Let's Prepare for technical interview (part-3)

Mechanical engineering interview questions:
Que: What is the difference between generator and invertor ?
Ans: An inverter is only effective if there is already a source of electrical energy. It cannot generate
its own. It can simply convert electrical energy that is already there. On the other hand, a traditional generator cannot make AC current into DC current.

Que: What is the difference between qualitative and quantitative research ?
Ans: Quantitative research involves gathering data that is absolute, such as numerical data, so that it can be examined in as unbiased a manner as possible.
Qualitative research may yield stories, or pictures, or descriptions of feelings and emotions. The interpretations given by research subjects are given weight in qualitative research, so there is no seeking to limit their bias. At the same time, researchers tend to become more emotionally attached to qualitative research, and so their own bias may also play heavily into the results.

Que: Difference between adsorption and absorption ?
Ans: In absorption, one substance (matter or energy) is taken into another substance. But in adsorption only the surface level interactions are taking place.

Que: Difference between liquid and aqueous ?
Ans:

• Liquid is a state of matter, while aqueous is a special type of liquid formed by dissolving a compound in water.
• All aqueous solutions are liquids, but not all liquids are aqueous solutions.

Que: Difference between gas and vapor ?
Ans:

• Vapour can turn back and forth into liquid and solid states but a gas cannot.
• Gases cannot be see while vapours are visible.
• Vapours settle down on ground while gases do not.

Que: Difference between boiling and melting point ?
Ans:

• The melting point is a defined for solids when it transfers from solid state to liquid state.
• The boiling point is defined for liquids for a state change from liquid to gas.
• Boiling point is highly dependent on the external pressure whereas the melting point is independent of the external pressure.

Que: why sound is faster in warm air ?
Ans: The speed of sound in air cair = 331.3 + (0.66 x T) m/s, where T is the temperature in °C.
         The speed of sound is proportional to gas temperature and inversely proportional to its
molar mass.
 Sound is transferred by collisions of molecules. Therefore sound waves will travel faster on warm air because collisions of molecules of air in warm air is greater.

Que: How Cavitation can be eliminated in a pump ?
Ans: 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.

Que: Which pump is more efficient centrifugal pump or reciprocating pump?
Ans: Centrifugal pump.

Because flow rate is higher compared to reciprocating pump. Flow is smooth and it requires less space to install. Lower initial cost and lower maintenance cost.

Que: Why centrifugal pump not called positive displacement type of pump ?
Ans: The centrifugal has varying flow depending on pressure or head, whereas the Positive Displacement pump has more or less constant flow regardless of pressure.

         Likewise viscosity is constant for positive displacement pump where centrifugal pump have up and down value because the higher viscosity liquids fill the clearances of the pump causing a higher volumetric efficiency. When there is a viscosity change in supply there is also greater loss in the system. This means change in pump flow affected by the pressure change.

        One more example is, positive displacement pump has more or less constant efficiency, where centrifugal pump has varying efficiency rate.

Que: Why centrifugal pump is called high discharge pump ?
Ans: Centrifugal pump is a kinetic device. The centrifugal pump uses the centrifugal force to push out the fluid. So the liquid entering the pump receives kinetic energy from the rotating impeller. The centrifugal action of the impeller accelerates the liquid to a high velocity, transferring mechanical (rotational) energy to the liquid. So it discharges the liquid in high rate. It is given in the following formulae:

Centrifugal force F= (M*V2)/R.
Where,
M-Mass
V-Velocity
R-Radius

Que: why cavitation will occur in centrifugal pump and not in displacement pump ?
Ans: 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 vapour due to increase in temperature in impeller. This air bubbles are transmitted to pump which forms cavitation.