One of specifications of steam boiler should be stated is feedwater temperature. Every steam boiler type has different feedwater temperature. For process boiler whose low capacity and does not need heating for its treatment, the feedwater temperature required is low approximately 85 C. But for power boiler which has high design pressure, the feedwater temperature required up to 270 C.
There are some differences of temperature feedwater will be supplied to steam boiler and it has some levels temperature. The differences depend on system and type of boiler such as for Heat Recovery Steam Generator (HRSG) which does not has heating cycling process for condensate, the feedwater temperature flow through HRSG about 30-40 C. For process boiler, the condensate output from condenser has temperature 85 C and the feedwater should be injected with sodium sulfite before distributed into steam boiler to prevent possibility pitting corrosion occur in the metal of pressure parts. The feedwater can be distributed to economizer first if engineer / designer want to increase efficiency boiler.
For higher temperature needed approximately 105 C, the feedwater is heated and the oxygen is removed in one equipment which named as deaerator. Deaerator can also help to increase feedwater temperature until 130-140 C. For the higher temperature again, a power plant system usually use High Pressure Heater (HP Heater) located after deaerator to increase temperature up to 160 C - 270 C.