EME Notes Refrigeration

Yogeesha H C

Lecture Notes on 10EME14/24

Refrigeration and Air Conditioning 1. Define Refrigeration and explain the principle of refrigeration Refrigeration is defined as a method of reducing the temperature of a system below that of the surroundings and maintaining it at the lower temperature by continuously extracting the heat from it. The principle of refrigeration is based on second law of thermodynamics. It states HOT BODY that heat doe snot flow from a low temperature body to a high temperature body without the help of an external work. In refrigeration process, the heat is Work Input Refrigerator continuously removed from a system at lower temperature and transfer it to the surroundings at a higher temperature. This operation according to second law of thermodynamics can only be performed by COLD BODY the aid of the external work. Therefore in a refrigeration system, power is to be supplied to remove heat continuously from the refrigerator to keep it cool at a temperature less than the surroundings. The refrigeration cycle is based on reversible carnot cycle. 2. State the refrigeration concepts and mention the uses of refrigeration The principle of refrigeration is based on the following concepts. 1. Heat flows from a system at higher temperature to another at lower temperature 2. Fluids by absorbing the heat, change from liquid phase to vapour phase and subsequently condense by giving off the heat 3. 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 4. Heat can flow from a system at low temperature to a system at higher temperature by the aid of external work done as per second law of thermodynamics. Uses of Refrigeration: 1. Ice making 2. Preservation of food products and medicines 3. Air conditioning 4. Transportation of food stuffs, diary products, flowers etc 5. Cooling of concrete for dams 6. Treatment of air for blast furnace 7. Special industrial processes such as chemicals, medicals, surgical aids, etc 8. Processing of food products, beverages, textiles, printing work, etc 3. Explain the parts of a refrigeration system Following are the parts of a refrigeration system 1. Evaporator: The evaporator is the heart of the refrigeration system, where the liquid refrigerant is evaporated by the absorption of the heat from the refrigerator cabinet in which the substances to be cooled are kept. It consists of copper tubing surrounding he freezing and cooling compartments.

Nagarjuna College of Engineering & Technology, Bangalore

Yogeesha H C Lecture Notes on 10EME14/24 2. Compressor or Pump : For circulating the refrigerant in the system, a compressor or a pump is used. They increase the pressure and therefore the temperature of the refrigerant. Generally these devices are driven by electric motor. 3. Condenser: A condenser is an appliance in which the heat from the refrigerant is rejected at a higher temperature to another medium, usually atmospheric air. In a condenser, the refrigerant vapour, gives off its latent heat to the air and consequently condenses into liquid form so that it can be recirculated in the refrigeration cycle. It consists of coil of tubes made of copper. 4. Expansion or Throttle valve : An expansion valve serves as a device to reduce the pressure and temperature of the liquid refrigerant before it passes to the evaporator. This also used to control the flow of refrigerant to the evaporator. EVAPORATOR

COMPRESSOR OR PUMP

EXPANSION VALVE

CONDENSOR 4. Explain the following terms 1) Refrigeration effect: The rate at which the heat is absorbed in a cycle of from the interior space to be cooled is called refrigeration effect. It is defined as the quantity of heat removed to the time taken. It is also called as the capacity of a refrigerator. 2) Ton of refrigeration: The capacity of the refrigeration system is expressed in ton of refrigeration which is the unit of refrigeration. A ton of refrigeration is defined as the quantity of heat absorbed in order to form one ton of ice in 24 hours when the initial temperature of water is zero degree centigrade. 1 ton of refrigeration = 210 kJ/min = 3.5 kW 3) Co-efficient of Performance(COP): The Co-efficient of Performance is defined as the ratio of heat absorbed in a system to the work supplied. 4) Ice making capacity: It is the ability of the refrigeration system to make ice. In other words, it is the capacity of refrigeration system to remove heat from water to make ice. 5) Relative COP: It is the ratio of actual COP to the theoretical COP of a refrigerator. Actual COP is measured during a test and theoretical COP is obtained by applying the laws of thermodynamics. 5. Mention the desirable properties of a good refrigerant and explain The desirable properties of a good refrigerant are as under. 1. Boiling point: An ideal refrigerant must have a low boiling point at atmospheric pressure. 2. Freezing point: The refrigerant must have a very low freezing point because the refrigerant should not freeze at low evaporator temperatures. 3. Latent heat of evaporation: It should be very high so that a minimum amount of refrigerant will accomplish the desired result. 4. Specific volume: should be very low and it reduces the size of the compressor. Nagarjuna College of Engineering & Technology, Bangalore

Yogeesha H C Lecture Notes on 10EME14/24 5. Specific heat of liquid and vapour: A good refrigerant must have a low specific heat in liquid state and high specific heat in vapour state. The low specific heat of the refrigerant helps in sub-cooling of the liquid and high specific heat of vapor helps in decreasing the superheating of the vapour. 6. Viscosity: must be very low as it improves the heat transfer rate and reduces the pumping pressure. 7. Non-toxic: should be non-toxic because any leakage increases the suffocation and poisons the atmosphere. 8. Non-corrosive: to prevent corrosion of the metallic parts 9. Chemical stability : should not decompose under operating conditions 10. Odourless: otherwise food preserved will loose their taste. 11. Leakage tests: any leakage must be easily detected by simple tests 12. Action with lubricating oil: should not react with lubricating oil 6. Name the most commonly used refrigerants and state their uses i) Ammonia: It is the most widely used refrigerant in vapour absorption system. It has normal boiling point of -33.3C. It is highly toxic, inflammable, irritating and corrosive. Hence not suitable for domestic applications. It is used in large industries for cold storage, ice plants, packaging plants, etc ii) Carbon Dioxide: It is non-toxic and non-flammable. Its normal boiling point is -77.6C. It is heavier than air and hence requires higher operating pressures. Due its low specific volume, the plant size is compact and hence used in marine applications. iii) Sulphur dioxide: It is colourless, suffocating and possesses irritating odour. Boiling point is -10C. Used in domestic refrigerators in olden days iv) Freon - 12: It is non-flammable, non-explosive, non.-corrosive and odourless. Boiling point -29.8C. used in small capacity plants such as domestic refrigerators, water coolers, air conditioners, etc v) Freon- 22: It has a boiling point of -40.8C. It is employed for large air conditioners, food freezing, food drying, etc 7. With a neat sketch, explain the working of Vapour Compression Refrigeration system The vapour compression refrigeration system is most commonly used in domestic refrigerators. In VCRS the refrigerant alternatively undergoes a change of phase from vapour to liquid and vice versa during a cycle. The system consists of evaporator, compressor, condenser and an expansion valve. The liquid refrigerant which is at low pressure and low temperature flows into the compressor. In the compressor the refrigerant is compressed and converted into a high pressure and high temperature. This high pressure and high temperature refrigerant then passes through the condenser where it is condensed into high pressure liquid refrigerant. The high pressure liquid refrigerant is then pass through the expansion valve where its pressure and temperature drops and it partly evaporates. It is then allowed into the evaporator at a controlled rate. In the evaporator, the partly liquid and vapour refrigerant is evaporated and converted into a low pressure vapour. During this process, the refrigerant absorbs its latent heat of vaporization from the material to be cooled. Thus the body is cooled in the evaporator by continuously extracting heat.

Nagarjuna College of Engineering & Technology, Bangalore

Yogeesha H C

Lecture Notes on 10EME14/24

8. With a neat sketch, explain the working of Vapour Absorption Refrigeration system

The vapour absorption refrigeration system us a heat operated system. It differs from the VCRS only in the manner in which the circulation of the refrigerant is achieved. In this system, the compressor is replaced by an absorber, a generator and a pump. The refrigerant used in this system must be highly soluble in the solution known as absorbent. The system uses ammonia as refrigerant and water as absorbent. Nagarjuna College of Engineering & Technology, Bangalore

Yogeesha H C Lecture Notes on 10EME14/24 The liquid refrigerant in the evaporator absorbs the heat from the medium to be cooled and it undergoes a change of phase from liquid to vapour. The low pressure vapour is then passed to the absorber. In the absorber, the low pressure ammonia vapour is dissolved in the weak ammonia solution producing strong ammonia solution at low pressure. The strong ammonia solution is then pumped to a generator through the heat exchanger at high pressure. While passing through the heat exchanger, the strong ammonia solution is warmed up by the hot weak ammonia solution flowing from the generator to absorber. The warm strong ammonia solution is heated by an external source in the generator. Due to the heating, the vapour gets separated from the solution. The vapour which is at high pressure and high temperature is condensed to low temperature in the condenser by cold water circulation. The high pressure liquid ammonia then passes through the expansion valve where it is expanded to low pressure and temperature. The low pressure and low temperature ammonia liquid again enters the evaporator where it absorbs the heat from the medium to be cooled and the cycle continues. 9. Compare Vapour Compression and Vapour Absorption refrigeration systems VCRS VARS Works using mechanical energy Works using heat energy Refrigerating capacity is less than 1000 tons Capacity is more than 100 tons Cop is much higher ( 4 to 10) COP is less than 2 Noisy due to compressor Pump noise is less Chances of leakage of refrigerant are more No leakage Maintenance and operating cost is more Less Smaller in size Larger in size Wear and tear are more Less Freon – 12 or any other refrigerant will be Only Ammonia is used as refrigerant used Refrigerant vapour is compressed Refrigerant vapour is absorbed and heated 10. What is air conditioning? Mention the application of air conditioning It is the process of controlling and maintaining the properties of air like temperature, humidity, purity, direction of flow etc in a closed space. It is the simultaneous control of temperature, humidity, cleanliness, and air motion of the confined space. The term air conditioning was coined by Stuart Crammer in 1905. Mr. Willis Carrier is considered as father of air conditioning worked extensively to promote controlled air in air conditioning systems. Applications: • Comfort applications: Aim to provide an indoor environment that remains relatively constant in a range despite changes in external conditions or in internal needs 1. In residential building 2. Institutional buildings 3. Commercial buildings 4. Transportation • Process Application: Aim to provide a suitable environment for a process being carried out, regardless of internal heat loads and external weather conditions. Applications include 1. Hospitals 2. Clean Rooms for production of ICs, pharmaceuticals, and the like in which high cleanliness and control of temperature is required

Nagarjuna College of Engineering & Technology, Bangalore

Yogeesha H C

Lecture Notes on 10EME14/24

3. Textile factories 4. Nuclear facilities 5. Food cooking and processing areas 6. Data processing centers, etc 11. State the principal of air conditioning and Mention its classification. An Air conditioning system draws the air from an indoor space to be cooled and cools it by refrigeration principles and discharges it back to the same indoor space that needs to be cooled. Such continuous cyclic drawal and recirculation of the cooled air keeps the indoor space at the required lower temperature for the desired purpose. Classification: Basis Comfort Season of the year

Equipment Arrangement

Tye 1. 2. 1. 2. 3. 1. 2. 3. 4.

Human Confort AC Industrial A C Winter AC Summer A C Year Round AC Unitary System Split System Central station system Combination system

12. Explain the following terms 1) Pyshrometry: It is the study of the properties of the moist air. The properties of air and water vapour mixture are called pyschometric properties. 2) Dry Air : The atmospheric air without the presence of water vapour is called dry air 3) Moist air : It is the mixture of dry air and water vapour 4) Dry Bulb Temperature: It is the temperature of the air measured using an ordinary thermometer. It is not affected by the water vapour present in the air 5) Wet Bulb temperature: It is the temperature measured by ordinary thermometer when its bulb is covered with wet cloth and exposed to air 6) Dew Point Temperature : The temperature at which the water vapour in the air begins to condense when the temperature of the air is continuously reduced. 7) Humidity: It is the quantity of water vapour present in the air. The absolute humidity is defined as the weight of water vapour contained in a given volume of air. The specific humidity is defined as the ratio of weight of water vapour to the total weight of air. 8) Relative Humidity: It the ratio of mass of water vapour present in a given volume of air at a given temperature to the mass of water vapour present in the same volume and temperature of the air when it is fully saturated. 13. With a neat sketch explain the construction and working of a room air conditioner The room conditioner unit consists of a vapour compression refrigeration system, an air filter and double motor shaft that drives a fan at one end and a blower at the other end. The room side and the outside of the unit are separated by an insulation partition wall within the casing. The blower sucks the warm air from the room to be cooled through the air filter and an evaporator. The refrigerant inside the evaporator coil gets vaporized by absorbing the heat from the warm air. The motor runs the blower to deliver the cooled air into the room. This air

Nagarjuna College of Engineering & Technology, Bangalore

Yogeesha H C Lecture Notes on 10EME14/24 mixes with the air present in the room, thereby bringing down the temperature and humidity level so as to maintain comfortable conditions. The refrigerant vapour from the evaporator is compressed in the compressor to high pressure and temperature. The high pressure vapour then enters the condenser where it is cooled by the outside atmospheric air circulated by a fan. The high pressure refrigerant is reduced to low pressure by passing it through an expansion valve. The low pressure ad temperature refrigerant again enters the evaporator where it absorbs heat from the room to be cooled and cycle continues. Desired temperature in the room can be maintained by controlling the operator of the compressor with the help of a thermostat. The heater and humidifiers may also be provided in the unit to achieve the required temperature and humidity level.

Nagarjuna College of Engineering & Technology, Bangalore