High temperature and pressure steam is used in turbines

[Impulse2]

I did thermodynamics I and II 50 yrs. ago so help me out here.
I note that high temperature and pressure steam is used in turbines and presumably the higher, the better the efficiency. My question is, is the temperature doing anything or is it only necessary to produce the high steam pressure from the boiler. If it was possible to produce the same pressure at a lower temperature would the result be the same? Is it only the kinetic energy of the steam that is driving the turbine blades?

 

[russ_watters]

The temperature is necessary because if it were much lower (there is probably some superheat), the steam would condense.

 

[Topher925]

The temperature is necessary because if it were much lower (there is probably some superheat), the steam would condense.

True, if the quality ratio is to high or to low it would destroy the turbine. But to answer the question with regards to temperature, yes it is doing something. There are many different types of turbines but for Rankin cycles there are turbines that generate work from a pressure drop and others that generate work from thermal expansion. If you ever have a chance to analyze a power plant you will see that not only is there a drop in pressure between the intake and exhaust of a turbine but also a significant temperature drop due to this thermal expansion. This is not do to heat transfer, as most turbines are insulated AFAIK.

This is also the main argument of why turbo chargers are more efficient than super chargers. Because turbos can create work from thermal expansion leading to lower final exhaust temperatures and are not just powered solely by the induced pressure from the pistons on the exhaust stroke.

 

[gmax137]

Superheated steam also has higher enthalpy than at saturation. Just as an example, saturated steam at 1000 psia (T=545F) has an enthalpy of 1193 Btu/lb. Superheated by 50 degrees F, (P=1000, T=595) and the enthalpy is 1245 Btu/lb. That "extra" 50 Btu/lb is available to work on the turbine on its way to the condenser, so more work out.

ps, Sorry to the SI fans out there for these obscure units.

 

[Enthalpy]

Hi there!

If (big IF, but that's so simpler!) you consider gases to be perfect, then the mechanical power you can extract from them in adiabatic expansion is the enthalpy variation which in turn depends only on the input to output temperature difference.

At identical pressures (or pressure ratio), a higher input temperature means more temperature drop. If the pressure ratio increases (that's the case of saturated steam AND of steam that shouldn't condensate) then you exploit even more of the input temperature.

That's why people like to have a high input temperature for any gas turbine. It means power and efficiency.

Now, steam is more complicated. You invest a lot of heat just to boil it, and this heat is lost if steam can't condensate in the turbine. So the more heat you add once water is gaseous, the lower the relative loss into boiling water first.

However, I learned as so many people did that steam shouldn't condensate in the turbine, or it will destroy it or lessen the efficiency. Completely wrong.

In PWR (nuclear reactors with pressurized water), the maximum achievable temperature is 374°C so water remains liquid around the fuel rods, and even less hot to have a margin and keep a good density for water. So steam temperature is uncomfortably low.

Guess what? Designers just decided to let steam condensate in the turbine. It means that pressure is higher, giving smaller turbines. It means that the condensing fraction of steam gives heat to the gaseous fraction, thus keeping it warmer than a gas would be when expanding - they get a part back from the heat invested in boiling first the water, and efficiency is better.

In the Westinghouse technology, saturated steam expands in the high pressure stage (and condensates partially there), then is dried (liquid is separated), heated again (by injecting some high pressure steam in it I think) and sent to the low pressure stage. Erosion doesn't look so horrible, as blades are simply made of a variant of X20Cr13 (Aisi 420) and last for a few years.