 Welcome back. So, in the last snippet we looked at an adiabatic turbine. We have already looked at an adiabatic compressor as far as its H s diagram goes. Now, we will discuss it in slightly greater detail. So, we will again draw the H s diagram for a compressor which is as follows. We draw the H s diagram and we show the two isobars. So, this time we realize that the inlet to the compressor is actually a lower pressure and we the compressor is used to pump the fluid from the lower pressure to the higher pressure and hence this is P i which is the inlet isobar. This is P e which is the exit isobar. This is the inlet state let us call it I. Ideally, we would have gone vertically ahead. Here, that is at the same entropy as the inlet we would have reached let us say it is e star that is our nomenclature is e star. And what happens is if it is an actual process you actually go in this fashion that is you go towards an increasing entropy and you reach the state e. So, this is the actual state, this is the ideal state e star is the state where the entropy is the same as that of the inlet whereas, e is the state where the entropy is more than that at the inlet. So, we can write down the same equation that we had done for the turbine. In this case, we will see that P e is greater than P i because this is a compressor. S e star of course, is equal to S i which is the inlet entropy and in this case S e that is the actual exit state is greater than S i which implies S e is greater than S e star. Let us put it as greater than or equal to because it could still be ideal in some cases. So, in this case in a compressor the enthalpy at the exit is higher than the enthalpy at the inlet. So, H e is definitely greater than H i. What of course, we can notice that because of the nature of the line because the nature of the isobar which keeps on increasing and increase in entropy implies that H e exact that is H e actual is greater than or equal to H e star. So, now, if we look at the difference H e minus H i because H e is greater than H e star we will have H e minus H i is greater than or equal to H e star minus H i. Of course, now if we look at our first law for open systems it is W dot S is equal to m dot H i minus H e. Now, H i is a smaller quantity than H e and we are going to get that W dot S is a number which is less than 0. It is a negative number which is correct which is what we expect the work is input into the system W dot S the shaft work is a negative quantity. If we look at the definition of W dot S because both quantities are negative and because H e star minus H i is a is of a larger magnitude if we consider W dot S actual we will realize that W dot S algebraically is a smaller quantity than W dot S star. This is algebraic both of these are negative and W dot S is a larger negative quantity than W dot S star. But if one looks at only the magnitude then W dot S that is the actual work input into the system is greater than or equal to W dot S star. So, in this case if you look only at the magnitude this is what is happening that the actual work input into the system is greater than the ideal work that is expected to be put into the system and this is what we expect we are trying to put in some work to get a particular result that is an ideal case we expect something and because of so called irreversibility and the process being non ideal we have to now put more work into the system and this is what is being seen here W dot S is greater than equal to W dot S star. And because of this nature we define the efficiency slightly differently to ensure that the efficiency is a number which is less than 1 and also you will realize that it is intuitively a correct way of looking at efficiency for a compressor. We define the isentropic efficiency NSC for the compressor as equal to W dot S star notice that now this is in the numerator upon W dot S and since both of them are negative the efficiency is a positive quantity and since magnitude wise W dot S star is less than W dot S this is the number which is less than or equal to 1 this is the so called isentropic efficiency of the compressor. Of course, you can also see that you could have written it as H star minus H i at an H e minus H i in this case. So, this is true for this case in general we will say that it is the ideal work that is expected to be input divided by the actual work that is input. So, this is how the efficiency is defined for a compressor for an adiabatic compressor and it is called isentropic efficiency for the same reason that we have based the ideal work input into the turbine based on an isentropic process. Thank you.