1. Introduction
Heat transfer is a science, which deals with the flow of heat from a higher temperature to lower temperature. Heat cannot be stored and it is defined as the energy in transit due to the difference in the temperatures of the hot and cold bodies. The study of heat transfer not only explains how the heat energy transports but also predicts about the rate of heat transfer. When a certain amount of water is evaporated or condensed, the amount of heat transferred in either of the processes is same. However, the rate of heat transfer in both the cases may be different.

At this point, it is very important to understand about the basic information that the phases of a substance (solid, liquid, and gas) are associated with its energy content. In the solid phase, the molecules or atoms are very closely packed to give a rigid structure (fig.1.1a). In the liquid phase, sufficient thermal energy is present, which keeps the molecules sufficiently apart and as a result the rigidity looses (fig.1.1b). In the gas phase, the presence of additional energy results in a complete separation and the molecules or atoms are free to move anywhere in the space (fig.1.1c). It must be noticed that whenever a change in phase occurs, a large amount of energy involves in the transition.

Fig. 1.1: Relative molecular distance of different phases of a substance at a fixed temperature (a) gas/vapour, (b) liquid, and (c) solid

As we are dealing with the heating and cooling of materials in almost our all the processes, the heat transfer is an indispensable part of any of the industries. Therefore, heat transfer is a common subject in many engineering disciplines, especially mechanical and chemical engineering. Study of heat transfer has a vital role in the chemical process industries. Chemical engineers must have a thorough knowledge of heat transfer principles and their applications.

There are three different modes in which heat may pass from a hot body to a cold one. These modes are conduction, convention, and radiation. It should be noted that the heat transfer takes place in combination of two or three modes in any of the real engineering application. In this chapter, we will briefly discuss about the different modes of heat transfer along with the various basic information that will help us as a building block for further study.