htn 2 Flashcards
Which of the following parameters of the fluid is not very
important, while deciding its route in a shell and tube heat
exchanger?
A. Corrosiveness & fouling characteristics
B. pressure
C. viscosity
D. density
D
Corrosiveness of steam condensate is due to the presence
of
A. CO2
B. dissolved O2
C. both (a) & (b)
D. neither (a) nor (b)
C
The equation, (NSt x N2/3Pr) = f/2, is the __________ analogy.
A. Colburn
B. Reynolds
C. Prandtl
D. none of these.
A
The driving potential for the crystal growth during crystallisation
is the __________ of the solution.
A. concentration
B. viscosity
C. super-saturation
D. Density
C
It
is not recommended to use a 1-2 shell and tube heat
exchanger for a particular heat duty, whenever the LMTD
correction factor is
A. 0.75
B. < 0.75
C. < 0.50
D. < 0.25
B
Kg of liquid evaporated per hour in an evaporator is defined as
its
A. capacity
B. economy
C. steam load
D. none of these
A
Pick out the correct statement.
A. 1 kcal/hr.m.°C is equal to 1 BTU/hr. ft.°F.
B. In steady state heat conduction, the only property of
the substance which determines the temperature
distribution, is the thermal conductivity.
C. In unsteady state heat conduction, heat flows in the
direction of temperature rise.
D. In heat transfer by forced convection, Grashoff
number is very important.
B
Open pan evaporators are preferred to be used, when the
solution to be concentrated is
A. scaling
B. highly viscous
C. corrosive
D. salty
B
Cpu/k is termed as the __________ number.
A. Grashoff
B. Nusselt
C. Prandtl
D. Stanton
C
The maximum heat transfer co-efficient from steam heating will
be attained when the steam is
A. supersaturated
B. saturated
C. wet
D. none of these
b
Kirchoff’s law is applicable to
A. monochromatic radiation only
B. total radiation only
C. both (a) and (b)
D. only volumes and not to surfaces
C
In a forward feed multiple effect evaporator, the pressure is
A. highest in last effect
B. lowest in last effect
C. same in all effects
D. dependent on the number of effects
B
In a co-current double pipe heat exchanger used for condensing
saturated steam over the inner tube, if the entrance and exit
conditions of the coolant are interchanged, then the rate of
condensation will
A. increase
B. decrease
C. remain unchanged
D. either increase or decrease; depends on the coolant
flow rate
C
Which of the following has maximum thermal conductivity?
A. Iron
B. Coal
C. Nitrogen
D. Tar
A
Which of the following situations can be approximated to a
steady state heat transfer system ?
A. A red hot steel slab (having outside surface
temperature as 1300°C) exposed to the atmosheric
air at 35°C.
B. 10 kg of dry saturated steam at 8 kgf/cm2 flowing
through a short length of stainless steel pipe exposed
to atmospheric air at 35°C.
C. Boiling brine kept in open vessel when the bottom
surface temperature of the vessel is maintained
constant at 180°C.
D. A sub-cooled refrigerant liquid at 8°C flowing at the
rate of 6 Kg/minute through a copper pipe exposed
to atmospheric air at 35°C.
B
Sensible heat absorbed by 1 lb of water when it is heated from
32 to 212°F may be around __________ BTU.
A. 180
B. 970
C. 3.97
D. data insufficient, can’t be predicted
A
The unit of conductance in SI unit is
A. W/m
B. W/m2
C. W/°K
D. W/m°K
C
In a shell and tube heat exchanger, square pitch compared to
triangular pitch
A. gives a higher shell side pressure drop.
B. gives a lower shell side pressure drop.
C. can pack more surface area into a shell of given
diameter.
D. none of these.
B
A wall has two layers of materials A and B; each made of a
different material. Both the layers have the same thickness. The
thermal conductivity of materialA is twice that of B. Under the
equilibrium, the temperature difference across the wall is 36°C.
The temperature difference across the layer A is ________ °C.
A. 6
B. 12
C. 18
D. 24
B
The film co-efficient between condensing vapour and metal wall
increases with
A. increasing temperature of the vapour.
B. decreasing temperature of the vapour.
C. increasing viscosity of the film of condensate.
D. increasing temperature drop.
A
Critical value of the __________ number governs the transition
from laminar to turbulent flow in free convection heat transfer.
A. Grashoff
B. Reynolds
C. both ‘a’ & ‘b’
D. Prandtl & Grashoff
D
The thermal boundary layer at NPr > 1
A. is thicker than hydrodynamic boundary layer.
B. is thinner than hydrodynamic boundary layer.
C. and the hydrodynamic boundary layer are identical.
D. disappears.
D
With increase in temperature, the thermal conductivity of a gas
A. increases.
B. decreases.
C. remains same.
D. may increase / decrease depending on type of gas.
A
In the free convection regime of pool boiling, the heat flux is
proportional to
A. Δt1/2
B. Δt2
C. Δt5/4
D. Δt
C
With increase in temperature, the thermal conductivity of fresh
lubricating oil
A. increases
B. decreases
C. remains unchanged
D. may increase or decrease; depends on its
composition
B
Fresh orange juice contains 12% (by weight) solids and the rest
water 90% of the fresh juice is sent to an evaporator to remove
water and subsequently mixed with the remaining 10% of fresh
juice. The resultant product contains 40% solids. The kg of
water removed from 1 kg fresh juice is
A. 0.4
B. 0.5
C. 0.6
D. 0.7
D
Nusselt number is the ratio of the temperature gradient at the
wall to
A. temperature difference
B. heat flux
C. that across the entire pipe
D. none of these
C
The Stefan-Boltzman constant depends on the
A. medium
B. temperature
C. surface
D. none of these
D
The units of resistance to heat transfer is
A. J.m-2.K-1
B. J.m-1.K-1
C. W.m-2.K-1
D. W-1m2K
D
Thermal conductivity of a gas at low density, __________ with
increase in temperature.
A. decreases
B. increases
C. remains unchanged
D. may increase or decrease; depends on the gas
B
Boiling of milk in an open vessel is an example of _____ boiling.
A. film
B. sub-cooled
C. saturated nucleate
D. none of these
A
Mode of heat transfer involved in the cooling of air cooled
internal combustion engine is
A. conduction
B. natural convection
C. forced convection
D. none of these
B
Radiation heat losses from satisfactorily insulated high pressure
boilar may be about __________ percent.
A. 1
B. 7
C. 18
D. 26
B
Pick out the wrong statement.
A. In process heat exchangers, saturated steam is
preferred over the superheated steam.
B. The maximum is the emissive power of a surface at
a temperature T1 occurs at a wavelength of λ1. If the
surface temperature is halved, the maximum in the
emissive power would occur at a wavelength of 0.5
λ1.
C. When a vertical plate is heated in infinite air
environmental under natural convection conditions,
the velocity profile in air, normal to the plate, exhibits
a maximum.
D. A body at 925 K emits an energy of 1.42 x
1011σW/m2 (σ is the Stefan-Boltzman constant) in
the wavelength band between 3 μm to 4 μm. The
fraction of this energy in the total energy emitted over
the entire wavelength range is equal to emissivity.
B
Nusselt number is a function of Prandtl number and
__________ number of fluid in natural convection heat transfer.
A. Grashoff
B. Biot
C. Stantan
D. Reynolds
A
_ heat exchanger is used for chilling oil to be dewaxed.
A. U-tube
B. Double pipe
C. Fixed tube
D. Floating head
B
Pick out the wrong statement.
A. Economy of a multiple effect evaporator is not
influenced by the boiling point elevation.
B. Two identical cubes of iron and copper will have the
same heat content under the same conditions of
temperature.
C. Double pipe heat exchangers are mostly used in the
field of refrigeration.
D. Finned tube heat exchangers are suitable for heating
air by steam.
B
In sub-cooled boiling,
A. temperature of the heating surface is less than the
boiling point of the liquid.
B. temperature of the heating surface is more than the
boiling point of the liquid.
C. bubbles from heating surface are absorbed by the
mass of the liquid.
D. very large vapour space is necessary.
B
A __________ evaporator employs an annular downtake.
A. basket type
B. horizontal
C. long tube vertical
D. none of these
A
In natural convection heat transfer, the correlating parameter is
the
A. Graetz number
B. Eckert number
C. Grashoff number
D. Bond number
C
Steady state one dimensional heat flow by conduction as given
by Fourier’s low does not assume that
A. there is no internal heat generation.
B. boundary surfaces are isothermal.
C. material is anisotropic.
D. constant temperature gradient exists.
C
The left face of a one dimensional slab of thickness 0.2 m is
maintained at 80°C and the right face is exposed to air at 30°C.
The thermal conductivity of the slab is 1.2 W/m.K and the heat
transfer co-efficient from the right face is 10 W/m2.K. At steady
state, the temperature of the right face in °C is
A. 77.2
B. 71.2
C. 63.8
D. 48.7
D
In a shell and tube heat exchanger, the shell side fluid velocity
can not be changed by changing the
A. tube layout
B. tube diameter
C. tube pitch
D. no. of baffles
B
To reduce the tube side pressure drop for the same flow rate,
the heat exchanger recomended is
A. 1-2 heat exchanger
B. 1-1 heat exchanger
C. 3-2 heat exchanger
D. 2-4 heat exchanger
B
Heat transfer co-efficient (h) for a fluid flowing inside a clean pipe is given by ℎ = 0.023K/D(DVp/u)^0.8(Cpu/k)^0.4 . This is valid forthe value of NRe equal to
A. < 2100
B. 2100-4000
C. 4000
D. > 10000
D
Steam side heat transfer co-efficient in an evaporator is in the
range of __________ kcal/hr.m2°C.
A. 10-50
B. 100-500
C. 1000-1500
D. 5000-15000
d
Correction is applied to LMTD for __________ flow.
A. parallel
B. counter
C. cross
D. none of these
C
If h1 = inner film co-efficient and /h2 = outer film co-efficient,
then the overall heat transfer co-efficient is
A. always less than h1
B. always between h1 and h2
C. always higher than h2
D. dependent on metal resistance
B
One kilogram of water at 0°C is changed to superheated steam
of one atm pressure and 300° C. The major heat consumption
in the process will be to
A. heat the water from 0°C to 100°C.
B. evaporate the water.
C. to superheat the steam.
D. data insufficient, can’t be predicted
B
A composite wall consists of two plates A and B placed in series
normal to the flow of heat. The thermal conductivities are kA and
kB and the specific heat capacities are CPA and CPB for plates A
and B respectively. Plate B has twice the thickness of plate A.
At steady state, the temperature difference across plate A is
greater than that across plate B, when
A. CPA > CPB
B. CPA < CPB
C. kA < 0.5kB
D. kA>2 kB
C
For __________ Prandtl number values, the heat conduction
will be negligible in the buffer zone.
A. extremely low
B. low
C. high
D. No
C
he radiation heat flux from a heating element at a temperature
of 800°C, in a furnace maintained at 300°C is 8 kW/m2. The
flux, when the element temperature is increased to 1000°C for
the same furnace temperature is
A. 11.2 kW/m2
B. 12.0 kW/m2
C. 14.6 kW/m2
D. 16.5 kW/m2
D
Heat exchangers operating, when the asymptotic range is
reached,
A. provide very large heat transfer co-efficient.
B. results in making part of the heating surface inactive.
C. results in abruptly increased velocity.
D. none of these.
B
In case of a shell and tube heat exchanger, the minimum and
maximum baffle spacing is respectively (where, D = inside
diameter of the shell)
A. D/5 and D
B. D/2 and 2 D
C. D/4 and 2 D
D. D and 2 D
A
In case of heat transfer by conduction in a hollow cylinder,
__________ mean area is used to calculate the heat transfer
rate.
A. geometric
B. arithmetic
C. logarithmic
D. either (a), (b) or (c)
C
In a gas-liquid shell and tube heat exchanger, the
A. presence of a non-condensible gas decreases the
condensing film co-efficient.
B. gases under high pressure are routed through the
tube side, because high pressure gases are
corrosive in nature.
C. gases to be heated/cooled is normally routed through
the shell side, because the corrosion caused by the
cooling water or steam condensate remain localised
to the tubes.
D. all ‘a’, ‘b’ & ‘c’
D
Which is the most suitable for the concentration of foamy &
frothy liquors ?
A. Agitated film evaporator
B. Long tube vertical evaporator
C. Open pan evaporator
D. None of these
B
Boiling point elevation of an ideal solution
A. increases rapidly with temperature rise.
B. decreases rapidly with temperature rise
C. in independent of pressure.
D. both (b) and (c).
A
The rate of heat transfer is a product of overall heat transfer co
efficient, the difference in temperature and the
A. heating volume
B. heat transfer area
C. Nusselt number
D. none of these
B
Which of the following has the lowest overall heat transfer
coefficient?
A. Dowtherm
B. Molten sodium
C. Water
D. Air
D
Which of the following has the minimum absorptivity?
A. Aluminium foil
B. Coal dust
C. Refractory bricks
D. Iron plates
A
A black body when hot, emits heat radiation of __________
wavelengths.
A. small
B. large
C. all
D. one fixed
C
Vibrations in the tubes of a shell and tube heat exchanger is
induced due to the
A. flow of fluid on the tube and shell sides.
B. oscillations in the flow of shell/tube sides fluid.
C. vibrations transmitted through piping and/or
supports due to external reasons.
D. all (a), (b) and (c)
D
Heat transfer by conduction in the turbulent core of a fluid
flowing through a heated pipe is negligible, if the value of Prandtl
number is
A. 0.2
B. 0.4
C. 0.6
D. 0.8
C
The Nusselt number for fully developed (both thermally and
hydrodynamically) laminar flow through a circular pipe whose
surface temperature remains constant is
A. 1.66
B. 88.66
C. 3.66
D. dependent on NRe only
C
Sensible heat of hot industrial flue gases can not be recovered
by a/an
A. economiser
B. regenerator
C. ceramic recuperator
D. none of these
D
The critical radius of insulation for a spherical shell is (where, K
= thermal conductivity of insulating material h0 = heat transfer
coefficient at the outer surface)
A. K/h0
B. 2K/h0
C. h0/K
D. h0/2K
B
Radiator of an automobile engine is a __________ type of heat
exchanger.
A. co-current
B. cross-current
C. counter-current
D. direct contact
D
In pipe flow, heat is transferred from hot wall to the liquid by
A. conduction only.
B. forced convection only.
C. forced convection and conduction.
D. free and forced convection.
C
The value of Stefan-Boltazman constant in SI unit is
A. 5.6697 x 10-8W/m2.°K4
B. 0.1714 x 10-8W/m2.°K4
C. 5.6697 x 10-8kcal/m2 . °K4
D. 0.1714 x 10-8kcal/m2. °K4
A
The unit of heat transfer co-efficient is
A. BTU/hr. ft2°F
B. BTU/hr. °F. ft.
C. BTU/hr. °F
D. BTU/hr. ft
A
For turbulent flow in a tube, the heat transfer co-efficient is
obtained from the Dittus-Boelter correlation. If the tube diameter
is halved and the flow rate is doubled, then the heat transfer co
efficient will change by a factor of
A. 1
B. 1.74
C. 6.1
D. 37
C
The inner wall of a furnace is at a temperature of 700°C. The
composite wall is made of two substances, 10 and 20 cm thick
with thermal conductivities of 0.05 and 0.1 W.m-1.°C-1
respectively. The ambient air is at 30°C and the heat transfer
co-efficient between the outer surface of wall and air is 20 W.m
2.°C-1. The rate of heat loss from the outer surface in W.m-2is
A. 165.4
B. 167.5
C. 172.5
D. 175
A
Value of Nusselt number [Nu = (hD/k)] for the heat transfer by
conduction from a droplet or a spherical particle to a
surrounding stagnant film is
A. 0.5
B. 2
C. 10
D. 100
B
Low thermal conductivity of heat insulating materials is due to
its
A. dense structure.
B. high proportion of air space.
C. high specific heat.
D. none of these.
B
The most conducive surface for dropwise condensation to occur
is the __________ surface.
A. coated
B. oily
C. glazed & polished
D. smooth
B
In an interphase heat transfer process, the equilibrium state
corresponds to equality of temperature in the two phases, while
the condition for equilibrium in an interphase mass transfer
process is equality of
A. concentrations
B. chemical potentials
C. activity co-efficients
D. mass transfer co-efficients
A
Multiple effect evaporators are used to
A. increase the steam economy & decrease the
capacity.
B. increase the steam economy & the capacity.
C. decrease the steam economy & the capacity.
D. decrease the steam economy & increase the
capacity.
B
Calburn analogy is applicable for the value of Prandtl number
from
A. 0.001 to 1
B. 0.6 to 120
C. 0.5 to 5
D. 120 to 400
B
Pick out the correct equation.
A. jH = (St)(Pr)2/3 = f/2
B. jH = (St)(Pr)1/3 = f/2
C. jH = (St)2/3(Pr) = f/2
D. jH = (St)1/3(Pr) = f/2
A
Prandtl number is given by
A. cPU/a
B. hD/k
C. cPu/k
D. u/hcP
c
Small scale evaporation is done in a
A. heat exchanger
B. condenser
C. multiple effect evaporator
D. steam jacketed kettle
D
Heat exchanger tubes are never made of
A. plain carbon steel
B. stainless steel
C. lead
D. Copper
C
At constant temperature, the thermal conductivities of gases
__________ with rise in pressure.
A. decrease
B. increase
C. remain unchanged
D. may increase or decrease; depends on the pressure
A
Reason for operating an evaporator in multiple effect is to
secure
A. increased steam economy.
B. decreased steam consumption.
C. both (a) and (b).
D. increased capacity.
C
In a cooling tower, water becomes cool by
A. losing sensible heat.
B. heat transfer to surroundings.
C. vaporisation due to heat loss to air.
D. loosing latent heat.
A
In Fourier’s law, the proportionality constant is called the
A. heat transfer co-efficient
B. thermal diffusivity
C. thermal conductivity
D. Stefan-Boltzman constant
C
The heat transfer co-efficient in film type condensation is
__________ that for dropwise condensation.
A. greater than
B. lower than
C. is same as
D. half
B
Multiple effect evaporation accounts for
A. steam economy
B. lower operating costs
C. investment economy
D. none of these
A
Pick out the wrong statement.
A. Bubble size increases with the dynamic viscosity of
the liquid in case of nucleate pool boiling.
B. Thermal conductivity of a dry material is more than
that of the damp material.
C. Ratio of its capacity to economy equals the steam
consumption in kg/hr in an evaporator.
D. Vaporisation of organic substances in evaporator
mostly causes foam formation.
B
Duhring’s plot’ is of use in
A. extractive distillation
B. evaporation
C. leaching
D. absorption
B
Which has the lowest Prandtl number ?
A. Liquid metal
B. Aqueous solution
C. Water
D. Lube oil
A
Pick out the wrong statement.
A. The value of hydrostatic head increases with
increase in vacuum in the effect in a multiple effect
evaporator system.
B. Entering velocity of the liquid in the tubes of natural
circulation evaporators is in the range of 0.3 to 0.9
metre/second.
C. Duhring’s plot is used for calculating the
concentration of solution.
D. In a multiple effect evaporation system, the number
of effects is limited by the total boiling point rise.
C
Prandtl number for most of dry gases is about
A. 0.001
B. 0.72
C. 70
D. 150
B
Prandtl number is the reciprocal of
A. Thermal diffusivity/Momentum diffusivity
B. thermal diffusivity x Momentum
C. Thermal diffusivity x Mass diffusivity
D. Mass diffusivity x Momentum diffusivity
A
In a heat exchanger with steam outside the tubes, a liquid gets
heated to 45°C, when its flow velocity in the tubes is 2 m/s. If
the flow velocity is reduced to 1 m/s, other things remaining the
same, the temperature of the exit liquid will be
A. less than 45°C
B. more than 45°C
C. equal to 45°C
D. initially decreases and remains constant thereafter.
B
For a liquid in laminar flow through a very long tube, when the
exit fluid temperature approaches the wall temperature, the
equation to be used is
A. Nu = 0.023 Re0.8 . Pr 0.4
B. Nu = (n/2) Gz
C. Nu = (2/n) Gz
D. Nu = 2Gz0.5
C
In case of a supercooled solution, which is on the verge of
crystallisation, the free energy of the solution as compared to
that of the solid is
A. more
B. less
C. same
D. more or less; depends on the nature of solution
A
Maximum water velocity in tubes of a 1-2 shell and tube heat
exchanger may be around __________ metres/second.
A. 1
B. 10
C. 20
D. 30
A
Which of the following is the most widely used heat insulating
material for pipelines carrying steam ?
A. Tar dolomite bricks followed by asbestos.
B. Fireclay refractory followed by aluminium sheet.
C. Cotton followed by aluminium foil.
D. 85% magnesia cement and glass wool.
D
Heat transfer co-efficient equation for forced convection, Nu =
0.023 Re0.8 . Prn, is not valid, if the value of
A. n = 0.4 is used for heating.
B. n = 0.3 is used for cooling.
C. Reynolds number for the flow involved is > 10000.
D. Reynolds number for the flow involved is < 2100.
D
Which is the best tube arrangement (in a shell and tube heat
exchanger) if the fluids are clean and non-fouling?
A. Square pitch
B. Triangular pitch
C. Diagonal square pitch
D. None of these
B
In case of a multipass shell and tube heat exchanger, the
temperature drop in the fluid
A. is inversely proportional to the resistance across
which the drop occurs.
B. and the wall are proportional to individual
resistances.
C. and the wall is not related.
D. none of these.
B
The purpose of providing expansion bellows in the shell of
tubular exchanger is to
A. increase the heating load.
B. impart structural strength.
C. account for the uneven expansion of shell and tube
C
A graph between __________ is called Wilson plot.
A. 1/U vs. 1/V0.8
B. 1/V0.8 vs. U
C. V0.8 vs. U
D. 1/U vs. 1/V
A
Grashhoff number is given by
A. gD3.β.Δtρ2/μ2
B. gD2βΔtρ/μ2
C. gD2βΔtP2μ
D. gD3βΔtP2/μ
A
The overall heat transfer co-efficient for a shell and tube heat
exchanger for clean surfaces is U0 = 400 W/m2.K. The fouling
factor after one year of operation is found to be hd0 = 2000
W/m2.K. The overall heat transfer co-efficient at this time is
A. 1200W/m2.K
B. 894W/m2.K
C. 333W/m2.K
D. 287 W/m2.K
C
Steam economy in case of a triple effect evaporator will be
A. 1
B. < 1
C. > 1
D. between 0 and 1
C
In a laboratory test run, the rate of drying was found to be 0.5 x
10-3 kg/m2.s, when the moisture content reduced from 0.4 to 0.1
on dry basis. The critical moisture content of the material is 0.08
on a dry basis. A tray dryer is used to dry 100 kg (dry basis) of
the same material under identical conditions. The surface area
of the material is 0.04 m2/kg of dry solid. The time required (in
seconds) to reduce the moisture content of the solids from 0.3
to 0.2 (dry basis) is
A. 2000
B. 4000
C. 5000
D. 6000
C
In Biot number, the characteristic length used is the ratio of the
__________ of the solid.
A. volume to surface area
B. perimeter to surface area
C. surface area to volume
D. surface area to perimeter
A
Pick out the wrong statement.
A. ‘Solvates’ are chemical compounds formed by
solute with their solvents. When water is the solvent,
then it is called a ‘hydrate’.
B. In heat exchanger calculations (Δt)weighted is used
in place of Δt, when it involves more than one
sequence of heating or cooling i.e., desuperheating
& condensation or condensation & sub-cooling.
C. Heat transfer co-efficient during nucleate boiling is
not influenced by the agitation imparted.
D. In case of short tube vertical evaporators, area of
central downtake is about 50 to 100% of the total
tube cross-sectional area.
C
What is the geometric mean of two heat transfer areas A1 and
A2 ?
A. √A1A2
B. √A1+A2
C. 1/2√A1A2
D. 2 √A1*A2
A
Evaporation by thermocompression results in the
A. saving of steam.
B. realisation of multiple effect economy in a single
effect.
C. both (a) and (b).
D. none of these.
C
Forced circulation evaporators are normally used for
concentrating liquids having
A. scaling characteristics
B. high viscosity
C. both(a) & (b)
D. neither (a) nor (b)
C
