RANKINE CYCLE WITH REHEAT AND REGENERATION

We were discussing the concept of reheat cycle in steam power plant and also we have seen the concept of regeneration in Rankine cycle in our previous posts, where we have also tried to understand the various methods to increase the thermal efficiency of the steam power cycle or Rankine cycle.

If we analyse the effect of reheat cycle over the thermal efficiency of the Rankine cycle, we will conclude that thermal efficiency of the Rankine cycle will be improved by adopting the concept of reheat cycle but we must note that improvement in thermal efficiency of the cycle will be very small.

Concept of regenerative Rankine cycle with feedwater heater will provide better positive result in respect of increase in thermal efficiency of the Rankine cycle or in simple way we can say that thermal efficiency of the steam power cycle will be improved quite good by using the concept of regenerative Rankine cycle with feedwater heater as compared with reheat cycle.

These days, modern steam power units are operated with the Reheat-regenerative cycle and we will see here the basic concept of Reheat-regenerative cycle in this post.

Let us see here the basic block diagram and TS diagram also for steam power cycle with three stages of feedwater heating.

As we can see in block diagram, high pressure and high temperature steam enters to the high pressure turbine at state 1 and as we are also considering here the concept of regeneration hence we must note it here that all steam will not be expanded through the high pressure turbine up to pressure corresponding to state 3 but also certain quantity of steam will be extracted from the high pressure turbine and its state is displayed by state 2 in figure.

T-S diagram

Expanded steam from high pressure turbine will enter in to the reheater at state 3 and reheater, as shown in figure, will use the heat energy from boiler for reheating the steam coming from high pressure turbine. In simple way we can say that steam will be reheated in reheater coil tube by receiving heat energy from boiler. Steam will be heated in reheater up to a temperature of T₄ as displayed in figure by process 3-4.

Steam coming from reheater will enter to the low pressure turbine at state 4 and similarly as studied earlier certain quantity of steam will be extracted here from low pressure turbine before the complete expansion of steam up to the condenser pressure.

So as shown in figure, steam will be extracted from low pressure turbine at state 5 and state 6 and rest quantity of steam will be expanded up to the condenser pressure i.e. up to state 7.

Steam will enter to the condenser at state 7 and will go undergo condensation process during the process 7 to 8. Now working fluid will be pumped with the help of feed pump by process 8-9.

Now extracted steam at state 6 and working fluid (pumped by the feed pump WP1) at state 9 will exchange heat energy with each other in to the feedwater heater 3.

Therefore, working fluid will be heated in to the feedwater heater and we have displayed this process of heat energy addition to the working fluid in feedwater heater 9-10.

Similarly working fluid will secure heat energy during the process 11-12 and 13-14 in feewater heater 2 and feedwater 1 respectively.

Process 15-1 will indicate here the heat energy addition to the working fluid in boiler and we will consider this heat energy addition as input energy addition. Working fluid will also secure heat energy in reheater during the process 3-4 and hence heat energy added during the process 3-4 will also be taken as input heat energy.

However we have recently studied that heat energy will also be added to the working fluid during the process of feedwater heater but that heat energy addition will be internal as that heat energy addition will be done by the concept of regeneration process.

Work done by the turbine