1.0 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.
- The mass expands in physical dimension.
- The temperature increases.
- There may be change in phase (solid, liquid and vapour or gas) and vice versa.
- The specific density decreases (as the mass remains constant while the volume increases).
Since the refrigerant evaporates and condenses it is necessary that the operation is cyclic. Over and over again the process is repeated in order to produce more cooling. A refrigeration cycle is said to be completed when the refrigerant has moved from a point round the components and back to the starting point.
Worth of consideration is the effect of pressure. It is a linear and positive relationship with temperature, which is the basic property of transmission. An increase in pressure of a body induces internal stress on the body. Also pressure increase brings an increase in the heat capacity of a body. For instance, and increase in the atmospheric pressure of water increase its boiling point and vice versa. This explains the reason for refrigerant to be sealed and circulated under pressure. We can rightly say that pressure increase or decrease of a substance brings about increase or decrease of the temperature on the substance respectively.
Nevertheless, heat does not flow from a cool region to a hot one as in refrigeration without work being done against the natural principle. This gives rise to energy that is required which is artificially produced and at this we talk of Deep Freezer.
Deep freezer: It is an electron — mechanical appliance, which maintains the temperature of substance, space or materials as required by the user i.e. it produces artificial cooling. It operates on the principle of Reversed Carnot Cycle. That is to say, I t absorbs heat from a low temperature and rejects it to a high temperature region. The compressor provides the conversion of energy that is used in circulating the refrigerant round the circuit.
The crankshaft, which drives the piston though the connecting rod, is moved by the torque produced by electric motor. The pumping effect of the piston produces the high pressure on the refrigerant, which set it in motion. in the cause of circulation and attendant change of state, the refrigerant returns to the initial state and ready for another cycle.
In a deep freezer, the condenser performs function similar to that of the radiator in an automobile. It cools the hot refrigerant gas by giving out heat to the surrounding and changing to liquid. The liquid is delivered to the dryer, which traps moisture that can block the metering device. The liquid refrigerant passes though the metering device (capillar), which expands the liquid into the evaporator pipe, which is of larger cross sectional area. The pipe is laid on the inside of the Deep Freezer. A lot of insulation is done to isolate the refrigerated space from the surrounding and its heating effect. One fact remains that heat cannot be totally isolated from the refrigerated space. Yet it has to be reduced to the barest minimum. Other modification and appliances are put in place for convenience, efficiency and for security purposes.
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