Carnot and renkine steam cycle

[Micko]

Hello to all,
this is my first post on this forum. I'm electrical engineer (technical informatics) but in my spare time I'm interested in thermodynamics especially about steam cycle in power plant.
I know that condenser is meant to convert steam to water and here we have pure losses of energy. Can anyone explaine why vakkum in condenser is needed?
Thanks

 

[jasc15]

in the condenser, there is no loss in energy (look on the enthalpy/entropy diagram, the portion of the cycle from condenser to pump is a horizontal line). as far as why the condenser is at vacuum pressure (something like 1psi or 7kPa) it is to be able to extract as much energy from the working fluid while keeping it in the gaseous phase (water is still steam at 100F or 37C when the pressure is at 1psi) because obviously you can't have liquid in a turbine. hope that helps..

 

[Homer Simpson]

Cycle efficiancy goes up when vacuum pressure goes down. Its that delta P between turbine inlet (steam P) and outlet (CD P) that determines power. In summer when the lake temperature goes up, the condensing cooling water is warmer, therefore the steam condenses at a higher pressure and we lose megawatts. You could exhaust the turbine to atmosphere theoetically but you'ld be losing a lot of power (and water!). There is something like 1000:1 reduction in volume as steam collapses, and this explains why the vessel is under vaccuum. Also note that there is no air or other non condensable gasses in the condensor, just water.

 

[Micko]

Okey, thanks guys, I'll try to understand your answers. I know that efficiency is something like (T1-T2)/T1 and at first I thought that condenser is there to reduce T2 and makes efficiency better, but then I know that this goes to air and not to turbine, so that is probably wrong conclusion.
Can you suggest me where to find out about difference between enthropy and enthalpy?
It seems that I'm confused about Work (Energy) and Heat. In condenser heat is rejected but no work energy. How to explain that?

 

[russ_watters]

It can be useful to think of temp and pressure as two parts of the same thing (indeed, for some other thermodynamic cycles, efficiency is expressed in terms of pressure ratio). If the pressure in a vessel goes down, the temperature will follow and if pressure is lower at a point in the cycle, the boiling point is also lower.

 

[Micko]

It can be useful to think of temp and pressure as two parts of the same thing (indeed, for some other thermodynamic cycles, efficiency is expressed in terms of pressure ratio). If the pressure in a vessel goes down, the temperature will follow and if pressure is lower at a point in the cycle, the boiling point is also lower.

When you say: lower pressure, lower boiling point, is that conclusion from:
p*V/T = const?

 

[Homer Simpson]

http://canteach.candu.org/library/20043706.pdf

pretty late reply I know:zzz: , but check out the above... very related. esp. see pg's 8,9/13 which deal with thermo cycle of condenser a bit.