### Mechanical Interview Questions and Answers – Part 1

1. Why excess air is required to burn a fuel completely ?

Excess air is required to ensure adequate mixing of fuel and air, avoid smoke, minimize slagging in coal burning, and to ensure maximum steam output.

2. What is factor of evaporation ?

It is the ratio of heat change from feed water to steam and the heat of vaporization of steam at atmospheric pressure.

3. Materials which absorb neutrons and produce fissionable material are called ?

Fertile materials, such as U28 and Th22.

4. What is the process which produces fissionable material from a fertile material ?

Breeding.

5. What is burn up in connection with fissionable material ?

Burn up corresponds to destruction of fissionable material when it undergoes nuclear fission.

6. What kind of nuclear fuel is used in Narora, Kalapakkam and Rana Pratap Sagar nuclear plants ?

Natural uranium.

7. Which type of plant will you recommend for remote location if power is required in six to twelve months time ?

Diesel engine power plant.

8. Under what condition a nuclear reaction is said to be critical ?

For critical condition, the reaction should continue at a steady rate which is possible when rate of production of neutrons is same as the combined rate of absorption of neutrons and the rate of leakage of neutrons.

9. If a nuclear reactor generates more fuel that it consumes, it is called ______ ?

Fast breeder reactor.

10. Natural uranium contains only 0.7% U25 which is capable of switching chain nuclear reaction. How this percentage is increased ?

Percentage of U25 in natural uranium is increased by a process called uranium enrichment.

11. Why the vane passages are gradually increased in size in successive wheels in steam turbine ?

The velocity of steam decreases in successive stages and to accommodate same flow per unit time, the area of flow must increase.

12. At what pressure and temperature the density of water and steam is same ?

At 225 kg/cm2 and 74.6° C.

13. What is the latent heat of evaporation at critical point ?

Zero.

14. How much space steam will occupy at atmospheric pressure corresponding to 1 kg of water ?

100 times the space occupied by 1 kg of water.

15. What is meant by quality of steam ?

It refers to amount of unevaporated moisture in steam. If steam is perfectly dry, it’s quality is 100%.

16. What is diagram efficiency in steam turbine ?

Diagram efficiency = Change in K.E./kg / Energy suppled/kg.

17. What do you understand by effectiveness of heat exchanger ?

Effectiveness of heat exchanger = actual heat transfer/maximum possible heat transfer.

18. If superheat and condenser temperatures are unchanged and if pressure of steam is increased, then what happens to dryness fraction of steam after isentropic expansion ?

It will decrease.

19. As pressure of steam is increased, why reheating is necessary?

As pressure is increased, the dryness fraction of steam lowers on isentropic expansion. Therefore, steam has to be reheated after partial expansion so that dryness fraction remains within limits after expansion.

20. What is boundary layer thickness in free convection case ?

Velocity of fluid flow at solid surface is zero and it increases rapidly as distance from solid surface becomes greater, reaches a maximum value and then decreases to that of undisturbed fluid. The distance between the solid surface and point where boundary layer velocity equals undisturbed flow (edge) is called boundary layer thickness.

21. What do you understand by forced convection ?

When convection heat transfer occurs between a solid body and a fluid and where circulation of fluid is caused and controlled by some mechanical.

22. In radiative heat transfer, a gray surface is one whose emissivity is _____ ?

Independent of wavelength.

23. What is the effect of water injection and steam injection in gas turbine of heat rate and power output ?

Water injection results in higher mass flow rate through turbine section and as rich electrical output is increased. However it increases gas turbine heat rate because of the additional heat consumption required to vaporise the water.

Steam injection increases power output and decreases heat rate because of its higher energy entering the combustion zone.

24. Water/steam injection in gas turbines increases power output. What is the limit and reasons for same ?

Water/steam injection is not allowed beyond 5% of compressor flow to avoid flame out in combustion and to minimise operating cost and impact on inspection intervals.

25. What are the advantages and drawbacks of centrifugal compressor and axial flow compressors in gas turbines?

Centrifugal compressors have following advantages :

• short length due to high pressure ratio attained in a single stage.
• simple in design, easy to manufacture, rugged in construction, less costly.
• more reliable in operation under all operating condition.
• less susceptible to the effects of deposits left on flow path.
• less sensitive to the fouling of the flow path.
• Good efficiency over a wide range of operation.

Drawback is larger cross-section area for same capacity and its unsuitability for high pressure ratios.

Axial flow compressor is more popular due to high delivery capacity, high compression ratio due to many stages, high efficiency (85-90%), low cross sectional area. Its drawback is sophisticated design and appreciable length due to large number of stages, narrow operating range for good efficiency, higher weight and cost, high starting power.

26. What is the difference between pinch point and approach point in connection with heat recovery steam generator (IIRSG) ? How they affect performance of IIRSG ?

Pinch point is the temperature difference between the gas turbine exhaust leaving the evaporator section and the saturation temperature of steam at corresponding pressure (drum operat¬ing pressure). Approach point is the temperature difference between the temperature of steam corre¬sponding to drum operating pressure and water temperature leaving the economiser. Both these vari¬ables affect the steam production and the cost and effectiveness of HRSG.

If pinch point is lower, total heat recovered in HRSG in higher and steam generation is also high. However lowering pinch point requires more heat exchange surface and increase in cost and draft side loss. Its optimum value is 8-10°C.

Lower is approach temperature, higher is steam production in economiser due to flashing which needs to be avoided for long life of tubes. Higher approach temperature increases the surface in evaporator section and assures higher stability.

27. How the efficiency of heat recovery in IIRSG increased ?

It can be increased by generating steam at more than one pressure.

Further improvement is possible by heating make up water/condensate in HRSG, generating low pressure saturated steam or dearation steam in HRSG using a low pressure evaporator, preheating the make up water in a heat exchanger before it enters deaerator, and circulate more water than necessary through the economiser and recirculate excess to the deaerator.

28. NO.,, reduction in gas turbines is challenge to meet environmental regulations. What measures are taken in this regard ?

NO* in gas turbines is controlled by (a) injecting water/steam into combustor, (6) using selective catalytic redaction (SCR) and non-catalytic reduction system (NRS), and (c) better combustor design.

29. What is the difference between fuel NOx and thermal NOx ?

Fuel NO* is formed by the gas phase oxidation of char nitrogen (CN~ compounds) in the fuel and thermal NO* is formed by high temperature reaction between the nitrogen and oxygen in the combustion air.

30. How steam/water injection in combustion zone lowers NOx formation.

It lowers the flame and gas temperatures.

31. What are disadvantages of injecting steam/water in combustion zone in gas turbine ?

• Increased CO at lower loads
• Increased fuel consumption
• Loss of extensively treated water
• Increased complexity of engine control
• Short life of major turbine components.

32. What for water/steam is injected in combustion zones of a gas turbine ?

Water/steam is injected to limit the amount of NOx formed by lowering the flame and gas temperatures.

33. What are improved low NOx combustors ?

Dry low NOx combustors result in improved air/fuel mixing and reduced flame temperature. The main type are lean premixed combustors, rich/quench lean combustors and catalytic combustors. Variable guide vanes are used to control the fuel-air mixture. In one design combustion chamber is designed into a series of small, lean premixed chambers.

34. What is selective catalytic reduction (SCR) method for NO* control in gas turbines ?

SCR is a post combustion method in which a catalyst is installed in flue gas where temperature could be of the order of 15-400°C. It enables vaporised NH to react with NO* in exhaust gas to form N2 and H20. Vaporised NH is injected into exhaust gases before it passes through the catalyst bed. This process can achieve in excess of 90% NOx reduction.

35. What catalyst is used in SCR process for control of NOx in gas turbines ?

V2O5 supported by titanium or a mixture of titanium and silica is generally used as catalyst. Oxides of Ti and Mo are often incorporated as moderators.

36. Measure to minimise formation of NOx inhibit complete combustion and this results in formation of CO and voltatile organic compound (VOCs). How these are controlled ?

Catalytic oxidation, a post combustion treatment at 470°-600° temperature is used to reduce emission of CO and VOCs.

37. It is usual practice to report performance characteristics of gas turbine at ISO conditions. What are these ?

ISO conditions for GT performance are : Ambient dry bulb temperature : 15°C, Relative humidity – 60%. Ambient berometric pressure 1.0 kg/cm2.

38. Which parameters influence the heat rate of gas turbines and how ?

Various parameters affecting heat rate of GT are :

(i) Lower the gas turbine inlet temperature, lower the heat rate.
(ii) Higher the compression ratio, lower the heat rate.
(iii) Higher the turbine firing temperature, lower the heat rate.
(iv) Lower the exhaust temperature, lower the heat rate.
(v) Exhaust gas flow, inlet pressure loss, exhaust pressure loss also affect heat rate.

39. Which parameters influence the performance of gas turbines ?

Ambient conditions, inlet/exhaust pressure losses, fuels, and water/steam injection flow rates’influence the performance of gas turbines.

40. Why the efficiency of superheat Rankine cycle is higher than that of simple Rankine cycle ?

Because the mean temperature of heat addition is higher for superheat cycle.

41. A Carnot cycle is to be designed to attain efficiency of 0.75. If temperature of high temperature reservoir is 727°C, then low temperature reservoir will have to be maintained at______°C ?

– 2°C.

42. A gas having a negative Joule-Thompson coefficient, when throttled, will become …. ?

hotter.

43. What is boundary layer thickness in turbulent boundary layer ?

In ease of turbulent boundary layer, boundary layer thickness is defined as the distance from solid surface at which the fluid velocity is 99% of the undisturbed free stream velocity. Near the surface of solid, there is laminar flow which changes to buffer zone and finally turbulent region.

44. Stirling cycle with regenerative arrangement and Carnot cycle operate within same temperature limits. What can be said about thermal efficiency of these two ?

Thermal n of both will be same.

45. How does the stagnation temperature behave along a streamline in adiabatic flow with friction ?

It remains constant.

46. What is the frequency of secondary imbalance in four-stroke engine ?

Two times the engine speed.

47. What would be the order of boundary layer in pipe flow (i) laminar (ii) transition, and (iii) fully turbulent ?

In laminar flow, boundary layer extends right from surface upto centre. In fully turbulent it may extend 5-10% of radius only whereas in transition flow, boundary layer may be from 50-60% of radius of pipe.

48. In general how the boundary layer and local film coefficient of heat transfer are related ?

In general, the thinner the boundary layer, higher the value of local heat transfer film coefficient.

49. Out of constant volume and constant pressure line, which line on T-S diagram has higher slope ?

Constant volume.

50. A heat engine and a refrigeration cycle operate between two temperature limits T1 and T2(T1 > T2). The product of efficiency of heat engine and COP of refrigeration cycle will be ?

T2/TX.

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 ?

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 ?

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 ?

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 ?

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 ?
Ans: .

56. Why a refrigeration compressor designed to operate with R22 refrigerant can’t be operated with R12 ?

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 ?

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 ?

Humidification in summer and dehumidification in winter.

59. What is the name given to combined process of cooling and humidifying ?

Evaporative Cooling.

60. Which refrigerant is used for ice plant and transport refrigeration ?

Ammonia and CO2 respectively.

61. Which cycle is used in vapour compression and gas cycle refrigerantion system ?

Brayton cycle, and Bell-Coleman cycle.

62. In what connection Wilson line and Willan’s line used ?

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 ?

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 ?

10, 12.5 and 16 respectively.

65. Name pressure compounded and velocity compounded turbine.

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 ?

Logarithmic and hyperbolic.

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 ?

Compressibility effects usually become significant at gas flow velocities greater than 0.5 times the velocity of sound.

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

Deaeration process.

70. We often come across use of jet pumps in process industry ? What is the reason for this ?

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 ?

When the shear stress is direcly proportional to velocity gradient.

72. Why alcohol can’t be used in diesel engine ?

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 ?

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 ?

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 ?

Speed

76. How you define the emissivity of a body ?

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 ?

When the emissivity of material does not change with temperature, it is called gray body.

78. Define fin efficiency for extended heat transfer surfaces.

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 ?

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.

80. What do you understand by microbial desulphurisation ?

Sulphur from coal can be removed by physical, chemical and microbial meAns: 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 ?

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 ?

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 ?

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 ?

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 ?

By using throttling calorimeter.

86. What is the difference between isentropic process and throttlinglprocess ?

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.

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 ?

Exergy.

89. Does the area on PV diagram for a process represent work for both reversible and non-reversible process ?

No. Same is true for only reversible process.

90. Why work is considered as high grade of energy and heat as low grade energy ?

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 ?

When there is relative motion between them.

92. Under what condition a number of forces acting at a point are in equilibrium ?

When sum of the resolved parts in any two perpendicular directions is zero.

93. What happens to two equal and opposite coplanar couples ?

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 ?

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 ?

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 ?

It will increase.

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

I section.

98. What does the area under the stress-strain curve represent ?

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 ?

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 _____?

doubled.