ABSTRACT

Mechanical Engineering Department of the named Institute has many non – operational malhiues. Sequel to the fact, maintenance was poorly excuted which resulted to the breakdown deep freezer.Quality and effectiveness of machine maintenance were well observed. Also, a minimized maintenance and a highly successful repair was adopted.In other words during the repair and rehabilitation of the facility Deep Freezer, appropriate tools, cost and quality of materials were well considered.

CHAPTER ONE

INTRODUCTION 

Refrigeration is the process of removing heat from a space or region and maintaining this low temperature. The practical application of the process is to produce or maintain the temperature below the ambient. For a deep freezer, the temperature of the space is reduces up to the freezing point of the substances in the freezing space. Cooling agent, which may be bear to the material or space being cooled in direct contact with the material being cooled depending on the required final eff, such 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 its work by evaporating and condensing when it loses the absorbed heat to return to its original state (liquid or gaseous as the case may be). This change of state due to heat is called latent heat. Generally, when a mass or body absorbs heat from its surroundings, the following physical changes can take place.1. The body will expand or increase in physical dimension as the body resonates within a wider range.2. There is a phase change as the body change state four solid to liquid and consequently to gas and vice versa. 3. The temperature of the body increases as the molecules gain move internal hinetic energy.4. The specific density of the body decrease, as the mass remains constant while the volume increases. All these result tot he refrigerant evaporating into the surrounding space/atmosphere and needs to be physically re-cycled tot he original condition for another turn of refrigeration over and over again, the processes are repeated and move cooling achieved. Moreover, the latent heat leads to the change in phase of the refrigerant. A refrigeration cycle is to have been completed when the refrigerants have attained full metamorphic circulation from the compressor, round, and ball to the compressor again. Some commonly used refrigerants are refrigerant – 12, ammonia etc. Another area worthy of consideration in refrigeration is the pressure effect, it has a direct positive relationship with temperature, which is the basic property of the transmission. Adding pressure to the body induces internal stress of 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 bring about a 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 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.Nevertheless, she whole of the refrigerating conditions explained above is abstract in the natural realms. Heat does not flow a cold region as in the refrigeration process without work being done against the natural principle. Hence, energy is required, which is artificially produced and at this we now talk of refrigeration. A deep freezer can be defined as an Electro – mechanical appliance used to maintain the temperature of a substance at freezing point. It can also be said to be an Electro – mechanical appliance used in carrying out artificial cooling of a substance up to its freezing point. 

It operates with the principle of the reversed carnot cycle, in other woods, heat is received from a low temperature region and is rejected at the high temperature region. The temperature at which heat is absorbed is kept constant in order to enhance the efficiency of the system. The energy used in pumping the refrigerant and the refrigeration cycle comes from the compressor, which can be called the heat of the deep freezer. The system makes use of both electrical and mechanical components. The torque for driving the crankshaft and consequently the piston is produced by an electric motor (it makes use of the magnetic effects of current). Similarly, the pumping effect of the piston creates the high pressure, which induces the refrigeration. This act sets the refrigerant in motion, the refrigerant goes through some physical metamorphosis after which it comes back to its initial state and gets ready for another cycle.In the deep freezer, the condenser performs a function similar to the function of the radiator in automobiles is condensed (i.e., changed from its gaseous state to the liquid state) and during this process heat is given off to the surrounding atmosphere. 

The metering device expands the refrigerant gas as it passes through the capillary tube and suddenly expands abruptly into another pipe of larger diameter. The evaporator consists of several conclusion of copper tubing. The copper tubing can either be wound internally or externally. This is actually where the cooling / freezing action of the deep freezer unit takes place.However, a lot of insulation is done to isolate the space for refrigeration from the surrounding environment and its effects. This is achieved by the following measures. 1. The body was properly lagged with a good heat insulator.2. The inside and outside are brightly coloured to effect light and heat reflection. 3. The condenser tubing is positioned externally at the back of the deep freezer and a very poor heat conductor – material like fibre – glass, cotton – wool, silk etc. are used to shield away its heating effect from the refrigerating space.

LAWS OF REFRIGERATION 

All refrigerating systems depend on five thermal laws:1. Fluids absorb heat while changing from a liquid state to a vapour state and give up heat in changing from a vapour to a liquid.2. The temperature at which a change of state occurs is constant during the change provided the pressure remains constant. 3. Heat flows only from a body, which is at a higher temperature to a body which is at a lower temperature (hot to cold).4. Metallic parts of the evaporating and condensing units use, which have a high heat conductivity (copper, brass aluminum).5. Heat energy and other forces of energy are interchangeable. For example, electricity may be converted to heat, heat to electrical energy and heat to mechanical energy.