Mar 202013
 

Reheat Rankine Cycle is essentially a modification of simple rankine cycle. In reheat rankine cycle, the following improvements are made to increase the efficiency of rankine cycle.

  • Lowering the condenser pressure
  • Increasing the temperature of steam while entering the turbine
  • Large variation in pressure between boiler and condenser
  • Implementation of reheat and regenerative system in the cycle

What is Reheat Rankine cycle?

In simple rankine cycle, after the isentropic expansion in turbine , steam is directly fed into condenser for condensation process. (Refer this article for better understanding). But in reheat system, two turbines (high pressure turbine and low pressure turbine) are employed for improving efficiency. Steam, after expansion from high pressure turbine, is sent again to boiler and heated till it reaches superheated condition. It is then left to expand in low pressure turbine to attain condenser pressure.

h-s diagram of Reheat Rankine Cycle:

Reheat Rankine cycle can be understood well if you refer the following h-s diagram:

Reheat Rankine Cycle h-s diagram

Processes in Reheat Rankine Cycle:

Six processes take place in reheat Rankine cycle. They are explained in detail below:

Process: 1-2 (high pressure turbine)

Here, dry saturated steam from the boiler is allowed to expand in a turbine isentropically i.e., Entropy remains constant.

Let h1 be the enthalpy of steam entering the turbine

Let h2 be the enthalpy of steam leaving the turbine

Finally, the workdone by turbine is given by

W= h1 −  h2

Calculation of h1 and  h2 :

Using the pressure and temperature values at point 1, values for entropy (S1) and enthalpy (h1) can be calculated from superheated steam table or from Mollier diagram for steam.

After finding h1 and S1 ,dryness fraction (x2) can be calculated using the formula given below,

\(S_{1} = S_{2} = S_{f2} + x_{2} \times S_{fg2}\)

Substitute the value of x2 in the following equation to find h2,

\(h_{2} = h_{f2} + x_{2} \times  h_{fg2}\)

Process: 2-3 (boiler)

The expanded steam is made to attain the required temperature i.e., reheated in low pressure boiler at constant pressure level. The enthalpy (h3) and entropy (S3) are calculated by the same method that we followed in process 1 to 2.

Process: 3-4 (Low pressure Turbine)

After attaining required temperature, steam is passed into low pressure turbine to carry out the remaining expansion. The enthalpy (h4) and entropy (S4) values are calculated by the same method that we followed in process 1 to 2 .

 Process: 4-5 (condenser)

After expansion in turbine, steam is passed into condenser to preform condensation process. Here the remaining heat in the steam is rejected into atmosphere.

Let h4 be the enthalpy of steam entering the condenser.

Let h5 be the enthalpy of water leaving the condenser.

Heat rejected from condenser is given by

QR  =  h4  − h5  J     or     QR  =  h2  − hf4 J

hf4 = h4  (Since, the output from condenser is a fluid and graphically, the enthalpy at point 4 and point 5 are same.)

 Process: 5-6 (Pump)

Water from the condenser is pumped into the boiler using an external pump. During this process, pressure increases P5  to P6 isentropically (The enthalpy and temperature of water also increase due to pump work).

Let P5 ,h5 be the pressure and enthalpy at stage 5 respectively.

Let P6 ,h6 be the pressure and enthalpy at stage 6 respectively.

The work done by pump is given by

Wp = h6 – h5 = Vf5 (P6 – P4)  × 100 J

Note :

All the values of pressure here are substituted in N/m² and all values of enthalpy are substituted in Joules.

Process: 6-1 (boiler)

Here the saturated water from the pump is heated by using a constant heat source (such as furnace). The input saturated water is heated till it reaches super-heated condition. Temperature and enthalpy of saturated water raise to a great extent, but its pressure remains constant. The change of phase from liquid to vapour occurs in boiler.

Let h6 be the enthalpy of saturated water entering the boiler.

Let h1 be the enthalpy of super-heated steam coming out of boiler.

Heat supplied is given by

QS = h − h6 J

h6 can be calculated by means of pump workdone formula.        

h6 = h5 + Wp J

All processes can be understood well if you refer the h-s diagram above.

Efficiency of Reheat Rankine Cycle:

As we know, efficiency is the ratio between output and input. Here, the output is workdone and input is heat energy.

Net workdone =  workdone in turbine (both H.P. turbine and L.P. turbine)  + workdone in pump .

Net heat transfer = heat supplied in boiler + heat rejected in condenser.

Efficiency of reheat Rankine cycle is given by:

\(\eta_{reheat} = \frac{\left ( h_{1}-h_{2} \right )+\left ( h_{3}-h_{4} \right )-W_{P}}{h_{1}-\left ( h_{5}+W_{p} \right )+\left ( h_{3}-h_{2} \right )}\)

It is 30 to 40% greater than simple Rankine cycle.

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I am a mechanical engineering student. I am interested in writing articles, designing cars, engines and bikes.

  • hardik

    why we reheat up to point 3 only not more then that?

    • Akshat Pal

      As we know that our turbine can take vapours and withstand a temp of T1 point. So, we usually heat upto the Temp at T1 (So that the high temp of vapours does not damage the blades of turbine).

  • Ankit

    Why the temperature of Reheat steam is greater than the Main steam?