The invention discloses a composite
phase change heat exchanger with a medium-pressure and low-pressure
evaporator, which comprises the medium-pressure and low-pressure
evaporator and a steam header which are communicated by an ascending
pipe and a descending
pipe, a water feeding
pipe and a steam outputting pipe are arranged on the steam header, an upper section of the composite
phase change heat exchanger is positioned in a wind channel of an original
air preheater, and a lower section of the composite
phase change heat exchanger is positioned in a back
flue channel of the
air preheater; an
automatic control device of the composite phase change heat exchanger adjusts and controls the heat exchange volume of the
evaporator and the wall temperature of the lower section of the composite phase change heat exchanger. The composite phase change heat exchanger not only directly produces low-pressure or medium-pressure steam by using the recycled heat through a steam generating device and provides heat usage as a
heat supply, but also controls the
thermal equilibrium among the heat exchange volumes of the
flue gas, the medium-pressure and low-pressure evaporator, a
coal economizer and the wall of the
air preheater by adjusting the steam flow of the steam outputting pipe of the steam header through the
automatic control device, and consequently leads the
flue gas temperature at the outlet to be higher than a
flue gas acid
dew point on the lower section wall surface of the composite phase change heat exchanger, avoids
moisture condensation and achieves the purpose of utilizing
waste heat of the
flue gas to the utmost degree.