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Interview Questions Mechanical Engineering Part-6 :: Saudi Aramco Standard

Mechanical Engineering Interview Questions Part-6

51. A refrigerator and a heat pump are working on the reversed Carnot cycle between the same temperature limits. How COP of refrigerator and heat pump related ?
Answer: COP of refrigerator = COP of heat pump -1.

52. For a given set of operating pressure limits of a Rankine cycle, for which cycle the efficiency will be highest ?
Answer: For regenerative cycle which approaches Carnot cycle. 1

53. A steam pipe is to be insulated by two different insulating materials of same thickness. What arrangement is preferred ?
Answer: Material with lower thermal conductivity should be used for inner layer and material with higher thermal conductivity for the outer layer.

54. Why steam is in open space and water inside tube in condensers used in power plants ?
Answer: Overall heat transfer coefficient can be increased by increasing velocity of water in tube. Further steam needs more space due to higher specific volume.

55. A condenser of a refrigeration system rejects heat at a rate of 60 kW, while its compressor consumes a power of 15 kW. What will be the coefficient of performance of this system ?
Answer: .

56. Why a refrigeration compressor designed to operate with R22 refrigerant can’t be operated with R12 ?
Answer: Condensing pressure of R22 at any given temperature is higher than that of R12.

57. Leakage of Freon-12 refrigerant can not be easly detected, still it is preferred for air-conditioning. Why ?
Answer: Leakage of Freon-12 can’t attain total concentration but it has all desirable characteristics as good refrigerant.

58. In which season the process of humidification and in which season dehumidifi-cation is used in air-conditioning ?
Answer: Humidification in summer and dehumidification in winter.

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59. What is the name given to combined process of cooling and humidifying ?
Answer: Evaporative Cooling.

60. Which refrigerant is used for ice plant and transport refrigeration ?
Answer: Ammonia and CO2 respectively.

61. Which cycle is used in vapour compression and gas cycle refrigerantion system ?
Answer: Brayton cycle, and Bell-Coleman cycle.

62. In what connection Wilson line and Willan’s line used ?
Answer: Wilson line represents saturation line on the Mollier diagram. Willan’s line is connected with determination of frictional power in IC engines.

63. How the characteristics of fuel like front end volatility, mid-range volatility and tail end volatility affect S.I. engines ?
Answer: These characteristics affect SI engines in cold starting, icing of carburettor and dilution in crankcase respectively.

64. What would be the approximate air fuel ratio at time of cold start, part load operation, full load, and idling ?
Answer: 10, 12.5 and 16 respectively.

65. Name pressure compounded and velocity compounded turbine.
Answer: Rateau and Curtis respectively.

66. What is the nature of curve in case of transfer of heat by conduction through a cylindrical wall and through a spherical wall ?
Answer: Logarithmic and hyperbolic.

67. What is radiosity ?
Answer: It is the rate at which radiation leaves a surface.

68. When the compressibility effects become significant in heat transfer problems in high speed flow ?
Answer: Compressibility effects usually become significant at gas flow velocities greater than 0.5 times the velocity of sound.

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69. What is the name given to process of removing non condensables in steam and other vapour cycles ?
Answer: Deaeration process.

70. We often come across use of jet pumps in process industry ? What is the reason for this ?
Answer: Jet pumps are easy to maintain and can be used to transport gases, liquids and mixtures of both.

71. When it can be said that a fluid is Newtonian ?
Answer: When the shear stress is direcly proportional to velocity gradient.

72. Why alcohol can’t be used in diesel engine ?
Answer: Alcohol has low cetane number and thus its ignition is prevented by compression.

73. Out of 2 stroke SI engine, 4 stroke SI engine, and 4 stroke CI engine, how the brake thermal efficiency behaves if these are to be used for road vehicles ?
Answer: 4 stroke CI engine has highest brake thermal efficiency followed by 4 stroke SI engine and the 2 stroke SI has lowest brake thermal efficiency.

74. A jet engine is used to drive a vehicle. When its mechanical efficiency will be maximum ?
Answer: It will be maximum when vehicle speed approaches the relative velocity of gases at nozzle exit.

75. The knocking tendency in a S.I. engine can be reduced by increasing ?
Answer: Speed

76. How you define the emissivity of a body ?
Answer: Emissivity of a body is the ratio of the emitted radiant energy flux density to the emitted radiant energy flux density of a blackbody at the same temperature.

77. What is gray body ?
Answer: When the emissivity of material does not change with temperature, it is called gray body.

78. Define fin efficiency for extended heat transfer surfaces.
Answer: Fin efficiency is defined as the ratio of the mean temperature difference from surface to fluid divided by the temperature difference from fin to fluid at the base or root of the fin.

79. What are the harmful effects of sulphur in coal used for power generation ?
Answer: Sulphur in coal gives rise to number of ecological problems ranging from acid mine drainage to ash waste problems and air pollution due to SO2 emission. Emission of S02 is the cause of acid rain and is responsible for severe environmental damage.

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80. What do you understand by microbial desulphurisation ?
Answer: Sulphur from coal can be removed by physical, chemical and microbial meAnswer: Microbial means has a number of advantages over other methods. Thiobalilius ferroxidans is the most widely used micro organism studied for coal desulphurisation. This bacteria can remove 90-98% of pyritic sulphur from coal. The reaction, of course, is slow and may take several days to several weeks to complete the reaction.

81. What causes rotation of steam turbine rotor ?
Answer: Rotation of turbine rotor is caused by centrifugal force created due to change of direction of fast flowing jet of steam.

82. What do you understand by the term coagulation ?
Answer: Impurities in water are usually in finely divided state which takes long time to settle and these usually pass through the filtering media. By adding certain chemicals (coagulants), gelatinous substances are formed which cause small particles to coalesce into groups large enough to be checked during filtering.

83. What is equivalent evaporation in a boiler plant ?
Answer: It is the amount of water that would be evaporated from water at 100°C to steam at 100°C by the same amount of heat which was actually absorbed by water and steam under operating conditions.

84. What is meant by balanced draft in boiler ?
Answer: Balanced draft refers to combination of forced and induced draft fans controlled to keep the furnace under slightly vacuum condition.

85. How is dryness fraction of steam measured ?
Answer: By using throttling calorimeter.

86. What is the difference between isentropic process and throttlinglprocess ?
Answer: In isentropic process, heat transfer takes place and in throttling process, enthalpy before and after the process is same.

87. What is the difference between pyrometer, pyranometer and pyrheliometer.
Answer: Pyrometer measures temperature by radiation principle. Pyranometer is used to measure total hemispherical solar radiation. Pyrheliometer is used to measure beam intensity by collimating the radiation.

88. What is the name given to useful part of available energy which is available for conversion to useful work ?
Answer: Exergy.

89. Does the area on PV diagram for a process represent work for both reversible and non-reversible process ?
Answer: No. Same is true for only reversible process.

90. Why work is considered as high grade of energy and heat as low grade energy ?
Answer: While work can be completely con¬verted to heat, heat can’t be converted fully to work.

91. When coluomb friction comes into existence between two surfaces ?
Answer: When there is relative motion between them.

92. Under what condition a number of forces acting at a point are in equilibrium ?
Answer: When sum of the resolved parts in any two perpendicular directions is zero.

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93. What happens to two equal and opposite coplanar couples ?
Answer: They balance each other.

94. To slide a heavy block over a rough floor by a rope with minimum force by a man, at what angle the rope should be inclined with the level of floor ?
Answer: He should incline the rope with horizontal at angle of friction between floor and block.

95. A body is dropped from a certain height. If same body were to move down a smooth inclined plane from same height, which parameters will be same on reaching the ground ?
Answer: Velocity, kinetic energy and momentum.

96. A test specimen is stressed slightly beyond the yield point and then unloaded. What happens to its yield strength ?
Answer: It will increase.

97. Out of circular, square, channel, I sections, which is most economical for a component subjected to bending ?
Answer: I section.

98. What does the area under the stress-strain curve represent ?
Answer: Energy required to cause failure.

99. A tension member of certain diameter is to be replaced by a square bar of the same material. Side of square compared to diameter will be ?
Answer: Smaller.

100. A circular bar subjected to tension is designed for a particular safety factor. If both load and diameter are doubled, then factor of safety will be _____?
Answer: doubled.

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