Thermodynamics: Steam Power Plant

[maskedup]

This is not homework question, I'm just curious.
Question:

What is the purpose of a condenser in a steam power plant? I read that a condenser increases efficiency, but how?

Since steam from the turbine exit ideally reduce only in temperature (but maintain the same entropy) why is there a need to condense the steam to water first, only to be heated again?

Why can't the steam from the turbine exit be fed straight to the boiler to be superheated?Wouldn't this increase efficiency?

Thanks in advance.=)

 

[Danger]

Short answer: I don't know.
Long answer: I don't think that most boilers are equipped to superheat steam. If they were, then why not do it the first time around?
This is a question that would probably be best answered by Russ.

 

[vlado_skopsko]

It was a long time since i had contact with that, but i will try to answer.
One of the coefficients of efficiency, and most important one is dynamic coefficient.
And it can rise with rising the enthalpy on entrance (with rising of p0 and t0) or
lowering the enthalpy on exit (lowering the pressure of the steam on the exit and creating a vacuum)
So with a condenser we are creating vacuum on exit and with that increasing the efficiency of the engine. The vacuum is made with lowering the specific volume of the steam with its condensing.
And its easier like that the water to be transported to the boiler with pumps.

Superheaters are used for more efficient use of the "coal burner" so gases with lower temperature leave the chimney.

 

[gmax137]

Why can't the steam from the turbine exit be fed straight to the boiler to be superheated? Wouldn't this increase efficiency?

Well, in real plants 'some' of the steam is diverted and used to heat the feedwater, and this is done to increase the cycle efficiency. (google 'extraction steam' and 'feedwater heater'). The optimum design is determined by the efficiency gain versus the cost of additional heaters and extraction piping.

As mentioned above, condensing the steam also allows use of a pump (rather than a compressor) to return the feedwater to the boiler.

 

[maskedup]

Thanks for the prompt response.

From what i understand, steam power plants uses a Rankine cycle. Having said that, one of the practical ways to increase efficiency is to superheat the steam. Steam that are left in the boiler is further heated up to superheated state, which carries more energy.

Steam at the turbine outlet is already low in temperature and pressure (refer to rankine cycle T-s diagram).

In real plants, to prevent heat wastage, waste heat recovery methods are implemented to save cost and energy.

Can't vacuum created by the condenser (mentioned by vlado_skopsko) be achieved by a pump and a one way valve?

My real question is, can a steam power plant run without a condenser? that is, by pumping steam from the turbine outlet straight to the boiler.
If so, why is it not practiced? Is it because of drastic drop in efficiency?
If not, why? Why is the conversion of steam back to water crucial to the cycle?

 

[QuantumPion]

It would be difficult/impractical to design a heat exchanger capable to transferring that much energy by steam convection. Boiling water absorbs a lot of energy compared to just heating it.

 

[Dark_Karbon]

Hello guys... all of you people are just forgetting basic principle behind the working of Rankine cycle. There are 2 things why steam is needed to be condensed-

1) To increase efficiency. (More the temp. difference, more the efficiency)
2) Boiler feed pumps cannot operate with steam (esp. wet steam), so steam has to be condensed first to water and only then it can be pumped to boiler drum.

Comment if I am wrong