rankine cycle

Ideal rankine cycle:

1-2: Isentropic expansion of superheated steam in the turbine.

2-3: Condensation in the condenser which converts steam into water. It is the constant pressure and constant
temperature heat rejection from steam which causes condensation. The volume is reduced about 1000 time in the condenser, thus saving huge amount of mechanical work.

rankine cycle
ideal rankine cycle

3-4: Constant entropy pressure rise in the pump.

4-5: There is constant pressure heat supply in the economizer or with feed water heating.

5-6: Heat supply at constant temperature and pressure in the Boiler.

6-1: The steam is superheated at constant pressure.

4-5-6-1 ; This is the line of constant pressure.

5-6: Is the line of constant Temperature.

After expansion inside the turbine, the dryness fraction is kept more than 0.85, as high water content causes damage like rust etc.

To reduce mechanical work for pumping, we cool until saturated liquid is obtained and no steam content remains.

Actual rankine cycle:

In actual Rankine cycle, the processes are deviated from those of Ideal Rankine cycle.
Solid lines are showing ideal processes and dotted lines are showing actual processes.

rankine cycle
actual rankine cycle

1’-2’: Actual adiabatic expansion of superheated steam in the turbine. Turbine is surrounded by insulating material such as fiber glass or asbestos to prevent heat loss.

2’-3’: Actual process of condensation.

3’-4’: Adiabatic rise in pressure in the pump.

S’2 is higher than S2 and S4 is higher than S4, showing that entropy is generated in these processes.

Isentropic Efficiency
 = Actual work/ Ideal work when expansion is isentropic.

Unfortunately, the entropy is generated in cases, the expansion and the compression.

4’-5’: Heating in the economizer.

5’-6’: Actual process of heating in the boiler.

6’-1’: Super-heating process in the super heater.

It must be mentioned here that all the process in the Rankine Cycle are control volume as mass is entering and leaving and also the heat and work.

Ideal rankine Cycle is a Cycle in which all the thermodynamic processes go ideally without losses and there is no entropy generation during any of the process.

In the figure below, the actual rankine cycle is shown on the T-S diagram.

Reheat rankine cycle:

The drawback of having high pressure in the boiler is the increase of water content at the blades steam turbine. To avoid this problem, reheat Rankine cycle is used.
The diagram for the Reheat Rankine cycle is shown.

reheat rankine cycle
reheat rankine cycle

We can enhance the thermal efficiency of the steam turbine power plant by reheating the steam coming from high pressure turbine, approximately to same temperature as the inlet of the first turbine and then passing it from an another turbine operating at low pressure, called low pressure turbine. By this arrangement, the dryness fraction at the exit of low pressure turbine is increase which is desirable to increase the thermal efficiency of the Plant.

The various processes in Reheat Rankine Cycle are:

1-2: Isentropic Expansion of superheated steam in the high pressure turbine.

2-3: Constant pressure reheating in the Re-heater.

3-4: Isentropic expansion in low pressure turbine.

4-5: Heat rejection in the condenser at constant temperature and pressure.

5-6: Isentropic pressure rise in the Pump.

6-7: Heating of subcooled liquid

7-1: Constant Pressure Reheating in the Boiler.

Heat supplied to the Boiler = h1 – h6 = Q1

Heat rejected to the condenser = h4 – h5 = Q2

Total Work output = Wt1 + Wt2 = Work done in low pressure turbine + Work done in high pressure turbine = (h1 – h2) + (h3 – h4)

Total Heat supplied = heat supplied in the Boiler + Heat supplied in the Re-heater = (h1 -h6) + (h3 – h2)

Thermal efficiency = Net work output/ Total heat supplied = {(h1 – h2) + (h3 – h4) – (h6 – h5)}/ {(h1 -h2) + (h3 – h2)}.

Combined Cycle Power Plants

The combined cycle power plant is power plant which consists of two power cycles.

1.Brayton Cycle

2.Rankine Cycle

Brayton Cycle:

In Brayton cycle Gas turbine is powered by gas to spin the generator and produce electrical power. In this cycle Gas turbine also can be power by furnace oil or High speed diesel. The efficiency of brayton cycles gas turbines is up to 40%.

Rankine Cycle:

The other cycle is called HRSG (Heat recovery steam generation). In this process, steam is generated from the exhaust of gas turbine and this is fed to steam turbine to spin the attached generator. The temperature of exhaust is too high near 650c which is enough to generate useable steam. overall efficiency of power plant is increased from 40% to 54%. There are many other accessories for combined cycle power plant which work with parallel as like auxiliary pumps for fuel, water , lube oil, air intake etc.

rankine cycle  for detailed procedure.

rankine cycle is cycle of steam power plant which is explained cycle by cycle.

A steam turbine power plant converts the energy of the fuel into the shaft work continuously and ultimately from the shaft work, electricity is produced.  In the boiler, there is combustion of fuel (fossil fuel) like oil, natural gas, and coal or fissile fuel such as Uranium or Thorium to produce steam.

In the combustion process, heat generated is supplied to the boiler which is a heat exchanger. Boiler is a tube separating two fluids by a walls and thermal conductivity of this wall is very high. In the Nuclear Power plant, the furnace is replaced by Nuclear reactor.

The steam is produced at very high temperature and pressure which expands in steam turbine to produce Mechanical energy and the electrical energy.

Where
Mf     = Mass of fuel

C.V   = Calorific value

Wp    = Work of  the Pump

Q1     = Heat Supplied

Q2     = Heat rejected

Wt is the shaft work, which is form of mechanical energy.  Steam coming from the turbine is fed to the condenser where condensation of the steam takes place. Here heat is extracted from steam to convert it into liquid. Steam emitted from the turbine has both liquid and vapor phases.

River or see water can be used to extract heat from steam and there is change of phase from vapor to liquid. If no river or see is present in nearby area, cooling tower is needed to supply cold water to condenser. The pressure inside the condenser is 10% of the atmospheric pressure. So, a pump is used to raise the pressure from condenser pressure to boiler pressure.  Therefore steam turbine power plant work in a sequence.

B —–> T ——-> C ——> P  ——> B

And the cycle is going on repeatedly.

Co-Generation:

The production of electric power and heat in a single unit is called co-generation. Co-generation is applicable to the industries like paper, Textile, sugar, chemical, cement etc.

Advantages of Cogeneration:

Isothermal process is maintained by steam as it is the best fluid to maintain constant temperature.  The constant process is maintained by using its latent heat of vaporization.

Process is good for safety point of view. For example in inflammable environment, e.g in oil refinery, the direct heat can blow up the whole plant. Steam use for this process is safe.

Feasibility of the plant must be seen before choosing a cogeneration plant.

There are two types of cogeneration cycle.

A.    Topping cycle:
B.    Bottoming cycle:

Topping cycle:

It is cycle where main emphasis is on the production of electricity. High grade steam is used to generate power and then low grade steam is used in industrial process. Topping cycle finds its application in process industry.

topping cycle
topping cycle

 

 

 

 

 

Bottoming Cycle:

Here the steam generated by the steam generator is mainly utilized in the Industrial process and then the steam from the discharge of industrial process is used to generate electricity.

bottoming cycle
bottoming cycle

In the cement industry, this type of cycle is applicable as the steam required there should be high grade. ( High temperature and pressure)