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Multiple choice question for engineering

Set 1

1. The purpose of governing in steam turbines is to
a) reheat the steam and improve its quality
b) maintain the speed of the turbine
c) reduce the effective heat drop
d) completely balance against end thrust

View Answer

Answer: b [Reason:] The purpose of governing in steam turbines is to maintain the speed of the turbine.

2. The ratio of the work done on the blades per kg of steam to the total energy supplied per stage per kg of steam is called
a) nozzle efficiency
b) stage efficiency
c) mechanical efficiency
d) blading efficiency

View Answer

Answer: b [Reason:] The ratio of the work done on the blades per kg of steam to the total energy supplied per stage per kg of steam is called Stage Efficiency.

3. A nozzle is said to be a divergent nozzle
a) when the cross-section of the nozzle increases continuously from entrance to exit
b) when the cross-section of the nozzle decreases continuously from entrance to exit
c) when the cross-section of the nozzle first decreases from entrance to throat and then increases from its throat to exit
d) none of the mentioned

View Answer

Answer: a [Reason:] A nozzle is said to be a divergent nozzle when the cross-section of the nozzle increases continuously from entrance to exit.

4. The ratio of the isentropic heat drop to the heat supplied, is called
a) stage efficiency
b) reheat factor
c) internal efficiency
d) rankine efficiency

View Answer

Answer: d [Reason:] The ratio of the isentropic heat drop to the heat supplied is called Rankine efficiency.

5. The reheat factor depends upon
a) exit pressure
b) turbine stage efficiency
c) initial pressure and superheat
d) all of the mentioned

View Answer

Answer: d [Reason:] The factors on which the reheat factor depends are exit pressure, turbine stage efficiency, initial pressure & superheat.

6. The critical pressure ratio for initially dry saturated steam is
a) 0.546
b) 0.577
c) 0.586
d) 0.578

View Answer

Answer: b [Reason:] The critical pressure ratio for initially dry saturated steam is 0.577.

7. In order to reduce the rotor speed of an impulse turbine, the method employed is
a) pressure compounding
b) velocity compounding
c) pressure-velocity compounding
d) all of the mentioned

View Answer

Answer: d [Reason:] In order to reduce the rotor speed of an impulse turbine, the methods employed are Pressure compounding, Velocity compounding, Pressure-Velocity compounding, etc.

8. Which of the following statement is wrong?
a) The flowing the convergent portion of the nozzle is sub-sonic
b) The critical pressure gives the velocity of steam at the throat equal to the velocity of sound
c) The flow in the divergent portion of the nozzle is super-sonic
d) To increase the velocity of steam above sonic velocity (super-sonic) by expanding steam below the critical pressure, the divergent portion for the nozzle is not necessary

View Answer

Answer: d [Reason:] In order to increase the velocity of steam above sonic velocity by expanding steam below the critical pressure, the divergent portion for the nozzle is necessary.

9. In an impulse turbine, steam expands
a) wholly in nozzle
b) partly in the nozzle and partly in blades
c) wholly in blades
d) none of the mentioned

View Answer

Answer: a [Reason:] In an impulse turbine, steam expands wholly in nozzle.

10. A steam nozzle converts
a) heat energy of steam into potential energy
b) potential energy into heat energy of steam
c) heat energy of steam into kinetic energy
d) kinetic energy into heat energy of steam

View Answer

Answer: c [Reason:] A steam nozzle converts heat energy of steam into kinetic energy.

11. The process of draining steam from the turbine, at certain points during its expansion and using this steam for heating the feed water in feed water heaters and then supplying it to the boiler is known as
a) regenerative heating
b) reheating of steam
c) bleeding
d) none of the mentioned

View Answer

Answer: c [Reason:] The process of draining steam from the turbine, at certain points during its expansion and using this steam for heating the feed water in feed water heaters and then supplying it to the boiler is known as Bleeding.

12. If the critical pressure ratio for steam is 0.546, then the steam is initially
a) wet
b) dry saturated
c) superheated
d) none of the mentioned

View Answer

Answer: c [Reason:] Generally, when the critical pressure ratio for steam is 0.546, then the steam is said to be superheated.

13. A steam turbine, in which a part of the steam after partial expansion, is used for process heating and the remaining steam is further expanded for power generation, is known as
a) pass out turbine
b) impulse turbine
c) low pressure turbine
d) high pressure turbine

View Answer

Answer: a [Reason:] A steam turbine, in which a part of the steam after partial expansion, is used for process heating and the remaining steam is further expanded for power generation, is known as Pass out Turbine.

14. A stage, in reaction turbine, is represented by
a) each row of blades
b) number of entries of steam
c) number of exits of steam
d) none of the mentioned

View Answer

Answer: a [Reason:] Each row of blades represents each stage in a reaction turbine.

15. Which of the following statement is correct?
a) The pressure of steam, in reaction turbines, is increased in fixed blades as well as in moving blades
b) The efficiency of steam turbine is greater than the steam engines
c) A flywheel is a must for steam turbine.
d) The turbine blades do not change the direction of steam issuing from the nozzle

View Answer

Answer: b [Reason:] The efficiency of steam turbines is greater than that of a steam engine.

Set 2

1. The cycle efficiency ________________ with the _____________ of condenser pressure.
a) decreases, decrease
b) increases, decrease
c) remains same, decrease
d) does not vary, decrease

View Answer

Answer: b [Reason:] As the decrease in condenser pressure mainly depends on the available cooling water temperature & thus on the climatic conditions of the place. Hence, cycle efficiency being a function of condenser pressure has an inverse relation with the condenser pressure.

2. What is the relation between cooling water temperature (tc1) & condenser pressure (pc)?
a) tc1 is directly proportional to pc
b) tc1 is directly proportional to 2xpc
c) tc1 is directly proportional to 5xpc
d) tc1 is directly proportional to 6xpc

View Answer

Answer: a [Reason:] Cooling water temperature is directly proportional to condenser pressure 1xpc.

3. An increase in inlet steam temperature ___________ the heat rate.
a) lowers
b) increases
c) does not have any effect on
d) temperature has no significance in heat rate

View Answer

Answer: a [Reason:] An increase in inlet steam temperature, i.e. an increase in superheat at constant inlet steam pressure & condenser pressure gives a steady improvement in cycle efficiency & lowers the heat rate due to an increase in the mean temperature of heat addition.

4. What is the effect of increasing superheat at constant inlet steam pressure & condenser pressure on cycle efficiency?
a) Steady decrement in cycle efficiency
b) Steady improvement in cycle efficiency
c) No effect on cycle efficiency
d) None of the mentioned

View Answer

Answer: b [Reason:] An increase in inlet steam temperature, i.e. an increase in superheat at constant inlet steam pressure & condenser pressure gives a steady improvement in cycle efficiency & lowers the heat rate due to an increase in the mean temperature of heat addition.

5. What is the effect of increasing superheat at constant inlet steam pressure & condenser pressure on mean temperature of heat addition?
a) increases
b) decreases
c) remains same
d) none of the mentioned

View Answer

Answer: a [Reason:] An increase in inlet steam temperature, i.e. an increase in superheat at constant inlet steam pressure & condenser pressure gives a steady improvement in cycle efficiency & lowers the heat rate due to an increase in the mean temperature of heat addition.

6. What is the effect of increasing steam inlet temperature on turbine internal efficiency?
a) it remains same
b) it increases
c) it decreases
d) none of the mentioned

View Answer

Answer: b [Reason:] Increasing the inlet steam temperature decreases the wetness of the steam in the later stages of the turbine & improves the turbine efficiency.

7. What is the percentage drop in the ultimate strength of unalloyed steels when the steam temperature is raised from 400-500 degree Centigrade?
a) 20%
b) 30%
c) 40%
d) 60%

View Answer

Answer: b [Reason:] The ultimate strength of unalloyed steels is 30% when the steam temperatures are in the range 400-500 degree Centigrade. Alloying can increase this strength.

8. Apart from alloying, name a material that is used instead of ferritic steels to increase the ultimate strength of unalloyed steels.
a) Chromium
b) Molybdenum
c) Austenitic steel
b) None of the mentioned

View Answer

Answer: c [Reason:] The use of Austenitic steels increases the ultimate strength of unalloyed steels at high temperatures. The temperature limits are in the range 538-565 degree Centigrade.

9. Which of these cases will have a higher cycle efficiency?
a) Condenser Pressure= 25 mm of Hg; Cooling Water Temperature= 12 degree Centigrade
b) Condenser Pressure= 50 mm of Hg; Cooling Water Temperature= 27 degree Centigrade
c) Condenser Pressure= 75 mm of Hg; Cooling Water Temperature= 36 degree Centigrade
d) Condenser Pressure= 100 mm of Hg; Cooling Water Temperature= 45 degree Centigrade

View Answer

Answer: a [Reason:] In identical steam conditions cycle efficiency is inversely proportional to cooling water temperature.

10. For the given values of efficiency, inlet steam pressure and quality of steam, pressure limits can be determined by ?
a) Mollier Charts
b) Gantt Charts
c) Both of the mentioned
d) None of the mentioned

View Answer

Answer: a [Reason:] The pressure limits can be readily determined by drawing the corresponding expansion line of the turbine om a Mollier Diagram.

Set 3

1. In a glass rube type water indicator for a boiler, one end of the tube is connected to water space and the other end is connected to
a) chimney
b) water space
c) steam space
d) super heater

View Answer

Answer: c [Reason:] In a glass rube type water indicator for a boiler, one end of the tube is connected to water space and the other end is connected to the steam space.

2. The draught produced by a steam jet issuing from a nozzle placed in the ashpit under the fire grate of the furnace is called
a) chimney draught
b) induced steam jet draught
c) forced steam jet draught
d) none of the mentioned

View Answer

Answer: b [Reason:] The draught produced by a steam jet issuing from a nozzle placed in the ashpit under the fire grate of the furnace is called induced steam jet draught.

3. Willian’s line for the steam engine is a straight line relationship between the steam consumption per hour and
a) brake power
b) indicated power
c) pressure of steam
d) efficiency

View Answer

Answer: b [Reason:] Willian’s line for the steam engine is a straight line relationship between the steam consumption per hour and indicated power.

4. The natural draught is produced by
a) chimney
b) centrifugal fan
c) steam jet
d) none of the mentioned

View Answer

Answer: a [Reason:] The natural draught is produced by chimney.

5. Which of the following statement is wrong?
a) Water tube boilers are internally fired
b) Locomotive boiler is a water tube boiler
c) La-mont boiler is a low pressure water tube boiler
d) All of the mentioned

View Answer

Answer: d [Reason:] All the mentioned statement are wrong as water-tube boilers are externally fired, locomotive boiler isn’t a water-tube boiler & La-mont boiler is a high pressure boiler.

6. An air preheater is installed
a) between the economiser and chimney
b) before the superheater
c) before the economiser
d) none of the mentioned

View Answer

Answer: a [Reason:] The installation of an air pre-heater is between the economiser & the chimney.

7. Which of the following statement is correct for a compound steam engine?
a) The forces in the working parts are increased as the forces are distributed over more parts
b) The cost of the engine, for the same power and economy, is more than that of a simple steam engine
c) The temperature range per cylinder is increased, with corresponding increase in condensation
d) The ratio of expansion is reduced, thus reducing the length of stroke

View Answer

Answer: d [Reason:] When the ratio of expansion is reduced, the length of stroke is reduced in a compound steam engine.

8. The length of shell of a Locomotive boiler is
a) 2m
b) 3m
c) 4m
d) 6m

View Answer

Answer: c [Reason:] The shell of a locomotive boiler has a length of 4m.

9. The function of a flywheel is
a) to convert reciprocating motion of the piston into rotary motion
b) to prevent fluctuation of speed
c) to keep the engine speed uniform at all load conditions
d) to convert rotary motion of the crankshaft into to and fro motion of the valve rod

View Answer

Answer: b [Reason:] Flywheel is employed to prevent fluctuation of speed.

10. The shell diameter of a Locomotive boiler is?
a) 1.5m
b) 1m
c) 2m
d) 2.5m

View Answer

Answer: a [Reason:] The shell diameter of a Locomotive boiler is 1.5 m.

Set 4

1. From the concept of kinetic theory, mean travel velocity of the gas molecules is prescribed by the relation
a) V = (3M/GT) 1/2
b) V = (3GT/M) 1/2
c) V = (6T/GM) 1/2
d) V = (3GT/2M) 1/2
Where,
G = Universal gas constant
M = Molecular weight of the gas
T = Absolute temperature

View Answer

Answer: b [Reason:] Heat transfer by conduction in gases occurs through transport of the kinetic energy of molecular motion resulting from the random movement and collisions of the molecules.

2. Low temperature insulation are used when the enclosure is at a temperature lower than the ambient temperature. Which one of the following is not a low temperature insulation?
a) Asbestos
b) Cork
c) Cattle hair
d) Slag wool

View Answer

Answer: a [Reason:] Asbestos is a high temperature insulation which are used when it is desired to prevent an enclosure at a temperature higher than the ambient from losing heat to its surroundings.

3. The value of Lorenz number in 10-8 W ohms/K2 is
a) 2.02
b) 2.35
c) 2.56
d) 2.45

View Answer

Answer: d [Reason:] Lorenz no. = k/α T

4. For liquids, thermal conductivity is governed by the relation
a) k = A c p p7/3/M1/3
b) k = 2A c p p4/3/M2/3
c) k = A c p p4/3/M1/3
d) k = A c p p8/3/M1/3
Where,
CP = Specific heat at constant pressure
M = Molecular weight of the gas
p = Mass density
A = Area

View Answer

Answer: c [Reason:] Area doesn’t depends on the nature of liquid but on the quantity (Ac p) is nearly constant for all liquids.

5. Consider the following parameters
(i) Composition
(ii) Density
(iii) Porosity
(iv) Structure
Then, thermal conductivity of glass wool varies from sample to sample because of variation is
a) 1 and 2
b) 1, 2, 3 and 4
c) 1 and 3
d) 1, 2 and 3

View Answer

Answer: b [Reason:] There is a variation due to all above options.

6. The thermal conductivity and the electrical conductivity of a metal at absolute temperature are related as
a) k/σ T
b) k/σ
c) k σ/T
d) k/T

View Answer

Answer: a [Reason:] It is defined as the ratio of thermal conductivity to the product of electrical conductivity and temperature.

7. The relation Ϫ2 t =0 is referred to as
a) Poisson’s equation
b) Fourier heat conduction equation
c) Solution for transient conduction
d) Laplace equation

View Answer

Answer: d [Reason:] In the absence of internal heat generation or release of energy within the body, equation reduces to Laplace equation.

8. The unit of thermal diffusivity is
a) m2/hr-K
b) kcal/m2-hr
c) m2/hr
d) m/hr-K

View Answer

Answer: c [Reason:] The quantity α = k/pc is called thermal diffusivity.

9. To effect a bond between two metal plates, 2.5 cm and 15 cm thick, heat is uniformly applied through the thinner plate by a radiant heat source. The bonding must be held at 320 K for a short time. When the heat source is adjusted to have a steady value of 43.5 k W/m2, a thermocouple installed on the side of the thinner plate next to source indicates a temperature of 345 K. Calculate the temperature gradient for heat conduction through thinner plate. In the diagram, the upper plate is 2.5 cm thick while the lower is 15 cm thick.
heat-transfer-interview-questions-answers-q9
a) – 1000 degree Celsius/m
b) – 2000 degree Celsius/m
c) – 3000 degree Celsius/m
d) – 4000 degree Celsius/m

View Answer

Answer: a [Reason:] Temperature gradient = d t/d x = – 1000 degree Celsius/m. Also, q/A = – k (t2– t 1)/δ.

10. The diffusion equations
Ɏ2t + q g = (1/α) (d t/d r)
Governs the temperature distribution under unsteady heat flow through a homogenous and isotropic material. The Fourier equation follows from this expression when
a) Temperature doesn’t depends on time
b) There is no internal heat generation
c) Steady state conditions prevail
d) There is no internal heat generation but unsteady state condition prevails

View Answer

Answer: d [Reason:] In unsteady state condition, there is no internal heat generation.

Set 5

1. The relevant boundary conditions in case of heat dissipation from a fin insulated at the tip are
a) t = t 0 at x = 0 and d t/d x = 0 at x = 0
b) t = t 0 at x = 0 and d t/d x = 0 at x = 1
c) t = t 0 at x = 1 and d t/d x = 0 at x = 1
d) t = t 0 at x = infinity and d t/d x = 0 at x = infinity

View Answer

Answer: b [Reason:] It should be at x = 0 and x = 1 respectively.

2. The temperature distribution in case of fin insulated at the tip is given by
a) t – t 0/t 0 – t a = cos h m (3 – x)/cos ml
b) t – t 0/t 0 – t a = cos h m (2 – x)/sin h ml
c) t – t 0/t 0 – t a = cos h m (l – x)/cos h ml
d) t – t 0/t 0 – t a = cos m (l – x)/sin ml

View Answer

Answer: c [Reason:] It should contain cos h term and (1 – x) term.

3. The rate of heat transfer from the fin in case of fin insulated at the tip is
a) (h P k A) 1/2 (t 0 – t a) tan h ml
b) (h k A) 1/2 (t 0 – t a) tan h ml
c) (h P A) 1/2 (t 0 – t a) tan h ml
d) (h P k) 1/2 (t 0 – t a) tan h ml

View Answer

Answer: a [Reason:] It should contains all the terms i.e. h, A, P, k.

4. “Fin is insulated at the tip”. What does that mean?
a) Less heat is transferred from the tip
b) Heat will transferred from tip only
c) More heat is transferred from the tip
d) No heat is transferred from the tip

View Answer

Answer: d [Reason:] The fin is of finite length with the tip insulated and so no heat is transferred from the tip.

5. Find the heat transfer rate from a hot surface for 6 fins of 10 cm length? The base temperature of the fin is maintained at 200 degree Celsius and the film is exposed to a convection environment at 15 degree Celsius with convective coefficient 25W/square m K. Each fin has cross-sectional area 2.5 square centimeter and is made of a material having thermal conductivity 250W/m K
a) 120.34W
b) 130.18W
c) 145.46W
d) 165.43W

View Answer

Answer: b [Reason:] n = 6 and l = 10 cm = 0.1 m, ml = 0.4472, Q = 6[(250) (2.5 * 10– 4) (4.472) (200 – 15) tan h (0.4472) = 130.18W.

6. An array of 10 fins of anodized aluminum (k = 180W/m K) is used to cool a transistor operating at a location where the ambient conditions correspond to temperature 35 degree Celsius and convective coefficient 12W/square m K. The distance AB is 3 mm, EF is 0.4 mm. The length of the fin is 5 mm and has its base at 60 degree Celsius. Find the power dissipated by the fin array?
heat-transfer-interview-questions-answers-experienced-q6
a) 8.673W
b) 1.432W
c) 0.786W
d) 0.128W

View Answer

Answer: c [Reason:] P = 2(3 + 0.4) = 6.8 mm, A = (3) (0.4) = 1.2 square meter, m = 19.44 per meter. So, Q = k A m (t 0 – t a) tan h ml = 0.0786, therefore heat loss from the array of 10 fins = (0.0786) (10) = 0.786W.

7. An electronic semiconductor device generates 0.16 k J/hr of heat. To keep the surface temperature at the upper safe limit of 75 degree Celsius, it is desired that the heat generated should be dissipated to the surrounding environment which is at 30 degree Celsius. The task is accomplished by attaching aluminum fins, 0.5 square mm and 10 mm to the surface. Work out the number of fins if thermal conductivity of fin material is 690W/m K and the heat transfer coefficient is 45k J/square m hr K. Neglect the heat loss from the tip of the fin
a) 4
b) 3
c) 2
d) 1

View Answer

Answer: a [Reason:] P = 2(0.5 + 0.5) = 2 mm, A = (0.5) (0.5) = 0.25 square meter. m = 22.85 per meter, so Q = k A m (t 0 – t a) tan h ml = 39.77 * 10 -3 k J/hr per fin. So number of fins = 0.16/39.77 * 10 -3 = 4.02.

8. A rod of 10 mm diameter and 80 mm length with thermal conductivity 16W/ m K protrudes from a surface at 160 degree Celsius. The rod is exposed to air at 30 degree Celsius with a convective coefficient of 25W/square m K. How does the heat flow from this rod get affected if the same material volume is used for two fins of the same length? Assume short fin with insulated end
a) 12.25 %
b) 25.6 %
c) 23.4 %
d) 21.2 %

View Answer

Answer: d [Reason:] Case 1 – m1 = 25 per meter, m1 l = 25 * 0.08 = 2. Therefore, Q1 = 3.935W Case 2 – d =0.00707 m, m2 = 29.73 per meter, m2 l = 2.378. Therefore, Q2 = 2.385W % increase in heat flow = 4.77 – 3.935/3.935 = 0.21.

9. Two rods A and B of the same length and diameter protrude from a surface at 120 degree Celsius and are exposed at air at 25 degree Celsius. The temperatures measured at the end of the rods are 50 degree Celsius and 75 degree Celsius. If thermal conductivity of material A is 20W/ m K, calculate it for B
a) 31.13W/m K
b) 41.13W/m K
c) 51.13W/m K
d) 61.13W/m K

View Answer

Answer: c [Reason:] α/ α0 = t – t a/t 0 – t a = 1/cos h ml. For rod A, cos h m1 l =3.8. Similarly for rod 2, cos h m2 l = 1.9, m1/m2 = 1.599. So k2 = k1 (1.599)2 = 51.13W/m K.

10. An centrifugal pump which circulates a hot liquid metal at 500 degree Celsius is driven by a 3600 rpm electric motor. The motor is coupled to the pump impeller by a horizontal steel shaft of dia 25 mm. Let us assume the motor temperature as 60 degree Celsius with the ambient air at 25 degree Celsius, what length of shaft should be specified between the motor and the pump? It may be presumed that the thermal conductivity of the shaft material is 35W/m K, and that the convective film coefficient between the steel shaft and the ambient air is 15.7W/square m K
a) 38.96 cm
b) 54.76 cm
c) 23.76 cm
d) 87.43 cm

View Answer

Answer: a [Reason:] 60 – 25/500 – 25 = cos h m (l – l)/cos h ml = 1/cos h ml, so ml = 3.3. For a circular shaft of diameter d, P/A = 4/d, m = 8.47 per meter. So, l = 3.3/8.47 = 38.96 cm.