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Chemical Engineering
Heat Transfer
Quiz 8
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Q.1
As per Kirchoffs law, the ratio of the total radiating power to the absorptivity of a body depends on the
Temperature of the body only
Wavelength of monochromatic radiation
Both A and B
Nature of material of body
Q.2
With increase in temperature, the thermal conductivity of fresh lubricating oil
Increases
Decreases
Remains unchanged
May increase or decrease; depends on its composition
Q.3
The Stefan-Boltzman constant depends on the
Medium
Temperature
Surface
None of these
Q.4
Pick out the wrong statement.
Heat transfer from a hot body to cold body by the emission of heat waves is called radiation
Filmwise condensation takes place on non-wettable surfaces
The boiling point of a solution is affected by liquid head as well as boiling point elevation
None of these
Q.5
'Fouling factor' used in the design of a multipass shell and tube heat exchanger is a
Non-dimensional factor
Factor of safety
Conversion factor for individual film heat transfer co-efficient to overall heat transfer co-efficient
None of these
Q.6
In a gas-liquid shell and tube heat exchanger, the
Presence of a non-condensible gas decreases the condensing film co-efficient
Gases under high pressure are routed through the tube side, because high pressure gases are corrosive in nature
Gases to be heated/cooled is normally routed through the shell side, because the corrosion caused by the cooling water or steam condensate remain localised to the tubes
All of the above
Q.7
Pick out the wrong statement.
In drying a solid containing moisture above the critical moisture content the number of degrees of freedom is 2
Sherwood number in mass transfer corresponds to Nusselt number in heat transfer and Schmidt number to Prandtl number
Forced convection is relatively more effective in increasing the rate of mass transfer, if Schmidt number is larger
Hot gases at moderate pressure are usually in the shell side of shell and tube heat exchangers. At higher pressure, however, it is customary to put gas in the tube side
Q.8
Multipass heat exchangers are used
Because of simplicity of fabrication
For low heat load
To obtain higher heat transfer co-efficient and shorter tube
To reduce the pressure drop
Q.9
In a parallel flow heat exchanger, if the outlet temperature of hot and cold fluids are the same, then the log mean temperature difference (LMTD) is
Minimum
Maximum
Zero
Infinity
Q.10
Low thermal conductivity of heat insulating materials is due to its
Dense structure
High proportion of air space
High specific heat
None of these
Q.11
Presence of a non-condensing gas in a condensing vapour
Increases the rate of condensation
Decreases thermal resistance
Is desirable to increase the film co-efficient
None of these
Q.12
In SI units, thermal conductivity is expressed in
Watt/m.°K
Watt/m². °K
Watt/m². °K
Watt/m4. °K
Q.13
With increase in temperature, the thermal conductivity of most liquids
Increases
Decreases
Remains same
First increases upto a certain temperature and then becomes constant
Q.14
Steam economy in case of a triple effect evaporator will be
1
< 1
> 1
Between 0 and 1
Q.15
Kirchoff's law is applicable to
Monochromatic radiation only
Total radiation only
Both A and B
Only volumes and not to surfaces
Q.16
In case of __________ boiling, the liquid temperature is below the saturation temperature and the boiling takes place in the vicinity of the heated surface.
Nucleate
Local
Pool
Saturated
Q.17
__________ heat exchanger is the most suitable, when the temperature of shell side fluid is much higher than that of tube side.
Single pass, fixed tube sheet
U-tube
Three pass, fixed tube sheet
None of these
Q.18
Which of the following has the highest thermal conductivity?
Brick
Air
Water
Silver
Q.19
Absorptivity of a perfect black body is unity. Which of the following has maximum absorptivity?
Aluminium foil
Refractory bricks
Iron plate
Coke breeze
Q.20
As the difference between the wall temperature and bulk temperature increases, the boiling heat transfer co-efficient
Continues to increase
Continues to decrease
Goes through a minimum
Goes through a maximum
Q.21
Which of the following has the lowest overall heat transfer co-efficient?
Dowtherm
Molten sodium
Water
Air
Q.22
The local surface conductance for laminar film condensation on vertical surface is (where, t = film thickness)
∝ t
∝ 1/t
∝ √t
Independent of 't'
Q.23
Heat waves
Can not pass through vacuum
Travel in straight line
Can be reflected by a mirror
Both B and C
Q.24
Which of the following parameters is increased by use of finned tube in a multipass shell and tube heat exchanger?
Tube side pressure drop and the heat transfer rate
Convective heat transfer co-efficient
Effective tube surface area for convective heat transfer
All of the above
Q.25
A __________ evaporator employs an annular downtake.
Basket type
Horizontal
Long tube vertical
None of these
Q.26
During crystallisation, formation of crystal can occur in __________ solution only.
Saturated
Supersaturated
Undersaturated
All of the above
Q.27
Prandtl number is the ratio of
Momentum diffusivity to mass diffusivity
Momentum diffusivity to thermal diffusivity
Thermal diffusivity to mass diffusivity
Thermal diffusivity to momentum diffusivity
Q.28
Pick out the wrong statement.
Bubble size increases with the dynamic viscosity of the liquid in case of nucleate pool boiling
Thermal conductivity of a dry material is more than that of the damp material
Ratio of its capacity to economy equals the steam consumption in kg/hr in an evaporator
Vaporisation of organic substances in evaporator mostly causes foam formation
Q.29
Pick out the correct statement.
Higher is the temperature of the radiating body, higher is the wavelength of radiation
Logarithmic mean area is used for calculating the heat flow rate through a thick walled cylinder
The wavelength corresponding to maximum mono-chromatic emissive power increases with rise in temperature
Solid angle subtended by the finite surface at the radiating element is called the angle of incidence
Q.30
Steady state one dimensional heat flow by conduction as given by Fourier's low does not assume that
There is no internal heat generation
Boundary surfaces are isothermal
Material is anisotropic
Constant temperature gradient exists
Q.31
Steam traps are provided in steam carrying pipelines to
Condense steam
Release excess steam pressure by bleeding steam
Remove condensate and inert gases
None of these
Q.32
Overall heat transfer co-efficient for cooling of hydrocarbons by water is about
50 -100 Kcal/hr.m².°C
50 -100 W/m² . °K
50 -100 BTU/hr. ft.2°F
1000 - 1500 BTU/hr. ft.2°F
Q.33
Radiant energy received by a body is proportional to (where, d = the distance between the object emitting radiation and that receiving it.)
√d
d
d
2
d
1.5
Q.34
Heat transfer by natural convection is enhanced in system with
High viscosity
High co-efficient of thermal expansion
Low temperature gradients
Low density change with temperature
Q.35
A black body when hot, emits heat radiation of __________ wavelengths.
Small
Large
All
One fixed
Q.36
Which characteristic of a fluid is not important in deciding its route in a shell and tube heat exchanger?
Corrosiveness
Fouling characteristic
Viscosity
None of these
Q.37
The heat flux in the nucleate boiling regimes is proportional to (where, ΔT = excess temperature)
(ΔT)²
(ΔT)⁴
(ΔT)³
√(ΔT)
Q.38
For small temperature difference, the heat transfer rate as per Newton's law of cooling is proportional to (where, Δt = excess temperature)
Δt
Δt²
Δt³
√Δt
Q.39
Analogy between mass and heat transfer is not applicable in case of
Same velocity profile or equal eddy diffusivities
Thermal or pressure mass diffusion
Viscous heating or chemical reaction
Both B and C
Q.40
Heat transfer rate per unit area is called
Thermal conductivity
Heat flux
Heat transfer co-efficient
Thermal diffusivity
Q.41
Water is normally used as a coolant in the heat exchange equipments mainly because ofits
Abundance & high heat capacity
Low density
Low viscosity
High fluidity
Q.42
The overall heat transfer co-efficient for a shell and tube heat exchanger for clean surfaces is U0 = 400 W/m².K. The fouling factor after one year of operation is found to be hd0 = 2000 W/m².K. The overall heat transfer co-efficient at this time is
1200W/m².K
894W/m².K
333W/m².K
287 W/m².K
Q.43
Forced circulation evaporators are normally used for concentrating liquids having
Scaling characteristics
High viscosity
Both A & B
Neither A nor B
Q.44
In sub-cooled boiling,
Temperature of the heating surface is less than the boiling point of the liquid
Temperature of the heating surface is more than the boiling point of the liquid
Bubbles from heating surface are absorbed by the mass of the liquid
Very large vapour space is necessary
Q.45
A dilute aqueous solution is to be concentrated in an evaporator system. High pressure steam is available. Multiple effect evaporator system is employed, because
Total heat transfer area of all the effects is -less than that in a single effect evaporator system
Total amount of vapor produced per Kg of feed steam in a multiple effect system is much higher than in a single effect
Boiling point elevation in a single effect system is much higher than that in any effect in a multieffect system
Heat transfer co-efficient in a single effect is much lower than that in any effect in a multieffect system
Q.46
The critical radius of insulation for cylindrical pipe is (where, h
i
= heat transfer coefficient at inside of the pipe )
K/h
0
2K/h
0
h
i
/K
2h
i
/K
Q.47
The purpose of providing bleed points in the evaporator is to
Admit the feed
Remove the product
Facilitate removal of non-condensable gases
Create vacuum
Q.48
Maximum water velocity in tubes of a 1-2 shell and tube heat exchanger may be around __________ metres/second.
1
10
20
30
Q.49
At what value of Prandtl number, the hydrodynamic and thermal boundary layers are identical?
0.5
1
1.5
80
Q.50
In an interphase heat transfer process, the equilibrium state corresponds to equality of temperature in the two phases, while the condition for equilibrium in an interphase mass transfer process is equality of
Concentrations
Chemical potentials
Activity co-efficients
Mass transfer co-efficients
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