Adding low-temperature economizer in flue duct of a boiler's tail section to use flue gas to heat condensed water is of great significance for reducing power plant exhaust gas temperature.After the lowtemperature economizer was installed

the extraction steam turns back to the steam turbine to continue expanding

which increases the power output of the steam turbine

decreases the turbine heat rate and the gross coal consumption rate.Moreover

the steam turbine exhaust grows

the condenser vacuum declines

the condensed water amount and flue gas flow resistance increases

the auxiliaries' power consumption increases and the unit heat economic efficiency declines.To solve this problem

by taking the TRL working condition of a supercritical 600 MW direct air-cooling units as an example

this paper carry out thermal efficiency calculation for the unit before and after the low-temperature economizer was installed

using the habitual thermal balance method and considering the operation performance of the steam turbine stages under varying load conditions caused by waste heat utilization.The results show that

after the low-temperature economizer was installed

the exhaust pressure increased by 1.62 kPa and the auxiliaries power consumption increased by 293.854 kW.However

the comprehensively calculated turbine heat rate decreased by25.711kJ/(kW·h)

and the gross coal consumption rate reduced by 0.960g/(kW·h)

indicating adding low-temperature economizer achieved good energy saving results.