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THERMAL POWER PLANT (Part 2-The process and principle)
The process
and principle
The basic
entities in the power plant are Boiler, Turbine , Condenser , pump. Here, the
principle is explained with the help of Temperature and Entropy (TS) curve.
Starting at
1, the water at room temperature is boiled at constant temperature in the
boiler. This process has a T constant for it is the boiling of water which
takes at a constant temperature. Here we are increasing the entropy of by phase
change and the energy is sorted in the form of latent heat also. As you might assume, the
temperature is not 100 ’ c the boiling
point is also high. As shown in the figure the temperature is constant and the
line is parallel to S axis. All this process happens in a boiler. The end
product is steam.
Once you
have steam, you cannot immediately pump it to the turbine to extract energy
because once it enters turbine and starts condensing forming water droplets without
giving much energy. The point here is, just a phase change won’t work out for
energy transfer from coal in boiler to turbine. The steam must be heated to
higher temperatures. This phase is called super heater and this phases mainly
executed in ‘super heater’. This super heater will be located in between boiler
and the turbine. The source of heat for the super heater is the hot flue gases
obtained in the boiler after burning coal. This explains the phase 2-3 in the
T-S curve. Note that , in this process, neither the temperature nor the entropy
remains constant.
Now, in the
next process, the super heated steam is allowed to expand in the turbine. As,
the high pressure steam is allowed through a small nozzle ,steam acquires kinetic
energy. This kinetic energy of the steam will exert required force on the
turbine blades. The turbine
is designed well to receive maximum force from the steam. This process is a
constant entropy process. The steam is allowed inside the turbine until water
droplets begin to form. In practice, formation of water drops is strictly prohibited
for the water drops will impinge the turbine blades and cause corrosion. Now
the output of the turbine is low pressure and low temperature steam. This
accounts for the phase 3-4 of the cycle.
In this
phase of condenser, the heat of steam is exchanged with a heat exchanger,
essentially water. The steam now turns into water and this is processed again
and sent into boiler for the next cycle. The heat exchanger gets heated up and
this needs to be cooled for further use. Hence the heat of exchanger is
dissipated in atmosphere through large cooling towers. A lot of energy from the
entire system remains unused in this 4-5 . The analysis can be obtained in the
efficiency discussion at the end of the tutorial.
As you have
seen throughout the process, water needs to be flown from one part to other.
The necessary draught is created by the pump. The step 5-6 is a pump which is
used to circulate water. During this process, a little temperature change can
be observed. Finally, the cooled water cannot be directly sent into boiler.
Because the boiler is at higher temperature, it causes irregularly expansion which
results in collapse of the boiler. Hence, the water should be heated to higher
temperature. This is done in economizer which uses heat from flue gases.
MY FURTHER BLOG PART 3 CONTAINS ALL ABOUT CONSTRUCTION AND WORKING OF PARTS AND PLANT
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