We were discussing “

*Rankine cycle*” in our previous post, where we have seen the various components of Rankine cycle and its basic operations also. A steam power plant works on the principle of Rankine cycle and hence we can say that a steam power plant will be designed in such a way that process of each component of power plant will follow the process of Rankine cycle.
Today we will see here the basic concept of actual
Rankine cycle process or actual vapour cycle process with the help of this
post.

###
**Actual
Rankine cycle or actual vapour cycle process**

Let us first draw here the theoretical Rankine cycle
or ideal Rankine cycle.

Processes of an actual Rankine cycle will be
slightly different with the processes of an ideal Rankine cycle, because in
practice there will be various irreversibilities in various components of
Rankine cycle. Heat losses to surrounding and friction will be the main causes
of irreversibilities and we will also discuss here the deviation of actual
vapour cycle with the ideal Rankine cycle.

###
**Turbine
losses **

Let us consider the turbine first. During discussion
of the ideal Rankine cycle, we have considered that the process through turbine
will be reversible and adiabatic and therefore we have shown the process 1-2 as
isentropic process and it is displayed in figure by a vertical line showing
that entropy will be constant.

When we consider the actual vapour cycle process,
1-2 process will not be vertical or 1-2 process will not be isentropic. Pressure
drop because of friction and loss of heat energy to surrounding are the most
important causes of irreversibilities.

We can insulate the turbine in order to reduce the
heat loss and hence we can also reduce the irreversibilities due to thermal
dissipation. We can insulate the turbine quite enough and we can minimise the
loss of heat energy. In some cases we can not say that the loss of heat energy is
nill even with proper insulation also or in other way we can say that there
will be some loss of heat energy.

We can not avoid the irreversibilities due to
friction as we can never say that there will be frictionless flow through the
turbine and hence the irreversibilities due to friction must be considered
during drawing the actual vapour cycle.

*T-S diagram of actual vapour cycle*

Now let us consider the process 1-2, process 1-2
line will not be vertical but also it will move towards right side as shown in
figure above because according to the principle of increase in entropy there
will be increment in entropy during this process.

Actual work done by turbine, W

_{T}= h_{1}-h^{1}_{2}
Ideal work done by turbine, W

_{T, Actual}= h_{1}-h_{2}
As we know that friction will be present during the process
of expansion through the turbine and therefore friction will be converted in
terms of intermolecular energy and this intermolecular energy will increase the
temperature and hence enthalpy will also be increased.

Therefore
h

^{1}_{2}> h_{2}
We can also say that actual work done by turbine
will be less as compared to the ideal work done by turbine.

Let us see here the turbine efficiency, η

_{T}
η

_{T}= (h_{1}-h^{1}_{2})/( h_{1}-h_{2})###
**Pump
losses **

Similarly, we can see the deviation or changes in
process 3-4. Process 3-4 indicates the ideal process for working fluid flowing
through feed pump. In practical, process 3-4

^{1}will be the process for working fluid flowing through feed pump.
Now we will see the ideal work required by the feed
pump and also actual work required by the feed pump here.

Actual work required by the feed pump, W

_{P}= h^{1}_{4}– h_{3}
Ideal work required by the feed pump, W

_{P, Actual}= h_{4}-h_{3}
As we can easily observe that h

^{1}_{4}> h_{4}and therefore actual work required by the feed pump will be greater as compared to the ideal work required by the feed pump.
Ratio of ideal work required by the feed pump to the
actual work required by the feed pump will be termed as efficiency of feed pump.

Let us see here the feed pump efficiency, η

_{P}
η

_{T}= (h_{4}-h_{3})/( h^{1}_{4}– h_{3})
So, we have considered here the two important
components i.e. turbine and feed pump of rankine cycle and we have also seen
here the losses and deviation in curve too.

###
**Boiler
**

Let us consider the third important component of
rankine cycle i.e. Boiler. We have discussed earlier that heat addition will be
done at constant pressure in boiler. There will also be loss in pressure in
boiler too because of the presence of friction. But we must note it here that pressure
drop in boiler will be very less as compared to the boiler pressure.

We can neglect this pressure drop as it will be very less as compared to the boiler pressure, but we have considered this pressure drop also and it is displayed in PV diagram of actual Rankine cycle.

We can neglect this pressure drop as it will be very less as compared to the boiler pressure, but we have considered this pressure drop also and it is displayed in PV diagram of actual Rankine cycle.

*PV diagram of actual Rankine cycle*

If we insulate the boiler in order to avoid the loss
of heat energy, then there will be approximate adiabatic process. We can not
say that there will be no loss of heat energy, there will be surely some amount
of heat energy which will be lost to the
surrounding even with optimum insulation of boiler too.

In simple way we can say that, in practical case or
in actual vapour cycle, working fluid at the inlet of turbine will suffer with
slightly pressure drop due to friction and also suffer with slightly
temperature drop due to heat loss.

###
**Condenser
losses**

Losses in condenser will be quite less and it will
consist loss of pressure and cooling of condensate below the saturation
temperature.

Do you have any suggestions? Please write in comment
box.

We will see another topic 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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