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The Rankine cycle is a Thermodynamic cycle. Like other thermodynamic cycles, the maximum Efficiency of the Rankine cycle is given by calculating the maximum efficiency of the Carnot Cycle . It is named after William John Macquorn Rankine , a Scottish polymath.

This article will deal with the Rankine cycle from an Engineering point of view.


PROCESSES OF THE RANKINE CYCLE


- S diagram of a Rankine cycle, showing both ideal and Non-ideal processes.]]

There are four processes in the Rankine cycle, each changing the state of the working fluid. These states are identified by number in the diagram above.

  • Process 4-1: First, the working fluid is pumped (ideally Isentropically ) from low to high pressure by a Pump . Pumping requires a power input (for example mechanical or electrical).

  • Process 1-2: The high pressure liquid enters a boiler where it is heated at constant pressure by an external heat source to become a superheated vapor. Common heat sources for power plant systems are Coal , Natural Gas , or Nuclear Power .

  • Process 2-3: The superheated vapor expands through a Turbine to generate power output. Ideally, this expansion is isentropic. This decreases the temperature and pressure of the vapor.

  • Process 3-4: The vapor then enters a Condenser where it is cooled to become a Saturated Liquid . This liquid then re-enters the pump and the cycle repeats.



DESCRIPTION

Rankine cycles describe the operation of steam Heat Engines commonly found in Power Generation Plants . In such vapor power plants, power is generated by alternately Vaporizing and Condensing a working fluid (in many cases water, although Refrigerants such as ammonia may also be used).

The working fluid in a Rankine cycle follows a closed loop and is re-used constantly. Steam seen billowing from power plants is evaporating cooling water, not working fluid.


VARIABLES







\dot{Q}_{in}heat input rate (energy per unit time)
\dot{m} Mass Flow Rate (mass per unit time)
\dot{W}mechanical Power used by or provided to the system (energy per unit time)
\etathermodynamic efficiency of the process (power used for turbine per heat input, unitless)
h_1, h_2, h_3, h_4these are the "specific Enthalpies " at indicated points on the T-S diagram



EQUATIONS

Each of the first four equations are easily derived from the Energy and Mass Balance for a control volume. The fifth equation defines the Thermodynamic Efficiency of the cycle as the ratio of net power output to heat input.