Set No: 1 III B.Tech. II Semester Supplementary Examinations, April/May 2013 REFRIGERATION AND AIR CONDITIONING (Mechanical Engineering) Time: 3 Hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks Note: The following Tables are permitted Refrigeration and Psychrometric Charts, Steam Tables 1. a) What are the advantages of dense air refrigeration system over an open air refrigerating system? b) A dense air refrigeration system of 10 tonnes capacity works between 4 bar and 16 bar. The air leaves the cold chamber at 0 0 C and discharges at 25 0 C to the expansion cylinder after air cooler. The expansion and compression cylinders are double acting. The mechanical efficiency of compressor and expander are 85% and 80% respectively. The compressor speed is 250 rpm and has a stroke of 250 mm. Determine: (i) C.O.P. (ii) Power required, and (iii) Bore of compression and expansion cylinders. Assume isentropic compression and expansion as polytropic with n = 1.25 2. a) Sketch the T-s and p-h diagrams for the vapour compression refrigeration cycle when the vapour after compression is dry saturated. b) A refrigeration plant of 8 TR capacity has its evaporation temperature of -8 0 C and condenser temperature of 30 0 C. The refrigerant is sub cooled by 5 0 C before entering into the expansion valve and vapour is superheated by 6 0 C before leaving the refrigerator. The suction pressure drop is 0.2 bar in the suction valve and discharge pressure drop is 0.1 bar in the discharge valve. If the refrigerate used as R 12 find out the C.O.P. of the plant and theoretical power required for the compressor. Assume compression is isentropic. Use p h chart for calculation 3. a) Write short note on: (i) Ozone depletion (ii) Global warming b) A two cylinder R 12 reciprocating compressor has a bore 80 mm and stroke 60 mm. The compressor runs at 1450 rpm. The condensing temperature is 40 0 C. Find the mass of refrigerant circulated per minute and refrigerating capacity of the compressor when evaporating temperature is (i) 5 0 C and (ii) 0 0 C. Neglect clearance. 4. a) Draw a neat diagram of lithium bromide water absorption system and explain its working. List the major field application of this system. b) In absorption refrigeration system, heating, cooling and refrigeration take place at the temperatures of 150 C, 30 C, and -20 C. (i) Find the theoretical COP of the system. (ii) If the heating temperature is increased to 200 C and refrigeration temperature is decreased to 40 C, find the percentage change in theoretical C.O.P 1 of 2
Set No: 1 5. a) Explain the various components of Steam Jet Refrigeration system and clearly discuss the function of each component. b) Define the figure of merit related to thermo electric refrigeration system and explain its effect on COP of the system. 6. a) Derive the relationship relative humidity (φ) and degree of saturation (µ) in the form φ p 1 1 s p Where p is atmospheric pressure and p s is the saturation pressure at temperature of the mixture b) The moist air enters a chamber at 5 0 C dry bulb temperature and 2.5 0 C wet bulb temperature at the rate of 90 m 3 /min. The barometric pressure is 1.01325 bar. While passing through the chamber, the air absorbs sensible heat at the rate of 40.7 kw and picks up 40 kg/h of saturated steam at 110 0 C. Determine the dry bulb and wet bulb temperatures of the leaving air. 7. a) Why ventilation is required? Explain why different ventilation standards for different purposes are recommended? b) The outdoor summer design condition for a bank for 100 persons at a place is 35 0 C dry bulb temperature and 24 0 C wet bulb temperature. The required inside conditions are 24 0 C dry bulb temperature and 50% RH. The room sensible heat gain is 58 kw and the room latent heat is 15 kw. The ventilation requirement per person is 0.54 m 3 /min. The by-pass factor is 0.15. Determine: (i) Grand total heat (ii) Effective sensible heat factor (iii) Apparatus dew point; and (iv) volume flow rate of dehumidified air 8. a) Explain why heat pump is most efficient when used for heating purposes? And draw the arrangement of heat pump when it is used for year round air conditioning b) Define the following (i) Fan total pressure (ii) Fan air power, and (iii) Fan total efficiency 2 of 2
Set No: 2 III B.Tech. II Semester Supplementary Examinations, April/May 2013 REFRIGERATION AND AIR CONDITIONING (Mechanical Engineering) Time: 3 Hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks Note: The following Tables are permitted Refrigeration and Psychrometric Charts Steam Tables 1. a) Discuss the applications of refrigeration and types of refrigeration? b) A simple evaporative air refrigeration system is used for an aeroplane to take 20 tonnes of refrigeration load. The ambient air conditions are 20 0 C and 0.9 bar. The ambient air is rammed isentropically to a pressure of 1 bar. The air leaving the main compressor at pressure 3.5 bar is first cooled in the heat exchanger having effectiveness of 0.6 and then in the evaporator where its temperature is reduced by 5 0 C. The air from the evaporator is passed through the cooling turbine and then it is supplied to the cabin which is to be maintained at a temperature of 25 0 C and at a pressure of 1.05 bar. If the internal efficiency of the compressor is 80% and that of cooling turbine is 75%, determine: (i) Mass of air bled off the main compressor; (ii) Power required for the refrigerating system; and (iii) C.O.P. of the refrigerating system 2. a) Explain the functions of parts of vapour compression system and discuss merits and demerits of vapour compression over air refrigeration system b) A vapour compression works on a simple saturation cycle with R 12 as the refrigerant which operates between the condenser temperature of 40 0 C and an evaporator temperature of -5 0 C. For the modified cycle, the evaporator temperature is changed to -10 0 C and other operating conditions are the same as the original cycle. Compare the power requirement for both cycles. Both systems develop 15 tonnes of refrigeration. 3. a) What are the advantages and disadvantages of centrifugal compressors over reciprocating compressors? b) Explain the working with neat diagram of following types of condensers (i) Shell and tube condenser (ii) Shell and coil condenser 4. a) Draw a neat line diagram of Electro - Lux refrigerator and explain its working principle. What is the important role of hydrogen in this refrigeration system? b) In a refrigeration system, the heating in generator is carried out by using steam at 3 bar absolute pressure and 85% dry. The refrigeration temperature is -10 0 C. The condensation of the refrigerant is carried out at 30 0 C using water. Determine (i) The maximum possible COP of the system. (ii) If the steam leaves the generator as saturated water at some pressure, determine the quantity of steam required to run plant of 20 tonnes refrigeration plant. Assume relative COP = 0.4 1 of 2
Set No: 2 5. a) Derive the formula for finding out the quantity of steam required per ton of refrigeration in case of steam refrigeration system b) Explain with the help of sketch the working of Ranque Hilsch tube 6. a) Define relative humidity, specific humidity, and dew point temperature and describe a theoretical method for determining their values b) Atmospheric air at a dry bulb temperature of 16 0 C and 25% relative humidity passes through a furnace and then through a humidifier, in such a way that the final dry bulb temperature is 30 0 C and 50% relative humidity. Find the heat and moisture added to the air. Also determine the sensible heat factor of the process. 7. a) Define the term Effective Temperature and explain its importance in air conditioning system b) An air conditioned apace is maintained at 27 0 C dry bulb temperature and 50% relative humidity. The ambient conditions are 40 0 C dry bulb temperature and 27 0 C wet bulb temperature. The space has a sensible heat gain of 14 kw. The air is supplied to the space at 7 0 C saturated. Calculate: (i) Mass of moist air supplied to the space in kg/h; (ii) Latent heat gain of space in kw; and (iii) Cooling load of air washer in kw if 30% percent of air supplied to the space is fresh, remainder being recirculated. 8. a) Discuss the common considerations for humidification of air in air conditioning b) What is the function of a fan in an air-conditioning system and compare the characteristics of forward curved and backward curved fans 2 of 2
Set No: 3 III B.Tech. II Semester Supplementary Examinations, April/May 2013 REFRIGERATION AND AIR CONDITIONING (Mechanical Engineering) Time: 3 Hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks Note: The following Tables are permitted Refrigeration and Psychrometric Charts, Steam Tables 1. a) Explain with suitable sketch the Bootstrap air refrigeration system b) An open air-refrigeration machine works between 1 bar and 10 bar. It produces 20 tons of refrigeration. Temperature of air leaving the cooler and refrigerators are 300 K and 250 K. Assuming expansion and compression processes follow the law pv 1.38 = constant, find (i) COP of system (ii) HP per ton 2. a) How does an actual vapour compression cycle differ from that of theoretical cycle? b) A vapour compression plant using R-12 operates between 35 0 C condensing temperature and 5 0 C evaporation temperature with saturated vapour leaving the evaporator. The plant consists of twin cylinder, single acting compressor with 100 mm diameter and 120 mm stroke running at 300 rpm. The volumetric efficiency is 85% and the mechanical efficiency is 90%. Assuming isentropic compression, determine: (i) C.O.P. of system (ii) Power required; and (iii) Tonnage capacity of the plant 3. a) Write a short note on methods of defrosting b) A single stage, single acting reciprocating compressor has a bore of 200 mm and a stroke of 300 mm. It receives vapour refrigerant at 1 bar and delivers it at 5.5 bar. If the compression and expansion follows the law pv 1.3 =constant and clearance volume is 5% of the stroke volume, determine: (i) The power required to drive the compressor, if it runs at 500 rpm; and (ii) The volumetric efficiency of the compressor 4. a) Explain Electrolux refrigeration system. How the system is operated to obtain different pressures in the cycle without pump b) In a vapour absorption system, the heat is supplied to the generator by condensing steam at 3 bar and 85% dry. The temperature in the evaporator is to be maintained at -10 0 C. If the cooling water rejects heat at 30 0 C in the condenser, find the maximum C.O.P. of the system. When the refrigeration load is 10 tonnes and the actual C.O.P. is 40% of the maximum C.O.P., find the mass of steam required per hour 5. a) Under what situation is steam jet refrigeration system recommended. What are its limitations? Can it be used for obtaining sub-zero temperatures? b) Explain with the help of sketch the working of Vortex tube 1 of 2
Set No: 3 6. a) Explain various psychrometric processes? With a sketch Explain cooling with dehumidification process b) The pressure and temperature of a mixture of dry air and water vapour are 736 mm of Hg and 21 0 C. The dew point temperature of the mixture is 15 0 C. Determine the following using steam tables: (i) Partial pressure of water vapour (ii) Relative humidity (iii) Specific humidity (iv) Enthalpy of mixture per kg of dry air (v) Specific volume of the mixture per kg of dry air. 7. a) Discuss the factors that determine human comfort b) A room has a sensible heat gain of 24 kw and a latent heat gain of 5.2 kw and it has to be maintained at 26 0 C DBT and 50% RH. 180 m 3 /min of air is delivered to the room. Determine the state of supply of air. 8. a) Discuss about the working of ceiling diffusers and their types with a neat sketches b) Explain the various types of axial flow fans 2 of 2
Set No: 4 III B.Tech. II Semester Supplementary Examinations, April/May 2013 REFRIGERATION AND AIR CONDITIONING (Mechanical Engineering) Time: 3 Hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks Note: The following Tables are permitted Refrigeration and Psychrometric Charts, Steam Tables 1. a) Why artificial cooling is required in aero planes? And what are the different systems used for cooling the aero planes? b) A cold storage plant is required to store 20 tonnes of fish. The fish is supplied at a temperature of 30 0 C. The specific heat of fish above freezing point is 2.93 kj/kg-k. The specific heat of fish below freezing point is 1.26 kj/kg-k. The fish is stored in cold storage which is maintained at -8 0 C. The freezing point of fish is -4 0 C. The latent heat of fish is 235 kj/kg. If the plant requires 75 kw to drive it, find: (i) The capacity of the plant, and (ii) Time taken to achieve cooling. Assume actual C.O.P. of the plant as 0.3 of the Carnot C.O.P 2. a) Why is Reversed Carnot cycle not a practical cycle? Explain the modifications done to reversed Carnot cycle in the Reversed Brayton cycle. b) A refrigeration plant of 8 TR capacity has its evaporation temperature of -8 0 C and condenser temperature of 30 0 C. The refrigerant is sub cooled by 5 0 C beore entering into the expansion valve and vapour is superheated by 6 0 C beore leaving the refrigerator. The suction pressure drop is 0.2 bar in the suction valve and discharge pressure drop is 0.1 bar in the discharge valve. If the refrigerant used as R 12 find out the C.O.P. of the plant and theoretical power required for the compressor. Assume compression is isentropic. Use p h chart for calculation 3. a) Derive an expression for clearance volumetric efficiency of reciprocating compressor and also discuss the factors affecting the volumetric efficiency b) Describe, with neat sketches, the working of (i) Shell and coil condenser; and (ii) Shell and tube condenser 4. a) What is the function of the following components in an absorption system: (i) Absorber (ii) Rectifier b) In an absorption type refrigerator, the heat is supplied to NH 3 generator by condensing steam at 2 bar and 90% dry. The temperature in the refrigerator is to be maintained at 5 0 C. Find the maximum C.O.P. possible. If the refrigeration load is 20 tonnes and actual C.O.P. is 70% of the maximum C.O.P., find the mass of steam required per hour. Take temperature of the atmosphere as 30 0 C 1 of 2
Set No: 4 5. a) The steam jet refrigeration system is usually not economical above 35 0 C condensing temperatures, discuss the statement. b) Derive the formula for finding out the quantity of steam required per ton of refrigeration in case of steam refrigeration system 6. a) Define the following: (i) Specific humidity (ii) Absolute humidity (iii) Relative humidity (iv) Dew Point Temperature b) Air at 40 0 C dry bulb temperature and 15% relative humidity is passed through the adiabatic humidifier at the rate of 200 m 3 /min. The outlet conditions of air are 25 0 C dry bulb temperature and 20 0 C wet bulb temperature. Find (i) Dew point temperature; (ii) Relative humidity of exit air; and (iii)amount of water vapour added to the air per minute 7. a) Distinguish clearly between heat stroke, heat exhaustion and heat cramp b) The following data refer to air conditioning of a public hall: Outdoor conditions = 40 0 C DBT, 20 0 C WBT Required comfort conditions = 20 0 C DBT, 50% RH Seating capacity of hall = 1000 Amount of outdoor air supplied = 0.3 m 3 /min/person If the required condition is achieved first by adiabatic humidifying and then cooling, find: (i) The capacity of the cooling coil and surface temperature of the coil if the by-pass factor is 0.25; and (ii) The capacity of the humidifier and its efficiency 8. a) Define specific speed for a centrifugal fan. Derive its expression b) What are the different methods of humidifying the air? Explain the working of any one of the atomizing the water type humidifier. 2 of 2