Calculation Basis of Part Load

 

FW Heater TTD and DCA

ES_Rankine assumes that FW Heater TTD and DCA at part load are same with design load.

In actual operation, those at part load are different from Design, because the heating surface of FW heaters is same with Design while heat load is different.

However, the effect of TTD and DCA to cycle performance is too small to take time for TTD and DCA calculation by iteration.   So, ES_Rankine uses the same TTD and DCA figures with Design for part load.

 

Turbine Steam Pressure

Since turbine is analyzed as a nozzle, turbine steam pressure at part load is calculated by the equation below.

In case of turbine inlet pressure, the mass flow rate to be used in the equation below is turbine inlet flow, while that of turbine bleed pressure is the flow rate flowing to the following stage group.   

Please note that the pressure calculation basis for calculation of turbine bleed pressure is "Flow to Following (Stage) Group".   That is, inlet flow to the stage shell minus bleeding flow.

 

P_Tbn_PLoad = P_Tbn_Design x (MFlow_Tbn_PLoad / MFlow_Tbn_Design)^2 x (SVol_Tbn_PLoad / SVol_Tbn_Design)

wherein :

 

- P_Tbn_PLoad

: Turbine absolute pressure at Part Load

- P_Tbn_Design

: Turbine absolute pressure at Design Load

- MFlow_Tbn_PLoad

: Turbine mass flow rate at Part Load

 (Turbine inlet flow in case of turbine inlet pressure, flow to following group in case of turbine bleed pressure)

- MFlow_Tbn_Design

: Turbine mass flow rate at Design Load

 (Turbine inlet flow in case of turbine inlet pressure, flow to following group in case of turbine bleed pressure)

- SVol_Tbn_PLoad

: Steam specific volume at Part Load

 (Turbine inlet specific volume in case of turbine inlet pressure, turbine shell specific volume in case of turbine bleed pressure)

- SVol_Tbn_Design

: Steam specific volume at Design Load

 (Turbine inlet specific volume in case of turbine inlet pressure, turbine shell specific volume in case of turbine bleed pressure)

 

Reheater and Pipe Pressure Drop

Reheater and pipe pressure drop at part load is calculated by Darcy equation below.

dP_Pipe_PLoad = dP_Pipe_Design x (MFlow_Pipe_PLoad / MFlow_Pipe_Design)^2 x (SVol_Pipe_PLoad / SVol_Pipe_Design)

wherein :

 

- dP_Pipe_PLoad

: Reheater or pipe pressure drop at Part Load

- dP_Pipe_Design

: Reheater or pipe pressure drop at Design Load

- MFlow_Pipe_PLoad

: Mass flow rate of reheater or pipe at Part Load

- MFlow_Pipe_Design

: Mass flow rate of reheater or pipe at Design Load

- SVol_Pipe_PLoad

: Inlet steam specific volume of reheater or pipe at Part Load

- SVol_Pipe_Design

: Inlet steam specific volume of reheater or pipe at Design Load

 

BFP(Boiler Feed Water Pump) Discharge Pressure and Pump Efficiency

Variable Speed BFP

BFP discharge pressure of variable speed BFP at Part Load is calculated according to the following equation.

P_BFPDisPR_PLoad = P_HPTbn_PLoad x Pct_BFPDisPR / 100

wherein :

 

- P_BFPDisPR_PLoad

: BFP discharge pressure at Part Load

- P_HPTbn_PLoad

: (HP) turbine inlet steam pressure at Part Load

- Pct_BFPDisPR

: BFP Discharge Press % input in "Pump" tab of DESIGN window

 

Constant Speed BFP

BFP discharge pressure of constant speed BFP at Part Load is calculated according to the following equation.

P_BFPDisPR_PLoad = P_HPTbn_Design x Pct_BFPDisPR / 100 x PLoad_H-Q_Pct_Builtin / 100

wherein :

 

- P_BFPDisPR_PLoad

: BFP discharge pressure at Part Load

- P_HPTbn_Design

: (HP) turbine inlet steam pressure at Part Load

- Pct_BFPDisPR

: BFP Discharge Press % input in "Pump" tab of DESIGN window

- PLoad_H-Q_Pct_Builtin

: Head rise % read from a built-in, standard H-Q curve

 

BFP Efficiency

For variable speed BFP, pump efficiency at design load is used in part load as it is.   In case of constant speed BFP, pump efficiency at part load is calculated based on design efficiency, but considering efficiency variation at part load according to a built-in, standard pump efficiency curve.

 


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