Comparison between Reheat & Regenerative Rankine Cycle

This article gives you the basic comparison between reheat Rankine cycle and regenerative Rankine cycle. This comparison will help you to find out distinguishable features of reheat Rankine cycle and regenerative Rankine cycle. Prerequisite before reading this article is the basic idea of Rankine cycle, PV and TS diagram of reheat Rankine cycle and regenerative Rankine cycle. The differences of reheat cycle compared to regenerative cycle are discussed as follows.

All the steam turbine thermal power plants what we are seeing today is centered around the qualitative thermodynamics of Rankine cycle, which is the vapor standard cycle for the same. It consists of two reversible adiabatic process two isobaric processes and the working fluid undergoes a phase change. The topic of our discussion, that is regeneration and reheating is a method of increasing efficiency of Rankine cycle. Note that reheat need not give higher efficiency and it is primarily attached to the power plant in order to improve the dryness fraction of the steam obtainable at turbine outlet. 

Reheat Rankine Cycle

• Basic arrangement behind reheating is that, after initial expansion of steam in the turbine, the same steam is further expanded in another turbine with the help of a reheater.

• The main purpose of reheating in Rankine cycle is to increase the dryness fraction of steam after the expansion in the turbine which indirectly allows more boiler pressure to be used to increase the efficiency.

• The efficiency of Rankine cycle may or may not be improved by reheating. It depends on whether reheating has increased the mean temperature of heat addition to the cycle. If mean temperature of heat addition increased due to reheating, then it will improve the thermal efficiency.

• By deploying a reheater, the specific fuel consumption decreases because the mass of steam required will decrease due to reheating.

• The size of the boiler decreases because of the above reason.

• The specific volume of steam reduces or there is a reduction in steam volume and heat to the condenser due to the increase in boiler pressure which can be attained with the help of reheating.

• A small condenser is only required because of the above reasons.

• Dryness fraction increases.

• Pumping power required is less compared to regenerative cycle as the pump has to carry comparatively less amount of feed water.

• There is more gain of work in turbine compared to normal Rankine cycle as it is obvious from the TS and HS diagram.

• Only one stage reheating is generally preferred. More than two reheats result in more cost and complication.

• The minimum capacity of the plant to utilize reheat economically is 10 MW.

• Life of the plant increases due to reheating as it reduces the overall temperature gradient in the power plant which induces thermal stresses.

• There are chances for overheating of blades at light loads.

Regenerative Rankine Cycle

• Ideal regeneration utilizes the concept of Non-extraction feed heating where the efficiency of the same is equal to the Carnot efficiency. However, in actual regeneration, a part of a steam is bleed at certain stages of expansion and its efficiency is less than Carnot efficiency.

• The main purpose of regenerative Rankine cycle is to improve the efficiency of Rankine cycle by increasing the mean temperature of heat addition.

• Specific steam consumption increases in the boiler to compensate bleeding from the turbine.

• The size of the boiler is increased compared to the reheat Rankine cycle because of the above reason. 

• Steam flow to the condenser decreases due to bleeding, so here also a small condenser is only required. 

• Due to regeneration, there is no change in dryness fraction of steam.

• Pumping power in the regenerative cycle is high compared to Rankine cycle as it deploys more feed pumps to circulate water compared to reheat Rankine cycle.

• Turbine output drops or loss of work in turbine will happen due to bleeding.

• Normally 5 to 6 feed heaters are used in the regenerative plant.

• Infinite feed heating cycle tends to achieve Carnot efficiency but it is not practically possible. 

• A number of feed heaters will increase the optimum feed temperature and finally results in evaporation of the feed lines.