HEAT TRANSFER INDIABIX Flashcards
Crystal size in a continuous crystalliser depends upon the
rate of heat transfer.
degree of turbulence.
degree of supersaturation.
all (a), (b) and (c).
all (a), (b) and (c).
Pick out the wrong statement.
A Bubble size increases with the dynamic viscosity of the liquid in case of nucleate pool boiling.
B Thermal conductivity of a dry material is more than that of the damp material. C Ratio of its capacity to economy equals the steam consumption in kg/hr in an evaporator.
D Vaporisation of organic substances in evaporator mostly causes foam formation.
B
A backward feed multiple effect evaporator is better than forward feed for concentrating cold feed, because it provides
A higher economy
B lower capacity
C both (a) & (b)
D lower economy
A
Which of the following is concerned with both heat and mass transfer?
A Lewis relationship
B Nusselt number
C Kutateladze number
D Froude number
A
Black liquor generated during paper manufacture is concentrated in
a single effect evaporator.
single effect evaporator followed by a crystalliser.
multiple effect evaporator.
multiple effect evaporators followed by a crystalliser.
multiple effect evaporator.
When vaporisation takes place directly at the heating surface, it is called ———
film boiling
nucleate boiling
vapour binding
none of these
nucleate boiling
In the equation Q = UAΔt; Δt is ——-
geometric mean temperature difference.
arithmetic mean temperature difference.
logarithmic mean temperature difference.
the difference of average bulk temperatures of hot and cold fluids.
logarithmic mean temperature difference.
For an ideal black body——
absorptivity = 1
reflectivity = 1
emissivity = 0 transmissivity = 1
absorptivity = 1
What is the unit of thermal conductivity?
Kcal/hr. m2 °C
Kcal/hr.m.°C
Kcal/hr.m
Kcal/hr. °C
Kcal/hr.m.°C
Economy of a multiple effect evaporator depends upon the ————
heat balance consideration rate of heat transfer
both (a) and (b)
neither (a) nor (b)
heat balance consideration
In case of parallel flow heat exchanger, the lowest temperature theoretically attainable by the hot fluid is __________ the outlet temperature of the cold fluid.
equal to
more than
less than
either more or less than (depending upon the fluid)
equal to
Planck’s distribution law is valid for __________ bodies. black
white
coloured
all (a), (b) & (c)
coloured
An equipment which converts the latent or sensible heat of one fluid into the latent heat of vaporisation of another, is called a
boiler
heat exchanger recuperator
regenerator
heat exchanger
Finned tube heat exchangers ————
give larger area per tube.
use metal fins of low thermal conductivity.
facilitate very large temperature drop through tube wall.
are used for smaller heat load.
give larger area per tube.
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 ———
convection becomes important
conduction becomes important
radiation becomes important
sub-cooled boiling occurs
radiation becomes important
Temperature profile in steady state heat transfer is —–
asymptotic
hyperbolic
parabolic
linear
linear
In a shell and tube heat exchanger, ———
the temperature drops in the two fluids and the wall are proportional to individual resistances.
the temperature drop is inversely proportional to the resistance across which the drop occurs.
there is no relationship between temperature drop and resistance.
the relationship is not generalised.
the temperature drops in the two fluids and the wall are proportional to individual resistances.
What is the steam economy in case of a single effect evaporator system?
1 >1 <1 0.1
<1
Value of Prandtl number for water ranges from
1 to 2 5 to 10 100 to 500 1000 to 2000
5 to 10
Crystallisation of solids from a homogeneous solution is a/an __________ process. exothermic
mildly endothermic
highly endothermic
none of these
exothermic
A __________ surface has the maximum thermal emissivity out of the following.
black & smooth
black & rough
white & smooth
white & rough
black & rough
Mechanical recompression evaporation is used in the production of
alcohol
distilled water
salt
fruits jam
distilled water
Baffles are provided in heat exchangers to increase the
fouling factor
heat transfer area
heat transfer co-efficient heat transfer rate
heat transfer co-efficient
Heat transfer by __________ may not necessarily require the presence of a medium.
conduction
natural convection
forced convection
radiation
Radiation
Which one gives the monochromatic emissive power for black body radiation?
Planck’s law
Kirchhoffs law
Wien’s law Stefan-Boltzman law
Planck’s law
Dietus-Boelter equation used for the determination of heat transfer co-efficient is valid - - - -
for fluids in laminar flow.
for fluids in tubulent flow.
when Grashhoff number is very important.
for liquid metals.
for fluids in tubulent flow.
The range of electromagnetic spectrum important in heat transfer by radiation is __________ microns.
0.38-0.78
0.5-50
100-1000
5-50
0.5-50
A 2-4 heat exchanger involves ——
only counter-flow of fluids.
only parallel-flow of fluids.
both counter and parallel-flow of the fluids.
smaller pressure drop compared to 1-2 exchanger.
both counter and parallel-flow of the fluids.
Which of the following forced convection heat transfer equation accounts for the liquid viscosity effect for viscous liquids? ——
Dittus-Boeltier equation Sieder-Tate equation
Nusselt equation
None of these
Sieder-Tate equation
Heat transfer in the laminar sublayer in case of a liquid flowing through a pipe, is mostly by —————-
eddies current
conduction
convection
none of these
conduction
Reynold’s analogy states that (where, St = Stanton number f = friction factor)
St = f/2
St = f/4
St = 4f
St = f^1/2
St = f/2
What is the thermal conductivity of a perfect heat insulator?
zero
one
∞
between 0 and ∞
zero
Fouling factor’ used in the design of a multipass shell and tube heat exchanger is a
non-dimensional factor. factor of safety.
conversion factor for individual film heat transfer co-efficient to overall heat transfer co-efficient.
none of these.
factor of safety
Prandtl and Reynolds analogy are same, when Prandtl number is
0.5
1
> 2
1.5
1
Heat transfer occurs by natural convection because change in temperature causes difference in
viscosity
density
thermal conductivity
heat capacity
density
The ratio of kinematic viscosity to thermal diffusivity is called the __________ number.
Peclet
Prandtl
Stanton
Nusselt
Prandtl
If average heat transfer co-efficient is ha and the local coefficient at the end of the plate is hl then in case of heat transfer to a fluid flowing over a flat plate, heated over its entire length
ha = hl
ha = 2hl
ha = 0.5 hl
ha = 0.75 hl
ha = 2hl
jH factor for heat transfer is not a function of the __________ number.
Reynolds
Nusselt
Grashoff
both (b) & (c)
both (b) & (c)
The Fourier number (defined as a.t/L2) is used in the analysis of problem involving heat transfer by
forced convection
natural convection transient conduction
steady state conduction
transient conduction
The energy radiated from a surface Q at absolute temperature T is related as
Q ∝ T^2
Q ∝ T^4
Q ∝ T^3
none of these
Q ∝ T^4
In case of a vertical tube evaporator, with increase in the liquor level, the
capacity of the evaporator is decreased.
capacity of the evaporator is increased.
the temperature drop increases.
both (b) and (c).
capacity of the evaporator is decreased.
The ratio of total radiating power to the absorptivity of the body depends upon the __________ as per Kirchoffs law.
wavelength of radiation
nature of the body temperature of the body none of these
temperature of the body
For gases, the thermal conductivity increases with temperature rise. For liquids, with increase in concentration, its thermal conductivity generally
decreases
increases
remains unchanged increases exponentially
decreases
In a liquid-liquid heat exchanger, for the same process temperature, the ratio of the LMTD in parallel flow to the LMTD in counter flow is always
< 1
> 1
1
∞
< 1
With increase in temperature, the thermal conductivity of steel
increases
decreases
remains unchanged
increases exponentially
decreases
The main function of baffles provided in a shell and tube heat exchanger is to facilitate the cleaning of
outer tube surface. enhance turbulence.
hold the tubes in position.
all ‘a’, ‘b’ & ‘c’.
enhance turbulence.
In a single effect evaporator, the economy is
1
< 1
> 1
none of these
< 1
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
0
1
>1
<1
<1
Which of the following is not concerned with the heat transfer?
Brinkman number Stanton number
Schmidt number
Peclet number
Schmidt number
Among liquids, water has a comparatively high value of thermal conductivity, due to its
low density
high viscosity
partial ionisation
dense structure
partial ionisation
__________ paint has the maximum absorption coefficient.
Black
Yellow
White
Grey
Black
The critical radius ‘r’ of insulation on a pipe is given by
r = 2k/h
r = k/h
r = k/2h
r = h/k
r = k/h
In a multipass shell and tube heat exchanger, tube side return pressure loss is equal to __________ the velocity head.
twice
four times
square root of
square of
four times
Which of the following is correct?
Rate = Driving force x Resistance
Driving force = Rate x Resistance
Resistance = Driving force x Rate
Rate = Resistance/Driving force
Driving force = Rate x Resistance
As the difference between the wall temperature and bulk temperature increases, the boiling heat transfer co-efficient
continues to increase.
continues to decrease.
goes through a minimum.
goes through a maximum.
goes through a minimum.
Heat transfer rate per unit area is called thermal conductivity
heat flux
heat transfer
co-efficient
thermal diffusivity
heat flux
For a given ambient air temperature with increase in the thickness of insulation of a hot cylindrical pipe, the rate of heat loss from the surface would
decrease
increase
first decrease and then increase
first increase and then decrease
first increase and then decrease
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
minimum
maximum
zero
infinity
zero
For what value of Prandtl number, the Col-burn analogy is valid?
0.06 to 120
0.6 to 120
1 to 103
1 to 50
0.6 to 120
In a forward feed multiple effect, the pressure build up will be
least at the inlet of the first effect.
least at the outlet of the last effect.
highest at the inlet of the last effect.
highest at the outlet of the last effect.
least at the outlet of the last effect.
Film boiling is usually not desired in commercial equipments, because
the heat transfer rate is low in view of the large temperature drop.
it is difficult to maintain.
it is not economic.
none of these.
the heat transfer rate is low in view of the large temperature drop.
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
3
0.33
1
1.33
1
The variation of thermal conductivity of a metal with temperature is often correlated using an expression of the form K = K0 + at. where, K is the thermal conductivity and T is the temperature (in °K). The units of ‘a’ in SI system will be
W/m.k
W/m
W/m.k^2
none, ‘a’ is just a number
W/m.k^2
Film condensation is promoted on a/an __________ surface.
oily
coated
clean & smooth
dirty
clean & smooth
Dietus-Boelter equation cannot be used for molten metals mainly due to its very low
Prandtl number
Grashoff number
thermal conductivity viscosity
Prandtl number
Which of the following accessories is provided in the vapor line of an evaporator for removing the entrained liquid?
Bleed point
Vent
Catchall
Baffle
Catchall
If Prandtl number is greater than the Schmidt number, then the
thermal boundary layer lies inside the concentration boundary layer.
concentration boundary layer lies inside the thermal boundary layer.
thermal & concentration boundary layers are of equal thickness.
hydrodynamic(i.e., momentum)boundary layer is thicker than the other two.
thermal boundary layer lies inside the concentration boundary layer.
“Multiple effect evaporators are commonly used in the manufacture of
P. Paper
Q. Superphosphate
R. Sugar
S. Fats”
P and Q
P and R
P and S
R and S
P and S
Which of the following has the highest thermal conductivity?
Brick
Air
Water
Silver
Silver
Which of the following is unimportant in forced convection?
Reynolds number
Prandtl number
Grashoff number
none of these
Grashoff number
