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Heat Transfer
Quiz 4
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Q.1
What is the thermal conductivity of a perfect heat insulator?
Zero
One
∞
Between 0 and ∞
Q.2
Crystal size in a continuous crystalliser depends upon the
Rate of heat transfer
Degree of turbulence
Degree of supersaturation
All of the above
Q.3
For heat flow through very thick walled cylinder, use____mean radius.
Arithmetic
Logarithmic
Geometric
Either A or C
Q.4
The ratio of kinematic viscosity to thermal diffusivity is called the __________ number.
Peclet
Prandtl
Stanton
Nusselt
Q.5
The main function of baffles provided in a shell and tube heat exchanger is to
Facilitate the cleaning of outer tube surface
Enhance turbulence
Hold the tubes in position
All of the above
Q.6
The maximum heat transfer co-efficient from steam heating will be attained when the steam is
Supersaturated
Saturated
Wet
None of these
Q.7
At Pr > 1, conduction in an ordinary fluid flowing through a heated pipe is limited to the
Buffer zone
Turbulent core
Both A and B
Viscous sub-layer
Q.8
When vaporisation takes place directly at the heating surface, it is called
Film boiling
Nucleate boiling
Vapour binding
None of these
Q.9
Which is the most suitable for the con-certration of highly concentrated solution?
Open pan evaporation
Long tube vertical evaporator
Agitated film evaporator
None of these
Q.10
Heat flux increases with temperature drop beyond the Leiden frost point in the plot of heat flux vs. temperature drop for a boiling liquid, because
Convection becomes important
Conduction becomes important
Radiation becomes important
Sub-cooled boiling occurs
Q.11
For a laminar flow of fluid in a circular tube, 'h₁' is the convective heat transfer co-efficient at velocity 'V₁'. If the velocity is reduced by half and assuming the fluid properties are constant, the new convective heat transfer co-efficient is
1.26 h₁
0.794 h₁
0.574 h₁
1.741 h₁
Q.12
Tube expansion allowances exist in __________ heat exchanger.
Multipass fixed tube sheet
U-tube
Single pass fixed tube sheet
None of these
Q.13
In case of a shell and tube heat exchanger, the minimum and maximum baffle spacing is respectively (where, D = inside diameter of the shell)
D/5 and D
D/2 and 2 D
D/4 and 2 D
D and 2 D
Q.14
In a shell and tube heat exchanger, the height of 25 percent cut baffles is equal to (where, D = inside diameter of shell)
0.25 D
0.75 D
0.50 D
None of these
Q.15
Tube pitch is the __________ of tube diameters and the clearances.
Sum
Difference
Ratio
None of these
Q.16
The average heat transfer co-efficient for laminar film condensation on vertical surface is inversely proportional to (where, ΔT = Temperature drop across condensate film )
(ΔT)
2
√ΔT
(ΔT)
1/4
(ΔT)
3/2
Q.17
(N
Gr
x N
Pr
) is called the __________ number.
Graetz
Reyleigh
Nusselt
Stanton
Q.18
The rate of emission of radiation by a body does not depend upon the
Wavelength of radiation
Surface temperature of the body
Nature of the surface
Shape and porosity of the body
Q.19
In a multiple effect evaporator, the effect of boiling point elevation is to
Reduce the capacity
Reduce the economy
Increase the economy
None of these
Q.20
Pick out the wrong statement.
The emissivity of a surface decreases, if it gets corroded by atmospheric environment
The emissivity of a surface increases with increase in surface roughness
The emissivity of a polished surface is quite low
The emissivity of a non-metallic surface decreases with increase in the temperature
Q.21
For the same heat transfer area and the terminal conditions, the ratio of the capacities of a single effect evaporator to a triple effect evaporator is
3
0.33
1
1.33
Q.22
200 kg of solids (on dry basis) is subjected to a drying process for a period of 5000 seconds. The drying occurs in the constant rate period with the drying rate as, Nc = 0.5 x 10⁻³ kg/m².s. The initial moisture content of the solid is 0.2 kg moisture/kg dry solid. The interfacial area available for drying is 4 m²/1000 kg of dry solid. The moisture content at the end of the drying period is (in kg moisture/kg dry solid)
0.5
0.05
0.1
0.15
Q.23
It is desired to concentrate a 20% salt solution (20 kg of salt in 100 kg of solution) to a 30% salt solution in an evaporator. Consider a feed of 300 kg/min at 30°C. The boiling point of the solution is 110°C, the latent heat of vaporisation is 2100 kJ/kg and the specific heat of the solution is 4 kJ/kg.K. The rate at which the heat has to be supplied in (kJ/min) to the evaporator is
3.06 x l0⁵
6.12 x 10⁵
7.24 x 10⁵
9.08 x 10⁵
Q.24
If h₁ = inner film co-efficient and /h₂ = outer film co-efficient, then the overall heat transfer co-efficient is
Always less than h₁
Always between h₁ and h₂
Always higher than h₂
Dependent on metal resistance
Q.25
Mode of heat transfer involved in the cooling of air cooled internal combustion engine is
Conduction
Natural convection
Forced convection
None of these
Q.26
A BTU/hr.ft.2 °F is equal to
1 kcal/hr. m²°C
4.88 kcal/hr. m.2°C
1 kcal/hr. m².°K
None of these
Q.27
A dephlegmator is a
Total condenser
Vacuum evaporator
Partial condenser
Double pipe heat exchanger
Q.28
Vacuum is generally maintained in the vapour space of an evaporator mainly to
Get economical temperature difference by using moderate pressure steam
Facilitate forward feeding in multiple effect evaporation
Concentrate heat sensitive materials
Achieve very high concentration of the final product
Q.29
Prandtl and Reynold's analogies are identical for Prandtl number value of
0
0.5
1
5
Q.30
The thermal radiative flux from a surface of emissivity = 0.4 is 22.68 kW/m². The approximate surface temperature (K) is
(Stefan-Boltzman constant = 5.67xl0⁻⁸ W/m².K⁴)
1000
727
800
1200
Q.31
Which of the following is concerned with both heat and mass transfer?
Lewis relationship
Nusselt number
Kutateladze number
Froude number
Q.32
An ejector is used to
Increase pressure
Increase temperature
Remove condensate
None of these
Q.33
An insulator should have
Low thermal conductivity
High thermal conductivity
Less resistance to heat flow
A porous structure
Q.34
Shell side pressure drop in a shell and tube heat exchanger does not depend upon the
Baffle spacing & shell diameter
Tube diameter & pitch
Viscosity, density & mass velocity of shell side fluid
None of these
Q.35
Evaporation of 1kg of water from a solution in a single effect evaporator requires about __________ kg of steam.
0.4 - 0.6
1-1.3
1.8-2
2 - 2.4
Q.36
Stefan-Boltzman law which describes the radiation heat transfer states that, it is proportional to (where, t = temperature in °C T = absolute temperature in ° K )
t⁴
T⁴
1/t⁴
1/T⁴
Q.37
Fqra cold viscous feed, backward feed gives __________ than forward feed.
A higher capacity
A lower capacity
Lower economy
None of these
Q.38
In a multipass shell and tube heat exchanger, tube side return pressure loss is equal to __________ the velocity head.
Twice
Four times
Square root of
Square of
Q.39
Terminal point temperature differences between fluids in case of a heat exchanger is termed as
Approach
Log mean temperature difference
Arithmetic mean temperature difference
Geometric mean temperature difference
Q.40
Nusselt number for full developed, laminar, constant property flow in a pipe at uniform heat flux is
0.72
4.364
18
83
Q.41
Value of Nusselt number [Nu = (hD/k)] for the heat transfer by conduction from a droplet or a spherical particle to a surrounding stagnant film is
0.5
2
10
100
Q.42
In case of vertical tube evaporator, with increase in liquor level, the overall heat transfer co-efficient
Increases
Decreases
Is not affected
May increase or decrease; depends on the feed
Q.43
Dietus-Boelter equation cannot be used for molten metals mainly due to its very low
Prandtl number
Grashoff number
Thermal conductivity
Viscosity
Q.44
A hollow sphere and a solid sphere of the same material and equal radii are heated to the same temperature. In this case,
The cooling rate will be the same for the two spheres and hence the two spheres will have equal temperatures at any instant
Both the spheres will emit equal amount of radiation per unit time in the beginning
Both will absorb equal amount of radiation from the surrounding in the beginning
Both B & C
Q.45
The film thickness for laminar film condensation on vertical surface __________ from top to bottom.
Cummulatively increases
Cummulatively decreases
Remains constant
And the surface conductance increase
Q.46
The overall resistance for heat transfer through a series of flat resistance, is the __________ of the resistances.
Average
Geometric mean
Product
Sum
Q.47
Viscosity of gases __________ with increase in temperature.
Increase very rapidly
Increase slowly
Decrease slowly
Remain unaffected
Q.48
Thermal conductivity of a gas at low density, __________ with increase in temperature.
Decreases
Increases
Remains unchanged
May increase or decrease; depends on the gas
Q.49
For large heat transfer area requirement, shell and tube heat exchanger is preferred, because it
Occupies smaller space
Is more economical
Is easy to operate and maintain
All of the above
Q.50
At constant temperature, the thermal conductivities of gases __________ with rise in pressure.
Decrease
Increase
Remain unchanged
May increase or decrease; depends on the pressure
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