11481

Rankine Cycle

In a Rankine cycle, high-pressure liquid water (1) enters a boiler where it is heated to saturation temperature (2), vaporized, and superheated (3). The superheated steam is fed to a turbine, where it expands and generates mechanical work. The steam exits the turbine at a lower pressure and temperature as either superheated steam or steam with a vapor quality (4). After the steam is condensed (5), a pump compresses the liquid water to high pressure (1). Select " diagram" to see the cycle on a pressure-enthalpy diagram. Use sliders to select the inlet pressure to the turbine and one of four outlet pressures . Use a slider to vary the turbine efficiency , which is the ratio of work produced by the turbine to the work produced by a reversible turbine between the same inlet and outlet pressures. The irreversible turbine pathway () is the dashed black line on the diagram; the reversible turbine pathway is the orange dashed line. Select "Rankine cycle" to view a schematic of the cycle, and select "turbine" to show the inlet and outlet conditions for the turbine and the work generated.

SNAPSHOTS

  • [Snapshot]
  • [Snapshot]

DETAILS

The inlet temperature to the turbine, (°C) is set; you can vary the inlet and outlet pressures (MPa). From these conditions the enthalpy (kJ/kg) and entropy (kJ/[kg K]) are obtained from the superheated steam tables.
The superscript denotes reversibility. A reversible steam turbine is isentropic, so , where is the entropy of the steam exiting a reversible turbine.
If at , then the reversible outlet state is superheated.
If at , then the outlet steam is at saturation temperature and consists of liquid and vapor. The quality of the steam is:
,
where is the fraction of vapor (quality), and the superscripts and refer to saturated liquid and saturated vapor.
Depending on the quality of the exiting steam, is obtained from either the saturated steam tables or the superheated steam tables.
If , and is found in the superheated steam tables at and .
If , and .
The change in enthalpy for the reversible turbine is the work for the reversible process.
The turbine efficiency indicates the turbine irreversibility and is the ratio of irreversible work to reversible work. For the irreversible turbine:
.
The cycle efficiency is:
,
where is the net work (kJ/kg), and is heat added (kJ/kg).
The screencast video at [1] shows how to use this Demonstration, and the video at [2] explains the cycle.
References
[2] Power Cycle Introduction [Video]. (Mar 15, 2012) www.youtube.com/watch?v=BA77fu3zAbs.
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