Double-effect heat exchanger for fresh air system

The double-effect heat exchanger for the fresh air system comprises a heat exchange cabinet, a plurality of heat exchange pipes are installed in the heat exchange cabinet in parallel, the two ends of each heat exchange pipe are connected to an air inlet pipe A and a collecting pipe through branch pipes respectively, and the collecting pipe is connected to a heat exchange channel A. An air inlet pipe B is arranged at the end, away from the air inlet pipe A, of the heat exchange cabinet. An exhaust pipe is arranged on the side close to the air inlet pipe A and connected to a heat exchange channel B. The heat exchange channel A and the heat exchange channel B are arranged in parallel and provided with coincident sections, and a plurality of semiconductor heat exchange pieces are distributed between the coincident sections. By means of the two-step heat exchange mode, natural heat exchange is achieved, the thermoelectric refrigeration piece is further introduced, refrigeration and heating can be switched, the hea