The transfer of heat within a fluid as the result of mixing of the warmer and cooler portions of the fluid is convection. For example, air in contact with the hot plates of a radiator in a room rises and cold air is
drawn off the floor of the room. The room is heated by convection. It is the mixing of the warmer and cooler portions of the fluid that conducts the heat from the radiator on one side of a room to the other side. Another example is a bucket of water placed over a flame. The water at the bottom of the bucket becomes heated and less dense than before due to thermal expansion. It rises through the colder upper portion of the bucket transferring its heat by mixing as it rises.
A good example of convection in a process application is the transfer of heat from a fire tube to a liquid, as in an oil treater. A current is set up between the cold and the warm parts of the water transferring the heat from the surface of the fire tube to the bulk liquid.
This type of heat transfer may be described by an equation that is similar to the conduction equation. The rate of flow of heat is proportional to the temperature difference between the hot and cold liquid, and the heat transfer area. It is expressed:
The proportionality constant, h, is influenced by the nature of the fluid and the nature of the agitation and is determined experimentally. If agitation does not exist, h is only influenced by the nature of the fluid and is called the film coefficient.