According to the principle of chemical engineering, various unit operations are divided into three transfer processes: momentum transfer, heat transfer and mass transfer. Three-transmission analogy is to study and analyze three transmission processes in fluid flow by analogy, and it is the main method to explain "three-transmission" in chemical engineering principles.

1. Analogy of transmission essence (1) Momentum transmission

Momentum transfer is due to the unequal velocity between fluid layers, and momentum will be transferred from high velocity to low velocity. (2) Heat transfer

Heat transfer refers to the transfer of heat from high temperature to low temperature due to the different temperatures in the fluid. (3) mass transfer

Mass transfer is due to the concentration difference of substances in the fluid, and substances will transfer from high concentration to low concentration.

Most of these three transfer phenomena in fluid are caused by the random motion of fluid particles. If there is a temperature difference inside the fluid, there must be heat transfer at the same time as momentum transfer; Similarly, if there is a concentration difference in the fluid, there will also be mass transfer. If there is no momentum transfer, the heat and mass transfer is mainly caused by the random movement of molecules, and its transfer rate is slow. In order to improve the transfer rate, it is necessary to do work on the fluid to make it flow.