# Performance Comparison of 500 MW Coal-Fired Thermal Power Plants with Final Feedwater Temperatures
## 3 Results and Discussion
### Final Feedwater Temperature and Feedwater System  
To raise the final feedwater temperature under the condition of the same TTD, which was the main design factor of the feedwater heater, the saturated steam pressure on the shell side of the No. 8 feedwater heater was increased. To increase the shell side saturated steam pressure, the pressure of the HPT extraction steam to the shell side of feedwater heater No. 8 should be increased. Therefore, to raise the final feedwater temperature, the HPT extraction steam pressure supplied to feedwater heater No. 8 should be increased. Figure 3 shows the relationship between the pressure of the HPT extraction steam and the final feedwater temperature. As shown in Figure 3, under the condition that TTD and the pressure drop ratio of the extraction steam pipe were constant, the final feedwater temperature and the HPT extraction steam pressure were proportional. In the definition of _TTD_, the final feedwater temperature was a function of the saturated steam temperature according to the pressure of the feedwater heater shell. As shown in Figure 4, the flow rate increment of feedwater supplied to the boiler and the final feedwater temperature were proportional in all cases. As the temperature of the main steam or the temperature of reheat steam was increased, the change rate of the feedwater flow rate increment according to the final feedwater temperature was decreased. Figure 5 shows the final feedwater temperature and the flow rate increment of the HPT extraction steam. In all cases, the Pearson product-moment correlation coefficient between the final feedwater temperature and the flow rate increment of HPT extraction steam was over +0.9996. As shown in Figure 5, the flow rate increment of the HPT extraction steam and the final feedwater temperature were proportional and strongly correlated. If the final feedwater temperature increased by 1 \({}^{\circ}\)C, the flow rate increment of the HPT extraction steam increased by 4.92 t/h in case 1, 4.82 t/h in case 2, 4.53 t/h in case 3, 4.45 t/h in case 4, and 4.21 t/h in case 5. Considering cases 1-4, in which the main steam temperature was identical, the change rate of the flow rate increment of the HPT extraction steam according to the final feedwater temperature decreased as the reheat temperature increased. Considering cases 2-5, in which the reheat temperature was identical, the change rate of the flow rate increment of the HPT extraction steam according to the final feedwater temperature decreased as the reheat temperature increased. As the temperature of the main steam or the temperature of reheat steam increased, the change rate of the HPT extraction steam flow rate increment according to the final feedwater temperature decreased. Figure 3: Final feedwater temperature versus HPT extraction steam pressure. The heat duty increments in feedwater heater No. 8 versus the final feedwater temperature are shown in Figure 6. The temperature of the feedwater supplied to the tube side of feedwater heater No. 8 was identical. As the final feedwater temperature and the feedwater flow rate increased, the heat duty of feedwater heater No. 8 increased. Similar to the change rate of the HPT extraction steam flow rate increment, the temperature of the main steam or the temperature of reheat steam increased, and the change rate of the heat duty increment of feedwater heater No. 8 according to the final feedwater temperature decreased. Figure 4: Feedwater flow increment versus final feedwater temperature. Figure 5: HPT extraction steam increment flow versus final feedwater temperature. The specific enthalpy increments in HPT extraction steam versus final feedwater temperature are shown in Figure 7. In all cases, the Pearson product-moment correlation coefficient between the final feedwater temperature and the specific enthalpy increment of HPT extraction steam was +1.0. As shown in Figure 7, the final feedwater temperature and the specific enthalpy increment of HPT extraction steam were proportional and had a positive linear relationship.