Refrigerator is a mechanical device which is used to maintain the temperature of a given space lower than the temperature of surroundings. Air conditioner is a mechanical device which is used to control the total environment of a confined space. Both of these devices work on the same basic principle of transferring heat from a low temperature region to a high temperature region and follows the second law of thermodynamics, which states that “heat cannot flow from a low temperature region to a high temperature unless aided by some external work”. In both of the above devices, heat has to be extracted from a low temperature region and transferred to a high temperature region. Hence, according to second law of thermodynamics, some external work or energy is to be added to the system.
Refrigeration Effect
Refrigeration is defined as the process of maintaining the temperature of a confined space at low temperature by continuously removing heat from the low temperature space and transferring it to high temperature surroundings. The low temperature space is known as the refrigerated chamber and a high temperature surroundings is the atmosphere. The medium which carries heat from the low temperature region to a high temperature region is called the Refrigerant. But according to second law of thermodynamics some external work or energy must be added to the system to achieve the refrigeration effect. In most of the refrigerators the work is supplied in the form of mechanical energy.
Coefficient of Performance (COP)
It is the term used to indicate the efficiency or performance of a refrigerator and is defined as the ratio of amount of heat extracted from the refrigerator chamber or amount of heat rejected to the atmosphere to the net work input to the system.
Refrigeration Effect
Refrigeration is defined as the process of maintaining the temperature of a confined space at low temperature by continuously removing heat from the low temperature space and transferring it to high temperature surroundings. The low temperature space is known as the refrigerated chamber and a high temperature surroundings is the atmosphere. The medium which carries heat from the low temperature region to a high temperature region is called the Refrigerant. But according to second law of thermodynamics some external work or energy must be added to the system to achieve the refrigeration effect. In most of the refrigerators the work is supplied in the form of mechanical energy.
Coefficient of Performance (COP)
It is the term used to indicate the efficiency or performance of a refrigerator and is defined as the ratio of amount of heat extracted from the refrigerator chamber or amount of heat rejected to the atmosphere to the net work input to the system.
Unit of Refrigeration
The capacity of a refrigerator, that is the rate of heat removal from the refrigerated space, is expressed in terms of Tons of refrigeration. One American ton (2000 pounds) is taken as the standard in the refrigeration practice.
One ton of refrigeration is the capacity to freeze one ton of water at 00C in 24 hours or it 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.
The capacity of a refrigerator, that is the rate of heat removal from the refrigerated space, is expressed in terms of Tons of refrigeration. One American ton (2000 pounds) is taken as the standard in the refrigeration practice.
One ton of refrigeration is the capacity to freeze one ton of water at 00C in 24 hours or it 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.
Applications of Refrigeration
1. Ice making
2. Preservation of food products (perishables) and medicines
3. Air - conditioning
4. Transportation of foodstuff, dairy products, flowers, etc.,
5. Special industrial processes for chemicals, petroleum, medicals, surgical aids, etc.,
6. Cryogenics
7. Processing of food products, beverages, textiles, oil refining, photographic materials, etc.,
1. Ice making
2. Preservation of food products (perishables) and medicines
3. Air - conditioning
4. Transportation of foodstuff, dairy products, flowers, etc.,
5. Special industrial processes for chemicals, petroleum, medicals, surgical aids, etc.,
6. Cryogenics
7. Processing of food products, beverages, textiles, oil refining, photographic materials, etc.,
Refrigerants
Refrigerant is the medium which carries heat from the low temperature region to high temperature region. The refrigerant undergoes phase change, from liquid state to vopour state as it absorbs heat from the space to be cooled and back to liquid state as the heat is rejected to the atmosphere, in one cycle of operation. For the maximum performance of the refrigerator, selection of good refrigerant is a must and it should possess some ideal properties.
Classification of Refrigerants:
Primary refrigerants: Directly take part in the refrigeration system.
Refrigerant is the medium which carries heat from the low temperature region to high temperature region. The refrigerant undergoes phase change, from liquid state to vopour state as it absorbs heat from the space to be cooled and back to liquid state as the heat is rejected to the atmosphere, in one cycle of operation. For the maximum performance of the refrigerator, selection of good refrigerant is a must and it should possess some ideal properties.
Classification of Refrigerants:
Primary refrigerants: Directly take part in the refrigeration system.
Examples: Ammonia, Carbon dioxide, Sulphur dioxide, Methyl chloride, Ethyl chloride etc.
Secondary refrigerants: First cooled by primary refrigerants and then used for cooling purposes
Examples: Brine solution, water, air, etc.
Most commonly used refrigerants are:
v Ammonia - in vapour absorption refrigerator
v Carbon dioxide - in marine refrigerators
v Sulphur dioxide - in household refrigerators
v Methyl chloride - in small scale refrigeration and domestic refrigerators
v Freon-12 - in domestic vapour compression refrigerators
v Freon-22 - in Air Conditioners
Properties of refrigerants
There exists no ideal refrigerant which can be used under all operating conditions. Hence the selection of a proper refrigerant for a particular application is important and its properties must match the requirements of the application. For example, refrigerant used in the refrigerators used for preserving edibles should be non-toxic. Similarly refrigerants used in nuclear applications should be non-flammable.
Thermodynamic properties
Boiling temperature: The boiling temperature of the refrigerant should be low so that it vaporizes at very low temperatures that exist inside the freezing chamber.
Freezing temperature: Refrigerants should have very low freezing point to avoid their freezing in the evaporator tubes mounted inside the freezing chamber.
Evaporator and Condenser pressure: The evaporator is called the low pressure side and the condenser is the high pressure side. Both these pressures should be positive, means above the atmospheric pressure and nearer to it. The positive pressures are necessary in order to prevent leakage of air and moisture into the refrigerating system. It also helps in early detection of leakages.
Latent heat of vaporization: The refrigerant should have high latent heat of vaporization so that it absorbs maximum amount of heat from the evaporator chamber. This improves the coefficient of performance of the refrigerator resulting in high refrigerating effect per kg of refrigerant circulated.
Critical Temperature: Critical temperature of a refrigerant is the highest temperature at which it can be condensed to liquid. It should be above the highest condensation temperature that might be encountered in the system.
Specific Volume: Specific Volume of a refrigerant should be low so that it occupies less volume upon vaporization. Higher the specific volume the size of the compressor should be large which requires more power for its operation and also increases the cost of the device.
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.
Chemical Properties
1. Toxicity: The refrigerant used should be non-toxic. This property of the refrigerant is of prime importance in domestic refrigerators, wherein leakage of refrigerant will cause the poisoning of items. Among the various refrigerants Ammonia and Sulphur Dioxide are highly toxic and are not used domestic refrigeration.
2. Flammability: The refrigerant should have high ignition temperature so that when compressed to high temperature and pressure should not catch fire.
Physical Properties
1. Stability and Inertness: A good refrigerant should not decompose at high temperature. Some refrigerants disintegrate due to reaction with metals. In order to avoid this, the refrigerant should be inert with respect to all materials used in the refrigeration system.
2. Corrosiveness: A refrigerant should be non corrosive. The Freon group of refrigerants is non corrosive practically with all metals. Ammonia is used only with iron or steel. Sulphur dioxide is non corrosive to all metals in the absence of water because in presence of water it reacts and forms sulphuric acid.
3. Viscosity: The refrigerants should have low viscosity both in the liquid and vapor states. The heat transfer through condenser and evaporator is improved at low viscosities.
4. Specific Volume: The specific volume of the refrigerant must be very low so that it occupies less volume upon vaporization. Higher the specific volume, the size of the compressor should be large, which requires more power for its operation and also increases the cost of the device.
Safe working properties
1. Toxicity: A good refrigerant should be non-toxic. Any leakage of the toxic refrigerant increases suffocation and poisons the atmosphere or any food items stored.
2. Corrosiveness: A good refrigerant should be non-corrosive to prevent the corrosion of the metallic parts of the refrigerators.
3. Chemical Stability: An ideal refrigerant must not decompose under operating conditions.
4. 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.
Other properties
1. Coefficient of Performance (COP): The COP of a refrigerant must be high so that the energy spent in refrigeration will be less.
2. Odour: A good refrigerant must be odourless, otherwise some food stuff such as meat, and butter, etc. may lose their taste.
3. Leakage: The refrigerant must be such that any leakage can be detected by simple tests.
4. Action with lubricating Oil: A good refrigerant must not react with the lubricating oil used in lubricating the parts of the compressor.
5. Flammability: The refrigerant should have high ignition temperature so that when compressed to high temperature and pressure, it should not catch fire.
6. Cost: the cost of refrigerant is not so important in small refrigerating units but it is very important in capacity refrigerating systems like industrial and commercial. The ammonia being cheapest is widely in large industrial plants such as cold storages and ice plants.
Desirable Properties for an ideal refrigerant
Positive evaporating pressures
Moderately low condensing pressure
Relatively high critical temperature
Low freezing point
High latent heat of vaporization
Low cost of refrigerant
Inertness and stability
High heat transfer characteristics
Non toxicity and odourless
Non inflammable non explosive
Low viscosity
Low boiling point
Low specific volume
Low specific heat capacity
Non-corrosive to metals
Types of refrigerants:
Secondary refrigerants: First cooled by primary refrigerants and then used for cooling purposes
Examples: Brine solution, water, air, etc.
Most commonly used refrigerants are:
v Ammonia - in vapour absorption refrigerator
v Carbon dioxide - in marine refrigerators
v Sulphur dioxide - in household refrigerators
v Methyl chloride - in small scale refrigeration and domestic refrigerators
v Freon-12 - in domestic vapour compression refrigerators
v Freon-22 - in Air Conditioners
Properties of refrigerants
There exists no ideal refrigerant which can be used under all operating conditions. Hence the selection of a proper refrigerant for a particular application is important and its properties must match the requirements of the application. For example, refrigerant used in the refrigerators used for preserving edibles should be non-toxic. Similarly refrigerants used in nuclear applications should be non-flammable.
Thermodynamic properties
Boiling temperature: The boiling temperature of the refrigerant should be low so that it vaporizes at very low temperatures that exist inside the freezing chamber.
Freezing temperature: Refrigerants should have very low freezing point to avoid their freezing in the evaporator tubes mounted inside the freezing chamber.
Evaporator and Condenser pressure: The evaporator is called the low pressure side and the condenser is the high pressure side. Both these pressures should be positive, means above the atmospheric pressure and nearer to it. The positive pressures are necessary in order to prevent leakage of air and moisture into the refrigerating system. It also helps in early detection of leakages.
Latent heat of vaporization: The refrigerant should have high latent heat of vaporization so that it absorbs maximum amount of heat from the evaporator chamber. This improves the coefficient of performance of the refrigerator resulting in high refrigerating effect per kg of refrigerant circulated.
Critical Temperature: Critical temperature of a refrigerant is the highest temperature at which it can be condensed to liquid. It should be above the highest condensation temperature that might be encountered in the system.
Specific Volume: Specific Volume of a refrigerant should be low so that it occupies less volume upon vaporization. Higher the specific volume the size of the compressor should be large which requires more power for its operation and also increases the cost of the device.
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.
Chemical Properties
1. Toxicity: The refrigerant used should be non-toxic. This property of the refrigerant is of prime importance in domestic refrigerators, wherein leakage of refrigerant will cause the poisoning of items. Among the various refrigerants Ammonia and Sulphur Dioxide are highly toxic and are not used domestic refrigeration.
2. Flammability: The refrigerant should have high ignition temperature so that when compressed to high temperature and pressure should not catch fire.
Physical Properties
1. Stability and Inertness: A good refrigerant should not decompose at high temperature. Some refrigerants disintegrate due to reaction with metals. In order to avoid this, the refrigerant should be inert with respect to all materials used in the refrigeration system.
2. Corrosiveness: A refrigerant should be non corrosive. The Freon group of refrigerants is non corrosive practically with all metals. Ammonia is used only with iron or steel. Sulphur dioxide is non corrosive to all metals in the absence of water because in presence of water it reacts and forms sulphuric acid.
3. Viscosity: The refrigerants should have low viscosity both in the liquid and vapor states. The heat transfer through condenser and evaporator is improved at low viscosities.
4. Specific Volume: The specific volume of the refrigerant must be very low so that it occupies less volume upon vaporization. Higher the specific volume, the size of the compressor should be large, which requires more power for its operation and also increases the cost of the device.
Safe working properties
1. Toxicity: A good refrigerant should be non-toxic. Any leakage of the toxic refrigerant increases suffocation and poisons the atmosphere or any food items stored.
2. Corrosiveness: A good refrigerant should be non-corrosive to prevent the corrosion of the metallic parts of the refrigerators.
3. Chemical Stability: An ideal refrigerant must not decompose under operating conditions.
4. 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.
Other properties
1. Coefficient of Performance (COP): The COP of a refrigerant must be high so that the energy spent in refrigeration will be less.
2. Odour: A good refrigerant must be odourless, otherwise some food stuff such as meat, and butter, etc. may lose their taste.
3. Leakage: The refrigerant must be such that any leakage can be detected by simple tests.
4. Action with lubricating Oil: A good refrigerant must not react with the lubricating oil used in lubricating the parts of the compressor.
5. Flammability: The refrigerant should have high ignition temperature so that when compressed to high temperature and pressure, it should not catch fire.
6. Cost: the cost of refrigerant is not so important in small refrigerating units but it is very important in capacity refrigerating systems like industrial and commercial. The ammonia being cheapest is widely in large industrial plants such as cold storages and ice plants.
Desirable Properties for an ideal refrigerant
Positive evaporating pressures
Moderately low condensing pressure
Relatively high critical temperature
Low freezing point
High latent heat of vaporization
Low cost of refrigerant
Inertness and stability
High heat transfer characteristics
Non toxicity and odourless
Non inflammable non explosive
Low viscosity
Low boiling point
Low specific volume
Low specific heat capacity
Non-corrosive to metals
Types of refrigerants:
A wide variety of refrigerants are commercially available and the selection of proper refrigerant depends on the particular application. Few of the most commonly used refrigerants and their properties are discussed below:
Chlorofluoro carbons (CFCs): It is a group of refrigerants, which are commercially known as Freons. Some of the refrigerants belonging to this group are given below:
Dichlorodifluoromethane (CCl2F2): It is available with the commercial name of R-12 or Freon-12. It is a colorless, odorless liquid with boiling point of -290C at atmospheric pressure. It is non-toxic, non-corrosive, non-irritating and non-flammable. It is the widely used among all the CFCs. It is very widely used in industrial and commercial applications such as refrigerators, freezers, water coolers, room & window air conditioning units etc.
Trichloromonofluoromethane (CCl3F): Its commercial name is freon-11 or R-11. It is stable, non-flammable and non-toxic. Due to its low operating pressures this refrigerant is exclusively used in large centrifugal compressor systems.
Dichloromonofluoromethane (CHCl2F): It is commercially known as Freon-21or R-21. It is mainly used in centrifugal compressor systems for relatively high temperature refrigeration requirements.
Methyl Chloride (CH3Cl): Its commercial name is R-40. It is flammable and explosive when mixed with air in concentrations from 8.1 to 17.2%. In the pure state it is non-corrosive and in the presence of moisture it is corrosive. This is used in domestic units with both reciprocating and rotary compressors and in commercial units with reciprocating compressors.
Ammonia (NH3): Ammonia is one of the oldest and most widely used of all the refrigerants. Its boiling temperature at atmospheric pressure is -33.30C and freezing point is -780C. The low boiling point makes it possible to have refrigeration at temperatures considerably below 00C without using pressures below atmospheric in the evaporator. The major drawback of ammonia is its toxicity. It is extensively used in cold storage, warehouse plants, ice-cream manufacture, ice plants, beverage industry, food freezing plants etc. Its commercial name is R-717.
Carbon Dioxide (CO2): It is non-toxic, non-flammable and non-irritating. Because of its very low efficiency, it is used in household units, but is used in some industrial applications and aboard ships. Its commercial name is R-744.
Sulphur Dioxide (SO2): This is produced by the combustion of sulphur in air. It is very stable with high critical temperature, non-flammable and non-explosive. In earlier days it was widely used in household and small commercial units. It is available with the trade name of R-764.
Types of Refrigeration Systems
1. Air Refrigeration
2. Vapour Compression Refrigeration
3. Vapour Absorption Refrigeration
Basic Components of a Refrigerating Unit
All the refrigerating units will have the following four (4) basic components.
Evaporator
Compressor / Pump
Condenser
Throttle or Expansion valve
Evaporator:
Chlorofluoro carbons (CFCs): It is a group of refrigerants, which are commercially known as Freons. Some of the refrigerants belonging to this group are given below:
Dichlorodifluoromethane (CCl2F2): It is available with the commercial name of R-12 or Freon-12. It is a colorless, odorless liquid with boiling point of -290C at atmospheric pressure. It is non-toxic, non-corrosive, non-irritating and non-flammable. It is the widely used among all the CFCs. It is very widely used in industrial and commercial applications such as refrigerators, freezers, water coolers, room & window air conditioning units etc.
Trichloromonofluoromethane (CCl3F): Its commercial name is freon-11 or R-11. It is stable, non-flammable and non-toxic. Due to its low operating pressures this refrigerant is exclusively used in large centrifugal compressor systems.
Dichloromonofluoromethane (CHCl2F): It is commercially known as Freon-21or R-21. It is mainly used in centrifugal compressor systems for relatively high temperature refrigeration requirements.
Methyl Chloride (CH3Cl): Its commercial name is R-40. It is flammable and explosive when mixed with air in concentrations from 8.1 to 17.2%. In the pure state it is non-corrosive and in the presence of moisture it is corrosive. This is used in domestic units with both reciprocating and rotary compressors and in commercial units with reciprocating compressors.
Ammonia (NH3): Ammonia is one of the oldest and most widely used of all the refrigerants. Its boiling temperature at atmospheric pressure is -33.30C and freezing point is -780C. The low boiling point makes it possible to have refrigeration at temperatures considerably below 00C without using pressures below atmospheric in the evaporator. The major drawback of ammonia is its toxicity. It is extensively used in cold storage, warehouse plants, ice-cream manufacture, ice plants, beverage industry, food freezing plants etc. Its commercial name is R-717.
Carbon Dioxide (CO2): It is non-toxic, non-flammable and non-irritating. Because of its very low efficiency, it is used in household units, but is used in some industrial applications and aboard ships. Its commercial name is R-744.
Sulphur Dioxide (SO2): This is produced by the combustion of sulphur in air. It is very stable with high critical temperature, non-flammable and non-explosive. In earlier days it was widely used in household and small commercial units. It is available with the trade name of R-764.
Types of Refrigeration Systems
1. Air Refrigeration
2. Vapour Compression Refrigeration
3. Vapour Absorption Refrigeration
Basic Components of a Refrigerating Unit
All the refrigerating units will have the following four (4) basic components.
Evaporator
Compressor / Pump
Condenser
Throttle or Expansion valve
Evaporator:
This is the low temperature region where in the items to be preserved are kept. It is also known as Refrigeration chamber or Freezing chamber. It houses the small diameter metallic tubes called evaporator tubes through which the refrigerant flows. As the refrigerant flows through these tubes, absorbs heat from the chamber and vaporizes. These vapours are then passed through the condenser.
Compressor / Pump:
Compressor / Pump:
The vapours coming out of the evaporator tubes are made to flow through the compressor where in the vapours are compressed to high pressure and temperature. In the compressor work is done on the vapours to enable the dissipation of energy from low temperature vapours to the high temperature surroundings in the condenser, according to second law of thermodynamics. Compressor is used only in vapour compression type refrigerators. In case of the vapour absorption type refrigerators, a pump is used to increase the pressure of the vapours. Condenser:
The dry saturated vapours coming out of the compressor passes through the condenser tubes wherein the vapours dissipate heat to the surrounding cooling medium, air or water, and converted into liquid refrigerant.
Throttle valve:
Throttle valve:
Throttle valve is a capillary device which reduces the pressure of the liquid refrigerant coming out of the condenser tubes to that of the initial pressure inside the evaporator chamber. Form the throttle valve the liquid refrigerant enters the evaporator chamber to start the new cycle.
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