Heat Transfer Flashcards
1
Q
Crystal size in a continuous crystalliser depends upon the
a. rate of heat transfer.
b. degree of turbulence.
c. degree of supersaturation.
d. all (a), (b) and (c).
A
all (a), (b) and (c).
2
Q
Air is best heated with steam in a heat exchanger of
A
double pipe type with fin on air side.
3
Q
Black liquor generated during paper manufacture is concentrated in a
A
multiple effect evaporator.
4
Q
When vaporisation takes place directly at the heating surface, it is called
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)
A
C/E
6
Q
Reynold’s analogy states that
A
Nst α f
7
Q
With increase in temperature, the thermal conductivity of non-metallic amorphous solids
A
increases
8
Q
Fourier’s law applies to the heat transfer by
A
conduction
9
Q
Shell side pressure drop in a shell and tube heat exchanger does not depend upon the
a. baffle spacing & shell diameter.
b. tube diameter & pitch.
c. viscosity, density & mass velocity of shell side fluid.
d. none of these.
A
none of these.
10
Q
In the equation Q = UAΔt; Δt is
A
logarithmic mean temperature difference.
11
Q
With increase in porosity, the thermal conductivity of a solid substance
A
decreases
12
Q
For an ideal black body
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
A
34.4
14
Q
Fouling factor
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
A
decreases
16
Q
The steam ejector is used to
A
create vacuum.
17
Q
For shell and tube heat exchanger, with increasing heat transfer area, the purchased cost per unit heat transfer area
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
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
A
32
20
Q
In a heat exchanger, floating head is provided to
A
relieve stresses caused by thermal expansion.
21
Q
Leidenfrost point is a term concerned with the
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
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
A
I^2 R
24
Q
Pick out the wrong statement.
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.
highly turbulent
26
The actual temperature drop across the heating surface in an evaporator depends on the
a. feed.
b. depth of liquid over heating surface.
c. pressure difference between steam chest and vapour space.
d. 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?
a. Dowtherm
b. mercury
c. liquid metal (e.g. molten sodium) d
. 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
a. increases the rate of condensation.
b. decreases thermal resistance.
c. is desirable to increase the film co-efficient.
d. none of these.
none of these.
29
With the increase of temperature, the Col-burn jH factor
increases.
30
In case of evaporators, liquid entrainment results primarily due to
foaming of the solution.
31
Baffles in the shell side of a shell and tube heat exchanger
increase the shell side heat transfer co-efficient.
32
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 outside and steam inside.
33
The unit of heat transfer co-efficient in SI unit is
W/m2°K
34
The number of kg vaporised per kg of steam fed to the evaporator is defined as
economy
35
Pick out the wrong statement.
a. In case of heat transfer by purely forced convection, GR/Re 2 ≤ 1.
b. The equivalent diameter of heat transfer for a duct of square crosssection (having each side as 'x') is equal to 4x.
c. Distillation process is not the same as evaporation.
d. 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.
36
Economy of a multiple effect evaporator is not influenced much by the
boiling point elevations
37
Increasing the liquor level in the evaporator results in the
decreased capacity.
38
The equivalent diameter for pressure drop is __________ that for heat transfer.
smaller than
39
Controlling heat transfer film co-efficient is the one, which offers __________ resistance to heat transfer.
the largest
40
Pick out the wrong statement.
a. The controlling resistance in case of heating of air by condensing steam is in the air film.
b. 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.
c. In case of a 1 - 2 shell and tube heat exchanger, the LMTD correction factor value increases sharply, when a temperature cross occurs.
d. 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.
41
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)^1/4
42
Bulk of the convective heat transfer resistance from a hot tube surface to the fluid flowing in it, is
mainly confined to a thin film of fluid near the surface.
43
Which of the following is concerned with both heat and mass transfer?
a. Lewis relationship
b. Nusselt number
c. Kutateladze number
d. Froude number
Lewis relationship
44
A backward feed multiple effect evaporator is better than forward feed for concentrating cold feed, because it provides
higher economy
45
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
less than that of the uninsulated steam pipe.
46
Which area is used in case of heat flow by conduction through a cylinder?
Logarithmic mean area
47
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 not in physical contact with each other.
48
The dimensionless group in mass transfer that is equivalent to Prandtl number in heat transfer is
Schmidt number.
49
Finned tube heat exchangers
give larger area per tube.
50
The absorptivity of a grey body at a given temperature __________ with increasing wavelength of radiation.
may increase or decrease ; depends on the material
51
Which one gives the monochromatic emissive power for black body radiation?
Planck's law
52
For what value of Prandtl number, St = f/2?
1
53
In a shell and tube heat exchanger,
the temperature drops in the two fluids and the wall are proportional to individual resistances.
54
Dietus-Boelter equation used for the determination of heat transfer co-efficient is valid
for fluids in tubulent flow.
55
The equation, Nst = (f/2)/(1+5(NPr - 1)(sqrt*f/2)), corrosponds to __________ analogy.
Prandtl
56
For large heat transfer area requirement, shell and tube heat exchanger is preferred, because it
a. occupies smaller space.
b. is more economical.
c. is easy to operate and maintain.
d. all (a), (b) and (c).
all (a), (b) and (c).
57
Out of the following four assumptions used in the derivation of the equation for LMTD [LMTD = (Δt1 - Δt2)/(ln(Δt1/Δt2))], which one is subject to the largest deviation in practice ?
Constant rate of fluid flow.
58
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
radiation becomes important
59
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.)
a. t
b. 1/ρ
c. 1/A
d. all (a), (b) & (c)
all (a), (b) & (c)
60
Temperature profile in steady state heat transfer is
linear
61
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
increases the overall heat transfer coefficient.
62
When vaporisation takes place through a blanketting film of gas, the phenomenon is termed as __________ boiling.
film
63
Minimum recommended baffle spacing in a shell and tube heat exchanger is about (where, D = shell diameter)
0.2 D
64
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
65
The Nusselt number for fully developed (both thermally and hydrodynamically) laminar flow through a circular pipe, where the wall heat flux is constant, is
4.36
66
What is the unit of thermal conductivity?
Kcal/hr.m.°C
67
The thermal radiation emitted by a body is proportional to T^n, where T is its absolute temperature. The value of 'n' is exactly 4 for
all bodies
68
It is not preferable to use superheated steam in evaporators, because of its very
low film co-efficient
69
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
70
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).
71
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^1/3
72
The range of electromagnetic spectrum important in heat transfer by radiation is __________ microns.
0.5-50
73
Use of transverse baffles in a shell and tube heat exchanger is done to increase the
a. rate of heat transfer.
b. flow velocity.
c. turbulence of shell side fluid.
d. all (a), (b) and (c).
all (a), (b) and (c).
74
The actual temperature drop across the heating surface of an evaporator depends on the
a. liquid depth over the heating surface.
b. solution being evaporated.
c. pressure difference between the steam chest and the vapor space above the boiling liquid.
d. all (a), (b) and (c)
all (a), (b) and (c)
75
Economy of a multiple effect evaporator depends upon the
heat balance consideration
76
The Graetz number is concerned with the
heat transfer in laminar flow.
77
Solid angle subtended by the finite surface at the radiating element is
called the angle of vision.
78
Pick out the wrong statement.
The emissivity of a surface decreases, if it gets corroded by atmospheric environment.
79
In forced convection, the heat transfer depends on
Re, Pr
80
With increase in temperature, the thermal conductivity of most liquids
decreases
81
Heat transfer by __________ may not necessarily require the presence of a medium.
radiation
82
Pick out the correct statement.
a. Higher is the temperature of the radiating body, higher is the wavelength of radiation.
b. Logarithmic mean area is used for calculating the heat flow rate through a thick walled cylinder.
c. The wavelength corresponding to maximum mono-chromatic emissive power increases with rise in temperature.
d. 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.
83
In a shell and tube type heat exchanger, the floating tube bundle heat arrangement is used
in high range of temperature differences.
84
Stefan-Boltzman law applies to __________ body.
black
85
The film co-efficient is decreased due to the presence of non-condensing gases in the vapors. The film co-efficient of superheated vapor as compared to that of saturated vapor is
some
86
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
35C
87
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
88
An equipment which converts the latent or sensible heat of one fluid into the latent heat of vaporisation of another, is called a
heat exchanger
89
Which of the following is directly concerned with the heat transfer?
Grashoff number
90
Pick out the wrong statement:
a. With change in temperature, the radiant energy emitted by a black body remains unchanged.
b. Absorptivity of a body approaches unity in case of diffuse reflection.
c. Absorptivity of a perfectly black body is unity.
d. 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.
91
Planck's distribution law is valid for __________ bodies.
coloured
92
For hot feed, forward feeding as compared to backward feeding results in __________ economy.
increased
93
Radiation energy is emitted by all the substances, which are above
0°K
94
The interchange factor for radiation heat transfer from surface 'x' to surface 'y' in case of an infinite parallel planes with emis-sivities εx & εy is given by
(εx + εy)/(εx + εy - εx . εy)
95
What is the steam economy in case of a single effect evaporator system?
<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 = 2hl
97
The Fourier number (defined as a.t/L^2) is used in the analysis of problem involving heat transfer by
transient conduction
98
The energy radiated from a surface Q at absolute temperature T is related as
Q ∝ T^4
99
Baffles are provided in heat exchangers to increase the
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)
0.1
101
In case of a vertical tube evaporator, with increase in the liquor level, the
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.
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
104
A 2-4 heat exchanger involves
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
106
With increase in temperature, the thermal conductivity of steel
decreases
107
Which of the following forced convection heat transfer equation accounts for the liquid viscosity effect for viscous liquids?
Sieder-Tate equation
108
The main function of baffles provided in a shell and tube heat exchanger is to
enhance turbulence.
109
In a single effect evaporator, the economy is
< 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
<1
111
Which of the following is not concerned with the heat transfer?
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
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
114
Crystallisation of solids from a homogeneous solution is a/an __________ process.
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
24000
116
Value of Prandtl number for water ranges from
5 to 10
117
Heat transfer in the laminar sublayer in case of a liquid flowing through a pipe, is mostly by
conduction
118
Among liquids, water has a comparatively high value of thermal conductivity, due to its
partial ionisation
119
Reynold's analogy states that (where, St = Stanton number f = friction factor)
St = f/2
120
For evaporation of viscous solution in a multiple effect evaporator, the prefered feeding scheme is
backward
121
The sum of reflectivity and absorptivity for an opaque body is equal to
1
122
The film thickness for laminar film condensation on vertical surface __________ from top to bottom.
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
124
The local surface conductance for laminar film condensation on vertical surface is (where, t = film thickness)
∝ 1/t
125
Heat transfer co-efficient (h1) for liquids increases with
increasing temperature.
126
A diathermaneous substance __________ the thermal radiation completely.
transmits
127
A hot body will radiate heat most rapidly, if its surface is
black & rough
128
Evaporation of 1kg of water from a solution in a single effect evaporator requires about __________ kg of steam.
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
0.025
130
A __________ surface has the maximum thermal emissivity out of the following.
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^2
132
Mechanical recompression evaporation is used in the production of
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.
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
0.45
135
The purpose of providing a 'catchall' in the vapor line of an evaporator is to
arrest the entrained liquid
136
What is the thermal conductivity of a perfect heat insulator?
zero
137
Fouling factor' used in the design of a multipass shell and tube heat exchanger is a
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^-0.2
139
Prandtl and Reynolds analogy are same, when Prandtl number is
1
140
Heat transfer occurs by natural convection because change in temperature causes difference in
density
141
The ratio of kinematic viscosity to thermal diffusivity is called the __________ number.
Prandtl
142
The critical radius 'r' of insulation on a pipe is given by
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.
144
If the baffle spacing in a shell and tube heat exchanger increases, then the Reynolds number of the shell side fluid
decreases.
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 white (with aluminium paint).
146
__________ paint has the maximum absorption coefficient.
Black
147
What is Nusselt number?
hD/k
148
Steam traps are provided in steam carrying pipelines to
remove condensate and inert gases.
149
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
straight line
150
Radiation heat transfer rates does not depend upon the
a. type of absorbing surface.
b. distance between the heat source and the object receiving the heat.
c. surface area and temperature of the heat source.
d. none of these.
none of these.
151
Which of the following is correct?
a. Rate = Driving force x Resistance
b. Driving force = Rate x Resistance
c. Resistance = Driving force x Rate
d. Rate = Resistance/Driving force
Driving force = Rate x Resistance
152
Convective heat transfer, in which heat is transferred by movement of warmed matter is described by
Newton's law of cooling
153
As the difference between the wall temperature and bulk temperature increases, the boiling heat transfer co-efficient
goes through a minimum.
154
Heat transfer rate per unit area is called
heat flux
155
Log mean temperature difference (LMTD) cannot be used, if
a. heat transfer co-efficient over the entire heat exchanger is not constant.
b. there exists an unsteady state.
c. the heat capacity is not constant and there is a phase change.
d. none of these.
none of these.
156
In a multipass shell and tube heat exchanger, tube side return pressure loss is equal to __________ the velocity head.
four times
157
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
first increase and then decrease
158
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
zero
159
For what value of Prandtl number, the Col-burn analogy is valid?
0.6 to 120
160
In a forward feed multiple effect, the pressure build up will be
least at the outlet of the last effect.
161
Film boiling is usually not desired in commercial equipments, because
the heat transfer rate is low in view of the large temperature drop.
162
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
1
163
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.k2
164
Film condensation is promoted on a/an __________ surface.
clean & smooth
165
Dietus-Boelter equation cannot be used for molten metals mainly due to its very low
Prandtl number
166
If Prandtl number is greater than the Schmidt number, then the
thermal boundary layer lies inside the concentration boundary layer.
167
Which of the following accessories is provided in the vapor line of an evaporator for removing the entrained liquid?
Catchall
168
Rate of heat transfer by vaporisation from pools of water is affected by the
a. nature of heating surface and distribution of bubbles.
b. surface tension of water.
c. viscosity of water.
d. all (a), (b) and (c).
all (a), (b) and (c).
169
Multiple effect evaporators are commonly used in the manufacture of
P. Paper Q. Superphosphate
R. Sugar S. Fats
P and S
170
Which of the following has the highest thermal conductivity?
a. Brick
b. Air
c. Water
d. Silver
Silver
171
Which of the following is unimportant in forced convection?
a. Reynolds number
b. Prandtl number
c. Grashhoff number
d. none of these
Grashoff number
172
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 shell side.
173
In a multiple effect evaporator, the effect of boiling point elevation is to
reduce the capacity
174
Dropwise condensation occurs on __________ surfaces.
contaminated cooling
175
For specified tube outside diameter, higher BWG means higher
cross-sectional area
176
Pick out the wrong statement.
a. Swenson-Walker crystalliser is a batch crystalliser.
b. Super saturation of the solution is the driving potential for a crystal growth.
c. The liquor left after the removal of crystals from a solution is called mother liquor.
d. The first stage of crystal formation is called nucleation.
Swenson-Walker crystalliser is a batch crystalliser.
177
Electromagnetic radiations propagate in vacuum with a velocity of __________ metre/second.
3 x 10^8
178
Heat flux is the time rate of heat transfer per unit
area
179
Film boiling occurs at __________ pressure.
very high
180
Heat flux through several resistances in series in analogous to the current flowing through several
resistances in series.
181
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
constant temperature equal to that of the base
182
Vacuum is generally maintained in the vapour space of an evaporator mainly to
get economical temperature difference by using moderate pressure steam.
183
Double pipe heat exchangers are used
when heat transfer area required is very low, i.e (100-200 ft2).
184
Pick out the wrong statement.
a. Heat transfer by radiation can not occur across an absolute volume.
b. 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.
c. Propagation velocity for travel of heat radiation through vacuum is equal to the velocity of the light.
d. 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.
185
Which type of heat exchanger is preferred for heavy heat loads ?Which type of heat exchanger is preferred for heavy heat loads?
Series and parallel set of shell and tube
186
Pick out the wrong statement.
a. Reciprocal of the resistance to heat flow is called thermal conductance.
b. Unit of thermal conductance is W/°K.
c. Thermal conductance of a wall of thickness 'L', thermal conductivity 'k' and heat flow area 'A' is kL/A.
d. none of these.
Thermal conductance of a wall of thickness 'L', thermal conductivity 'k' and heat flow area 'A' is kL/A.
187
While the total emissivity of a perfect black body is unity, the same for a real body is
between 0 and 1
188
The Grashoff number is defined as the ratio of the
buoyancy to viscous forces.
189
According to Reynolds analogy, Stanton number is equal to (where, f = Fanning friction factor)
f/2
190
The absorptivity of a body is equal to its emissivity
under thermal equilibrium.
191
Absorptivity and refletivity of a perfect black body are respectively
1 and 0
192
In thermal radiation for a black body (where, ε is emissivity and α is absorptivity)
α = 1; ε = 1
193
Pick out the wrong statement.
a. Superheated steam is preferably not used for process heating because of its low heat transfer film co-efficient.
b. In a shell and tube heat exchanger, the shell pressure drop is maximum for orifice baffles.
c. S.I. unit of fouling factor is Watt/m2.°K.
d. 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.
194
When warm and cold liquids are mixed, the heat transfer is mainly by
convection
195
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?
Sphere
196
Vent pipes are provided in a condenser to
remove non-condensable gases.
197
Heat transfer by natural convection is enhanced in system with
high co-efficient of thermal expansion.
198
In case of __________ boiling, the bubbles formed on a submerged hot surface get absorbed in the mass of the liquid.
pool
199
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
200
In forced convection, the Nusselt number is a function of
Re and Pr
201
At what value of Prandtl number, the hydrodynamic and thermal boundary layers are identical?
1
202
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 amount of vapor produced per Kg of feed steam in a multiple effect system is much higher than in a single effect.
203
Heat transfer efficiency leading of energy conservation in a heat exchanger can be achieved by
keeping the heat transfer surface clean.
204
Trap is used to remove __________ from steam pipe lines.
condensate
205
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
160
206
Tube pitch is the __________ of tube diameters and the clearances.
sum
207
With increase in the distance between the heat source and the object receiving the heat, the radiation heat transfer
decreases
208
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.
less
209
Natural convection is characterised by
Grashoff number
210
Nusselt number for full developed, laminar, constant property flow in a pipe at uniform heat flux is
4.364
211
Thickness of thermal boundary layer is more compared to that of hydrodynamic boundary layer, when the value of Prandtl number is
< 1
212
The critical radius of insulation for cylindrical pipe is (where, hi = heat transfer coefficient at inside of the pipe)
K/h0
213
The heat flux in the nucleate boiling regimes is proportional to (where, ΔT = excess temperature)
(ΔT)^3
214
A body is called grey if the monochromatic emissivity of the body is
same for all wavelengths
215
In an extended surface heat exchanger, fluid having lower co-efficient
flows outside the tubes.
216
In a shell and tube heat exchanger, the tube side heat transfer co-efficient just at the entrance of the tube is
infinity.
217
Grashoff number, which is defined as g . β(Ts - T∞) ρ2. L3/μ2, is proportional to the ratio of buoyancy force to __________ force.
viscous
218
Pick out the wrong statement.
a. By increasing the number of shell passes, the temperature cross in a shell and tube heat exchanger can be prevented.
b. An underdesigned steam trap will back up the condensate instead of discharging it out.
c. Steam condensate is corrosive because of the presence of dissolved oxygen in it.
d. 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.
219
Graetz number is given by
mCp/kL
220
For a cold dilute feed to produce thick viscous liquor, backward feeding as compared to forward feeding results in
increased economy
221
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.
linear
222
In regenerative air preheater (as practised in heating of coke ovens), the heat is transferred
by heating an intermediate material (like chequor bricks) and then heating the air from this hot material.
223
__________ equation relates the thermal conductivity of a solid to its temperature.
Lee's
224
For small temperature difference, the heat transfer rate as per Newton's law of cooling is proportional to (where, Δt = excess temperature)
Δt
225
Pick out the wrong statement.
a. The condensing film co-efficient is about 3 times lower for vertical condenser as compared to the equivalent horizontal condenser for identical situation.
b. Film co-efficient for vaporisation decreases as a result of vapor binding.
c. In industrial practice, sub-cooling of condensate is required, when the condensate is a volatile liquid and is to be transferred for storage.
d. 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.
226
Viscous & heat sensitive liquids are concentrated in __________ evaporators.
agitated film
227
1 BTU/hr.ft.°F is equal to __________ kcal/hr. m.°C.
1.49
228
Heat transfer by radiation between two bodies at T1 & T2 and in an ambient temperature of Ta°C depends on
a.T1 - T2
b. T1 - Ta
c. T2 - Ta
d. none of these
none of these
229
The ratio of momentum diffusivity to thermal diffusivity is the __________ number.
Prandtl
230
For a perfectly transparent surface (like gases), the
transmissivity = 1
231
For a fluid flowing in an annulus space, the wetted perimeter for heat transfer and pressure drop are
different
232
During crystallisation, formation of crystal can occur in __________ solution only.
supersaturated
233
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
234
An insulator should have
low thermal conductivity.
235
In a backward feed multiple effect evaporator
feed flows from low pressure to high pressure.
236
Choose the correct equation.
a. Nu = (Re) (Pr) (Gz)
b. Nu = (Re) (Pr) (St)
c. Nu = (Re)(Pr)
d. Nu = (Pr)(St)
Nu = (Re) (Pr) (St)
237
In case of heat flow by conduction for a cylindrical body with an internal heat source, the nature of temperature distribution is
parabolic
238
For heat flow through very thick walled cylinder, use____mean radius.
logarithmic
239
Indirect contact heat exchangers are preferred over direct contact heat exchangers, because
there is no risk of contamination.
240
Water is normally used as a coolant in the heat exchange equipments mainly because ofits
abundance & high heat capacity
241
Which of the following has maximum thermal conductivity at the same temperature?
a. Steel
b. Petrol
c. Air
d. All have the same conductivity
Steel
242
Maximum heat transfer rate is obtained in __________ flow.
turbulent
243
In case of __________ boiling, the liquid temperature is below the saturation temperature and the boiling takes place in the vicinity of the heated surface. a. nucleate b. local c. pool d. saturated
local
244
Terminal point temperature differences between fluids in case of a heat exchanger is termed as
approach
245
Fqra cold viscous feed, backward feed gives __________ than forward feed.
a higher capacity
246
Walls of a cubical oven are of thickness l, and they are made of material of thermal conductivity k. The temperature inside the oven is 100°C and the inside heat transfer co-efficient is 3k/l. If the wall temperature on the outside is held at 25°C, what is the inside wall temperature in degree centigrade ?
81.25
247
Overall heat transfer co-efficient of a particular tube is U1. If the same tube with some dirt deposited on either side has coefficient U2, then
U1 = U2
248
In SI units, thermal conductivity is expressed in
Watt/m.°K
249
Boiling point of a solution according to Duhring's rule is a linear function of the __________ of water.
boiling point (at the same pressure)
250
At what value of Prandtl number, conduction is negligible in the turbulent core of a fluid flowing through a heated pipe ?
> 0.6
251
A sphere of radius 'R1' is enclosed in a sphere of radius 'R2'. The view (or shape) factor for radiative heat transfer of the outer sphere with respect to the inner sphere is
R2/(R1+R2)
252
For a laminar flow of fluid in a circular tube, 'h1' is the convective heat transfer coefficient 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
0.794 h1
253
The main purpose of providing fins on heat transfer surface is to increase the
heat transfer area.
254
A BTU/hr.ft.2 °F is equal to
4.88 kcal/hr. m.2°C.
255
Which of the following has the lowest Prandtl number ?
a. Molten sodium (a coolant used in fast breeder reactor)
b. Water
c. Transformer oil
d. Dilute H2SO4
Molten sodium (a coolant used in fast breeder reactor)
256
In SI units, fouling factor is expressed in
m2°K/W
257
Extremely large or small volumes of fluids are generally best routed through the shell side of a shell and tube heat exchanger, because of the
flexibility possible in the baffle arrangement.
258
All analogy equations connecting friction factor and heat transfer co-efficient apply only to
wall or skin friction
259
Electro-magnetic spectrum range, which is important for radiation varies from __________ microns.
0.5 to 50
260
Heat transfer rate described by Fourier's law will decrease, if the __________ increases.
thickness
261
The capacity of double-effect evaporator is less than half of the capacity of two single effects, each of which is operating over same terminal temperature difference, when
solution has an elevation of boiling point.
262
Peclet number (Pe) is given by
Pe = Re.Pr
263
Nusselt number is the ratio of the
heat flux at the wall to that across the entire pipe.
264
Prandtl number is the ratio of
momentum diffusivity to thermal diffusivity.
265
Forced circulation evaporators are useful for the concentration of viscous, salting and scale forming liquors. Which of the following is a forced circulation evaporator?
Agitated film evaporator
266
"The ratio of the total emissive power to the absorptivity for all bodies is same at thermal equilibrium". This is __________ law.
Kirchoffs
267
The advantage of using a 1 - 2 shell and tube heat exchanger over a 1 - 1 shell and tube heat exchanger is
higher tube side heat transfer co-efficient.
268
For a multipass shell and tube heat exchanger, the LMTD correction factor is always
< 1
269
Tube expansion allowances exist in __________ heat exchanger.
U-tube
270
Pick out the wrong statement.
a. In drying a solid containing moisture above the critical moisture content the number of degrees of freedom is 2.
b. Sherwood number in mass transfer corresponds to Nusselt number in heat transfer and Schmidt number to Prandtl number.
c. Forced convection is relatively more effective in increasing the rate of mass transfer, if Schmidt number is larger.
d. 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.
Forced convection is relatively more effective in increasing the rate of mass transfer, if Schmidt number is larger.
271
Agitated film evaporator is suitable for concentrating __________ liquids.
viscous
272
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 tempeature follows __________ law.
parabolic
273
Intermittant tube cleaning is possible to be done in case of a __________ evaporator.
basket type
274
Arithmetic mean area can be used in heat transfer problem to calculate the heat flow by conduction through a cylinder which is
thin walled having the value of Ao/Ai< 2.
275
Three solid objects of the same material and of equal mass-a sphere, a cylinder (length = diameter) and a cube are at 500°C initially. These are dropped in a quenching bath containing a large volume of cooling oil each attaining the bath temperature eventually. The time required for 90% change in temperature is the smallest for
cube
276
Overall heat transfer co-efficient for cooling of hydrocarbons by water is about
50 -100 BTU/hr. ft.2°F
277
Absorptivity of a perfect black body is unity. Which of the following has maximum absorptivity?
coke breeze
278
Thermal diffusivity is given by
k/p.Cp
279
At what value of Prandtl number, the hydrodynamic and thermal boundary layers of a fluid flowing over a heated plate will be identical?
1
280
In a forward feed multiple effect evaporator unit
a. viscosity of liquid is highest in first effect.
b. transfer from effect to effect is done by pumps.
c. no pump is required to withdraw the product from the last effect.
d. none of these.
d. none of these.
281
__________ chart is known as transient heat conduction chart.
Dirhing's
282
Choose the most important factor on which the heat conducted through a wall in a unit time will depend on?
a. Thickness of the wall.
b. Area of the wall perpendicular to heat flow.
c.Material of the wall.
d. Temperature difference between the two surfaces of the wall.
Temperature difference between the two surfaces of the wall.
283
Which of the following has the minimum thermal conductivity?
a. Nitrogen
b. Steel
c. Carbon black
d. Tar
Nitrogen
284
In a heat exchanger, one transfer unit means
a section of the exchanger in which change in temperature of one stream equals the average driving force in the section.
285
For condensation of pure vapors, if the heat transfer co-efficients in filmwise and drop-wise condensation are respectively hf and hd, then
hf < hd
286
Pick out the wrong statement:
a. The capacity of an evaporator is reduced by the boiling point elevation.
b. Corrosive liquid is normally passed through the tubes in a shell and tube heat exchanger.
c. Steam jet ejector is used for vapor compression in a thermal recompression evaporator.
d. Heat sensitive materials should be concentrated in high pressure evaporators.
Heat sensitive materials should be concentrated in high pressure evaporators.
287
For concentrating an aqueous solution of a material like anhydrous Na2SO4, whose solubility decreases with rise in temperature, the most suitable evaporator is a __________ evaporator.
vacuum
288
Circulation pump is located below the evaporater to
create more suction head.
289
The statement that "maximum wavelength of radiation is inversly proportional to the temperature" is __________ law.
Wien's displacement
290
Steam condensate is recovered by steam traps and recycled for use as boiler feed water, because of its low
a. hardness
b. dissolved soilds content
c. suspended solids content
d. all (a), (b) and(c)
all (a), (b) and(c)
291
The rate of emission of radiation by a body does not depend upon the
shape and porosity of the body.
292
Duhring rule is important in solving problems on
evaporation
293
In a shell and tube heat exchanger, the height of 25 percent cut baffles is equal to (where, D = inside diameter of shell)
0.75 D
294
Multiple effect evaporation is generally recommended, when the
large scale evaporation of liquor is needed
295
The inside heat transfer co-efficient in case of turbulent flow of liquid in the tube side in a 1-2 shell and tube heat exchanger is increased by __________ times, when the number of tube passes is increased to 8.
4^0.8
296
__________ heat exchanger is the most suitable, when the temperature of shell side fluid is much higher than that of tube side.
U-tube
297
In which mode of heat transfer, the Biot number is important?
Transient heat conduction
298
_________ paint has the minimum absorption co-efficient.
White lead
299
Asymptotic conditions is reached, when for a fluid flowing in laminar flow through a long tube
exit fluid temperature = wall temperature.
300
1000 kg of wet solids are to be dried from 60% to 20% moisture (by weight). The mass of moisture removed in kg is
400
301
Which is the most suitable for the con-certration of highly concentrated solution?
a. Open pan evaporation
b. Long tube vertical evaporator
c. Agitated film evaporator
d. None of these
Open pan evaporation
302
Unsteady state heat conduction occurs, when
temperature distribution is dependent on time.
303
What is the absorptivity of a black body?
1
304
Mode of heat transfer in which the fluid moves under the influence of changes in fluid pressure produced by external work is called
forced convection
305
Baffle spacing
should be less than the inside diameter of the shell.
306
The ratio of velocity head to tube side return loss in case of a multipass shell and tube heat exchanger is
1/4
307
The advantage of backward feed multiple effect evaporators over forward feed units is that
most concentrated liquid is at highest temperature.
308
Which of the following is the most controlling factor for the rate of bubble detachment from the hot solid surface?
a. Liquid density
b. Liquid viscosity
c. Hot surface temperature
d. Interfacial tension
Interfacial tension
309
Which characteristic of a fluid is not important in deciding its route in a shell and tube heat exchanger?
a. Corrosiveness
b. Fouling characteristic
c. Viscosity
d. None of these
None of these
310
Thermal conductivities of most of the liquids __________ with rise in temperature.
decreases
311
A metal ball of radius 0.1 m at a uniform temperature of 90°C is left in air at 30°C. The density and the specific heat of the metal are 3000 kg/m3 and 0.4 kJ/kg.K respectively. The heat transfer co-efficient is 50 W/m2.K Neglecting the temperature gradients inside the ball, the time taken (in hours) for the ball to cool to 60°C is
0.15
312
The purpose of floating head in a heat exchanger is to
avoid buckling of tubes.
313
1000 Kg of liquid at 30°C in a well stirred vessel has to be heated to 120°C, using immersed coils carrying condensing steam at 150°C. The area of the steam coils is 1.2 m2 and the overall heat transfer co-efficient to the liquid is 1500 W/m2.°C. Assuming negligible heat loss to the surrounding and specific heat capacity of the liquid to be 4 kJ/kg.°C, the time taken for the liquid to reach desired temperature will be
51 min
314
Overall thermal resistance for conductive heat transfer through a series of flat resistances is equal to the
sum of all resistances.
315
Double pipe heat exchangers are preferably useful, when
requirement of heat transfer area is low.
316
Removal of __________ heat is involved in the condensation of a vapor under saturated conditions.
latent
317
The rate of heat transfer from a vertical plate by natural convection depends upon the temperature differences (ΔT) between wall and outside bulk. The proportionality is given as
(ΔT)^3/4
318
If air (a non-condensing gas) is present in a condensing vapor stream, it will __________ the condensation rate of vapor.
decrease
319
Prandtl and Reynold's analogies are identical for Prandtl number value of
1
320
The outlet temperature of cooling water in a heat exchanger is generally not allowed to exceed above 50°C in industrial practice mainly to avoid
excessive corrosion
321
LMTD can't be used as such without a correction factor for the
a. multipass heat exchanger.
b. baffled heat exchanger.
c. condensation of mixed vapour in a condenser.
d. all (a) (b) and (c).
a. multipass heat exchanger.
322
The type of liquor circulation system to be ' employed in evaporators (viz. natural or forced circulation) is determined mainly by the __________ of the liquid.
viscosity
323
The overall resistance for heat transfer through a series of flat resistance, is the __________ of the resistances.
sum
324
Thermal diffusivity is the most important in heat transfer by
conduction
325
(NGr x NPr) is called the __________ number.
Reyleigh
326
A dephlegmator is a
partial condenser
327
A tank painted with which of the following coloured paints, would heat up maximum by radiation from sun?
Black paint
328
The separation of liquid droplets from the vapor is done by a/an __________ , in the evaporators.
entrainment separator
329
Fouling factor for a heat exchanger is given by (where, U1 = heat transfer co-efficient of dirty surface U2 = heat transfer co-efficient of clean surface)
1/U1 - 1/U2
330
If a single tube pass heat exchanger is converted to two pass, then for the same flow rate, the pressure drop per unit length in tube side will
increase by 2^16
331
A steel sphere of radius 0.1 m at 400°K is immersed in an oil at 300°K. If the centre of the sphere reaches 350°K in 20 minutes, how long will it take for a 0.05 m radius steel sphere to reach the same temperature (at the centre) under identical conditions ? Assume that the conductive heat transfer co-efficient is infinitely large.
5 minutes
332
Fluid motion in the natural convection heat transfer between a solid surface and a fluid in contact with it, results from the
none of these.
333
Evaporator tubes are generally
vertical
334
An ejector is used to
a. increase pressure
b. increase temperature
c. remove condensate
d. none of these
d. none of these
335
In a boiling curve, the peak heat flux is called the __________ point.
burnout
336
Pick out the wrong statement.
a. Heat transfer from a hot body to cold body by the emission of heat waves is called radiation:
b. Filmwise condensation takes place on non-wettable surfaces.
c. The boiling point of a solution is affected by liquid head as well as boiling point elevation.
d. none of these.
Filmwise condensation takes place on non-wettable surfaces.
337
h.D/K is called the __________ number.
Nusselt
338
Prandtl number is the ratio of
momentum diffusivity to thermal diffusivity
339
Conductance is given by(where, x = thickness, A = heat flow area, K = thermal conductivity.)
KA/x
340
Heat flux, as defined in heat flow is analogous to __________ in electricity flow.
current
341
The wavelength at which the maximum monochromatic emissive power occurs for a black body, is (where, T = absolute temperature of the black body)
α 1/T
342
In a shell and tube heat exchanger, putting a longitudinal baffle across the shell, forces the shell side fluid to pass __________ through the heat exchanger.
twice
343
When the ratio of the Grashoff number and to the square of Reynolds number is one, the dominant mechanism of heat transfer is
mixed convection (both free and forced)
344
The purpose of providing bleed points in the evaporator is to
facilitate removal of non-condensable gases
345
Water always boils when its
saturated vapour pressure equals the external pressure on its surface.
346
A multiple effect evaporator as compared to a single effect evaporator of the same capacity has
higher steam economy
347
Which is the most suitable for cold viscous feed?
Backward feed
348
Dropwise condensation is promoted on a/an __________ surface.
oily
349
At Pr > 1, conduction in an ordinary fluid flowing through a heated pipe is limited to the
viscous sub-layer
350
In a single evaporator system, the steam economy __________ by creating vacuum in the evaporator.
increases
351
(NRe.NPr)(D/L)is called the __________ number
Graetz
352
Nucleate boiling is promoted
on rough surfaces
353
In forced circulation, the heating element is injected
internally
354
If all the conditions and dimensions are same, then the ratio of velocity through the tubes of a double pass heat exchanger to that through the single pass heat exchanger is
2
355
The equation, Nst = f/2, is the __________ analogy.
Reynolds
356
Boiling point elevation for a strong and concentrated solution is found by Duhring's rule, which states that at the same pressure, the boiling point of a solution is a linear function of the __________ of pure water.
boiling point
357
Rate of crystallisation does not depend upon the
a. extent of supersaturation.
b. turbulence within the solution.
c. number and active surface area of the crystals.
d. none of these.
none of these.
358
Stefen's block body radiation law can also be derived from __________ law.
Planck's
359
Multipass heat exchangers are used
to obtain higher heat transfer co-efficient and shorter tube.
360
The characteristic dimensionless groups for heat transfer to a fluid flowing through a pipe in laminar flow are
Nu, Gz
361
Condensing film co-efficient for steam on horizontal tubes ranges from 5000 to 15000 Kcal/hr.m2 .°C. Condensation of vapor is carried out inside the tube in a shell and tube heat exchanger, when the
condensate is corrosive in nature.
362
When does the heat generated by fluid friction becomes appreciable compared to the heat transferred between the fluids?
At high fluid velocity
363
The Sieder-Tate correlation for heat transfer in turbulent flow in pipe gives Nu α Re^0.8, where, Nu is the Nusselt number and Re is the Reynolds number for the flow. Assuming that this relation is valid, the heat transfer co-efficient varies with the pipe diameter (D) as
(D)^-0.2
364
A long iron rod initially at a temperature of 20°C has one end dipped in boiling water (100°C) at time, t = 0. The curved surface of the rod is insulated so that heat conduction is one dimensional in the axial direction. The temperature at a distance 100 mm from the dipped end becomes 40°C at time, t = 200 s. The same temperature is achieved at a distance of 200 mm from the dipped end at time
t = 800 s
365
LMTD for evaporators & condensers for a given terminal parameters & set of conditions for counterflow is equal to that for parallel flow. In such heat exchangers, with one of the fluids condensing or evaporating, the surface area required is the least in the __________ flow.
a. parallel
b. mixed
c. counterflow
d. same in either 'a', b' or 'c'
same in either 'a', b' or 'c'
366
Thermal diffusivity of a material
a. has the unit m2 /sec.
b. is defined as K/ρ . Cp.
c. is the ratio of thermal conductivity to thermal capacity.
d. all (a), (b) and (c).
all (a), (b) and (c).
367
Viscosity of gases __________ with increase in temperature.
increase slowly
368
What is the emissivity of a black body?
1
369
Steam is routed through the tube in case of a __________ evaporator.
horizontal tube
370
Dropwise condensation of steam on cooling surface is promoted
an oily surface.
371
Conduction occurs in the buffer zone for a fluid flowing through a heated pipe, only when Prandtl number is
0.1
372
In a heat exchanger, the rate of heat transfer from the hot fluid to the cold fluid
varies directly as the area and the LMTD.
373
With increase in temperature, the total emissivity of conductors
increases
374
Loss of heat from untagged steam pipe to the ambient air is by
a. conduction
b. convection
c. radiation
d. all (a), (b) & (c)
all (a), (b) & (c)
375
Fruit juice (a heat sensitive material) can be concentrated in a __________ evaporator.
falling film
376
Tubes are held between top and bottom tube sheets in Calenderia type evaporator by keeping
both the tube sheets fixed.
377
A single pass air heater is connected to a two pass unit. For the air flow rate and other conditions remaining the same, the film heat transfer co-efficient for air will vary in the ratio of
2^0.8
378
Heat sensitive materials can be concentrated in an evaporator employing
vacuum
379
A concentric double pipe heat exchanger as compared to the shell and tube heat exchanger for the same heat load requires
more space.
380
In a shell and tube heat exchanger, floating head is used for
large temperature differentials.
381
Steam trap is used to
remove water resulting from partial condensation of steam.
382
The average heat transfer co-efficient over the entire length of the plate (ha) and the local heat transfer co-efficient (hL), in case of heat transfer over a flat plate in lminar zone is related as
ha = 2hL
383
Which of the following parameters is increased by use of finned tube in a multipass shell and tube heat exchanger ?
a. Tube side pressure drop and the heat transfer rate.
b. Convective heat transfer co-efficient.
c. Effective tube surface area for convective heat transfer.
d. All (a) (b) and (c).
d. All (a) (b) and (c).
384
Maximum heat transfer rate is achieved in __________ flow.
turbulent
385
Wavelength corresponding to the maximum energy is inversely proportional to the absolute temperature. This is __________ law.
Wien's
386
Three material A, B and C of equal thick-nes and of thermal conductivity of 20, 40 & 60 kcal/hr. m. °C respectively are joined together. The temperature outside of A and C are 30°C and 100°C respectively. The interface between B and C will be at a temperature of __________ °C.
70
387
Which of the following parameters of the fluid is not very important, while deciding its route in a shell and tube heat exchanger?
density
388
View factor is important in heat transfer by
radiation
389
The equation, (NSt x N2/3Pr) = f/2, is the __________ analogy.
Colburn
390
The driving potential for the crystal growth during crystallisation is the __________ of the solution.
super-saturation
391
jH factor for heat transfer depends upon the __________ number.
Reynolds
392
Kg of liquid evaporated per hour in an evaporator is defined as its
capacity
393
Pick out the correct statement.
a. 1 kcal/hr.m.°C is equal to 1 BTU/hr. ft.°F.
b. In steady state heat conduction, the only property of the substance which determines the temperature distribution, is the thermal conductivity.
c. In unsteady state heat conduction, heat flows in the direction of temperature rise.
d. In heat transfer by forced convection, Grashoff number is very important.
In steady state heat conduction, the only property of the substance which determines the temperature distribution, is the thermal conductivity.
394
Open pan evaporators are preferred to be used, when the solution to be concentrated is
highly viscous
395
Cp.μ/K is termed as the __________ number.
Prandtl
396
The maximum heat transfer co-efficient from steam heating will be attained when the steam is
saturated
397
If heat transfer rate varies with the time, it is termed as
a. forced convection
b. steady state conduction
c. monochromatic radiation
d. none of these
d. none of these
398
In a forward feed multiple effect evaporator, the pressure is
lowest in last effect
399
In a co-current double pipe heat exchanger used for condensing saturated steam over the inner tube, if the entrance and exit conditions of the coolant are interchanged, then the rate of condensation will
remain unchanged
400
Which of the following situations can be approximated to a steady state heat transfer system?
a. A red hot steel slab (having outside surface temperature as 1300°C) exposed to the atmosheric air at 35°C.
b. 10 kg of dry saturated steam at 8 kgf/cm2 flowing through a short length of stainless steel pipe exposed to atmospheric air at 35°C.
c. Boiling brine kept in open vessel when the bottom surface temperature of the vessel is maintained constant at 180°C.
d. A sub-cooled refrigerant liquid at 8°C flowing at the rate of 6 Kg/minute through a copper pipe exposed to atmospheric air at 35°C.
10 kg of dry saturated steam at 8 kgf/cm2 flowing through a short length of stainless steel pipe exposed to atmospheric air at 35°C.
401
Sensible heat absorbed by 1 lb of water when it is heated from 32 to 212°F may be around __________ BTU.
180
402
The unit of conductance in SI unit is
W/°K
403
In a shell and tube heat exchanger, square pitch compared to triangular pitch
gives a higher shell side pressure drop.
404
Which of the following has maximum thermal conductivity?
Iron
405
A wall has two layers of materials A and B; each made of a different material. Both the layers have the same thickness. The thermal conductivity of materialA is twice that of B. Under the equilibrium, the temperature difference across the wall is 36°C. The temperature difference across the layer A is __________ °C.
12
406
The film co-efficient between condensing vapour and metal wall increases with
increasing temperature of the vapour.
407
Critical value of the __________ number governs the transition from laminar to turbulent flow in free convection heat transfer.
Prandtl & Grashoff
408
The thermal boundary layer at NPr > 1
is thinner than hydrodynamic boundary layer.
409
In the free convection regime of pool boiling, the heat flux is proportional to
Δt^5/4
410
With increase in temperature, the thermal conductivity of fresh lubricating oil
decreases
411
Fresh orange juice contains 12% (by weight) solids and the rest water 90% of the fresh juice is sent to an evaporator to remove water and subsequently mixed with the remaining 10% of fresh juice. The resultant product contains 40% solids. The kg of water removed from 1 kg fresh juice is
0.7
412
Nusselt number is the ratio of the temperature gradient at the wall to
that across the entire pipe
413
The Stefan-Boltzman constant depends on the
a. medium
b. temperature
c. surface
d. none of these
none of these
414
The units of resistance to heat transfer is
W-1m2K
415
Thermal conductivity of a gas at low density, __________ with increase in temperature.
increases
416
Nusselt number is a function of Prandtl number and __________ number of fluid in natural convection heat transfer.
Grashoff
417
__________ heat exchanger is used for chilling oil to be dewaxed.
Double pipe
418
It is not recommended to use a 1-2 shell and tube heat exchanger for a particular heat duty, whenever the LMTD correction factor is
< 0.75
419
Pick out the wrong statement.
a. Economy of a multiple effect evaporator is not influenced by the b. boiling point elevation.
b. Two identical cubes of iron and copper will have the same heat content under the same conditions of temperature.
c. Double pipe heat exchangers are mostly used in the field of refrigeration.
d. Finned tube heat exchangers are suitable for heating air by steam.
Two identical cubes of iron and copper will have the same heat content under the same conditions of temperature.
420
In sub-cooled boiling,
temperature of the heating surface is more than the boiling point of the liquid.
421
With increase in temperature, the thermal conductivity of a gas
increases.
422
A __________ evaporator employs an annular downtake.
basket type
423
Radiation heat losses from satisfactorily insulated high pressure boilar may be about __________ percent.
7
424
In natural convection heat transfer, the correlating parameter is the
Grashoff number
425
Boiling of milk in an open vessel is an example of __________ boiling.
film
426
Pick out the wrong statement.
a. In process heat exchangers, saturated steam is preferred over the superheated steam.
b. The maximum is the emissive power of a surface at a temperature T1 occurs at a wavelength of λ1. If the surface temperature is halved, the maximum in the emissive power would occur at a wavelength of 0.5 λ1.
c. When a vertical plate is heated in infinite air environmental under natural convection conditions, the velocity profile in air, normal to the plate, exhibits a maximum.
d. A body at 925 K emits an energy of 1.42 x 1011σW/m2 (σ is the Stefan-Boltzman constant) in the wavelength band between 3 μm to 4 μm. The fraction of this energy in the total energy emitted over the entire wavelength range is equal to emissivity.
The maximum is the emissive power of a surface at a temperature T1 occurs at a wavelength of λ1. If the surface temperature is halved, the maximum in the emissive power would occur at a wavelength of 0.5 λ1.
427
Mode of heat transfer involved in the cooling of air cooled internal combustion engine is
natural convection
428
Steady state one dimensional heat flow by conduction as given by Fourier's low does not assume that
material is anisotropic.
429
The left face of a one dimensional slab of thickness 0.2 m is maintained at 80°C and the right face is exposed to air at 30°C. The thermal conductivity of the slab is 1.2 W/m.K and the heat transfer co-efficient from the right face is 10 W/m2.K. At steady state, the temperature of the right face in °C is
48.7
430
In a shell and tube heat exchanger, the shell side fluid velocity can not be changed by changing the
tube diameter
431
To reduce the tube side pressure drop for the same flow rate, the heat exchanger recomended is
1-1 heat exchanger
432
Steam side heat transfer co-efficient in an evaporator is in the range of __________ kcal/hr.m2°C.
5000 - 150000
433
Correction is applied to LMTD for __________ flow.
cross
434
If h1 = inner film co-efficient and /h2 = outer film co-efficient, then the overall heat transfer co-efficient is
always between h1 and h2
435
One kilogram of water at 0°C is changed to superheated steam of one atm pressure and 300° C. The major heat consumption in the process will be to
evaporate the water.
436
For __________ Prandtl number values, the heat conduction will be negligible in the buffer zone.
high
437
The radiation heat flux from a heating element at a temperature of 800°C, in a furnace maintained at 300°C is 8 kW/m2. The flux, when the element temperature is increased to 1000°C for the same furnace temperature is
16.5 kW/m2
438
Heat exchangers operating, when the asymptotic range is reached,
results in making part of the heating surface inactive.
439
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
440
In case of heat transfer by conduction in a hollow cylinder, __________ mean area is used to calculate the heat transfer rate.
logarithmic
441
In a gas-liquid shell and tube heat exchanger, the
a. presence of a non-condensible gas decreases the condensing film co-efficient.
b. gases under high pressure are routed through the tube side, because high pressure gases are corrosive in nature.
c. 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.
d. all 'a', 'b' & 'c'
all 'a', 'b' & 'c'
442
Which is the most suitable for the concentration of foamy & frothy liquors?
Long tube vertical evaporator
443
Boiling point elevation of an ideal solution
increases rapidly with temperature rise.
444
The rate of heat transfer is a product of overall heat transfer co-efficient, the difference in temperature and the
heat transfer area
445
Which of the following has the lowest overall heat transfer co-efficient?
a. Dowtherm
b. Molten sodium
c. Water
d. Air
Air
446
Which of the following has the minimum absorptivity?
a. Aluminium foil
b. Coal dust
c. Refractory bricks
d. Iron plates
Aluminum foil
447
A black body when hot, emits heat radiation of __________ wavelengths.
all
448
A composite wall consists of two plates A and B placed in series normal to the flow of heat. The thermal conductivities are kA and kB and the specific heat capacities are CPA and CPB for plates A and B respectively. Plate B has twice the thickness of plate A. At steady state, the temperature difference across plate A is greater than that across plate B, when
kA < 0.5kB
449
Vibrations in the tubes of a shell and tube heat exchanger is induced due to the
a. flow of fluid on the tube and shell sides.
b. oscillations in the flow of shell/tube sides fluid. c. vibrations transmitted through piping and/or supports due to external reasons.
d. all (a), (b) and (c)
all (a), (b) and (c)
450
Heat transfer by conduction in the turbulent core of a fluid flowing through a heated pipe is negligible, if the value of Prandtl number is
0.6
451
The Nusselt number for fully developed (both thermally and hydrodynamically) laminar flow through a circular pipe whose surface temperature remains constant is
3.66
452
Sensible heat of hot industrial flue gases can not be recovered by a/an
a. economiser
b. regenerator
c. ceramic recuperator
d. none of these
none of these
453
The critical radius of insulation for a spherical shell is (where, K = thermal conductivity of insulating material h0 = heat transfer coefficient at the outer surface)
2K/h0
454
Radiator of an automobile engine is a __________ type of heat exchanger.
direct contact
455
In pipe flow, heat is transferred from hot wall to the liquid by
forced convection and conduction
456
The value of Stefan-Boltazman constant in SI unit is
5.6697 x 10-8W/m2.°K4
457
The unit of heat transfer co-efficient is
BTU/hr. ft2°F
458
For turbulent flow in a tube, the heat transfer co-efficient is obtained from the Dittus-Boelter correlation. If the tube diameter is halved and the flow rate is doubled, then the heat transfer co-efficient will change by a factor of
6.1
459
The inner wall of a furnace is at a temperature of 700°C. The composite wall is made of two substances, 10 and 20 cm thick with thermal conductivities of 0.05 and 0.1 W.m-1.°C-1 respectively. The ambient air is at 30°C and the heat transfer co-efficient between the outer surface of wall and air is 20 W.m-2.°C-1. The rate of heat loss from the outer surface in W.m-2is
165.4
460
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
2
461
Low thermal conductivity of heat insulating materials is due to its
high proportion of air space
462
The most conducive surface for dropwise condensation to occur is the __________ surface.
oily
463
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
464
Calburn analogy is applicable for the value of Prandtl number from
0.6 to 120
465
Pick out the correct equation.
jH = (St)(Pr)^2/3 = f/2
466
Multiple effect evaporators ar used to
increase the steam economy & the capacity.
467
Prandtl number is given by
(Cp · μ)/k
468
Small scale evaporation is done in a
steam jacketed kettle
469
Heat exchanger tubes are never made of
lead
470
At constant temperature, the thermal conductivities of gases __________ with rise in pressure.
decrease
471
In a cooling tower, water becomes cool by
loosing sensible heat.
472
In Fourier's law, the proportionality constant is called the
thermal conductivity
473
The heat transfer co-efficient in film type condensation is __________ that for dropwise condensation.
lower than
474
Pick out the wrong statement.
a. Bubble size increases with the dynamic viscosity of the liquid in case of nucleate pool boiling.
b. Thermal conductivity of a dry material is more than that of the damp material.
c. Ratio of its capacity to economy equals the steam consumption in kg/hr in an evaporator.
d. Vaporisation of organic substances in evaporator mostly causes foam formation.
Thermal conductivity of a dry material is more than that of the damp material.
475
Duhring's plot' is of use in
evaporation
476
Which has the lowest Prandtl number?
a. Liquid metal
b. Aqueous solution
c. Water
d. Lube oil
Liquid metal
477
Pick out the wrong statement.
a. The value of hydrostatic head increases with increase in vacuum in the effect in a multiple effect evaporator system.
b. Entering velocity of the liquid in the tubes of natural circulation evaporators is in the range of 0.3 to 0.9 metre/second.
c. Duhring's plot is used for calculating the concentration of solution.
d. In a multiple effect evaporation system, the number of effects is limited by the total boiling point rise.
Duhring's plot is used for calculating the concentration of solution.
478
Prandtl number for most of dry gases is about
0.72
479
Prandtl number is the reciprocal of
Thermal diffusivity/Momentum diffusivity
480
In a heat exchanger with steam outside the tubes, a liquid gets heated to 45°C, when its flow velocity in the tubes is 2 m/s. If the flow velocity is reduced to 1 m/s, other things remaining the same, the temperature of the exit liquid will be
more than 45°C
481
For a liquid in laminar flow through a very long tube, when the exit fluid temperature approaches the wall temperature, the equation to be used is
Nu = 2/pi x Gz
482
Multiple effect evaporation accounts for
steam economy
483
In case of a supercooled solution, which is on the verge of crystallisation, the free energy of the solution as compared to that of the solid is
more
484
Maximum water velocity in tubes of a 1-2 shell and tube heat exchanger may be around __________ metres/second.
1
485
In Biot number, the characteristic length used is the ratio of the __________ of the solid.
volume to surface area
486
Which of the following is the most widely used heat insulating material for pipelines carrying steam?
a. Tar dolomite bricks followed by asbestos.
b. Fireclay refractory followed by aluminium sheet.
c. Cotton followed by aluminium foil.
d. 85% magnesia cement and glass wool.
85% magnesia cement and glass wool.
487
Heat transfer co-efficient equation for forced convection, Nu = 0.023 Re^0.8 . Pr^n, is not valid, if the value of
Reynolds number for the flow involved is < 2100.
488
Which is the best tube arrangement (in a shell and tube heat exchanger) if the fluids are clean and non-fouling?
Triangular pitch
489
In case of a multipass shell and tube heat exchanger, the temperature drop in the fluid
and the wall are proportional to individual resistances.
490
The purpose of providing expansion bellows in the shell of tubular exchanger is to
account for the uneven expansion of shell and tube bundles.
491
A graph between __________ is called Wilson plot.
1/U Vs. 1/V^0.8
492
Grashhoff number is given by
gD^3.β.Δtρ2/μ^2
493
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
333W/m2.K
494
In a laboratory test run, the rate of drying was found to be 0.5 x 10-3 kg/m2.s, when the moisture content reduced from 0.4 to 0.1 on dry basis. The critical moisture content of the material is 0.08 on a dry basis. A tray dryer is used to dry 100 kg (dry basis) of the same material under identical conditions. The surface area of the material is 0.04 m2/kg of dry solid. The time required (in seconds) to reduce the moisture content of the solids from 0.3 to 0.2 (dry basis) is
5000
495
Pick out the wrong statement.
a. 'Solvates' are chemical compounds formed by solute with their solvents. When water is the solvent, then it is called a 'hydrate'.
b. In heat exchanger calculations (Δt)weighted is used in place of Δt, when it involves more than one sequence of heating or cooling i.e., desuperheating & condensation or condensation & sub-cooling.
c. Heat transfer co-efficient during nucleate boiling is not influenced by the agitation imparted.
d. In case of short tube vertical evaporators, area of central downtake is about 50 to 100% of the total tube cross-sectional area.
Heat transfer co-efficient during nucleate boiling is not influenced by the agitation imparted.
496
What is the geometric mean of two heat transfer areas A1 and A2?
sqrt*(A1 . A2)
497
Steam economy in case of a triple effect evaporator will be
> 1
