Run for Finding Optimum Enthalpy Rise

This run of ES_Rankine is for finding optimum final feed water temperature that results in the least heat rate.

If infinite number of feed water heaters is installed, turbine cycle heat rate decreases as much as final feed water temperature increases, even though heat rate improvement rate decreases as final feed water temperature increases.

However, turbine cycle with finite number of feed water heaters must have an optimum final feed water temperature that results in the least heat rate.   Since installation of infinite number of feed water heaters is impossible, we may have a question about what is optimum final feed water temperature when number of feed water heater has been fixed.   This run of ES_Rankine is to answer to the question.

If final feed water temperature is fixed, the least heat rate can be achieved when enthalpy rise in each heater is equal.   For this reason, final feed water enthalpy is used in this kind of analysis instead of temperature, and equal enthalpy rise in each heater is assumed for analysis purpose.   ES_Rankine uses same terms and same assumption for this run.

 

100% Enthlapy Rise

The maximum turbine bleed pressure for 100% enthalpy rise is selected as the lower of the two below.

- (HP) Turbine Inlet Steam Pressure divided by Minimum Pressure Ratio of (Turbine) Stage input in "Turbine" tab of DESIGN window

- Turbine bleed pressure that results in 370 oC saturation temperature in shell side of final feed water heater

Even in super-critical steam pressure turbine cycle, turbine bleed pressure for 100% enthalpy rise is selected not to exceed critical steam pressure.

 

Enthalpy Ris Max. and Min. %

Max. is 1% and Min. is 100%.

 

Rise Step %

Rise step should be greater than 0%.   

If User inputs 80% for Start, 90% for End and 5% for Step, run is performed for 3 cases of 80%, 85% and 90%, and finds the enthalpy rise % having the least heat rate.

Number of run should not exceed 100.   If number of run exceeds 100, split the case into several to have 100 cases or less for each run.

 

Equal Enthalpy Rise in Each Feed Water Heater

For this run, ES_Rankine assumes equal enthalpy rise in each feed water heater, which results in the best cycle efficiency.

If turbine bleed pressure so selected is in between HP turbine exhaust pressure and Reheat turbine inlet pressure, i.e. cold reheat line bleed, the bleed pressure is changed to HP turbine exhaust pressure automatically.

Same is true for turbine bleed pressure in between IP turbine exhaust pressure and LP turbine inlet pressure, i.e. LP crossover line bleed, in which the bleed pressure is changed to IP turbine exhaust pressure automatically.

 

Minimum Pressure Ratio of (Turbine) Stage

In this run, fulfillment of the Minimum Pressure Ratio of Stage input of "Turbine" tab of DESIGN window is NOT checked.   

Therefore, after this run, it is recommended to run again for the least heat rate case with "Run for User Input" option button selected in order to check the fulfillment of the Minimum Pressure Ratio of Stage.   Please note that the least heat rate case run is shown as the last run.

 

Example of 1000MW Class Cycle Analysis

Execute Smart Run by clicking "Smart Run & Add to List" button of "Start" tab in DESIGN window, after selection of "1000 MW Class Reheat Ultra-Supercritical" item in ENGSOFT List combo box.

 

Select "FW Heater" tab and then select "Run for Optimum Enthalpy Run".   Input "Start", "End" and "Step" values and click "Run" button for execution.   Execution takes time because this run has 100 cases.

After run execution, click "Chart..." and "Text Output..." buttons to see results.

 

The starting heat balance generated by Smart Run has the following design conditions, which are current industry practice.

-

Turbine Inlet Steam

: 246 bara, 600 oC (3500 psig, 1112 oF)

-

Reheat Steam

: 46 bara, 600 oC (650 psig, 1112 oF)

-

FW Heaters

: 8 (HPT Bleed 1, Cold Reheat Bleed : 1, IPT Bleed 1, IPT Exhaust Bleed : 1, LPT Bleed : 4)

-

Final FW Temperature

: 295 oC

-

BFP Driver

: Steam Turbine (IPT Exhaust Steam Driven)

-

Cycle Gross Heat Rate

: 7414.2 kJ/kWh

 

The heat rate step-down points of (A), (B) and (C) shown in the Chart output below, are due to change of bleeding points from reheat turbine inlet(hot steam) to cold reheat line(cold steam) that results in more bleeding steam flow and better cycle efficiency.   

According to the results, the best cycle efficiency can be obtained at 324.7 oC final feed water temperature, while current industry practice uses 295 oC final feed water temperature.   One thing to note is that cycle efficiency difference between the two is 0.13%, not so big.   When considering cycle efficiency gain between the two, current industry practice of 295 oC is acceptable.

 

< Chart Output >

< Text Output >

 

 


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