DESIGN

FW Heaters Tab

This tab is for selection and input of the design conditions for Design Heat Balance.

Among inputs and selections of this tab, automatically reflected to "Start" tab are "FW Heater Quantity" combo box, "BFP Heater Nr." text box, "Heater Nr. from Cold Reheat" text box and "Heater Nr. from IP Tbn. Exhaust" text box.

On the other hand, the "Type", "T FW Out", "TTD", "DCA", "dP Ext" and "P Tbn Ext" input values of this tab are automatically reflected to "Graphic" tab.

FW Heater Quantity combo box

To select quantity of feed water heaters.   The maximum quantity is 10.

 

"P Tbn Ext" Input check box

To select whether design feed water outlet temperatures of heaters are defined by feed water outlet temperature itself or turbine bleed pressure.

If this check box is checked, User has to define the feed water outlet temperatures by turbine bleed pressure.

 

BFP Heater Nr. text box

To input the feed water heater number, under which BFP(Boiler Feed Water Pump) is located.   The BFP heater should be Contact type.

Only one Contact type feed water heater should exist in a heat balance diagram for correct calculation.

Calculation of more than one Contact type feed water heater is possible, but the results may not be correct.

 

Heater Nr. from Cold Reheat text box

To input the feed water heater number, of which heating steam is supplied from cold reheat line.   If no such heater exists, please input "0".

 

Heater Nr. from IP Tbn. Exhaust text box

To input the feed water heater number, of which heating steam is supplied from IP turbine exhaust, i.e. LP crossover line.

ES_Rankine assumes that IP and LP turbine exist independently when condenser flow is more than 2 flows.   If condenser flow is single flow, there is no LP turbine and no LP crossover line.

 If no such heater exists, please input "0".

 

Nr.  text box

To show feed water heater numbers.   The lowest pressure heater is Nr. 1 and the higher pressure the higher number.

 

Type combo box

To select type of feed water heaters.    Three types, Contact, Flash and Pumped, are provided for selection.

 

T FW Out text box

To input feed water outlet temperature of heaters.

If "P Tbn Ext" check box is not checked, these text boxes are activated and ready for input.   If the case, "P Tbn Ext" values are automatically calculated as below.

- Calculate the saturation temperature inside of heater shell by add TTD value to T FW Out value.

- Calculate the saturation pressure corresponding to the saturation temperature inside of heater shell.

- Calculate "P Tbn Ext." value by dividing the saturation pressure by (1 - dP Ext % / 100).

 

TTD text box

To input TTD(Terminal Temperature Difference) values of feed water heaters.

TTD is the saturation pressure inside of heater shell minus the feed water outlet temperature of the heater.

According to HEI(Heat Exchange Institute) Standard, USA, TTD of Close Feed Water Heater is recommended not to be lower than 2 oF if the heater has no desuperheating zone.   Flash and Pumped type heaters are the Closed Feed Water Heater.

TTD of Contact type feed water heater is said as "0".    When User selects Contact type heaters, TTD value of the heater is input as "0" automatically.

 

DCA text box

To input DCA(Drain Cooling Approach) values of feed water heaters.

DCA is the drain temperature out of heater minus the feed water temperature entering heater.

According to HEI(Heat Exchange Institute) Standard, USA, DCA of Close Feed Water Heater equipped with Internal Drain Subcooler is recommended not to be lower than 10 oF.

If the lower DCA is required, HEI recommends to use Separate(External) Drain Subcooler instead of Internal Drain Subcooler.

If no drain cooler is installed, please input "0" or a very large value that results in heater drain temperature greater than heater shell-side saturation temperature.

 

 dP Ext text box

To input the pressure drop % value of feed water heater bleed line.

The pressure drop % is from turbine bleed stage to heater shell, not bleed pipe pressure drop only.  % is pressure drop over turbine bleed stage absolute pressure.

Pressure drop of feed water heater bleed line consists of two parts.   One is from turbine bleed stage to turbine bleed flange outlet, and the other is from turbine bleed flange outlet to heater shell.   The first one is normally given by turbine manufacturer and the second is decided by architectural engineering engineer.   The value of first one is in the range of 2 to 3 percent.   It varies depending on turbine manufacturers.

If engineering engineer selects 4% as the second pressure drop and turbine manufacturer provides the first one as 2%, then "dP Ext" value shall be 6%(= 4% + 2%).

 

P Tbn Ext text box

To input turbine bleed stage pressure.   Please note that this pressure is turbine internal stage pressure, not turbine bleed flange outlet pressure.

If "P Tbn Ext" Input check box is checked, these text boxes are activated and ready for input.   If the case, "T FW Out" text boxes are inactivated and automatically calculated according to the reverse procedure described above in "T FW Out" text box explanation.

 

Flow text box

To show the turbine bleed flow rate for each heater calculated.   This is not for User input.

 

Run for User Input option button

To select the run mode that User have to input feed water heater design conditions.   This run mode is default run mode.

 

Run for xx % Enthalpy Rise of Maximum (Same Enthalpy Rise in Each Heater) option button

To select the run mode that calculates a heat balance having xx % of enthalpy rise of the maximum enthalpy rise applicable.

The calculation is done based on the following conditions.

- Enthalpy rise in each feed water heater is same.

- Turbine stage minimum pressure ratio input by User is ignored.

 

The maximum enthalpy rise applicable depends on the turbine inlet steam pressure.   

Heat transfer in feed water heater is better in saturation steam status.   In this concern, feed water heating above critical temperature is not recommended.  Moreover, even in advanced ultra-supercritical turbine cycle, the least heat rate is achieved at the final feed water temperature under critical temperature.

ES_Rankine selects lower of the two below as the turbine bleed pressure for maximum enthalpy rise in feed water heating.

- (HP) Turbine inlet steam pressure divided by turbine stage minimum pressure ratio

- Pressure that results in 370 oC saturation temperature at final feed water heater shell

 

Run for Finding Optimum Enthalpy Rise option button

To select the run mode that calculates a range of "Run for xx % Enthalpy Rise of Maximum" in order to find out the optimum(i.e. the least heat rate) enthalpy rise.  

The range should be input by User in the text boxes of the Optimum Enthalpy Rise Run frame.   User should define the range by Start %, End % and Step %.   After range calculation, ES_Rankine finds the least heat rate case and shows it as the last run.   Results of range run are shown as chart and text output.

In regenerative rankine cycle with finite feed water heater quantity, cycle thermal efficiency does not always increase as final feed water temperature increases.   It has peak point at a feed water temperature, and after that point cycle thermal efficiency starts to decrease.

On the other hand, if they have same final feed water temperature, the best cycle thermal efficiency can be achieved when the enthalpy rise in each feed water heater is identical.

This run mode is useful when engineers performs cycle studies.

The chart below is the Chart output of this run for 850 MW advanced ultra-supercritical turbine cycle from 1% to 100% for 1% step enthalpy rise.

The maximum quantity of run is 100.   If the run quantity is over 100, then a warning message pops up for change of inputs.   If User wants detailed search, then run it section by section.

For example, run for 1% to 100% for 1% step first, then run for 86% to 88% for 0.1% step if the least heat rate occurs at 87% enthalpy rise in the first run.

For information, abrupt heat rate drops in point (A), (B) and (C) occurs because a turbine bleeding point for a heater changed from IP turbine inlet section to cold reheat line, by which turbine bleed flow increases abruptly due to lower steam temperature of cold heat line than the IP turbine inlet section.   This gives an idea to bleed from cold reheat line as much as possible if bleed pressure for feed water heating is near cold reheat line pressure.

 

 

Reheating AEEP Enthalpy Level from H1 text box

Reheating AEEP Enthalpy Level is calculated by the following equation.

Reheat AEEP Enthalpy Level % = (H1 - HPTbn_Exh_AEEP) / (H1 - RhtTbn_Exh_AEEP) x 100

 wherein;

 

- H1

: HP turbine inlet steam enthalpy

- HPTbn_Exh_AEEP

: AEEP (Available Energy End Point) at HP turbine exhaust

- RhtTbn_Exh_AEEP

: AEEP at reheat condensing turbine exhaust

Reheating AEEP enthalpy level from H1 is an useful method to judge adequacy of reheat pressure in view of cycle thermal efficiency.

Generally the optimum, i.e. the maximum, reheat cycle thermal efficiency occurs at around 30% reheating AEEP enthalpy level.

 


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