Why is steam condensed in thermal power cycle?

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.

 

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'

 

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.

 

Condensing the steam also creates a vacuum, which "pulls" the steam through the turbine.

 

Although it appears as though a vacuum pulls the steam, there is no 'tension' in the steam.

 

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.

 

If you want to build a cycle where the fluid is re-used, you have to return it to its original state (in this case, liquid water). That means the steam has to be condensed.

You can build cycles that use a gas throughout (example, a stirling cycle). These have no condenser; but they still need a heat sink for the 'cold end.'

If you don't re-use the fluid (like say, in a jet engine) you can just blow the turbine exhaust to atmosphere.

 

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.

 

Right, precisely why I put the "pulls" in quotes. In the end it's just a pressure difference.

 

If I'm understanding your question, you want to know why there has to be a condenser in the steam cycle? The answer is that the cycle doesn't close if you don't reject heat to a low-temperature sink. There's a lot of additional hardware in the diagram you linked, but fundamentally the steam cycle comprises

1) a pump that takes cool liquid water and raises the pressure,
2) a boiler that adds heat to the liquid, boiling and superheating it,
3) a turbine that extracts shaft work from the superheated steam and exhausts a low-pressure, high quality liquid-vapor mixture, and
4) a condenser that takes the turbine exhaust and returns it to the pump in the cool, liquid state again.

Without the condenser the cycle wouldn't close. The pump, designed to handle cool liquid, would cavitate when fed vapor. Even if you designed a pump to handle liquid and vapor, the cycle would spiral to higher and higher temperatures because no heat would be rejected.

However, if you push on it, this is a very subtle question. Is it possible to design a cyclic heat engine that takes heat from a single source at some fixed temperature and produces usable work, rejecting no heat? The answer is no -- that would permit violations of the second law of thermodynamics, as it turns out, because the heat reservoir would continually lose entropy and the heat engine would continue in a cyclic process forever. So the second law keeps condenser manufacturers in business.

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