Heat Transfer Flashcards
(239 cards)
1
Q
Crystal size in a continuous crystalliser depends upon the ____.
rate of heat transfer.
degree of turbulence.
degree of supersaturation.
all (a), (b) and (c).
A
all (a), (b) and (c).
2
Q
Air is best heated with steam in a heat exchanger of ____.
plate type.
double pipe type with fin on steam side.
double pipe type with fin on air side.
shell and tube type.
A
double pipe type with fin on air side.
3
Q
Black liquor generated during paper manufacture is concentrated in a ____.
single effect evaporator.
single effect evaporator followed by a crystalliser.
multiple effect evaporator.
multiple effect evaporators followed by a crystalliser.
A
multiple effect evaporator.
4
Q
When vaporisation takes place directly at the heating surface, it is called
film boiling
nucleate boiling
vapour binding
none of these
A
nucleate boiling
5
Q
Steam consumption in kg/hr in case of an evaporator is given by (where, C & E are capacity the economy of the evaporator respectively)
C/E
E/C
CE
1/CE
A
C/E
6
Q
Reynold’s analogy states that
Nst α f
Nst α NRe
NNu α f
NRe α f
A
Nst α f
7
Q
With increase in temperature, the thermal conductivity of non-metallic amorphous solids
decreases
increases
remains constant
first decreases upto certain temperature and then increases
A
increases
8
Q
Fourier’s law applies to the heat transfer by
convection
radiation
conduction
all (a), (b) & (c)
A
conduction
9
Q
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.
A
none of these.
10
Q
In the equation Q = UAΔt; Δt is
geometric mean temperature difference.
arithmetic mean temperature difference.
logarithmic mean temperature difference.
the difference of average bulk temperatures of hot and cold fluids.
A
logarithmic mean temperature difference.
11
Q
With increase in porosity, the thermal conductivity of a solid substance
increases
decreases
remains unchanged
may increase or decrease; depends on the solid
A
decreases
12
Q
For an ideal black body
absorptivity = 1
reflectivity = 1
emissivity = 0
transmissivity = 1
A
absorptivity = 1
13
Q
In Joule’s experiment, an insulated container contains 20 kg of water initially at 25°C. It is stirred by an agitator, which is made to turn by a slowly falling body weighing 40 kg through a height of 4 m. The process is repeated 500 times. The acceleration due to gravity is 9.8 ms-2. Neglecting the heat capacity of agitator, the temperature of water (in °C) is
40.5
34.4
26.8
25
A
34.4
14
Q
Fouling factor
is a dimensionless quantity.
does not provide a safety factor for design.
accounts for additional resistances to heat flow.
none of these.
A
accounts for additional resistances to heat flow.
15
Q
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
A
decreases
16
Q
The steam ejector is used to
remove condensate from the steam pipelines.
create vacuum.
superheat the steam.
none of these.
A
create vacuum.
17
Q
For shell and tube heat exchanger, with increasing heat transfer area, the purchased cost per unit heat transfer area
increases
decreases
remains constant
passes through a maxima
A
passes through a maxima
18
Q
The thermal efficiency of a reversible heat engine operating between two given thermal reservoirs is 0.4. The device is used either as a refrigerator or as a heat pump between the same reservoirs. Then the coefficient of performance as a refrigerator (COP)R and the co-efficient of performance as a heat pump (COP)HP are ____.
(COP)R = (COP)HP = 0.6
(COP)R = 2.5; (COP)HP = 1.5
(COP)R = 1.5; (COP)HP = 2.5
(COP)R = (COP)HP = 2.5
A
(COP)R = 1.5; (COP)HP = 2.5
19
Q
Hot water (0.01 m3 /min) enters the tube side of a counter current shell and tube heat exchanger at 80°C and leaves at 50°C. Cold oil (0.05 m3/min) of density 800 kg/m3 and specific heat of 2 kJ/kg.K enters at 20°C. The log mean temperature difference in °C is approximately ___.
32
37
45
50
A
32
20
Q
In a heat exchanger, floating head is provided to
facilitate cleaning of the exchanger.
increase the heat transfer area.
relieve stresses caused by thermal expansion.
increase log mean temperature gradient.
A
relieve stresses caused by thermal expansion.
21
Q
Leidenfrost point is a term concerned with the ___.
condensation of the saturated vapor on a cold surface.
concentration of a corrosive solution by evaporation.
heat transfer between two highly viscous liquids.
boiling of a liquid on a hot surface.
A
boiling of a liquid on a hot surface.
22
Q
A process stream of dilute aqueous solution flowing at the rate of10 Kg.s-1 is to be heated. Steam condensate at 95°C is available for heating purpose, also at a rate of 10 Kg.s-1. A 1 - 1 shell and tube heat exchanger is available. The best arrangement is ___.
counter flow with process stream on shell side.
counter flow with process stream on tube side.
parallel flow with process stream on shell side.
parallel flow with process stream on tube side.
A
counter flow with process stream on shell side.
23
Q
Heat produced when a steady state current, I passes through an electrical conductor having resistance, ‘R’ is
IR
I²R
IR²
I²R²
A
I²R
24
Q
Pick out the wrong statement.
Orifice baffles are never used in a shell and tube heat exchanger.
Pressure drop on the shell side of a heat exchanger depends upon tube pitch also.
In a horizontal tube evaporator, surface blanketing by air is avoided.
Split ring type and pull through type floating heads are two commonly used floating heads is heat exchangers.
A
Orifice baffles are never used in a shell and tube heat exchanger.
25
Convective heat transfer co-efficient in case of fluid flowing in tubes is not affected by the tube length/diameter ratio, if the flow is in the __________ zone.
laminar
transition
both 'a' & 'b'
highly turbulent
highly turbulent
26
The actual temperature drop across the heating surface in an evaporator depends on the
feed.
depth of liquid over heating surface.
pressure difference between steam chest and vapour space.
all (a), (b) and (c).
all (a), (b) and (c).
27
Which of the following is not used as a medium for high temperature heating ?
Dowtherm
mercury
liquid metal (e.g. molten sodium)
fused salts (e.g., an eutectic mixture of 53% KNO3, 40% NaNO2and 7% NaNO3)
mercury
28
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.
none of these.
29
Kirchoff s law applies to __________ radiation.
total
monochromatic
both (a) & (b)
neither (a) nor (b)
both (a) & (b)
30
With the increase of temperature, the Col-burn jH factor
increases.
decreases.
remains unchanged.
may increase or decrease ; depending on temperature.
increases.
31
In case of evaporators, liquid entrainment results primarily due to
high vacuum in the evaporator.
high evaporation rate.
foaming of the solution.
high heat transfer rate.
foaming of the solution.
32
Baffles in the shell side of a shell and tube heat exchanger
increase the cross-section of the shell side liquid.
force the liquid to flow parallel to the bank.
increase the shell side heat transfer co-efficient.
decrease the shell side heat transfer co-efficient.
increase the shell side heat transfer co-efficient.
33
Air is to be heated by condensing steam. Two heat exchangers are available (i) a shell and tube heat exchanger and (ii) a finned tube heat exchanger. Tube side heat transfer area are equal in both the cases. The recommended arrangement is ____.
finned tube heat exchanger with air inside and steam outside.
finned tube heat exchanger with air outside and steam inside.
shell and tube heat exchanger with air inside tubes and steam on shell side.
shell and tube heat exchanger with air on shell side and steam inside tubes.
finned tube heat exchanger with air outside and steam inside.
34
The unit of heat transfer co-efficient in SI unit is
J/M2°K
W/m2°K
W/m°K
J/m°K
W/m2°K
35
The number of kg vaporised per kg of steam fed to the evaporator is defined as ____.
capacity
rate of evaporation
economy
rate of vaporisation
economy
36
Pick out the wrong statement.
In case of heat transfer by purely forced convection, GR/Re2 ≤ 1.
The equivalent diameter of heat transfer for a duct of square cross-section (having each side as 'x') is equal to 4x.
Distillation process is not the same as evaporation.
The effectiveness of nucleate boiling depends basically on the ease with which the bubbles are formed and detached from the heating surface.
The equivalent diameter of heat transfer for a duct of square cross-section (having each side as 'x') is equal to 4x.
37
Economy of a multiple effect evaporator is not influenced much by the _____.
boiling point elevations
temperature of the feed
rate of heat transfer
ratio of the weight of the thin liquor to thick liquor
boiling point elevations
38
Increasing the liquor level in the evaporator results in the
decreased capacity.
increase in liquor film co-efficient.
decreased effect of hydrostatic head.
increased true temperature drop.
decreased capacity.
39
The equivalent diameter for pressure drop is __________ that for heat transfer.
smaller than
greater than
equal to
not related with
smaller than
40
Controlling heat transfer film co-efficient is the one, which offers __________ resistance to heat transfer.
no
the least
the largest
lower
the largest
41
The heat flux (from outside to inside) across an insulating wall with thermal conductivity, K= 0.04 W/m.°K and thickness 0.16m is 10 W/m2. The temperature of the inside wall is - 5°C. The outside wall temperature is
25°C
30°C
35°C
40°C
35°C
42
Thermal conductivity of a conducting solid material depends upon its
temperature
porosity
both (a) & (b)
neither (a) nor (b)
both (a) & (b)
43
Pick out the wrong statement.
The controlling resistance in case of heating of air by condensing steam is in the air film.
The log mean temperature difference (LMTD) for counter flow and parallel flow can be theoretically same when any one of the fluids (hot or cold fluid) passes through the heat exchanger at constant temperature.
In case of a 1 - 2 shell and tube heat exchanger, the LMTD correction factor value increases sharply, when a temperature cross occurs.
Phase change in case of a pure fluid at a given pressure from liquid to vapor or vice-versa occurs at saturation temperature.
In case of a 1 - 2 shell and tube heat exchanger, the LMTD correction factor value increases sharply, when a temperature cross occurs.
44
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
(ΔT)^1/4
45
Bulk of the convective heat transfer resistance from a hot tube surface to the fluid flowing in it, is
in the central core of the fluid.
uniformly distributed throughout the fluid.
mainly confined to a thin film of fluid near the surface.
none of these.
mainly confined to a thin film of fluid near the surface.
46
Which of the following is concerned with both heat and mass transfer ?
Lewis relationship
Nusselt number
Kutateladze number
Froude number
Lewis relationship
47
A backward feed multiple effect evaporator is better than forward feed for concentrating cold feed, because it provides
higher economy
lower capacity
both (a) & (b)
lower economy
higher economy
48
A 10 cm dia steam pipe, carrying steam at 180°C, is covered with an insulation (conductivity = 0.6 W/m.°C). It losses heat to the surroundings at 30°C. Assume a heat transfer co-efficient of 0.8 W/m2.°C for heat transfer from surface to the surroundings. Neglect wall resistance of the pipe and film resistance of steam. If the insulation thickness is 2 cms, the rate of heat loss from this insulated pipe will be ____.
greater than that for uninsulated steam pipe.
less than that of the uninsulated steam pipe.
equal to that of the uninsulated steam pipe.
less than the steam pipe with 5 cms insulation.
less than that of the uninsulated steam pipe.
49
Which area is used in case of heat flow by conduction through a cylinder ?
Logarithmic mean area
Arithmetic mean area
Geometric mean area
None of these
Logarithmic mean area
50
Heat transfer by conduction results due to the transfer of free electrons, kinetic energy & vibrational energy from one molecule to another. Conduction heat transfer can not take place
between two bodies in physical contact with each other.
between two bodies not in physical contact with each other.
from one part of a body to the another part of the same body.
both 'b' & 'c'
between two bodies in physical contact with each other.
51
The dimensionless group in mass transfer that is equivalent to Prandtl number in heat transfer is
Nusselt number.
Sherwood number.
Schmidt number.
Stanton number.
Schmidt number.
52
Finned tube heat exchangers ____.
give larger area per tube.
use metal fins of low thermal conductivity.
facilitate very large temperature drop through tube wall.
are used for smaller heat load.
give larger area per tube.
53
The absorptivity of a grey body at a given temperature __________ with increasing wavelength of radiation.
increases
decreases
remains constant
may increase or decrease ; depends on the material
may increase or decrease ; depends on the material
54
Which one gives the monochromatic emissive power for black body radiation ?
Planck's law
Kirchhoffs law
Wien's law
Stefan-Boltzman law
Planck's law
55
For what value of Prandtl number, St = f/2?
1.5
1
> 1
< 1
1
56
In a shell and tube heat exchanger, ____
the temperature drops in the two fluids and the wall are proportional to individual resistances.
the temperature drop is inversely proportional to the resistance across which the drop occurs.
there is no relationship between temperature drop and resistance.
the relationship is not generalised.
the temperature drops in the two fluids and the wall are proportional to individual resistances.
57
Dietus-Boelter equation used for the determination of heat transfer co-efficient is valid
for fluids in laminar flow.
for fluids in tubulent flow.
when Grashhoff number is very important.
for liquid metals.
for fluids in tubulent flow.
58
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 (a), (b) and (c).
all (a), (b) and (c).
59
Out of the following four assumptions used in the derivation of the equation for LMTD , which one is subject to the largest deviation in practice ?
Constant overall heat transfer co-efficient.
Constant rate of fluid flow.
Constant specific heat.
No partial phase change in the system.
Constant rate of fluid flow.
60
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
radiation becomes important
61
Resistance to heat flow by conduction is proportional to (where, t & ρ are thickness & density of the material respectively and A = area normal to the direction of heat flow. )
t
1/ρ
1/A
all (a), (b) & (c)
all (a), (b) & (c)
62
Temperature profile in steady state heat transfer is
asymptotic
hyperbolic
parabolic
linear
linear
63
For the same heat load and mass flow rate in the tube side of a shell and tube heat exchanger, one may use multipass on the tube side, because it
decreases the pressure drop.
decreases the outlet temperature of cooling medium.
increases the overall heat transfer coefficient.
none of these.
increases the overall heat transfer coefficient.
64
When vaporisation takes place through a blanketting film of gas, the phenomenon is termed as __________ boiling.
pool
nucleate
transition
film
film
65
Minimum recommended baffle spacing in a shell and tube heat exchanger is about (where, D = shell diameter)
0.2 D
0.5 D
0.66 D
0.80 D
0.2 D
66
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 )
t^4
T^4
1/t^4
1/T^4
T^4
67
The Nusselt number for fully developed (both thermally and hydrodynamically) laminar flow through a circular pipe, where the wall heat flux is constant, is
2.36
4.36
120.36
dependent on NRe only
4.36
68
What is the unit of thermal conductivity ?
Kcal/hr. m2 °C
Kcal/hr.m.°C
Kcal/hr.m
Kcal/hr. °C
Kcal/hr.m.°C
69
The thermal radiation emitted by a body is proportional to Tn, where T is its absolute temperature. The value of 'n' is exactly 4 for
black painted bodies only
all bodies
polished bodies only
a black body
all bodies
70
It is not preferable to use superheated steam in evaporators, because of its very
high temperature
high pressure
low film co-efficient
none of these
low film co-efficient
71
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 10^5
6.12 x 10^5
7.24 x 10^5
9.08 x 10^5
3.06 x 10^5
72
LMTD for counterflow and prallel flow heat exchanger will be the same, when the ____.
cold fluid is heated to a certain temperature by condensing steam (isothermal fluid).
outlent temperature of both the hot and cold fluid are same.
outlet temperature of hot fluid is less than the outlet temperature of the cold fluid.
none of these.
cold fluid is heated to a certain temperature by condensing steam (isothermal fluid).
73
For flow over a flat plate, the ratio of thermal boundary layer thickness, 'xt' and hydrodynamic boundary layer thickness 'x' is equal to (where, NPr = Prandtl number)
NPr
NPr^1/3
NPr^-1
NPr^-1/3
NPr^1/3
74
The range of electromagnetic spectrum important in heat transfer by radiation is __________ microns.
0.38-0.78
0.5-50
100-1000
5-50
0.5-50
75
Use of transverse baffles in a shell and tube heat exchanger is done to increase the
rate of heat transfer.
flow velocity.
turbulence of shell side fluid.
all (a), (b) and (c).
all (a), (b) and (c).
76
The actual temperature drop across the heating surface of an evaporator depends on the
liquid depth over the heating surface.
solution being evaporated.
pressure difference between the steam chest and the vapor space above the boiling liquid.
all (a), (b) and (c)
all (a), (b) and (c)
77
Economy of a multiple effect evaporator depends upon the ____.
heat balance consideration
rate of heat transfer
both (a) and (b)
neither (a) nor (b)
heat balance consideration
78
The Graetz number is concerned with the ____.
mass transfer between a gas and a liquid.
absorption with chemical reaction.
heat transfer in turbulent flow.
heat transfer in laminar flow.
heat transfer in laminar flow.
79
Solid angle subtended by the finite surface at the radiating element is _____.
called the view factor.
called the angle of vision.
proportional to the square of the distance between surfaces.
expressed in terms of radians.
called the angle of vision.
80
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.
The emissivity of a surface decreases, if it gets corroded by atmospheric environment.
81
In forced convection, the heat transfer depends on
Re, Pr
Re, Gr
mainly Gr
Re only
Re, Pr
82
With increase in temperature, the thermal conductivity of most liquids
increases
decreases
remains same
first increases upto a certain temperature and then becomes constant
decreases
83
Heat transfer by __________ may not necessarily require the presence of a medium.
conduction
natural convection
forced convection
radiation
radiation
84
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.
Logarithmic mean area is used for calculating the heat flow rate through a thick walled cylinder.
85
In a shell and tube type heat exchanger, the floating tube bundle heat arrangement is used _____.
in low range of temperature differences.
in high range of temperature differences.
because of its low cost.
to prevent corrosion of the tube bundles.
in high range of temperature differences.
86
Stefan-Boltzman law applies to __________ body.
black
white
grey
any colour
black
87
A perfect black body is a perfect __________ of radiation.
absorber
emitter
both (a) & (b)
neither (a) nor (b)
both (a) & (b)
88
In case of parallel flow heat exchanger, the lowest temperature theoretically attainable by the hot fluid is __________ the outlet temperature of the cold fluid.
equal to
more than
less than
either more or less than (depending upon the fluid)
equal to
89
An equipment which converts the latent or sensible heat of one fluid into the latent heat of vaporisation of another, is called a
boiler
heat exchanger
recuperator
regenerator
heat exchanger
90
Which of the following is directly concerned with the heat transfer ?
Strouhal number
Sherwood number
Euler number
Grashoff number
Grashoff number
91
Pick out the wrong statement:
With change in temperature, the radiant energy emitted by a black body remains unchanged.
Absorptivity of a body approaches unity in case of diffuse reflection.
Absorptivity of a perfectly black body is unity.
Value of Stefan-Boltzman constant is 4.876 x 10-8 KCal/m2.hr.°K4.
With change in temperature, the radiant energy emitted by a black body remains unchanged.
92
Planck's distribution law is valid for __________ bodies.
black
white
coloured
all (a), (b) & (c)
coloured
93
For hot feed, forward feeding as compared to backward feeding results in __________ economy.
increased
decreased
no effect on
none of these
increased
94
Radiation energy is emitted by all the substances, which are above
0°K
0°C
100°C
room temperature
0°K
95
What is the steam economy in case of a single effect evaporator system ?
1
>1
< 1
0.1
< 1
96
If average heat transfer co-efficient is ha and the local coefficient at the end of the plate is hl then in case of heat transfer to a fluid flowing over a flat plate, heated over its entire length
ha = hl
ha = 2hl
ha = 0.5 hl
ha = 0.75 hl
ha = 2hl
97
The Fourier number (defined as a.t/L2) is used in the analysis of problem involving heat transfer by
forced convection
natural convection
transient conduction
steady state conduction
transient conduction
98
The energy radiated from a surface Q at absolute temperature T is related as
Q ∝ T^2
Q ∝ T^4
Q ∝ T^3
none of these
Q ∝ T^4
99
Baffles are provided in heat exchangers to increase the ____.
fouling factor
heat transfer area
heat transfer co-efficient
heat transfer rate
heat transfer co-efficient
100
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
0.1
101
In case of a vertical tube evaporator, with increase in the liquor level, the
capacity of the evaporator is decreased.
capacity of the evaporator is increased.
true temperature drop increases.
both (b) and (c).
capacity of the evaporator is decreased.
102
The ratio of total radiating power to the absorptivity of the body depends upon the __________ as per Kirchoffs law.
wavelength of radiation
nature of the body
temperature of the body
none of these
temperature of the body
103
For gases, the thermal conductivity increases with temperature rise. For liquids, with increase in concentration, its thermal conductivity generally ____.
decreases
increases
remains unchanged
increases exponentially
decreases
104
A 2-4 heat exchanger involves
only counter-flow of fluids.
only parallel-flow of fluids.
both counter and parallel-flow of the fluids.
smaller pressure drop compared to 1-2 exchanger.
both counter and parallel-flow of the fluids.
105
In a liquid-liquid heat exchanger, for the same process temperature, the ratio of the LMTD in parallel flow to the LMTD in counter flow is always
< 1
> 1
1
∞
< 1
106
With increase in temperature, the thermal conductivity of steel
increases
decreases
remains unchanged
increases exponentially
decreases
107
Which of the following forced convection heat transfer equation accounts for the liquid viscosity effect for viscous liquids ?
Dittus-Boeltier equation
Sieder-Tate equation
Nusselt equation
None of these
Sieder-Tate equation
108
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 'a', 'b' & 'c'.
enhance turbulence.
109
In a single effect evaporator, the economy is
1
< 1
> 1
none of these
< 1
110
LMTD correction factor which is to be applied for a cross-flow heat exchanger increases with increase in the number of shell passes. Its value for a single pass cross flow heat exchanger is
0
1
>1
<1
<1
111
Which of the following is not concerned with the heat transfer?
Brinkman number
Stanton number
Schmidt number
Peclet number
Schmidt number
112
Latent heat absorbed by 1 lb of water at 212°F, when it is changed to steam at 212°F, may be around __________ BTU.
180
970
3.97
none of these
180
113
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
1000
114
Crystallisation of solids from a homogeneous solution is a/an __________ process.
exothermic
mildly endothermic
highly endothermic
none of these
exothermic
115
A multiple effect evaporator has a capacity to process 4000 kg of solid caustic soda per day, when it is concentrating from 10% to 25% solids. The water evaporated in kg per day is
6000
24000
60000
48000
24000
116
Value of Prandtl number for water ranges from
l to 2
5 to 10
100 to 500
1000 to 2000
5 to 10
117
Heat transfer in the laminar sublayer in case of a liquid flowing through a pipe, is mostly by
eddies current
conduction
convection
none of these
conduction
118
Among liquids, water has a comparatively high value of thermal conductivity, due to its
low density
high viscosity
partial ionisation
dense structure
partial ionisation
119
Reynold's analogy states that (where, St = Stanton number f = friction factor)
St = f/2
St = f/4
St = 4f
St = f1/2
St = f/2
120
For evaporation of viscous solution in a multiple effect evaporator, the prefered feeding scheme is
forward
backward
parallel
none of these
backward
121
The sum of reflectivity and absorptivity for an opaque body is equal to
0.5
1
0
2
1
122
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
cummulatively increases
123
In case of a multipass shell and tube heat exchanger, providing a baffle on the shell side __________ the heat transfer rate.
increases
decreases
does not affect
may increase or decrease, depends on the type of baffle
increases
124
The local surface conductance for laminar film condensation on vertical surface is (where, t = film thickness)
∝ t
∝ 1/t
∝ t
independent of 't'
∝ 1/t
125
Heat transfer co-efficient (h1) for liquids increases with
increasing temperature.
decreasing temperature.
decreasing Reynolds number.
none of these.
increasing temperature.
126
A diathermaneous substance __________ the thermal radiation completely.
absorbs
reflects
transmits
none of these
transmits
127
A hot body will radiate heat most rapidly, if its surface is
white & rough
black & rough
white & polished
black & polished
black & rough
128
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
1-1.3
129
A metal wire of 0.01 m dia and thermal conductivity 200 W/m.K is exposed to a fluid stream with a convective heat transfer coefficient of 100 W/m2.K. The Biot number is
5.6
0.025
3.5
0.0035
0.025
130
A __________ surface has the maximum thermal emissivity out of the following.
black & smooth
black & rough
white & smooth
white & rough
black & rough
131
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
√d
132
Mechanical recompression evaporation is used in the production of _____.
alcohol
distilled water
salt
fruits jam
distilled water
133
A measure of the extent to which viscous heating is important relative to the heat flow resulting from the impressed temperature difference is represented by the __________ number.
Condensation
Grashoff
Stantan
Brinkman
Brinkman
134
Out of 100 kcal/second of incident radiant energy on the surface of a thermally transparent body, 300 kcal/second is reflected back. If the transmissivity of the body is 0.25, the emissivity of the surface will be
0.35
0.45
0.55
0.85
0.45
135
The purpose of providing a 'catchall' in the vapor line of an evaporator is to
create vacuum
regulate the vapor flow
vent the non-condensible gases
arrest the entrained liquid
arrest the entrained liquid
136
What is the thermal conductivity of a perfect heat insulator ?
zero
one
∞
between 0 and ∞
zero
137
'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.
factor of safety.
138
A fluid is flowing inside the inner tube of a double pipe heat exchanger with diameter 'd'. For a fixed mass flow rate, the tube side heat transfer co-efficient for turbulent flow conditions is proportional to
d^0.8
d^-0.2
d^-1
d^-1.8
d^-0.2
139
Prandtl and Reynolds analogy are same, when Prandtl number is
0.5
1
> 2
1.5
1
140
Heat transfer occurs by natural convection because change in temperature causes difference in
viscosity
density
thermal conductivity
heat capacity
density
141
The ratio of kinematic viscosity to thermal diffusivity is called the __________ number.
Peclet
Prandtl
Stanton
Nusselt
Prandtl
142
The critical radius 'r' of insulation on a pipe is given by
r = 2k/h
r = k/h
r = k/2h
r = h/k
r = k/h
143
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.
temperature of the body only.
144
If the baffle spacing in a shell and tube heat exchanger increases, then the Reynolds number of the shell side fluid
remains unchanged.
increases.
increases or decreases depending on number of shell passes.
decreases.
decreases.
Increasing spacing decreases crossflow velocity which decreases Nre
145
The heat transfer by radiation from a mild steel surface is to be reduced by reducing the emissivity of the surface. This can be best achieved by
painting the surface black.
painting the surface white (with aluminium paint).
giving the surface a mirror finish.
roughening the surface.
painting the surface white (with aluminium paint).
146
__________ paint has the maximum absorption coefficient.
Black
Yellow
White
Grey
Black
147
What is Nusselt number ?
Cp*μ/k
hD/k
h*Cp/μ
Cp*μ/h
hD/k
148
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.
remove condensate and inert gases.
149
The Dietus-Boelter equation for convective leat transfer cannot be used for:
low Reynold's number
very low Grashoff number
molten metals
all (a), (b) and (c)
all (a), (b) and (c)
150
A hot liquid is kept in a big room. The logorithm of the numerical value of the temperature difference between the liquid and the room is plotted against time. The plot will be very nearly a/an
ellipse
straight line
parabola
circular arc
parabola
151
Radiation heat transfer rates does not depend upon the
type of absorbing surface.
distance between the heat source and the object receiving the heat.
surface area and temperature of the heat source.
none of these.
none of these.
152
Which of the following is correct ?
Rate = Driving force x Resistance
Driving force = Rate x Resistance
Resistance = Driving force x Rate
Rate = Resistance/Driving force
Driving force = Rate x Resistance
153
Convective heat transfer, in which heat is transferred by movement of warmed matter is described by
Fourier's law
Newton's law of cooling
Fick's law
none of these
Newton's law of cooling
154
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.
goes through a minimum.
155
Heat transfer rate per unit area is called
thermal conductivity
heat flux
heat transfer co-efficient
thermal diffusivity
heat flux
156
Log mean temperature difference (LMTD) cannot be used, if
heat transfer co-efficient over the entire heat exchanger is not constant.
there exists an unsteady state.
the heat capacity is not constant and there is a phase change.
none of these.
none of these.
157
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
four times
158
For a given ambient air temperature with increase in the thickness of insulation of a hot cylinderical pipe, the rate of heat loss from the surface would
decrease
increase
first decrease and then increase
first increase and then decrease
first increase and then decrease
159
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
zero
160
For what value of Prandtl number, the Col-burn analogy is valid ?
0.06 to 120
0.6 to 120
1 to 103
l to 50
0.6 to 120
161
In a forward feed multiple effect, the pressure build up will be
least at the inlet of the first effect.
least at the outlet of the last effect.
highest at the inlet of the last effect.
highest at the outlet of the last effect.
least at the outlet of the last effect.
162
Film boiling is usually not desired in commercial equipments, because
the heat transfer rate is low in view of the large temperature drop.
it is difficult to maintain.
it is not economic.
none of these.
the heat transfer rate is low in view of the large temperature drop.
163
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
1
164
The variation of thermal conductivity of a metal with temperature is often correlated using an expression of the form K = K0 + at. where, K is the thermal conductivity and T is the temperature (in °K). The units of 'a' in SI system will be
W/m.k
W/m
W/m.k2
none, 'a' is just a number
W/m.k2
165
Film condensation is promoted on a/an __________ surface.
oily
coated
clean & smooth
dirty
clean & smooth
166
Dietus-Boelter equation cannot be used for molten metals mainly due to its very low
Prandtl number
Grashoff number
thermal conductivity
viscosity
Prandtl number
167
If Prandtl number is greater than the Schmidt number, then the ______.
thermal boundary layer lies inside the concentration boundary layer.
concentration boundary layer lies inside the thermal boundary layer.
thermal & concentration boundary layers are of equal thickness.
hydrodynamic(i.e., momentum)boundary layer is thicker than the other two.
thermal boundary layer lies inside the concentration boundary layer.
168
Which of the following accessories is provided in the vapor line of an evaporator for removing the entrained liquid ?
Bleed point
Vent
Catchall
Baffle
Catchall
169
Rate of heat transfer by vaporisation from pools of water is affected by the
nature of heating surface and distribution of bubbles.
surface tension of water.
viscosity of water.
all (a), (b) and (c).
all (a), (b) and (c).
170
Multiple effect evaporators are commonly used in the manufacture of ______.
P. Paper Q. Superphosphate
R. Sugar S. Fats
P and Q
P and R
P and S
R and S
P and S
171
Which of the following has the highest thermal conductivity ?
Brick
Air
Water
Silver
Silver
172
Which of the following is unimportant in forced convection ?
Reynolds number
Prandtl number
Grashhoff number
none of these
Grashhoff number
173
Steam is to be condensed in a shell and tube heat exchanger, 5 m long with a shell diameter of 1 m. Cooling water is to be used for removing the heat. Heat transfer co-efficient for the cooling water, whether on shell side or tube side is the same. The best arrangement is ______.
vertical heat exchanger with steam on tube side.
vertical heat exchanger with steam on shell side.
horizontal heat exchanger with steam on tube side.
horizontal heat exchanger with steam on shell side.
vertical heat exchanger with steam on shell side.
174
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
reduce the capacity
175
Dropwise condensation occurs on __________ surfaces.
clean and dirt free
smooth clean
contaminated cooling
polished
contaminated cooling
176
For specified tube outside diameter, higher BWG means higher
tube thickness
cross-sectional area
weight per unit length
none of these
cross-sectional area
177
Pick out the wrong statement.
Swenson-Walker crystalliser is a batch crystalliser.
Super saturation of the solution is the driving potential for a crystal growth.
The liquor left after the removal of crystals from a solution is called mother liquor.
The first stage of crystal formation is called nucleation.
Swenson-Walker crystalliser is a batch crystalliser.
178
In case of a vertical tube evaporator, with increase in the liquor level, the __________ is increased.
velocity of circulation
liquor-film co-efficient
both (a) and (b)
neither (a) and (b)
neither (a) and (b)
179
Electromagnetic radiations propagate in vacuum with a velocity of __________ metre/second.
3 x l0^5
3 x l0^8
3 x l0^10
3 x l0^12
3 x l0^8
180
Heat flux is the time rate of heat transfer per unit
length
area
volume
none of these
area
181
Film boiling occurs at __________ pressure.
atmospheric
sub-atmospheric
negative
very high
very high
182
Heat flux through several resistances in series in analogous to the current flowing through several
resistances in parallel.
capacitors in series.
resistances in series.
none of these.
resistances in series.
183
Extended heat transfer surface like fins are used to increase the heat transfer rate. Fin efficiency is defined as the ratio of heat transferred across the fin surface to the theoretical heat transfer across an equal area held at the
surrounding temperature.
average temperature of the fin.
temperature of the fin end.
constant temperature equal to that of the base
constant temperature equal to that of the base
184
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.
get economical temperature difference by using moderate pressure steam.
185
Double pipe heat exchangers are used
when heat transfer area required is very high.
when heat transfer area required is very low, i.e (100-200 ft2).
because it occupies less floor area.
because it is less costly.
when heat transfer area required is very low, i.e (100-200 ft2).
186
Nusselt number (for forced convection heat transfer) is a function of the __________ number.
Prandtl
Reynolds
both (a) & (b)
neither (a) nor (b)
both (a) & (b)
187
Pick out the wrong statement.
Heat transfer by radiation can not occur across an absolute volume.
In case of a shell and tube heat exchanger, the pressure drop through the shell is proportional to the number of times the fluid crosses the bundle between baffles.
Propagation velocity for travel of heat radiation through vacuum is equal to the velocity of the light.
The amount of heat involved in the condensation or vaporisation of 1 kg of a fluid is the same.
Heat transfer by radiation can not occur across an absolute volume.
188
Which type of heat exchanger is preferred for heavy heat loads ?
Double pipe
Plate fine
Series and parallel set of shell and tube
None of these
Series and parallel set of shell and tube
189
Pick out the wrong statement.
Reciprocal of the resistance to heat flow is called thermal conductance.
Unit of thermal conductance is W/°K.
Thermal conductance of a wall of thickness 'L', thermal conductivity 'k' and heat flow area 'A' is kL/A.
none of these.
Thermal conductance of a wall of thickness 'L', thermal conductivity 'k' and heat flow area 'A' is kL/A.
190
While the total emissivity of a perfect black body is unity, the same for a real body is
0
1
> 1
between 0 and 1
between 0 and 1
191
The Grashoff number is defined as the ratio of the
buoyancy to inertial forces.
buoyancy to viscous forces.
inertial to viscous forces.
buoyancy to surface tension forces.
buoyancy to viscous forces.
192
According to Reynolds analogy, Stanton number is equal to (where, f = Fanning friction factor)
2f
f
f/2
f/4
f/2
193
The absorptivity of a body is equal to its emissivity
at a particular temperature.
for circular bodies.
under thermal equilibrium.
none of these.
under thermal equilibrium.
194
Absorptivity and refletivity of a perfect black body are respectively
1 and 0
0 and 1
1 and ∞
0 and 0.5
1 and 0
195
In thermal radiation for a black body (where, ε is emissivity and α is absorptivity)
α = 1; ε ≠ 1
α ≠ 1, ε = 1
α ≠ 1, ε ≠ 1
α = 1; ε = 1
α = 1; ε = 1
196
Pick out the wrong statement.
Superheated steam is preferably not used for process heating because of its low heat transfer film co-efficient.
In a shell and tube heat exchanger, the shell pressure drop is maximum for orifice baffles.
S.I. unit of fouling factor is Watt/m2.°K.
Longitudinal fins are used in extended surface heat exchangers, when the direction of fluid flow is parallel to the axis of the tube.
S.I. unit of fouling factor is Watt/m2.°K.
197
When warm and cold liquids are mixed, the heat transfer is mainly by
conduction
convection
radiation
both(a) & (c)
convection
198
A cube, sphere & a thin circular plate (all having same mass and made of same material) are all heated to 300°C and allowed to cool in natural air. Which one will cool the slowest ?
Cube
Plate
Sphere
All will cool at the same rate
Sphere
199
Vent pipes are provided in a condenser to
remove non-condensable gases.
purge the condenser.
facilitate easy cleaning of tubes.
none of these.
remove non-condensable gases.
200
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.
high co-efficient of thermal expansion.
201
In case of __________ boiling, the bubbles formed on a submerged hot surface get absorbed in the mass of the liquid.
nucleate
pool
low pressure
none of these
pool
202
A body cools down from 75°C to 70°C in 10 minutes. It will cool down from 70° C to 65° C in __________ minutes.
10
>10
<10
either (b) or (c), depends on the mass of the body
>10
203
In forced convection, the Nusselt number is a function of
Re and Pr
Re and Gr
Pr and Gr
Re and Sc
Re and Pr
204
At what value of Prandtl number, the hydrodynamic and thermal boundary layers are identical ?
0.5
1
1.5
80
1
205
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.
total amount of vapor produced per Kg of feed steam in a multiple effect system is much higher than in a single effect.
206
Heat transfer efficiency leading of energy conservation in a heat exchanger can be achieved by
keeping the heat transfer surface clean.
enhancing the fluid pumping rate.
increasing the tube length.
none of these.
keeping the heat transfer surface clean.
207
Trap is used to remove __________ from steam pipe lines.
steam
condensate
non-condensables
none of these
condensate
208
A composite flat wall of a furnace is made of two materials 'A' and 'B'. The thermal conductivity of 'A' is twice of that of material 'B', while the thickness of layer of 'A' is half that of B. If the temperature at the two sides of the wall are 400 and 1200°K, then the temperature drop (in °K) across the layer of material 'A' is
125
133
150
160
160
209
Tube pitch is the __________ of tube diameters and the clearances.
sum
difference
ratio
none of these
sum
210
With increase in the distance between the heat source and the object receiving the heat, the radiation heat transfer
decreases
increases
increases exponentially
remains unaffected
decreases
211
The equivalent diameter for the annulus of a double pipe heat exchanger, whose inner pipe has fins on the outside is __________ compared to the same size pipes without fins.
more
less
same
unpredictable
less
212
Natural convection is characterised by
Grashhoff number
Peclet number
Reynolds number
Prandtl number
Grashhoff number
213
Nusselt number for full developed, laminar, constant property flow in a pipe at uniform heat flux is
0.72
4.364
18
83
4.364
214
Thickness of thermal boundary layer is more compared to that of hydrodynamic boundary layer, when the value of Prandtl number is
1
< 1
> 1
> 5
< 1
215
The critical radius of insulation for cylindrical pipe is (where, hi = heat transfer coefficient at inside of the pipe )
K/h0
2K/h0
hi/K
2hi/K
K/h0
216
The heat flux in the nucleate boiling regimes is proportional to (where, ΔT = excess temperature)
(ΔT)^2
(ΔT)^4
(ΔT)^3
√(ΔT)
(ΔT)^3
217
In a 1-1 cocurrent heat exchanger, if the tube side fluid outlet temperature is equal to the shell side fluid outlet temperature, then the LMTD is
∞
0
equal to the difference between hot and cold fluids inlet temperature.
equal to the difference between hot fluid inlet temperature and cold fluid outlet temperature.
0
218
A body is called grey if the monochromatic emissivity of the body is
zero
unity
same for all wavelengths
different for all wavelengths
same for all wavelengths
219
In an extended surface heat exchanger, fluid having lower co-efficient
flows through the tube.
flows outside the tubes.
can flow either inside or outside the tubes.
should not be used as it gives very high pressure drop.
flows outside the tubes.
220
In a shell and tube heat exchanger, the tube side heat transfer co-efficient just at the entrance of the tube is
infinity.
zero.
same as average heat transfer co-efficient for tube side.
none of these.
infinity
221
Grashoff number, which is defined as g . β(Ts - T∞) ρ2. L3/μ2, is proportional to the ratio of buoyancy force to __________ force.
viscous
elastic
inertial
none of these
viscous
222
Pick out the wrong statement.
By increasing the number of shell passes, the temperature cross in a shell and tube heat exchanger can be prevented.
An underdesigned steam trap will back up the condensate instead of discharging it out.
Steam condensate is corrosive because of the presence of dissolved oxygen in it.
Film boiling is desirable in commercial equipments due to high heat transfer rate at low temperature drop.
Film boiling is desirable in commercial equipments due to high heat transfer rate at low temperature drop.
223
Graetz number is given by
mCp/kL
kL/mCp
mCp/kμ
kL/mCp
mCp/kL
224
For a cold dilute feed to produce thick viscous liquor, backward feeding as compared to forward feeding results in ______.
increased economy
decreased economy
lower capacity
no effect on economy
increased economy
225
If the thermal conductivity of a wall material is independent of temperature, the steady state temperature distribution in the very large thin plane wall having steady, uniform surface temperature follows __________ law.
hyperbolic
parabolic
linear
logarithmic
linear
226
In regenerative air preheater (as practised in heating of coke ovens), the heat is transferred
through a metallic wall.
by direct contact of hot flue gas with air.
by heating an intermediate material (like chequor bricks) and then heating the air from this hot material.
none of these.
by heating an intermediate material (like chequor bricks) and then heating the air from this hot material.
227
_________ equation relates the thermal conductivity of a solid to its temperature.
Antonie
Kopp's
Lee's
Kistyakowski
Lee's
228
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
Δt
229
Pick out the wrong statement.
The condensing film co-efficient is about 3 times lower for vertical condenser as compared to the equivalent horizontal condenser for identical situation.
Film co-efficient for vaporisation decreases as a result of vapor binding.
In industrial practice, sub-cooling of condensate is required, when the condensate is a volatile liquid and is to be transferred for storage.
Overall heat transfer co-efficient in a heat exchanger is controlled by the value of the film co-efficient, which is higher.
Overall heat transfer co-efficient in a heat exchanger is controlled by the value of the film co-efficient, which is higher.
230
Viscous & heat sensitive liquids are concentrated in __________ evaporators.
open pan
long tube
agitated film
none of these
agitated film
231
1 BTU/hr.ft.°F is equal to __________ kcal/hr. m.°C.
1.49
1
4.88
none of these
1.49
232
Heat transfer by radiation between two bodies at T1 & T2 and in an ambient temperature of Ta°C depends on
T1 - T2
T1 - Ta
T2 - Ta
none of these
none of these
233
The ratio of momentum diffusivity to thermal diffusivity is the __________ number.
Prandtl
Nusselt
Stanton
Grashoff
Prandtl
234
For a perfectly transparent surface (like gases), the
absorptivity = 0
transmissivity = 1
reflectivity = 0
all (a), (b) & (c)
transmissivity = 1
235
For a fluid flowing in an annulus space, the wetted perimeter for heat transfer and pressure drop are
same
different
never different
linearly related
different
236
During crystallisation, formation of crystal can occur in __________ solution only.
saturated
supersaturated
undersaturated
all (a), (b) and (c)
supersaturated
237
An evaporator while concentrating an aqueous solution from 10 to 40% solids evaporates 30000 kg of water. The amount of solids handled by the system in kg is
4000
9000
4600
3000
4000
238
An insulator should have
low thermal conductivity.
high thermal conductivity.
less resistance to heat flow.
a porous structure.
low thermal conductivity.
239
In a backward feed multiple effect evaporator
feed is introduced in the first effect.
feed flows from low pressure to high pressure.
no pumps are required between successive effects.
none of these.
feed flows from low pressure to high pressure.
