(3) Heat Transfer Flashcards
1
Q
Crystal size in a continuous crystalliser depends upon the
A
2
Q
Heat waves
A
3
Q
Air is best heated with steam in a heat exchanger of
A
4
Q
Which of the following is generally considered as opaque surface towards radiations?
A
5
Q
Black liquor generated during paper manufacture is concentrated in a
A
6
Q
When vaporisation takes place directly at the heating surface, it is called
A
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
8
Q
Reynold’s analogy states that
A
9
Q
With increase in temperature, the thermal conductivity of non-metallic amorphous solids
A
10
Q
Fourier’s law applies to the heat transfer by
A
11
Q
Shell side pressure drop in a shell and tube heat exchanger does not depend upon the
A
12
Q
In the equation Q = UAΔt; Δt is
A
13
Q
With increase in porosity, the thermal conductivity of a solid substance
A
14
Q
For an ideal black body
A
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
16
Q
Fouling factor
A
17
Q
In case of vertical tube evaporator, with increase in liquor level, the overall heat transfer co-efficient
A
18
Q
The steam ejector is used to
A
19
Q
For shell and tube heat exchanger, with increasing heat transfer area, the purchased cost per unit heat transfer area
A
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
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
22
Q
In a heat exchanger, floating head is provided to
A
23
Q
Leidenfrost point is a term concerned with the
A
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
25
Heat produced when a steady state current, I passes through an electrical conductor having resistance, 'R' is
26
Pick out the wrong statement.
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.
28
The actual temperature drop across the heating surface in an evaporator depends on the
29
Which of the following is not used as a medium for high temperature heating?
30
Presence of a non-condensing gas in a condensing vapour
31
Kirchoff s law applies to __________ radiation.
32
With the increase of temperature, the Col-burn jH factor
33
In case of evaporators, liquid entrainment results primarily due to
34
Baffles in the shell side of a shell and tube heat exchanger
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
36
The unit of heat transfer co-efficient in SI unit is
37
The number of kg vaporised per kg of steam fed to the evaporator is defined as
38
Pick out the wrong statement.
39
Economy of a multiple effect evaporator is not influenced much by the
40
Increasing the liquor level in the evaporator results in the
41
The equivalent diameter for pressure drop is __________ that for heat transfer.
42
Controlling heat transfer film co-efficient is the one, which offers __________ resistance to heat transfer.
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
44
Thermal conductivity of a conducting solid material depends upon its
45
Pick out the wrong statement.
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)
47
Bulk of the convective heat transfer resistance from a hot tube surface to the fluid flowing in it, is
48
Which of the following is concerned with both heat and mass transfer?
49
A backward feed multiple effect evaporator is better than forward feed for concentrating cold feed, because it provides
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
51
Which area is used in case of heat flow by conduction through a cylinder?
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
53
The dimensionless group in mass transfer that is equivalent to Prandtl number in heat transfer is
54
Finned tube heat exchangers
55
The absorptivity of a grey body at a given temperature __________ with increasing wavelength of radiation.
56
Which one gives the monochromatic emissive power for black body radiation?
57
For what value of Prandtl number, St = f/2?
58
In a shell and tube heat exchanger,
59
Dietus-Boelter equation used for the determination of heat transfer co-efficient is valid
60
The equation, Nst = (f/2)/(1+5(NPr - 1)(sqrt*f/2)), corrosponds to __________ analogy.
61
For large heat transfer area requirement, shell and tube heat exchanger is preferred, because it
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 ?
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
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.)
65
Temperature profile in steady state heat transfer is
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
67
When vaporisation takes place through a blanketting film of gas, the phenomenon is termed as __________ boiling.
68
Minimum recommended baffle spacing in a shell and tube heat exchanger is about (where, D = shell diameter)
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)
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
71
What is the unit of thermal conductivity?
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
73
It is not preferable to use superheated steam in evaporators, because of its very
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
75
LMTD for counterflow and prallel flow heat exchanger will be the same, when the
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)
77
The range of electromagnetic spectrum important in heat transfer by radiation is __________ microns.
78
Use of transverse baffles in a shell and tube heat exchanger is done to increase the
79
The actual temperature drop across the heating surface of an evaporator depends on the
80
Economy of a multiple effect evaporator depends upon the
81
The Graetz number is concerned with the
82
Solid angle subtended by the finite surface at the radiating element is
83
Pick out the wrong statement.
84
In forced convection, the heat transfer depends on
85
With increase in temperature, the thermal conductivity of most liquids
86
Heat transfer by __________ may not necessarily require the presence of a medium.
87
Pick out the correct statement.
88
In a shell and tube type heat exchanger, the floating tube bundle heat arrangement is used
89
Stefan-Boltzman law applies to __________ body.
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
91
A perfect black body is a perfect __________ of radiation.
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.
93
An equipment which converts the latent or sensible heat of one fluid into the latent heat of vaporisation of another, is called a
94
Which of the following is directly concerned with the heat transfer?
95
Pick out the wrong statement:
96
Planck's distribution law is valid for __________ bodies.
97
For hot feed, forward feeding as compared to backward feeding results in __________ economy.
98
Radiation energy is emitted by all the substances, which are above
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
100
What is the steam economy in case of a single effect evaporator system?
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
102
jH factor for heat transfer is not a function of the __________ number.
103
The Fourier number (defined as a.t/L2) is used in the analysis of problem involving heat transfer by
104
The energy radiated from a surface Q at absolute temperature T is related as
105
Baffles are provided in heat exchangers to increase the
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)
107
In case of a vertical tube evaporator, with increase in the liquor level, the
108
The ratio of total radiating power to the absorptivity of the body depends upon the __________ as per Kirchoffs law.
109
For gases, the thermal conductivity increases with temperature rise. For liquids, with increase in concentration, its thermal conductivity generally
110
A 2-4 heat exchanger involves
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
112
With increase in temperature, the thermal conductivity of steel
113
Which of the following forced convection heat transfer equation accounts for the liquid viscosity effect for viscous liquids?
114
The main function of baffles provided in a shell and tube heat exchanger is to
115
In a single effect evaporator, the economy is
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
117
Which of the following is not concerned with the heat transfer?
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.
119
In counter flow compared to parallel flow,
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)
121
Crystallisation of solids from a homogeneous solution is a/an __________ process.
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
123
Value of Prandtl number for water ranges from
124
Heat transfer in the laminar sublayer in case of a liquid flowing through a pipe, is mostly by
125
Among liquids, water has a comparatively high value of thermal conductivity, due to its
126
Reynold's analogy states that (where, St = Stanton number f = friction factor)
127
For evaporation of viscous solution in a multiple effect evaporator, the prefered feeding scheme is
128
The sum of reflectivity and absorptivity for an opaque body is equal to
129
The film thickness for laminar film condensation on vertical surface __________ from top to bottom.
130
In case of a multipass shell and tube heat exchanger, providing a baffle on the shell side __________ the heat transfer rate.
131
The local surface conductance for laminar film condensation on vertical surface is (where, t = film thickness)
132
Heat transfer co-efficient (h1) for liquids increases with
133
A diathermaneous substance __________ the thermal radiation completely.
134
A hot body will radiate heat most rapidly, if its surface is
135
Evaporation of 1kg of water from a solution in a single effect evaporator requires about __________ kg of steam.
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
137
Analogy between mass and heat transfer is not applicable in case of
138
A __________ surface has the maximum thermal emissivity out of the following.
139
Radiant energy received by a body is proportional to (where, d = the distance between the object emitting radiation and that receiving it.)
140
Mechanical recompression evaporation is used in the production of
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.
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
143
The purpose of providing a 'catchall' in the vapor line of an evaporator is to
144
What is the thermal conductivity of a perfect heat insulator?
145
Fouling factor' used in the design of a multipass shell and tube heat exchanger is a
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
147
Prandtl and Reynolds analogy are same, when Prandtl number is
148
Heat transfer occurs by natural convection because change in temperature causes difference in
149
The ratio of kinematic viscosity to thermal diffusivity is called the __________ number.
150
The critical radius 'r' of insulation on a pipe is given by
151
As per Kirchoffs law, the ratio of the total radiating power to the absorptivity of a body depends on the
152
If the baffle spacing in a shell and tube heat exchanger increases, then the Reynolds number of the shell side fluid
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
154
__________ paint has the maximum absorption coefficient.
155
What is Nusselt number?
156
Steam traps are provided in steam carrying pipelines to
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)
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
159
Radiation heat transfer rates does not depend upon the
160
Which of the following is correct?
161
Convective heat transfer, in which heat is transferred by movement of warmed matter is described by
162
As the difference between the wall temperature and bulk temperature increases, the boiling heat transfer co-efficient
163
Heat transfer rate per unit area is called
164
Log mean temperature difference (LMTD) cannot be used, if
165
In a multipass shell and tube heat exchanger, tube side return pressure loss is equal to __________ the velocity head.
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
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
168
For what value of Prandtl number, the Col-burn analogy is valid?
169
In a forward feed multiple effect, the pressure build up will be
170
Film boiling is usually not desired in commercial equipments, because
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
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
173
Film condensation is promoted on a/an __________ surface.
174
Dietus-Boelter equation cannot be used for molten metals mainly due to its very low
175
If Prandtl number is greater than the Schmidt number, then the
176
Which of the following accessories is provided in the vapor line of an evaporator for removing the entrained liquid?
177
Rate of heat transfer by vaporisation from pools of water is affected by the
178
Multiple effect evaporators are commonly used in the manufacture of
P. Paper Q. Superphosphate
R. Sugar S. Fats
179
Which of the following has the highest thermal conductivity?
180
Which of the following is unimportant in forced convection?
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
182
In a multiple effect evaporator, the effect of boiling point elevation is to
183
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
184
Dropwise condensation occurs on __________ surfaces.
185
For specified tube outside diameter, higher BWG means higher
186
Pick out the wrong statement.
187
In case of a vertical tube evaporator, with increase in the liquor level, the __________ is increased.
188
Electromagnetic radiations propagate in vacuum with a velocity of __________ metre/second.
189
Heat flux is the time rate of heat transfer per unit
190
Film boiling occurs at __________ pressure.
191
Heat flux through several resistances in series in analogous to the current flowing through several
192
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
193
Vacuum is generally maintained in the vapour space of an evaporator mainly to
194
Double pipe heat exchangers are used
195
Nusselt number (for forced convection heat transfer) is a function of the __________ number.
196
Pick out the wrong statement.
197
Which type of heat exchanger is preferred for heavy heat loads ?Which type of heat exchanger is preferred for heavy heat loads?
198
Pick out the wrong statement.
199
While the total emissivity of a perfect black body is unity, the same for a real body is
200
The Grashoff number is defined as the ratio of the
201
According to Reynolds analogy, Stanton number is equal to (where, f = Fanning friction factor)
202
The absorptivity of a body is equal to its emissivity
203
Absorptivity and refletivity of a perfect black body are respectively
204
In thermal radiation for a black body (where, ε is emissivity and α is absorptivity)
205
Pick out the wrong statement.
206
When warm and cold liquids are mixed, the heat transfer is mainly by
207
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?
208
Vent pipes are provided in a condenser to
209
Heat transfer by natural convection is enhanced in system with
210
In case of __________ boiling, the bubbles formed on a submerged hot surface get absorbed in the mass of the liquid.
211
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.
212
In forced convection, the Nusselt number is a function of
213
At what value of Prandtl number, the hydrodynamic and thermal boundary layers are identical?
214
A dilute aqueous solution is to be concentrated in an evaporator system. High pressure steam is available. Multiple effect evaporator system is employed, because
215
Heat transfer efficiency leading of energy conservation in a heat exchanger can be achieved by
216
Trap is used to remove __________ from steam pipe lines.
217
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
218
Tube pitch is the __________ of tube diameters and the clearances.
219
With increase in the distance between the heat source and the object receiving the heat, the radiation heat transfer
220
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.
221
Natural convection is characterised by
222
Nusselt number for full developed, laminar, constant property flow in a pipe at uniform heat flux is
223
Thickness of thermal boundary layer is more compared to that of hydrodynamic boundary layer, when the value of Prandtl number is
224
The critical radius of insulation for cylindrical pipe is (where, hi = heat transfer coefficient at inside of the pipe)
225
The heat flux in the nucleate boiling regimes is proportional to (where, ΔT = excess temperature)
226
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
227
A body is called grey if the monochromatic emissivity of the body is
228
In an extended surface heat exchanger, fluid having lower co-efficient
229
In a shell and tube heat exchanger, the tube side heat transfer co-efficient just at the entrance of the tube is
230
Grashoff number, which is defined as g . β(Ts - T∞) ρ2. L3/μ2, is proportional to the ratio of buoyancy force to __________ force.
231
Pick out the wrong statement.
232
Graetz number is given by
233
For a cold dilute feed to produce thick viscous liquor, backward feeding as compared to forward feeding results in
234
Economy of an evaporator is influenced by the
235
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.
236
In regenerative air preheater (as practised in heating of coke ovens), the heat is transferred
237
__________ equation relates the thermal conductivity of a solid to its temperature.
238
For small temperature difference, the heat transfer rate as per Newton's law of cooling is proportional to (where, Δt = excess temperature)
239
Pick out the wrong statement.
240
Viscous & heat sensitive liquids are concentrated in __________ evaporators.
241
1 BTU/hr.ft.°F is equal to __________ kcal/hr. m.°C.
242
Heat transfer by radiation between two bodies at T1 & T2 and in an ambient temperature of Ta°C depends on
243
The ratio of momentum diffusivity to thermal diffusivity is the __________ number.
244
For a perfectly transparent surface (like gases), the
245
For a fluid flowing in an annulus space, the wetted perimeter for heat transfer and pressure drop are
246
During crystallisation, formation of crystal can occur in __________ solution only.
247
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
248
An insulator should have
249
In a backward feed multiple effect evaporator
250
Choose the correct equation.
251
In a forward feed multiple effect evaporator unit
252
In case of heat flow by conduction for a cylindrical body with an internal heat source, the nature of temperature distribution is
253
For heat flow through very thick walled cylinder, use____mean radius.
254
Indirect contact heat exchangers are preferred over direct contact heat exchangers, because
255
Water is normally used as a coolant in the heat exchange equipments mainly because of its
256
The rate of emission of radiation by a body does not depend upon the
257
Which of the following has maximum thermal conductivity at the same temperature?
258
Maximum heat transfer rate is obtained in __________ flow.
259
In case of __________ boiling, the liquid temperature is below the saturation temperature and the boiling takes place in the vicinity of the heated surface.
260
The ratio of velocity head to tube side return loss in case of a multipass shell and tube heat exchanger is
261
Terminal point temperature differences between fluids in case of a heat exchanger is termed as
262
LMTD can't be used as such without a correction factor for the
263
Fqra cold viscous feed, backward feed gives __________ than forward feed.
264
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 ?
265
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
266
In SI units, thermal conductivity is expressed in
267
Boiling point of a solution according to Duhring's rule is a linear function of the __________ of water.
268
The overall resistance for heat transfer through a series of flat resistance, is the __________ of the resistances.
269
At what value of Prandtl number, conduction is negligible in the turbulent core of a fluid flowing through a heated pipe?
270
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
271
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
272
The main purpose of providing fins on heat transfer surface is to increase the
273
A BTU/hr.ft.2 °F is equal to
274
Which of the following has the lowest Prandtl number?
275
In SI units, fouling factor is expressed in
276
An ejector is used to
277
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
278
All analogy equations connecting friction factor and heat transfer co-efficient apply only to
279
Electro-magnetic spectrum range, which is important for radiation varies from __________ microns.
280
The characteristic dimensionless groups for heat transfer to a fluid flowing through a pipe in laminar flow are
281
Heat transfer rate described by Fourier's law will decrease, if the __________ increases.
282
A black body does not __________ radiation.
283
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
284
Peclet number (Pe) is given by
285
Nusselt number is the ratio of the
286
Prandtl number is the ratio of
287
Which of the following parameters is increased by use of finned tube in a multipass shell and tube heat exchanger?
288
Forced circulation evaporators are useful for the concentration of viscous, salting and scale forming liquors. Which of the following is a forced circulation evaporator?
289
"The ratio of the total emissive power to the absorptivity for all bodies is same at ther-mal equilibrium". This is __________ law.
290
View factor is important in heat transfer by
291
The advantage of using a 1 - 2 shell and tube heat exchanger over a 1 - 1 shell and tube heat exchanger is
292
For a multipass shell and tube heat exchanger, the LMTD correction factor is always
293
If heat transfer rate varies with the time, it is termed as
294
Pick out the wrong statement.
295
Tube expansion allowances exist in __________ heat exchanger.
296
Pick out the wrong statement.
297
Agitated film evaporator is suitable for concentrating __________ liquids.
298
The thickness of condensate layer in filmwise condensation depends on the
299
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.
300
Intermittant tube cleaning is possible to be done in case of a __________ evaporator.
301
Harmonic mean temperature difference is given by
302
Arithmetic mean area can be used in heat transfer problem to calculate the heat flow by conduction through a cylinder which is
303
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
304
Overall heat transfer co-efficient for cooling of hydrocarbons by water is about
305
Absorptivity of a perfect black body is unity. Which of the following has maximum absorptivity?
306
Thermal diffusivity is given by
307
At what value of Prandtl number, the hydrodynamic and thermal boundary layers of a fluid flowing over a heated plate will be identical?
308
Fourier's law of heat conduction applies to __________ surfaces.
309
__________ chart is known as transient heat conduction chart.
310
Choose the most important factor on which the heat conducted through a wall in a unit time will depend on?
311
Which of the following has the minimum thermal conductivity?
312
In a heat exchanger, one transfer unit means
313
For condensation of pure vapors, if the heat transfer co-efficients in filmwise and drop-wise condensation are respectively hf and hd, then
314
Pick out the wrong statement:
315
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.
316
Circulation pump is located below the evaporater to
317
The statement that "maximum wavelength of radiation is inversly proportional to the temperature" is __________ law.
318
Steam condensate is recovered by steam traps and recycled for use as boiler feed water, because of its low
319
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
320
Duhring rule is important in solving problems on
321
In a shell and tube heat exchanger, the height of 25 percent cut baffles is equal to (where, D = inside diameter of shell)
322
Multiple effect evaporation is generally recommended, when the
323
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.
324
__________ heat exchanger is the most suitable, when the temperature of shell side fluid is much higher than that of tube side.
325
In which mode of heat transfer, the Biot number is important?
326
_________ paint has the minimum absorption co-efficient.
327
Asymptotic conditions is reached, when for a fluid flowing in laminar flow through a long tube
328
1000 kg of wet solids are to be dried from 60% to 20% moisture (by weight). The mass of moisture removed in kg is
329
Which is the most suitable for the con-certration of highly concentrated solution?
330
Unsteady state heat conduction occurs, when
331
What is the absorptivity of a black body?
332
Mode of heat transfer in which the fluid moves under the influence of changes in fluid pressure produced by external work is called
333
Baffle spacing
334
Boiling point elevation of a solution of NaOH
335
The advantage of backward feed multiple effect evaporators over forward feed units is that
336
Which of the following is the most controlling factor for the rate of bubble detachment from the hot solid surface?
337
Which characteristic of a fluid is not important in deciding its route in a shell and tube heat exchanger?
338
Thermal conductivities of most of the liquids __________ with rise in temperature.
339
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
340
The purpose of floating head in a heat exchanger is to
341
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
342
Overall thermal resistance for conductive heat transfer through a series of flat resistances is equal to the
343
Double pipe heat exchangers are preferably useful, when
344
Removal of __________ heat is involved in the condensation of a vapor under saturated conditions.
345
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
346
If air (a non-condensing gas) is present in a condensing vapor stream, it will __________ the condensation rate of vapor.
347
Prandtl and Reynold's analogies are identical for Prandtl number value of
348
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
349
In evaporators, lowering the feed temperature
350
The type of liquor circulation system to be ' employed in evaporators (viz. natural or forced circulation) is determined mainly by the __________ of the liquid.
351
In case of surface condensers, a straight line is obtained on plotting __________ on an ordinary graph paper.
352
Thermal diffusivity is the most important in heat transfer by
353
(NGr x NPr) is called the __________ number.
354
A dephlegmator is a
355
A tank painted with which of the following coloured paints, would heat up maximum by radiation from sun?
356
The separation of liquid droplets from the vapor is done by a/an __________ , in the evaporators.
357
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)
358
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
359
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.
360
Fluid motion in the natural convection heat transfer between a solid surface and a fluid in contact with it, results from the
361
Evaporator tubes are generally
362
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
363
In a boiling curve, the peak heat flux is called the __________ point.
364
Pick out the wrong statement.
365
h.D/K is called the __________ number.
366
Prandtl number is the ratio of
367
Conductance is given by(where, x = thickness, A = heat flow area, K = thermal conductivity.)
368
Heat flux, as defined in heat flow is analogous to __________ in electricity flow.
369
The wavelength at which the maximum monochromatic emissive power occurs for a black body, is (where, T = absolute temperature of the black body)
370
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.
371
When the ratio of the Grashoff number and to the square of Reynolds number is one, the dominant mechanism of heat transfer is
372
The purpose of providing bleed points in the evaporator is to
373
Water always boils when its
374
A multiple effect evaporator as compared to a single effect evaporator of the same capacity has
375
Which is the most suitable for cold viscous feed?
376
Dropwise condensation is promoted on a/an __________ surface.
377
At Pr > 1, conduction in an ordinary fluid flowing through a heated pipe is limited to the
378
In a single evaporator system, the steam economy __________ by creating vacuum in the evaporator.
379
(NRe.NPr)(D/L)is called the __________ number
380
Nucleate boiling is promoted
381
In forced circulation, the heating element is injected
382
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
383
The equation, Nst = f/2, is the __________ analogy.
384
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.
385
Rate of crystallisation does not depend upon the
386
Stefen's block body radiation law can also be derived from __________ law.
387
Multipass heat exchangers are used
388
A hollow sphere and a solid sphere of the same material and equal radii are heated to the same temperature. In this case,
389
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
390
When does the heat generated by fluid friction becomes appreciable compared to the heat transferred between the fluids?
391
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
392
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
393
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.
394
Thermal diffusivity of a material
395
Viscosity of gases __________ with increase in temperature.
396
What is the emissivity of a black body?
397
Steam is routed through the tube in case of a __________ evaporator.
398
Dropwise condensation of steam on cooling surface is promoted
399
Conduction occurs in the buffer zone for a fluid flowing through a heated pipe, only when Prandtl number is
400
In a heat exchanger, the rate of heat transfer from the hot fluid to the cold fluid
401
With increase in temperature, the total emissivity of conductors
402
Loss of heat from untagged steam pipe to the ambient air is by
403
Fruit juice (a heat sensitive material) can be concentrated in a __________ evaporator.
404
Tubes are held between top and bottom tube sheets in Calenderia type evaporator by keeping
405
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
406
Heat sensitive materials can be concentrated in an evaporator employing
407
A concentric double pipe heat exchanger as compared to the shell and tube heat exchanger for the same heat load requires
408
In a shell and tube heat exchanger, floating head is used for
409
Steam trap is used to
410
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
411
What is the logarithmic mean of r1 and r2 ?
412
Maximum heat transfer rate is achieved in __________ flow.
413
Wavelength corresponding to the maximum energy is inversely proportional to the absolute temperature. This is __________ law.
414
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.
415
Which of the following parameters of the fluid is not very important, while deciding its route in a shell and tube heat exchanger?
416
Corrosiveness of steam condensate is due to the presence of
417
The equation, (NSt x N2/3Pr) = f/2, is the __________ analogy.
418
The driving potential for the crystal growth during crystallisation is the __________ of the solution.
419
jH factor for heat transfer depends upon the __________ number.
420
Kg of liquid evaporated per hour in an evaporator is defined as its
421
Pick out the correct statement.
422
Open pan evaporators are preferred to be used, when the solution to be concentrated is
423
Cp.μ/K is termed as the __________ number.
424
The maximum heat transfer co-efficient from steam heating will be attained when the steam is
425
Kirchoff's law is applicable to
426
In a forward feed multiple effect evaporator, the pressure is
427
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
428
Which of the following situations can be approximated to a steady state heat transfer system?
429
Sensible heat absorbed by 1 lb of water when it is heated from 32 to 212°F may be around __________ BTU.
430
The unit of conductance in SI unit is
431
In a shell and tube heat exchanger, square pitch compared to triangular pitch
432
Which of the following has maximum thermal conductivity?
433
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.
434
The film co-efficient between condensing vapour and metal wall increases with
435
Critical value of the __________ number governs the transition from laminar to turbulent flow in free convection heat transfer.
436
The thermal boundary layer at NPr > 1
437
In the free convection regime of pool boiling, the heat flux is proportional to
438
With increase in temperature, the thermal conductivity of fresh lubricating oil
439
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
440
Nusselt number is the ratio of the temperature gradient at the wall to
441
The Stefan-Boltzman constant depends on the
442
The units of resistance to heat transfer is
443
Thermal conductivity of a gas at low density, __________ with increase in temperature.
444
Nusselt number is a function of Prandtl number and __________ number of fluid in natural convection heat transfer.
445
__________ heat exchanger is used for chilling oil to be dewaxed.
446
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
447
Pick out the wrong statement.
448
In sub-cooled boiling,
449
With increase in temperature, the thermal conductivity of a gas
450
A __________ evaporator employs an annular downtake.
451
Radiation heat losses from satisfactorily insulated high pressure boilar may be about __________ percent.
452
In natural convection heat transfer, the correlating parameter is the
453
Boiling of milk in an open vessel is an example of __________ boiling.
454
Pick out the wrong statement.
455
Mode of heat transfer involved in the cooling of air cooled internal combustion engine is
456
Steady state one dimensional heat flow by conduction as given by Fourier's low does not assume that
457
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
458
In a shell and tube heat exchanger, the shell side fluid velocity can not be changed by changing the
459
To reduce the tube side pressure drop for the same flow rate, the heat exchanger recomended is
460
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
461
Steam side heat transfer co-efficient in an evaporator is in the range of __________ kcal/hr.m2°C.
462
Correction is applied to LMTD for __________ flow.
463
If h1 = inner film co-efficient and /h2 = outer film co-efficient, then the overall heat transfer co-efficient is
464
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
465
For __________ Prandtl number values, the heat conduction will be negligible in the buffer zone.
466
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
467
Heat exchangers operating, when the asymptotic range is reached,
468
In case of a shell and tube heat exchanger, the minimum and maximum baffle spacing is respectively (where, D = inside diameter of the shell)
469
In case of heat transfer by conduction in a hollow cylinder, __________ mean area is used to calculate the heat transfer rate.
470
In a gas-liquid shell and tube heat exchanger, the
471
Which is the most suitable for the concentration of foamy & frothy liquors?
472
Boiling point elevation of an ideal solution
473
The rate of heat transfer is a product of overall heat transfer co-efficient, the difference in temperature and the
474
Which of the following has the lowest overall heat transfer co-efficient?
475
Which of the following has the minimum absorptivity?
476
A black body when hot, emits heat radiation of __________ wavelengths.
477
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
478
Vibrations in the tubes of a shell and tube heat exchanger is induced due to the
479
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
480
The Nusselt number for fully developed (both thermally and hydrodynamically) laminar flow through a circular pipe whose surface temperature remains constant is
481
Sensible heat of hot industrial flue gases can not be recovered by a/an
482
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)
483
Radiator of an automobile engine is a __________ type of heat exchanger.
484
In pipe flow, heat is transferred from hot wall to the liquid by
485
The value of Stefan-Boltazman constant in SI unit is
486
The unit of heat transfer co-efficient is
487
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
488
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
489
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
490
Low thermal conductivity of heat insulating materials is due to its
491
The most conducive surface for dropwise condensation to occur is the __________ surface.
492
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
493
Calburn analogy is applicable for the value of Prandtl number from
494
Pick out the correct equation.
495
Multiple effect evaporators ar used to
496
Prandtl number is given by
497
Small scale evaporation is done in a
498
Heat exchanger tubes are never made of
499
At constant temperature, the thermal conductivities of gases __________ with rise in pressure.
500
Reason for operating an evaporator in multiple effect is to secure
501
In a cooling tower, water becomes cool by
502
In Fourier's law, the proportionality constant is called the
503
The heat transfer co-efficient in film type condensation is __________ that for dropwise condensation.
504
Pick out the wrong statement.
505
Duhring's plot' is of use in
506
Which has the lowest Prandtl number?
507
Pick out the wrong statement.
508
Prandtl number for most of dry gases is about
509
Prandtl number is the reciprocal of
510
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
511
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
512
Multiple effect evaporation accounts for
513
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
514
Maximum water velocity in tubes of a 1-2 shell and tube heat exchanger may be around __________ metres/second.
515
In Biot number, the characteristic length used is the ratio of the __________ of the solid.
516
Which of the following is the most widely used heat insulating material for pipelines carrying steam?
517
Heat transfer co-efficient equation for forced convection, Nu = 0.023 Re^0.8 . Pr^n, is not valid, if the value of
518
Which is the best tube arrangement (in a shell and tube heat exchanger) if the fluids are clean and non-fouling?
519
In case of a multipass shell and tube heat exchanger, the temperature drop in the fluid
520
The purpose of providing expansion bellows in the shell of tubular exchanger is to
521
A graph between __________ is called Wilson plot.
522
Grashhoff number is given by
523
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
524
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
525
Pick out the wrong statement.
526
What is the geometric mean of two heat transfer areas A1 and A2?
527
Evaporation by thermocompression results in the
528
Forced circulation evaporators are normally used for concentrating liquids having
529
Steam economy in case of a triple effect evaporator will be
