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Why is this done? Wikipedia says that condensing the steam makes the cycle 'more efficient.' How is it more efficient? Why can't the de-energized steam be transferred to the boiler without condensing it?
The work done by the boiler (heat of vaporization) can be extracted elsewhere to take advantage of it. That's why steam-based industrial processes are so common. These extraction processes (such as turbines) may be made much more efficient by adding post-process condensers to the line, before returning the condensate to the boiler's feedwater system.
When the steam exits a turbine, it should still be a vapor, so that the end-stages of the turbine blades don't get eroded by contact with fast-moving liquid. This is enhanced by adding a condensation stage to the turbine discharge, so that the steam can remain in the vapor stage longer.Maybe I'm completely misunderstanding something.
When the 'steam' exits a turbine, is it still in the vapor phase, or is it a liquid?
0. The steam exiting the turbine is steam (vapor) at very low pressure (like, below atmospheric pressure)
1. condensing the steam creates a vacuum on the downstream side of the turbine.
2. condensing the steam allows you to use a pump (rather than, say, a fan) to push it back into the boiler
3. (most important but hardest to understand) a thermal plant works by catching some of the energy as heat 'moves' from a hot place to a colder place. If you didn't condense the steam you wouldn't have a 'colder' place. Look up 'Carnot cycle' on Google/wiki
4. oddly enough, it does increase the efficiency (in the thermodynamic sense of the word, see 'Carnot cycle' above) to send some (but just some) of the steam back to the boiler (actually it is used to heat the liquid water before it goes into the boiler). Lookup 'Rankine cycle' and 'feedwater heater'
Although it appears as though a vacuum pulls the steam, there is no 'tension' in the steam.Condensing the steam also creates a vacuum, which "pulls" the steam through the turbine.
It takes much less work/energy to pump a given mass of liquid than would be to blow an amount of steam/vapor. The pumping also pressurizes and adds momentum to the liquid.Oops, I should have made my original post more clear.
I understand that passing steam through a turbine removes some of its energy. I wanted to know why thermal plants have a separate condenser after the turbine (eg, item #8 in this diagram http://en.wikipedia.org/wiki/File:PowerStation2.svg )
But you say it's simply to make the steam condensate easier to transfer back to the boiler?
Which would be incredibly expensive for a boiler/T-G set. That feedwater has to be demineralized, heated, treated with additives to inhibit corrosion and the production of precipitates, etc. Condensate is very valuable, and its loss would quickly make many steam systems uneconomical.If you don't re-use the fluid (like say, in a jet engine) you can just blow the turbine exhaust to atmosphere.
Although it appears as though a vacuum pulls the steam, there is no 'tension' in the steam.
Oops, I should have made my original post more clear.
I understand that passing steam through a turbine removes some of its energy. I wanted to know why thermal plants have a separate condenser after the turbine (eg, item #8 in this diagram http://en.wikipedia.org/wiki/File:PowerStation2.svg )
But you say it's simply to make the steam condensate easier to transfer back to the boiler?