CHAPTER ONE
INTRODUCTION
Refrigeration is a branch of science that deals with the process of removing heat from a substance or space in order to make it cooler. Refrigerator is defined as a device that is used in cooling the internal temperature below the room temperature (that is between 250C to 300C). Generally, however, for a space or substance to be cooler, it must loss that heat to another. Also, for a space or substance to get hotter, it must absorb heat from another, which must be at higher temperature. For both process to occur, heat must be absorbed or lost, thus heat is the characteristics agent of heating and cooling, consequently, for heat flow there is absorbs at a lower temperature region and rejected at a higher temperature region, that is the quantity being determined by the temperature gradient of the two regions. The modes of heat transmission are conduction concretion and radiation of which conduct convection and radiation, of which conduction and convection are extensively involved, in domestic refrigeration.
In refrigeration process, there is always a body employed as the heat absorber or indirect contact with the space or substance being cooled depending on the required final effect. Such cooling agents is known as refrigerant, which is known as the refrigerant, which is circulated around the evaporator that id high temperature region) and condensing region (that is higher temperature) in order to maintain a constant refrigeration process. It does it work be evaporating (when it absorbs heat up to the boiling point temperature) and by condensing when it losses the absorbed heat to return to its original liquid state, in the system. These heat when absorbed, may be classified as sensible heat or latent heat depending on the its physical effect on the refrigerants. From stage 4 to 1 is freezer O0c to 260C or 260C to O0C and then the thermostat actuates and adds heat to the refrigerant and this is sucked into the compressor then the refrigerant is in a vapour stagets enable it work and at this, there is increase in temperature and pressure.
Therefore, process 1 to 2 that is in vapour compressor work increasing in temperature and pressure. Process 2 to 3 (Q23) heat rejection. At the refrigerator is condensed by the condenser and by the condenser and it reduced to liquid. At point 3 the refrigerant gas is liquid. At process 3 to 4 isentropic work expansion (-w34) from 3 to 4 it is not liquid. At 4, the starting point of the evaporator and it is at lower temperature and pressure when it reaches –260C the thermostat stops because any further cooling has affect on the heat cycle. All these result to the refrigerant evaporating into the surrounding space or atmosphere and needs to be physically recycled to original condition. For another turn of refrigeration. Over and over again, these processes are repeated and more cooling achieved. Moreover, the sensible heat leads to the increase in the temperature of the refrigerant while the leant heat changes refrigerant from one phase to another (solid to liquid to gas). A refrigerant cycle is said to have been completed when the refrigerant has undertaken a turn of its circulation that is from the compressor 6o the condenser to the evaporator 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 heat transmission.
Pressure is force per unit area and can be used to increase the heat capacity of body, thus, increase in the atmospheric pressure a of a substance brings about relative increase in the boiling point of that substance. also, sudden expansion of a compressed gas brings about an effective cooling and this explains the reason why refrigerant gases are sealed and circulated under pressure. On the other hand, a reduction in atmospheric pressure of a substance brings about a comparative decrease in the boiling point of that substance. In all, pressure increase and decrease respectively of that substance. Nonetheless, the whole of the refrigeration conditions explained above is abstract in the natural realm. Heat does not flow from a cold region to a hot region, as in the refrigeration process without work being done against the natural principles hence, energy is required, which is artificially produced and at this, we now talk of refrigerators.
THE REFRIGERATOR
This is an Electro-mechanical appliance for carrying out the artificial cooling process. It operates with the principles of fevered carnot cycle, that is it takes heat from a low temperature region and rejects it at a high temperature region. The temperature at which heat is absorbed (low) as well as that at which the heat is rejected (high) are kept constant and by so doing a net negative work is done on the system.
REFRIGERATOR OPERATION DIAGRAM
T2
High temperature Condenser
Q2
W (work is done on the system) that is work input
Compressor
Q1
Low temperature Evaporator (Copper tubbing)
The energy for the pumping of the refrigerant round the refrigeration cycle comes from the compressor, which is the heart of refrigeration. It makes use of both mechanical and electrical components.
COMPONENTS OF REFRIGERATOR
1. Compressor
2. Condenser
3. Metering device of expansion value
4. Evaporator
5. Dryer or drier strainer
6. Capillary tube
7. Thermostat
DIAGRAM OF A REFRIGERATOR CYCLE
The touring for driving the crankshaft and consequently, the piston is produced by an electric motor, (which make use of the magnetic effects of current). The pumping effect of piston capacities the high-pressure which induces to the refrigeration. This, thus, set in motion the refrigerant, which after some physical metamorphosis, returns to the former state and continue another cycle. In the refrigerator, the condensers (as the name implies) condense the refrigerant and give out heat to the (surrounding atmosphere, while condenser fin is use circulation of air around). The metering device expands the gas as it posses through the capillary and suddenly expands into another pipe of large cross section. The evaporator consists of twining copper tubing, which is where the actual cooling action of the refrigerator takes place.
However, a lot of insulation is done to isolate the refrigerating space from the surrounding environment ant its effects. This is achieved by the following measures. 1. The body is made of or lined with poor heat conductors. 2. The inside and the outside are brightly coloured to effect light or heat reflection. 3. The condenser tubing is positioned externally at the back of the refrigerator and very poor heat conducting materials like: fibre glass, cotton wool, silk etc is used to shield away its heating effect from a system refrigerating process. Note: It is a fact that heat cannot be totally isolated from a system, all these measures are taken to reduce its effect to the barest minimum. Other appliances and modifications are put in place for convenience, efficiency, and security purpose.
