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


PROCESSES OF THE RANKINE CYCLE


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 to the right.

  • Process 1-2: The working fluid is pumped from low to high pressure, as the fluid is a liquid at this stage the pump requires little input energy.

  • Process 2-3: The high pressure liquid enters a boiler where it is heated at constant pressure by an external heat source to become a dry saturated vapour. Common heat sources for power plant systems are Coal , Natural Gas , or Nuclear Power .

  • Process 3-4: The dry saturated vapour expands through a Turbine generating power output usually orders of magnitude greater than the power required by the pump. This decreases the temperature and pressure of the vapour and some condensation may occur.

  • Process 4-1: The wet vapour then enters a Condenser where it is cooled at a constant low pressure to become a Saturated Liquid . It is fully condensed to a liquid to minimise the work required by the pump.



In an ideal Rankine cycle the pump and turbine would be Isentropic , i.e. the pump and turbine would generate no entropy and hence maximise the net work output. Processes 1-2 and 3-4 would be represented by vertical lines on the Ts diagram and more closely resemble the Ts diagram of the Carnot cycle.

The exposed Rankine cycle can also prevent vapour overheating , which reduces the amount of liquid condensed after the expansion in the turbine.


DESCRIPTION


Rankine cycles describe the operation of steam Heat Engines most commonly found in Power Generation Plants . By taking advantage of the Phase Change of water (Mercury, ammonia and many other substances have also been used) the cycle can almost achieve iso-thermal heat addition and rejection. The cycle is sometimes referred to as a practical Carnot Cycle as, when an efficient turbine is used, the Ts diagram will begin to closely resemble the Carnot cycle.

In Gas Turbines a significant fraction of the work generated by the turbine will go to driving the compressor and so limits net work output and efficiency. The Rankine cycle on the other hand does not face this problem. By condensing the steam to water, the work required by the pump will only consume approximately 1% of the turbine power and so give a much higher efficiency. As liquids are far less compressible they require only a fraction of the energy needed to compress a gas to the same pressure.

The efficiency of a Rankine cycle is usually limited by the working fluid. Without the pressure going Super Critical the temperature range the cycle can operate over is quite small, giving a maximum possible efficiency ( Carnot Efficiency ) of around 50%. For this reason the Rankine cycle is often used as a bottoming cycle in Combined Cycle Gas Turbine power stations.

The working fluid in a Rankine cycle follows a closed loop and is re-used constantly. Water Vapour seen billowing from power plants is evaporating cooling water, not working fluid. (Note that Steam is invisible until it comes in contact with cool, saturated air, at which point it condenses and forms the white billowy clouds seen leaving Cooling Towers ).


VARIABLES







\dot{Q}Heat flow rate to/from the system(energy per unit time)
\dot{m} Mass Flow Rate (mass per unit time)
\dot{W}Mechanical Power consumed/provided to the system (energy per unit time)
\etaThermodynamic efficiency of the process (net power output per heat input, dimensionless)
h_1, h_2, h_3, h_4The "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. As the work required by the pump is often around 1% of the turbine work output equation 5 can be simplified.