CHAPTER ONE

REFRIGERATION

Refrigeration is defined as a process of removing heat from a substance or space in other to make it cooler.  More so, it is the branch of science which deals with the process of reducing and maintaining the temperature of the surrounding.

Generally, for a space to be cooler, it must lose heat to another substance.  Also for a space or material to get hotter, it must absorb heat from another substance.  Thus, heat is the characteristic agent of heating and cooling.  Consequently, for heat to flow, there must be a medium or mode for the transmission.  This always follows from high temperature region to low temperature region/bodies.

The modes of heat transmission are conduction, convection and radiation.  Conduction and convection are extensively involved in domestic refrigeration.

In refrigeration process, there is always a body employed as the heat absorber  or the cooling agent which may be near to the material or space being cooled or space being cooled or in direct contact with the material being cooled depending on the required final effect such a cooling agent is known as the Refrigerant.  The refrigerant circulates around the evaporating and condensing regions.  In other to maintain a constant refrigeration process it does it work by evaporating and condensing.  When it loses the absorbed heat to return to its original liquid or solid rate, as the case may be classified a s sensible heat or latent heat, (depending on the physical effect on the refrigerants.  Generally, when a mass or body absorbs heat the following physical changes takes place.

1.      The mass expands or increases in physical dimensions as the molecules resonate within a wider range

2.      The temperature increase (as the molecules gain more internal lunatic energy)

3.      There is a change in phase (as the mass changes from solid to liquid and consequently to gas and vice versa)

4.      The specific density decreases (as the mass remain constant while the volume increase) etc.

All these result to the refrigerant evaporating into the surrounding space/atmosphere and needs to be physically re-cycled to the original condition for another turn of refrigeration.  Over and over again, the processes are repeated and more cooling achieved.  Moreover, the sensible heat leads to the increase in the temperature of the refrigerant from one phase to another phase (solid – liquid – gas).  A refrigeration cycle is said to have been completed when this refrigerant has undertaken all its metamorphic circulation from the compressor, round, and back to the compressor again.

Another area worthy of consideration in refrigeration is the pressure effect.  It has a linear and positive relationship with temperature, which is the basic property of the transmission.  Adding pressure to the body induces internal stress on the molecular structure of the body.  Pressure can be used to increase the heat capacity of a body, thus, increase in the atmospheric pressure of a substance brings about a relative increase in the boiling point of that substance.  Also, sudden expansion or a compressed gas brings about an effective cooling and this explains the reasons why refrigerant gases are sealed and circulated under pressure. On the other hand, a reduction in the atmospheric pressure of a substance brings about a comparative decrease in the boiling point of that substance, in all, pressure increase and decrease of a particular substance causes temperature increase and decrease respectively of that substance.

Never the less, the whole of the refrigeration conditions explained above in abstract in the natural realms.   Heat does not flow from a cold region, as in the refrigeration process without work done against the natural principle.  Hence, energy is required, which is artificially produced and at this we now talk of refrigeration.

THE REFRIGERATOR

A refrigerator is an electro mechanical appliance, which is used in maintaining the temperature of substance as required by the user, or it is an electro mechanical appliance used in carrying on artificial cooling.  It operates with the principle of reversed carnet cycle.  In other words, heat is absorbed from a low temperature region and rejected to a high temperature region.  The temperature at which heat is absorbed are kept at constant.

The energy for the pumping of the refrigerant  round the refrigeration cycle comes from the compressor (that of the refrigerator).  It makes use of other mechanical and electric al components.  The torque for driving the crankshaft and consequently the piston is produced by an electric motor (it makes use of magnetic effects of current).  Similarly, the pumping effect of the piston creates the high pressure which induces to the refrigeration.  This thus, sets in motion the refrigerants, which after some physical metamorphosis returns to the former state ready   for another cycle.

 In the refrigerator, the condenser performs a function similar to that of the radiator in an automobile.  It is the cooling coil for the hot refrigerants gas.  In the condenser, the refrigerant is condensed and expelled to the surrounding atmosphere, the metering device expands the gas as another pipe passes through the capillary tube and suddenly expands into another pipe of  larger cross section, the evaporator consist of convolutions of a copper tubing which is where the actual cooling action of the refrigerator takes place.

However, a hot        of insulation is done to isolate t he refrigeration space from the surrounding environment and its effects.  This is achieved by the following measures.

i.        The body was made up or lagged   with poor heat conductor.

ii.       The inside and the outside are brightly coloured to effect light and heat reflection

iii.      The condenser tubing is positioned externally at the back of the refrigerator and a very poor heat conducting materials like fibre glass