ME6404 THERMAL ENGINEERING
LTPC 3104 UNIT I GAS POWER CYCLES 12 Otto, Diesel, Dual, Brayton cycles, Calculation of mean effective pressure, and air Standard efficiency - Actual and theoretical PV diagram of four stroke and two stroke engines. UNIT II INTERNAL COMBUSTION ENGINES 12 Classification - Components and their function - Valve timing diagram and port Timing diagram - Comparison of two stroke and four stroke engines – Carburetor system, Diesel pump and injector system. Performance calculation - Comparison of petrol and diesel engine - Lubrication system and Cooling system - Battery and Magneto Ignition System – Formation of exhaust emission in SI and CI engines. UNIT III STEAM NOZZLES AND TURBINES 12 Flow of steam through nozzles, shapes of nozzles, effect of friction, critical pressure ratio, supersaturated flow, Impulse and Reaction principles, compounding, Velocity diagram for simple and multi-stage turbines, speed regulations –Governors. UNIT IV AIR COMPRESSOR 12 Classification and working principle of various types of compressors, work of compression with and without clearance, Volumetric efficiency, Isothermal efficiency and Isentropic efficiency of reciprocating compressors, Multistage air compressor and inter cooling –work of multistage air compressor. UNIT V REFRIGERATION AND AIR CONDITIONING 12 Vapour compression refrigeration cycle- super heat, sub cooling – Performance calculations working principle of vapour absorption system, Ammonia –Water, Lithium bromide –water systems (Description only) - Alternate refrigerants – Comparison between vapour compression and absorption systems - Air conditioning system: Types, Working Principles - Psychrometry, Psychrometric chart - Cooling Load calculations - Concept of RSHF, GSHF, ESHF -(Use of standard thermodynamic tables, Mollier diagram, Psychrometric chart and refrigerant property tables are permitted in the examination)
TOTAL: 60 PERIODS TEXT BOOKS: 1. Sarkar, B.K,”Thermal Engineering” Tata McGraw-Hill Publishers, 2007 2. Kothandaraman.C.P., Domkundwar.S,Domkundwar. A.V., “A course in thermal engineering,”Dhanpat Rai &sons ,Fifth edition, 2002 REFERENCES: 1. Rajput. R. K., “Thermal Engineering” S.Chand Publishers , 2000 2. Arora.C.P,”Refrigeration and Air Conditioning ,” Tata McGraw-Hill Publishers 1994 3. Ganesan V..” Internal Combustion Engines” , Third Edition, Tata Mcgraw-Hill 2007 4. Rudramoorthy, R, “Thermal Engineering “,Tata McGraw-Hill, New Delhi,
SUBJECT CODE: ME 6404 SUBJECT NAME: THERMAL ENGINEERING UNIT I GAS POWER CYCLES PART – A [1 MARK] 1. Otto cycle consists of sets of processes [a] adiabatic and constant volume. [b] adiabatic and constant pressure. [c] Isothermal and constant pressure. [d] Isothermal and constant volume. 2. Otto cycle is a theoretical cycle, on which [a] only petrol engine run. [b] only diesel engine run. [c] only gas engines run. [d] petrol and gas engine runs. 3. Diesel cycle consists of [a] two adiabatic and two constant volume process. [b] two adiabatic and two constant pressure processes. [c] two adiabatic, one constant pressure and one constant volume processes. [c] two isothermal, one constant pressure and one constant volume processes. 4. For the same compression ratio, the efficiency of diesel cycle as compared to Otto cycle is [a] less [b] more [c] equal [d] none of the above. 5. The efficiency of diesel cycle approaches Otto cycle efficiency with [a] increase in cut-off. [b] decrease in cut-off. [c] zero cut-off. [d] constant cut-off. 6. With decrease in cut-off, the efficiency of diesel cycle [a] increases [b] decreases [c] remains constant [d] none of the above. 7. Brayton cycle consists of sets of processes [a] isentropics and constant volume. [b] isentropics and constant pressure. [c] isothermal and constant pressure. [d] isothermal and constant volume. 8. Brayton cycle can not be used in reciprocating engines for same adiabatic compression ratio and work output because [a] it requires large air-fuel ratio. [b] it is less efficient. [c] large volume of low pressure air cannot be efficiently handled. [d] all of the above. 9. The cycle generally used for gas turbines is [a] Rankine cycle. [b] Carnot cycle. [c] Otto cycle. [d] Brayton cycle. 10. The duel combustion cycle consists of two adiabatic processes and [a] two constant volume and one constant pressure processes. [b] one constant volume and two constant pressure processes. [c] one constant volume and one constant pressure processes. [d] two constant volume and two constant pressure processes. 11. For the same compression ratio, the efficiency of dual combustion cycle as compared to Otto cycle is [a] more [b] less [c] equal [d] none of the above. 12. For the same compression ratio, the efficiency of dual combustion cycle as compared to diesel cycle is [a] more [b] less [c] equal [d] none of the above. 13. The air standard diesel cycle is less efficient than the Otto cycle for the [a] same compression ratio and heat addition [b] same pressure and heat addition. 33
[c] same rpm and cylinder dimensions. 14.
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[d] same pressure and compression ratio.
With reference to air standard Otto and the same Diesel cycles, which of the following statements are true? [a] For a given compression ratio and the same state of air before compression. Diesel cycle is less efficient than on Otto cycle. [b] For a given compression ratio and the same state of air before compression. Diesel cycle is more efficient than on Otto cycle. [c] The efficiency of a diesel cycle increases with an increase in the cut-off ratio. [d] None of the above. 15. The Otto cycle operates with volumes of 40cm3 and 400 cm3 at top dead center [TDC] and bottom dead centre [BDC] respectively. If the power output is 100 kW, what is heat input, in kJ/s? Assume γ = 1.4 [a] 166 [b] 145 [c] 110 [d] 93 16. Thermal efficiency of a standard Otto cycle for a compression ratio 5.5, will be [a] 25% [b] 50% [c]75% [d]100% 17. Air standard efficiency of an I.C. engine depends on [a] speed [b] compression ratio [c] fuel [d] all of the above 18. Efficiency of a diesel cycle will approach to Otto cycle, when [a] diesel engine will operate at high speed [b] cut off period of diesel cycle is reduced to zero. [c] diesel fuel is balance with petrol. [d] none of these. 19. For same maximum pressure and temperature [a] Otto cycle is more efficient than diesel cycle. [b] Diesel cycle is more efficient than Otto cycle. [c] Dual cycle is more efficient than Otto and diesel cycles. [d] Dual cycle is less efficient than Otto and Diesel cycles. 20. For constant maximum pressure and heat input, the air standard efficiency of gas power cycles is in the order [a] Diesel cycle, Dual cycle, Otto cycle [b] Otto cycle, Diesel cycle, Dual cycle [c] Dual cycle, Otto cycle, Diesel cycle [d] Diesel cycle, Otto cycle, Dual cycle. ANSWER 2 3 4 d
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PART B 21.
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[2 MARKS]
Define mean effective pressure of Otto cycle. [AU., May/Jun. 2011,2012] The mean pressure developed during one cycle of operation is called as mean effective pressure. In other words it is the ratio of work done to the swept volume. Mean Effective Pressure {Pm] = Work done/Swept volume. Mention the thermodynamic processes involved in Diesel cycle. [AU., May/Jun. 44
2011] 1. Isentropic compression. 2. Constant pressure heat addition. 3. Isentropic expansion, and 4. Constant volume heat rejection. 23. What is a thermodynamic cycle? [OCT’ 97] Thermodynamic cycle is defined as the series of processes performed on the system, so that the system attains its original state. 24. What are the assumptions made for air standard cycle analysis? [Anna Univ. Nov’02, May ’03, Apr’05 & May/June 2013] 1. The working medium is a perfect gas throughout i.e., it follows the law рν =mRT 2. The working medium does not undergo any chemical change throughout the cycle. 3. The compression and expansion processes are reversible adiabatic i.e., There are no loss or gain of entropy. 4. Kinetic and potential energies of the working fluid are neglected. 5. The operation of the engine is frictionless. 6. Heat is supplied and rejected in a reversible manner. 25. Mention the various processes of the Brayton cycle. [OCT’ 96] 1. Isentropic compression, 2. Constant pressure heat supplied, 3. Isentropic expansion, and 4. Constant pressure heat rejection. 26. Mention the various processes of dual cycle. [APR’96] 1. Isentropic compression, 2. Constant volume heat addition, 3. Constant pressure heat addition, 4. Isentropic expansion, and 5. Constant volume heat rejection. 27. Define air standard cycle efficiency. [OCT’96, & OCT’97] Air standard efficiency is defined as the ratio of work done by the cycle to the heat supplied to the cycle. 28. Which cycle is more efficient with respect to the same compression ratio? [OCT’95] For the same compression ratio, Otto cycle is more efficient than diesel cycle. 29. For the same compression ratio and heat supplied, state the order of decreasing air standard efficiency of Otto, diesel and dual cycle. [APR’98, OCT’98] ή Otto > ή Dual > ή Diesel 30. Name the factors that affect air standard efficiency of Diesel cycle. [APR’97] Compression ratio and cut-off ratio 31. What is the effect of cut-off ratio on the efficiency of diesel cycle when the compression ratio is kept constant? [Anna Univ. APR’03] When cut-off ratio of diesel cycle increases, the efficiency of cycle is decreased when compression ratio is kept constant and vice versa. 32. What are all the modifications are carried out in Brayton cycle? Why? In Brayton we incorporate [i] Regenerator [ii] Reheater and [iii] Intercooler, because of increasing thermal efficiency of the cycle. PART C [16 MARKS] 33. A Six-cylinder petrol engine has a compression ratio of 5:1. The clearance volume of each cylinder is 110CC. It operates on the four-stroke constant volume cycle and the indicated efficiency ratio referred to air standard efficiency is 0.56. At the speed of 2400rpm, it 55
consumes 10kg if fuel per hour. The calorific value of fuel is 44000KJ/kg. Determine the average indicated mean effective pressure. [APR’95] 34. An air standard dual cycle has a compression ratio of 16 and compression begins at 1bar and 0 50 C. The maximum pressure is 70bar. The heat transferred to air at constant pressure is equal to heat transferred at constant volume. Find the temperature at all cardinal points, cycle efficiency and mean effective pressure. Take Cp = 1.005kJ/kgK; Cv = 0.718kJ/kgK. [MAY’03] 35. In an oil engine working on dual cycle, the heat supplied at constant pressure is twice that of heat supplied at constant volume. The compression and expansion ratios are 8 and 5.3. The 0 pressure and temperature at the beginning of cycle are 0.93 bar and 27 C. Find the efficiency of the cycle and mean effective pressure. Take Cp = 1.005kJ/kgK; Cv = 0.718kJ/kgK. [Anna Univ. May/June 2013] 0 36. The pressure, temperature and volume of air at the beginning of dual cycle are 1.03bar, 35 C and 150liters respectively. The volume after compression is 10liters 42kJ of heat is added to constant volume and 63kJ at constant pressure. Determine air standard efficiency, clearance and cut off percentages. [APR’98] 0 37. In a Brayton cycle, the air enters the compressor at 1 bar and 25 C. The pressure of air 0 leaving the compressor is 3 bar and temperature at turbine inlet is 650 C. Determine per kg of air [i] Cycle efficiency [ii] Heat supplied to air [iii] Work input [iv] Heat rejected in the cooler and [v] Temperature of air leaving the turbine. [MAY’03] 38. In an air standard diesel cycle, the pressure and volume at the beginning of compression are 3 100kPa and 0.03m respectively. Pressure after Isentropic compression is 4.2MPa and after isentropic expansion is 200kPa. Determine [i] Compression ratio [ii] Cut-off ratio [iii] Expansion ratio and [iv] Cycle efficiency. Assume γ= 1.4, Cv = 0.718kJ/kgK. [NOV’04] 39. Consider a stationary power plant operating on an ideal Brayton cycle. The pressure ratio of 0 the cycle is 8 and the gas temperature at the compressor inlet and turbine inlet are 27 C & 0 1027 C respectively. Determine the following: [i] Gas temperature at the compressor and turbine exit, [ii] Back work ratio, and [iii] Thermal efficiency. Assume pr1 = 1.386 and pr3 = 330.9. Where pr is the relative pressure. [APR’05] 40. Explain a Diesel cycle. [Anna Univ. Apr. 05] 41. Derive an expression for the thermal efficiency of an ideal diesel cycle. [Anna Univ. Apr. 05] 42. Show the dual cycle on p-v and T-s diagrams and derive and expression for its efficiency. 43. What are the differences between Otto, diesel and dual cycles? 44. Show that the efficiency of the diesel cycle is always lower than the efficiency of the Otto cycle for same compression ratio. 45. Derive an expression for air standard efficiency of Brayton cycle in terms of [i] compression ratio and [ii] the pressure ratio. 46. Why the Brayton cycle is most suitable for gas turbine power plant? 47. Derive an expression for air the air standard efficiency of diesel cycle. Explain why the efficiency of Otto cycle is more than that of the diesel cycle for the same compression ratio. [Anna Univ. May/June 2013] 48. [a] The maximum and minimum temperature of an otto cycle are limited to 1200 K and 300 K. Find the maximum work done by the cycle per kg of fluid if air is used as working fluid. Find also the air standard efficiency. [8] [b] Discuss in detail the actual pv diagram of a two stroke SI engine. [8] [AU., Nov./Dec. 2010] 49. [a] The mean effective pressure of an ideal diesel cycle is 8 bar. The initial pressure is 1 bar and compression ratio is 12. Determine the cut off ratio and air standard efficiency of the 66
cycle. [8] [b] Discuss the effects of operating variables on cycle analysis. [8] [AU., Nov./Dec. 2010] 50. The minimum pressure and temperature in an Otto cycle are 100 Kpa and 27°C. The amount of heat added to the air per cycle is 1500 KJ/kg. [a] Determine the pressure and temperature at all points of the air standard Otto cycle. [b] Also calculate the specific work and thermal efficiency of the cycle for a compression ratio of 8:1. Take for air: Cv = 0.72 KJ/kg K and γ = 1.4. [AU., May/June 2011] 51. An IC engine operating on the dual cycle the temperature of the working fluid [air] at the beginning of compression is 27°C. The ratio of the maximum and minimum pressure of the cycle is 70 and compression ratio is 15. The amounts of heat added at constant volume and constant pressure are equal. Compute the air standard thermal efficiency of cycle. State three main reasons why the actual thermal efficiency is different from the theoretical value. [AU., May/June 2011] UNIT II
INTERNAL COMBUSTION ENGINES
PART A [1 MARKS] 52. In internal combustion [I.C.] engines, combustion of fuel takes place. [a] outside the cylinder. [b] inside the cylinder. [c] not in the cyclinder. [d] none of the above. 53. In I.C. engines, the power developed inside the cylinder is known as [a] break horse power. [b] indicated horse power. [c] pumping power. [d] none of the above. 54. The power spend in suction and exhaust stroke is known as [a] break horse power. [b] indicated horse power. [c] net indicated horse power. [d] pumping power. 55. The power available at the shaft of an I.C. engine is known as [a] break horse power. [b] indicated horse power. [c] net indicated horse power. [d] pumping power. 56. In a four stroke cycle engine, the operations are in sequence [a] suction, compression, expansion and exhaust. [b] suction, expansion, compression and exhaust. [c] expansion, compression, suction and exhaust. [d] compression, expansion, suction and exhaust. 57. In a four stroke cycle, the minimum temperature inside the engine cylinder occurs at the [a] beginning of suction stroke. [b] end of suction stroke. [c] beginning of exhaust stroke. [d] end of exhaust stroke. 58. Fly wheel used in two stroke cycle engine as compared to four stroke cycle engine is [a] heavier [b] lighter [c] same in weight [d] none of the above. 59. Thermal efficiency of a two stroke cycle engine as compare to four stroke cycle engine is [a] more [b] less [c] same [d] none of the above. 60. Two stroke cycle engine uses [a] petrol and air [b] diesel and air [c] same [d] both [a] and [b] 61. Stoichiometric ratio is [a] chemically correct air-fuel ratio by weight. [b] chemically correct air-fuel ratio by volume. 77
[c] actual air-fuel ratio for maximum efficiency. [d] none of the above.
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62. An air-fuel ratio of 8:1 means [a] excess air [b] excess fuel. [c] chemically correct mixture. [d] none of the above. 63. The petrol engine works on [a] Otto cycle. [b] Joule cycle [c] Rankine cycle [d] Carnot cycle. 64. In a four stroke cycle petrol engine [a] the inlet valve opens at 20º before top dead centre and closes at 40º after bottom dead centre. [b] the compression starts at 40º after bottom dead centre and ends at 30º before top dead centre. [c] the charge is ignited at 30º before top dead centre. [d] all of the above. 65. In petrol engine, the charge is ignited with [a] spark plug. [b] Compression [c] both [a] and [b] above. [d] none of the above. 66. In a four stroke cycle petrol engine, the charge is compressed when [a] inlet valve is closed. [b] exit valve is closed. [c] both inlet and exit valves are closed. [d] both inlet and exit valves are open. 67. In petrol engines, the sparking increase [a] pressure [b] volume. [c] temperature of the products of combustion. [d] both [a] and [b] above [e] none of the above. 68. The ignition takes place due to heat produced in the engine cylinder at the end of compression, in [a] petrol engine [b] diesel engine [c] steam engine [d] none of the above. 69. In I.C. engine, removing the burnt gases from combustion chamber of engine cylinder is known as [a] scavenging [b] supercharging [c] detonation [d] polymerization. 70. Higher compression ratio in diesel engine results in [a] higher pressure [b] lower pressure [c] same pressure [d] none of the above. 71. In a spark ignition engine, supercharging [a] increases knocking tendency. [b] decreases knocking tendency. [c] does not effect knocking tendency. [d] none of the above. ANSWER 52 53 54 55 56 57 58 59 60 61 62 63 64
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PART B [2 MARKS] 72. Name the basic thermodynamic cycles of the two types of internal combustion reciprocating engines. [Anna Univ. Apr’03] Otto cycle in S.I. engines and diesel cycle in C.I. engines. 73. Define the terms as applied to reciprocating I.C. engines. “Mean effective pressure” and “Compression ratio”. [Anna Univ. Nov’04] Mean effective pressure: It is defined as the algebraic sum of the mean pressure acting on the piston during one complete cycle. Compression ratio: Same as previous question. 99
74. What is meant by highest useful compression ratio? [APR’97] The compression ratio which gives maximum efficiency is known as highest useful compression ratio. 75. Why compression ratio of petrol engines is low while diesel engines have high compression ratio? [[OCT’98] Since fire point of petrol is less as compared to diesel, petrol engine has low compression ratio. 76. Compare the thermal efficiency of petrol engines with diesel engines. Give reasons. [APR’00] Thermal efficiency of diesel engine is greater than petrol engine. This is due to high compression ratio. 77. Why the actual cycle efficiency is much lower than the air standard cycle efficiency? List and explain the major losses in an actual engine. [Anna Univ. Nov’03] Theoretically, the compression and expansion are followed adiabatically. Butt in actual cycle it is not so. Because of the heat and pressure losses are involved. Actual area on p-v diagram per cycle is less than theoretical because of lower pressure rise and pumping losses. Major losses in an actual engine: Heat rejected to the cooling water Heat carried away by exhaust gas Heat loss due to radiation. 78. What is splash lubrication? [AU., May/Jun. 2011] In this system, oil is stored in the crank case. A small scoop is attached with the big end of connecting rod. When the crank is rotated, the scoop dips in the oil and splashes the oil. The oil is splashed on cylinder wall, connecting rod ends and valve mechanisms. 79. Mention different types of fuel injection systems in C.I. Engines. [AU., May/Jun. 2011] a] Air injection system b] Airless or solid injection [i] Common rail system [ii] Individual pump system. 80. What do you mean by scavenging in I.C. Engines? [Anna Univ. Apr.’03 & May/June 2013] The process of removing the burnt gases from the combustion chamber of engine cylinder is known as Scavenging. 81. Define Cetane number. [Anna Univ. Apr.’03] The property that quantities the ignition delay is called as Cetane number, 82. What is the purpose of a thermostat in an engine cooling system? [Anna Univ. Apr.’03] A thermostat valve is used in the water-cooling system to regulate the circulation of water in system to maintain the normal working temperature of the engine parts during the different operating conditions. 83. Explain exhaust blow down in case of IC engines. [Anna Univ. Nov.’03] The opening of the exhaust valve during the expansion stroke itself, before the piston reaches BDC which enables the exhaust gas to leave under pressure is known as exhaust blow down.
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84. List out the effects of detonation. [Anna Univ. Nov/Dec’2011] The impact on the engine components and structures may cause failure and creates undesirable noise which is always objectionable. The lack of control of combustion process leads to pre ignition and local over-heating. Therefore, piston may be changed by overheating. The pressure differences in the combustion chamber cause the gas to vibrate and scrub the chamber walls causing increased loss of heat to the coolant. Detonation results in increased carbon deposits on the wall of the cylinder. Due to increase in the rate of heat transfer, the power output as well as efficiency of the engine will decrease. 85. What do we feel the necessity of cooling an IC engine? [Anna Univ. May/June’2012] 0 When the air-fuel mixture is ignited and combustion takes place at about 2500 C for producing power inside an engine the temperature of the cylinder, cylinder head, piston and value, continuous to raise when the engine runs. It these parts are not cooled by some means then they likely to get damaged and even melted. The piston may cease inside the cylinder. To prevent this, the temperature of the parts around the combustion, chamber is maintained 0 0 as 200 C to 250 C. Too much of cooling will lower the thermal efficiency of the engine. Hence the purpose of cooling is to keep the engine at its most efficient operating temperature at all engine speeds and all driving conditions. 86. What catalytic converter does? [Anna Univ. May/June 2013] The diesel engine catalytic convertor is a pure oxidation catalytic converter. It oxidizes HC and CO into water and CO2. It cannot reduce NO2. PART C [16 MARKS] 87. A simple jet carburetor is required to supply 5.5 kg of air per minute and 0.6 kg of fuel per 3 0 minute. The density of fuel is 750kg/m . The air is initially at 1 bar and 30 C. Calculate the throat diameter of the choke for a flow velocity of 95m/s. The velocity coefficient is taken as 0.78. If the pressure drop across the fuel metering orifice is 0.76 of that at the choke, calculate orifice diameter assuming Cdf = 0.62. 88. The throat diameter of a carburetor is 80mm and nozzle diameter is 6mm. The Cda = 0.85 and Cdf = 0.7. The nozzle lip is 6mm. The pressure difference causing the flow is 0.1bar. Find [a] Air-fuel ratio supplied by the carburetor neglecting nozzle lip. [b] Air-fuel ratio considering nozzle lip and [c] The minimum velocity of air required to start the fuel flow. 3 3 Neglect air-compressibility. Take ρa = 1.2kg/m and ρf = 750kg/m 89. Air fuel ratio of a mixture supplied to an engine by a carburetor is 13. The fuel consumption of the engine is 7.5kg/hr. The diameter of the venture is 20mm. Find the diameter of fuel 3 nozzle if the lip of the nozzle is 4mm. Take the following data: ρf = 750kg/m , Cda = 0.80, 0 Cdf = 0.7 and atmospheric pressure = 1.013bar and temperature = 27 C. 90. The venture of a simple carburetor has a throat diameter of 20mm and the coefficient of flow is 0.8. The fuel orifice has a diameter of 1.14mm and coefficient of fuel flow is 0.65. The gasoline surface is 5mm below the throat, calculate [i] The air-fuel ratio for a pressure drop of 0.08bar when the nozzle lip is neglected. [ii] The air fuel ratio when the nozzle lip is taken into account. [iii] The minimum velocity of air or critical air velocity required to start the fuel flow when the nozzle lip is provided. Assume the density of air and fuel to be 1.2kg/m3 and 750kg/m3 respectively. [Anna Univ. Nov.’03]
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91. A single cylinder four stroke diesel engine, having a swept volume of 750cm is tested at 300rpm. When a braking torque of 65N-m is applied, the mean effective pressure is 1100 2 kN/m . Calculate the brake power and mechanical efficiency of the engine. 92. In a laboratory experiment, the following observations were noted during the test of a four stroke S.I. engine. Area of Indicator diagram = 510mm2 Length of indicator diagram = 55mm Spring index = 1.25bar/mm Diameter of the piston = 120mm Length of the stroke = 180mm Engine rpm = 480rpm Effective brake load =25kg Effective brake radius = 0.45m Determine [i] Indicated m.e.p. [ii] Indicated power, [iii] Brake power and [iv] Mechanical efficiency. 93. A rope brake has a brake wheel diameter of 750mm and the diameter of the rope is 8mm. The dead load on the brake is 275N and spring balance reads 35N. If the engine rpm is 480, find the brake power developed. 94. A six-cylinder petrol engine has a compression ratio of 5 to 1. The clearance volume for each cylinder is 110CC. It operates on the four-stroke constant volume cycle and the indicated thermal efficiency ratio referred to the air standard cycle is 0.56. At a speed 2400rev/min it consumed 10kg of fuel per hour, the energy of combustion being 44MJ/kg. Determine the average indicated mean effective pressure in the cylinder. [MU, April 95] 95. A four cylinder diesel engine works on four-stroke cycle has a cylinder bore of 90mm and a stroke of 150mm. The crank speed is 370rpm, and fuel consumption is 15kg/hr, having a calorific value of 39000kJ/kg. The indicated mean effective pressure is 5bar. If the compression ratio is 14 and cut off ratio is 2.3. Calculate the relative efficiency. Taking γ = 1.4. 96. The following observations were taken during a test on a single cylinder four – stroke cycle engine having a bore of 300 mm and stroke of 450mm. 0 Ambient air temperature = 22 C Fuel Consumption = 11kg/hr, CV of fuel = 42000kJ/kg Engine speed = 300rpm Mean effective pressure = 6 bar Net brake load = 1.0 kN. Brake drum diameter =2m Quantity of Jacket cooling water = 590kg/hr 0 Temperature of entering cooling water = 22 C 0 Temperature of leaving cooling water = 70 C Quantity of air as measured = 225kg/hr Specific heat of exhaust gas = 1005kJ/kgK 0 Exhaust gas temperature = 405 C Rope diameter = 2 cm Determine indicated power, brake power, mechanical efficiency and draw a heat balance sheet on hour basis. [Anna Univ. May/June 2013] 97. Draw the typical port-timing diagram of a two stroke SI engine and explain the salient points. [Anna Univ. Apr.’03] 98. [i] Explain the construction and working of a fuel injector with a neat sketch [8].
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[ii] Draw and explain the Port Timing diagram of two stroke cycle diesel engine. [8] [Anna Univ. May/June 2013] 99. Explain how knocking takes place in diesel engines and discuss the various methods of controlling it. Compare the knocking in diesel engines with that of the petrol engines. [Anna Univ. Nov.’03] 100. Discuss the significance of various factors affecting flame speed in SI engines. [Anna Univ. Nov.’03]. 101. [a] Discuss in detail the various types of fuel supply systems of a IC engine. [8] [b] Compare and contrast petrol and diesel engine. [8] [AU. Nov/Dec. 2010] 102. [a] Discuss with neat sketches the various types of lubricating systems employed for an IC engine. [8] [b] The petrol used in a SI engine contains 85% C and 15% H2. The amount of air supplied per kg of fuel is 14 kg. Assume all H2 is burned, no carbon is deposited and exhaust does not contain free O2, find [i] mass of carbon burning to CO2, [ii] mass of each of the gases in the wet exhaust air contains 23% O2 and 77% N2 by mass. [8] [AU. Nov/Dec. 2010] 103. Explain why cooling is necessary in I.C. engine. With neat sketches describe the working of water cooling system used for multi-cylinder engine. Why should a should a pump and thermostat be provided in the cooling system on an engine? [AU. May/June 2011] 104. A six cylinder 4 stroke S.I. engine having a piston displacement of 700cm3 per cylinder developed 78 kW at 3200 r.p.m. and consumed 27 kg of petrol per hour. The calorific value of petrol is 44 MJ/kg. Estimate [a] The volumetric efficiency of the engine if the air – fuel ratio is 12 and intake air is at 0.9 bar, 32°C [b] The brake thermal efficiency [c] The brake torque. For air, R = 0.287 kJ/kg K. [AU. May/June 2011] UNIT III 105.
106. 107. 108.
STEAM NOZZLES AND TURBINES
PART A [1 MARK] When the steam flows through a nozzle a. pressure, velocity and temperature decreases b. pressure, velocity and temperature increases c. pressure and temperature decreases while velocity increases. d. none of the above. The expansion of steam in a nozzle follows a. Rankine cycle b. Carnot cycle c. Joule cycle d. Stirling cycle If the cross-section of the nozzle decreases continuously from entrance to exit, the nozzle is known as a. convergent nozzle b. divergent c. convergent-divergent d. none of the above, If the cross-section of the nozzle increases continuously from entrance to exit, the nozzle is known as a. convergent nozzle b. divergent nozzle c. convergent-divergent nozzle d. none of the above. 22 22
109. 110. 111. 112. 113.
114. 115. 116. 117. 118. 119. 120. 121. 122. 123. 124.
Steam enters the steam nozzle at a. Low pressure and low velocity. b. High pressure and high velocity. c. High pressure and low velocity. d. Low pressure and high velocity. The pressure, at which the steam leaves the nozzle is known as a. back pressure b. forward pressure c. steam pressure d. none of the above. Due to friction in the nozzle, dryness fraction of steam a. increases b. decreases c. remains same d. none of the above. Nozzle is designed for maximum a. discharge b. pressure at outlet c. both [a] and [b] above d. none of the above. A steam nozzle is said to be choked, when a. throat gets clogged b. pressure at the throat is less than the exit pressure. c. pressure at the throat is equal to the stream supply pressure. d. steam flow through nozzle ceases. Critical pressure is the pressure of steam at a. inlet of steam nozzle. b. throat of steam nozzle. c. exit of steam nozzle. d. none of the above. The action of steam used in steam turbines is a. static b. dynamic c. pressure d. both [a] and [b] above. The turbine blades are a. straight b. curved c. circular d. none of the above. Steam turbines as compared to reciprocating steam engines a. develops higher speeds b. has more efficiency c. consumes less steam d. all of the above. Steam turbines are used for a. electric generation b. direct drive of fans, compressors and pumps c. large marine propulsion d. all of the above. Internal loss in a steam turbine is a. nozzle loss b. blade friction loss c. wheel friction loss d. all of the above In an impulse turbine, steam expands a. fully in nozzle b. fully in blades c. partly in nozzle and partly in blades d. none of the above. In reaction turbine, pressure on the two sides of the moving blades a. increases b. decreases c. remains same d. none of the above For a given horsepower, an impulse turbine as compared to a reaction turbine has a. equal number of rows b. less number of rows c. more number of rows d. none of the above. In reaction turbine, a stage is represented by a. each row of blades b. number of casing c. number of steam exits d. none of the above. In pressure compounded impulse turbine, the ratio of blade velocity to steam velocity, a. increases b. decreases c. remains constant d. none of the above.
23 23
ANSWER 105 106
c
a
107 108
a
b
109 110 111
c
a
a
112
113
114
115
116
117
118
a
b
b
b
b
d
d
PART B 125.
126.
127.
128.
119 120
b
a
121
a
122 123 124
c
a
[2 MARKS]
What are the various types of nozzles and their functions? [Anna Univ. Apr.’04 & Apr.’05] Nozzle is a duct of varying cross-sectional area in which the velocity increases with the corresponding drop in pressure. 1. Convergent Nozzle In convergent nozzle, the cross sectional area decreases from the inlet section to the outlet section. 2. Divergent Nozzle In divergent nozzle, the cross sectional area increases from the inlet section to the outlet section. 3. Convergent-Divergent nozzle When the cross section of a nozzle first decreases form the inlet section to throat and then increases from its throat to outlet section. It is called a convergent-divergent nozzle. Define nozzle efficiency. [AU., May/June 2011, Nov/Dec’2011] Nozzle efficiency: It is defined as the ratio of actual enthalpy drop to the isentropic enthalpy drop. Nozzle efficiency =Actual enthalpy drop/Isentropic enthalpy drop. What are the effects of friction on the flow through a steam nozzle? [Anna Univ. Apr.’03 & Nov.’04] a. The expansion is no more isentropic and enthalpy drop is reduced thereby resulting in lower exit velocity. b. The final fraction of the steam is increased as the part of the kinetic energy gets converted into heat due to friction and absorbed by steam with in increase in enthalpy. c. The specific volume of steam is increased as the steam becomes drier due to this frictional reheating. Explain the phenomenon of super saturated expansion in steam nozzle. [Anna Univ. Apr.’03] [OR] What is metastable flow? [Anna Univ. Nov.’03, Nov/Dec.2010 & May/June 2013] When the supersaturated steam is expanded in the nozzle, the condensation should occur in the nozzle. Since the steam has a great velocity, the condensation does not take place at the expected rate. So the equilibrium between the liquid and vapour phase is delayed and the steam continues to expand in a dry state. The steam in such set of condition is said to be supersaturated or metastable flow.
24 24
c
129.
130.
131. Sl. No. 1. 2. 3. 4. 5. 132.
133. 134.
135.
What are the conditions that produce super saturation of steam in nozzles? [Anna Univ. Nov.’04] When the superheated steam expands in the nozzle, the condensation will occur in the nozzle. Since, the steam has more velocity; the condensation will not take place at the expected rate. So, the equilibrium between the liquid and vapour phase is delayed and the steam continues to expand in a dry state. The steam in such set of condition is said to be supersaturated or metastable flow. What are the effects of super saturation in a steam nozzle? [Anna Univ. Nov.’02] The following effects in a nozzle on steam, in which super saturation occurs, may be summarized as follows. [i] The dryness fraction of the steam is increased. [ii] Entropy and specific volume of the steam are increased. [iii] Exit velocity of the steam is reduced. What is the fundamental difference between the operation of impulse and reaction steam turbines? [Anna Univ. Nov.’03 & Apr.’04] Impulse Turbine Reaction turbine It consists of nozzles and moving It consists of fixed blades and moving blades blades. Pressure drop occurs only in nozzles Pressure drop occurs in fixed as well as not in moving blades. moving blades Steam strikes the blade with kinetic Steam passes over the moving blades energy. with pressure and kinetic energy. It has constant blade channels area. It has varying blade channels area. Due to more pressure drop per blade, Number of stages required is more due number of stages required is less. to more pressure drop. Explain the need of compounding in steam turbines. [Anna Univ. Apr.’03, May/June 2011] [OR] Explain the purpose of compounding in steam turbines. [Anna Univ. Nov.’04] In simple impulse turbine, the expansion of steam from the boiler pressure to condenser pressure takes place in a single stage turbine. The velocity of steam at the exit of turbine is very high. Hence, there is a considerable loss of kinetic energy [i.e. about 10 to 12%]. Also the speed of the rotor is very high [i.e. up to 30000rpm]. There are several methods of reducing this speed to lower value. Compounding is a method of absorbing the jet velocity in stages when the steam flows over moving blades. Define degree of reaction. [Anna Univ. Apr.’04] It is defined as the ratio of isentropic heat drop in the moving blades to isentropic heat drop in the entire stage of the reaction turbine. What for a governor is used? [AU. Nov/Dec. 2010] The governor is used to regulate the supply of steam to the turbine in such a way that the speed of the turbine is maintained as for as possible a constant under varying load conditions. What are the different methods of governing steam turbines? [Anna Univ. Nov.’04 & May/June 2013] a. Throttle governing 25 25
136.
137.
b. Nozzle control governing c. By-pass governing d. Combination of throttle and nozzle governing or throttle and by-pass governing. What are the different losses involved in steam turbines? [Anna Univ. May/June’2012] 1. Losses in regulating valves, 2. Losses due to steam friction, 3. Losses due to mechanical friction, 4. Losses due to leakage, 5. Residual velocity losses, 6. Carry over losses, 7. Losses due to wetness of steam, and 8. Losses due to radiation. What are advantages of velocity compound impulse turbine. Anna. Univ.Nov/Dec’2011] a. It’s initial cost is less because of few numbers of stages. b. Less space is required. c. The system is reliable and easy to start. d. There is need of strong casing due to low pressure. PART C
[16 MARKS]
138. Steam at 10.5bar and 0.95dryness is expanded through a convergent divergent nozzle. The pressure of steam leaving the nozzle is 0.85bar. Find [i] Velocity of steam at throat for maximum discharge, [ii] The area at exit, [iii] Steam discharge if the throat area is 1.2cm2. Assume the flow is isentropic and there are no friction losses. Take n = 1.135. [Anna Univ. Apr.’03] 139. Dry saturated steam at 2.8bar is expanded through a convergent nozzle to 1.7bar. The exit 2 area is 3cm . Calculate the exit velocity and mass flow rate for [i] Isentropic expansion and [ii] Super saturated flow. [Anna Univ. Apr.’03] 140. Dry saturated steam at a pressure of 8bar enters a convergent divergent nozzle and leaves it at a pressure of 1.5bar. If the steam flow process is isentropic and if the corresponding expansion index is 1.135, find the ratio of cross sectional area at exit and throat for maximum discharge. [Anna Univ. Oct.’02] 0 141. Steam enters a group of convergent-divergent nozzles at 21bar and 270 C; the discharge pressure of nozzle is 0.07bar. The expansion is equilibrium throughout and the loss of friction in convergent portion of the nozzle is negligible, but the loss by friction in the divergent section of the nozzle is equivalent to 10% of enthalpy drop available in that section. Calculate the throat and exit area to discharge 14kg/s of steam. [Anna Univ. Oct.’02 & Nov.’03] 142. The following data refer to a single stage impulse turbine: Isentropic nozzle entropy drop 0 = 200kJ/kg, Nozzle efficiency = 90%, Nozzle angle 25 Ratio of blade speed to whirl component of steam speed = 0.5. Blade coefficient = 0.9. The velocity of steam entering the nozzle 30m/s. Find [i] The blade angles at the inlet and outlet if the steam enters the blade without shock and leaves the blade in the axial direction [ii] Blade efficiency [iii] Power developed and [iv] Axial thrust if the steam flow rate is 10kg/s. [Anna Univ. Apr.’03] 143. Steam enters the blade row of an impulse turbine with a velocity of 600m/s at an angle of 0 25 to the plane of rotation of the blades. The mean blade speed is 250m/s. The blade 0 angle at the exit side is 30 . The blade friction loss is 10%. Determine [i] The blade angle
26 26
144.
145.
146. 147.
148. 149. 150. 151. 152. 153. 154.
155. 156.
157.
at inlet [ii] The work done per kg of steam [iii] The diagram efficiency [iv] The axial thrust per kg of steam per sec. [Anna Univ. Nov.’03] At a particular stage of a reaction turbine, the mean blade speed is 60m/s and the steam 0 pressure is 3.5bar with a temperature of 175 C. The identical fixed and moving blades 0 0 have inlet angles of 30 and outlet angle 20 . Determine [i] the blade height if it is 1/10 of the blade ring diameter for a flow rate of 13.5kg/s. [ii] The power developed by a pair and [iii] The specific enthalpy drop if the stage efficiency is 85%. [Anna Univ. Apr.’04] 0 Steam at a 3bar with 10 C superheat is passed through a convergent nozzle. The velocity of steam entering the nozzle is 91.5m/s. The backpressure is 1.5bar. Assuming nozzle efficiency of 90%, determine the area of the nozzle at exit. Discharge though the nozzle is 0 limited to 0.45kg/sec. Take Cps [superheated steam] = 2.2kJ/kg C. [Anna Univ. Nov.’04] A convergent-divergent adiabatic steam nozzle is supplied with steam at 10bar and 2500C. The discharge pressure is 1.2bar. Assuming that the nozzle efficiency is 100% and initial velocity of steam is 50m/s, find the discharge velocity. [Anna Univ. Nov.’04] 2 Dry saturated steam at 10bar is expanded in a nozzle to 0.4bar. The throat area is 7cm and the inlet velocity is negligible. Determine the mass flow and the exit area. Assume isentropic flow and take the index n = 1.135 for dry saturated steam. [Anna Univ. Apr.’05] Define critical pressure and critical temperature ratios of a nozzle. [Anna Univ. Apr.’05] Explain the supersaturated expansion of steam in a nozzle with h-s diagram. [Anna Univ. Apr.’04] Describe the construction of the combined velocity triangle of an impulse turbine and derive an expression for the power developed by the turbine. [Anna Univ. Nov.’04] Explain the pressure-velocity compounding with a neat sketch. [Anna Univ. Nov.’04] Define the following terms: [i] Diagram efficiency. [ii] Stage efficiency. [Anna Univ. Nov.’04] Obtain the relationship between area, velocity and pressure in nozzle flow. [Anna Univ. Apr’04] [a] Explain various types of nozzles and their distinguishing features [8] [b] Steam expands from 40 bar and specific volume of 0.0749 m3/kg to a pressure of 20 bar in a nozzle. Steam remains superheated throughout. Determine the exit area of cross section. [8] [AU., Nov/Dec. 2010] [a] Derive an expression for critical pressure ratio. [8] [b] Compare the throttle and nozzle control governing in steam turbines. [8] [AU. Nov/Dec. 2010] Dry saturated steam at a pressure of 8 bar enters a convergent-divergent nozzle and leaves it at a pressure of 1.5 bar. If the flow is isentropic, and the corresponding expansion index is 1.135; find the ratio of cross-sectional area at exit and throat for maximum discharge. [AU., May/June 2011] A simple impulse turbine has a mean blade speed of 200 m/s. The nozzles are inclined at 20° to the plane of rotation of the blades. The steam velocity from nozzles is 600 m/s. The turbine uses 3500 kg/hr of steam. The absolute velocity at exit is along the axis of the turbine. Determine : a] The inlet and exit angles of the blades b] The power output of then turbine 27 27
c] The diagram efficiency d] The axial thrust [per kg steam per second] Assume the inlet and outlet angles to be equal. [AU. May/June 2011] 158. [i] What are the effects of friction in a nozzle? Explain. [8] [ii] A convergent – divergent nozzle is required to discharge 2kg of steam per second. 0 The nozzle is supplied with steam at 7 bar and 180 C and discharge takes place against a back pressure of 1 bar. The expansion up to throat is isentropic and the frictional resistance between the throat and exit is equivalent to 63kJ/kg of steam. Taking approach velocity of 75m/s and throat pressure of 4 bar, estimate [1] Suitable areas for the throat and exit and [2] Overall efficiency of nozzle based on the enthalpy drop between the actual inlet pressure and temperature and the exit pressure. [8] [Anna. Univ. May/June 2013] 159. [i] The velocity of steam, leaving the nozzle of an impulse turbine is 1000 m/s and the 0 nozzle angle is 20 . The blade velocity is 350 m/s and the blade velocity of coefficient is 0.85. Assuming no losses due to shock at inlet, calculate for a mass flow of 1.5kg/s and symmetrical blading. [1] Blade inlet angle [3] [2] Driving force on the wheel [3] [3] Axial thrust on the wheel and [3] [4] Power developed by the turbine [3] [ii] Differentiate between impulse and reaction turbine? [4] [Anna Univ. May/June 2013] UNIT IV
AIR COMPRESSOR PART A
[1 MARK]
160. To increase the pressure of air or gas, machine driven by prime mover is used. This is known as a. steam turbine b. gas turbine c. compressor d. I.C. engine. 161. A compressor is used a. in gas turbine plants b. in starting and super charging I.C. engine. c. in pneumatic drills. d. all of the above. 162. Ratio of the absolute discharged pressure to the absolute inlet pressure of air, is known as a. compression ratio b. expansion ratio c. compressor capacity d. compressor efficiency. 163. Compression ratio for the compressor is always a. more than 1.0 b. less than 1.0 c. equal to 1.0 d. zero 164. Compressor capacity is a. volume of air delivered b. volume of air sucked c. both [a] and [b] above. d. none of the above. 165. Performance of air compressor at high altitudes as compared to that at sea level is a. better b. inferior c. same d. depend on type of compressor. 166. Ratio of work done per cycle to the stroke volume of the compressor is know as a. compression ratio b. compressor capacity c. mean effective pressure d. compressor efficiency. 28 28
167. As the compression ratio increases, the volumetric efficiency of air compressor a. increases b. decreases c. remains constant d. none 168. Clearance ratio is the ratio of a. swept volume to clearance volume b. clearance volume to swept volume c. clearance volume to cylinder volume d. swept volume to cylinder volume. 169. Ratio of the isentropic work to actual work required for compressing air for the same pressure ratio, is known as a. isothermal efficiency b. adiabatic efficiency c. mechanical efficiency d. volumetric efficiency. 170. Work done on a compressor will be minimum, when air is taken from a. atmosphere b. source of low temperature air c. source of high temperature air d. a source at 0ºC. 171. In a compressor, during compression and delivery of air, work is done a. on the piston b. by the piston c. both [a] and [b] above d. none of the above. 172. The temperature of air at the beginning of the compression stroke as compared to atmospheric temperature is a. more b. less c. equal d. none of the above. 173. Compressor which compresses air from atmospheric pressure to desired discharge pressure, in a single operation, is known as a. single stage compressor b. single acting compressor c. rotary compressor d. none of the above. 174. In a single stage, single acting reciprocating air compressor without clearance volume, work done is minimum, when the compression is a. isothermal b. adiabatic c. polytropic d. any of the above. 175. Clearance ratio for a single stage compressor is a. 1.5 to 3% b. 2 to 10% c. 10 to 15% d. 20 to 30% 176. A multi stage air compressor as compared to single stage compressor a. gives more uniform torque b. reduce cost of compressor c. reduces work done per kg of air d. improves volumetric efficiency for given pressure ratio e. all of the above. 177. In multi-stage compressor with intercooler, compression obtained is a. isothermal b. adiabatic c. polytropic d. none of the above. 178. In multi-stage compression, perfect inter cooling means temperature of air at inlet to subsequent stages as compared to initial temperature is a. more b. less c. same d. none of the above. 179. In multi-stage compressor, for minimum work, the work done on a. first stage should be least b. last stage should be least c. all stages should be equal d. none of the above. ANSWER 160 161
c
d
162 163
a
a
164 165 166
a
b
c
167 168 169
b
b
b
170 171
b
29 29
b
172 173
a
a
174 175
a
b
176
c
177 178 179
a
c
c
PART B
[2 MARKS]
180. Classify the various types of air-compressors. [Anna Univ. Dec.’03] a.According to the design and principle of operation. i] Reciprocating compressors. ii] Rotary compressors. b. According to the action i] Single acting compressors. ii] Double acting compressors. c.According to the number of stages i] Single stage compressors. ii] Multistage compressors. d. According to the pressure limit i] Low pressure compressors. ii] Medium pressure compressors. iii] High pressure compressors. e.According to the capacity 3 i] Low capacity compressors [Volume delivered 0.15 m /s or less]. 3 3 ii] Medium capacity compressors [Volume delivered 0.15m /s to 5m /s]. 3 iii] High capacity compressors [Volume delivered is above 5m /s]. 181. Indicate the applications of reciprocating compressors in industry. [Anna Univ. Nov.’04] The applications of compressed air are as follows: 1. Pneumatic brakes, 2. Pneumatic drills, 3. Pneumatic jacks, 4. Pneumatic lifts, 5. Spray painting, 6. Shop cleaning, 7. Injecting fuel in diesel engines, 8. Supercharging internal combustion engines, 9. Refrigeration, and air conditioning systems. 182. What are the advantages of multi stage compression with inter cooling over single stage compression for the same pressure ratio? [Anna Univ. May’03, Nov.’02 & Apr.’04] a.The work done per kg of air is reduced in multistage compression with inter cooler as compared with single stage compression for the same delivery pressure. b.It improves the volumetric efficiency for the given pressure ratio. c.The size of the cylinders [i.e. high pressure and low pressure] may be adjusted to suit the volume and the pressure of the air. d.It reduces the leakage loss considerably. e. It gives more uniform torque and hence, a smaller size flywheel is required. f. It provides effective lubrication because of lower operating temperature. g.It reduces the cost of the compressor. 183. What is meant by free air delivered? [Anna Univ. Dec.’03,Nov/Dec’11] The free air delivered is the actual volume delivered at the stated pressure reduced to 3 intake pressure and temperature and expressed in m /min. 184. What for inter cooling is used in compressors? [AU. Nov/Dec. 2010] An inter cooling is a simple heat exchanger. It exchanges the heat of compressed air from the low-pressure compressor to the circulating water before the air enters to the highpressure compressor. The purpose of inter cooling is to minimize the work of compression.
30 30
185. Define mechanical efficiency and isothermal efficiency of a reciprocating compressor. [AU. May/June 2011] Mechanical Efficiency: Mechanical efficiency is defined as the ratio between brake power to the indicated power. Mechanical Efficiency = Brake power/Indicated power. Isothermal Efficiency: Isothermal efficiency is defined as the ratio between isothermal work to the actual work of the compressor. Isothermal Efficiency = Isothermal work/Actual work. 186. What is the difference between perfect inter cooling and imperfect inter cooling? [AU. May/June 2011] Perfect cooling: When the temperature of air leaving the inter cooler is equal to the original atmospheric air temperature, then inter cooling is known as perfect inter cooling. Imperfect Inter cooling: When the temperature of air leaving the inter cooling is more than original atmospheric air temperature, then inter cooling is known as imperfect inter cooling. 187. What factors limit the delivery pressure in a reciprocating compressor? [MU, Oct.’97] a. To obtain high delivery pressure, the size of the cylinder will be large. b. Temperature of air. 188. Why clearance is necessary and what is its effect on the performance of reciprocation compressor? [MU, Apr.’98 & Oct.’99] When the piston reaches top dead center in the cylinder, there is a dead space between piston top and cylinder head. This space is known as clearance space and the volume occupied by this space is known as clearance volume. 189. Give the expression for work done for a two-stage compressor with perfect inter cooling. [MU – April 98 & Anna Univ. May/June 2013] [n-1/2n] W = [2n/n-1]p1V1{[p3/p1] – 1} 190. What are the factors that affect the volumetric efficiency of a reciprocating compressor? [MU, Apr.’98] a. Clearance volume. b. Compression ratio. 191. Discuss the effect of clearance upon the performance of an air compressor. [Anna Univ. May’03, Nov.’01] The volumetric efficiency of air compressor increases with decrease in clearance of the compressor. The free air delivered by the compressor is increased by decreasing the clearance volume. 192. Give two merits of rotary compressor over reciprocating compressor. [MU, Apr.’98, Apr.’00] a. Rotary compressor gives uniform delivery of air when compared to reciprocating compressor. b. Rotary compressors are small in size for the same discharge as compared with reciprocating compressors. c. Lubricating system is more complicated in reciprocating compressor where as it is very simple in rotary compressor. d. 31 31
193. Give two examples for positive displacement rotary compressor? 1. Roots blower 2. Van blower 194. List out the application of compressed air[Anna Univ. May/June’2012] Compressed air is mostly used in pneumatic brakes, pneumatic drills, pneumatic jacks, pneumatic lifts, spray painting, shop cleaning, injecting fuel in diesel engines, supercharging, internal combustion engines, refrigeration and air conditioning systems 195. Define volumetric efficiency of a reciprocating compressor. [Anna Univ. May/June 2013] Volumetric efficiency is defined as the ratio of volume of free air sucked into the compressor per cycle to the stroke volume of the cylinder. ηvol = Volume of free air taken per cycle / Stroke volume of the cylinder. PART C
[16 MARKS] 3
0
196. A single stage reciprocating air compressor takes 1m of air per minute at 1bar and 15 C 1.3 and delivers it at 7bar. The law of compression is pV = constant, calculate the indicated power. Neglect clearance. If the speed of compressor is 300rpm and stroke to bore ratio is 1.5, calculate the cylinder dimensions. Find the power required if the mechanical efficiency of compressor is 85% and motor transmission efficiency is 90%. [NOV’02] 3 197. A single acting single stage-reciprocating compressor takes 1m of air per minute at 0 1.35 1.013bar and 17 C and delivers it at 7bar. The law of compression is pV = constant. Clearance is neglected. Compressor runs at 300rpm. Stroke to bore ratio is 1.5. Mechanically efficiency compressor is 85% and motor transmission efficiency is 90%. Calculate mass or air delivered per minute, indicated power, bore and stroke, and the motor power. [Anna Univ. may 2004] 198. The free air delivery of a single cylinder single stage reciprocating air compressor is 3 2.5m /min. The ambient air is at STP conditions and delivery pressure is 7bar. The clearance volume is 5% of the stroke volume and law of compression and expansion is 1.25 pV = C. If L = 1.2D and the compressor runs at 150rpm, determine the size of the cylinders. [DEC’03] 199. A Single stage single –acting compressor delivers 15m3 of free air per minute from 1bar to 8bar. The speed of compressor is 300rpm. Assuming that compression and expansion 1.3 th follow the law pV = constant and clearance is 1/16 of swept volume, find the diameter and stroke of the compressor. Take L/D = 1.5. The temperature and pressure of air at the suction are same as atmospheric air. [DEC’03] 200. A single stage double acting compressor has a free air delivery [F.A.D] of 14m3/min 0 measured at 1.013bar and 15 C. The pressure and temperature in the cylinder during 0 induction are 0.95bar and 32 C respectively. The delivery pressure is 7bar and index of compression and expansion, n= 1.3. The clearance volume is 5% of the swept volume. Calculate the indicated power required and the volumetric efficiency. [Anna Univ. Apr.’04] 3 201. A three-stage air-compressor delivers 5.2m of free air per minute. The suction pressure 0 0 and temperature are 1bar and 30 C. The pressure and temperature are 1.03bar and 20 C at 0 the free air condition. The air is cooled at 30 C after each stage of compression. The delivery pressure of the compressor is 150bar. The R.P.M. of the compressor is 300. The clearance of L.P., I.P., and H.P. cylinders are 5% of the respective strokes. The index of 32 32
202.
203. 204. 205. 206. 207. 208.
209.
210.
211.
compression and re-expansion in all stages is 1.35. Neglecting pressure losses, find the B.P. of the motor required to run the compressor if the mechanical efficiency is 80%. [Anna Univ. Nov.’04] Consider a single-acting 2 stage reciprocating air compressor running at 300rpm. Air is compressed at a rate of 4.5kg/min from 1.013bar and 288K through a pressure ratio of 9 to 1. Both the stages have the same pressure ratio and the index of compression and expansion in both stages is 1.3. Assume a complete inter cooling, find the indicated power and the cylinder swept volumes required. Assume that the clearance volumes of both stages are 5% of their respective swept volumes. [Anna Univ. Apr.’05] Explain with the help of a neat sketch the principle of operation of a reciprocating air compressor. [Anna Univ. Apr.’04] With the help of a neat sketch explain the principles of operation of a centrifugal compressor. [Anna Univ. Apr.’04 & Nov.’04] Compare reciprocating and rotary air compressors. [Anna Univ. Apr.’04] Discuss the merits and demerits of rotary and reciprocating compressors. [Anna Univ. Dec.’03 & Nov.’04] With the help of schematic and p-V diagrams, explain the working of a vane type compressor. [Anna Univ. Apr.’05] [a] Show that in a reciprocating air compressor, with perfect inter cooling, the work done for compressing air is rejected to cooling medium. [b] With the aid of P-V diagrams discuss the reasons for the use of multi stage compressors. [AU. Nov/Dec.2010] [a] Compare reciprocating and rotary compressors and discuss. [b] What are the advantages of multi stage compressors over single stage compressors? Derive the condition of minimum work with complete inter cooling in a two stage compressor. [AU. May/June 2011] 3 A two stage single acting compressor takes in air at the rate of 0.2 m /s the intake pressure and temperature are 0.1 MPa and 16°C. The air is pressed to a final pressure of 0.7 MPa. The intermediate pressure is ideal and inter cooling is perfect. The compression index in both the stages is 1.25 and the compressor runs at 600 r.p.m. Neglecting clearance, determine: [a] The intermediate pressure [b] The total volume of each cylinder [c] The power required to drive the compressor and [d] The rate of heat rejection in the inter cooler. Take Cp = 1.005 KJ/kg K and R = 0.287 KJ/Kg K. [AU. May/June 2011] [a] Explain the working of a single stage single acting reciprocating compressor with a neat sketch and p-v diagram. [b] A single stage double acting reciprocating air compressor is required to deliver 14 m3 of air minute measured at 1.013 bar and 15°C. The delivery pressure is 7 bar and the seed 300 r.p.m. Take the clearance volume as 5% of the swept volume with the compression and expansion index of n= 1.3. Calculate [i] Swept volume of the cylinder [ii] The delivery temperature [iii] Indicated power. [AU. May/June 2011]
33 33
212. A single acting reciprocating air compressor has a piston diameter of 200mm and a stroke of 300mm and runs at 350rpm. Air is drawn at 1.1 bar pressure and is delivered at 8 bar 1.35 pressure. The law of compression is pV = constant and clearance volume is 6% of the stroke volume. Determine the mean effective pressure and the power required to drive the compressor. [Anna Univ. May/June 2013] 213. Derive the work done by a two stage reciprocating air compressor with inter cooler and derive the condition for minimum work input and the expression for minimum work required for two stage reciprocating compressor? [Anna Univ. May/June 2013] UNIT V
REFRIGERATION AND AIR CONDITIONING
PART A [1 MARK] 214. A refrigeration system a. removes heat from a high temperature body and transfers it to a low temperature body b. removes heat from a low temperature body and transfer it to high temperature body c. rejects energy to a low temperature body d. rejects energy from a high temperature body 215. Coefficient of performance is always a. more than unity b. less than unity c. equal to unity d. none of the above 216. Coefficient of performance of a domestic refrigerator as compared to that of an air conditioner is generally a. more b. less c. same d. can not be compared 217. Ratio of actual coefficient of performance to theoretical coefficient of performance is known as a. refrigeration efficiency b. relative coefficient of performance c. critical coefficient of performance d. none of the above 218. Air refrigeration works on a. Bell- Coleman cycle b. reversed Carnot cycle c. Carnot cycle d. both (a) and (b) above 219. In a refrigerator system heat rejected as compared to heat absorbed is a. more b. less c. equal d. none of the above 220. An vapour absorption refrigeration a. gives quiet operation b. cools below 0ºC c. requires little power d. all of the above. 221. A refrigerant should have low a. boiling point b. specific heat of liquid c. specific volume of vapour d. all of the above. 222. Net refrigerating effect for a refrigerant is proportional a. directly to latent heat of refrigerant b. directly to square of latent heat of refrigerant c. to latent heat of refrigerant d. none of the above. 223. Refrigerant with lowest freezing point temperature is a. ammonia b. Freon – 11 c. Freon – 12 d. Freon - 22 224. Air conditioning includes a. cooling b. removal of air impurities c. heating d. dehumidifying e. all of the above.
34 34
225. Ratio of mass of water vapour associated with unit mass of dry air to the mass of water vapour associated with unit mass of dry air at the same temperature is known as a. specific humidity b. relative humidity c. absolute humidity d. degree of saturation 226. Ratio of specific humidity of moist air to the specific humidity of saturated air at the same temperature is known as a. relative humidity b. absolute humidity c. humidity ratio d. degree of saturation 227. Ratio of water vapour to the mass of dry air in a given mixture is known as a. relative humidity b. absolute humidity c. humidity ratio d. degree of saturation 228. Mass water vapour present in 1 kg of dry is known as a. relative humidity b. absolute humidity c. humidity ratio d. percentage humidity 229. As warm air cools, its relative humidity a. increases b. degreases c. remains same d. none 230. Dew point is the temperature at which a. ice starts melting in presence of air without external aid b. condensation of water vapour in air starts c. vapourisation of water vapour in air starts d. none of the above 231. Temperature of air recorded by an ordinary thermometer is known as a. wet bulb temperature b. dry bulb temperature c. dew point temperature d. saturation temperature 232. The wet bulb depression indicates a. relative humidity b. absolute humidity c. percentage humidity d. specific humidity 233. In a psychometric chart, horizontal and uniformly spaced lines indicated a. dry bulb temperature b. wet bulb temperature c. dew point temperature d. specific humidity ANSWER 214 215
b
a
216 217
b
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218 219 220
d
a
a
221 222 223
d
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d
PART B
224 225
c
d
226 227 228 229
d
c
c
b
230
b
231 232 233
a
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234. Define tons of refrigeration and COP. [Anna Univ. May’04,Nov/Dec’11] A tonne of refrigeration is defined as the quantity of heat required to be removed from one tonne of water [1000kg] at 00C to convert that into ice at 00C in 24hours. It actual practice, 1 tonne of refrigeration = 210kJ/min = 3.5kW 235. Define Relative Humidity. [Anna Univ. May/June 2013] It is defined as the actual mass of water vapour in a given volume to the saturated mass of water in same volume and temperature.
a
d
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Mass of water vapour in a given volume Relative Humidity, [Φ] = ----------------------------------------------------------------------------Saturated mass of water vapour in same volume of temperature The pressure and temperature conditions of air at the suction of compressor are [MU, Oct.’96] Slightly less than atmospheric. How does the actual vapor compression cycle differ from that of the ideal cycle? [Anna Univ. Apr.’05] a. In actual cycles, pressure losses occur in both condenser and evaporator. b. Friction losses occur in compressor. Name four important properties of a good refrigerant. [MU-Apr.’99, Apr.’95 & Oct.’97] a. Low boiling point. b. High critical temperature & pressure. c. Low specific heat of liquid. What is the function of the throttling valve in vapour compression refrigeration system? [MU-OCT’99] The function of throttling valve [Expansion valve] is to allow the liquid refrigerant under high pressure and temperature to pass at controlled rate after reducing its pressure and temperature. Explain unit of Refrigeration. [MU-Apr’95] Unit of refrigeration is expressed in terms of ‘tonne of refrigeration’. A tonne of refrigeration is defined as the quantity of heat required to be removed from one 0 0 tonne of water [1000kg] at 0 C to convert that into ice 0 C in 24 hours. What are the effect pf superheat and sub cooling on the vapour compression cycle? [Anna Univ. May’03] Superheating increases the refrigeration effect and COP may be increased or decreased. But sub cooling always increase the COP of the refrigeration and also decrease the mass flow rate of refrigerant. What are the properties of good refrigerant? [Anna Univ. May 03, Oct.’02 & Apr’05[EEE]] An ideal refrigerant should possess the following desirable properties. 1. The refrigerant should have low freezing point. 2. It must have high critical pressure and temperature to avoid large power requirements. 3. It should have low-specific volume to reduce the size of the compressor. 4. It should be nonflammable, non-explosive, non-toxic and non-corrosive. 5. It should give high C.O.P. in the working temperature range. This is necessary to reduce the running cost of the system. 6. It must have low specific heat and high latent heat. 7. It should be odorful for leak detection. 8. It should be of low cost. What are the advantages and disadvantages of air refrigeration system? [Anna Univ. May’03] Advantages: a.The refrigerant used namely air is cheap and easily available.
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b. There is no danger of fire or toxic effects due to leakages. c.The weight to tonne of refrigeration ratio is less as compared to other systems. Disadvantages: 1. The quantity of refrigerant used per tonne of refrigeration is high as compared to other systems. 2. The COP of the system is very low. Therefore running cost is high. 3. The danger of frosting at the expander valves is more as the air contains moisture content. What are the advantages of vapour compression refrigeration system over air refrigeration system? [Anna Univ. Dec.’04] a.The quantity of refrigerant used per tonne of refrigeration is high as compared to other systems. b.The COP of the system is very low. Therefore running cost is high. c.The danger of frosting at the expander valves is more as the air contains moisture content. Define refrigerant. [OCT.’96] Any substance capable of absorbing heat from another required substance can be used as refrigerant. State any two advantages of vapour absorption system over compression system. [AU. Nov./Dec. 2010] a. No need of electric power. b. Wear and Tear is less. c. Tonne capacity is high. d. There is no leakage of refrigerant. e. Space requirement is less. What is GSHF? [AU. Nov./Dec. 2010] GSHF = TSH/GTH = TSH/[TLH+TSH] TSH = Total sensible heat load. GTH = Grand total heat load. TLH = Total latent heat load. GSHF = Grand sensible heat factor. Mention the desirable properties of refrigerant. [AU. May/June 2011] a. The refrigerant should have low freezing point. b. It must have high critical pressure and temperature to avoid large power requirements. c. It should have low-specific volume to reduce the size of the compressor. d. It should be non flammable, non-explosive, non-toxic and non-corrosive. e. It should give high C.O.P in the working temperature range. This is necessary to reduce the running cost of the system. f. It must have low specific heat and high latent heat. g. It should be odorful for leak detection. h. It should be of low cost. Define sensible heating and sensible cooling process. [AU., May/June 2011] Sensible heat process: In sensible heating process, air is heated at constant specific humidity. It means, heating is done without adding moisture. During this process, dry bulb temperature is increased. Sensible cooling process: In sensible cooling process, air is cooled at constant specific humidity. During this process, the dry bulb temperature is reduced.
250. What is humidification and dehumidification? Humidification is defined as the process of adding moisture at constant dry bulb temperature. Dehumidification is defined as the process of removing moisture at constant dry bulb temperature. 251. Enumerate the components of cooling load estimate. [AU., May/June 2012] a. Heat flow through the exterior walls,ceilings,floors, doors and windows. b. Heat by solar radiation c. Heat received from the occupants d. Heat received by infietrated air etc.. PART C
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252. A 5tonne refrigeration plant uses R12 as refrigerant. It enters the compressor at -5 C as 0 saturated vapour. Condensation takes place at 32 C and there is no under cooling of refrigerant liquid. Assuming isentropic compression, determine COP of the plant, Mass flow ofrefrigerant, power required to rum the compressor in kW. The properties of R-12 are given in table. [Anna Univ. Nov.’02] TC
0
P[bar]
32 -5
7.85 2.61
Enthalpy kJ/kg hf hg 130.5 264.5 -249.3 0
Entropy kJ/kgK sg 1.542 1.557
0
253. A refrigerator works between -7 C and 27 C. The vapour is dry at the end of adiabatic compression. Assuming there is no under cooling determine [i] The C.O.P [ii] Power of the compressor to remove a heat load of 12140kJ/hr. the properties of refrigerant are given table. [Anna Univ. May 03] Sensible heat, Entropy of Entropy of Latent heat 0 [hf] Liquid [sf] Vapour [sg] TC [hfg] kJ/kg [kJ/kgK kJ/kgK kJ/kgK -7 -29.3 1297.9 -0.109 4.748 27 117.23 1172.3 0.427 4.333 254. A vapour compression refrigeration system using R12 has a condensing temperature of 0 0 50 C and evaporating temperature of 0 C. The refrigeration capacity is 7tons. The liquid leaving the condenser is saturated liquid and compression is isentropic. The vapour leaving the evaporator is dry saturated. Assume that enthalpy at the end of isentropic compression = 210kJ/kg. Determine: [i] The refrigeration flow rate. [ii] The power required to run the compressor. [iii] The heat rejected in the plant. [iv] COP of the system. The properties of R12 are listed below: [Anna Univ. Nov.’03 & Dec.’04] Pressure hf hg [sf] sg Temp 0 [bar] [kJ/kg] [kJ/kg kJ/kgK [kJ/kgK] [ C] 50 12.199 84.868 206.298 0.3034 0.6792 0 0.086 36.022 187.397 0.1418 0.6960
0 255. 28 tonnes of ice from and 0 C is produced per day in an ammonia refrigerator. The 0 0 temperature range in the compressor is from 25 C to -15 C. The vapour is dry and saturated at the end of compression performance of 62% of theoretical, Calculate the power required to drive the compressor. The properties of ammonia are given in the following table: [Anna Univ. Apr.’04] Enthalpy [kJ/kg] Entropy [kJ/kgK] Temperature Liquid Vapour Liquid Vapour 25 100.4 1319.22 0.3473 4.4852 -15 -54.56 1304.99 -2.1338 5.0585 256. Air enters the compressor of an aircraft system at 100kPa, 277K and is compressed to 300kPa with an isentropic efficiency of 72%. After being cooled to 328K at constant pressure in a heat exchanger the air then expands in a turbine to 100kPa with an isentropic efficiency of 78%. The low temperature air absorbs a cooling load of 3tons of refrigeration at constant pressure before re-entering the compressor. Which is driven by the turbine. Assuming air as ideal gas, find the COP, the driving power required and the air mass flow rate. [Anna Univ. May.’05] 0 257. An ammonia refrigerator produces 20tons of ice per day from and at 0 C. The 0 0 condensation and evaporation takes at 20 C and -20 C respectively. The temperature of the 0 vapour at the end of isentropic compression is 50 C and there is no under cooling of the liquid. The actual COP is 70% of the theoretical COP. Determine [i] The rate of NH3 circulation and [ii] The size of single acting compressor when running at 240rpm assuming L = D and volumetric efficiency of 80%. Take latent heat of ice is 335kJ/kg, specific heat 0 of superheated vapour is 2.8kJ/kg-K and specific volume of saturated vapour at -20 C is 3 0.624m /kg. Use the following properties of Ammonia: [Anna Univ. Apr.’05 (EEE)] Sat. Temp. Enthalpy [kJ/kg] Entropy [kJ/kgK] 0 ( C) hf hg sf sg 20 274.98 1461.58 1.0434 5.0919 -20 89.72 1419.05 0.3682 5.6204 0
258. [i] An office is to be air-conditioned for 50 staff when the outdoor conditions are 30 C 3 DBT and 75%RH if the quantity of air supplied is 0.4m /min/person, find the following: [1] Capacity of the cooling coil in tones of refrigeration. [4] [2] Capacity of the heating coil in kW. [4] [3] Amount of water vapour removed per hour. [4] 0 Assume that required air inlet conditions are 20 C DBT and 60% RH air is conditioned first by cooling and dehumidifying and then by heating. [ii] Describe the factors that affect human comfort. [4] [Anna Univ. Apr.’04 & May/June 2013] 0 3 259. An air-vapour mixture at 0.1 MPa, 30 C, 80% RH has a volume of 50m , Calculate the specific humidity, dew point temperature, wet bulb temperature, mass of dry air and mass of water vapour. [Anna Univ. Apr.’05] 260. With a neat flow diagram, explain the working of a vapour compression refrigeration system. [MU, Apr’96, Apr.’98 & Anna Univ. Apr.’04 & Apr.’05] 261. What are the desirable properties of good refrigerants? [Anna Univ. Apr.’05]
SSCET
MECH / V SEM
QB
262. With a neat sketch, discuss briefly the ammonia absorption refrigeration cycle. [Anna Univ. Apr.’05] 263. Discuss the advantages and disadvantages of vapour absorption refrigeration system over vapour compression system. [Anna Univ. Dec.’03] 264. Define RSHF and ASHF. [Anna Univ. Dec.’03] 265. Draw a neat diagram of air conditioning system required in winter season. Explain the working of different components in the circuit. Is it possible to use steam for such air conditioning system. [Anna Univ. Dec.’03 & apr.’03] 266. Describe the working of summer air conditioning system suitable for hot and wet weather and for hot and dry weather with simple component diagrams. [Anna Univ. Apr.’03 & Apr.’04] 267. [a] Explain with a neat sketch the working principle of lithium bromide refrigeration system . [b] An R – 12 system is operating at conditions such that the vaporizing temperature is 15° C and the condensing temperature is 40°C. If it is assumed that no sub cooling of the liquid occurs so that the temperature of liquid at the refrigerant control is also 40°C, find the following [i] the refrigerating effect per kilogram [ii] the mass of refrigerant circulated in kilograms per second per kilowatt [iii] the mass of refrigerant circulated per second for a ton system. [AU., Nov/Dec.2010] 268. Write short notes on the following. [i] GSHF [ii] RSHF. [AU., Nov/Dec.2010] 269. Explain vapour absorption refrigeration system with a neat sketch. Also bring out the difference between vapour compression and absorption refrigeration systems. [AU., May/June 2011] 270. An air-conditioning is to be designed for a small office for winter conditions. Out-door conditions 10°C DBT and 8°C WBT Required indoor conditions 20°C DBT and 60% R.H. Amount of air circulation 0.3 m3/min/person Seating capacity of the office 50 The required condition is achieved first by heating and then by adiabatic humidifying. Find the following: [a] Heating capacity of the coil in kW and the surface temperature required if the bypass factor of the coil is 0.32. [b] The capacity of the humidifier. [AU. May/June 2011] 272. [i] What are the properties of a good refrigerant? [4] 0 [ii] An ammonia refrigerator produces 30 tons of ice at 0 C in a day of 24 hours. The 0 0 temperature range in the compressor is from 25 C to -15 C. The vapour is dry saturated at the end of compression. Assume a COP of 60% Theoretical value. Calculate the power required to drive the compressor. Assume latent heat of ice is 335kJ/kg. For properties of NH3, refer the table below. [12]
SSCET
Temperature [0C]
hf kJ/kgk 25 298.9 -15 112.34 [Anna Univ. May/June 2013]
MECH / V SEM hg Sf kJ/kgk kJ/kgk 1465.8 1.124 1426.5 0.4572
Sg kJ/kgk 5.039 5.549
QB
