What Are the Advantages and Disadvantages of the Superheated Steam Cycle?

[ksle82]

does anyone knows what are the advantages and disadvantages of using a superheated steam cycle (i.e. effect of turbine blade)?

 

[turbo]

Steam carries a lot of usable energy in the form of the latent heat of vaporization. This is the part of the heat that does the work, if it is exploited properly. Generally, the steam is produced in a boiler and is extracted from the steam drum as saturated steam at the boiler's operating pressure - it is then is then superheated in another set of tubes, then it is desuperheated to a specific temperature target by injecting feedwater into the steam. The amount of superheat needed is determined by the pressure and temperature of the steam at the turbine. The turbine is designed such that at each stage of buckets and blades the steam undergoes a pressure drop, keeping the steam near superheat temperature at each stage drop. In big turbines, there may be a few extraction stages, too, so that the turbine can supply 150# and 50# steam for other processes, for instance. Big turbines generally operate at below atmospheric pressure in the final stages, thanks to hogging extractors and condensers, so the steam will condense at less than 100 deg C.

Now, you don't want to send "wet" steam to a turbine - the erosion on the buckets and blades is destructive. Giving the touchy nature of the vibration sensors in the big turbines, it would probably trip pretty quickly, anyway.

 

[Astronuc]

For a thermodynamic cycle, the higher the temperature, the greater the thermal efficiency (think Carnot efficiency), and the higher the pressure for a given condenser pressure, the more work one can extract from the turbine set.

In a large power plant, steam is passed to a high pressure (HP) turbine, then an intermediate pressure (IP) turbine and finally a low pressure (LP) turbine set. In between turbine stages are moisture separators and reheaters. For instance, the condensation from the HP stage can go to reheat the steam out of the IP stage before it goes to the LP stage. In addition, other condensation is mixed directly or indirectly with the recirculating condensate from the condenser to increase the temperature of the feedwater to the boiler, thus reducing the heat input to raise the water back to maximum temperature.

That's the positive side. The downside to higher temperature is the corrosion of the metals in the hotter regions. As water temperature increases, so does it corrosion (chemical) potential, and so does the corrosion potential for any metal. Oxidation kinetics increases with temperature, and the morphology of the oxide is different (less adherent), because among other effects, differential thermal expansion of metal components and their oxide (corrosion) leads to stress in the metal/oxide interface. If the oxide sloughs off, then erosion can be a problem. Erosion is also a problem at higher fluid velocities.

Higher pressures mean thicker walled piping and pressure vessels in order to maintain a given level of stress. The cost goes up, not only for more material but more fabrication (e.g. welding). Thicker wall mean greater inspection effort, and likely more surveillance during operation.

 

[turbo]

Darn, Astonuc, I thought I gave the OP WAY too much to chew on. I expected that he might be cursing my name at this point, but he may be cursing yours now. ;-) BTW, it is important to point out that you cannot simply send really hot steam through any particular system and expect to extract work out of it. You have to have the steam near the condensation point, and you have to have a mechanical system that is optimized to take advantage of the heat value in that phase transition.

 

[ksle82]

Thanks a lot guys for the superb explanations. Keep up the good work.

 

[Cyrus]

Yes, they gave you an answer of superb quality

 

[Astronuc]

I gave a short answer too!

Actually, this is a very interesting and relevant question because one of the 7 types of advanced (Gen IV) reactors is a 'supercritical' light water reactor! This is much more aggressive than just superheated steam.

See - Supercritical-Water-Cooled Reactor (SCWR)
http://nuclear.inl.gov/gen4/scwr.shtml

The Supercritical-Water-Cooled Reactor (SCWR) system is a high-temperature, high-pressure water-cooled reactor that operates above the thermodynamic critical point of water (374 degrees Celsius, 22.1 MPa, or 705 degrees Fahrenheit, 3208 psia).

Thermodynamically, it would be great! BUT, there are numerous technical challenges. We already have significant technical problems with the current technology, particularly in corrosion and aging. Pushing up the operating temperature (and pressures) pushes materials and systems closer to technical/physical limits.

We didn't quantify the degree of superheat, but one can look on the P-T diagram and see what the implications are for superheat.

Another advantage of superheat is the dry steam - which is great for the turbines in the sense that it avoids the droplets (condensation) which can damage the blades by impact. But as steam cools, somewhere (in the succession of blade stages) the droplets will form.

 

[turbo]

I gave a short answer too!

That WAS a short answer considering the concepts you were covering.