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.

Excellent….

Super

Why power of all the 6 cycles are taken as compressor power not taken in amount of expander power.

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

Thnx sir…