For the purpose of exploring the technology to improve the power supply efficiency of in-service subcritical units

the feasibility of enhancing the steam temperature for in-service subcritical units was studied

by taking the No.4 unit in Huaneng Yueyang Power Plant as the demonstration. The method of check calculation and determination for the steam temperature improvement scheme was studied

the thermal efficiency and key operation parameters of the unit were tested before and after the steam temperature was increased. With detailed analysis and verification

the main steam and reheat steam temperatures were respectively improved by 5 ℃ without large scale retrofit of the equipments

and of the turbine heat rate and coal consumption decreased by 21.2~23.5 k J/(k W·h) and 0.84~0.94 g/(k W·h)

respectively. The research shows that it is possible to improve the operation temperatures of in-service units to enhance the unit efficiency. The safety of the heating surface in the boiler should be emphasized. A technology to improve the efficiency of an in-service unit has been explored and can be applied in other units.