Effect of Thermodynamics Variables on Efficiency and Output of Rankine Cycle

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Effect of Thermodynamics Variables on Efficiency and Output of Rankine Cycle

Following are the thermodynamic variables which effect the efficiency and work output of a Rankine cycle :
  1. Superheating of steam
  2. Boiler pressure or pressure of steam at inlet to turbine
  3. Steam pressure at exhaust or condenser pressure or back pressure of steam. The influence of each is being discussed below.

Effect of Superheating of Steam :

  1. Figure A shows the effect of superheating of steam before entering the turbine for the given boiler pressure, p, and condenser pressure, P.
  2. It should be noted the work output increases by an amount equal to the shaded area (3-3′- 4-4).
  3. The heat supplied is increased by an amount equal to area under the process curve (3-3) i.e. area (3-3-b-a).
  4. However, the ratio of increase in work output to increase in heat supplied to steam is more than the ratio of workdone to heat supplied for Rankine cycle (1-2-3-4) due to which the cycle efficiency increases.
  5. Alternately, the cycle efficiency increases because the superheating of steam increases the mean temperature of heat addition process in the boiler while the mean temperature of heat rejection process remains the same.
Effect of Superheating
Figure A

Other advantages and considerations of superheating of steam are :
  1. It reduces the moisture content in turbine expansion process which reduces the corrosion of blades caused by moisture content of steam.
  2. Maximum temperature of superheated steam used is fixed from metallurgical considerations i.e. the materials used for various components of the turbine and the pipe lines, valves etc. and their ability to withstand high temperatures.
  3. Usually the maximum temperatures of steam used are limited to 650°C from metallurgical considerations.
  4. The dryness fraction of steam at the end of expansion of steam should not be below 0.85 to avoid the corrosion of turbine blades and to reduce the inertia force on blades due to moisture content in the steam.
  5. The condenser pressures are fixed based on practical considerations. In case of steam turbines the minimum pressure is limited to 0.05 to 0.1 bar pressure.
Effect of Boiler Pressure
Figure B
Effect of Increase in Boiler Pressure :
  1. Two Rankine cycles (1-2-3-4) and (1-2-3-4) having the same maximum temperature, Tz = T’, but at the different maximum pressures p and p’ respectively are shown in Fig B.
  2. The condenser pressure is same in both the cases.
  3. As a result the work done increases by the area shown by the oblique hatching and decreases by the area shown by horizontal hatching due to the increased pressure of the cycle.
  4. These two workdone (areas) are approximately the same but the heat rejected in the condenser decreases by the area (4-4-C-B).
  5. Hence, the Rankine efficiency increases with the increase in the maximum pressure of the cycle.
Effect of Increase in Condenser Pressure Two :
  1. Rankine cycles (1-2-3-4) at condenser pressure, P4 and the cycle (1-2-3-4′) with increased condenser pressure, p’4 are shown in Figure C.
  2. Net workdone is reduced equal to an area (1-2-2-1-4-4-1) due to the increased condenser pressure and the heat supplied during the cycle reduces by the area equal (A-2-2-B).
  3. These two areas are nearly equal.
  4. The net result is the decrease in efficiency of the cycle.
  5. It should be expected because the temperature of the heat rejection process during the cycle increases with the increased condenser pressure.
Effect of Condenser Pressure
Figure C

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