IBX - Heat Transfer Sec. 7 to 11 Flashcards
Intermittant tube cleaning is possible to be done in case of a __________ evaporator.
basket type
horizontal tube
calendria
none of these
basket type
Harmonic mean temperature difference is given by
sqrt(ΔT1.ΔT2)
(2.ΔT1.ΔT2)/(ΔT1+ΔT2)
(2.ΔT1.ΔT2)/(ΔT1-ΔT2)
(ΔT1-ΔT2)/(ΔT1.ΔT2)
B. (2.ΔT1.ΔT2)/(ΔT1+ΔT2)
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.
thick walled.
having the value of Ao/Ai > 2.
both (b) and (c).
thin walled having the value of Ao Ai/< 2.
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
cylinder
sphere
equal for all the three
cube
Overall heat transfer co-efficient for cooling of hydrocarbons by water is about
50 -100 Kcal/hr.m2.°C
50 -100 W/m2 . °K
50 -100 BTU/hr. ft.2°F
1000 - 1500 BTU/hr. ft.2°F
50 -100 BTU/hr. ft.2°F
Absorptivity of a perfect black body is unity. Which of the following has maximum absorptivity ?
aluminium foil
refractory bricks
iron plate
coke breeze
coke breeze
Thermal diffusivity is given by
k/ (ρ. Cp)
(ρ. Cp)/k
(Cp. µ)/a
µ/hCp
k/ (ρ. Cp)
At what value of Prandtl number, the hydrodynamic and thermal boundary layers of a fluid flowing over a heated plate will be identical?
1
< 1
> 1
none of these
1
Fourier’s law of heat conduction applies to __________ surfaces.
isothermal
non-isothermal
both (a) and (b)
neither (a) and (b)
both (a) and (b)
__________ chart is known as transient heat conduction chart.
Dirhing’s
Heisler’s
Mollier’s
Cox
Dirhing’s
Choose the most important factor on which the heat conducted through a wall in a unit time will depend on?
Thickness of the wall.
Area of the wall perpendicular to heat flow.
Material of the wall.
Temperature difference between the two surfaces of the wall.
Temperature difference between the two surfaces of the wall.
Which of the following has the minimum thermal conductivity ?
Nitrogen
Steel
Carbon black
Tar
Nitrogen
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.
The size of the exchanger in which heat transfer rate is 1 kcal/hr.
Both (a) and (b)
None of these
A section of the exchanger in which change in temperature of one stream equals the average driving force in the section.
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
hf > hd
hf < hd
hf could be greater or smaller than hd
hf < hd
Pick out the wrong statement:
The capacity of an evaporator is reduced by the boiling point elevation.
Corrosive liquid is normally passed through the tubes in a shell and tube heat exchanger.
Steam jet ejector is used for vapor compression in a thermal recompression evaporator.
Heat sensitive materials should be concentrated in high pressure evaporators.
Heat sensitive materials should be concentrated in high pressure evaporators.
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.
high pressure
vacuum
backward feed
none of these
vacuum
Circulation pump is located below the evaporater to
avoid cavitation.
avoid frequent priming.
create more suction head.
none of these.
create more suction head.
The statement that “maximum wavelength of radiation is inversly proportional to the temperature” is __________ law.
Stefan-Boltzman’s
Planck’s
Wien’s displacement
none of these
Wien’s displacement
Steam condensate is recovered by steam traps and recycled for use as boiler feed water, because of its low
hardness
dissolved soilds content
suspended solids content
all (a), (b) and(c)
all (a), (b) and(c)
Duhring rule is important in solving problems on
distillation
crystallisation
evaporation
humidification
evaporation
In a shell and tube heat exchanger, the height of 25 percent cut baffles is equal to (where, D = inside diameter of shell)
0.25 D
0.75 D
0.50 D
none of these
0.75 D
Multiple effect evaporation is generally recommended, when the
large scale evaporation of liquor is needed.
corrosive liquids are to be concentrated.
fuel is cheaply available.
evaporation on small scale is to be done.
large scale evaporation of liquor is needed.
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.
2^0.8
4^0.8
4^0.4
2^0.4
4^0.8
__________ heat exchanger is the most suitable, when the temperature of shell side fluid is much higher than that of tube side.
Single pass, fixed tube sheet
U-tube
Three pass, fixed tube sheet
none of these
U-tube
In which mode of heat transfer, the Biot number is important ?
Transient heat conduction
Natural convection
Forced convection
Radiation
Transient heat conduction
__________ paint has the minimum absorption co-efficient.
Black
White lead
Grey
Light cream
White lead
Asymptotic conditions is reached, when for a fluid flowing in laminar flow through a long tube
exit-fluid temperature > wall temperature.
exit fluid temperature < wall temperature.
exit fluid temperature = wall temperature.
Graetz number > 100.
exit fluid temperature = wall temperature.
1000 kg of wet solids are to be dried from 60% to 20% moisture (by weight). The mass of moisture removed in kg is
520
200
400
500
400
Which is the most suitable for the con-certration of highly concentrated solution?
Open pan evaporation
Long tube vertical evaporator
Agitated film evaporator
None of these
Open pan evaporation
Unsteady state heat conduction occurs, when
temperature distribution is independent of time.
temperature distribution is dependent on time.
heat flows in one direction only.
three dimensional heat flow is concerned.
temperature distribution is dependent on time
What is the absorptivity of a black body ?
1
0
0.78
0.95
1
Mode of heat transfer in which the fluid moves under the influence of changes in fluid pressure produced by external work is called
radiation
natural convection
forced convection
conduction
forced convection
Baffle spacing
is not the same as baffle pitch.
should be less than one fifth the diameter of the shell.
should be less than the inside diameter of the shell.
none of these.
should be less than the inside diameter of the shell.
Boiling point elevation of a solution of NaOH
increases rapidly with temperature rise.
is almost independent of temperature.
is almost independent of pressure.
both (b) and (c).
both (b) and (c).
The advantage of backward feed multiple effect evaporators over forward feed units is that
heat sensitive material can be handled.
there is no additional cost of pumping.
most concentrated liquid is at highest temperature.
equal heat transfer co-efficients exist in various effects.
most concentrated liquid is at highest temperature
Which of the following is the most controlling factor for the rate of bubble detachment from the hot solid surface ?
Liquid density
Liquid viscosity
Hot surface temperature
Interfacial tension
Interfacial tension
Which characteristic of a fluid is not important in deciding its route in a shell and tube heat exchanger ?
Corrosiveness
Fouling characteristic
Viscosity
None of these
None of these
Thermal conductivities of most of the liquids __________ with rise in temperature.
increases
decreases
remains unchanged
may increase or decrease; depends on the liquid
decreases
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
555
55.5
0.55
0.15
0.15
The purpose of floating head in a heat exchanger is to
avoid buckling of tubes.
provide support for tubes.
decrease the pressure drop.
facilitate its lengthening, if needed.
avoid buckling of tubes.
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
15 min
22 min
44 min
51 min
51 min
Overall thermal resistance for conductive heat transfer through a series of flat resistances is equal to the
maximum resistance in the series.
sum of all resistances.
average of all resistances.
minimum resistance presents in the series.
sum of all resistances.
Double pipe heat exchangers are preferably useful, when
high viscosity liquid is to be cooled.
requirement of heat transfer area is low.
overall heat transfer co-efficient is very high.
a corrosive liquid is to be heated.
requirement of heat transfer area is low.
Removal of __________ heat is involved in the condensation of a vapor under saturated conditions.
super
sensible
latent
both(b) & (c)
latent
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)^1/4
(ΔT)^1/2
(ΔT)^5/4
(ΔT)^3/4
(ΔT)^3/4
If air (a non-condensing gas) is present in a condensing vapor stream, it will __________ the condensation rate of vapor.
increase
decrease
not affect
increase the condensing film co-efficient as well as
decrease
Prandtl and Reynold’s analogies are identical for Prandtl number value of
0
0.5
1
5
1
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
its evaporation loss
excessive corrosion
uneconomic LMTD
decrease in heat exchanger efficiency
excessive corrosion
In evaporators, lowering the feed temperature
increases the heating area required.
reduces the economy.
both (a) and (b).
decreases the heating area required.
reduces the economy
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
density
thermal conductivity
corrosive nature
viscosity
In case of surface condensers, a straight line is obtained on plotting 1/U vs __________ on an ordinary graph paper.
1/ v̅^-0.8
v̅^-0.8
v̅^-2
1/ v̅^2
1/ v̅^-0.8
Thermal diffusivity is the most important in heat transfer by
conduction
radiation
condensation
natural convection
conduction
(NGr x NPr) is called the __________ number.
Graetz
Reyleigh
Nusselt
Stanton
Reyleigh
A dephlegmator is a
total condenser
vacuum evaporator
partial condenser
double pipe heat exchanger
partial condenser
A tank painted with which of the following coloured paints, would heat up maximum by radiation from sun?
Yellow paint
White paint
Black paint
Grey paint
Black paint
The separation of liquid droplets from the vapor is done by a/an __________ , in the evaporators.
steam ejector
entrainment separator
compressor
vacuum pump
entrainment separator
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)
U1 - U2
(1/U1)-(1/U2)
(1U2)-(1/U1)
U2 - U1
(1/U1)-(1/U2)
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 1.8 times
decrease by 2^2
increase by 2^16
remain unchanged
increase by 2^16
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
10 minutes
20 minutes
40 minutes
5 minutes
Fluid motion in the natural convection heat transfer between a solid surface and a fluid in contact with it, results from the
Existence of thermal boundary layer.
Temperature gradient produced due to density difference
Buoyancy of the bubbles produced at active nucleation site.
None of these.
None of these.
Evaporator tubes are generally
horizontal
vertical
inclined
random
vertical
For a counter current heat exchanger with Tih = 80°C, T°c = 60°C, T°h = 50°C and Tic = 30°C, and the temperature difference between the two streams being the same everywhere along Z, the direction of flow of hot fluid. The temperature profile should satisfy
(d^2T/dZ^2) > 0
(d^2T/dZ^2) = 0
(d^2T/dZ^2) < 0
dT/dZ = 0
(d^2T/dZ^2) = 0
In a boiling curve, the peak heat flux is called the __________ point.
Nusselt
Leidenfrost
boiling
burnout
burnout
Pick out the wrong statement.
Heat transfer from a hot body to cold body by the emission of heat waves is called radiation:
Filmwise condensation takes place on non-wettable surfaces.
The boiling point of a solution is affected by liquid head as well as boiling point elevation.
none of these.
Filmwise condensation takes place on non-wettable surfaces.
h.D/K is called the __________ number.
Nusselt
Peclet
Rayleigh
Grashoff
Nusselt
Prandtl number is the ratio of
mass diffusivity to thermal diffusivity.
momentum diffusivity to thermal diffusivity.
thermal diffusivity to mass diffusivity.
thermal diffusivity to momentum diffusivity.
momentum diffusivity to thermal diffusivity.
Conductance is given by(where, x = thickness, A = heat flow area, K = thermal conductivity.)
x/KA
KA/x
K/Ax
A/Kx
KA/x
Heat flux, as defined in heat flow is analogous to __________ in electricity flow.
current
voltage
resistance
none of these
current
The wavelength at which the maximum monochromatic emissive power occurs for a black body, is (where, T = absolute temperature of the black body)
αT
α 1/T
α T4
independent of T
α 1/T
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.
once
twice
thrice
four times
twice
When the ratio of the Grashoff number and to the square of Reynolds number is one, the dominant mechanism of heat transfer is
Free convection
Entry length problem in laminar forced conduction (developing thermal boundary layer)
Mixed convection (both free and forced)
Forced convection
Mixed convection (both free and forced)
The purpose of providing bleed points in the evaporator is to
admit the feed
remove the product
facilitate removal of non-condensable gases
create vacuum
facilitate removal of non-condensable gases
Water always boils when its
Temperature reaches 100° C.
Vapour pressure equals 76 cm of Hg.
Saturated vapour pressure equals the external pressure on its surface.
Saturated vapour pressure is less than the atmospheric pressure.
Saturated vapour pressure equals the external pressure on its surface.
A multiple effect evaporator as compared to a single effect evaporator of the same capacity has
lower heat transfer area.
lower steam economy.
higher steam economy.
higher solute concentration in the product.
higher steam economy.
Which is the most suitable for cold viscous feed?
Forward feed
Backward feed
Mixed feed
Parallel feed
Backward feed
Dropwise condensation is promoted on a/an __________ surface.
glazed
oily
coated
smooth
oily
At Pr > 1, conduction in an ordinary fluid flowing through a heated pipe is limited to the
buffer zone
turbulent core
both (a) and (b)
viscous sub-layer
viscous sub-layer
In a single evaporator system, the steam economy __________ by creating vacuum in the evaporator.
increases
decreases
remains constant
may increase or decrease, depends on the vacuum
increases
(Nre . NPr) (D/L) is called the ______ number
Peclet
Stanton
Graetz
none of these
Graetz
Nucleate boiling is promoted
on polished surfaces
on rough surfaces
in the absence of agitation
none of these
on rough surfaces
In forced circulation, the heating element is injected
internally
externally
both (a) and (a)
neither (a) nor (b)
internally
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
1
2
1/2
4
2
The equation, Nst = f/2, is the __________ analogy.
Colburn
Reynolds
Prandtl
none of these
Reynolds
Boiling point elevation for a strong and concentrated solution is found by Duhring’s rule, which states that at the same pressure, the boiling point of a solution is a linear function of the __________ of pure water.
boiling point
dynamic viscosity
kinematic viscosity
density
boiling point
Rate of crystallisation does not depend upon the
extent of supersaturation.
turbulence within the solution.
number and active surface area of the crystals.
none of these.
none of these.
Stefen’s block body radiation law can also be derived from __________ law.
Kirchoffs
Planck’s
Fourier’s
none of these
Planck’s
Multipass heat exchangers are used
because of simplicity of fabrication.
for low heat load.
to obtain higher heat transfer co-efficient and shorter tube.
to reduce the pressure drop.
to obtain higher heat transfer co-efficient and shorter tube.
A hollow sphere and a solid sphere of the same material and equal radii are heated to the same temperature. In this case,
The cooling rate will be the same for the two spheres and hence the two spheres will have equal temperatures at any instant.
Both the spheres will emit equal amount of radiation per unit time in the beginning.
Both will absorb equal amount of radiation from the surrounding in the beginning.
Both(b) & (c).
Both(b) & (c).
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
Higher condensing film co-efficient is desired.
Condensate is corrosive in nature.
Lower pressure drop through the exchanger is desired.
Temperature of the incoming vapor is very high.
Condensate is corrosive in nature.
When does the heat generated by fluid friction becomes appreciable compared to the heat transferred between the fluids ?
At high fluid velocity
At low velocity
When fluid flows past a smooth surface
None of these
At high fluid velocity
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)^-1.8
(D)^-0.2
(D)^0.2
(D)^1.8
(D)^-0.2
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 = 283 s
t = 356 s
t = 400 s
t = 800 s
t = 800 s
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.
parallel
mixed
counterflow
same in either ‘a’, b’ or ‘c’
same in either ‘a’, b’ or ‘c’
Thermal diffusivity of a material
has the unit m2 /sec.
is defined as K/ρ . Cp.
is the ratio of thermal conductivity to thermal capacity.
all (a), (b) and (c).
all (a), (b) and (c).
Viscosity of gases __________ with
increase in temperature.
increase very rapidly
increase slowly
decrease slowly
remain unaffected
increase slowly
What is the emissivity of a black body ?
1
0
0.90
0.5
1
Steam is routed through the tube in case of a __________ evaporator.
basket type
horizontal tube
short tube vertical
long tube vertical
horizontal tube
Dropwise condensation of steam on cooling surface is promoted
an oily surface.
when both the steam and the tube are clean.
only in presence of air.
none of these.
an oily surface
Conduction occurs in the buffer zone for a fluid flowing through a heated pipe, only when Prandtl number is
0.1
>1
< 1
l
0.1
