneration type of cycle is recommended. For high Mach numbers if you look at 1.6Mac
number here you can see that the value of dart is close to 0 but value of dart for bootstrap
is on positive side of temperature positive temperature some 10 or 20 degree centigrade
temperature, now for very high velocity aircrafts for very high velocity aircrafts we can go
for reduced ambient type of cooling system you can see the reduced ambient type of cooling
system is very stable up to. Let us say 2 or 2.2 MEK we can comfortably
go with the system but the aircraft has to fly on very high speed let us say to 2 Mac
or 2.5 Mac in that case the best one is bootstrap with erupted evaporative cooling, so bootstrap
air refresher cycle in combination to evaporative cooling is best suited for high velocity aircrafts. Now irreversibility’s in a refrigeration
system because till now we have considered all processes as a reversible process. But in actual practice none of the process
is a reversible process almost all the processes are irreversible process. Let us start with the state 0. Where air is coming with a very high velocity
towards the aircraft state 0 and it get compressed to state 1 during ramming action, if we apply
the first law for open system thenh1=ho+Co2/2, h1 – ho=C02 /2, now as you know the change
in enthalpy in sensible heating or sensible cooling is because here it is a adiabatic
compression and we have already derived in adiabatic compression the change in enthalpy
is CP(T1 – T2) sorry T1 – To CPDT is equal to C12/2, now T1 – TO=C2/2CP or we can write
as Co2/2 ?/ ? – 1. R because CP=? / ? -1.R the further if we
divide the entire equation by T goes so T1 by Tu. Shall be equal to. 1 + Cu2 two ?RTo divided by ? – 1 right and
then To by T1 is equal to 1+ ? – 1 by 2 and this ?RT can always take as can always be
taken as sonic velocity at this particular temperature and this ratio will give you. To by T1 sorry T1 by T1 this is T1 by To so
here also it is T1 by T o is equal to 1 + ? – 1 by 2 M2 from this equation if we know
the value of Mac number suppose the aircraft is moving with M is equal to 1 let us say
the Sonic aircraft M is equal to 1 so immediately we can find that T1 by To is going to be equal
to 1 + ? – 1 by 2 or it is going to be ? + 1 by 2 but in actual practice this process
does not take place instead of pressurizing up to P once from Po to P1 the pressure rises
up to P1 – only so entire stagnation pressure is not retained but part of the stagnation
pressure is attained. But the temperature at this point this is
0.1 so instead of getting point 1 in the process in the irreversible process we get point 1
– now temperature of point 1 dash is equal toT1 so T1 dash is equal toT1. But the pressure of at this state is less
than the pressure of it the state 1 and there is a pressure recovery factor pressure recovery
factor PRF pressure recovery factor is P1- – Po divided by P1 -Po after in stating state
1′ the air goes to the compressor now inside the compressor ideally there is an isentropic
process or reversible adiabatic process there is no heat transfer during this process and
we get state 2 when in actual practice it is a irreversible process due to irreversibility
it is no longer a vertical line or entropy does not remain constant there is a change
in the entropy and we get state 2′. If you look at here the temperature at state
2′ is greater than temperature at state 2 this is due to irreversibility in the process
and there is a term which is known as polytrophic efficiency, so polytrophic efficiency of the
compressor can be expressed as. T2-T1/T2′-T1 definitely in this process, process
1 to 2 energy consumption is less in comparison to the process 1′ to 2′ in process 1′ to 2
the energy consumption is less in comparison to the actual process that is 1′ to 2′ this
polytrophic efficiency for a rotary compressor is approximately of the order of 90% it varies
in fact but normally it is between 85 to 90% after the process attaining process 2′ cooling
of air takes place it is supposed to be at a constant pressure p2 but in actual practice
there is always a pressure drop. So instead of p2 there is the pressure p2′,
p2′ and temperature let us say temperature 3 is attained this is state 3, so state 3
is attained at a constant it is supposed to be a constant pressure process. But since the gas passes through the heat
exchanger and it passes through the pipes also there is a certain there pressure drop
during this process so instead of getting temperature in state 3 at pressure 2 we attain
it at the pressure 2′ which is slightly later than which is slightly less than the pressure
2. After attaining the state 3 the expansion
takes place I am taking the simple cycle expansion takes place and during this expansion
also if the expansion takes place inside the turbine so this is an ideal process but in
actual practice it is no longer a vertical line it is no longer a constant entropy process
again the during this process the entropy increases they increase in entropy during
this process due to irreversibility is present during this process. And instead of getting state 4 we get state
4 – and temperature of state for – is higher than the temperature of a state 4 or after
expansion the temperature is higher in case of irreversible process in comparison to the
reversible expansion and here also the efficiency of the turbine poly tropic efficiency poly
tropic efficiency of the turbine can be expressed as P 4 – – T 3 /T4 – T3, so it is if you
look at these two equations this is ideal temperature drop in compressor ideal temperature
rise actual temperature rise this is ideal temperature rise actual temperature rise. Ratio of these two will give the poly tropic
efficiency of the compressor in case of turbine it is reverse actual temperature drop divided
by ideal temperature drop so this is the this expression is for the poly traffic efficiency
of the turbine if you are using bootstrap cycle in case of bootstrap cycle again some
pressure drop will be here and again during the compression process there will be in reverse
abilities or they will be rise in entropy so in bootstrap cycle also again the same
type of phenomena takes place. So in a bootstrap cycle again the pressure
this compression is the secondary compressor again it is a it is not a reversible process
and change in entropy takes place that is why temperature at this state may be let us
say the state is 5 t5 and p5 _ so t5_>T 5 and in this case also the efficiency of turbine
is in the range of 85 to 90%, so while dealing with the real-life problems we have to we
have to take into the consideration with the efficiency of all the components in an air
refrigeration system. Normally the efficiency of the rotary machines
because in the air efficient system rotary machines are used so normally the efficiencies
poly tropic efficiencies of the rotary machines are higher than the efficiency of reciprocating
machines, for example if I take reciprocating compressor, so for a reciprocating compressor,
the 80% efficiency is very high efficiency. Normally it lies between 70%and 80%, the reason
being that if let us take example of axial flow turmoil or axial flow compressor, in
axial flow compressor there is no change in the direction of the fluid, straightaway passes
through the compressor since there is no change in the direction of the fruit flow, that is
why the losses are minimum. So highest efficiencies attain with the axial
flow compresses, now if we replace axial flow compressor and it is always close to 90 % or
greater than 90 %, if we close replace the axial flow compressor with the centrifugal
compressor, in the centrifugal compressor the direction of the flow of the fluid is
like this, there is an axial entry of the fluid in the centrifugal compressor and there
is a real exit. And change in the direction of the fluid is
approximately 90 degree, approximately 90 degree that is why in centrifugal compressors
the losses during the flow are more than in the case of axial flow compresses, and efficiency
of this compressor is slightly less than the efficiency of an axial flow compressor, and
if you go to the recalculating compressor the efficiency is minimum. However in most of the cases in air refrigeration
system, axial flow compresses are used because in air refrigeration systems high bulk of
fluid has to be handled, if very high bulk of fluid has to be handled, the axial flow
compressor are the best kind of compressors, so here with this I and the discussions on
the air refrigeration cycles, in the next class we will try to solve one numerical on
a refrigeration cycle.

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  1. Lectures are good but better explanation could've been given in terms of math and physics sir thank you…!!

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