Because the specific volume of steam is more for steam
compared to water.so if we are not condensing it,the
entire power generated by the turbine will be utilised only
to pump the steam.Also as per second law of
thermodynamics,there must be a source and a sink.Here
boiler acts as a source and condenser as a sink.
In the last stage of LP cylinder the out coming steam has
higher specific volume. It is not possible to keep the large
size pipe to travel the steam from one place to another.
hence we go for condensation process. Also further addition
or makeup of DM water can be done only in condenser. Hence
we go for condensation.
after last stage, the pressure & temperature of the steam
is very less. so that it can't travel further any area.
so that we have to condensate that steam and pump it to
boiler reheater. With less pressure steam can't travel
upto boiler height. That is why we r changing the phase
from steam to liquid. We can pump the liquids very easily
than steam. Another point is in condenser, there are lot
of non-condensable gases will present in the media. Those
gases we have to remove. If u reheat the same steam, due
to these non-condensable gases, the boiler tubes may
damage. So we required to pass this liquid to deaerator
According to Second law of thermodynamics, There should be a
source and sink. Here Boiler acts as a source and condensor
acts as a sink.
If the Steam is not condensed, The power generated by the
turbine will be used to drive the pump to pump the fluid to
a higher pressure. Instead if we use a boiler it converts
the wet steam into a super heated steam. So, Both source a
nd sink are must.
ANSWER: According to Kelvin- Plank's statement,Steam has to
There are many reasons for not reheating steam
1.According to thermodynamics law there should be heat source and sink.Boiler as source and condensor as sink.
2.There are non condensable gases which are to be removed otherwise they will damage boiler tubes.
3.As thermal power plant works on rankine cycle so as much pressure and temperature drop is there as improved will be efficiency and in case of condensor there is vaccumm being created by ejector so pressure drop is substantial.
4.WE can use steam from some intermediate stage (Bleed) to heat feed water and deaeration or for reheating in boiler .
the steam again condensed into water in order to avoid
arrosion and also to maintain constant temperature...if it
is reheated the desired temperature is crossed which inturn
efffects the material of the turbine
if we directly use the steam to reheat then turbine will not give the rated/designed efficiency because some energy go with the steam. another cost of transfer will be high according to power consumption.