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# Multiple choice question for engineering

## Set 1

1. A cable of 10 mm outside is to be laid in an atmosphere of 25 degree Celsius (h = 12.5 W/m2 degree) and its surface temperature is likely to be 75 degree Celsius due to heat generated within it. How would the heat flow from the cable be affected if it is insulated with rubber having thermal conductivity k = 0.15 W/m degree?
a) 43.80 W per meter length
b) 53.80 W per meter length
c) 63.80 W per meter length
d) 73.80 W per meter length

Answer: b [Reason:] Q = 2 π d t/ (1/k) log e(r c/r 0) = 53.80 W per meter length.

2. Chose the correct one with respect to critical radius of insulation
a) There is more heat loss i.e. conductive
b) There occurs a decrease in heat flux
c) Heat loss increases with addition of insulation
d) Heat loss decreases with addition of insulation

Answer: c [Reason:] For a pipe heat loss is more at the critical radius.

3. A heat exchanger shell of outside radius 15 cm is to be insulated with glass wool of thermal conductivity 0.0825 W/m degree. The temperature at the surface is 280 degree Celsius and it can be assumed to remain constant after the layer of insulation has been applied to the shell. The convective film coefficient between the outside surface of glass wool and the surrounding air is estimated to be 8 W/m2 degree. What is the value of critical radius?
a) 9.31 mm
b) 10.31 mm
c) 11.31 mm
d) 12.31 mm

Answer: b [Reason:] Critical radius of insulation = k/h = 0.0825/8 = 0.01031 m = 10.31 mm.

4. For an object i.e. spherical the value of critical radius would be
a) 2k/3h
b) 3k/h
c) 2k/h
d) k/h

Answer: c [Reason:] It depends on variation of angle with layers of insulation.

5. Maximum value of critical radius is
a) 0.01 m
b) 0.04 m
c) 0.06 m
d) 0.0001 m

Answer: a [Reason:] K for common insulating material is 0.05 W/ m degree.

6. An electric cable of aluminum (k = 240 W/ m degree) is to be insulated with rubber (k = 6 W/ square meter degree). If the cable is in air (h = 6 W/square meter degree). Find the critical radius?
a) 80 mm
b) 160 mm
c) 40 mm
d) 25 mm

Answer: d [Reason:] Critical radius = 0.15/6 = 0.025 m = 25 mm.

7. The value of critical radius in case of cylindrical hollow object is
a) 2k/h
b) 2h/k
c) k/h
d) h/k

Answer: c [Reason:] Unit is meter.

8. A wire of radius 3 mm and 1.25 m length is to be maintained at 60 degree Celsius by insulating it by a material of thermal conductivity 0.175 W/m K. The temperature of surrounding is 20 degree Celsius with heat transfer coefficient 8.5 W/ m2 K. Find percentage increase in heat loss due to insulation?
a) 134.46 %
b) 124.23 %
c) 100.00 %
d) 12.55 %

Answer: a [Reason:] Q = 8.5 (2 π 0.003 1.25) (60 – 20) = 8.01 W. % increase = (18.78 – 8.01/8.01) (100) = 134.46 %.

9. A pipe of outside diameter 20 mm is to be insulated with asbestos which has a mean thermal conductivity of 0.1 W/m degree. The local coefficient of convective heat to the surroundings is 5 W/square meter degree. Find the critical radius of insulation for optimum heat transfer from pipe?
a) 10 mm
b) 20 mm
c) 30 mm
d) 40 mm

Answer: b [Reason:] Critical radius of insulation = k/h0 = 0.1/5 = 0.02 m = 20 mm.

10. For insulation to be properly effective in restricting heat transmission, the pipe radius r0 will be
d) Greater than or equal to critical radius

Answer: d [Reason:] Addition of insulating material doesn’t always decrease in the heat transfer rate.

## Set 2

1. Which of the following have a continuous spectrum?
a) Solids
b) Liquids
c) Gases
d) Solids and liquids

Answer: d [Reason:] They emit and absorb radiant energy of all wavelength.

2. Which of the following is/are known as selective absorbers?
a) Solids
b) Liquids
c) Gases and vapors
d) Vapors

Answer: c [Reason:] They emit and absorb radiant energy in definite parts of the spectrum called bands.

3. Which layers participated in the process of thermal radiation through solids and liquids?
a) 1 micron to 1 mm thick
b) 2 micron to 2 mm thick
c) 1 micron to 0.5 mm thick
d) 2 micron to 4.57 mm thick

Answer: a [Reason:] Gases possess a much small radiating power and all their volumes participate in the radiation.

4. Some of the gases have low emissive power and absorptivity. They are considered practically diathermanous. Which of the following option is true?
a) Ammonia
b) Helium
c) Carbon-dioxide
d) Methane

Answer: b [Reason:] Helium is monoatomic gas and is extremely inert to thermal radiation.

5. Consider an enclosure formed by three surfaces having the following values of shape factors, emissivities and temperatures

Surface 1 i.e. curved cylindrical has an emissivity 0.75 and temperature 800 K
Surface 2 i.e. closing disc has an emissivity 0.8 and temperature 700 K
Surface 3 i.e. closing disc has an emissivity 0.8 and temperature 700 K
The closing flat discs are 25 mm in diameter and they have interspacing distance equal to 100 mm. If the shape factor between these two identical discs is 0.05, calculate the net rate of radiant heat flow from the curve surface to each of the closing end surface
a) 1.561 W
b) 2.561 W
c) 3.561 W
d) 4.561 W

Answer: c [Reason:] F 21 + F 23 + 1 and A 1 = 0.00785 m2, A2 = 0.000491 m2. Then from reciprocity theorem, F 12 = 0.0594. Q12 = F 12 A 1 σ b (T 14 – T 2 4) = 3.561 W.

6. Consider a beam of monochromatic radiation at wavelength λ that enters a layer of absorbing gas. As the beam passes through the gas layer, its intensity
a) Decreases
b) Increases
c) Become twice
d) Remains same

Answer: a [Reason:] The decrease is given by d I λ X = – K λ I λ X d x.

7. When a gas or vapor is in the process of oxidation and combustion, it is called as
a) Oxidation
b) Flame
c) Reduction
d) Combustion

Answer: b [Reason:] It is known as flame.

8. The monochromatic absorption coefficient depends upon
(i) Temperature
(ii) Time
(iii) Pressure
(iv) Wavelength
Identify the correct option
a) 1, 2 and 4
b) 2, 3 and 4
c) 1 and 2
d) 1, 3 and 4

Answer: d [Reason:] It depends upon the state of gas and the wavelength.

9. Which one of the following is not extremely inert to thermal radiation?
a) Orgon
b) Oxygen
c) Methane
d) Nitrogen

Answer: c [Reason:] Methane is a polyatomic gas.

10. Consider a beam of monochromatic radiation at wavelength λ that enters a layer of absorbing gas. As the beam passes through the gas layer, its intensity is given by
a) d I λ X = – K λ I λ X d x
b) d I λ X = – 2 K λ I λ X d x
c) d I λ X = – 3 K λ I λ X d x
d) d I λ X = – 4 K λ I λ X d x

Answer: a [Reason:] Its intensity gets reduced.

## Set 3

1. Consider two bodies, one absolutely back and the other non-black and let these be at same temperature. Which one of the following statement is correct?
a) Black body radiates less intensively than a non-black body
b) Non-black body radiates less intensively than a black body
d) None of them will radiates

Answer: b [Reason:] The radiation spectrum for a non-black body may be similar or radically different from that of a black body.

2. When Stefan-Boltzmann law is applied to a black body, it takes the form
a) E = σ T
b) E = σ T 2
c) E = σ T 3
d) E = σ T 4

Answer: d [Reason:] The constant is different for different bodies.

3. When the emissivity of non-black surface is constant at all temperatures and throughout the entire range of wavelength, the surface is called
a) Gray body
b) Transparent body
c) Opaque bodies
d) Perfect black body

Answer: a [Reason:] The radiation spectrum for a grey body, though reduced in vertical scale, is continuous and identical to the corresponding curve for a perfectly black surface.

4. The emissivity of the gray surface may be expressed as
a) σ /2 σ b
b) σ / 3 σ b
c) σ / σ b
d) ½ σ / σ b

Answer: c [Reason:] It is the ratio of radiating coefficient to that of black body radiating coefficient.

5. If a black body at 1000 K and a gray body at 1250 K emit the same amount of radiation, what should be the emissivity of the gray body?
a) 0.3096
b) 0.4096
c) 0.5096
d) 0.6096

Answer: b [Reason:] E = (T b/T G) 4 = 0.4096.

6. The radiant heat transfer from a plate of 2.5 cm2 area at 1250 K to a very cold enclosure is 5.0 W. Determine the emissivity of the plate at this temperature
a) 0.444
b) 0.344
c) 0.244
d) 0.144

Answer: d [Reason:] Emissivity = E/σ A T4 = 0.144.

7. A 100 W light bulb has a tungsten filament (emissivity = 0.30) which is required to operate at 2780 K. If the bulb is completely evacuated, calculate the minimum surface area of the tungsten filament
a) 0.98 * 10 -4 m2
b) 1.98 * 10 -4 m2
c) 2.98 * 10 -4 m2
d) 3.98 * 10 -4 m2

Answer: a [Reason:] E = (Emissivity) σ A T4. So, A = 0.98 * 10 -4 m2.

8. The monochromatic emissivity € of a diffuse surface at 1600 K varies with wavelength in the following manner
€ = 0.4 for 0 < λ < 2
= 0.8 for 2 < λ < 5
Determine the total emissivity

a) 0.5558
b) 0.5568
c) 0.5578
d) 0.5588

Answer: c [Reason:] Total emissivity = 0.4 (-0.3181 – 0.0000) + 0.8 (0.8563 – 0.3118) = 0.5578.

9. For a hemisphere solid angle is measured as
a) Radian and its maximum value is π
b) Degree and its maximum value is 180 degree
c) Steradian and its maximum value is 2π
d) Steradian and its maximum value is π

10. A gray body (E = 0.8) emits the same amount of heat as a black body at 1075 K. Find out the required temperature of the gray body
a) 1146.72 K
b) 1136.72 K
c) 1126.72 K
d) 1116.72 K

Answer: b [Reason:] T b4 = E T g4.

## Set 4

1. Engineering problems of practical interest are involved with heat exchange between two or more surfaces, and this exchange is strongly dependent upon
(ii) Temperature levels
(iii) Surface geometrics
Identify the correct statements
a) 1 and 2
b) 1 and 3
c) 2 and 3
d) 1, 2 and 3

Answer: d [Reason:] For black surface, it is necessary to determine what portion of radiation emitted by one will be intercepted by the other.

2. The fraction of the radiative energy that is diffused from one surface element and strikes the other surface directly with no intervening reflections is called
(ii) Geometrical factor
(iii) Configuration factor
a) 1 only
b) 2 only
c) 1, 2 and 3
d) 3 only

Answer: c [Reason:] Radiation shape factor, geometrical factor and configuration factor are all same.

3. The interchange factor is also known as
a) Equivalent emissivity
d) Shape factor

Answer: a [Reason:] The interchange factor is also known as equivalent emissivity.

4. For the same type of shapes, the value of radiation shape factor will be higher when
a) Surfaces are closer
b) Surfaces are larger and held closer
c) Surfaces are moved further apart
d) Surfaces are smaller and held closer

Answer: b [Reason:] Obviously the value of radiation shape factor will be higher when surfaces are larger and held closer.

5. A thin shield of emissivity E 3 on both sides is placed between two infinite parallel plates of emissivities E 1 and E 2 and temperatures T 1 and T 2. If E 1 = E 2 = E 3, then the fraction radiant energy transfer without shield takes the value
a) 0.25
b) 0.50
c) 0.75
d) 1.25

Answer: b [Reason:] The ratio of radiant energy transfer without and with shield is given by (1/E 1 + 1/E 2 – 1)/ [(1/E 1 + 1/E 3 – 1) + (1/E 3 + 1/E 2 – 1)].

6. The grey body shape factor for radiant heat exchange between a small body (emissivity = 0.4) in a large enclosure (emissivity = 0.5) is
a) 0.1
b) 0.2
c) 0.4
d) 0.5

Answer: c [Reason:] (F) 12 = 1/ (1 – E 1 + 1 + 0).

7. Two long parallel surfaces, each of emissivity 0.7 are at different temperatures and accordingly have radiation exchange between them. It is desired to reduce 75% of this radiant heat transfer by inserting thin parallel shields of equal emissivity 0.7 on both sides. What should be the number of shields?
a) 2
b) 4
c) 1
d) 3

Answer: d [Reason:] Without shields/with shield = 1/N + 1.

8. An enclosure consists of four surfaces 1, 2, 3 and 4. The view factors for radiation heat transfers are
F 11 = 0.1
F 12 = 0.4
F 13 = 0.25
The surface areas A 1 and A 2 are 4 m2 and 2 m2. The view factor F 41 is
a) 0.50
b) 0.75
c) 0.1
d) 0.25

Answer: a [Reason:] F 11 + F 12 + F 13 + F 14 = 1.

9. The value of shape factor depends on how many factors?
a) 4
b) 1
c) 2
d) 3

Answer: c [Reason:] Geometry and orientation.

10. Find the shape factor F 12 for the arrangement shown in the figure. The areas A 1 and A 2 are perpendicular but do not share the common edge

a) 0.03
b) 0.04
c) 0.05
d) 0.06

Answer: b [Reason:] A 5 = A 1 + A 3 and A 6 = A 2 + A 4. The sequence of the solution is, A 5 F 56 = A 1 F 16 + A 3 F 36.

## Set 5

1. Capacity ratio is defined as the product of
a) Mass and temperature
b) Mass and specific heat
c) Specific heat and temperature
d) Time and temperature

Answer: b [Reason:] The product mass and specific heat of a fluid flowing in a heat exchanger is known a s capacity ratio.

2. A single pass shell and tube heat exchanger, consisting of a bundle of 100 tubes (inner diameter 25 mm and thickness 2 mm) is used for heating 28 kg/s of water from 25 degree Celsius to 75 degree Celsius with the help of a steam condensing at atmospheric pressure on the shell side with condensing heat transfer coefficient 5000 W/m2 degree. Make calculation for overall heat transfer coefficient based on the inner area. Take fouling factor on the water side to be 0.002 m2 degree/W per tube and neglect effect of fouling factor on the shell side and thermal resistance of the tube wall

a) 647.46 W/m2 degree
b) 747.46 W/m2 degree
c) 847.46 W/m2 degree
d) 947.46 W/m2 degree

Answer: c [Reason:] Q = m c c c (t c2 – t c1). Re = 7394, Pr = 3.53 and Nu = 47.41. I/U = I/h I + R + r i /(r 0) (h 0).

3. Which of the following is not associated with heat exchanger?
a) Fouling
b) NTU
c) Capacity ratio

Answer: d [Reason:] The correction factor i.e. Mc Adam’s is associated with laminar film condensation on a vertical plate.

4. The engine oil at 150 degree Celsius is cooled to 80 degree Celsius in a parallel flow heat exchanger by water entering at 25 degree Celsius and leaving at 60 degree Celsius. Estimate the exchanger effectiveness
a) 0.56
b) 0.66
c) 0.76
d) 0.86

Answer: a [Reason:] Effectiveness = (t h 1 – t h 2) C h /C MIN (t h 1 – t c 2).

5. Consider the above problem, if the fluid flow rates and the inlet conditions remain unchanged, workout the lowest temperature to which the oil may be cooled by increasing length of the exchanger
a) 46.62 degree Celsius
b) 56.62 degree Celsius
c) 66.62 degree Celsius
d) 76.62 degree Celsius

Answer: c [Reason:] Effectiveness = 1 – [exponential [- NTU (1 – C)]/1 + C].

6. In a surface condenser, the water flowing through a series of tubes at the rate of 200 kg/hr is heated from 15 degree Celsius to 75 degree Celsius. The steam condenses on the outside surface of tubes at atmospheric pressure and the overall heat transfer coefficient is estimated at 860 k J/m2 hr degree. Find the effectiveness of the heat exchanger. At the condensing pressure, stream has saturation temperature 0f 100 degree Celsius and the latent heat of vaporization is 2160 k J/kg. Further, the steam is initially just saturated and the condensate leaves the exchanger without sub-cooling i.e. only latent heat of condensing steam is transferred to water. Take specific heat of water as 4 k J/kg K
a) 0.224
b) 0.706
c) 2.224
d) 3.224

Answer: b [Reason:] Effectiveness = 1 – exponential (- NTU) and Effectiveness = C h (t h 1 – t h 2)/C MIN (t h 1 – t c 2).

7. Consider the above problem, find the tube length. Let the diameter of tube is 25 mm
a) 14.5 m
b) 15.5 m
c) 16.5 m
d) 17.5 m

Answer: a [Reason:] NTU = U (π d l)/C.

8. For evaporators and condensers, for the given conditions, the logarithmic mean temperature difference for parallel flow is
a) Does not depend on counter flow
b) Smaller than counter flow
c) Greater than counter flow
d) Equal to counter flow

Answer: d [Reason:] The temperature of one of the fluid remains constant during the flow passage.

9. Water (specific heat = 4 k J/kg K) enters a cross flow exchanger (both fluids unmixed) at 15 degree Celsius and flows at the rate of 7.5 kg/s. It cools air (C P = 1 k J/kg K) flowing at the rate of 10 kg/s from an inlet temperature of 120 degree Celsius. For an overall heat transfer coefficient of 780 k J/m2 hr degree and the surface area is 240 m2, determine the NTU
a) 4.2
b) 5.2
c) 6.2
d) 7.2