CHAPTER ONE
INTRODUCTION
Refrigeration is the process of removing heat from a substance or space in order to make it cool. It is defined also as a branch of science that deals with the process of reducing and maintaining the temperature of a space, substance or material below the temperature of the surrounding.
For a space, substance or material to be cooled, it has to loose heat to another substance. On the other hand, for a material or substance to get hot, it has to absorb heat from another substance or material. Therefore heat is the agent of refrigeration process. For heat to flow there must be medium or mode to heat transfer. It is also a known phenomenon that heat flows from high temperature regions to low temperature regions by conduction, convention or radiation. However, in freezer and domestic refrigeration’s generally conduction and convention are employed as modes of heat transfer.
Refrigerant is the cooling agent in refrigeration. It makes indirect contact with the substance to be cooled. It is circulated in refrigeration circuit and the attendant change of state from liquid to vapour brings about the necessary cooling that is required in the system, which also involves latent or sensible heats.
Generally, when a mass absorbs heat, the following physical change takes place.
1.                 The mass expands in physical dimension.
2.                 The temperature increases.
3.                 There may be change in phase (solid, liquid and vapour or gas) and vice versa.
4.                 The specific density decreases (as the mass remains constant while the volume increases).