Here I'm going to show you what reversible and irreversible processes, both compression
and expansion, look like on an entropy- enthalpy diagram. And also use those diagrams to help
us understand behavior and the differences between reversible and irreversible processes.
So these would be, for example, running a compressor for a compression process or using
a turbine to get work out of an expansion process. So let's look at, first, compression.
So on an enthalpy- entropy diagram I'm showing two constant pressure lines. These are not
going to be straight lines, but on this scale, for what we're interested in, they are approximately
straight. We are going to enter at a lower pressure we are going to leave at a higher
pressure. So pressure 2 is higher than pressure 1. So if we look at the reversible process,
adiabatic, that's the constant entropy process, we increase the enthalpy which means we increase
the temperature when we compress something. We have a higher enthalpy at the same entropy
means a higher temperature. So we can see the work we have to do on the gas to compress
it. Reversible work is the same as isentropic work. We are adding energy to increase the
pressure and we're increasing the enthalpy. The work then, reversible, is just the final
enthalpy minus the initial enthalpy. So now we want to compare this to an irreversible
process which of course will correspond to a real process. So here I'm showing in blue
a real process. So now the entopy increases, so S2 is greater than S1. We are going to
the same pressure but notice, at a constant pressure line it means that the enthalpy is
higher. So enthalpy 2 for the irreversible process is greater than enthalpy 2 for a reversible
process. So we are at a higher temperature, and a higher enthalpy, and the same pressure
corresponds to a higher temperature. Now I'm showing you the work for the real process,
the irreversible process. Notice that the work for the real process, the irreversible
process, is greater than the work for the reversible process. We have to put in more
work to get to the same pressure and therefore end up at a higher temperature for adiabatic
compression that is irreversible. So temperature 2, irreversible is greater than temperature
2 reversible. Now we want to look at the same type behavior but for expansion instead of
compression. Meaning starting at high pressure, going to low pressure. So again we are looking
at the same plot, but now our starting pressure is pressure 1. Pressure 1 is greater than
pressure 2. If we expand reversibly, adiabatically, that's constant entropy. We go to lower pressure,
we go to lower entropy, and therefore our final temperature is less than our initial
temperature. So we can get the work the same way from this plot. The reversible work that
we get out is shown here, so work reversible, which is isentropic, is our final enthalpy
minus the initial enthalpy and this number is less than zero. We are removing energy
is work is negative. Now if we compare this to a real process and the entropy is going
to increase. So on blue we are showing a real process, an irreversible process. going to
the same final pressure but now notice that S2 irreversible is greater than S2 reversible,
notice that the enthalpy is higher for the irreversible process. So H2 for the irreversible
process is higher than H2 for the reversible. Which means we actually get less work out
of the process. Delta H is negative so delta H irreversible, absolute value, is less than
delta H reversible, absolute value. So we're going to end up at a higher temperature, we're
at a higher enthalpy. So for example and ideal gas only depends on temperature- higher temperature
means higher enthalpy. So we compare, now, the two work values. So notice now the work
for the real process, the irreversible process, and we use absolute value here, is less than
the work for the reversible process. And I'm using absolute value here because work is
negative. So real process, to go to the same pressure, we end at a higher temperature,
we get less energy out of the system. So when we have expansion we are getting less work
out absolute value, when we are doing compression, irreversibly, we have to put more energy in
for the irreversible process.