Rankine Cycle

Basis for all large scale steam power plants. Named after William John Rankine. Also an ideal case.

When compared with the Carnot cycle:

• Consumes more heat
• Produces more work output
• Less efficient

Heat transfer is not entirely isothermal: latent heat transfer is also there. Less efficient

The Carnot cycle cannot be used in steam power cycles. Because:

• Stopping the condensation process at a specific point is practically difficult or rather impossible task.
  There is a particular dryness fraction associated with the stopping state. Stopping the condensation process to have that percentage of saturated vapor and that percentage of saturated liquid is a very difficult process.
• 2-phase compression is difficult.
  During this compression process there is a greater chance for the liquid phase and the vapor phase to separate. Therefore the compressor has to compress a 2-phase solution and that is not that easy. Also, since vapor is highly compressible, the work input required for the compressor is also quite high.

Rankine power cycle overcomes these issues by allowing the condensation process to go down to the saturated liquid point, or in other words: allowing full condensation.

Superheat

When the steam is further heated than saturated vapour.

Increases average temperature of heat transfer, efficiency, dryness fraction. Avoids water droplet erosion.

Still the efficiency is less than the Carnot cycle efficiency.

Water droplet erosion

The continuous impact of water droplets onto the surface of the blade at high speeds causing impact pressure leads to cracking of the grain structure of the blade material.

Practiclly, the exit turbine has to have a dryness fraction which is greater than 0.9 to avoid water droplet erosion.