Based on the transformation that the driven method of the induced draft fan(IDF) for a subcritical 600 MW unit was changed from motor driven to steam driven

the thermal economy of five schemes for steam-driven IDF with different steam sources were analyzed.The results showed that

using the regenerative steam to drive the IDF would reduce the absolute electric efficiency

but the auxiliary power consumption rate decreased greater

so the coal consumption rate of the power supply for the whole plant was reduced.If not considering the piping efficiency's variation

taking scheme Ⅲ as an example

the drop amount of the standard coal consumption was increased from 1.556 g/(kW·h) to 2.026 g/(kW·h)

namely the economic benefit of this modification(motor-driven to steam-driven) was over estimated.For scheme Ⅲ with considering the piping efficiency's variation

the standard coal consumption rate for the whole plant was reduced by about 1.556 g/(kW·h).Assuming that the unit runs for 7 000 h per year

this scheme can totally save standard coal of 6 535.2 t.Scheme Ⅲ is superior to other four schemes.