Regenerative Reheat Rankine Cycle

Regenerative Rankine Cycle

The reason why the regenerative Rankine Cycle has better efficiency than non-regenerative Rankine cycle can be explained by the following three ways.

 

1) According to the second law of thermodynamics, reversible process has the best efficiency.  In order to approach to reversible process, temperature difference for heat transfer should be minimized as much as possible, and all processes should be isentropic process or isothermal process.

In Simple Rankine cycle which do not use feed water heating by turbine bleed steam, heat transfer in boiler occurs between high temperature of boiler flue gas and low temperature of condenser condensate of approximately 30 oC that introduced to the boiler directly from condenser.   This means that irreversible heat transfer having big temperature difference occurs at boiler and cycle efficiency becomes poor according to the second law of thermodynamics explained above.

In order to reduce this high irreversibility, used is regenerative Rankine cycle, in which the feed water temperature introduced into boiler becomes higher by heating using turbine bleed steam.

 

2) Heat rate of heat engine cycle is expressed by the following equation.

Heat Rate = Heat Supplied / (Heat Supplied - Heat Rejected)

If Heat Supplied from boiler is 4 and Heat Rejected at condenser is 3 in simple Rankine cycle, the heat rate of the cycle is 4 as below.

Heat Rate = 4 / (4 - 3) = 4

Under the condition that turbine exhaust flow to condenser is same, if a part of turbine inlet steam is extracted and used for feed water heating, the extracted steam returns to boiler in the form of feed water after producing power at turbine, and then boiler should supply more heat to turbine cycle by heating the feed water formed by extracted steam.   

This means that Heat Rejected is same but Heat Suppled increases.  If Heat Suppled increases to 5 from 4, Heat Rate decreases to 2.5 from 4 as below, and cycle efficiency is improved.

Heat Rate = 5 / (5 - 3) = 2.5

 

3) The power produced by extracted steam from turbine for feed water heating is generated by 100% efficiency.

That is because, after generating power in turbine, the extracted steam go out of turbine cycle without heat rejection.   In case of condensing steam, a part of heat is rejected in condenser and the power generated by condensing steam is less efficient as much as heat rejected in condenser.

Therefore, the greater the power generated by the extracted steam, the better cycle efficiency.

 

Optimum Final Feed Water Temperature exists.

Regenerative Rankine cycle efficiency with finite number of feed water heaters does not always increase as the final feed water temperature increases.

In the explanation 2) above, turbine cycle efficiency increases as much as Heat Supplied increases by heating the turbine extracted steam flow in boiler.   However, as feed water temperature to boiler increases, Heat Supplied per unit mass flow in boiler decreases.   By this phenomenon, at a feed water temperature, the decreasing rate of Heat Supplied per unit mass flow by higher feed water temperature to boiler, exceeds the increasing rate of the turbine extracted steam flow for feed water heating.    From this temperature, heat rate of turbine cycle starts to decrease.

ES_Rankine has "Optimum Enthalpy Rise Run Function" that finds the optimum final feed water temperature.

 

Reheat Rankine Cycle

Efficiency of thermodynamic cycle increases as much as heat supply temperature increases.    Reheat Ranking cycle makes the heat supply temperature increase to have better efficiency.

Besides, reheat cycle reduces moisture content of turbine last stage blade that results in reduction of moisture loss and improvement of turbine efficiency.   If same moisture content is maintained in the last stage blade, turbine inlet steam pressure of reheat Rankine cycle can be increased so that turbine cycle efficiency can be improved.

If reheat Rankine cycle is used, cycle efficiency is improved by approximately 4 to 6%, while turbine inlet steam flow increase by approximately 15%.   The reason why the cycle efficiency is not improved as much as turbine inlet steam flow increases, is due to more heat to be supplied for reheat process.

 

Reference Document : Steam Turbine and Their Cycles, written by J. Kenneth Salisbury, General Electric Company, Schenectady, New York, USA, Original Edition 1950


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