Condensers In Thermal Power Stations

Pls refer to my post http://cr4.globalspec.com/thread/69519#comment740821

post #10

Pumping water would be easier and less energy consuming i guess rather than compressing steam again to send back to boiler... But the reason could be more theoretical..

Latent heat.

If you return steam to the boiler, you can only superheat it, and thus not "send out" much more added energy than the return steam already has.

Since the boiler is under high pressure, to pump back steam into the boiler, it would require a lot of energy, given by pressure x volume.

By condensation, the volume of steam is greatly reduced (by 819 times).

So, pumping water back to the boiler use a lot less energy.

GA first correct answer.

It improves the efficiency of the turbines and steam engines as that makes the outlet virtually a vacuum. I don't have any figures to show how low the pressure actually is as I have simply forgotten it during the last 38 years.....

The one I've seen in operation had an outlet pressure of 0.04 bar (absolute). In fact it used a very small amount of steam to decrease the outlet pressure by means of a "steam ejector" - basically a steam-operated vacuum pump.

Not all steam turbines operate on the same principal, though. You can also have counter-pressure turbines (not sure whether that's how they're called in English), where you have fairly high pressures at the outlet - and then you use the energy left on the outlet steam to power something else ... sometimes an industrial process, and sometimes even a lower-pressure condensating steam turbine. Anyhow, condensating the steam is always a required step before sending it back to the boiler, because both of the latent heat (condensating allows you to extract more of the energy steam conveys) and of the innefficiency of compressing a gas.

Although the steam is condensed, it is still hot water (nominally over 90°C) when it is recirculated to the boilers. So the energy required to reconvert it back to steam only raises it a few degrees as liquid, plus the latent heat of vaporisation, plus any super-heating. Power stations are already optimised to achieve the minimum costs for both condensation and reheating.

The answer is pressure.
The steam loses its pressure when it condenses, and will return via gravity to the inlet on the boiler. The entire system is pressurized. Condensation lowers the pressure in the volume of the low pressure side of the system and it is the high pressure steam coming from the boiler that is trying to get to the lower pressure area on the other side of the workload. Condensation, and gravity, and a much smaller pipe and pump them move the liquid back to the boiler in a closed path system.
Condensation increases the volumetric efficiency of the closed path steam system.

OOps! I just reread the OP and realized I need more coffee...
You are right! Why not? Perhaps my original "answer" is part of the correct answer. How do you move all of that heat back to input side of the turbine? Somehow, the heat must be concentrated and reintroduced to the boiler. In the closed system that is what happens, and only the energy needed to go from near boiling to boiling is lost to the condenser.
My guess is that it is efficiency. It takes as much to re-compress the steam as steam to put it back into the boiler as it took to make it steam and then expand it in the workload of the turbine. You are better off letting it go. But why condense it if you are letting it go? I would say it is the final stage that creates a greater difference in pressure at the low pressure side of the workload. In a piston type steam system, there is no need for a condenser. But to a turbine, the need is absolute. Without the lower pressure on the output side, there is less difference between the high side and the low.
Ok, that's enough thinking for a Sunday!