www bookspar com Website for Students VTU NOTES
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS REFRIGERATION 1
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www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 4 REFRIGERATION v. 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. v. 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. v. 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.
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 5 v. 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. v. 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.
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 6 • 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.
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 7 • Both Refrigerator & Air conditioner 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 ”.
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS • In both 8 Refrigerator & Air conditioner, heat has to be extracted from a low temperature region and transferred to a high temperature region. • According to second law of thermodynamics, some external work input to the system is required
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 9 Applications of Refrigeration Ice making Preservation of food products (perishables) and medicines Air - conditioning Transportation of foodstuff, dairy products, flowers, etc. , Special industrial processes for chemicals, petroleum, medicals, surgical aids, etc. , Cryogenics Processing of food products, beverages, textiles, oil refining, photographic materials, etc. ,
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 10 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.
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 11 • The low temperature space is known as the Refrigerated chamber. • The 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.
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 12 • 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.
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 13 Unit of Refrigeration: Tons of refrigeration v. 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. v. The capacity of a refrigeration system is expressed in Tons of refrigeration which is the unit of refrigeration.
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 14 v. A ton of refrigeration is defined as the quantity of heat absorbed to convert one ton of water at 00 C to one ton of ice at the same temperature in 24 hours. v. One American ton (2000 pounds) is taken as the standard in the refrigeration practice. In S. I. System, 1 Ton of Refrigeration = 210 k. J/min = 3. 5 k. W
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 15 Coefficient of Performance (COP) of a Refrigerator v. 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 v. The performance indicator of a refrigerator COP can be improved by increase in the amount of heat absorbed from a system and by minimizing the amount of work supplied.
Refrigerants www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 16 v. Refrigerants are heat carrying mediums, 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 of Refrigerants: Refrigerants (i) Primary refrigerants (ii) Secondary refrigerants
Refrigerants www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 17 v. Primary refrigerants: Directly take part in the refrigeration system Examples: Ammonia, Carbon dioxide, Sulphur dioxide, Methyl chloride, Ethyl chloride etc. v. Secondary refrigerants: first cooled by primary refrigerants and then used for cooling purposes Examples: Brine solution, water, air, etc.
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 18 Common Refrigerants Most commonly used refrigerants are: v Ammonia v Carbon dioxide v Sulphur dioxide v Methyl chloride - vapour absorption refrigerator - marine refrigerators - household refrigerators - small scale refrigeration and domestic refrigerators v Freon-12 - domestic vapour compression refrigerators v Freon-22 - Air Conditioners
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 19 Most commonly used refrigerants Chloro Fluoro Carbons (CFCs): Group of refrigerants which are commercially known as Freons. Some of the refrigerants belonging to this group are: Dichlorodifluoromethane (CCl 2 F 2): Available with the commercial name of R-12 or Freon 12. It is very widely used in industrial and commercial applications such as refrigerators, freezers, water coolers, room and window air conditioning units etc. ,
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 20 Trichloromonofluoromethane (CCl 3 F): Commercial name is Freon-11 or R-11. Due to its low operating pressures, this refrigerant is exclusively used in large centrifugal compressor systems Dichloromonofluoromethane (CCl 2 F): Commercially known as Freon-21 or R-21. It is mainly used in centrifugal compressor systems for relatively high temperature refrigeration requirements.
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 21 Methyl Chloride (CH 3 Cl): Commercial name is R- 40 and is used in domestic and commercial units. Ammonia (NH 3): One of the oldest and most widely used of all the refrigerants. The major drawback of ammonia is its toxicity. Widely used in cold storage, warehouse plants, ice cream manufacture, ice plants, beverage industry, food freezing plants etc. , Commercial name is R-717.
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS Desirable Properties of Refrigerant 22 Thermodynamic Properties: v. Boiling point: An ideal refrigerant must have low boiling temperature at atmospheric pressure. v. Freezing point: It must have a very low freezing point because the refrigerant should not freeze at low evaporator temperatures. v. 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.
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 23 v. Evaporator and condenser pressure: To avoid the leakage of the atmospheric air and to enable the detection of the leakage of refrigerant, both the evaporator and condenser pressures should be slightly above the atmospheric pressure. v. 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. v. 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.
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 24 Physical Properties v. 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. v. 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.
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS : Safe Working Properties : 25 v. 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. v. Corrosiveness: A good refrigerant should be non- corrosive to prevent the corrosion of the metallic parts of the refrigerators. v. Chemical Stability: An ideal refrigerant must not decompose under operating conditions. v. 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.
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 26 Other Properties v. Coefficient of Performance (COP): The COP of a refrigerant must be high so that the energy spent in refrigeration will be less. v. Odour: A good refrigerant must be odourless, otherwise some food stuff such as meat, butter, etc. loses their taste. v. Leakage: The refrigerant must be such that any leakage can be detected by simple tests.
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 27 v. Action with Lubricating Oil: A good refrigerant must not react with the lubricating oil used in lubricating the parts of the compressor. v. Flammability: The refrigerant should have high ignition temperature so that when compressed to high temperature and pressure, it should not catch fire.
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 28 Cost : The cost of refrigerant is not so important in small refrigerating units, but is very important in large refrigerating systems, like industrial and commercial. The ammonia being cheapest, is widely in large industrial plants such as cold storages and ice plants.
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 29 Desirable properties for an ideal refrigerant Positive evaporating pressures Moderately low condensing pressure Relatively high critical temperature Low freezing point, low viscosity, low sp. vol. High latent heat of vaporization Low cost of refrigerant Inertness and stability High heat transfer characteristics Non toxicity and odour less Non flammability
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 30 Principle of Refrigeration Heat flows from a system at higher to another at lower temperature Heat can flow from a system at low temperature to a system at higher temperature by the aid of external work as per the 2 nd law of Thermodynamics.
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 31 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 depend on its pressure. of a fluid
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 32 Types of Refrigeration Systems Air Refrigeration Vapour Compression Refrigeration Vapour Absorption Refrigeration
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 33 Basic Components of a Refrigerating Unit Four (4) basic components: Evaporator Compressor / Pump (in Vapour absorption refrigeration systems) Condenser Throttle Valve or Expansion Valve
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 34 Major Components in Vapour Compression Cycle
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www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 36 Vapour Compression Refrigeration system v. 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. v. 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.
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 37 v. 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. v. The delivery side of the compressor is connected, to a condenser, which in turn connected to a throttle valve.
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 38 Vapour compression refrigeration system Condenser
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www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 40 Vapour compression refrigeration system
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS Vapour compression refrigeration system 41
v. Evaporator www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 42 • It is the heart of the refrigerator • Liquid refrigerant is evaporated by the absorption of heat from the refrigerator cabinet (where the substances which have to be cooled are kept) • The evaporator consists of metal tubes which surround 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).
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 43 Evaporator : - Low temperature region, where in the items to be preserved are kept. Also known as Refrigeration chamber or Freezing chamber or cooling unit or chilling unit and houses the small diameter metallic tubes called evaporator tubes through which the refrigerant flows. As the refrigerant flows through these tubes, it absorbs heat from the chamber and vaporizes. This vapours are then passed through the condenser. Compressor / Pump: Pump - Vapours coming out of the evaporator tubes 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. 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.
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 44 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 or Expansion 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. From the throttle valve, the liquid refrigerant enters the evaporator chamber to start the new cycle.
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 45 v. 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.
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 46 Vapour Absorption Refrigeration Flow Diagram of Vapour Absorption Refrigeration System
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 47 Vapour absorption refrigeration system In addition to evaporator, condenser and throttle valve consists of Absorber, generator (Separator) and a pump (No compressor) compressor The refrigerant used in these systems is Ammonia.
Working www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 48 Refrigerant ammonia passes through the Evaporator tubes and absorbs heat from the freezing chamber and evaporates. Low pressure and low temperature ammonia vapours are then passed through the Absorber containing cold water. Vapours dissolve in cold water to form a strong ammonia solution. The strong ammonia solution is pumped by a Pump into the Separator ( Generator), Generator where it is heated by an Electric heating coil.
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 49 Due to this heat addition the ammonia dissolved in water vapourises and gets separated from the solution. These high pressure high temperature ammonia vapours pass through the condenser and gives off heat to the cooling medium (cold water circulating around the tubes) and condensed to liquid state. The liquid ammonia is collected in the receiver. The high pressure, medium temperature liquid refrigerant is then passed through the throttle valve, where it expands with reduction in the pressure and temperature. Enters the evaporator kept in the storage room and cycle repeats.
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 50 Comparison between Vapour Compression and Vapour Absorption Refrigeration Vapour Compression System Vapour Absorption System Refrigerant vapour is compressed & works on mechanical energy Refri. vapour is absorbed & heated. Works on heat energy Refrigeration capacity is less than 1000 tons Refrigeration capacity is greater than 1000 tons COP is much higher (4 to 10) COP is less than or equal to 2 Smaller in size Larger Noisy due to compressor Pump noise is less Refilling of refrigerant is simple Difficult Chances of leakage of refrigerant are more Less Wear and tear are more Wear and tear are less Refrigerant: Freon - 12 Refrigerant: Ammonia Maintenance and operating costs are high Less
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS Air - Conditioning (A/C) 51
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 52 Air - Conditioning (A/C) Air-conditioning may be defined as the process of treating air with respect to temperature, humidity and cleanliness and distribute it as per the requirement of the conditioned space. Properly conditioned air results in greater comfort to the occupants in public places, offices and factories. Industrially it can be beneficial in controlling product quality, texture and uniformity.
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS Classification of Air - Conditioning (A/C) Based on comfort: Human Comfort Air Conditioning Industrial Air Conditioning Based on Season: Winter Air Conditioning system Summer Air Conditioning system Year Round Air Conditioning system 53
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 54 Psychometric Properties Specific Humidity or Humidity Ratio: Ratio of mass of water vapour to mass of dry air present in a given volume of mixture Absolute humidity: Mass of water vapour present in a given volume of air Relative Humidity: Ratio of the actual amount of water vapour (moisture) in the air to the maximum possible amount of water vapour (moisture) the air can hold at the prevailing temperature. The saturated air will have a relative humidity of 100%. Effective Temperature: Temperature of saturated air at which a person would experience the same feeling of comfort as experienced in the actual unsaturated environment
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 55 Applications of Air Conditioners Industrial applications: To control chemical and bio-chemical reactions. Examples: Photographic industry, textile, printing, machine tool industry etc. Commercial applications: Theatres, hotels, departmental stores, banks etc. , Transport applications: Automobiles, trains, aircraft, ships etc. , Special applications: T. V. centres, computer centres, automatic telephone exchange buildings, hospitals, etc. ,
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 56 Principle of A. C. • An A. C. continuously draws the air from an indoor space to be cooled and cools it by the refrigeration principles and discharges it back into the same indoor space that needs to be cooled
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 57 Wall Outside Room (inside) M-Motor, C-Compressor, F 1 - Condenser Fan, F 2 - Evaporator Fan Room / Window Air-Conditioner
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 58 Room / Window Air-Conditioner
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 59 Main components of Room / Window Type Air Conditioner : Compressor, (ii) Condenser, (iii) Air filter, (iv) Drier, (v) Capillary tube, (vi) Evaporator, (vii) Fans, (viii) Thermostat, (ix) Motor, (x) Heater (i)
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 60
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS Room / Window Air-Conditioner 61
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www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS A/C - Cycle 63
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 64 v. 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. v. Expansion (Throttle) Valve : 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. v. 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.
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 65 • REFRIGERATION AND AIR CONDITIONING • Introduction • 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 said 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 is input to the system. This principle is illustrated in the following block diagram.
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 66 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. COP = Q/W, where Q = Heat extracted from the system, k. W W = Work input to the system, k. W
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 67 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 ton of refrigeration is the capacity to freeze one ton of water at 00 C in 24 hours. 1 ton of refrigeration = 50. 4 kcal/min = 210 k. J/min = 3. 516 k. J/s
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 68 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 vapour 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.
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 69 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. Various properties of refrigerants can be grouped under three categories, as explained below:
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS • • 70 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.
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 71 Chemical Properties • 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. • Flammability: The refrigerant should have high ignition temperature so that when compressed to high temperature and pressure should not catch fire.
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 72 • Physical Properties • 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. • Corrosiveness: A refrigerant should be non-corrosive. The Freon group of refrigerants are 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. • 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. • 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.
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 73 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. • Non flammability.
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 74 • Types of refrigerants • A wide variety of refrigerants are commercially available and the selection of proper refrigerant depends on the particular application. The most important factor to be considered is the saturation pressure and temperature relationships.
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 75 • 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 (CCl 2 F 2): It is available with the commercial name of R-12 or Freon-12. It is a colorless, odourless liquid with boiling point of -290 C 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 and window air conditioning units etc. • Trichloromonofluoromethane (CCl 3 F): Its commercial name is freon-11 or R-11. It is stable, non-flammable and non-toxic. The latent heat at -150 C is 195 k. J/kg. The boiling point at atmospheric pressure is 23. 770 C. Due to its low operating pressures this refrigerant is exclusively used in large centrifugal compressor systems. • Dichloromonofluoromethane (CHCl 2 F): It is commercially known as Freon-21 or R-21. It has a boiling point of 90 C at atmospheric pressure. It is mainly used in centrifugal compressor systems for relatively high temperature refrigeration requirements.
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS • • 76 Methyl Chloride (CH 3 Cl): Its commercial name is R-40. Its boiling point at atmospheric pressure is -23. 70 C and the usual condenser pressure is 5 to 6. 8 bar. The latent heat of vaporization at -150 C 423. 5 k. J/kg. 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 (NH 3): Ammonia is one of the oldest and most widely used of all the refrigerants. Its boiling temperature at atmospheric pressure is -33. 30 C and freezing point is -780 C. The low boiling point makes it possible to have refrigeration at temperatures considerably below 00 C without using pressures below atmospheric in the evaporator. Its latent heat of evaporation is 1315 k. J/kg at -150 C. 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.
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS • • 77 Carbon Dioxide (CO 2): It is non-toxic, non-flammable and non-irritating. The boiling point of this refrigerant is as low as -73. 60 C and at -150 C, a pressure well over 20. 7 bar is required to prevent its evaporation. At a condenser temperature of +300 C, a pressure of approximately 70 bar is required to liquefy the gas. 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 (SO 2): This is produced by the combustion of sulphur in air. The boiling [point of SO 2 is -100 C at atmospheric pressure. The condensing pressure varies in the range of 4. 1 bar and 6. 2 bar under normal operating conditions. The latent heat is 396 k. J/kg at -150 C. It is very stable with high critical temperature, non-flammable and non-explosive. In earlier days it was widely used in household and
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 78 Basic Components of a Refrigerating Unit All the refrigerating units will have the following four basic components. • Evaporator • Compressor / Pump • Condenser • Throttle Valve or Expansion Valve
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 79
www. bookspar. com | Website for Students | VTU NOTES | QUESTION PAPERS 80 • Vapour absorption refrigeration system, in addition to evaporator, condenser and throttle valve consists • • • of absorber, heat exchanger, generator and a pump. There exists no compressor as in the case of a vapour compression refrigerator. The refrigerant used in these systems is ammonia. The refrigerant ammonia, as it passes through the evaporator tubes, absorbs heat from the freezing chamber and evaporates. These low pressure and low temperature ammonia vapours are then passed through the absorber containing cold water. Here the vapours dissolve in cold water to form a strong ammonia solution. The strong ammonia solution is pumped into the heat exchanger where it gains temperature due to heat exchange between the cold strong ammonia solution and the warm weak ammonia solution. This pre-heated, high pressure, strong ammonia solution flows through the generator where the solution is heated up by the heating coils mounted inside the generator. Due to this heat addition the ammonia dissolved in water vapourises and gets separated from the solution leaving behind a weak ammonia solution. These high pressure high temperature ammonia vapours pass through the condenser and gives off heat to the cooling medium due to its high saturation temperature. The warm weak ammonia solution left out in the generator chamber flows into the heat exchanger due to gravity. Inside the heat exchanger the warm weak ammonia solution exchanges heat with the cold and strong ammonia solution and becomes cold, weak ammonia solution. This cold, weak ammonia solution is passed into the absorber where it is ready to absorb the ammonia vapours coming out of the evaporator. The high pressure, medium temperature liquid refrigerant is then passed through the throttle valve where it expands and the pressure and temperature reduces to those values needed in the evaporator. In the case of vapour absorption refrigeration systems the transfer of heat from a low temperature region to a high temperature region takes place by adding the necessary external work through the pump. The pump increases the pressure of the strong ammonia solution and thereby increasing the saturation temperature above the temperature existing in the condenser. For the heat to be rejected in the condenser the temperature of the vapours should be higher than the temperature of cooling medium. So to increase the temperature of the solution it is heated up in the generator and during heating ammonia vapours are separated out. The ammonia vapours condense to liquid as it exchanges heat with the cooling medium in the condenser, due to high saturation temperature of the vapours.
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