ht Flashcards
Crystal size in a continuous crystalliser depends upon the
a. rate of heat transfer.
b. degree of turbulence.
c. degree of supersaturation.
d. all (a), (b) and (c).
all a b c
Air is best heated with steam in a heat exchanger of
a. plate type.
b. double pipe type with fin on steam side.
c. double pipe type with fin on air side.
d. shell and tube type.
double pipe with fin on AIR side
Which of the following is generally considered as opaque surface towards
radiations?
a. Gases
b. Solids
c. Liquids
d. Both (b) and (c)
both b and c
Black liquor generated during paper manufacture is concentrated in a
a. single effect evaporator.
b. single effect evaporator followed by a crystalliser.
c. multiple effect evaporator.
d. multiple effect evaporators followed by a crystalliser.
multiple effect evap only
When vaporisation takes place directly at the heating surface, it is called
a. film boiling
b. nucleate boiling
c. vapour binding
d. none of these
nucleate
. Steam consumption in kg/hr in case of an evaporator is given by
(where, C & E are capacity the economy of the evaporator respectively)
a. C/E
b. E/C
c. CE
C/E
Reynold’s analogy states that
a. Nst α f
b. Nst α NRe
c. NNu α f
d. NRe α f
a. Nst α f
With increase in temperature, the thermal conductivity of non-metallic
amorphous solids
a. decreases
b. increases
c. remains constant
d. first decreases upto certain temperature and then increases
INCREASES
Fourier’s law applies to the heat transfer by
a. convection
b. radiation
c. conduction
d. all (a), (b) & (c)
CONDUCTION
Shell side pressure drop in a shell and tube heat exchanger does not
depend upon the
a. baffle spacing & shell diameter.
b. tube diameter & pitch.
c. viscosity, density & mass velocity of shell side fluid.
d. none of these.
NONE OF THESE
In the equation Q = UAΔt; Δt is
a. geometric mean temperature difference.
b. arithmetic mean temperature difference.
c. logarithmic mean temperature difference.
d. the difference of average bulk temperatures of hot and cold fluids.
logarithmic mean temperature difference
With increase in porosity, the thermal conductivity of a solid substance
DECREASES
For an ideal black body
a. absorptivity = 1
b. reflectivity = 1
c. emissivity = 0
d. transmissivity = 1
absorptivity = 1
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. 40.5
b. 34.4
c. 26.8
d. 25
34.4
Fouling factor
a. is a dimensionless quantity.
b. does not provide a safety factor for design.
c. accounts for additional resistances to heat flow.
d. none of these.
accounts for additional resistances to heat flow.
In case of vertical tube evaporator, with increase in liquor level, the overall
heat transfer co-efficient
a. increases
b. decreases
c. is not affected
d. may increase or decrease; depends on the feed
DECREASES
he steam ejector is used to
a. remove condensate from the steam pipelines.
b. create vacuum.
c. superheat the steam.
d. none of these
CREATE VACUUM
For shell and tube heat exchanger, with increasing heat transfer area, the
purchased cost per unit heat transfer area
a. increases
b. decreases
c. remains constant
d. passes through a maxima
PASSES THROUGH A MAXIMA
The thermal efficiency of a reversible heat engine operating between two
given thermal reservoirs is 0.4. The device is used either as a refrigerator or as
a heat pump between the same reservoirs. Then the coefficient of
performance as a refrigerator (COP)R and the co-efficient of performance as a
heat pump (COP)HP are
a. (COP)R = (COP)HP = 0.6
b. (COP)R = 2.5; (COP)HP = 1.5
c. (COP)R = 1.5; (COP)HP = 2.5
d. (COP)R = (COP)HP = 2.5
(COP)R = 1.5; (COP)HP = 2.5
Hot water (0.01 m3 /min) enters the tube side of a counter current shell and
tube heat exchanger at 80°C and leaves at 50°C. Cold oil (0.05 m3/min) of
density 800 kg/m3 and specific heat of 2 kJ/kg.K enters at 20°C. The log mean
temperature difference in °C is approximately
a. 32
b. 37
c. 45
d. 50
32
In a heat exchanger, floating head is provided to
a. facilitate cleaning of the exchanger.
b. increase the heat transfer area.
c. relieve stresses caused by thermal expansion.
d. increase log mean temperature gradient.
relieve stresses caused by thermal expansion.
Leidenfrost point is a term concerned with the
a. condensation of the saturated vapor on a cold surface.
b. concentration of a corrosive solution by evaporation.
c. heat transfer between two highly viscous liquids.
d. boiling of a liquid on a hot surface.
boiling of a liquid on a hot surface.
A process stream of dilute aqueous solution flowing at the rate of10 Kg.s
1 is to be heated. Steam condensate at 95°C is available for heating purpose,
also at a rate of 10 Kg.s-1. A 1 - 1 shell and tube heat exchanger is available.
The best arrangement is
a. counter flow with process stream on shell side.
b. counter flow with process stream on tube side.
c. parallel flow with process stream on shell side.
d. parallel flow with process stream on tube side.
counter flow with process stream on shell side.
Heat produced when a steady state current, I passes through an electrical
conductor having resistance, ‘R’ is
a. IR
b. I2R
c. IR2
d. I2R2
I2R
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.
HIGHLY TURBULENT
The actual temperature drop across the heating surface in an evaporator
depends on the
a. feed.
b. depth of liquid over heating surface.
c. pressure difference between steam chest and vapour space.
d. all (a), (b) and (c).
ALL A B C
Which of the following is not used as a medium for high temperature
heating ?
a. Dowtherm
b. mercury
c. liquid metal (e.g. molten sodium)
d. fused salts (e.g., an eutectic mixture of 53% KNO3, 40%
NaNO2and 7% NaNO3)
MERCURY
Presence of a non-condensing gas in a condensing vapour
a. increases the rate of condensation.
b. decreases thermal resistance.
c. is desirable to increase the film co-efficient.
d. none of these.
NONE OF THESE
With the increase of temperature, the Col-burn jH factor
a. increases.
b. decreases.
c. remains unchanged.
d. may increase or decrease ; depending on temperature.
INCREASES
In case of evaporators, liquid entrainment results primarily due to
a. high vacuum in the evaporator.
b. high evaporation rate.
c. foaming of the solution.
d. high heat transfer rate.
FOAMING OF THE SOLUTION
Baffles in the shell side of a shell and tube heat exchanger
a. increase the cross-section of the shell side liquid.
b. force the liquid to flow parallel to the bank.
c. increase the shell side heat transfer co-efficient.
d. decrease the shell side heat transfer co-efficient.
INCREASE THE SHELL SIDE HEAT TRANSFER COEFFICIENT
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 inside and steam outside.
b. finned tube heat exchanger with air outside and steam inside.
c. shell and tube heat exchanger with air inside tubes and steam on
shell side.
d. shell and tube heat exchanger with air on shell side and steam
inside tubes.
finned tube heat exchanger with air OUTSIDE and steam inside.
The unit of heat transfer co-efficient in SI unit is
a. J/M2°K
b. W/m2°K
c. W/m°K
d. J/m°K
W/m2°K
The number of kg vaporised per kg of steam fed to the evaporator is
defined as
ECONOMY
Pick out the wrong statement.
a. In case of heat transfer by purely forced convection, GR/Re 2 ≤ 1.
b. The equivalent diameter of heat transfer for a duct of square cross
section (having each side as ‘x’) is equal to 4x.
c. Distillation process is not the same as evaporation.
d. The effectiveness of nucleate boiling depends basically on the
ease with which the bubbles are formed and detached from the
heating surface
The equivalent diameter of heat transfer for a duct of square cross
section (having each side as ‘x’) is equal to 4x.
Economy of a multiple effect evaporator is not influenced much by the
a. boiling point elevations
b. temperature of the feed
c. rate of heat transfer
d. ratio of the weight of the thin liquor to thick liquor
BOILING POINT ELEVATIONS
ncreasing the liquor level in the evaporator results in the
a. decreased capacity.
b. increase in liquor film co-efficient.
c. decreased effect of hydrostatic head.
d. increased true temperature drop.
DECREASED CAPACITY
The equivalent diameter for pressure drop is __________ that for heat
transfer.
a. smaller than
b. greater than
c. equal to
d. not related with
GREATER THAN
Controlling heat transfer film co-efficient is the one, which offers
__________ resistance to heat transfer.
a. no
b. the least
c. the largest
THE LARGEST
he heat flux (from outside to inside) across an insulating wall with thermal
conductivity, K= 0.04 W/m.°K and thickness 0.16m is 10 W/m2. The
temperature of the inside wall is - 5°C. The outside wall temperature is
a. 25°C
b. 30°C
c. 35°C
d. 40°C
35
Pick out the wrong statement.
a. The controlling resistance in case of heating of air by condensing
steam is in the air film.
b. The log mean temperature difference (LMTD) for counter flow and
parallel flow can be theoretically same when any one of the fluids
(hot or cold fluid) passes through the heat exchanger at constant
temperature.
c. In case of a 1 - 2 shell and tube heat exchanger, the LMTD
correction factor value increases sharply, when a temperature
cross occurs.
d. Phase change in case of a pure fluid at a given pressure from
liquid to vapor or vice-versa occurs at saturation temperature.
n case of a 1 - 2 shell and tube heat exchanger, the LMTD
correction factor value increases sharply, when a temperature
cross occurs.
The average heat transfer co-efficient for laminar film condensation on
vertical surface is inversely proportional to (where, ΔT = Temperature drop
across condensate film )
a. (ΔT)2
b. ΔT
c. (ΔT)1/4
d. (ΔT)3/
T1/4
Bulk of the convective heat transfer resistance from a hot tube surface to
the fluid flowing in it, is
a. in the central core of the fluid.
b. uniformly distributed throughout the fluid.
c. mainly confined to a thin film of fluid near the surface.
d. none of these.
MAINLY CONFINED TO A THIN FILM OF FLUID NEAR THE SURFACE
Which of the following is concerned with both heat and mass transfer ?
a. Lewis relationship
b. Nusselt number
c. Kutateladze number
d. Froude number
LEWIS RELATIONSHIP
A backward feed multiple effect evaporator is better than forward feed for
concentrating cold feed, because it provides
HIGHER ECONOMY
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. greater than that for uninsulated steam pipe.
b. less than that of the uninsulated steam pipe.
c. equal to that of the uninsulated steam pipe.
d. less than the steam pipe with 5 cms insulation.
Less than that of the uninsulated steam pipe.
Which area is used in case of heat flow by conduction through a cylinder ?
a. Logarithmic mean area
b. Arithmetic mean area
c. Geometric mean area
d. None of these
LOGARITMIC MEAN AREA
The dimensionless group in mass transfer that is equivalent to Prandtl
number in heat transfer is
a. Nusselt number.
b. Sherwood number.
c. Schmidt number.
d. Stanton number.
SCHMIDT
Finned tube heat exchangers
a. give larger area per tube.
b. use metal fins of low thermal conductivity.
c. facilitate very large temperature drop through tube wall.
d. are used for smaller heat load.
GIVE LARGER AREA PER TUBE
The absorptivity of a grey body at a given temperature __________ with
increasing wavelength of radiation.
a. increases
b. decreases
c. remains constant
d. may increase or decrease ; depends on the material
may increase or decrease ; depends on the material
Which one gives the monochromatic emissive power for black body
radiation ?
PLANCKS LAW
For what value of Prandtl number, St = f/2?
1
In a shell and tube heat exchanger,
a. the temperature drops in the two fluids and the wall are
proportional to individual resistances.
b. the temperature drop is inversely proportional to the resistance
across which the drop occurs.
c. there is no relationship between temperature drop and resistance.
d. the relationship is not generalised.
the temperature drops in the two fluids and the wall are
proportional to individual resistances
Dietus-Boelter equation used for the determination of heat transfer co
efficient is valid
a. for fluids in laminar flow.
b. for fluids in tubulent flow.
c. when Grashhoff number is very important.
d. for liquid metals.
fluids in TURBULENT FLOW
For large heat transfer area requirement, shell and tube heat exchanger is
preferred, because it
a. occupies smaller space.
b. is more economical.
c. is easy to operate and maintain.
d. all (a), (b) and (c).
ALL A B C
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 overall heat transfer co-efficient.
b. Constant rate of fluid flow.
c. Constant specific heat.
d. No partial phase change in the system.
Constant rate of fluid flow.
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. convection becomes important
b. conduction becomes important
c. radiation becomes important
d. sub-cooled boiling occurs
radiation becomes important
Resistance to heat flow by conduction is proportional to (where, t & ρ are
thickness & density of the material respectively and A = area normal to the
direction of heat flow. )
a. t
b. 1/ρ
c. 1/A
d. all (a), (b) & (c)
ALL A B C
Temperature profile in steady state heat transfer is
a. asymptotic
b. hyperbolic
c. parabolic
d. linear
LINEAR
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
a. decreases the pressure drop.
b. decreases the outlet temperature of cooling medium.
c. increases the overall heat transfer coefficient.
d. none of these.
INCREASES THE OVERALL HEAT TRANSFER COEFFICIENT
When vaporisation takes place through a blanketting film of gas, the
phenomenon is termed as __________ boiling.
a. pool
b. nucleate
c. transition
d. film
FILM
Minimum recommended baffle spacing in a shell and tube heat exchanger
is about (where, D = shell diameter)
a. 0.2 D
b. 0.5 D
c. 0.66 D
d. 0.80 D
0.2D
Stefan-Boltzman law which describes the radiation heat transfer states
that, it is proportional to (where, t = temperature in °C T = absolute
temperature in ° K )
a. t4
b. T4
c. 1/t4
d. 1/T4
T4
The Nusselt number for fully developed (both thermally and
hydrodynamically) laminar flow through a circular pipe, where the wall heat flux
is constant, is
a. 2.36
b. 4.36
c. 120.36
d. dependent on NRe only
4,36
What is the unit of thermal conductivity ?
a. Kcal/hr. m2 °C
b. Kcal/hr.m.°C
c. Kcal/hr.m
d. Kcal/hr. °C
Kcal/hr.m.°C
The thermal radiation emitted by a body is proportional to Tn, where T is its
absolute temperature. The value of ‘n’ is exactly 4 for
a. black painted bodies only
b. all bodies
c. polished bodies only
d. a black body
ALL BODIES
It is not preferable to use superheated steam in evaporators, because of
its very
a. high temperature
b. high pressure
c. low film co-efficient
d. none of these
LOW FILM COEFFICIENT
It is desired to concentrate a 20% salt solution (20 kg of salt in 100 kg of
solution) to a 30% salt solution in an evaporator. Consider a feed of 300
kg/min at 30°C. The boiling point of the solution is 110°C, the latent heat of
vaporisation is 2100 kJ/kg and the specific heat of the solution is 4 kJ/kg.K.
The rate at which the heat has to be supplied in (kJ/min) to the evaporator is
a. 3.06 x l05
b. 6.12 x 105
c. 7.24 x 105
d. 9.08 x 105
3.06 x l05
LMTD for counterflow and prallel flow heat exchanger will be the same,
when the
a. cold fluid is heated to a certain temperature by condensing steam
(isothermal fluid).
b. outlent temperature of both the hot and cold fluid are same.
c. outlet temperature of hot fluid is less than the outlet temperature of
the cold fluid.
d. none of these.
cold fluid is heated to a certain temperature by condensing steam
(isothermal fluid).
For flow over a flat plate, the ratio of thermal boundary layer thickness, ‘xt’
and hydrodynamic boundary layer thickness ‘x’ is equal to (where, NPr =
Prandtl number)
a. NPr
b. NPr 1/3
c. NPr-1
d. NPr-1/3
NPr 1/3
The range of electromagnetic spectrum important in heat transfer by
radiation is __________ microns.
a. 0.38-0.78
b. 0.5-50
c. 100-1000
d. 5-50
0.5-50
Use of transverse baffles in a shell and tube heat exchanger is done to
increase the
a. rate of heat transfer.
b. flow velocity.
c. turbulence of shell side fluid.
d. all (a), (b) and (c).
ALL A B C
The actual temperature drop across the heating surface of an evaporator
depends on the
a. liquid depth over the heating surface.
b. solution being evaporated.
c. pressure difference between the steam chest and the vapor space
above the boiling liquid.
d. all (a), (b) and (c)
ALL A B C
Economy of a multiple effect evaporator depends upon the
a. heat balance consideration
b. rate of heat transfer
c. both (a) and (b)
d. neither (a) nor (b)
HEAT BALANCE CONSIDERATION
he Graetz number is concerned with the
a. mass transfer between a gas and a liquid.
b. absorption with chemical reaction.
c. heat transfer in turbulent flow.
d. heat transfer in laminar flow.
IN LAMINAR FLOW
Solid angle subtended by the finite surface at the radiating element is called
angle of vision
Pick out the wrong statement.
a. The emissivity of a surface decreases, if it gets corroded by
atmospheric environment.
b. The emissivity of a surface increases with increase in surface
roughness.
c. The emissivity of a polished surface is quite low.
d. The emissivity of a non-metallic surface decreases with increase in
the temperature.
the emissivity of a surface decreases, if it gets corroded by
atmospheric environment.
In forced convection, the heat transfer depends on
a. Re, Pr
b. Re, Gr
c. mainly Gr
d. Re only
Re, Pr
With increase in temperature, the thermal conductivity of most liquids
decreases
Heat transfer by __________ may not necessarily require the presence of
a medium
radiation
Pick out the correct statement.
a. Higher is the temperature of the radiating body, higher is the
wavelength of radiation.
b. Logarithmic mean area is used for calculating the heat flow rate
through a thick walled cylinder.
c. The wavelength corresponding to maximum mono-chromatic
emissive power increases with rise in temperature.
d. Solid angle subtended by the finite surface at the radiating element
is called the angle of incidence.
Logarithmic mean area is used for calculating the heat flow rate
through a thick walled cylinder.
In a shell and tube type heat exchanger, the floating tube bundle heat arrangement is used
in low range of temperature differences.
in high range of temperature differences.
because of its low cost.
to prevent corrosion of the tube bundles.
in high range of temperature differences.
Stefan-Boltzman law applies to __________ body.
black
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
a. more
b. less
c. some
d. either more or less ; depends on the nature of vapor
some
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
An equipment which converts the latent or sensible heat of one fluid into
the latent heat of vaporisation of another, is called a
a. boiler
b. heat exchanger
c. recuperator
d. regenerator
heat exchanger
Which of the following is directly concerned with the heat transfer ?
a. Strouhal number
b. Sherwood number
c. Euler number
d. Grashoff number
grashof
Pick out the wrong statement:
a. With change in temperature, the radiant energy emitted by a black
body remains unchanged.
b. Absorptivity of a body approaches unity in case of diffuse
reflection.
c. Absorptivity of a perfectly black body is unity.
d. Value of Stefan-Boltzman constant is 4.876 x 10-8 KCal/m2.hr.°K4.
With change in temperature, the radiant energy emitted by a black
body remains unchanged.
Planck’s distribution law is valid for __________ bodies.
a. black
b. white
c. coloured
d. all (a), (b) & (c)
COLORED
For hot feed, forward feeding as compared to backward feeding results in
__________ economy.
a. increased
b. decreased
c. no effect on
d. none of these
INCREASED
Radiation energy is emitted by all the substances, which are above
0K
What is the steam economy in case of a single effect evaporator system ?
< 1
- 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
b. ha = 2hl
c. ha = 0.5 hl
d. ha = 0.75 hl
ha = 2hl
- The Fourier number (defined as a.t/L2) is used in the analysis of problem involving
heat transfer by
a. forced convection
b. natural convection
c. transient conduction
d. steady state conduction
transient conduction
the energy radiated from a surface Q at absolute temperature T is related as
a. Q ∝ T2
b. Q ∝ T4
c. Q ∝ T3
d. none of these
b. Q ∝ T4
Baffles are provided in heat exchangers to increase the
a. fouling factor
b. heat transfer area
c. heat transfer co-effient
c. heat transfer co-effient
7.200 kg of solids (on dry basis) is subjected to a drying process for a period of 5000
seconds. The drying occurs in the constant rate period with the drying rate as, Nc = 0.5
x 10-3 kg/m2.s. The initial moisture content of the solid is 0.2 kg moisture/kg dry solid.
The interfacial area available for drying is 4 m2/1000 kg of dry solid. The moisture
content at the end of the drying period is (in kg moisture/kg dry solid)
a. 0.5
b. 0.05
c. 0.1
d. 0.15
0.1
the ratio of total radiating power to the absorptivity of the body depends upon the
__________ as per Kirchoffs law.
a. wavelength of radiation
b. nature of the body
c. temperature of the body
d. 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
a. decreases
b. increases
c. remains unchanged
d. increases exponentially
decreases
A 2-4 heat exchanger involves
a. only counter-flow of fluids.
b. only parallel-flow of fluids.
c. both counter and parallel-flow of the fluids.
d. smaller pressure drop compared to 1-2 exchanger.
both counter and parallel-flow of the fluids.
- 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
With increase in temperature, the thermal conductivity of steel
a. increases
b. decreases
c. remains unchanged
d. increases exponentially
decreases
Which of the following forced convection heat transfer equation accounts for the
liquid viscosity effect for viscous liquids ?
a. Dittus-Boeltier equation
b. Sieder-Tate equation
c. Nusselt equation
a. None of these
Sieder-Tate
The main function of baffles provided in a shell and tube heat exchanger is to
a. facilitate the cleaning of outer tube surface.
b. enhance turbulence.
c. hold the tubes in position.
d. all ‘a’, ‘b’ & ‘c’.
enhance turbulence
In a single effect evaporator, the economy is
a. 1
b. < 1
c. >1
d. 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
a. 0
b. 1
c. >1
d. <1
<1
Which of the following is not concerned with the heat transfer?
a. Brinkman number
b. Stanton number
c. Schmidt number
d. Peclet number
schmidt
Latent heat absorbed by 1 lb of water at 212°F, when it is changed to steam at 212°F,
may be around __________ BTU.
a. 180
b. 970
c. 3.97
d. none of these
180
the thermal radiative flux from a surface of emissivity = 0.4 is 22.68 kW/m2. The
approximate surface temperature (K) is
(Stefan-Boltzman constant = 5.67xl0-8 W/m2.K4)
a. 1000
b. 727
c. 800
d. 1200
1000
Crystallisation of solids from a homogeneous solution is a/an __________ process.
exothermic
A multiple effect evaporator has a capacity to process 4000 kg of solid caustic soda
per day, when it is concentrating from 10% to 25% solids. The water evaporated in kg
per day is
a. 6000
b. 24000
c. 60000
d. 48000
24000
Value of Prandtl number for water ranges from
a. l to 2
b. 5 to 10
c. 100 to 500
d. 1000 to 2000
5 to 10
Heat transfer in the laminar sublayer in case of a liquid flowing through a pipe, is
mostly by
a. eddies current
b. conduction
c. convection
d. none of thes
conduction
Among liquids, water has a comparatively high value of thermal conductivity, due to
its
a. low density
b. high viscosity
c. partial ionisation
d. dense structure
partial ionization
For evaporation of viscous solution in a multiple effect evaporator, the prefered
feeding scheme is
a. forward
b. backward
c. parallel
d. none of these
backward
The sum of reflectivity and absorptivity for an opaque body is equal to
a. 0.5
b. 1
c. 0
d. 2
1
The film thickness for laminar film condensation on vertical surface __________ from
top to bottom.
a. cummulatively increases
b. cummulatively decreases
c. remains constant
d. and the surface conductance increase
cumulatively increases
In case of a multipass shell and tube heat exchanger, providing a baffle on the shell
side __________ the heat transfer rate.
a. increases
b. decreases
c. does not affect
d. may increase or decrease, depends on the type of baffle
increases
The local surface conductance for laminar film condensation on vertical surface is
(where, t = film thickness)
a. ∝ t
b. ∝ 1/t
c. ∝ t
d. independent of ‘t’
∝ 1/t
Heat transfer co-efficient (h1) for liquids increases with
a. increasing temperature.
b. decreasing temperature.
c. decreasing Reynolds number.
d. none of these.
increasing temperature
- A diathermaneous substance __________ the thermal radiation completely.
a. absorbs
b. reflects
c. transmits
d. none of these
transmits
A hot body will radiate heat most rapidly, if its surface is
black and rough
Evaporation of 1kg of water from a solution in a single effect evaporator requires about __________ kg of steam.
a. 0.4 - 0.6
b. 1-1.3
c. 1.8-2
d. 2 - 2.4
1-1.3
A metal wire of 0.01 m dia and thermal conductivity 200 W/m.K is exposed to a fluid
stream with a convective heat transfer coefficient of 100 W/m2.K. The Biot number is
a. 5.6
b. 0.025
c. 3.5
d. 0.0035
0.025
- A __________ surface has the maximum thermal emissivity out of the following.
a. black & smooth
b. black & rough
c. white & smooth
d. white & rough
black and rough
Radiant energy received by a body is proportional to (where, d = the distance between
the object emitting radiation and that receiving it.)
a. (d)^0.5
b. d
c. d2
d. d1.5
d2
Mechanical recompression evaporation is used in the production of
a. alcohol
b. distilled water
c. salt
d. fruits jam
distilled water
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 __________
brinkman number
Out of 100 kcal/second of incident radiant energy on the surface of a thermally
transparent body, 300 kcal/second is reflected back. If the transmissivity of the body is
0.25, the emissivity of the surface will be
a. 0.35
b. 0.45
c. 0.55
d. 0.85
0.45
- The purpose of providing a ‘catchall’ in the vapor line of an evaporator is to
a. create vacuum
b. regulate the vapor flow
c. vent the non-condensible gases
d. arrest the entrained liquid
arrest the entrained liquid
- What is the thermal conductivity of a perfect heat insulator ?
0
- ‘Fouling factor’ used in the design of a multipass shell and tube heat exchanger is a
a. non-dimensional factor.
b. factor of safety.
c. conversion factor for individual film heat transfer co-efficient to overall heat transfer
co-efficient.
d. none of these.
factor of safety
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
a. d0.8
b. d-0.2
c. d-1
d. d-1.8
b. d-0.2
Prandtl and Reynolds analogy are same, when Prandtl number is
a. 0.5
b. 1
c. >2
d. 1.5
1
Heat transfer occurs by natural convection because change in temperature causes
difference in
a. viscosity
b. density
c. thermal conductivity
d. heat capacity
density
The ratio of kinematic viscosity to thermal diffusivity is called the __________
number.
a. Peclet
b. Prandtl
c. Stanton
d. Nusselt
prandtl
- The critical radius ‘r’ of insulation on a pipe is given by
r = k/h
- If the baffle spacing in a shell and tube heat exchanger increases, then the Reynolds
number of the shell side fluid
decreases
The heat transfer by radiation from a mild steel surface is to be reduced by reducing
the emissivity of the surface. This can be best achieved by
a. painting the surface black.
b. painting the surface white (with aluminium paint).
c. giving the surface a mirror finish.
d. roughening the surface.
painting the surface white (with aluminium paint).
______ paint has the maximum absorption coefficient.
a. Black
b. Yellow
c. White
d. Grey
black
Steam traps are provided in steam carrying pipelines to
a. condense steam.
b. release excess steam pressure by bleeding steam.
c. remove condensate and inert gases.
d. none of thes
. remove condensate and inert gases.
.
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
a. ellipse
b. straight line
c. parabola
d. circular arc
straight line
Radiation heat transfer rates does not depend upon the
a. type of absorbing surface.
b. distance between the heat source and the object receiving the heat.
c. surface area and temperature of the heat source.
d. none of these.
none of these
Which of the following is correct ?
a. Rate = Driving force x Resistance
b. Driving force = Rate x Resistance
c. Resistance = Driving force x Rate
d. Rate = Resistance/Driving force
Driving force = Rate x Resistance
convective heat transfer, in which heat is transferred by movement of warmed matter
is described by
a. Fourier’s law
b. Newton’s law of cooling
c. Fick’s law
d. none of these
Newton’s law of cooling
As the difference between the wall temperature and bulk temperature increases, the
boiling heat transfer co-efficient
a. continues to increase.
b. continues to decrease.
c. goes through a minimum.
d. goes through a maximum.
goes through a minimum
- Heat transfer rate per unit area is called
a. thermal conductivity
b. heat flux
c. heat transfer co-efficient
d. thermal diffusivity
heat flux
Log mean temperature difference (LMTD) cannot be used, if
a. heat transfer co-efficient over the entire heat exchanger is not constant.
b. there exists an unsteady state.
c. the heat capacity is not constant and there is a phase change.
d. none of these.
none of these
- In a multipass shell and tube heat exchanger, tube side return pressure loss is equal to
__________ the velocity head.
a. twice
b. four times
c. square root of
d. square of
four times
- 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
a. decrease
b. increase
c. first decrease and then increase
d. first increase and then decrease
first increase 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
a. minimum
b. maximum
c. zero
d. infinity
zero
- For what value of Prandtl number, the Col-burn analogy is valid ?
a. 0.06 to 120
b. 0.6 to 120
c. 1 to 103
d. l to 50
0.6 to 120
- In a forward feed multiple effect, the pressure build up will be
a. least at the inlet of the first effect.
b. least at the outlet of the last effect.
c. highest at the inlet of the last effect.
d. 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
a. the heat transfer rate is low in view of the large temperature drop.
b. it is difficult to maintain.
c. it is not economic.
d. 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
a. 3
b. 0.33
c. 1
d. 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
a. W/m.k
b. W/m
c. W/m.k2
d. none, ‘a’ is just a number
W/mk2
- Film condensation is promoted on a/an __________ surface.
a. oily
b. coated
c. clean & smooth
d. dirty
clean and smooth
- Dietus-Boelter equation cannot be used for molten metals mainly due to its very low
a. Prandtl number
b. Grashoff number
c. thermal conductivity
d. viscosity
prandtl
- If Prandtl number is greater than the Schmidt number, then the
a. thermal boundary layer lies inside the concentration boundary layer.
b. concentration boundary layer lies inside the thermal boundary layer.
c. thermal & concentration boundary layers are of equal thickness.
d. hydrodynamic(i.e., momentum)boundary layer is thicker than the other two.
thermal boundary layer lies inside the concentration boundary layer.
Which of the following accessories is provided in the vapor line of an evaporator for
removing the entrained liquid ?
a. Bleed point
b. Vent
c. Catchall
d. Baffle
catchall
Rate of heat transfer by vaporisation from pools of water is affected by the
a. nature of heating surface and distribution of bubbles.
b. surface tension of water.
c. viscosity of water.
d. all (a), (b) and (c).
all
Multiple effect evaporators are commonly used in the manufacture of
P. Paper Q. Superphosphate
R. Sugar S. Fats
a. P and Q
b. P and R
c. P and S
d. R and S
P and S
Which of the following has the highest thermal conductivity ?
a. Brick
b. Air
c. Water
d. Silver
Silver
Which of the following is unimportant in forced convection ?
a. Reynolds number
b. Prandtl number
c. Grashhoff number
d. none of these
Grashoff number
- Steam is to be condensed in a shell and tube heat exchanger, 5 m long with a shell
diameter of 1 m. Cooling water is to be used for removing the heat. Heat transfer co
efficient for the cooling water, whether on shell side or tube side is the same. The best
arrangement is
a. vertical heat exchanger with steam on tube side.
b. vertical heat exchanger with steam on shell side.
c. horizontal heat exchanger with steam on tube side.
d. horizontal heat exchanger with steam on shell side.
vertical hex with steam on SHELL side
- In a multiple effect evaporator, the effect of boiling point elevation is to
a. reduce the capacity
b. reduce the economy
c. increase the economy
d. none of thes
reduce the CAPACITY
Dropwise condensation occurs on __________ surfaces.
a. clean and dirt free
b. smooth clean
c. contaminated cooling
d. polished
Contaminated cooling
- For specified tube outside diameter, higher BWG means higher
a. tube thickness
b. cross-sectional area
c. weight per unit length
d. none of these
cross-sectional area
Pick out the wrong statement.
a. Swenson-Walker crystalliser is a batch crystalliser.
b. Super saturation of the solution is the driving potential for a crystal growth.
c. The liquor left after the removal of crystals from a solution is called mother liquor.
d. The first stage of crystal formation is called nucleation.
Swenson-Walker crystalliser is a batch crystalliser.
Electromagnetic radiations propagate in vacuum with a velocity of __________
metre/second.
a. 3 x l05
b. 3 x l08
c. 3 x l010
d. 3 x l012
b. 3 x l08
Heat flux is the time rate of heat transfer per unit
a. length
b. area
c. volume
d. none of these
area
Film boiling occurs at __________ pressure.
a. atmospheric
b. sub-atmospheric
c. negative
d. very high
very high
Heat flux through several resistances in series in analogous to the current flowing
through several
a. resistances in parallel.
b. capacitors in series.
c. resistances in series.
d. none of these.
resistance in series
Extended heat transfer surface like fins are used to increase the heat transfer rate. Fin
efficiency is defined as the ratio of heat transferred across the fin surface to the
theoretical heat transfer across an equal area held at the
a. surrounding temperature.
b. average temperature of the fin.
c. temperature of the fin end.
d. constant temperature equal to that of the base
constant temperature equal to that of the base
Vacuum is generally maintained in the vapour space of an evaporator mainly to
a. get economical temperature difference by using moderate pressure steam.
b. facilitate forward feeding in multiple effect evaporation.
c. concentrate heat sensitive materials.
d. achieve very high concentration of the final product.
get economical temperature difference by using moderate pressure steam.
Double pipe heat exchangers are used
a. when heat transfer area required is very high.
b. when heat transfer area required is very low, i.e (100-200 ft2).
c. because it occupies less floor area.
d. because it is less costly.
when heat transfer area required is very low, i.e (100-200 ft2).
Pick out the wrong statement.
a. Heat transfer by radiation can not occur across an absolute volume.
b. In case of a shell and tube heat exchanger, the pressure drop through the shell is
proportional to the number of times the fluid crosses the bundle between baffles.
c. Propagation velocity for travel of heat radiation through vacuum is equal to the
velocity of the light.
d. The amount of heat involved in the condensation or vaporisation of 1 kg of a fluid is
the same.
Heat transfer by radiation can not occur across an absolute volume.
Which type of heat exchanger is preferred for heavy heat loads ?
a. Double pipe
b. Plate fine
c. Series and parallel set of shell and tube
d. None of these
series and parallel set of shell and tube
Pick out the wrong statement.
a. Reciprocal of the resistance to heat flow is called thermal conductance.
b. Unit of thermal conductance is W/°K.
c. Thermal conductance of a wall of thickness ‘L’, thermal conductivity ‘k’ and heat flow
area ‘A’ is kL/A.
d. none of these.
Thermal conductance of a wall of thickness ‘L’, thermal conductivity ‘k’ and heat flow
area ‘A’ is kL/A.
While the total emissivity of a perfect black body is unity, the same for a real body is
a. 0
b. 1
c. >1
d. between 0 and 1
between 0 and 1
The Grashoff number is defined as the ratio of the
a. buoyancy to inertial forces.
b. buoyancy to viscous forces.
c. inertial to viscous forces.
d. buoyancy to surface tension forces.
buoyancy to viscous forces
According to Reynolds analogy, Stanton number is equal to (where, f = Fanning
friction factor)
a. 2f
b. f
c. f/2
d. f/4
f/2
The absorptivity of a body is equal to its emissivity
a. at a particular temperature.
b. for circular bodies.
c. under thermal equilibrium.
d. none of these
under thermal equilibrium
- Absorptivity and refletivity of a perfect black body are respectively
a. 1 and 0
b. 0 and 1
c. 1 and ∞
d. 0 and 0.5
1 and 0
- In thermal radiation for a black body (where, ε is emissivity and α is absorptivity)
a. α = 1; ε ≠ 1
b. α ≠ 1, ε = 1
c. α ≠ 1, ε ≠ 1
d. α = 1; ε = 1
d. α = 1; ε = 1
Pick out the wrong statement.
a. Superheated steam is preferably not used for process heating because of its low heat
transfer film co-efficient.
b. In a shell and tube heat exchanger, the shell pressure drop is maximum for orifice
baffles.
c. S.I. unit of fouling factor is Watt/m2.°K.
d. Longitudinal fins are used in extended surface heat exchangers, when the direction of
fluid flow is parallel to the axis of the tube.
c. S.I. unit of fouling factor is Watt/m2.°K.
- 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 ?
a. Cube
b. Plate
c. Sphere
d. All will cool at the same rate
Cube
Vent pipes are provided in a condenser to
a. remove non-condensable gases.
b. purge the condenser.
c. facilitate easy cleaning of tubes.
d. none of these.
remove non-condensable gases.
- Heat transfer by natural convection is enhanced in system with
a. high viscosity.
b. high co-efficient of thermal expansion.
c. low temperature gradients.
d. 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.
a. nucleate
b. pool
c. low pressure
d. 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.
a. 10
b. >10
c. <10
d. either (b) or
> 10
In forced convection, the Nusselt number is a function of
a. Re and Pr
b. Re and Gr
c. Pr and Gr
d. Re and Sc
Re and PR
At what value of Prandtl number, the hydrodynamic and thermal boundary layers are
identical ?
a. 0.5
b. 1
c. 1.5
d. 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
a. total heat transfer area of all the effects is -less than that in a single effect evaporator
system.
b. total amount of vapor produced per Kg of feed steam in a multiple effect system is
much higher than in a single effect.
c. boiling point elevation in a single effect system is much higher than that in any effect
in a multieffect system.
d. 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
a. keeping the heat transfer surface clean.
b. enhancing the fluid pumping rate.
c. increasing the tube length.
d. none of these.
keeping the heat transfer surface clean.
Trap is used to remove __________ from steam pipe lines.
a. steam
b. condensate
c. non-condensables
d. 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
a. 125
b. 133
c. 150
d. 160
160
Tube pitch is the __________ of tube diameters and the clearances.
a. sum
b. difference
c. ratio
d. none of these
sum
- With increase in the distance between the heat source and the object receiving the heat,
the radiation heat transfer
a. decreases
b. increases
c. increases exponentially
d. 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.
a. more
b. less
c. same
d. unpredictable
less
Natural convection is characterised by
a. Grashhoff number
b. Peclet number
c. Reynolds number
d. Prandtl number
grashoff
Nusselt number for full developed, laminar, constant property flow in a pipe at
uniform heat flux is
a. 0.72
b. 4.364
c. 18
d. 83
4.364
Thickness of thermal boundary layer is more compared to that of hydrodynamic
boundary layer, when the value of Prandtl number is
a. 1
b. < 1
c. >1
d. >5
<1
The critical radius of insulation for cylindrical pipe is (where, hi = heat transfer
coefficient at inside of the pipe )
a. K/h0
b. 2K/h0
c. hi/K
d. 2hi/K
a. K/h0
The heat flux in the nucleate boiling regimes is proportional to (where, ΔT = excess
temperature)
a. (ΔT)2
b. (ΔT)4
c. (ΔT)3
d. (ΔT)^0.5
(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
a. ∞
b. 0
c. equal to the difference between hot and cold fluids inlet temperature.
d. equal to the difference between hot fluid inlet temperature and cold fluid outlet
temperature.
0
body is called grey if the monochromatic emissivity of the body is
a. zero
b. unity
c. same for all wavelengths
d. different for all wavelengths
c. same for all wavelengths
in an extended surface heat exchanger, fluid having lower co-efficient
a. flows through the tube.
b. flows outside the tubes.
c. can flow either inside or outside the tubes.
d. 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
a. infinity.
b. zero.
c. same as average heat transfer co-efficient for tube side.
d. none of these.
infinity
Grashoff number, which is defined as g . β(Ts - T∞) ρ2. L3/μ2, is proportional to the
ratio of buoyancy force to __________ force.
a. viscous
b. elastic
c. inertial
d. none of these
none of these
Pick out the wrong statement.
a. By increasing the number of shell passes, the temperature cross in a shell and tube heat
exchanger can be prevented.
b. An underdesigned steam trap will back up the condensate instead of discharging it out.
c. Steam condensate is corrosive because of the presence of dissolved oxygen in it.
d. Film boiling is desirable in commercial equipments due to high heat transfer rate at
low temperature drop
By increasing the number of shell passes, the temperature cross in a shell and tube heat
exchanger can be prevented.
For a cold dilute feed to produce thick viscous liquor, backward feeding as compared
to forward feeding results in
a. increased economy
b. decreased economy
c. lower capacity
d. no effect on economy
increased economy
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.
a. hyperbolic
b. parabolic
c. linear
d. logarithmic
linear
- In regenerative air preheater (as practised in heating of coke ovens), the heat is
transferred
a. through a metallic wall.
b. by direct contact of hot flue gas with air.
c. by heating an intermediate material (like chequor bricks) and then heating the air from
this hot material.
d. none of these.
by heating an intermediate material (like chequor bricks) and then heating the air from
this hot material.
_________ equation relates the thermal conductivity of a solid to its temperature.
a. Antonie
b. Kopp’s
c. Lee’s
d. Kistyakowski
Lee’s
- For small temperature difference, the heat transfer rate as per Newton’s law of cooling
is proportional to (where, Δt = excess temperature)
a. Δt
b. Δt2
c. Δt3
d. Δt^0.5
Δt
Pick out the wrong statement.
a. The condensing film co-efficient is about 3 times lower for vertical condenser as
compared to the equivalent horizontal condenser for identical situation.
b. Film co-efficient for vaporisation decreases as a result of vapor binding.
c. In industrial practice, sub-cooling of condensate is required, when the condensate is a
volatile liquid and is to be transferred for storage.
d. 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.
Viscous & heat sensitive liquids are concentrated in __________ evaporators.
a. open pan
b. long tube
c. agitated film
d. none of these
agitated film
- 1 BTU/hr.ft.°F is equal to __________ kcal/hr. m.°C.
a. 1.49
b. 1
c. 4.88
d. 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
a. T1 - T2
b. T1 - Ta
c. T2 - Ta
d. none of these
none of these
the ratio of momentum diffusivity to thermal diffusivity is the __________ number.
a. Prandtl
b. Nusselt
c. Stanton
d. Grashoff
Prandtl
for a perfectly transparent surface (like gases), the
a. absorptivity = 0
b. transmissivity = 1
c. reflectivity = 0
d. all (a), (b) & (c)
transmissivity = 1
For a fluid flowing in an annulus space, the wetted perimeter for heat transfer and
pressure drop are
a. same
b. different
c. never different
d. linearly related
different
During crystallisation, formation of crystal can occur in __________ solution only.
a. saturated
b. supersaturated
c. undersaturated
d. 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
a. 4000
b. 9000
c. 4600
d. 3000
4000
An insulator should have
a. low thermal conductivity.
b. high thermal conductivity.
c. less resistance to heat flow.
d. a porous structure
low thermal conductivity
In a backward feed multiple effect evaporator
a. feed is introduced in the first effect.
b. feed flows from low pressure to high pressure.
c. no pumps are required between successive effects.
d. none of these
feed flows from low pressure to high pressure
