**Basic heat transfer of plate heat exchangers**

The heat exchange between the cold and hot media in the plate heat exchanger is mostly realized by convection heat transfer. Since the plate heat exchanger is an ideal liquid-liquid exchange device, there is no radiation heat exchange. Convective heat transfer is the basic way of heat transfer.For heat transfer in working conditions, the liquid always comes into contact with the wall. Therefore, the transfer of heat is caused by the constant movement of the fluid (convection); the second is the heat conduction caused by the fluid and the wall surface and the temperature difference surface existing throughout the fluid. This simultaneous convection and heat transfer process is called convective heat transfer.

Because the flow of the two media is different, the convective heat transfer will be very different. Convective heat transfer can be divided into two categories, one is natural convection heat transfer, and the flow heat transfer caused by the difference in density caused by the difference in temperature between the two media;Another major category is forced convection heat transfer, which is the heat exchange generated by the fluid flowing under the external force of a pump or a fan, such as the heat exchange generated by the heat medium flowing under the pump and flowing inside the pipe.

In general, when the flow is forced, the flow rate of the liquid is higher than that at the free flow, so the convection heat exchange system in the forced flow is higher. For example, the free-flow heat transfer coefficient of air is about 5~25W/(m2.°C), and its forced flow heat transfer is 10~100W (m2.°C).

There are many factors affecting the heat transfer of the medium, such as the physical properties of the medium (specific heat, thermal conductivity, density, viscosity, etc.), the shape and size of the plate heat exchanger, and the flow pattern of the fluid will affect the convective heat transfer. For the convenience of heat transfer calculation, the following formula for calculating the amount of convective heat transfer is given: Q = α (tw - tf) A or q = α (tw - tf). It can be seen from the formula that the factors affecting convective heat transfer are attributed to the convective heat transfer coefficient, while the numerical value of the convective heat transfer system also reflects the strength of convective heat transfer.