The heat exchanger has different thermal characteristics due to different gas flow modes. Under the condition that the temperature of exhaust gas entering the heat exchanger is the same, the air can be preheated to a higher temperature by counter current than by forward current. Under the same other conditions, the heat transfer of countercurrent heat exchanger is larger than that of downstream heat exchanger, and its structure is more compact than that of downstream heat exchanger. From the working conditions of the heat exchanger wall, the forward flow type is more favorable. Because the temperature of the heat exchanger wall is almost the same for both ends of the parallel flow heat exchanger, its maximum temperature is lower than that of the counter flow heat exchanger, the heat exchanger is not easy to be deformed or damaged, and the material requirements are not high. On the counter flow heat exchanger, the wall temperature at the high temperature end is close to the exhaust gas population temperature, and the temperature difference between the two ends is large. This puts forward higher requirements for the material of the wall, and due to the large temperature difference at both ends, it is easy to deform and damage. In practical application, most heat exchangers adopt countercurrent mode.