System and method for fluid medium preheating

A method for operating a feedwater preheating system (10) for a heat recovery steam generator (HRSG) comprising a water separation unit (22), an evaporator (16), and a feedwater preheater (FWPH, 14) attached to a FWPH inlet line (18) and a FWPH outlet line (20) is provided. The method comprises feeding water from a condenser to the FWPH (14) along the FWPH inlet line(18), heating the water in the FWPH (14), feeding the water from the FWPH (14) to the evaporator (16) along an FWPH outlet line (20), boiling water in the evaporator (16) and feeding a resulting steam and water mixture to the water separation unit (22), monitoring an LP approach ([delta]T) of the water separation unit (22), setting a set value of the LP approach, when the LP approach is below the set value, directing some water to bypass the FWPH (14) from the FWPH inlet line (18) to the FWPH outlet line (20). Further features of the invention include methods of monitoring the LP approach and apparatus to carry out the method. 本发明提供操作用于余热蒸汽发生器(HRS