Summary: Ideal Verses Actual In Rankine Cycle

Ideal Verses Actual In Rankine-Cycle

Rankine-cycle is a known mechanical-cycle which is being commonly used in the power plants for converting the pressure energy of steam into mechanical energy using steam turbines. Major components of it are rotating steam turbine and boiler pump and stationary condenser and boiler. Boiler is used for heating the water for generation of steam at required pressure and temperature as per the requirement of the turbine for power generation. Turbine exhaust is directed to the radial or axial flow condenser for condensing the steam to condensate and recycled back to the boiler through boiler pumps for heating again.
The efficiency of the ideal Rankine cycle as described in the earlier section is close to
1-a and Fig 1-b represents the Rankine cycle on P-v and T-s diagram

Rankine Cycle Representation are as follows on P-v & T-s diagrams:

Ideal Rankine Cycle 1-2’-b-3’-4’-1
Actual Rankine Cycle 1-2-b-3-4-1

Critical Point ( CP) is in centre of the curve as shown in Fig 1-a & 1-b above. The curved lines on the left side of the CP are saturated- liquid lines and the region/area to the left of these lines are called as sub-cooled liquid regions.
Similarly curved lines on the right side of the CP are saturated- vapour lines and the region/area to the right of these lines are called as super-heat vapour regions
Energy Analysis of Ideal Rankine Cycle

All components of rankine cycle (Boiler, turbine, condenser and pump) are examples of steady flow process and to be analysed accordingly. Energy balance for the Ideal cycle is as follows:
Ideal Rankine Cycle Components Heat Work
Boiler feed Pump
Wpump-in ‘q=0 Wpump-in = ‘h2’ - h1 or Wpump-in = v(p2’-p1)
Boiler ‘qin = (h3’ - h2’) W=0

Turbine ‘q=0 Wturbine-out = (h3’ – h4’)

Condenser ‘qout = (h4’ – h1’) W=0

Thermal efficiency of Ideal Rankine cycle (Wnet/qin)
Because of the fluid friction pressure drop in the boiler circuitry, the pressure of the steam leaving the boiler will be at somewhat lower pressure. Also the steam has to be conveyed to steam turbine via steam piping which also accounts for further pressure drop. So the steam which reaches the turbine stop valve will be at lower pressure than that of the boiler discharge pressure and the same is represented by 3’ ( Fig-1a) in the actual rankine cycle instead of 3 in the ideal rankine