As we know that largest source of
production of electric power in the world is steam power plants and these steam
power plants work on the principle of Rankine cycle or we can say that steam
power units are designed to work on Rankine cycle.
We will be able to reduce the
consumption of fuels or in other words we can say that we will have more work
output from the Rankine cycle, if we can figure out the methods for increasing
the efficiency of the Rankine cycle and therefore we have already discussed the various methods for increasing the efficiency of the rankine cycle in our recent post.
Let us first draw here one simple
Rankine cycle to refresh the basic concept of heat addition process and heat
rejection process in simple Rankine cycle and then we will go ahead to
understand the basic concept of regenerative Rankine cycle with feedwater
heater.
As we have studied in our previous post that heat energy will be added in simple Rankine cycle during the process of
4-1 and heat energy will be rejected during the process of 2-3. Let us recall
the Carnot cycle, we have discussed there that heat energy will be added
isothermally and heat energy will be rejected isothermally too. Here in simple
Rankine cycle, heat energy will be rejected isothermally but heat energy will
not be added to the working fluid isothermally.
We can see from simple Rankine cycle
that heat energy will be added partially at constant temperature and rest of
heat energy will be added to the working fluid at varying temperature and this
is the main deviation of Rankine cycle with the concept of Carnot cycle.
Hence we will have lower efficiency for
a Rankine cycle as compared with the Carnot cycle efficiency. So we will have
one term known as mean temperature of heat addition i.e. Tm1. we have already discussed the concept of mean temperature of heat addition in our post "Concept of regeneration in Rankine cycle".
Therefore, Efficiency of a simple
rankine cycle will be calculated by following formula.
η= 1-[T2 / Tm1]
In order to increase the efficiency of
the Rankine cycle we will have to increase the mean temperature of heat
addition i.e. Tm1. Mean temperature of heat addition in Rankine
cycle could be increased by increasing the amount of heat energy supplied at
high temperature by the process of increasing superheat, by using reheat or by
using higher temperature and higher pressure of steam.
In simple, if we can increase the mean
temperature of heat addition in Rankine cycle by increasing the maximum
temperature of the Rankine cycle i.e. T1.
We have seen the basic concept of regeneration in Rankine cycle in our previous
post for increasing the mean temperature of heat addition and hence efficiency
of the Rankine cycle.
We must note it here that the solution
of increasing the efficiency of the Rankine cycle by the concept of
regeneration is practically not suitable because it will be quite difficult to
design the heat exchanger suitable for above solution and also at the final
stage of the turbine there will be increment in the content of moisture in the
steam.
So let us see the regenerative Rankine cycle with feedwater heater with the help of this post
If we consider the case of practical
regeneration process for a Rankine cycle, high temperature and high pressure
steam will enter in to the turbine at state 1. Entire steam will not be
expanded up to the condenser pressure through the turbine but also certain
quantity of the steam will be extracted through various points from turbine and
will be used for heating the feedwater and rest quantity of steam will be
expanded up to the condenser pressure.
Device where feedwater will be heated by
the concept of regeneration will be termed as feedwater heater. A feedwater heater is
basically defined as one type of heat exchanger where heat will be transferred from
the steam to the feedwater by the process of mixing the two fluid streams i.e.
open feedwater heaters or without mixing them i.e. closed feedwater heaters.
Let us see the block diagram of regenerative Rankine cycle with feedwater heater
Let us see the following block diagram, high
temperature and high pressure steam will be entered in to the turbine at state
1 and will be expanded through the turbine up to condenser pressure at state 2 but
we must note it here that entire steam will not be expanded up to the condenser
pressure through the turbine but also certain quantity of the steam will be
extracted through various points from turbine and will be used for heating the
feedwater and rest quantity of steam will be expanded up to the condenser
pressure at state 2 as shown in following figure.
In this case we have used single
feedwater heater as shown in figure and state of the extracted steam displayed
here by the state 5. Let if 1 Kg of steam is entering to the turbine from
boiler and as we have discussed that certain quantity of the steam will be
extracted hence let Y kg of steam is extracted from the turbine at state 5 as
shown in figure and therefore (1-Y) kg of steam will be expanded through the
turbine up to the condenser pressure.
Expanded steam will process through
condenser as usual and will reach to state 3. At state 3, working fluid will be
in liquid state and here working fluid will enter to the feed pump where feed
pump will pressurize it and will deliver at state 4.
Working fluid, at state 4, will enter to
the feedwater heater also termed as heat exchanger where working fluid (condensate
from condenser) will be mixed with extracted steam from the turbine. Process
will be controlled in such a fashion that discharge of feedwater heater or heat
exchanger will be in the state of saturated liquid state and it will be
displayed here by the state 6 as shown in figure.
Working fluid at the state 6 will enter in
to the feed pump and feed pump will pressurize the working fluid up to the
boiler pressure at state 7 and working fluid will enter in to the boiler at
state 7 as shown in figure.
Heat energy addition to the working
fluid will be carried out in the boiler during the process 7 to 1. Working fluid
will enter in to the boiler at state 7 and will be heated to state 1 as
displayed in block diagram and TS diagram too.
Let us see the process briefly here as displayed in TS diagram
Process 1-2: Expansion of steam through
the turbine with constant entropy
Process 2-3: Condensation process through
the condenser at constant pressure
Process 3-4: Pressurization of the working
fluid by the feed pump I up to the pressure at which working fluid will enter
in to the feedwater heater for mixing with the extracted steam
Process 4-6: Working fluid at state 4
and extracted steam at state 5 will be entered in to the feedwater heater and
hence process 4-6 will be termed as heat energy addition to the working fluid internally
in the heat exchanger or feedwater heater.
Process 6-7: Pressurization of the working
fluid by the feed pump II up to the boiler pressure at which working fluid will
enter in to the boiler for heat energy addition
Process 7-1: Heat energy addition to the
working fluid at constant pressure during the process 7-1
Work done by the turbine
WT = (h1-h5) + (1-Y) (h5-h2)
Net work from the cycle
W Net = WT – (WP1
+ WP2)
Let us ignore the pump works as it will
be quite small as compared to the work done by the turbine and therefore we can
say that net work from the cycle will be equivalent to the work from the
turbine.
W Net = (h1-h5) + (1-Y) (h5-h2)
Net input heat energy
Q =h1-h7
We can also see that during the process 4-6,
heat energy will be added but we must note it here that this heat energy
addition will be done by the extracted steam in the heat exchanger or feedwater
heater and hence this is an internal heat addition and hence we do not have to
write this addition of energy as input energy.
Efficiency of the cycle with
regeneration with single feedwater heater will be calculated as mentioned here
η= [(h1-h5) + (1-Y) (h5-h2)]/ (h1-h7)
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 P. K.
Das
Image courtesy: Google
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