(3) Heat Transfer_F Flashcards
(524 cards)
1
Q
Crystal size in a continuous crystalliser depends upon the
A
rate of heat transfer
degree of turbulence
degree of supersaturation
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 ___________________
C - capacity of the evaporator
E - economy of the evaporator
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 depend upon the
A
baffle spacing & shell diameter.
tube diameter & pitch.
viscosity, density & mass velocity of shell side fluid.
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
Fouling factor
A
accounts for additional resistances to heat flow.
14
Q
In case of vertical tube evaporator, with increase in liquor level, the overall heat transfer co-efficient
A
decreases
15
Q
The steam ejector is used to
A
create vacuum.
16
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
17
Q
In a heat exchanger, floating head is provided to
A
relieve stresses caused by thermal expansion.
18
Q
Leidenfrost point is a term concerned with the
A
boiling of a liquid on a hot surface.
19
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.
20
Q
Heat produced when a steady state current, I passes through an electrical conductor having resistance, ‘R’ is
A
I^2R
21
Q
Pick out the wrong statement.
A
Orifice baffles are never used in a shell and tube heat exchanger.
22
Q
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.
A
highly turbulent
23
Q
The actual temperature drop across the heating surface in an evaporator depends on the
A
feed
depth of liquid over heating surface.
pressure difference between steam chest and vapour space.
24
Q
Which of the following is not used as a medium for high temperature heating?
A
mercury
25
Presence of a non-condensing gas in a condensing vapour
none of these
(not)
increases the rate of condensation
decreases thermal resistance.
is desirable to increase the film co-efficient.
26
With the increase of temperature, the **Col-burn jH factor**
increases.
27
In case of evaporators, liquid entrainment results primarily due to
foaming of the solution.
28
Baffles in the shell side of a shell and tube heat exchanger
increase the shell side heat transfer co-efficient.
29
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.
30
The unit of heat transfer co-efficient in SI unit is
W/m2°K
31
**The number of kg vaporised per kg of steam fed** to the evaporator is defined as
economy
32
Pick out the wrong statement.
The equivalent diameter of heat transfer for a duct of square cross-section (having each side as 'x') is equal to 4x.
33
Economy of a multiple effect evaporator is not influenced much by the
boiling point elevations
34
**Increasing the liquor level** in the evaporator results in the
decreased capacity.
35
The equivalent diameter for pressure drop is __________ that for heat transfer.
smaller than
36
Controlling **heat transfer film co-efficient** is the one, which offers __________ resistance to heat transfer.
the largest
37
Pick out the wrong statement.
In case of a 1 - 2 shell and tube heat exchanger, the LMTD correction factor value increases sharply, when a temperature cross occurs.
38
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
39
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.
40
Which of the following is concerned with both heat and mass transfer?
Lewis relationship
41
A backward feed multiple effect evaporator is better than forward feed for concentrating cold feed, because it provides
higher economy
42
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.
43
Which area is used in case of **heat flow by conduction** through a **cylinder**?
Logarithmic mean area
44
**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.
45
The dimensionless group in mass transfer that is equivalent to **Prandtl number** in heat transfer is
Schmidt number.
46
Finned tube heat exchangers
give larger area per tube.
47
The **absorptivity of a grey body** at a given temperature ________ with **increasing wavelength of radiation**.
may increase or decrease ; depends on the material
48
Which one gives the **monochromatic emissive power** for black body radiation?
Planck's law
49
For what value of Prandtl number, St = f/2?
1
50
In a shell and tube heat exchanger,
the **temperature drops** in the two fluids and the **wall** are proportional to **individual resistances**.
51
**Dietus-Boelter equation** used for the determination of heat transfer co-efficient is valid
for fluids in tubulent flow.
52
The equation, Nst = (f/2)/(1+5(NPr - 1)(sqrt*f/2)), corrosponds to __________ analogy.
Prandtl
53
For **large heat transfer area requirement,** shell and tube heat exchanger is preferred, because it
occupies smaller space.
is more economical.
is easy to operate and maintain.
54
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.
55
**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
56
Resistance to heat flow by conduction is proportional to (where, t & ρ are thickness & density of the material respectively and A = area normal to the direction of heat flow.)
t
1/ρ
1/A
57
Temperature profile in steady state heat transfer is
linear
58
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.
59
When vaporisation takes place through a **blanketting film of gas**, the phenomenon is termed as __________ boiling.
film
60
**Minimum** recommended **baffle spacing** in a shell and tube heat exchanger is about (where, D = shell diameter)
0.2D
61
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
62
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
63
What is the unit of thermal conductivity?
Kcal/hr.m.°C
64
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
65
It is not preferable to use **superheated steam** in evaporators, because of its very
low film co-efficient
66
**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).
67
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
68
The range of **electromagnetic spectrum** important in heat transfer by **radiation** is ________ **microns**.
0.5-50
69
Use of **transverse baffles** in a shell and tube heat exchanger is done to increase the
rate of heat transfer
solution being evaporated.
pressure difference between the **steam chest** and the **vapor space** above the boiling liquid.
70
The **actual temperature drop** across the heating surface of an evaporator depends on the
liquid depth over the heating surface.
solution being evaporated.
pressure difference between the steam chest and the vapor space above the boiling liquid.
71
**Economy** of a multiple effect evaporator depends upon the
heat balance consideration
72
The **Graetz number** is concerned with the
heat transfer in laminar flow.
73
**Solid angle** subtended by the **finite surface** at the radiating element is
angle of vision
74
Pick out the wrong statement.
The emissivity of a surface decreases, if it gets corroded by atmospheric environment.
75
In forced convection, the heat transfer depends on
Re, Pr
76
With **increase in temperature**, the thermal conductivity of most **liquids**
decreases
77
Heat transfer by ________ may **not** necessarily require the **presence of a medium**.
radiation
78
Pick out the correct statement.
Logarithmic mean area is used for calculating the heat flow rate through a thick walled cylinder.
79
In a shell and tube type heat exchanger, the floating tube bundle heat arrangement is used
in high range of temperature differences.
80
Stefan-Boltzman law applies to __________ body.
black
81
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
82
In case of parallel flow heat exchanger, the lowest temperature theoretically attainable by the hot fluid is __________ the outlet temperature of the cold fluid.
83
An equipment which converts the latent or sensible heat of one fluid into the latent heat of vaporisation of another, is called a
84
Which of the following is directly concerned with the heat transfer?
85
Pick out the wrong statement:
86
Planck's distribution law is valid for __________ bodies.
87
For hot feed, forward feeding as compared to backward feeding results in __________ economy.
88
Radiation energy is emitted by all the substances, which are above
89
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
90
What is the steam economy in case of a single effect evaporator system?
91
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
92
jH factor for heat transfer is not a function of the __________ number.
93
The Fourier number (defined as a.t/L2) is used in the analysis of problem involving heat transfer by
94
The energy radiated from a surface Q at absolute temperature T is related as
95
Baffles are provided in heat exchangers to increase the
96
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)
97
In case of a vertical tube evaporator, with increase in the liquor level, the
98
The ratio of total radiating power to the absorptivity of the body depends upon the __________ as per Kirchoffs law.
99
For gases, the thermal conductivity increases with temperature rise. For liquids, with increase in concentration, its thermal conductivity generally
100
A 2-4 heat exchanger involves
101
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
102
With increase in temperature, the thermal conductivity of steel
103
Which of the following forced convection heat transfer equation accounts for the liquid viscosity effect for viscous liquids?
104
The main function of baffles provided in a shell and tube heat exchanger is to
105
In a single effect evaporator, the economy is
106
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
107
Which of the following is not concerned with the heat transfer?
108
Latent heat absorbed by 1 lb of water at 212°F, when it is changed to steam at 212°F, may be around __________ BTU.
109
In counter flow compared to parallel flow,
110
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)
111
Crystallisation of solids from a homogeneous solution is a/an __________ process.
112
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
113
Value of Prandtl number for water ranges from
114
Heat transfer in the laminar sublayer in case of a liquid flowing through a pipe, is mostly by
115
Among liquids, water has a comparatively high value of thermal conductivity, due to its
116
Reynold's analogy states that (where, St = Stanton number f = friction factor)
117
For evaporation of viscous solution in a multiple effect evaporator, the prefered feeding scheme is
118
The sum of reflectivity and absorptivity for an opaque body is equal to
119
The film thickness for laminar film condensation on vertical surface __________ from top to bottom.
120
In case of a multipass shell and tube heat exchanger, providing a baffle on the shell side __________ the heat transfer rate.
121
The local surface conductance for laminar film condensation on vertical surface is (where, t = film thickness)
122
Heat transfer co-efficient (h1) for liquids increases with
123
A diathermaneous substance __________ the thermal radiation completely.
124
A hot body will radiate heat most rapidly, if its surface is
125
Evaporation of 1kg of water from a solution in a single effect evaporator requires about __________ kg of steam.
126
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
127
Analogy between mass and heat transfer is not applicable in case of
128
A __________ surface has the maximum thermal emissivity out of the following.
129
Radiant energy received by a body is proportional to (where, d = the distance between the object emitting radiation and that receiving it.)
130
Mechanical recompression evaporation is used in the production of
131
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.
132
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
133
The purpose of providing a 'catchall' in the vapor line of an evaporator is to
134
What is the thermal conductivity of a perfect heat insulator?
135
Fouling factor' used in the design of a multipass shell and tube heat exchanger is a
136
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
137
Prandtl and Reynolds analogy are same, when Prandtl number is
138
Heat transfer occurs by natural convection because change in temperature causes difference in
139
The ratio of kinematic viscosity to thermal diffusivity is called the __________ number.
140
The critical radius 'r' of insulation on a pipe is given by
141
As per Kirchoffs law, the ratio of the total radiating power to the absorptivity of a body depends on the
142
If the baffle spacing in a shell and tube heat exchanger increases, then the Reynolds number of the shell side fluid
143
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
144
__________ paint has the maximum absorption coefficient.
145
What is Nusselt number?
146
Steam traps are provided in steam carrying pipelines to
147
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)
148
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
149
Radiation heat transfer rates does not depend upon the
150
Which of the following is correct?
151
Convective heat transfer, in which heat is transferred by movement of warmed matter is described by
152
As the difference between the wall temperature and bulk temperature increases, the boiling heat transfer co-efficient
153
Heat transfer rate per unit area is called
154
Log mean temperature difference (LMTD) cannot be used, if
155
In a multipass shell and tube heat exchanger, tube side return pressure loss is equal to __________ the velocity head.
156
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
157
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
158
For what value of Prandtl number, the Col-burn analogy is valid?
159
In a forward feed multiple effect, the pressure build up will be
160
Film boiling is usually not desired in commercial equipments, because
161
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
162
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
163
Film condensation is promoted on a/an __________ surface.
164
Dietus-Boelter equation cannot be used for molten metals mainly due to its very low
165
If Prandtl number is greater than the Schmidt number, then the
166
Which of the following accessories is provided in the vapor line of an evaporator for removing the entrained liquid?
167
Rate of heat transfer by vaporisation from pools of water is affected by the
168
Multiple effect evaporators are commonly used in the manufacture of
P. Paper Q. Superphosphate
R. Sugar S. Fats
169
Which of the following has the highest thermal conductivity?
170
Which of the following is unimportant in forced convection?
171
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
172
In a multiple effect evaporator, the effect of boiling point elevation is to
173
The rate of heat transfer through a pipe wall is given by, https://www.indiabix.com/_files/images/chemical-engineering/heat-transfer/271-499-1.png . For cylinder of very thin wall, q can be approximated by
174
Dropwise condensation occurs on __________ surfaces.
175
For specified tube outside diameter, higher BWG means higher
176
Pick out the wrong statement.
177
In case of a vertical tube evaporator, with increase in the liquor level, the __________ is increased.
178
Electromagnetic radiations propagate in vacuum with a velocity of __________ metre/second.
179
Heat flux is the time rate of heat transfer per unit
180
Film boiling occurs at __________ pressure.
181
Heat flux through several resistances in series in analogous to the current flowing through several
182
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
183
Vacuum is generally maintained in the vapour space of an evaporator mainly to
184
Double pipe heat exchangers are used
185
Nusselt number (for forced convection heat transfer) is a function of the __________ number.
186
Pick out the wrong statement.
187
Which type of heat exchanger is preferred for heavy heat loads ?Which type of heat exchanger is preferred for heavy heat loads?
188
Pick out the wrong statement.
189
While the total emissivity of a perfect black body is unity, the same for a real body is
190
The Grashoff number is defined as the ratio of the
191
According to Reynolds analogy, Stanton number is equal to (where, f = Fanning friction factor)
192
The absorptivity of a body is equal to its emissivity
193
Absorptivity and refletivity of a perfect black body are respectively
194
In thermal radiation for a black body (where, ε is emissivity and α is absorptivity)
195
Pick out the wrong statement.
196
When warm and cold liquids are mixed, the heat transfer is mainly by
197
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?
198
Vent pipes are provided in a condenser to
199
Heat transfer by natural convection is enhanced in system with
200
In case of __________ boiling, the bubbles formed on a submerged hot surface get absorbed in the mass of the liquid.
201
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.
202
In forced convection, the Nusselt number is a function of
203
At what value of Prandtl number, the hydrodynamic and thermal boundary layers are identical?
204
A dilute aqueous solution is to be concentrated in an evaporator system. High pressure steam is available. Multiple effect evaporator system is employed, because
205
Heat transfer efficiency leading of energy conservation in a heat exchanger can be achieved by
206
Trap is used to remove __________ from steam pipe lines.
207
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
208
Tube pitch is the __________ of tube diameters and the clearances.
209
With increase in the distance between the heat source and the object receiving the heat, the radiation heat transfer
210
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.
211
Natural convection is characterised by
212
Nusselt number for full developed, laminar, constant property flow in a pipe at uniform heat flux is
213
Thickness of thermal boundary layer is more compared to that of hydrodynamic boundary layer, when the value of Prandtl number is
214
The critical radius of insulation for cylindrical pipe is (where, hi = heat transfer coefficient at inside of the pipe)
215
The heat flux in the nucleate boiling regimes is proportional to (where, ΔT = excess temperature)
216
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
217
A body is called grey if the monochromatic emissivity of the body is
218
In an extended surface heat exchanger, fluid having lower co-efficient
219
In a shell and tube heat exchanger, the tube side heat transfer co-efficient just at the entrance of the tube is
220
Grashoff number, which is defined as g . β(Ts - T∞) ρ2. L3/μ2, is proportional to the ratio of buoyancy force to __________ force.
221
Pick out the wrong statement.
222
Graetz number is given by
223
For a cold dilute feed to produce thick viscous liquor, backward feeding as compared to forward feeding results in
224
Economy of an evaporator is influenced by the
225
If the thermal conductivity of a wall material is independent of temperature, the steady state temperature distribution in the very large thin plane wall having steady, uniform surface temperature follows __________ law.
226
In regenerative air preheater (as practised in heating of coke ovens), the heat is transferred
227
__________ equation relates the thermal conductivity of a solid to its temperature.
228
For small temperature difference, the heat transfer rate as per Newton's law of cooling is proportional to (where, Δt = excess temperature)
229
Pick out the wrong statement.
230
Viscous & heat sensitive liquids are concentrated in __________ evaporators.
231
1 BTU/hr.ft.°F is equal to __________ kcal/hr. m.°C.
232
Heat transfer by radiation between two bodies at T1 & T2 and in an ambient temperature of Ta°C depends on
233
The ratio of momentum diffusivity to thermal diffusivity is the __________ number.
234
For a perfectly transparent surface (like gases), the
235
For a fluid flowing in an annulus space, the wetted perimeter for heat transfer and pressure drop are
236
During crystallisation, formation of crystal can occur in __________ solution only.
237
An evaporator while concentrating an aqueous solution from 10 to 40% solids evaporates 30000 kg of water. The amount of solids handled by the system in kg is
238
An insulator should have
239
In a backward feed multiple effect evaporator
240
Choose the correct equation.
241
In a forward feed multiple effect evaporator unit
242
In case of heat flow by conduction for a cylindrical body with an internal heat source, the nature of temperature distribution is
243
For heat flow through very thick walled cylinder, use____mean radius.
244
Indirect contact heat exchangers are preferred over direct contact heat exchangers, because
245
Water is normally used as a coolant in the heat exchange equipments mainly because of its
246
The rate of emission of radiation by a body does not depend upon the
247
Which of the following has maximum thermal conductivity at the same temperature?
248
Maximum heat transfer rate is obtained in __________ flow.
249
In case of __________ boiling, the liquid temperature is below the saturation temperature and the boiling takes place in the vicinity of the heated surface.
250
The ratio of velocity head to tube side return loss in case of a multipass shell and tube heat exchanger is
251
Terminal point temperature differences between fluids in case of a heat exchanger is termed as
252
LMTD can't be used as such without a correction factor for the
253
Fqra cold viscous feed, backward feed gives __________ than forward feed.
254
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 ?
255
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
256
In SI units, thermal conductivity is expressed in
257
Boiling point of a solution according to Duhring's rule is a linear function of the __________ of water.
258
The overall resistance for heat transfer through a series of flat resistance, is the __________ of the resistances.
259
At what value of Prandtl number, conduction is negligible in the turbulent core of a fluid flowing through a heated pipe?
260
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
261
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
262
The main purpose of providing fins on heat transfer surface is to increase the
263
A BTU/hr.ft.2 °F is equal to
264
Which of the following has the lowest Prandtl number?
265
In SI units, fouling factor is expressed in
266
An ejector is used to
267
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
268
All analogy equations connecting friction factor and heat transfer co-efficient apply only to
269
Electro-magnetic spectrum range, which is important for radiation varies from __________ microns.
270
The characteristic dimensionless groups for heat transfer to a fluid flowing through a pipe in laminar flow are
271
Heat transfer rate described by Fourier's law will decrease, if the __________ increases.
272
A black body does not __________ radiation.
273
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
274
Peclet number (Pe) is given by
275
Nusselt number is the ratio of the
276
Prandtl number is the ratio of
277
Which of the following parameters is increased by use of finned tube in a multipass shell and tube heat exchanger?
278
Forced circulation evaporators are useful for the concentration of viscous, salting and scale forming liquors. Which of the following is a forced circulation evaporator?
279
"The ratio of the total emissive power to the absorptivity for all bodies is same at ther-mal equilibrium". This is __________ law.
280
View factor is important in heat transfer by
281
The advantage of using a 1 - 2 shell and tube heat exchanger over a 1 - 1 shell and tube heat exchanger is
282
For a multipass shell and tube heat exchanger, the LMTD correction factor is always
283
If heat transfer rate varies with the time, it is termed as
284
Pick out the wrong statement.
285
Tube expansion allowances exist in __________ heat exchanger.
286
Pick out the wrong statement.
287
Agitated film evaporator is suitable for concentrating __________ liquids.
288
The thickness of condensate layer in filmwise condensation depends on the
289
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.
290
Intermittant tube cleaning is possible to be done in case of a __________ evaporator.
291
Harmonic mean temperature difference is given by
292
Arithmetic mean area can be used in heat transfer problem to calculate the heat flow by conduction through a cylinder which is
293
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
294
Overall heat transfer co-efficient for cooling of hydrocarbons by water is about
295
Absorptivity of a perfect black body is unity. Which of the following has maximum absorptivity?
296
Thermal diffusivity is given by
297
At what value of Prandtl number, the hydrodynamic and thermal boundary layers of a fluid flowing over a heated plate will be identical?
298
Fourier's law of heat conduction applies to __________ surfaces.
299
__________ chart is known as transient heat conduction chart.
300
Choose the most important factor on which the heat conducted through a wall in a unit time will depend on?
301
Which of the following has the minimum thermal conductivity?
302
In a heat exchanger, one transfer unit means
303
For condensation of pure vapors, if the heat transfer co-efficients in filmwise and drop-wise condensation are respectively hf and hd, then
304
Pick out the wrong statement:
305
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.
306
Circulation pump is located below the evaporater to
307
The statement that "maximum wavelength of radiation is inversly proportional to the temperature" is __________ law.
308
Steam condensate is recovered by steam traps and recycled for use as boiler feed water, because of its low
309
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
310
Duhring rule is important in solving problems on
311
In a shell and tube heat exchanger, the height of 25 percent cut baffles is equal to (where, D = inside diameter of shell)
312
Multiple effect evaporation is generally recommended, when the
313
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.
314
__________ heat exchanger is the most suitable, when the temperature of shell side fluid is much higher than that of tube side.
315
In which mode of heat transfer, the Biot number is important?
316
_________ paint has the minimum absorption co-efficient.
317
Asymptotic conditions is reached, when for a fluid flowing in laminar flow through a long tube
318
1000 kg of wet solids are to be dried from 60% to 20% moisture (by weight). The mass of moisture removed in kg is
319
Which is the most suitable for the con-certration of highly concentrated solution?
320
Unsteady state heat conduction occurs, when
321
What is the absorptivity of a black body?
322
Mode of heat transfer in which the fluid moves under the influence of changes in fluid pressure produced by external work is called
323
Baffle spacing
324
Boiling point elevation of a solution of NaOH
325
The advantage of backward feed multiple effect evaporators over forward feed units is that
326
Which of the following is the most controlling factor for the rate of bubble detachment from the hot solid surface?
327
Which characteristic of a fluid is not important in deciding its route in a shell and tube heat exchanger?
328
Thermal conductivities of most of the liquids __________ with rise in temperature.
329
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
330
The purpose of floating head in a heat exchanger is to
331
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
332
Overall thermal resistance for conductive heat transfer through a series of flat resistances is equal to the
333
Double pipe heat exchangers are preferably useful, when
334
Removal of __________ heat is involved in the condensation of a vapor under saturated conditions.
335
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
336
If air (a non-condensing gas) is present in a condensing vapor stream, it will __________ the condensation rate of vapor.
337
Prandtl and Reynold's analogies are identical for Prandtl number value of
338
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
339
In evaporators, lowering the feed temperature
340
The type of liquor circulation system to be ' employed in evaporators (viz. natural or forced circulation) is determined mainly by the __________ of the liquid.
341
In case of surface condensers, a straight line is obtained on plotting __________ on an ordinary graph paper.
342
Thermal diffusivity is the most important in heat transfer by
343
(NGr x NPr) is called the __________ number.
344
A dephlegmator is a
345
A tank painted with which of the following coloured paints, would heat up maximum by radiation from sun?
346
The separation of liquid droplets from the vapor is done by a/an __________ , in the evaporators.
347
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)
348
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
349
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.
350
Fluid motion in the natural convection heat transfer between a solid surface and a fluid in contact with it, results from the
351
Evaporator tubes are generally
352
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
353
In a boiling curve, the peak heat flux is called the __________ point.
354
Pick out the wrong statement.
355
h.D/K is called the __________ number.
356
Prandtl number is the ratio of
357
Conductance is given by(where, x = thickness, A = heat flow area, K = thermal conductivity.)
358
Heat flux, as defined in heat flow is analogous to __________ in electricity flow.
359
The wavelength at which the maximum monochromatic emissive power occurs for a black body, is (where, T = absolute temperature of the black body)
360
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.
361
When the ratio of the Grashoff number and to the square of Reynolds number is one, the dominant mechanism of heat transfer is
362
The purpose of providing bleed points in the evaporator is to
363
Water always boils when its
364
A multiple effect evaporator as compared to a single effect evaporator of the same capacity has
365
Which is the most suitable for cold viscous feed?
366
Dropwise condensation is promoted on a/an __________ surface.
367
At Pr > 1, conduction in an ordinary fluid flowing through a heated pipe is limited to the
368
In a single evaporator system, the steam economy __________ by creating vacuum in the evaporator.
369
(NRe.NPr)(D/L)is called the __________ number
370
Nucleate boiling is promoted
371
In forced circulation, the heating element is injected
372
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
373
The equation, Nst = f/2, is the __________ analogy.
374
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.
375
Rate of crystallisation does not depend upon the
376
Stefen's block body radiation law can also be derived from __________ law.
377
Multipass heat exchangers are used
378
A hollow sphere and a solid sphere of the same material and equal radii are heated to the same temperature. In this case,
379
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
380
When does the heat generated by fluid friction becomes appreciable compared to the heat transferred between the fluids?
381
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
382
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
383
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.
384
Thermal diffusivity of a material
385
Viscosity of gases __________ with increase in temperature.
386
What is the emissivity of a black body?
387
Steam is routed through the tube in case of a __________ evaporator.
388
Dropwise condensation of steam on cooling surface is promoted
389
Conduction occurs in the buffer zone for a fluid flowing through a heated pipe, only when Prandtl number is
390
In a heat exchanger, the rate of heat transfer from the hot fluid to the cold fluid
391
With increase in temperature, the total emissivity of conductors
392
Loss of heat from untagged steam pipe to the ambient air is by
393
Fruit juice (a heat sensitive material) can be concentrated in a __________ evaporator.
394
Tubes are held between top and bottom tube sheets in Calenderia type evaporator by keeping
395
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
396
Heat sensitive materials can be concentrated in an evaporator employing
397
A concentric double pipe heat exchanger as compared to the shell and tube heat exchanger for the same heat load requires
398
In a shell and tube heat exchanger, floating head is used for
399
Steam trap is used to
400
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
401
What is the logarithmic mean of r1 and r2 ?
402
Maximum heat transfer rate is achieved in __________ flow.
403
Wavelength corresponding to the maximum energy is inversely proportional to the absolute temperature. This is __________ law.
404
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.
405
Which of the following parameters of the fluid is not very important, while deciding its route in a shell and tube heat exchanger?
406
Corrosiveness of steam condensate is due to the presence of
407
The equation, (NSt x N2/3Pr) = f/2, is the __________ analogy.
408
The driving potential for the crystal growth during crystallisation is the __________ of the solution.
409
jH factor for heat transfer depends upon the __________ number.
410
Kg of liquid evaporated per hour in an evaporator is defined as its
411
Pick out the correct statement.
412
Open pan evaporators are preferred to be used, when the solution to be concentrated is
413
Cp.μ/K is termed as the __________ number.
414
The maximum heat transfer co-efficient from steam heating will be attained when the steam is
415
Kirchoff's law is applicable to
416
In a forward feed multiple effect evaporator, the pressure is
417
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
418
Which of the following situations can be approximated to a steady state heat transfer system?
419
Sensible heat absorbed by 1 lb of water when it is heated from 32 to 212°F may be around __________ BTU.
420
The unit of conductance in SI unit is
421
In a shell and tube heat exchanger, square pitch compared to triangular pitch
422
Which of the following has maximum thermal conductivity?
423
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.
424
The film co-efficient between condensing vapour and metal wall increases with
425
Critical value of the __________ number governs the transition from laminar to turbulent flow in free convection heat transfer.
426
The thermal boundary layer at NPr > 1
427
In the free convection regime of pool boiling, the heat flux is proportional to
428
With increase in temperature, the thermal conductivity of fresh lubricating oil
429
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
430
Nusselt number is the ratio of the temperature gradient at the wall to
431
The Stefan-Boltzman constant depends on the
432
The units of resistance to heat transfer is
433
Thermal conductivity of a gas at low density, __________ with increase in temperature.
434
Nusselt number is a function of Prandtl number and __________ number of fluid in natural convection heat transfer.
435
__________ heat exchanger is used for chilling oil to be dewaxed.
436
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
437
Pick out the wrong statement.
438
In sub-cooled boiling,
439
With increase in temperature, the thermal conductivity of a gas
440
A __________ evaporator employs an annular downtake.
441
Radiation heat losses from satisfactorily insulated high pressure boilar may be about __________ percent.
442
In natural convection heat transfer, the correlating parameter is the
443
Boiling of milk in an open vessel is an example of __________ boiling.
444
Pick out the wrong statement.
445
Mode of heat transfer involved in the cooling of air cooled internal combustion engine is
446
Steady state one dimensional heat flow by conduction as given by Fourier's low does not assume that
447
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
448
In a shell and tube heat exchanger, the shell side fluid velocity can not be changed by changing the
449
To reduce the tube side pressure drop for the same flow rate, the heat exchanger recomended is
450
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
451
Steam side heat transfer co-efficient in an evaporator is in the range of __________ kcal/hr.m2°C.
452
Correction is applied to LMTD for __________ flow.
453
If h1 = inner film co-efficient and /h2 = outer film co-efficient, then the overall heat transfer co-efficient is
454
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
455
For __________ Prandtl number values, the heat conduction will be negligible in the buffer zone.
456
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
457
Heat exchangers operating, when the asymptotic range is reached,
458
In case of a shell and tube heat exchanger, the minimum and maximum baffle spacing is respectively (where, D = inside diameter of the shell)
459
In case of heat transfer by conduction in a hollow cylinder, __________ mean area is used to calculate the heat transfer rate.
460
In a gas-liquid shell and tube heat exchanger, the
461
Which is the most suitable for the concentration of foamy & frothy liquors?
462
Boiling point elevation of an ideal solution
463
The rate of heat transfer is a product of overall heat transfer co-efficient, the difference in temperature and the
464
Which of the following has the lowest overall heat transfer co-efficient?
465
Which of the following has the minimum absorptivity?
466
A black body when hot, emits heat radiation of __________ wavelengths.
467
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
468
Vibrations in the tubes of a shell and tube heat exchanger is induced due to the
469
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
470
The Nusselt number for fully developed (both thermally and hydrodynamically) laminar flow through a circular pipe whose surface temperature remains constant is
471
Sensible heat of hot industrial flue gases can not be recovered by a/an
472
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)
473
Radiator of an automobile engine is a __________ type of heat exchanger.
474
In pipe flow, heat is transferred from hot wall to the liquid by
475
The value of Stefan-Boltazman constant in SI unit is
476
The unit of heat transfer co-efficient is
477
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
478
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
479
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
480
Low thermal conductivity of heat insulating materials is due to its
481
The most conducive surface for dropwise condensation to occur is the __________ surface.
482
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
483
Calburn analogy is applicable for the value of Prandtl number from
484
Pick out the correct equation.
485
Multiple effect evaporators ar used to
486
Prandtl number is given by
487
Small scale evaporation is done in a
488
Heat exchanger tubes are never made of
489
At constant temperature, the thermal conductivities of gases __________ with rise in pressure.
490
Reason for operating an evaporator in multiple effect is to secure
491
In a cooling tower, water becomes cool by
492
In Fourier's law, the proportionality constant is called the
493
The heat transfer co-efficient in film type condensation is __________ that for dropwise condensation.
494
Pick out the wrong statement.
495
Duhring's plot' is of use in
496
Which has the lowest Prandtl number?
497
Pick out the wrong statement.
498
Prandtl number for most of dry gases is about
499
Prandtl number is the reciprocal of
500
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
501
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
502
Multiple effect evaporation accounts for
503
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
504
Maximum water velocity in tubes of a 1-2 shell and tube heat exchanger may be around __________ metres/second.
505
In Biot number, the characteristic length used is the ratio of the __________ of the solid.
506
Which of the following is the most widely used heat insulating material for pipelines carrying steam?
507
Heat transfer co-efficient equation for forced convection, Nu = 0.023 Re^0.8 . Pr^n, is not valid, if the value of
508
Which is the best tube arrangement (in a shell and tube heat exchanger) if the fluids are clean and non-fouling?
509
In case of a multipass shell and tube heat exchanger, the temperature drop in the fluid
510
The purpose of providing expansion bellows in the shell of tubular exchanger is to
511
A graph between __________ is called Wilson plot.
512
Grashhoff number is given by
513
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
514
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
515
Pick out the wrong statement.
516
What is the geometric mean of two heat transfer areas A1 and A2?
517
Evaporation by thermocompression results in the
518
Forced circulation evaporators are normally used for concentrating liquids having
519
Steam economy in case of a triple effect evaporator will be
520
Pressure drop on the shell side of a heat exchanger depends upon tube pitch also.
true
521
In a horizontal tube evaporator, surface blanketing by air is avoided.
true
522
Split ring type and pull through type floating heads are two commonly used floating heads is heat exchangers.
true
523
The emissivity of a surface increases with increase in surface .
roughness
524
The emissivity of a non-metallic surface decreases with increase in the .
temperature
