interview Q & A (Part-5)

1. What is the difference between scavenging and supercharging?

1.Scavenging is a process of flushing out burnt gases from engine cylinder by introducing fresh air into the cylinder before exhaust stroke ends. Supercharging is the process of supplying the higher mass of air by compressing the atmospheric air.

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

2. Because there is no heat transfer in this process.

3.What are two essential conditions of perfect gas?

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

4.Enthalpy and entropy are functions of one single parameter. Which is that?

4.Temperature.


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

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

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

6.Zero.

7.What is pitting? How it is caused?

7.Nonuniform corrosion over the entire metal surface, but occurring only in small pits is called pitting. It is caused by lack of uniformity in metal.

8. What is the difference between hard water and soft water?

8.Hard water contains an excess of scale forming impurities and soft water contains very little or no scale forming substances.

9.When maximum discharge is obtained in nozzle?

9.At the critical pressure ratio.

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

10.It is least when compression process approaches isothermal. For this purpose, attempts are made to cool the air during compression.

11.What is the difference between stalling and surging in rotary compressions?

11. Stalling is a local phenomenon and it occurs when How breaks away from the blades. Surging causes the complete breakdown of flow and as such it affects the whole machine.

12.What should be done to prevent a safety valve from sticking to its seat?

12.Safety valve should be blown off periodically so that no corrosion can take place on valve and valve seat.

13.Why large boilers are water tube type?

13.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.

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

14.Supercritical pressure boiler.

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15.Why boiler is purged every time before starting firing of fuel?

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

16.What is the principle of mechanical refrigeration?

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

17.What do you understand by timed cylinder lubrication?

17. For effective lubrication, lubricant oil needs to be injected between two piston rings when the piston is at bottom of stroke so that piston rides in oil during upward movement. This way lot of 

lubricant 

oil can be saved and used properly.

18.Why efficiency of gas turbines is lower compared to I.C. engines?

18.In gas turbines, 70% of the output of the gas turbine is consumed by the compressor. I.C. engines have much lower auxiliary consumption. Further combustion temperature of I.C. engines is much higher compared to the gas turbine.

19.Maximum combustion temperature in gas turbines is of the order of 1100 to 10°C whereas same is around 00°C in I.C. engine? Why ?

19.The high temperature in I.C. engine can be tolerated because it lasts for a fraction of the second but gas turbines have to face it continuously which metals can’t withstand.

20. What is the critical temperature of a refrigerant?

20.The critical temperature is the maximum temperature of a refrigerant which it can be condensed into liquid and beyond this, it remains gas irrespective of pressure applied.