Question Set 78

1.When maximum discharge is obtained in nozzle ?

Ans: At the critical pressure ratio.

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

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

3.What is the difference between stalling and surging in 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.

4.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.

5.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 compressor is higher.

6.What is the name given to portion of thermal energy to be necessarily rejected to environment ?

Ans: Anergy.

7.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.

8.What is caustic embrittlement ?

Ans: 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.

9.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.

10.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.

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