Question Set 11

Question No. 1

Why the electric motor of a fan with backward curved blades is never got overloaded under any condition?

Answer:                           

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.

Question No. 2

Why the work per kg of air flow in axial flow compressor is less compared to centrifugal compressor for same pressure ratio?

Answer:

Isentropic efficiency of axial flow compressor is higher.

Question No. 3

What is pitting? How it is caused?

Answer:

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

Question No. 4

How Cavitation can be eliminated by Pump?

Answer:

Cavitation means bubbles are forming in the liquid. To avoid Cavitation, we have to increase the Pump size to one or two inch to increase the pressure of the Suction Head, or decrease the Pump Speed.

Question No. 5

Which impurities form hard scale and which impurities soft scale?

Answer:

Sulphates and chlorides of lime and magnesium form hard scale, and carbonates of lime and magnesium form soft scale.

Question No. 6

What is the difference between hard water and soft water?

Answer:

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

Question No. 7

Which two elements in feed water can cause corrosion of tubes and plates in boiler?

Answer:

Acid and oxygen in feed water lead to corrosion.

Question No. 8

Why Cavitation will occur in Centrifugal Pump and not in Displacement Pump?

Answer:

The formation of cavities (or bubbles) is induced by flow separation, or non-uniform flow velocities, inside a pump casing. In centrifugal pumps the eye of the pump impeller is smaller than the flow area of pipe. This decrease in flow area of pump results in increase in flow rate. So pressure drop happened between pump suction and the vanes of the impeller. Here air bubbles or cavities are formed because of liquid vapor due to increase in temperature in impeller. This air bubbles are transmitted to pump which forms Cavitation.

Question No. 9

Why large boilers are water tube type?

Answer:

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 catastrophic.

Question No. 10

What type of boiler does not need a steam drum?

Answer:

Supercritical pressure boiler.