Heat transfer importance

There are several unit operations and unit processes which operate at high temperatures. Flow of heat is important to attain uniform temperature in the furnace chamber. Ideally the available heat must be utilized to raise the temperature of the reactants and products to the desired value, but some amount of heat is always lost to the surrounding. Loss of heat to the surrounding is loss of energy and one of the main objectives of an engineer is to minimize the heat losses. Fundamentals of heat transfer mechanisms are important to calculate the flow of heat and to design the most efficient flow path conforming to the process. Transfer of heat takes place by conduction, convection and radiation. In the following a brief account of these mechanisms is given. For the detailed description, number of books on heat transfer is available. Some references are given at the end of this lecture.

Conduction  

Conduction is the flow of heat through a body occurring without displacement of the particles which make up the body. Fourier’s law of heat conduction is the basic law which says that the rate of heat flow across a unit area  at steady stale is proportional to the temperature gradient perpendicular to the area. Heat flow in one dimension, i. e. in  is

(1)

  is the temperature gradient and A is the area which is assumed to be invariant along the heat flow path. The constant K is thermal conductivity of the material through which heat is flowing. The thermal conductivity of the material indicates the relative ease or difficulty of the transfer of heat through the material. K can vary from about  for gases to  for pure metals. Thermal conductivity depends, on temperature, bonding and structure of the material. Thermal conductivity for ceramic materials is lower than metals. Porosity in the material decrease K. Thermal conductivity of the material varies with the temperature. The variation of thermal conductivity with temperature can be described by

(2)