Mechanical Basic Interview Question and Answers Part 1

Basic of Mechanical Engineering Interview, viva ,Oral Question and Answers Part 1

Mechanical Engineering Interview Technical questions and answers with explanation for Job interview, competitive examinations and entrance tests.

1. What is Mechanical Engineering?

Mechanical Engineering is the branch of engineering which deals with the designing, constructing , maintaining and manufacturing various mechanical systems.

2. What is the difference between machine and engine?

Engine converts heat energy to Mechanical Energy but machine converts all forms of energy to mechanical energy except heat energy.

3. What are the names given to constant temperature, constant pressure, constant volume, constant internal energy, constant enthalpy, and constant entropy processes.

Isothermal, isochroic, isobaric, free expression, throttling and adiabatic processes respectively.

4. Why entropy change for a reversible adiabatic process is zero ?

Because there is no heat transfer in this process.

5. What are two essential conditions of perfect gas ?

It satisfies equation of state and its specific heats are constant.

6. What are turbo machines?

Turbo machines are devices which transfer energy to or from the fluid by the dynamic action of rotating blades.

7. Why rate of condensation is higher on a polished surface compared to rusty surface ?

Polished surface promotes drop wise condensation and does not wet the surface.

8. What is the difference between heat engine & heat pump?

Heat Engine produces work and transfers energy from high temp. to low temp source medium. In Heat pump external work is needed to transfer energy from low to high temp. medium.

9. What is the relationship between COP of heating and cooling ?

COP of heating is one(unity) more than COP of cooling.

10. How much is the work done in isochoric process ?

Zero.

11. When maximum discharge is obtained in nozzle ?

At the critical pressure ratio.

12. Under what condition the work done in reciprocating compressor will be least ?

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 surging in rotary compressions ?

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 ?

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 ?

Isentropic efficiency of axial flow compressor is higher.

16. What is the difference between scavenging and supercharging ?

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.

17. What is pitting ? How it is caused ?

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 ?

It is the actual physical change in metal that makes it extremely brittle and filled with minute cracks. It occurs particularly in the seams of rivetted joints and around the rivet holes.

19. Which impurities form hard scale and which impurities soft scale ?

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 ?

Hard water contains excess of scale forming impurities and soft water contains very little or no scale forming substances.
Acid and oxygen in feed water lead to corrosion.

22. What should be done to prevent a safety valve to stick to its seat ?

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 ?

Water tube boilers raise steam fast because of large heat transfer area and positive water circulation. Thus they respond faster to fluctuations in demand. Further single tube failure does not lead to catastrophy.

24. What type of boiler does not need a steam drum ?

Super-critical pressure boiler.

25. Why manholes in vessels are usually elliptical in shape ?

Elliptical shape has minimum area of opening and thus plate is weakened the least. Further it is very convenient to insert and take out the cover plate from elliptical opening.

26. Low water in boiler drum is unsafe because it may result in overheating of water tubes in furnace. Why it is unsafe to have high water condition in boiler drum ?

High drum level does not allow steam separation to be effective and some water can be carried over with steam which is not desirable for steam turbine.

27. Why boiler is purged everytime before starting firing of fuel ?

Purging ensures that any unburnt fuel in furnace is removed, otherwise it may lead to explosion.

28. What is the principle of mechanical refrigeration ?

Axis. A volatile liquid will boil under the proper conditions and in so doing will absorb heat from surrounding objects.

29. Why high latent heat of vaporisation is desirable in a refrigerant ?

A high latent heat of vaporisation of refrigerant results in small amount of refrigerant and thus lesser circulation system of refrigerant for same tonnage.

30. What is the critical temperature of a refrigerant ?

Critical temperature is the maximum temperature of a refrigerator which it can be condensed into liquid and beyond this it remains gas irrespective of pressure applied.