DHANALAKSHMI COLLEGE OF ENGINEERING Chennai 601 301. BASIC CIVIL AND MECHANICAL ENGINEERING UNIT V REFRIGERATION AND AIRCONDITIONING PART A 1. Define refrigeration. It is a science of providing and maintaining the temperature below that of the surrounding temperature. 2. Define Ton of Refrigeration Ton of refrigeration is defined as the quantity of heat to be extracted to produce one ton of ice at 0o C, within 24 hours when the initial condition of water is also at 0oC. In SI units 1 TR is equivalent to 210 kJ/in or 3.5 kW. 3. Define Coefficient of Performance (COP). Coefficient of performance is defined as the ratio between the quantity of heat extracted to Work done by the system. 4. What are the various applications of refrigeration? (i) Used in water coolers for supplying cold water. (ii)For preservation of food, vegetables, milk, ice cream etc., refrigeration system is used. (iii) For preservation of perishables like fish, mutton, chicken etc., 5. What are the various properties of refrigerants? (i) Low boiling point, low freezing point, high latent heat of evaporation. (ii) Low specific heat and low viscosity. (iii) Refrigerants should be easy to liquefy. (iv) Chemical stability (v) Non flammable.
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6. Differentiate between vapour compression refrigeration and vapour absorption refrigeration system. S.No.
Vapour compression refrigeration system
1 2 3 4 5
Smaller in use COP is higher System produces noise Maintenance cost is high Refrigerant used is R134 a
Vapour absorption refrigeration system Very large in size COP is lesser Silent in operation Maintenance cost is low Refrigerant used is ammonia
7. What are the various applications of air conditioning? (i) Airconditioning of houses, hotels, theatres etc., (ii) For comfort of passengers in cars, buses, trains, ships and aircrafts. (iii) Airconditioning is used textile industries, printing and machine tools etc. 8. Define the following. (i) Dry air (ii) Moist air (iii) Dry bulb temperature (iv) Wet bulb temperature. (i) Dry air : Air without water vapour or moisture (ii) Moist air : It is a mixture of dry air and moisture (iii) Dry bulb temperature : Actual temperature of a gas, measured by a standard mercury thermometer (iv) Wet bulb temperature : The temperature measured by a mercury thermometer, when the bulb is covered by a moistened cloth. 9. What are the various applications of centralized air conditioning? The central air conditioning system is adopted for large buildings, hotels, hospitals, cinema theatres etc. This sytem is used only for heavy loads of about 20 tons or more. 10. What is meant by dry ice refrigeration? Dry ice is solid carbon dioxide. It may be pressed into various sizes and shapes, blocks, or slabs. As it absorbs heat, it changes directly from a solid to a vapor. It does not go through the liquid state. This change from solid to vapor is called sublimation. At atmospheric pressure, solid carbon dioxide vaporizes at -109'F (-78°C).
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PART B 1. With a neat layout briefly explain the construction and working principle of a vapour compression refrigeration system.
Construction : Vapour compression refrigeration system contains a reciprocating vapour compressor, condenser, capillary tube (expansion device) and evaporator. Reciprocating vapour compressor compresses the incoming low pressure vapour refrigerant into high pressure vapour refrigerant. The function of an expansion device (capillary tube) is to reduce the pressure of high pressure liquid refrigerant into low pressure liquid refrigerant. Condenser and evaporator are the devices in which the phase of the refrigerant occurs. Working : 1. The low pressure refrigerant vapour coming out of the evaporator flows into the compressor. 2. The compressor is driven by a prime mover. 3. In the compressor the refrigerant vapour is compressed. 4. The high pressure refrigerant vapour from the compressor is then passed through the condenser. 5. The refrigerant gives out the heat it had taken in the evaporator (N) 6. The heat equivalent of work done on it (w) on the compressor. 7. This heat is carried by condenser medium which may be air or water. 8. The high pressure liquid refrigerant then enters the expansion valve. 9. This valve allows the high pressure liquid refrigerant to flow at a controlled rate into the evaporator. 10. While passing though this valve the liquid partially evaporates. Download this support notes from : www.arpradeep.tk
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11. Most of the refrigerant is vapourised only in the evaporator, at a low pressure. 12. In the evaporator the liquid refrigerant absorbs its latent heat of vapourisation from the material which is to be cooled. 13. Thus the refrigerating effect (N) is obtained. 14. Then the low pressure refrigerant enters the compressor and the cycle is repeated
2. With a neat layout, briefly explain about the construction and working principle of a vapour absorption refrigeration system.
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Construction :
The vapour absorption system consists of a condenser, an expansion valve and an evaporator. They perform the same as they do in vapour compression method. In addition to these, this system has an absorber, a heat exchanger, an analyser and a rectifier
Working : Dry ammonia vapour at low pressure passes in to the absorber from the evaporator. In the absorber the dry ammonia vapour is dissolved in cold water and strong solution of ammonia is formed. Heat evolved during the absorption of ammonia is removed by circulating cold water through the coils kept in the absorber. The highly concentrated ammonia (known as Aqua Ammonia) is then pumped by a pump to generator through a heat exchanger. In the heat exchanger the strong ammonia solution is heated by the hot weak solution returning from the generator to the absorber. In the generator the warm solution is further heated by steam coils, gas or electricity and the ammonia vapour is driven out of solution. The boiling point of ammonia is less than that of water. Hence the vapours leaving the generator are mainly of ammonia. The weak ammonia solution is left in the generator is called weak aqua. This weak solution is returned to the absorber through the heat exchanger. Ammonia vapours leaving the generator may contain some water vapour. If this water vapour is allowed to the condenser and expansion valve, it may freeze resulting in chocked flow. Analyser and rectifiers are incorporated in the system before condenser. The ammonia vapour from the generator passes through a series of trays in the analyser and ammonia is separated from water vapour. The separated water vapour returned to generator. Then the ammonia vapour passes through a rectifier. The rectifier resembles a condenser and water vapour still present in ammonia vapour condenses and the condensate is returned to analyser. The virtually pure ammonia vapour then passes through the condenser. The latent heat of ammonia vapour is rejected to the cooling water circulated through the condenser and the ammonia vapour is condensed to liquid ammonia. The high pressure liquid ammonia is throttled by an expansion valve or throttle valve. This reduces the high temperature of the liquid ammonia to a low value and liquid ammonia partly evaporates. Then this is led to the evaporator. In the evaporator the liquid fully vaporizes. The latent heat of evaporation is obtained from the brine or other body which is being cooled. The low pressure ammonia vapour leaving the evaporator again enters the absorber and the cycle is completed. This cycle is repeated again to provide the refrigerating effect.
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3. Differentiate between vapour compression refrigeration system and vapour absorption refrigeration system. S.No. 1.
2. 3.
Vapour compression system This system has more wear and tear and produces more noise due to the moving parts of the compressor. Electric power is needed to drive the system. Capacity of the system drops rapidly with lowered evaporator pressure.
4.
At partial loads performance is poor.
5.
Mechanical energy is supplied through compressor. Energy supplied is ¼ to ½ of the refrigerating effect. Charging of the refrigerating to the system is easy. Preventive measure is needed, since liquid refrigerant accumulated in the cylinder may damage to the cylinder.
6. 7. 8.
Vapour absorption system Only moving part in this system is an aqua pump. Hence the quieter in operation and less wear and tear Waste of exhaust steam may be used. No need of electric power. Capacity of the system decreases with the lowered evaporative pressure, by increasing the steam pressure in generator. At partial loads performance is not affected. Heat energy is utilized. Energy supplied is about one and half times the refrigerating effect. Charging of refrigerant is difficult. Liquid refrigerant has no bad effect on the system.
4. Draw the neat diagram of a window air conditioner and explain its working.
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Working : The low pressure vapour refigerant from the evaporator is sucked by compressor through the open inlet valve. The compressor compresses the vapour refrigerant. The high pressure and high temperature vapour refrigerant then flows to the condenser through the open outlet valve. In the condenser, the outside atmospheric temperature in summer being around 42o C, air is circulated by fan. After condensation, the high pressure liquid refrigerant formed passes through an expansion valve which reduces its pressure The low pressure refrigerant then enters the evaporator and evaporates, thus absorbing latent heat of vapourisation from the room air. The equipment which is used for evaporating the refrigerant is called evaporator. After evaporation, the refrigerant becomes vapour. The low pressure vapour is again passed to the compressor. Thus the cycle is repeated. A partition separates high temperature side of condenser, compressor and low temperature side of evaporator The quantity of air circulated can be controlled by the dampers. The moisture in the air passing over the evaporator coil is dehumidified and drips into the trays. The unit automatically stops when the required temperature is reached in the room. This is accomplished by the thermostat and control panel. 5. State the merits and demerits of a window air conditioner. Merits : A separate temperature control is provided in each room. Ducts are not required for distribution. Cost is less. Skilled technician is required for installation. Demerits: It makes noise. Large hole is made in the external wall or a large opening to be created in the window panel. This leads to insecurity to inmates. Air quantity cannot be varied.
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6. Draw the neat sketch, briefly explain the function of indoor unit and outdoor unit of a split type air conditioner.
In split air type air conditioner noise making components like compressor and condenser are mounted outside or away from room. Split type air conditioning system has two main components. (i) Outdoor Unit (ii) Indoor unit. The outdoor unit consists of compressor and condenser. The indoor unit consists of power cables, refrigerant tube and an evaporator mounted inside the room. Compressor is used to compress the refrigerant. The refrigerant moves between the evaporator and condenser through the circuit of tubing and fins in the coils. The evaporator and condenser are usually made of coil of copper tubes and surrounded by aluminium fins. The liquid refrigerant coming from the condenser evaporates in the indoor evaporator coil. During this process the heat is removed from the indoor unit air and thus, the room is cooled. Air return grid takes in the indoor air. Water is dehumidified out of air is drained through the drain pipe.
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The hot refrigerant vapour is passed to the compressor and then to the condenser where it becomes liquid. Thus the cycle is repeated. A thermostat is used to keep the room at a constant, comfortable temperature avoiding the frequent turning on off. 7. State the merits and demerits of a split type air conditioner. Merits : It is compact Upto four indoor AHU’s may be connected to one outdoor unit. It is energy and money saving. Duct is not used. Easier to install. It is noiseless, because rotary air compressor used is, kept outside. It is more efficient and powerful. It has the flexibility for zoning. Demerits : Initial cost is higher than window air conditioner Skilled technician is required for installation. Each zone or room requires thermostat to control the air cooling 8. Write briefly about comfort air conditioning. Due to the natural phenomenon of body heat disposal by evaporation of moisture from the human body and inflow of moisture from other sources, the humidity inside the room increases. The increased humidity causes difficulty in disposing of body heat. Also, the room temperature rises due to the heat dissipated from the human body and heat gains from light source and any other equipments. When the room temperature is high, it causes human discomfort. It has been found that for human comfort we need a dry bulb temperature of 20o C and 25o C and relative humidity of 60 percent in the room. Any air conditioning system should primarily be able to achieve the above conditions inside the room. References : www.phac-aspc.gc.ca/.../section3-eng.php Shanmugam G and Palanichamy M S, “Basic Civil and Mechanical Engineering”,Tata McGraw Hill Publishing Co., New Delhi, (1996). Venugopal K and Prahu Raja V, “Basic Mechanical Engineering”, Anuradha Publishers, Kumbakonam, (2000). Prepared by
A.R. pradeep Kumar Associate Professor/Mechanical Email :
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