The parallel bypass flue system for boiler air preheater is an important method for advanced utilization of boiler tail flue gas heat. The optimal capacity of parallel bypass flue is selected for optimal thermal efficiency of units. The formula of capacity and heat load of parallel bypass flue for boiler air preheater is derived based on the law of heat conservation. Combined with an ultra-supercritical 1 000 MW coal-fired unit

the capacity and heat load of bypass flue are calculated so as to select the optimal capacity of bypass flue. The results show that

the optimal capacity selection of bypass flue is closely related to the flue gas temperature at the outlet of the bypass flue

the average temperature difference between the flue gas and the cold air at the cold end of the air preheater

and the average air temperature at the outlet of the air heater. Under the same conditions

according to the calculation results of the average temperature difference at the cold end of the air preheater(40 ℃ and 50 ℃)

when the average air temperature at the outlet of the air heater is 50 ℃

the maximum heat load of the bypass flue is obtained

which is 51.601 MW and 43.261 MW

respectively. At this time

the optimal capacity of the bypass flue is 0.174 and 0.145

respectively. It can provide guidance for the design of parallel bypass flue system for boiler air preheater in thermal power plants.