Mechanical Engineering Interview Questions and Answers
1. What is the difference between scavenging and supercharging ?
Ans: Scavenging is process of flushing out burnt gases from engine cylinder by introducing fresh air in the cylinder before exhaust stroke ends. Supercharging is the process of supplying higher mass of air by compressing the atmospheric air.
2. What are the names given to constant temperature, constant pressure, constant volume, constant internal
energy, constant enthalpy, and constant entropy processes.
Ans: Isothermal, isochroic, isobaric, free expression, throttling and adiabatic processes respectively.
3. In a Rankine cycle if maximum steam pressure is increased keeping steam temperature and condenser pressure same, what will happen to dryness fraction of steamafter expansion ?
Ans: It will decrease.
4. Why entropy change for a reversible adiabatic process is zero ?
Ans: Because there is no heat transfer in this process.
5. What are two essential conditions of perfect gas ?
Ans: It satisfies equation of state and its specific heats are constant.
6. Enthalpy and entropy are functions of one single parameter. Which is that ?
Ans: Temperature.
7. Why rate of condensation is higher on a polished surface compared to rusty surface ?
Ans: Polished surface promotes drop wise condensation and does not wet the surface.
8. How much resistance is offered to heat flow by dropwise condensation ?
Ans: Nil
9. What is the relationship between COP of heating andcooling ?
Ans: COP of heating is one(unity) more than COP of cooling.
10. How much is the work done in isochoric process ?
Ans: Zero.
11. When maximum discharge is obtained in nozzle ?
Ans: At the critical pressure ratio.
12. Under what condition the work done in reciprocatingcompressor will be least ?
Ans: It is least when compression process approaches isothermal. For this purpose, attempts are made to cool the
air during compression.
13. What is the difference between stalling and surgingin rotary compressions ?
Ans: Stalling is a local phenomenon and it occurs when How breaks away from the blades. Surging causes complete breakdown of flow and as such it affects the whole machine.
14. Why the electric motor of a fan with backward curved blades is never got overloaded under any condition ?
Ans: The maximum power is consumed at about 70% of maximum flow in case’of fan with backward blades. For higher flow, power consumption gets lower.
15. Why the work per kg of air flow in axial flow compressor is less compared to centrifugal compressor for same pressure ratio ?
Ans: Isentropic efficiency of axial flow compressoris higher.
16. What is the name given to portion of thermal energyto be necessarily rejected to environment ?
Ans: Anergy.
17. What is pitting ? How it is caused ? Ans: Non uniform corrosion over the entire metal surface, but occuring only in small pits is called pitting. It is caused by lack of uniformity in metal.
18. What is caustic embrittlement ?
Ans: It is the actual physical change in metal thatmakes it extremely brittle and filled with minute cracks. It occurs particularly in the seams of rivetted jointsand around the rivet holes.
19. Which impurities form hard scale and which impurities soft scale ?
Ans: Sulphates and chlorides of lime and magnesium form hard scale, and carbonates of lime and magnesium form soft scale.
20. What is the difference between hard water and soft water ?
Ans: Hard water contains excess of scale forming impurities and soft water contains very little or no scale forming substances.
21. Which two elements in feed water can cause corrosion of tubes and plates in boiler ?
‘ Ans: Acid and oxygen in feed water lead to corrosion.
22. What should be done to prevent a safety valve to stick to its seat ?
Ans: Safety valve should be blown off periodically so that no corrosion can take place on valve and valve seat.
23. Why large boilers are water tube type ?
Ans: Water tube boilers raise steam fast because oflarge heat transfer area and positive water circulation. Thus they respond faster to fluctuations in demand.Further single tube failure does not lead to catastrophy.
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