The temperature of the primary heat medium used in the central heating of cities in my country is generally 130℃/70℃, and the temperature of the secondary heat medium is 95℃/70℃ or 85℃/60℃. In this case, the flow rate in the primary heat medium channel of the plate heat exchanger is only about half of the flow rate in the secondary heat medium channel. For a symmetrical flow channel, the flow rate of the primary heat medium is about 50% of the flow rate of the secondary heat medium, and the convective heat transfer coefficient between the fluid and the plates in the primary heat medium flow channel is about 70% of that in the secondary heat medium flow channel. about. Therefore, when the primary and secondary side media flow is large, asymmetric plate heat exchangers should be used. The convective heat transfer coefficient of the cold fluid and the hot fluid at the two sides of the plate determines the surface heat transfer coefficient of the plate heat exchanger, and is smaller than the smaller one of the cold and hot convective heat transfer coefficients.In order to make full use of the heat transfer performance of the plate heat exchanger, the convective heat transfer coefficient between the cold and hot fluid and the two sides of the plate should be appropriately increased. The selection and calculation of the plate heat exchanger is very complicated. To ensure the reasonable completion of the heat exchanger selection calculation, the influence of the flow velocity between the heat exchanger plates and the matching of the flow velocity between the cold and heat medium plates must be considered comprehensively. The flow rate, temperature difference, medium and physical parameters of the process must be within a reasonable range to ensure the full play of the superiority of the plate heat exchanger, so that the equipment can operate economically and reliably.