Heat Transfer Flashcards
1
Q
Crystal size in a continuous crystalliser depends upon the
A
all
2
Q
Heat waves
A
travel in straight line, can be reflected by a mirror.
3
Q
Air is best heated with steam in a heat exchanger of
A
double pipe type with fin on air side.
4
Q
Which of the following is generally considered as opaque surface towards radiations?
A
Solids, Liquids
5
Q
Black liquor generated during paper manufacture is concentrated in a
A
multiple effect evaporator.
6
Q
When vaporisation takes place directly at the heating surface, it is called
A
nucleate boiling
7
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
8
Q
Reynold’s analogy states that
A
Nst α f
9
Q
With increase in temperature, the thermal conductivity of non-metallic amorphous solids
A
increases
10
Q
Fourier’s law applies to the heat transfer by
A
conduction
11
Q
Shell side pressure drop in a shell and tube heat exchanger does not depend upon the
A
none of these.
12
Q
In the equation Q = UAΔt; Δt is
A
logarithmic mean temperature difference.
13
Q
With increase in porosity, the thermal conductivity of a solid substance
A
decreases
14
Q
For an ideal black body
A
absorptivity = 1
15
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
16
Q
Fouling factor
A
accounts for additional resistances to heat flow.
17
Q
In case of vertical tube evaporator, with increase in liquor level, the overall heat transfer co-efficient
A
decreases
18
Q
The steam ejector is used to
A
create vacuum.
19
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
20
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
21
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
22
Q
In a heat exchanger, floating head is provided to
A
relieve stresses caused by thermal expansion.
23
Q
Leidenfrost point is a term concerned with the
A
boiling of a liquid on a hot surface.
24
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.
25
Heat produced when a steady state current, I passes through an electrical conductor having resistance, 'R' is
I^2R
26
Pick out the wrong statement.
a.Orifice baffles are never used in a shell and tube heat exchanger.
b. Pressure drop on the shell side of a heat exchanger depends upon tube pitch also.
c. In a horizontal tube evaporator, surface blanketing by air is avoided.
d. Split ring type and pull through type floating heads are two commonly used floating heads is heat exchangers.
Orifice baffles are never used in a shell and tube heat exchanger.
27
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
28
The actual temperature drop across the heating surface in an evaporator depends on the
all
29
Which of the following is not used as a medium for high temperature heating?
mercury
30
Presence of a non-condensing gas in a condensing vapour
none of these.
31
Kirchoff s law applies to __________ radiation.
total, monochromatic
32
With the increase of temperature, the Col-burn jH factor
increases.
33
In case of evaporators, liquid entrainment results primarily due to
foaming of the solution.
34
Baffles in the shell side of a shell and tube heat exchanger
increase the shell side heat transfer co-efficient.
35
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.
36
The unit of heat transfer co-efficient in SI unit is
W/m2°K
37
The number of kg vaporised per kg of steam fed to the evaporator is defined as
economy
38
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.
39
Economy of a multiple effect evaporator is not influenced much by the
boiling point elevations
40
Increasing the liquor level in the evaporator results in the
decreased capacity.
41
The equivalent diameter for pressure drop is __________ that for heat transfer.
smaller than
42
Controlling heat transfer film co-efficient is the one, which offers __________ resistance to heat transfer.
the largest
43
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
35°C
44
Thermal conductivity of a conducting solid material depends upon its
temperature, porosity
45
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.
46
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
47
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.
48
Which of the following is concerned with both heat and mass transfer?
Lewis relationship
49
A backward feed multiple effect evaporator is better than forward feed for concentrating cold feed, because it provides
higher economy
50
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.
51
Which area is used in case of heat flow by conduction through a cylinder?
Logarithmic mean area
52
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.
53
The dimensionless group in mass transfer that is equivalent to Prandtl number in heat transfer is
Schmidt number.
54
Finned tube heat exchangers
give larger area per tube.
55
The absorptivity of a grey body at a given temperature __________ with increasing wavelength of radiation.
may increase or decrease ; depends on the material
56
Which one gives the monochromatic emissive power for black body radiation?
Planck's law
57
For what value of Prandtl number, St = f/2?
1
58
In a shell and tube heat exchanger,
the temperature drops in the two fluids and the wall are proportional to individual resistances.
59
Dietus-Boelter equation used for the determination of heat transfer co-efficient is valid
for fluids in tubulent flow.
60
The equation, Nst = (f/2)/(1+5(NPr - 1)(sqrt*f/2)), corrosponds to __________ analogy.
Prandtl
61
For large heat transfer area requirement, shell and tube heat exchanger is preferred, because it
all
62
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.
63
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
64
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.)
all
65
Temperature profile in steady state heat transfer is
linear
66
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.
67
When vaporisation takes place through a blanketting film of gas, the phenomenon is termed as __________ boiling.
film
68
Minimum recommended baffle spacing in a shell and tube heat exchanger is about (where, D = shell diameter)
0.2 D
69
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
70
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
71
What is the unit of thermal conductivity?
Kcal/hr.m.°C
72
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
73
It is not preferable to use superheated steam in evaporators, because of its very
low film co-efficient
74
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
75
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).
76
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
77
The range of electromagnetic spectrum important in heat transfer by radiation is __________ microns.
0.5-50
78
Use of transverse baffles in a shell and tube heat exchanger is done to increase the
all
79
The actual temperature drop across the heating surface of an evaporator depends on the
all
80
Economy of a multiple effect evaporator depends upon the
heat balance consideration
81
The Graetz number is concerned with the
heat transfer in laminar flow.
82
Solid angle subtended by the finite surface at the radiating element is
called the angle of vision.
83
Pick out the wrong statement.
a. The emissivity of a surface decreases, if it gets corroded by
atmospheric environment.
b. The emissivity of a surface increases with increase in surface
roughness.
c. The emissivity of a polished surface is quite low.
d. 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.
84
In forced convection, the heat transfer depends on
Re, Pr
85
With increase in temperature, the thermal conductivity of most liquids
decreases
86
Heat transfer by __________ may not necessarily require the presence of a medium.
radiation
87
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.
88
In a shell and tube type heat exchanger, the floating tube bundle heat arrangement is used
in high range of temperature differences.
89
Stefan-Boltzman law applies to __________ body.
black
90
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
91
A perfect black body is a perfect __________ of radiation.
absorber, emitter
92
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
93
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
94
Which of the following is directly concerned with the heat transfer?
Grashoff number
95
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.
96
Planck's distribution law is valid for __________ bodies.
coloured
97
For hot feed, forward feeding as compared to backward feeding results in __________ economy.
increased
98
Radiation energy is emitted by all the substances, which are above
0°K
99
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)
100
What is the steam economy in case of a single effect evaporator system?
<1
101
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
102
jH factor for heat transfer is not a function of the __________ number.
Nusselt, Grashoff
103
The Fourier number (defined as a.t/L2) is used in the analysis of problem involving heat transfer by
transient conduction
104
The energy radiated from a surface Q at absolute temperature T is related as
Q ∝ T^4
105
Baffles are provided in heat exchangers to increase the
heat transfer co-efficient
106
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
107
In case of a vertical tube evaporator, with increase in the liquor level, the
capacity of the evaporator is decreased.
108
The ratio of total radiating power to the absorptivity of the body depends upon the __________ as per Kirchoffs law.
temperature of the body
109
For gases, the thermal conductivity increases with temperature rise. For liquids, with increase in concentration, its thermal conductivity generally
decreases
110
A 2-4 heat exchanger involves
both counter and parallel-flow of the fluids.
111
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
112
With increase in temperature, the thermal conductivity of steel
decreases
113
Which of the following forced convection heat transfer equation accounts for the liquid viscosity effect for viscous liquids?
Sieder-Tate equation
114
The main function of baffles provided in a shell and tube heat exchanger is to
enhance turbulence.
115
In a single effect evaporator, the economy is
< 1
116
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
117
Which of the following is not concerned with the heat transfer?
Schmidt number
118
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
119
In counter flow compared to parallel flow,
LMTD is greater, less surface area is required for a given heat transfer rate
120
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
121
Crystallisation of solids from a homogeneous solution is a/an __________ process.
exothermic
122
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
123
Value of Prandtl number for water ranges from
5 to 10
124
Heat transfer in the laminar sublayer in case of a liquid flowing through a pipe, is mostly by
conduction
125
Among liquids, water has a comparatively high value of thermal conductivity, due to its
partial ionisation
126
Reynold's analogy states that (where, St = Stanton number f = friction factor)
St = f/2
127
For evaporation of viscous solution in a multiple effect evaporator, the prefered feeding scheme is
backward
128
The sum of reflectivity and absorptivity for an opaque body is equal to
1
129
The film thickness for laminar film condensation on vertical surface __________ from top to bottom.
cummulatively increases
130
In case of a multipass shell and tube heat exchanger, providing a baffle on the shell side __________ the heat transfer rate.
increases
131
The local surface conductance for laminar film condensation on vertical surface is (where, t = film thickness)
∝ 1/t
132
Heat transfer co-efficient (h1) for liquids increases with
increasing temperature.
133
A diathermaneous substance __________ the thermal radiation completely.
transmits
134
A hot body will radiate heat most rapidly, if its surface is
black & rough
135
Evaporation of 1kg of water from a solution in a single effect evaporator requires about __________ kg of steam.
1-1.3
136
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
137
Analogy between mass and heat transfer is not applicable in case of
thermal or pressure mass diffusion, viscous heating or chemical reaction.
138
A __________ surface has the maximum thermal emissivity out of the following.
black & rough
139
Radiant energy received by a body is proportional to (where, d = the distance between the object emitting radiation and that receiving it.)
d^2
140
Mechanical recompression evaporation is used in the production of
distilled water
141
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
142
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
143
The purpose of providing a 'catchall' in the vapor line of an evaporator is to
arrest the entrained liquid
144
What is the thermal conductivity of a perfect heat insulator?
zero
145
Fouling factor' used in the design of a multipass shell and tube heat exchanger is a
factor of safety.
146
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
147
Prandtl and Reynolds analogy are same, when Prandtl number is
1
148
Heat transfer occurs by natural convection because change in temperature causes difference in
density
149
The ratio of kinematic viscosity to thermal diffusivity is called the __________ number.
Prandtl
150
The critical radius 'r' of insulation on a pipe is given by
r = k/h
151
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.
152
If the baffle spacing in a shell and tube heat exchanger increases, then the Reynolds number of the shell side fluid
decreases.
153
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).
154
__________ paint has the maximum absorption coefficient.
Black
155
What is Nusselt number?
hD/k
156
Steam traps are provided in steam carrying pipelines to
remove condensate and inert gases.
157
The Dietus-Boelter equation for convective leat transfer
https://www.indiabix.com/_files/images/chemical-engineering/heat-transfer/245-165-1.png
cannot be used for (See picture for equation)
all
158
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
159
Radiation heat transfer rates does not depend upon the
none of these.
160
Which of the following is correct?
Driving force = Rate x Resistance
161
Convective heat transfer, in which heat is transferred by movement of warmed matter is described by
Newton's law of cooling
162
As the difference between the wall temperature and bulk temperature increases, the boiling heat transfer co-efficient
goes through a minimum.
163
Heat transfer rate per unit area is called
heat flux
164
Log mean temperature difference (LMTD) cannot be used, if
none of these.
165
In a multipass shell and tube heat exchanger, tube side return pressure loss is equal to __________ the velocity head.
four times
166
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
167
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
168
For what value of Prandtl number, the Col-burn analogy is valid?
0.6 to 120
169
In a forward feed multiple effect, the pressure build up will be
least at the outlet of the last effect.
170
Film boiling is usually not desired in commercial equipments, because
the heat transfer rate is low in view of the large temperature drop.
171
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
172
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
173
Film condensation is promoted on a/an __________ surface.
clean & smooth
174
Dietus-Boelter equation cannot be used for molten metals mainly due to its very low
Prandtl number
175
If Prandtl number is greater than the Schmidt number, then the
thermal boundary layer lies inside the concentration boundary layer.
176
Which of the following accessories is provided in the vapor line of an evaporator for removing the entrained liquid?
Catchall
177
Rate of heat transfer by vaporisation from pools of water is affected by the
all
178
Multiple effect evaporators are commonly used in the manufacture of
P. Paper Q. Superphosphate
R. Sugar S. Fats
P and S
179
Which of the following has the highest thermal conductivity?
Silver
180
Which of the following is unimportant in forced convection?
Grashoff number
181
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.
182
In a multiple effect evaporator, the effect of boiling point elevation is to
reduce the capacity
183
Dropwise condensation occurs on __________ surfaces.
contaminated cooling
184
For specified tube outside diameter, higher BWG means higher
cross-sectional area
185
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.
186
In case of a vertical tube evaporator, with increase in the liquor level, the __________ is increased.
neither
187
Electromagnetic radiations propagate in vacuum with a velocity of __________ metre/second.
3 x l08
188
Heat flux is the time rate of heat transfer per unit
area
189
Film boiling occurs at __________ pressure.
very high
190
Heat flux through several resistances in series in analogous to the current flowing through several
resistances in series.
191
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
192
Vacuum is generally maintained in the vapour space of an evaporator mainly to
get economical temperature difference by using moderate pressure steam.
193
Double pipe heat exchangers are used
when heat transfer area required is very low, i.e (100-200 ft2).
194
Nusselt number (for forced convection heat transfer) is a function of the __________ number.
Prandtl, Reynolds
195
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.
196
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
197
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.
198
While the total emissivity of a perfect black body is unity, the same for a real body is
between 0 and 1
199
The Grashoff number is defined as the ratio of the
buoyancy to viscous forces.
200
According to Reynolds analogy, Stanton number is equal to (where, f = Fanning friction factor)
f/2
201
The absorptivity of a body is equal to its emissivity
under thermal equilibrium.
202
Absorptivity and refletivity of a perfect black body are respectively
1 and 0
203
In thermal radiation for a black body (where, ε is emissivity and α is absorptivity)
α = 1; ε = 1
204
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.
205
When warm and cold liquids are mixed, the heat transfer is mainly by
convection
206
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
207
Vent pipes are provided in a condenser to
remove non-condensable gases.
208
Heat transfer by natural convection is enhanced in system with
high co-efficient of thermal expansion.
209
In case of __________ boiling, the bubbles formed on a submerged hot surface get absorbed in the mass of the liquid.
pool
210
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
211
In forced convection, the Nusselt number is a function of
Re and Pr
212
At what value of Prandtl number, the hydrodynamic and thermal boundary layers are identical?
1
213
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.
214
Heat transfer efficiency leading of energy conservation in a heat exchanger can be achieved by
keeping the heat transfer surface clean.
215
Trap is used to remove __________ from steam pipe lines.
condensate
216
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
217
Tube pitch is the __________ of tube diameters and the clearances.
sum
218
With increase in the distance between the heat source and the object receiving the heat, the radiation heat transfer
decreases
219
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
220
Natural convection is characterised by
Grashoff number
221
Nusselt number for full developed, laminar, constant property flow in a pipe at uniform heat flux is
4.364
222
Thickness of thermal boundary layer is more compared to that of hydrodynamic boundary layer, when the value of Prandtl number is
< 1
223
The critical radius of insulation for cylindrical pipe is (where, hi = heat transfer coefficient at inside of the pipe)
K/h0
224
The heat flux in the nucleate boiling regimes is proportional to (where, ΔT = excess temperature)
(ΔT)^3
225
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
226
A body is called grey if the monochromatic emissivity of the body is
same for all wavelengths
227
In an extended surface heat exchanger, fluid having lower co-efficient
flows outside the tubes.
228
In a shell and tube heat exchanger, the tube side heat transfer co-efficient just at the entrance of the tube is
infinity.
229
Grashoff number, which is defined as g . β(Ts - T∞) ρ2. L3/μ2, is proportional to the ratio of buoyancy force to __________ force.
viscous
230
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.
231
Graetz number is given by
mCp/kL
232
For a cold dilute feed to produce thick viscous liquor, backward feeding as compared to forward feeding results in
increased economy
233
Economy of an evaporator is influenced by the
feed temperature, number of effect
234
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
235
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.
236
__________ equation relates the thermal conductivity of a solid to its temperature.
Lee's
237
For small temperature difference, the heat transfer rate as per Newton's law of cooling is proportional to (where, Δt = excess temperature)
Δt
238
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.
239
Viscous & heat sensitive liquids are concentrated in __________ evaporators.
agitated film
240
1 BTU/hr.ft.°F is equal to __________ kcal/hr. m.°C.
1.49
241
Heat transfer by radiation between two bodies at T1 & T2 and in an ambient temperature of Ta°C depends on
none of these
242
The ratio of momentum diffusivity to thermal diffusivity is the __________ number.
Prandtl
243
For a perfectly transparent surface (like gases), the
transmissivity = 1
244
For a fluid flowing in an annulus space, the wetted perimeter for heat transfer and pressure drop are
different
245
During crystallisation, formation of crystal can occur in __________ solution only.
supersaturated
246
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
247
An insulator should have
low thermal conductivity.
248
In a backward feed multiple effect evaporator
feed flows from low pressure to high pressure.
249
Choose the correct equation.
Nu = (Re) (Pr) (St)
250
In a forward feed multiple effect evaporator unit
none of these.
251
In case of heat flow by conduction for a cylindrical body with an internal heat source, the nature of temperature distribution is
parabolic
252
For heat flow through very thick walled cylinder, use____mean radius.
logarithmic
253
Indirect contact heat exchangers are preferred over direct contact heat exchangers, because
there is no risk of contamination.
254
Water is normally used as a coolant in the heat exchange equipments mainly because of its
abundance & high heat capacity
255
The rate of emission of radiation by a body does not depend upon the
shape and porosity of the body.
256
Which of the following has maximum thermal conductivity at the same temperature?
Steel
257
Maximum heat transfer rate is obtained in __________ flow.
turbulent
258
In case of __________ boiling, the liquid temperature is below the saturation temperature and the boiling takes place in the vicinity of the heated surface.
local
259
The ratio of velocity head to tube side return loss in case of a multipass shell and tube heat exchanger is
45295
260
Terminal point temperature differences between fluids in case of a heat exchanger is termed as
approach
261
LMTD can't be used as such without a correction factor for the
all
262
Fqra cold viscous feed, backward feed gives __________ than forward feed.
a higher capacity
263
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
264
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
265
In SI units, thermal conductivity is expressed in
Watt/m.°K
266
Boiling point of a solution according to Duhring's rule is a linear function of the __________ of water.
boiling point (at the same pressure)
267
The overall resistance for heat transfer through a series of flat resistance, is the __________ of the resistances.
sum
268
At what value of Prandtl number, conduction is negligible in the turbulent core of a fluid flowing through a heated pipe?
> 0.6
269
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)
270
For a laminar flow of fluid in a circular tube, 'h1' is the convective heat transfer co-efficient at velocity 'V1'. If the velocity is reduced by half and assuming the fluid properties are constant, the new convective heat transfer co-efficient is
0.794 h1
271
The main purpose of providing fins on heat transfer surface is to increase the
heat transfer area.
272
A BTU/hr.ft.2 °F is equal to
4.88 kcal/hr. m.2°C.
273
Which of the following has the lowest Prandtl number?
Molten sodium (a coolant used in fast breeder reactor)
274
In SI units, fouling factor is expressed in
m2°K/W
275
An ejector is used to
none of these
276
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.
277
All analogy equations connecting friction factor and heat transfer co-efficient apply only to
wall or skin friction
278
Electro-magnetic spectrum range, which is important for radiation varies from __________ microns.
0.5 to 50
279
The characteristic dimensionless groups for heat transfer to a fluid flowing through a pipe in laminar flow are
Nu, Gz
280
Heat transfer rate described by Fourier's law will decrease, if the __________ increases.
thickness
281
A black body does not __________ radiation.
refract, reflect
282
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 the
solution has an elevation of boiling point.
283
Peclet number (Pe) is given by
Pe = Re.Pr
284
Nusselt number is the ratio of the
heat flux at the wall to that across the entire pipe.
285
Prandtl number is the ratio of
momentum diffusivity to thermal diffusivity.
286
Which of the following parameters is increased by use of finned tube in a multipass shell and tube heat exchanger?
all
287
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
288
"The ratio of the total emissive power to the absorptivity for all bodies is same at ther-mal equilibrium". This is __________ law.
Kirchoffs
289
View factor is important in heat transfer by
radiation
290
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.
291
For a multipass shell and tube heat exchanger, the LMTD correction factor is always
< 1
292
If heat transfer rate varies with the time, it is termed as
none of these
293
Pick out the wrong statement.
a. Fluid movement under the influence of buoyant forces resulting from change in
density takes place in case of natural convection.
b. The ratio NNu/NRe . Npr is called the Stanton number.
c. The Peclet number is a measure of the ratio of energy transport by convection to that
by conduction.
d. The Colbum jH factor for heat transfer is given by Nst Npr.
The Colbum jH factor for heat transfer is given by Nst Npr.
294
Tube expansion allowances exist in __________ heat exchanger.
U-tube
295
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.
296
Agitated film evaporator is suitable for concentrating __________ liquids.
viscous
297
The thickness of condensate layer in filmwise condensation depends on the
all
298
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
299
Intermittant tube cleaning is possible to be done in case of a __________ evaporator.
basket type
300
Harmonic mean temperature difference is given by
(2. ΔT1. ΔT2)/(ΔT1 + ΔT2)
301
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.
302
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
303
Overall heat transfer co-efficient for cooling of hydrocarbons by water is about
50 -100 BTU/hr. ft.2°F
304
Absorptivity of a perfect black body is unity. Which of the following has maximum absorptivity?
coke breeze
305
Thermal diffusivity is given by
k/p.Cp
306
At what value of Prandtl number, the hydrodynamic and thermal boundary layers of a fluid flowing over a heated plate will be identical?
1
307
Fourier's law of heat conduction applies to __________ surfaces.
isothermal, non-isothermal
308
__________ chart is known as transient heat conduction chart.
Dirhing's
309
Choose the most important factor on which the heat conducted through a wall in a unit time will depend on?
Temperature difference between the two surfaces of the wall.
310
Which of the following has the minimum thermal conductivity?
Nitrogen
311
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.
312
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
313
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.
314
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
315
Circulation pump is located below the evaporater to
create more suction head.
316
The statement that "maximum wavelength of radiation is inversly proportional to the temperature" is __________ law.
Wien's displacement
317
Steam condensate is recovered by steam traps and recycled for use as boiler feed water, because of its low
all
318
At steady state the temperature variation in a plane wall, made of two different solids I & II is shown below :
The thermal conductivity of material I
is smaller than that of II.
319
Duhring rule is important in solving problems on
evaporation
320
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
321
Multiple effect evaporation is generally recommended, when the
large scale evaporation of liquor is needed
322
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
323
__________ heat exchanger is the most suitable, when the temperature of shell side fluid is much higher than that of tube side.
U-tube
324
In which mode of heat transfer, the Biot number is important?
Transient heat conduction
325
_________ paint has the minimum absorption co-efficient.
White lead
326
Asymptotic conditions is reached, when for a fluid flowing in laminar flow through a long tube
exit fluid temperature = wall temperature.
327
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
328
Which is the most suitable for the con-certration of highly concentrated solution?
Open pan evaporation
329
Unsteady state heat conduction occurs, when
temperature distribution is dependent on time.
330
What is the absorptivity of a black body?
1
331
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
332
Baffle spacing
should be less than the inside diameter of the shell.
333
Boiling point elevation of a solution of NaOH
is almost independent of temperature, is almost independent of pressure
334
The advantage of backward feed multiple effect evaporators over forward feed units is that
most concentrated liquid is at highest temperature.
335
Which of the following is the most controlling factor for the rate of bubble detachment from the hot solid surface?
Interfacial tension
336
Which characteristic of a fluid is not important in deciding its route in a shell and tube heat exchanger?
None of these
337
Thermal conductivities of most of the liquids __________ with rise in temperature.
decreases
338
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
339
The purpose of floating head in a heat exchanger is to
avoid buckling of tubes.
340
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
341
Overall thermal resistance for conductive heat transfer through a series of flat resistances is equal to the
sum of all resistances.
342
Double pipe heat exchangers are preferably useful, when
requirement of heat transfer area is low.
343
Removal of __________ heat is involved in the condensation of a vapor under saturated conditions.
latent
344
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
345
If air (a non-condensing gas) is present in a condensing vapor stream, it will __________ the condensation rate of vapor.
decrease
346
Prandtl and Reynold's analogies are identical for Prandtl number value of
1
347
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
348
In evaporators, lowering the feed temperature
increases the heating area required, reduces the economy
349
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
350
In case of surface condensers, a straight line is obtained on plotting __________ on an ordinary graph paper.
1/ ¯v^-0.8
351
Thermal diffusivity is the most important in heat transfer by
conduction
352
(NGr x NPr) is called the __________ number.
Reyleigh
353
A dephlegmator is a
partial condenser
354
A tank painted with which of the following coloured paints, would heat up maximum by radiation from sun?
Black paint
355
The separation of liquid droplets from the vapor is done by a/an __________ , in the evaporators.
entrainment separator
356
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
357
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
358
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
359
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.
360
Evaporator tubes are generally
vertical
361
For a counter current heat exchanger with Tih = 80°C, T°c = 60°C, T°h = 50°C and Tic = 30°C, and the temperature difference between the two streams being the same everywhere along Z, the direction of flow of hot fluid. The temperature profile should satisfy
d^2T/dZ^2 = 0
362
In a boiling curve, the peak heat flux is called the __________ point.
burnout
363
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.
364
h.D/K is called the __________ number.
Nusselt
365
Prandtl number is the ratio of
momentum diffusivity to thermal diffusivity
366
Conductance is given by(where, x = thickness, A = heat flow area, K = thermal conductivity.)
KA/x
367
Heat flux, as defined in heat flow is analogous to __________ in electricity flow.
current
368
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
369
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
370
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)
371
The purpose of providing bleed points in the evaporator is to
facilitate removal of non-condensable gases
372
Water always boils when its
saturated vapour pressure equals the external pressure on its surface.
373
A multiple effect evaporator as compared to a single effect evaporator of the same capacity has
higher steam economy
374
Which is the most suitable for cold viscous feed?
Backward feed
375
Dropwise condensation is promoted on a/an __________ surface.
oily
376
At Pr > 1, conduction in an ordinary fluid flowing through a heated pipe is limited to the
viscous sub-layer
377
In a single evaporator system, the steam economy __________ by creating vacuum in the evaporator.
increases
378
(NRe.NPr)(D/L)is called the __________ number
Graetz
379
Nucleate boiling is promoted
on rough surfaces
380
In forced circulation, the heating element is injected
internally
381
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
382
The equation, Nst = f/2, is the __________ analogy.
Reynolds
383
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
384
Rate of crystallisation does not depend upon the
none of these.
385
Stefen's block body radiation law can also be derived from __________ law.
Planck's
386
Multipass heat exchangers are used
to obtain higher heat transfer co-efficient and shorter tube.
387
A hollow sphere and a solid sphere of the same material and equal radii are heated to the same temperature. In this case,
both the spheres will emit equal amount of radiation per unit time in the beginning, both will absorb equal amount of radiation from the surrounding in the beginning
388
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.
389
When does the heat generated by fluid friction becomes appreciable compared to the heat transferred between the fluids?
At high fluid velocity
390
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
391
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
392
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.
same in either parallel, mixed, or counterflow
393
Thermal diffusivity of a material
all
394
Viscosity of gases __________ with increase in temperature.
increase slowly
395
What is the emissivity of a black body?
1
396
Steam is routed through the tube in case of a __________ evaporator.
horizontal tube
397
Dropwise condensation of steam on cooling surface is promoted
an oily surface.
398
Conduction occurs in the buffer zone for a fluid flowing through a heated pipe, only when Prandtl number is
0.1
399
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.
400
With increase in temperature, the total emissivity of conductors
increases
401
Loss of heat from untagged steam pipe to the ambient air is by
all
402
Fruit juice (a heat sensitive material) can be concentrated in a __________ evaporator.
falling film
403
Tubes are held between top and bottom tube sheets in Calenderia type evaporator by keeping
both the tube sheets fixed.
404
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
405
Heat sensitive materials can be concentrated in an evaporator employing
vacuum
406
A concentric double pipe heat exchanger as compared to the shell and tube heat exchanger for the same heat load requires
more space.
407
In a shell and tube heat exchanger, floating head is used for
large temperature differentials.
408
Steam trap is used to
remove water resulting from partial condensation of steam.
409
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
410
What is the logarithmic mean of r1 and r2 ?
(r1-r2/ln(r1/r2))
411
Maximum heat transfer rate is achieved in __________ flow.
turbulent
412
Wavelength corresponding to the maximum energy is inversely proportional to the absolute temperature. This is __________ law.
Wien's
413
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
414
Which of the following parameters of the fluid is not very important, while deciding its route in a shell and tube heat exchanger?
density
415
Corrosiveness of steam condensate is due to the presence of
CO2, dissolved O2
416
The equation, (NSt x N2/3Pr) = f/2, is the __________ analogy.
Colburn
417
The driving potential for the crystal growth during crystallisation is the __________ of the solution.
super-saturation
418
jH factor for heat transfer depends upon the __________ number.
Reynolds
419
Kg of liquid evaporated per hour in an evaporator is defined as its
capacity
420
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.
421
Open pan evaporators are preferred to be used, when the solution to be concentrated is
highly viscous
422
Cp.μ/K is termed as the __________ number.
Prandtl
423
The maximum heat transfer co-efficient from steam heating will be attained when the steam is
saturated
424
Kirchoff's law is applicable to
monochromatic radiation only, total radiation only
425
In a forward feed multiple effect evaporator, the pressure is
highest in last effect
426
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
427
Which of the following situations can be approximated to a steady state heat transfer system?
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.
428
Sensible heat absorbed by 1 lb of water when it is heated from 32 to 212°F may be around __________ BTU.
180
429
The unit of conductance in SI unit is
W/°K
430
In a shell and tube heat exchanger, square pitch compared to triangular pitch
gives a higher shell side pressure drop.
431
Which of the following has maximum thermal conductivity?
Iron
432
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
433
The film co-efficient between condensing vapour and metal wall increases with
increasing temperature of the vapour.
434
Critical value of the __________ number governs the transition from laminar to turbulent flow in free convection heat transfer.
Prandtl & Grashoff
435
The thermal boundary layer at NPr > 1
is thinner than hydrodynamic boundary layer.
436
In the free convection regime of pool boiling, the heat flux is proportional to
Δt^5/4
437
With increase in temperature, the thermal conductivity of fresh lubricating oil
decreases
438
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
439
Nusselt number is the ratio of the temperature gradient at the wall to
that across the entire pipe
440
The Stefan-Boltzman constant depends on the
none of these
441
The units of resistance to heat transfer is
W-1m2K
442
Thermal conductivity of a gas at low density, __________ with increase in temperature.
increases
443
Nusselt number is a function of Prandtl number and __________ number of fluid in natural convection heat transfer.
Grashoff
444
__________ heat exchanger is used for chilling oil to be dewaxed.
Double pipe
445
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
446
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.
447
In sub-cooled boiling,
temperature of the heating surface is more than the boiling point of the liquid.
448
With increase in temperature, the thermal conductivity of a gas
increases.
449
A __________ evaporator employs an annular downtake.
basket type
450
Radiation heat losses from satisfactorily insulated high pressure boilar may be about __________ percent.
7
451
In natural convection heat transfer, the correlating parameter is the
Grashoff number
452
Boiling of milk in an open vessel is an example of __________ boiling.
film
453
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.
454
Mode of heat transfer involved in the cooling of air cooled internal combustion engine is
natural convection
455
Steady state one dimensional heat flow by conduction as given by Fourier's low does not assume that
material is anisotropic.
456
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
457
In a shell and tube heat exchanger, the shell side fluid velocity can not be changed by changing the
tube diameter
458
To reduce the tube side pressure drop for the same flow rate, the heat exchanger recomended is
1-1 heat exchanger
459
Heat transfer co-efficient (h) for a fluid flowing inside a clean pipe is given by
This is valid for the value of NRe equal to
> 10000
460
Steam side heat transfer co-efficient in an evaporator is in the range of __________ kcal/hr.m2°C.
5000-15000
461
Correction is applied to LMTD for __________ flow.
cross
462
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
463
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.
464
For __________ Prandtl number values, the heat conduction will be negligible in the buffer zone.
high
465
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
466
Heat exchangers operating, when the asymptotic range is reached,
results in making part of the heating surface inactive.
467
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
468
In case of heat transfer by conduction in a hollow cylinder, __________ mean area is used to calculate the heat transfer rate.
logarithmic
469
In a gas-liquid shell and tube heat exchanger, the
all
470
Which is the most suitable for the concentration of foamy & frothy liquors?
Long tube vertical evaporator
471
Boiling point elevation of an ideal solution
increases rapidly with temperature rise.
472
The rate of heat transfer is a product of overall heat transfer co-efficient, the difference in temperature and the
heat transfer area
473
Which of the following has the lowest overall heat transfer co-efficient?
Air
474
Which of the following has the minimum absorptivity?
Aluminum foil
475
A black body when hot, emits heat radiation of __________ wavelengths.
all
476
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
477
Vibrations in the tubes of a shell and tube heat exchanger is induced due to the
all
478
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
479
The Nusselt number for fully developed (both thermally and hydrodynamically) laminar flow through a circular pipe whose surface temperature remains constant is
3.66
480
Sensible heat of hot industrial flue gases can not be recovered by a/an
none of these
481
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
482
Radiator of an automobile engine is a __________ type of heat exchanger.
direct contact
483
In pipe flow, heat is transferred from hot wall to the liquid by
forced convection and conduction
484
The value of Stefan-Boltazman constant in SI unit is
5.6697 x 10-8W/m2.°K4
485
The unit of heat transfer co-efficient is
BTU/hr. ft2°F
486
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
487
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
488
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
489
Low thermal conductivity of heat insulating materials is due to its
high proportion of air space
490
The most conducive surface for dropwise condensation to occur is the __________ surface.
oily
491
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
492
Calburn analogy is applicable for the value of Prandtl number from
0.6 to 120
493
Pick out the correct equation.
a. jH = (St)(Pr)^2/3 = f/2
b. jH = (St)(Pr)^1/3 = f/2
c. jH = (St)^2/3(Pr) = f/2
d. jH = (St)^1/3(Pr) = f/2
jH = (St)(Pr)^2/3 = f/2
494
Multiple effect evaporators ar used to
increase the steam economy & the capacity.
495
Prandtl number is given by
(Cp · μ)/k
496
Small scale evaporation is done in a
steam jacketed kettle
497
Heat exchanger tubes are never made of
lead
498
At constant temperature, the thermal conductivities of gases __________ with rise in pressure.
decrease
499
Reason for operating an evaporator in multiple effect is to secure
increased steam economy, decreased steam consumption
500
In a cooling tower, water becomes cool by
loosing sensible heat.
501
In Fourier's law, the proportionality constant is called the
thermal conductivity
502
The heat transfer co-efficient in film type condensation is __________ that for dropwise condensation.
lower than
503
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.
504
Duhring's plot' is of use in
evaporation
505
Which has the lowest Prandtl number?
Liquid metal
506
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.
507
Prandtl number for most of dry gases is about
0.72
508
Prandtl number is the reciprocal of
Thermal diffusivity/Momentum diffusivity
509
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
510
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
511
Multiple effect evaporation accounts for
steam economy
512
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
513
Maximum water velocity in tubes of a 1-2 shell and tube heat exchanger may be around __________ metres/second.
1
514
In Biot number, the characteristic length used is the ratio of the __________ of the solid.
volume to surface area
515
Which of the following is the most widely used heat insulating material for pipelines carrying steam?
85% magnesia cement and glass wool.
516
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.
517
Which is the best tube arrangement (in a shell and tube heat exchanger) if the fluids are clean and non-fouling?
Triangular pitch
518
In case of a multipass shell and tube heat exchanger, the temperature drop in the fluid
and the wall are proportional to individual resistances.
519
The purpose of providing expansion bellows in the shell of tubular exchanger is to
account for the uneven expansion of shell and tube bundles.
520
A graph between __________ is called Wilson plot.
1/U Vs. 1/V^0.8
521
Grashhoff number is given by
gD^3.β.Δtρ2/μ^2
522
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
523
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
524
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.
525
What is the geometric mean of two heat transfer areas A1 and A2?
sqrt*(A1 . A2)
526
Evaporation by thermocompression results in the
saving of steam, realisation of multiple effect economy in a single effect
527
Forced circulation evaporators are normally used for concentrating liquids having
scaling characteristics, high viscosity
528
Steam economy in case of a triple effect evaporator will be
> 1
