Heat Transfer Part 2 Flashcards
The Grashoff number is defined as the ratio of the
buoyancy to inertial forces.
buoyancy to viscous forces.
inertial to viscous forces.
buoyancy to surface tension forces.
buoyancy to viscous forces.
According to Reynolds analogy, Stanton number is equal to (where, f = Fanning friction factor)
2f
f
f/2
f/4
f/2
The absorptivity of a body is equal to its emissivity
at a particular temperature.
for circular bodies.
under thermal equilibrium.
none of these.
under thermal equilibrium.
Absorptivity and refletivity of a perfect black body are respectively
1 and 0
0 and 1
1 and infinity
0 and 0.5
1 and 0
In thermal radiation for a black body (where, E is emissivity and a is absorptivity)
a = 1; E ≠ 1
a ≠ 1, E = 1
a ≠ 1, E ≠ 1
a = 1; E = 1
a = 1; E = 1
Pick out the wrong statement.
Superheated steam is preferably not used for process heating because of its low heat transfer film co-efficient.
In a shell and tube heat exchanger, the shell pressure drop is maximum for orifice baffles.
S.I. unit of fouling factor is Watt/m2. K.
Longitudinal fins are used in extended surface heat exchangers, when the direction of fluid flow is parallel to the axis of the tube.
S.I. unit of fouling factor is Watt/m2. K.
When warm and cold liquids are mixed, the heat transfer is mainly by
conduction
convection
radiation
both(a) & (c)
convection
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?
Cube
Plate
Sphere
All will cool at the same rate
Sphere
Vent pipes are provided in a condenser to
remove non-condensable gases.
purge the condenser.
facilitate easy cleaning of tubes.
none of these.
remove non-condensable gases.
vent remove non-condensable
Heat transfer by natural convection is enhanced in system with
high viscosity.
high co-efficient of thermal expansion.
low temperature gradients.
low density change with temperature.
high co-efficient of thermal expansion.
In case of __________ boiling, the bubbles formed on a submerged hot surface get absorbed in the mass of the liquid.
nucleate
pool
low pressure
none of these
pool
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.
10
>10
<10
either (b) or (c), depends on the mass of the body
> 10
In forced convection, the Nusselt number is a function of
Re and Pr
Re and Gr
Pr and Gr
Re and Sc
Re and Pr
At what value of Prandtl number, the hydrodynamic and thermal boundary layers are identical?
0.5
1
1.5
80
1
A dilute aqueous solution is to be concentrated in an evaporator system. High pressure steam is available. Multiple effect evaporator system is employed, because
total heat transfer area of all the effects is -less than that in a single effect evaporator system.
total amount of vapor produced per Kg of feed steam in a multiple effect system is much higher than in a single effect.
boiling point elevation in a single effect system is much higher than that in any effect in a multieffect system.
heat transfer co-efficient in a single effect is much lower than that in any effect in a multieffect system.
total amount of vapor produced per Kg of feed steam in a multiple effect system is much higher than in a single effect.
Heat transfer efficiency leading of energy conservation in a heat exchanger can be achieved by
keeping the heat transfer surface clean.
enhancing the fluid pumping rate.
increasing the tube length.
none of these.
keeping the heat transfer surface clean.
Trap is used to remove __________ from steam pipe lines.
steam
condensate
non-condensables
none of these.
condensate
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
125
133
150
160
160
q = (TL-TR)/R1+R2.
q = (1200 - 400)/(XA/KA+XB /KB),
q= 800/(0.5 XB /2KB + XB /KB),
q= 800/ (1.25 XB /KB)—> (1)
q = T at A/( XA/KA)—> (2)
Sub eq. (1) in eq. (2) then;
800/ (1.25 XB /KB) =T at A/( XA/KA),
T at A = 160.
Tube pitch is the __________ of tube diameters and the clearances.
sum
difference
ratio
none of these
sum
With increase in the distance between the heat source and the object receiving the heat, the radiation heat transfer
decreases
increases
increases exponentially
remains unaffected
decreases
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.
more
less
same
unpredictable
less
Natural convection is characterised by
Grashoff number
Peclet number
Reynolds number
Prandtl number
Grashoff number
Nusselt number for full developed, laminar, constant property flow in a pipe at uniform heat flux is
0.72
4.364
18
83
4.364
Thickness of thermal boundary layer is more compared to that of hydrodynamic boundary layer, when the value of Prandtl number is
1
< 1
> 1
> 5
< 1
thermal hydrodynamic Prandtl <1
The critical radius of insulation for cylindrical pipe is (where, hi = heat transfer coefficient at inside of the pipe)
K/h0
2K/h0
hi/K
2hi/K
K/h0
critical radius Kh0
The heat flux in the nucleate boiling regimes is proportional to (where, delta(T) = excess temperature)
(ΔT)^2
(ΔT)^4
(ΔT)^3
sqrt(ΔT)
(ΔT)^3
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
infinity
0
equal to the difference between hot and cold fluids inlet temperature.
equal to the difference between hot fluid inlet temperature and cold fluid outlet temperature.
0
A body is called grey if the monochromatic emissivity of the body is
zero
unity
same for all wavelengths
different for all wavelengths
same for all wavelengths
In an extended surface heat exchanger, fluid having lower co-efficient
flows through the tube.
flows outside the tubes.
can flow either inside or outside the tubes.
should not be used as it gives very high pressure drop.
flows outside the tubes.
In a shell and tube heat exchanger, the tube side heat transfer co-efficient just at the entrance of the tube is
infinity.
zero.
same as average heat transfer co-efficient for tube side.
none of these.
infinity.
Grashoff number, which is defined as g . beta(Ts - T(infinity)) rho2. L3/u2, is proportional to the ratio of buoyancy force to __________ force.
viscous
elastic
inertial
none of these.
viscous
Pick out the wrong statement.
By increasing the number of shell passes, the temperature cross in a shell and tube heat exchanger can be prevented.
An underdesigned steam trap will back up the condensate instead of discharging it out.
Steam condensate is corrosive because of the presence of dissolved oxygen in it.
Film boiling is desirable in commercial equipments due to high heat transfer rate at low temperature drop.
Film boiling is desirable in commercial equipments due to high heat transfer rate at low temperature drop.
wrong - film boiling is desirable
Graetz number is given by
mCp/kL
kL/mCp
mCp/ku
kL/mCp
mCp/kL
For a cold dilute feed to produce thick viscous liquor, backward feeding as compared to forward feeding results in
increased economy
decreased economy
lower capacity
no effect on economy
increased economy
Economy of an evaporator is influenced by the
steam pressure
feed temperature
number of effect
both(b) & (c)
both(b) & (c)
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.
hyperbolic
parabolic
linear
logarithmic
linear
In regenerative air preheater (as practised in heating of coke ovens), the heat is transferred
through a metallic wall.
by direct contact of hot flue gas with air.
by heating an intermediate material (like chequor bricks) and then heating the air from this hot material.
none of these.
by heating an intermediate material (like chequor bricks) and then heating the air from this hot material.
regenerative air preheater intermediate material
__________ equation relates the thermal conductivity of a solid to its temperature.
Antoine
Kopp’s
Lee’s
Kistyakowski
Lee’s
For small temperature difference, the heat transfer rate as per Newton’s law of cooling is proportional to (where, delta(t) = excess temperature)
delta(t)
delta(t)^2
delta(t)^3
sqrt(delta(t))
delta(t)
Pick out the wrong statement.
The condensing film co-efficient is about 3 times lower for vertical condenser as compared to the equivalent horizontal condenser for identical situation.
Film co-efficient for vaporisation decreases as a result of vapor binding.
In industrial practice, sub-cooling of condensate is required, when the condensate is a volatile liquid and is to be transferred for storage.
Overall heat transfer co-efficient in a heat exchanger is controlled by the value of the film co-efficient, which is higher.
Overall heat transfer co-efficient in a heat exchanger is controlled by the value of the film co-efficient, which is higher.
wrong - controlled by the value of film-coefficient,higher
Viscous & heat sensitive liquids are concentrated in __________ evaporators.
open pan
long tube
agitated film
none of these
agitated film
1 BTU/hr.ft. F is equal to __________ kcal/hr. m. C.
1.49
1
4.88
none of these
1.49
Heat transfer by radiation between two bodies at T1 & T2 and in an ambient temperature of Ta C depends on
T1 - T2
T1 - Ta
T2 - Ta
none of these
none of these
The ratio of momentum diffusivity to thermal diffusivity is the __________ number.
Prandtl
Nusselt
Stanton
Grashoff
Prandtl
For a perfectly transparent surface (like gases), the
transmissivity = 1
reflectivity = 0
all (a), (b) & (c)
transmissivity = 1
TT1 transparent transmissivity 1
For a fluid flowing in an annulus space, the wetted perimeter for heat transfer and pressure drop are
same
different
never different
linearly related
different
During crystallisation, formation of crystal can occur in __________ solution only.
saturated
supersaturated
undersaturated
all (a), (b) and (c)
supersaturated
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
4000
9000
4600
3000
4000
An insulator should have
low thermal conductivity.
high thermal conductivity.
less resistance to heat flow.
a porous structure.
low thermal conductivity.
In a backward feed multiple effect evaporator
feed is introduced in the first effect.
feed flows from low pressure to high pressure.
no pumps are required between successive effects.
none of these.
feed flows from low pressure to high pressure.
Choose the correct equation.
Nu = (Re) (Pr) (Gz)
Nu = (Re) (Pr) (St)
Nu = (Re)(Pr)
Nu = (Pr)(St)
Nu = (Re) (Pr) (St)
In a forward feed multiple effect evaporator unit
viscosity of liquid is highest in first effect.
transfer from effect to effect is done by pumps.
no pump is required to withdraw the product from the last effect.
none of these.
none of these.
In case of heat flow by conduction for a cylindrical body with an internal heat source, the nature of temperature distribution is
linear
hyperbolic
parabolic
none of these.
parabolic
For heat flow through very thick walled cylinder, use____mean radius.
arithmetic
logarithmic
geometric
either (a) or (c)
logarithmic
Indirect contact heat exchangers are preferred over direct contact heat exchangers, because
heat transfer co-efficient are high.
there is no risk of contamination.
there is no mist formation.
cost of equipment is lower.
there is no risk of contamination.
Water is normally used as a coolant in the heat exchange equipments mainly because of its
abundance & high heat capacity
low density
low viscosity
high fluidity
abundance & high heat capacity
The rate of emission of radiation by a body does not depend upon the
wavelength of radiation.
surface temperature of the body.
nature of the surface.
shape and porosity of the body.
shape and porosity of the body.
Which of the following has maximum thermal conductivity at the same temperature?
Steel
Petrol
Air
All have the same conductivity
Steel
Maximum heat transfer rate is obtained in __________ flow.
laminar
turbulent
creeping
transition region
turbulent
In case of __________ boiling, the liquid temperature is below the saturation temperature and the boiling takes place in the vicinity of the heated surface.
nucleate
local
pool
saturated
local
The ratio of velocity head to tube side return loss in case of a multipass shell and tube heat exchanger is
2
1/2
4
1/4
1/4
Terminal point temperature differences between fluids in case of a heat exchanger is termed as
approach
log mean temperature difference.
arithmetic mean temperature difference.
geometric mean temperature difference.
approach
terminal approach
LMTD can’t be used as such without a correction factor for the
multipass heat exchanger.
baffled heat exchanger.
condensation of mixed vapour in a condenser.
all (a) (b) and (c).
all (a) (b) and (c).
Fqra cold viscous feed, backward feed gives __________ than forward feed.
a higher capacity
a lower capacity
lower economy
none of these
a higher capacity
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 ?
35.5
43.75
81.25
48.25
81.25
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
U2 > U1
U1 > U2
U1 = dirt factor - U2
U1 = U2
In SI units, thermal conductivity is expressed in
Watt/m. K
Watt/m2. K
Watt/m2. K
Watt/m4. K
Watt/m. K
Boiling point of a solution according to Duhring’s rule is a linear function of the __________ of water.
boiling point (at the same pressure)
viscosity
density
thermal conductivity
boiling point (at the same pressure)
The overall resistance for heat transfer through a series of flat resistance, is the __________ of the resistances.
average
geometric mean
product
sum
sum
At what value of Prandtl number, conduction is negligible in the turbulent core of a fluid flowing through a heated pipe?
0.5
< 0.5
> 0.6
< 0.1
> 0.6
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
0
R2/(R1+R2)
1
(R1/R2)^2
R2/(R1+R2)
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
1.26 h1
0.794 h1
0.574 h1
1.741 h1
0.794 h1
The main purpose of providing fins on heat transfer surface is to increase the
temperature gradient.
mechanical strength of the equipment.
heat transfer area.
heat transfer co-efficient.
heat transfer area.
A BTU/hr.ft.2 F is equal to
1 kcal/hr. m2 C.
4.88 kcal/hr. m.2 C.
1 kcal/hr. m2. K.
none of these.
4.88 kcal/hr. m.2 C.
Which of the following has the lowest Prandtl number?
Molten sodium (a coolant used in fast breeder reactor)
Water
Transformer oil
Dilute H2SO4
Molten sodium (a coolant used in fast breeder reactor)
lowest Pr molten Na
In SI units, fouling factor is expressed in
m2 K/W
W/m2 K
m2 K
m K/W
m2 K/W
An ejector is used to
increase pressure
increase temperature
remove condensate
none of these
none of these
ejector none
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
less corrosion problems.
flexibility possible in the baffle arrangement.
low pressure drop.
high heat transfer co-efficient.
flexibility possible in the baffle arrangement.
All analogy equations connecting friction factor and heat transfer co-efficient apply only to
wall or skin friction
form friction
both (a) and (b)
turbulent flow
wall or skin friction
Electro-magnetic spectrum range, which is important for radiation varies from __________ microns.
1 to 100
0.5 to 50
10 to 100
100 to 1000
0.5 to 50
The characteristic dimensionless groups for heat transfer to a fluid flowing through a pipe in laminar flow are
Re, Gz
Nu, Pr
Nu, Pr, Re
Nu, Gz
Nu, Gz
NuGz Laminar
Heat transfer rate described by Fourier’s law will decrease, if the __________ increases.
thermal conductivity
thickness
temperature difference
heat transfer area
thickness
A black body does not __________ radiation.
absorb or emit
refract
reflect
both(b) & (c)
both(b) & (c)
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.
evaporators operate under vacuum.
evaporators opreate at atmospheric pressure.
none of these.
solution has an elevation of boiling point.
Peclet number (Pe) is given by
Pe = Re.Pr
Pe = Re/Pr
Pe = Pr/Re
Pe = Nu.Re
Pe = Re.Pr
Nusselt number is the ratio of the
temperature gradient of the wall to that across the entire pipe.
temperature difference to the temperature gradient at the wall.
heat flux at the wall to that across the entire pipe.
none of these
heat flux at the wall to that across the entire pipe.
Prandtl number is the ratio of
momentum diffusivity to mass diffusivity.
momentum diffusivity to thermal diffusivity.
thermal diffusivity to mass diffusivity.
thermal diffusivity to momentum diffusivity.
momentum diffusivity to thermal diffusivity.
Which of the following parameters is increased by use of finned tube in a multipass shell and tube heat exchanger?
Tube side pressure drop and the heat transfer rate.
Convective heat transfer co-efficient.
Effective tube surface area for convective heat transfer.
All (a) (b) and (c).
All (a) (b) and (c).
Forced circulation evaporators are useful for the concentration of viscous, salting and scale forming liquors. Which of the following is a forced circulation evaporator?
Long vertical evaporator
Horizontal tube evaporator
Agitated film evaporator
Calenderia vertical tube evaporator
Agitated film evaporator
Forced circulation Agitated film
“The ratio of the total emissive power to the absorptivity for all bodies is same at ther-mal equilibrium”. This is __________ law.
Kirchoffs
Planck’s
Wien’s displacement
Stefan-Boltzman
Kirchoffs
total emissive power/absorptivity Kirchoffs
View factor is important in heat transfer by
steady state conduction
natural convection
forced convection
radiation
radiation
RVF Radiation View Factor
The advantage of using a 1 - 2 shell and tube heat exchanger over a 1 - 1 shell and tube heat exchanger is
lower tube side pressure drop.
lower shell side pressure drop.
higher tube side heat transfer co-efficient.
higher shell side heat transfer co-efficient.
higher tube side heat transfer co-efficient.
For a multipass shell and tube heat exchanger, the LMTD correction factor is always
1
> 1
< 1
between 1 & 2
< 1
If heat transfer rate varies with the time, it is termed as
forced convection
steady state conduction
monochromatic radiation
none of these
none of these
Pick out the wrong statement.
Fluid movement under the influence of buoyant forces resulting from change in density takes place in case of natural convection.
The ratio NNu/NRe . Npr is called the Stanton number.
The Peclet number is a measure of the ratio of energy transport by convection to that by conduction.
The Colbum jH factor for heat transfer is given by Nst Npr.
The Colbum jH factor for heat transfer is given by Nst Npr.
wrong Colbum jH factor
Tube expansion allowances exist in __________ heat exchanger.
multipass fixed tube sheet
U-tube
single pass fixed tube sheet
none of these
U-tube
U-tube expansion allowance
Pick out the wrong statement.
In drying a solid containing moisture above the critical moisture content the number of degrees of freedom is 2.
Sherwood number in mass transfer corresponds to Nusselt number in heat transfer and Schmidt number to Prandtl number.
Forced convection is relatively more effective in increasing the rate of mass transfer, if Schmidt number is larger.
Hot gases at moderate pressure are usually in the shell side of shell and tube heat exchangers. At higher pressure, however, it is customary to put gas in the tube side.
Forced convection is relatively more effective in increasing the rate of mass transfer, if Schmidt number is larger.
wrong forced convection more effective schimdt larger
Agitated film evaporator is suitable for concentrating __________ liquids.
foaming
viscous
very thin
corrosive
viscous
The thickness of condensate layer in filmwise condensation depends on the
condensation rate
surface configuration
liquid flow rate from the surface
all (a), (b) and (c)
all (a), (b) and (c)
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
hyperbolic
linear
logarithmic
parabolic
uniform surface temp parabolic
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
√ΔT1. ΔT2
(2.ΔT1.ΔT2)/(ΔT1 + ΔT2)
(2.ΔT1.ΔT2)/(ΔT1 - ΔT2)
(ΔT1 -ΔT2)/(ΔT1.ΔT2)
(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?
aluminum foil
refractory bricks
iron plate
coke breeze
coke breeze
Thermal diffusivity is given by
k/p.Cp
p.Cp/k
Cp.u/a
u/hCp
k/p.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
DT Dirhing’s Transient (heat conduction)
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)
At steady state the temperature variation in a plane wall, made of two different solids I & II is shown below :
image
The thermal conductivity of material I
is smaller than that of II.
is greater than that of II.
is equal to that of II.
can be greater than or smaller than that of II.
is smaller than that of II.
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
25% cut 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
BTHC Biot 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.
unsteady heat conduction 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 (delta(T)) between wall and outside bulk. The proportionality is given as
(delta(T))^1/4
(delta(T))^1/2
(delta(T))^5/4
(delta(T))^3/4
(delta(T))^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.
both (a) and (b).
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/ v^-0.8
v^-0.8
v^-2
1/ v^-2
1/ v^-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
A total condenser
vacuum evaporator
partial condenser
douple pipe heat exchanger
partial condenser
dpc dephlegmator 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
1/U2 - 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
single converted 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.
Fluid motion natural convection 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.
wrong - filmwise non-wettable
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)
∝ T
∝ 1/T
∝ T^4
independent of T
∝ 1/T
maximum monochromatic emissive power - ∝ 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
DO dropwise 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
PSBB Planck’s Stefen’s block body
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 ? 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
(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/? . 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
gases - viscosity temperature directly proportional; slow increase of viscosity
liquids - viscosity temperature inversely proportional
What is the emissivity of a black body?
1
0
0.9
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
1
0.1
conduction buffer Pr 0.1
