CLASSIFICATION OF POWER PLANT CYCLE


Power plants cycle generally divided in to the following groups,
    1. Vapour Power Cycle
         (Carnot cycle, Rankine cycle, Regenerative cycle, Reheat cycle, Binary vapour cycle)
    2. Gas Power Cycles
        (Otto cycle, Diesel cycle, Dual combustion cycle, Gas turbine cycle.)

 CARNOT CYCLE

This cycle is of great value to heat power theory although it has not been possible to construct a practical plant on this cycle. It has high thermodynamics efficiency.
It is a standard of comparison for all other cycles. The thermal efficiency (η) of Carnot cycle is as
follows:
                            η = (T₁ – T₂)/T₁
where,      T₁ = Temperature of heat source
                 T₂ = Temperature of receiver

RANKINE CYCLE

Steam engine and steam turbines in which steam is used as working medium follow Rankine cycle. This cycle can be carried out in four pieces of equipment joint by pipes for conveying working medium as shown in Fig. 1.1. The cycle is represented on Pressure Volume P-V and S-T diagram as shown in Figs. 1.2 and 1.3 respectively.



       Efficiency of Rankine cycle
       = (H₁ – H₂)/ (H1 – Hw2)

where,
      H₁ = Total heat of steam at entry pressure
      H₂ = Total heat of steam at condenser 
                pressure(exhaust pressure)
      Hw2= Total heat of water at exhaust pressure




REHEAT CYCLE

In this cycle steam is extracted from a suitable point in the turbine and reheated generally to the original temperature by flue gases. Reheating is generally used when the pressure is high say above 100 kg/cm2. The various advantages of reheating are as follows:


(i) It increases dryness fraction of steam at exhaust so that blade erosion due to impact of water particles is reduced.
(ii) It increases thermal efficiency.
(iii) It increases the work done per kg of steam and this results in reduced size of boiler.

 The disadvantages of reheating are as follows:
(i) Cost of plant is increased due to the re-heater and its long connections.
(ii) It increases condenser capacity due to increased dryness fraction.

 Fig .1.4 shows flow diagram of reheat cycle. First turbine is high-pressure turbine and second  turbine is  low  pressure (L.P.) turbine. This cycle is shown on T-S (Temperature entropy) diagram (Fig. 1.5).

 If,
    H₁ = Total heat of steam at 1
    H₂ = Total heat of steam at 2
    H₃ = Total heat of steam at 3
    H₄ = Total heat of steam at 4
    Hw₄ = Total heat of water at 4

    Efficiency = {(H₁ – H₂) + (H₃ – H₄)}/{H₁ + (H₃ – H₂) – Hw₄}
REGENERATIVE CYCLE (FEED WATER HEATING)

 The process of extracting steam from the turbine at certain points during its expansion and using this steam for heating for feed water is known as Regeneration or Bleeding of steam. The arrangement of bleeding the steam at two stages is shown in Fig. 1.6.
Let,
      m₂ = Weight of bled steam at a per kg of feed water heated
      m₂ = Weight of bled steam at a per kg of feed water heated
      H₁ = Enthalpies of steam and water in boiler
      Hw₁ = Enthalpies of steam and water in boiler
      H₂, H₃ = Enthalpies of steam at points a and b
      t₂, t₃ = Temperatures of steam at points a and b

      H₄, Hw₄ = Enthalpy of steam and water exhausted to hot well.

 Work done in turbine per kg of feed water between entrance and a
                           = H₁ – H2₂
Work done between a and b = (1 – m₂)(H₂ – H₃)
Work done between b and exhaust = (1 – m₂ – m₃)(H₃ – H₄)
Total heat supplied per kg of feed water = H₁ – Hw₂
Efficiency (η) = Total work done/Total heat supplied

          = {(H1₁ – H₂) + (1 – m₂)(H₂ – H₃) + (1 – m₂ – m₃)(H₃ – H₄)}/(H₁ – Hw₂)
BINARY VAPOUR CYCLE

   In this cycle two working fluids are used. Fig. 1.7 shows Elements of Binary vapour power plant. The mercury boiler heats the mercury into mercury vapours in a dry and saturated state.

    These mercury vapours expand in the mercury turbine and then flow through heat exchanger where they transfer the heat to the feed water, convert it into steam. The steam is passed through the steam super heater where the steam is super-heated by the hot flue gases. The steam then expands in the steam turbine.
REHEAT-REGENERATIVE CYCLE
In steam power plants using high steam pressure reheat regenerative cycle is used. The thermal efficiency of this cycle is higher than only reheat or regenerative cycle. Fig. 1.8 shows the flow diagram of reheat regenerative cycle. This cycle is commonly used to produce high pressure steam (90 kg/cm²) to increase the cycle efficiency.




FORMULA SUMMARY

     1. Rankine efficiency
         = (H₁ – H₂)/(H₁ – Hw₂)
     2. Efficiency ratio or Relative efficiency
         = Indicated or Brake thermal efficiency/Rankine efficiency
     3. Thermal efficiency = 3600/m(H₁ – Hw₂), m = steam flow/kw hr

     4. Carnot efficiency = (T₁ – T₂)/T₁