By convection the complex heat transfer process is divided into a number of simpler processes: heat conduction, convection and radiation. Each of these heat transfer processes obeys its laws.
Heat conduction is the process of molecular heat transfer by microparticles (molecules, atoms, ions, etc.) in a medium with a non-uniform temperature distribution.
Convection is the process of heat transfer by displacing the macroscopic elements of a medium (molar volumes).
Radiation is the process of heat transfer from one body to another by electromagnetic waves (or quanta).
In technological facilities heat is as a rule transferred by two or three ways at a time. Such a combined process is referred to as heat transfer. The heat transfer process conditioned by the simultaneous action of convection and heat conduction is called