Heat Transfer (2) Flashcards
(235 cards)
1
Q
Black liquor generated during paper manufacture is concentrated in a
A
multiple effect evaporator.
2
Q
Reynold’s analogy states that
A
Nst α f
3
Q
When vaporisation takes place directly at the heating surface, it is called
A
nucleate boiling
4
Q
Shell side pressure drop in a shell and tube heat exchanger does not depend upon the
A
none of these.
5
Q
With increase in porosity, the thermal conductivity of a solid substance
A
decreases
6
Q
In Joule’s experiment, an insulated container contains 20 kg of water initially at 25°C. It is stirred by an agitator, which is made to turn by a slowly falling body weighing 40 kg through a height of 4 m. The process is repeated 500 times. The acceleration due to gravity is 9.8 ms-2. Neglecting the heat capacity of agitator, the temperature of water (in °C) is
A
34.4
7
Q
Fouling factor
A
accounts for additional resistances to heat flow.
8
Q
The steam ejector is used to
A
create vacuum.
9
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
10
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
11
Q
Presence of a non-condensing gas in a condensing vapour
A
none of these.
12
Q
In case of evaporators, liquid entrainment results primarily due to
A
foaming of the solution.
13
Q
Baffles in the shell side of a shell and tube heat exchanger
A
increase the shell side heat transfer co-efficient.
14
Q
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
A
finned tube heat exchanger with air outside and steam inside.
15
Q
Economy of a multiple effect evaporator is not influenced much by the
A
boiling point elevations
16
Q
Pick out the wrong statement.
A
The equivalent diameter of heat transfer for a duct of square cross-section (having each side as ‘x’) is equal to 4x.
17
Q
Controlling heat transfer film co-efficient is the one, which offers __________ resistance to heat transfer.
A
the largest
18
Q
Bulk of the convective heat transfer resistance from a hot tube surface to the fluid flowing in it, is
A
mainly confined to a thin film of fluid near the surface.
19
Q
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
A
less than that of the uninsulated steam pipe.
20
Q
The dimensionless group in mass transfer that is equivalent to Prandtl number in heat transfer is
A
Schmidt number.
21
Q
The absorptivity of a grey body at a given temperature __________ with increasing wavelength of radiation.
A
may increase or decrease ; depends on the material
22
Q
Dietus-Boelter equation used for the determination of heat transfer co-efficient is valid
A
for fluids in tubulent flow.
23
Q
Out of the following four assumptions used in the derivation of the equation for LMTD , which one is subject to the largest deviation in practice ?
A
Constant rate of fluid flow.
24
Q
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
A
radiation becomes important
25
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.
26
Minimum recommended baffle spacing in a shell and tube heat exchanger is about (where, D = shell diameter)
0.2 D
27
It is not preferable to use superheated steam in evaporators, because of its very
low film co-efficient
28
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).
29
The range of electromagnetic spectrum important in heat transfer by radiation is __________ microns.
0.5-50
30
Economy of a multiple effect evaporator depends upon the
heat balance consideration
31
The Graetz number is concerned with the
heat transfer in laminar flow.
32
Solid angle subtended by the finite surface at the radiating element is
called the angle of vision.
33
Pick out the wrong statement.
The emissivity of a surface decreases, if it gets corroded by atmospheric environment.
34
With increase in temperature, the thermal conductivity of most liquids
decreases
35
Heat transfer by __________ may not necessarily require the presence of a medium.
radiation
36
In a shell and tube type heat exchanger, the floating tube bundle heat arrangement is used
in high range of temperature differences.
37
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
38
An equipment which converts the latent or sensible heat of one fluid into the latent heat of vaporisation of another, is called a
heat exchanger
39
Planck's distribution law is valid for __________ bodies.
coloured
40
For hot feed, forward feeding as compared to backward feeding results in __________ economy.
increased
41
Baffles are provided in heat exchangers to increase the
heat transfer co-efficient
42
Which of the following forced convection heat transfer equation accounts for the liquid viscosity effect for viscous liquids ?
Sieder-Tate equation
43
Which of the following is not concerned with the heat transfer?
Schmidt number
44
LMTD correction factor which is to be applied for a cross-flow heat exchanger increases with increase in the number of shell passes. Its value for a single pass cross flow heat exchanger is
<1
45
Among liquids, water has a comparatively high value of thermal conductivity, due to its
partial ionisation
46
The film thickness for laminar film condensation on vertical surface __________ from top to bottom.
cummulatively increases
47
The local surface conductance for laminar film condensation on vertical surface is (where, t = film thickness)
∝ 1/t
48
Heat transfer co-efficient (h1) for liquids increases with
increasing temperature.
49
A diathermaneous substance __________ the thermal radiation completely.
transmits
50
Radiant energy received by a body is proportional to (where, d = the distance between the object emitting radiation and that receiving it.)
d>>2
51
Mechanical recompression evaporation is used in the production of
distilled water
52
A measure of the extent to which viscous heating is important relative to the heat flow resulting from the impressed temperature difference is represented by the __________ number.
Brinkman
53
The purpose of providing a 'catchall' in the vapor line of an evaporator is to
arrest the entrained liquid
54
A fluid is flowing inside the inner tube of a double pipe heat exchanger with diameter 'd'. For a fixed mass flow rate, the tube side heat transfer co-efficient for turbulent flow conditions is proportional to
d>>-0.2
55
Heat transfer occurs by natural convection because change in temperature causes difference in
density
56
A hot liquid is kept in a big room. The logorithm of the numerical value of the temperature difference between the liquid and the room is plotted against time. The plot will be very nearly a/an
straight line
57
Convective heat transfer, in which heat is transferred by movement of warmed matter is described by
Newton's law of cooling
58
Which of the following is correct ?
Driving force = Rate x Resistance
59
Log mean temperature difference (LMTD) cannot be used, if
none of these.
60
As the difference between the wall temperature and bulk temperature increases, the boiling heat transfer co-efficient
goes through a minimum.
61
In a forward feed multiple effect, the pressure build up will be
least at the outlet of the last effect.
62
For what value of Prandtl number, the Col-burn analogy is valid ?
0.6 to 120
63
In a multipass shell and tube heat exchanger, tube side return pressure loss is equal to __________ the velocity head.
four times
64
For a given ambient air temperature with increase in the thickness of insulation of a hot cylinderical pipe, the rate of heat loss from the surface would
first increase and then decrease
65
For the same heat transfer area and the terminal conditions, the ratio of the capacities of a single effect evaporator to a triple effect evaporator is
1
66
Dietus-Boelter equation cannot be used for molten metals mainly due to its very low
Prandtl number
67
Multiple effect evaporators are commonly used in the manufacture of
P. Paper Q. Superphosphate
R. Sugar S. Fats
P and S
68
Which of the following accessories is provided in the vapor line of an evaporator for removing the entrained liquid ?
Catchall
69
Which of the following is unimportant in forced convection ?
Grashhoff number
70
In a multiple effect evaporator, the effect of boiling point elevation is to
reduce the capacity
71
Dropwise condensation occurs on __________ surfaces.
contaminated cooling
72
For specified tube outside diameter, higher BWG means higher
cross-sectional area
73
Electromagnetic radiations propagate in vacuum with a velocity of __________ metre/second.
3 x l0^8
74
Film boiling occurs at __________ pressure.
very high
75
Pick out the wrong statement.
Heat transfer by radiation can not occur across an absolute volume.
76
The Grashoff number is defined as the ratio of the
buoyancy to viscous forces.
77
The absorptivity of a body is equal to its emissivity
under thermal equilibrium.
78
Pick out the wrong statement.
S.I. unit of fouling factor is Watt/m2.°K.
79
A cube, sphere & a thin circular plate (all having same mass and made of same material) are all heated to 300°C and allowed to cool in natural air. Which one will cool the slowest ?
Sphere
80
Vent pipes are provided in a condenser to
remove non-condensable gases.
81
In case of __________ boiling, the bubbles formed on a submerged hot surface get absorbed in the mass of the liquid.
pool
82
Trap is used to remove __________ from steam pipe lines.
condensate
83
Tube pitch is the __________ of tube diameters and the clearances.
sum
84
Natural convection is characterised by
Grashhoff number
85
Thickness of thermal boundary layer is more compared to that of hydrodynamic boundary layer, when the value of Prandtl number is
< 1
86
The critical radius of insulation for cylindrical pipe is (where, hi = heat transfer coefficient at inside of the pipe )
K/h0
87
The critical radius 'r' of insulation on a pipe is given by
r = k/h
88
The heat flux in the nucleate boiling regimes is proportional to (where, ΔT = excess temperature)
(ΔT)^3
89
A body is called grey if the monochromatic emissivity of the body is
same for all wavelengths
90
Grashoff number, which is defined as g . β(Ts - T∞) ρ2. L3/μ2, is proportional to the ratio of buoyancy force to __________ force.
viscous
91
If the thermal conductivity of a wall material is independent of temperature, the steady state temperature distribution in the very large thin plane wall having steady, uniform surface temperature follows __________ law.
linear
92
__________ equation relates the thermal conductivity of a solid to its temperature.
Lee's
93
For small temperature difference, the heat transfer rate as per Newton's law of cooling is proportional to (where, Δt = excess temperature)
Δt
94
Pick out the wrong statement.
Overall heat transfer co-efficient in a heat exchanger is controlled by the value of the film co-efficient, which is higher.
95
Heat transfer by radiation between two bodies at T1 & T2 and in an ambient temperature of Ta°C depends on
none of these
96
The ratio of momentum diffusivity to thermal diffusivity is the __________ number.
Prandtl
97
For a perfectly transparent surface (like gases), the
transmissivity = 1
98
For a fluid flowing in an annulus space, the wetted perimeter for heat transfer and pressure drop are
different
99
Choose the correct equation.
Nu = (Re) (Pr) (St)
100
In a forward feed multiple effect evaporator unit
none of these.
101
In case of __________ boiling, the liquid temperature is below the saturation temperature and the boiling takes place in the vicinity of the heated surface.
local
102
The ratio of velocity head to tube side return loss in case of a multipass shell and tube heat exchanger is
1/4
103
Terminal point temperature differences between fluids in case of a heat exchanger is termed as
approach
104
Overall heat transfer co-efficient of a particular tube is U1. If the same tube with some dirt deposited on either side has coefficient U2, then
U1 = U2
105
Boiling point of a solution according to Duhring's rule is a linear function of the __________ of water.
boiling point (at the same pressure)
106
In SI units, thermal conductivity is expressed in
Watt/m.°K
107
In SI units, fouling factor is expressed in
m2°K/W
108
An ejector is used to
none of these
109
Extremely large or small volumes of fluids are generally best routed through the shell side of a shell and tube heat exchanger, because of the
flexibility possible in the baffle arrangement.
110
All analogy equations connecting friction factor and heat transfer co-efficient apply only to
wall or skin friction
111
Electro-magnetic spectrum range, which is important for radiation varies from __________ microns.
0.5 to 50
112
The characteristic dimensionless groups for heat transfer to a fluid flowing through a pipe in laminar flow are
Nu, Gz
113
The capacity of double-effect evaporator is less than half of the capacity of two single effects, each of which is operating over same terminal temperature difference, when the
solution has an elevation of boiling point.
114
Peclet number (Pe) is given by
Pe = Re.Pr
115
Nusselt number is the ratio of the
heat flux at the wall to that across the entire pipe.
116
The advantage of using a 1 - 2 shell and tube heat exchanger over a 1 - 1 shell and tube heat exchanger is
higher tube side heat transfer co-efficient.
117
If heat transfer rate varies with the time, it is termed as
none of these
118
Pick out the wrong statement.
The Colbum jH factor for heat transfer is given by Nst Npr.
119
Pick out the wrong statement.
Forced convection is relatively more effective in increasing the rate of mass transfer, if Schmidt number is larger.
120
Agitated film evaporator is suitable for concentrating __________ liquids.
viscous
121
If the thermal conductivity of a wall material is independent of temperature, the steady state temperature distribution in the very large thin plane wall having steady, uniform surface tempeature follows __________ law.
parabolic
122
Intermittant tube cleaning is possible to be done in case of a __________ evaporator.
basket type
123
Arithmetic mean area can be used in heat transfer problem to calculate the heat flow by conduction through a cylinder which is
thin walled having the value of Ao Ai/< 2.
124
Three solid objects of the same material and of equal mass-a sphere, a cylinder (length = diameter) and a cube are at 500°C initially. These are dropped in a quenching bath containing a large volume of cooling oil each attaining the bath temperature eventually. The time required for 90% change in temperature is the smallest for
cube
125
Absorptivity of a perfect black body is unity. Which of the following has maximum absorptivity ?
coke breeze
126
__________ chart is known as transient heat conduction chart.
Dirhing's
127
Which of the following has the minimum thermal conductivity ?
Nitrogen
128
In a heat exchanger, one transfer unit means
a section of the exchanger in which change in temperature of one stream equals the average driving force in the section.
129
For condensation of pure vapors, if the heat transfer co-efficients in filmwise and drop-wise condensation are respectively hf and hd, then
hf < hd
130
Pick out the wrong statement:
Heat sensitive materials should be concentrated in high pressure evaporators.
131
For concentrating an aqueous solution of a material like anhydrous Na2SO4, whose solubility decreases with rise in temperature, the most suitable evaporator is a __________ evaporator.
vacuum
132
Circulation pump is located below the evaporater to
create more suction head.
133
The statement that "maximum wavelength of radiation is inversly proportional to the temperature" is __________ law.
Wien's displacement
134
In a shell and tube heat exchanger, the height of 25 percent cut baffles is equal to (where, D = inside diameter of shell)
0.75 D
135
__________ heat exchanger is the most suitable, when the temperature of shell side fluid is much higher than that of tube side.
U-tube
136
Asymptotic conditions is reached, when for a fluid flowing in laminar flow through a long tube
exit fluid temperature = wall temperature.
137
Which is the most suitable for the con-certration of highly concentrated solution?
Open pan evaporation
138
Unsteady state heat conduction occurs, when
temperature distribution is dependent on time.
139
Baffle spacing
should be less than the inside diameter of the shell.
140
The advantage of backward feed multiple effect evaporators over forward feed units is that
most concentrated liquid is at highest temperature.
141
Which characteristic of a fluid is not important in deciding its route in a shell and tube heat exchanger ?
None of these
142
Thermal conductivities of most of the liquids __________ with rise in temperature.
decreases
143
A metal ball of radius 0.1 m at a uniform temperature of 90°C is left in air at 30°C. The density and the specific heat of the metal are 3000 kg/m3 and 0.4 kJ/kg.K respectively. The heat transfer co-efficient is 50 W/m2.K Neglecting the temperature gradients inside the ball, the time taken (in hours) for the ball to cool to 60°C is
0.15
144
The purpose of floating head in a heat exchanger is to
avoid buckling of tubes.
145
1000 Kg of liquid at 30°C in a well stirred vessel has to be heated to 120°C, using immersed coils carrying condensing steam at 150°C. The area of the steam coils is 1.2 m2 and the overall heat transfer co-efficient to the liquid is 1500 W/m2.°C. Assuming negligible heat loss to the surrounding and specific heat capacity of the liquid to be 4 kJ/kg.°C, the time taken for the liquid to reach desired temperature will be
51 min
146
Removal of __________ heat is involved in the condensation of a vapor under saturated conditions.
latent
147
The rate of heat transfer from a vertical plate by natural convection depends upon the temperature differences (ΔT) between wall and outside bulk. The proportionality is given as
(ΔT)3/4
148
The outlet temperature of cooling water in a heat exchanger is generally not allowed to exceed above 50°C in industrial practice mainly to avoid
excessive corrosion
149
The type of liquor circulation system to be ' employed in evaporators (viz. natural or forced circulation) is determined mainly by the __________ of the liquid.
viscosity
150
Thermal diffusivity is the most important in heat transfer by
conduction
151
(NGr x NPr) is called the __________ number.
Reyleigh
152
A dephlegmator is a
partial condenser
153
A tank painted with which of the following coloured paints, would heat up maximum by radiation from sun ?
Black paint
154
The separation of liquid droplets from the vapor is done by a/an __________ , in the evaporators.
entrainment separator
155
If a single tube pass heat exchanger is converted to two pass, then for the same flow rate, the pressure drop per unit length in tube side will
increase by 2^16
156
A steel sphere of radius 0.1 m at 400°K is immersed in an oil at 300°K. If the centre of the sphere reaches 350°K in 20 minutes, how long will it take for a 0.05 m radius steel sphere to reach the same temperature (at the centre) under identical conditions ? Assume that the conductive heat transfer co-efficient is infinitely large.
5 minutes
157
Fluid motion in the natural convection heat transfer between a solid surface and a fluid in contact with it, results from the
none of these.
158
Evaporator tubes are generally
vertical
159
In a boiling curve, the peak heat flux is called the __________ point.
burnout
160
The wavelength at which the maximum monochromatic emissive power occurs for a black body, is (where, T = absolute temperature of the black body)
α 1/T
161
When the ratio of the Grashoff number and to the square of Reynolds number is one, the dominant mechanism of heat transfer is
mixed convection (both free and forced)
162
Water always boils when its
saturated vapour pressure equals the external pressure on its surface.
163
Dropwise condensation is promoted on a/an __________ surface.
oily
164
At Pr > 1, conduction in an ordinary fluid flowing through a heated pipe is limited to the
viscous sub-layer
165
In a single evaporator system, the steam economy __________ by creating vacuum in the evaporator.
increases
166
Nucleate boiling is promoted
on rough surfaces
167
In forced circulation, the heating element is injected
internally
168
If all the conditions and dimensions are same, then the ratio of velocity through the tubes of a double pass heat exchanger to that through the single pass heat exchanger is
2
169
Stefen's block body radiation law can also be derived from __________ law.
Planck's
170
Condensing film co-efficient for steam on horizontal tubes ranges from 5000 to 15000 Kcal/hr.m2 .°C. Condensation of vapor is carried out inside the tube in a shell and tube heat exchanger, when the
condensate is corrosive in nature.
171
The Sieder-Tate correlation for heat transfer in turbulent flow in pipe gives Nu α Re0.8, where, Nu is the Nusselt number and Re is the Reynolds number for the flow. Assuming that this relation is valid, the heat transfer co-efficient varies with the pipe diameter (D) as
(D))^-0.2
172
A long iron rod initially at a temperature of 20°C has one end dipped in boiling water (100°C) at time, t = 0. The curved surface of the rod is insulated so that heat conduction is one dimensional in the axial direction. The temperature at a distance 100 mm from the dipped end becomes 40°C at time, t = 200 s. The same temperature is achieved at a distance of 200 mm from the dipped end at time
t = 800 s
173
Viscosity of gases __________ with increase in temperature.
increase slowly
174
Steam is routed through the tube in case of a __________ evaporator.
horizontal tube
175
Conduction occurs in the buffer zone for a fluid flowing through a heated pipe, only when Prandtl number is
0.1
176
Tubes are held between top and bottom tube sheets in Calenderia type evaporator by keeping
both the tube sheets fixed.
177
A concentric double pipe heat exchanger as compared to the shell and tube heat exchanger for the same heat load requires
more space.
178
In a shell and tube heat exchanger, floating head is used for
large temperature differentials.
179
Steam trap is used to
remove water resulting from partial condensation of steam.
180
The average heat transfer co-efficient over the entire length of the plate (ha) and the local heat transfer co-efficient (hL), in case of heat transfer over a flat plate in lminar zone is related as
ha = 2hL
181
Maximum heat transfer rate is achieved in __________ flow.
turbulent
182
Three material A, B and C of equal thick-nes and of thermal conductivity of 20, 40 & 60 kcal/hr. m. °C respectively are joined together. The temperature outside of A and C are 30°C and 100°C respectively. The interface between B and C will be at a temperature of __________ °C.
70
183
Which of the following parameters of the fluid is not very important, while deciding its route in a shell and tube heat exchanger?
density
184
jH factor for heat transfer depends upon the __________ number.
Reynolds
185
Kg of liquid evaporated per hour in an evaporator is defined as its
capacity
186
Open pan evaporators are preferred to be used, when the solution to be concentrated is
highly viscous
187
In a co-current double pipe heat exchanger used for condensing saturated steam over the inner tube, if the entrance and exit conditions of the coolant are interchanged, then the rate of condensation will
remain unchanged
188
Which of the following situations can be approximated to a steady state heat transfer system ?
10 kg of dry saturated steam at 8 kgf/cm2 flowing through a short length of stainless steel pipe exposed to atmospheric air at 35°C.
189
The film co-efficient between condensing vapour and metal wall increases with
increasing temperature of the vapour.
190
The thermal boundary layer at NPr > 1
is thinner than hydrodynamic boundary layer.
191
In the free convection regime of pool boiling, the heat flux is proportional to
Δt^5/4
192
With increase in temperature, the thermal conductivity of fresh lubricating oil
decreases
193
Thermal conductivity of a gas at low density, __________ with increase in temperature.
increases
194
Nusselt number is a function of Prandtl number and __________ number of fluid in natural convection heat transfer.
Grashoff
195
__________ heat exchanger is used for chilling oil to be dewaxed.
Double pipe
196
Pick out the wrong statement.
Two identical cubes of iron and copper will have the same heat content under the same conditions of temperature.
197
In sub-cooled boiling,
temperature of the heating surface is more than the boiling point of the liquid.
198
In natural convection heat transfer, the correlating parameter is the
Grashoff number
199
Pick out the wrong statement.
The maximum is the emissive power of a surface at a temperature T1 occurs at a wavelength of λ1. If the surface temperature is halved, the maximum in the emissive power would occur at a wavelength of 0.5 λ1.
200
Steady state one dimensional heat flow by conduction as given by Fourier's low does not assume that
material is anisotropic.
201
In a shell and tube heat exchanger, the shell side fluid velocity can not be changed by changing the
tube diameter
202
To reduce the tube side pressure drop for the same flow rate, the heat exchanger recomended is
1-1 heat exchanger
203
Steam side heat transfer co-efficient in an evaporator is in the range of __________ kcal/hr.m2°C.
5000-15000
204
Correction is applied to LMTD for __________ flow.
cross
205
For __________ Prandtl number values, the heat conduction will be negligible in the buffer zone.
high
206
Heat exchangers operating, when the asymptotic range is reached,
results in making part of the heating surface inactive.
207
Which is the most suitable for the concentration of foamy & frothy liquors ?
Long tube vertical evaporator
208
Boiling point elevation of an ideal solution
increases rapidly with temperature rise.
209
Which of the following has the lowest overall heat transfer co-efficient ?
Air
210
A black body when hot, emits heat radiation of __________ wavelengths.
all
211
A composite wall consists of two plates A and B placed in series normal to the flow of heat. The thermal conductivities are kA and kB and the specific heat capacities are CPA and CPB for plates A and B respectively. Plate B has twice the thickness of plate A. At steady state, the temperature difference across plate A is greater than that across plate B, when
kA < 0.5kB
212
Heat transfer by conduction in the turbulent core of a fluid flowing through a heated pipe is negligible, if the value of Prandtl number is
0.6
213
The Nusselt number for fully developed (both thermally and hydrodynamically) laminar flow through a circular pipe whose surface temperature remains constant is
3.66
214
Sensible heat of hot industrial flue gases can not be recovered by a/an
none of these
215
The critical radius of insulation for a spherical shell is (where, K = thermal conductivity of insulating material h0 = heat transfer coefficient at the outer surface)
2K/h0
216
Radiator of an automobile engine is a __________ type of heat exchanger.
direct contact
217
For turbulent flow in a tube, the heat transfer co-efficient is obtained from the Dittus-Boelter correlation. If the tube diameter is halved and the flow rate is doubled, then the heat transfer co-efficient will change by a factor of
6.1
218
In an interphase heat transfer process, the equilibrium state corresponds to equality of temperature in the two phases, while the condition for equilibrium in an interphase mass transfer process is equality of
concentrations
219
Calburn analogy is applicable for the value of Prandtl number from
0.6 to 120
220
Multiple effect evaporators ar used to
increase the steam economy & the capacity.
221
Small scale evaporation is done in a
steam jacketed kettle
222
Heat exchanger tubes are never made of
lead
223
At constant temperature, the thermal conductivities of gases __________ with rise in pressure.
decrease
224
In a cooling tower, water becomes cool by
loosing sensible heat.
225
Pick out the wrong statement.
Thermal conductivity of a dry material is more than that of the damp material.
226
Which has the lowest Prandtl number ?
Liquid metal
227
Prandtl number for most of dry gases is about
0.72
228
For a liquid in laminar flow through a very long tube, when the exit fluid temperature approaches the wall temperature, the equation to be used is
Nu = 2/pi Gz
229
In a heat exchanger with steam outside the tubes, a liquid gets heated to 45°C, when its flow velocity in the tubes is 2 m/s. If the flow velocity is reduced to 1 m/s, other things remaining the same, the temperature of the exit liquid will be
more than 45°C
230
Multiple effect evaporation accounts for
steam economy
231
Maximum water velocity in tubes of a 1-2 shell and tube heat exchanger may be around __________ metres/second.
1
232
In case of a multipass shell and tube heat exchanger, the temperature drop in the fluid
and the wall are proportional to individual resistances.
233
The overall heat transfer co-efficient for a shell and tube heat exchanger for clean surfaces is U0 = 400 W/m2.K. The fouling factor after one year of operation is found to be hd0 = 2000 W/m2.K. The overall heat transfer co-efficient at this time is
333W/m2.K
234
In a laboratory test run, the rate of drying was found to be 0.5 x 10-3 kg/m2.s, when the moisture content reduced from 0.4 to 0.1 on dry basis. The critical moisture content of the material is 0.08 on a dry basis. A tray dryer is used to dry 100 kg (dry basis) of the same material under identical conditions. The surface area of the material is 0.04 m2/kg of dry solid. The time required (in seconds) to reduce the moisture content of the solids from 0.3 to 0.2 (dry basis) is
5000
235
Pick out the wrong statement.
Heat transfer co-efficient during nucleate boiling is not influenced by the agitation imparted.
