Temperature difference is the power of heat transfer. As long as there is temperature difference between two materials, they will carry out the heat or cold transfer spontaneously. The heat is always transferred from the fluid with higher temperature to the lower temperature fluid.
Although the heat transfer process of various heat exchangers is not identical, the heat transfer law is basically the same.
The following is an example of the most widely used dividing wall heat exchanger: heat is transferred from hot fluid to cold fluid through the wall and flows in the opposite direction. The whole heat transfer process is divided into three stages.
In the first stage, convection heat transfer: the heat flow body transmits heat to the wall surface. This process not only has heat transfer caused by the relative displacement of each part of the fluid, but also has heat conduction in the fluid, which is the comprehensive function of convection and heat conduction.
In the second stage, heat conduction: heat is transferred from wall 1 to wall 2 through the inner part of the wall. This process is a process of heat transfer by molecular collision inside the solid.
In the third stage, convection heat transfer: heat is transferred from wall 2 to cold fluid.
Therefore, the heat conduction process of heat exchanger is composed of convection heat conduction convection three stages and simultaneously.