1Dept. of Mech & Mfg. Engg.

REFRIGERATION

Refrigeration may be defined as the process of removing heat from a substance under controlled conditions and reducing and maintaining the temperature of a body below the temperature of its surroundings by the aid of external work.

Therefore in a refrigerator, power is to be supplied to remove the heat continuously from the refrigerator cabinet to keep it cool at a temperature less than the atmospheric temperature.

A medium called refrigerant continuously extracts the heat from the space within the refrigerator which is to be kept cool at temperatures less than the atmosphere and finally rejects to it to the surroundings.

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Principle of Refrigeration Heat flows from a system at higher temperature to another at

lower temperature.

Fluids by absorbing the heat, change from liquid phase to vapour phase and subsequently condense by giving off the heat.

The boiling and freezing temperatures of a fluid depend on its pressure. When a certain fluid at a very low pressure and temperature is compressed, even though its pressure increases it may still be in the condensed state itself if its temperature is not increased to the saturation temperature corresponding to the increased pressure.

Heat can flow from a system at low temperature to a system at higher temperature by the aid of external work as per the Second law of Thermodynamics.

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4Dept. of Mech & Mfg. Engg.

Unit of Refrigeration: Tons of refrigeration

In a refrigeration system, the rate at which the heat is absorbed in a cycle from the interior space to be cooled is called refrigerating effect.

The capacity of a refrigeration system is expressed in Tons of refrigeration which is the unit of refrigeration. A ton of refrigeration is defined as the quantity of heat absorbed to convert one ton of water at 00C to one ton of ice at the same temperature in 24 hours.

Here it should be noted that one American ton (2000 pounds) is taken as the standard in the refrigeration practice.

In S.I. System,

1 Ton of Refrigeration = 210 kJ/min = 3.5 kW

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Coefficient of Performance of a Refrigerator : COP

The performance of a refrigeration system is expressed by a factor known as Coefficient of Performance (COP). The COP of a refrigeration system is defined as the ratio of amount of heat absorbed or extracted in a system to the amount of work supplied.

Amount of heat absorbed COP = ------------------------------------------

Amount of work supplied

Therefore, the performance indicator of a refrigerator COP can be improved by increase in the amount of heat absorbed from a system and by minimize the amount of work supplied.

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Refrigerants.

The refrigerant is a heat carrying medium which during their cycle in the refrigeration system absorb heat from a low temperature system and reject the heat to a high temperature system.

Classification: Refrigerants are usually primary refrigerants and secondary refrigerants.

Primary refrigerants which directly take part in the refrigeration system. Ex: Ammonia, Carbon dioxide, Sulphur dioxide, Methyl chloride, Ethyl chloride etc.,

Secondary refrigerants are first cooled by primary refrigerants and then used for cooling purposes. Ex: Brine solution, Sodium chloride etc.,

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Common Refrigerants. The most commonly used refrigerants are: a)     Ammonia - in vapour absorption refrigerator. b)     Carbon dioxide - in marine refrigerators. c)     Sulphur dioxide - in household refrigerators. d)  Methyl chloride - in small scale refrigeration and domestic

refrigerators. e) Freon - 12 - in domestic vapour compression refrigerators. f) Freon-22 - in Air Conditioners.

Desirable Properties of Refrigerant:Thermodynamic Properties:

Boiling point: An ideal refrigerant must have low boiling temperature at atmospheric pressure.

Freezing point: It must have a very low freezing point because the refrigerant should not evaporator temperatures.

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Specific Heat: A good refrigerant must have low specific heat when it is in liquid state and high specific heat when it is vapourised. The low specific heat of the refrigerant helps in more heat absorption in the evaporator and high specific heat of the vapour helps in easy condensing. Both these desirable properties will increase the refrigerating effect.

Evaporator and condenser pressure: To avoid the leakage of the atmospheric air and also to enable the detection of the refrigerant, both the evaporator and condenser pressures should be slightly above the atmospheric pressure.

Latent heat of evaporation: This must be very high so that a minimum amount of refrigerant will give the desired result. In other words, it increases the refrigeration effect.

Critical temperature and pressure. The critical temperature of a refrigerant is the highest temperature at which it can be condensed to a liquid, regardless of a higher pressure and it should be above the highest condensing temperature.

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Physical Properties: Specific Volume: The specific volume of the refrigerant must

be very low. The lower specific volume of the refrigerant at the suction of the compressor reduces the size of the compressor.

Viscosity: The viscosity of a refrigerant at both the liquid and vapour states must be very low as it improves the heat transfer and reduces the pumping effort required.

Safe Working Properties: Toxicity: A good refrigerant should be non-toxic, because any

leakage of the toxic refrigerant increases suffocation and poisons the atmosphere or any food items stored

Corrosiveness: A good refrigerant should be non-corrosive to prevent the corrosion of the metallic parts of the refrigerators.

Chemical Stability: An ideal refrigerant must not decompose under operating conditions.

Miscibility: The ability of a refrigerant to mix with oil is called miscibility. The miscible refrigerants are advantageous from the heat transfer point of view.

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Other Properties: Coefficient of Performance: The coefficient of performance of

a refrigerant must be high so that the energy spent in refrigeration will be less.

Odour: A good refrigerant must be odourless; otherwise some food stuff such as meat, butter, etc. loses their taste.

Leakage: The refrigerant must be such that any leakage can be detected by simple tests.

Action with Lubricating Oil: A good refrigerant must not react with the lubricating oil used in lubricating the parts of the compressor.

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Vapour Compression Refrigeration system.

In a vapour compression refrigerator a vapour is used as refrigerant. It is circulated through the system in which it alternately evaporates and condenses, thus undergoing a change of phase from vapour to liquid and again liquid to vapour.

During evaporation it absorbs the latent heat from the refrigerated space and subsequently gives off heat while condensing. A vapour compression system makes use of mechanical energy supplied to the compressor to run the refrigerator.

It consists of an evaporator made of coiled tubes installed in the freezing compartment of the refrigerator and connected to the suction side of the compressor and a throttle valve.

The delivery side of the compressor is connected, to a condenser which in turn is connected to a throttle valve.

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Evaporator: As the name itself implies, the evaporator is the heart of the refrigerator where the liquid refrigerant is evaporated by the absorption of heat from the refrigerator cabinet in which the substances which have to be cooled are kept.

The evaporator consists of simply metal tubing which surrounds around the freezing and cooling compartments to produce the cooling effect required for freezing ice or lowering the temperature of perishables placed in the cooling compartment.

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Circulating system: This comprises of the mechanical devices such as compressors or pump necessary to circulate the refrigerant to undergo the refrigeration cycle. They increase the pressure and therefore the temperature of the refrigerant. Generally these devices are driven by the electrical energy input to the motor or pump.

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Condenser: It is an appliance in which the heat from the refrigerant is rejected at higher temperature to another medium, usually the atmospheric air or cooling water. In a condenser the refrigerant vapour gives off its latent heat to the cooling medium and condenses into liquid so that it can be expanded in the expansion device. The heat given off in the condenser includes the heat absorbed in the evaporator as well as the heat developed due to compression.

Expansion Device: An expansion device serves as a device to reduce the pressure suddenly and hence the temperature of the liquid refrigerant before it passes to the, evaporator. The liquid refrigerant from the condenser is passed through an expansion valve (throttle valve) where the pressure & temperature reduces.

The refrigerant at low pressure and temperature passing in the evaporator coiled tubes absorbs the heat from the contents in the freezing compartment and evaporates. This in turn lowers temperature in the freezing compartment.