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Interview Questions Mechanical Engineering Part-3 :: Saudi Aramco standard

 Mechanical Engineering Interview Questions Part-3

101. 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.

102. 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.

103. In a Rankine cycle if maximum steam pressure is increased keeping steam temperature and condenser pressure same, what will happen to dryness fraction of steam after expansion ?

It will decrease.

104. Why entropy change for a reversible adiabatic process is zero ?
Because there is no heat transfer in this process.

105. What are two essential conditions of perfect gas ?
It satisfies equation of state and its specific heats are constant.

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

107. 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.

108. How much resistance is offered to heat flow by drop wise condensation ?
Nil

109. What is the relationship between COP of heating and cooling ?
COP of heating is one(unity) more than COP of cooling.

110. How much is the work done in isochoric process ?
Zero.

111. When maximum discharge is obtained in nozzle ?
At the critical pressure ratio.

112. 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.

113. 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.

114. 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.

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115. 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.

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

117. 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.

118. 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.

119. 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.

120. 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.

121. Which two elements in feed water can cause corrosion of tubes and plates in boiler ?
Acid and oxygen in feed water lead to corrosion.

122. 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.

123. 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.

124. What type of boiler does not need a steam drum ?
Super-critical pressure boiler.

125. 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.

126. 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.

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127. Why boiler is purged every time before starting firing of fuel ?
Purging ensures that any unburnt fuel in furnace is removed, otherwise it may lead to explosion.

128. 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.

129. 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.

130. What is the critical temperature of a refrigerant ?
Critical temperature is the maximum temperature of a refrigerantrat which it can be condensed into liquid and beyond this it remains gas irrespective of pressure applied.

131. 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 ?
High temperature in I.C. engine can be tolerated because it lasts for a fraction of second but gas turbines have to face it continuously which metals can’t withstand.

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

133. What do you understand by timed cylinder lubrication ?
For effective lubrication, lub oil needs to be injected between two piston rings when piston is at bottom of stroke so that piston rides in oi during upward movement. This way lot of lub oil can be saved and used properly.

134. What is IIUCR in relation to petrol engine ?
HUCR is highest useful compression ratio at which the fuel can be used in a specific test engine, under specified operating conditions, without knocking.

135. In some engines glycerine is used in place of water for cooling of engine. Why ?
Glycerine has boiling point of 90°C which increases its heat carrying capacity. Thus weight of coolant gets reduced and smaller riadiator can be used.

136. Why consumption of lubricating oil is more in two-stroke cycle petrol engine than four-stroke cycle petrol engine ?
In two-stroke engine lub oil is mixed with petrol and thus some lub oil is blown out through the exhaust valves by scavenging and charging air. There is no such wastage in four stroke petrol engine.

137. As compression ratio increases, thermal n increases. How is thermal n affected by weak and rich mixture strength ?
Thermal n is high for weak mixture and it decreases as mixture strength becomes rich.

138. How engine design needs to be changed to burn lean mixture ?
Engine to burn lean mixture uses high compression ratio and the highly turbulent movement of the charge is produced by the geometry of the combustion chamber.

139. Horse power of I.C. engines can be expressed as RAC rating, SAE rating, or DIN rating. To which countries these standards belong ?
U.K., USA and Germany respectively.

140. What is the use of flash chamber in a vapour compression refrigeration cycle to improve the COP of refrigeration cycle ?
When liquid refrigerant as obtained from condenser is throttled, there are some vapours. These vapours if carried through the evaporator will not contribute to refrigerating effect. Using a flash chamber at some intermediate pressure, the flash vapour at this pressure can be bled off and fed back to the compression process. The throttling process is then carried out in stages. Similarly compression process is also done in two separate compressor stages.

141. Why pistons are usually dished at top ?
Pistons are usually hollowed at top to (i) provide greater spa’e for combustion, (ii) increase surface for flue gases to act upon, and (iii) better distribution of stresses.

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142. What is the function of thermostat in cooling system of an engine ?
Thermostat ensures optimum cooling because excessive cooling decreases the overall efficiency. It allows cooling water to go to radiator beyond a predetermined temperature.

143. What are the causes of failure of boiler tubes ?
Boiler tubes, usually are made from carbon steel and are subject to (a) high rates of heat transfer, (b) bending stresses due to uneven heating, especially at expanded or welded joints into headers or drums, (c) external erosion from burners and flue gas, (d) possible corrosion on the boiler side, and (e) occasional manufacturing defects.
Failure may occur due to following reasons :
(a) High thermal ratings may lead to rapid failure if the internal fluid flow is reduced for any reason. The resultant overheating leads to a failure by creep, characterised by the bulging of the tube with the eventual development of a longitudinal split.
(b) Fatigue cracking due to bending stresses occur. These are associated with change of section and/or weld undercut, where tubes are expanded or welded into headers.
(c) Failure may arise due to overstressing of a reduced section of metal.
(d) Sudden failure of the boiler tube due to corrosion arises from embrittlement of the carbon steel due to interaction between atomic hydrogen from the corrosion process and the iron carbide present in the steel.
(e) Defects in tube manufacture, although far from being a regular occurrence, can be a cause of serious trouble. Lamination in boiler tubes or score marks arising from the cold drawing of tubes, give rise to premature failure and may promote corrosion at these regions.

144. What are the causes of failure of superheater tubes ?
Superheater tubes are subjected to the most severe combination of stress, temperature and corrosive environment. In addition to high-temperature strength, resistance to corrosion is also important. For example, low-alloy ferritic steel such as -1/% Cr, 1% Mo would not be used at metal temperatures above 580°C because of inadequate resistance to corrosion and oxidation over a full service life of 100,000/150,000 hr.
Failures in superheater tubes may arise from :
(a) Prior fabrication history (b) Faulty heat treatment
(c) Consequences of welding (d) Overheating of the tube metal
(e) Gas-side corrosion (f) Stress corrosion (austenitic steels).

145. Why supercritical boilers use less amount of steel compared to non-supercritical boilers ?
Supercritical boilers do not head heavy drum for separation of steam from mixture of water and steam.

146. Out of electric heater and heat pump, which is economical in operation ?
Heat pump.

147. Which furnace burns low-ash fusion coal and retains most of the coal ash in the slag?
Cyclone furnace.

148. How the thickness of thermal boundary layer and thickness of hydrodynamic boundary layer related ?
Ratio of their thickness = (Prandtl number)-1/3.

149. What is the effect of friction on flow of steam through a nozzle ?
To decrease both mass flow rate and wetness of steam.

150. Why gas turbine power plant needs efficient compressor ?
Because a large portion of turbine work is eaten away by compressor and its inefficiency will affect net power output and cost of generation.

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