heat transfer Flashcards
Air is best heated with steam in a heat exchanger of
double pipe type with fin on air side
Black liquor generated during paper manufacture is concentrated in a
multiple effect evaporator.
When vaporisation takes place directly at the heating surface, it is called
nucleate boiling
Steam consumption in kg/hr in case of an evaporator is given by (where, C & E are capacity the economy of the evaporator respectively)
C/E
Reynold’s analogy states that
Nst α f
With increase in temperature, the thermal conductivity of non-metallic amorphous solids
increases
Fourier’s law applies to the heat transfer by
conduction
Shell side pressure drop in a shell and tube heat exchanger does not depend upon the
none of these.
In the equation Q = UAΔt; Δt is
logarithmic mean temperature difference.
With increase in porosity, the thermal conductivity of a solid substance
decreases
For an ideal black body
absorptivity = 1
Fouling factor
accounts for additional resistances to heat flow.
In case of vertical tube evaporator, with increase in liquor level, the overall heat transfer co-efficient
decreases
The steam ejector is used to
create vacuum.
For shell and tube heat exchanger, with increasing heat transfer area, the purchased cost per unit heat transfer area
passes through a maxima
In a heat exchanger, floating head is provided to
relieve stresses caused by thermal expansion.
Leidenfrost point is a term concerned with the
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
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
I2R
Pick out the wrong statement.
Orifice baffles are never used in a shell and tube heat exchanger.
Pressure drop on the shell side of a heat exchanger depends upon tube pitch also.
In a horizontal tube evaporator, surface blanketing by air is avoided.
Split ring type and pull through type floating heads are two commonly used floating heads is heat exchangers.
Orifice baffles are never used in a shell and tube heat exchanger.
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
Which of the following is not used as a medium for high temperature heating ?
mercury
With the increase of temperature, the Col-burn jH factor
increases.
In case of evaporators, liquid entrainment results primarily due to
foaming of the solution.
Baffles in the shell side of a shell and tube heat exchanger
increase the shell side heat transfer co-efficient.
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
finned tube heat exchanger with air outside and steam inside.
The unit of heat transfer co-efficient in SI unit is
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.
In case of heat transfer by purely forced convection, GR/Re2 ≤ 1.
The equivalent diameter of heat transfer for a duct of square cross-section (having each side as ‘x’) is equal to 4x.
Distillation process is not the same as evaporation.
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
boiling point elevations
Increasing the liquor level in the evaporator results in the
decreased capacity.
The equivalent diameter for pressure drop is __________ that for heat transfer.
smaller than
Controlling heat transfer film co-efficient is the one, which offers __________ resistance to heat transfer.
the largest
Pick out the wrong statement.
The controlling resistance in case of heating of air by condensing steam is in the air film.
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.
In case of a 1 - 2 shell and tube heat exchanger, the LMTD correction factor value increases sharply, when a temperature cross occurs.
Phase change in case of a pure fluid at a given pressure from liquid to vapor or vice-versa occurs at saturation temperature.
In 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 )
(ΔT)1/4
Bulk of the convective heat transfer resistance from a hot tube surface to the fluid flowing in it, is
mainly confined to a thin film of fluid near the surface.
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
less than that of the uninsulated steam pipe.
Which area is used in case of heat flow by conduction through a cylinder ?
Heat transfer by conduction results due to the transfer of free electrons, kinetic energy & vibrational energy from one molecule to another. Conduction heat transfer can not take place
Heat transfer by conduction results due to the transfer of free electrons, kinetic energy & vibrational energy from one molecule to another. Conduction heat transfer can not take place
between two bodies not in physical contact with each other.
The dimensionless group in mass transfer that is equivalent to Prandtl number in heat transfer is
Schmidt number.
Finned tube heat exchangers
give larger area per tube.
The absorptivity of a grey body at a given temperature __________ with increasing wavelength of radiation.
may increase or decrease ; depends on the material
Which one gives the monochromatic emissive power for black body radiation ?
Planck’s law
For what value of Prandtl number, St = f/2?
1
In a shell and tube heat exchanger,
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
for fluids in tubulent flow.
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 ?
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
radiation becomes important
Temperature profile in steady state heat transfer is
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
increases the overall heat transfer coefficient
When vaporisation takes place through a blanketting film of gas, the phenomenon is termed as __________ boiling.
film
Minimum recommended baffle spacing in a shell and tube heat exchanger is about (where, D = shell diameter)
0.2 D
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 )
T4
What is the unit of thermal conductivity ?
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
all bodies
It is not preferable to use superheated steam in evaporators, because of its very
low film co-efficient
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
3.06 x l05
LMTD for counterflow and prallel flow heat exchanger will be the same, when the
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)
NPr1/3
Economy of a multiple effect evaporator depends upon the
heat balance consideration
The Graetz number is concerned with the
heat transfer in laminar flow.
Solid angle subtended by the finite surface at the radiating element is
called the angle of vision.
Pick out the wrong statement.
The emissivity of a surface decreases, if it gets corroded by atmospheric environment.
The emissivity of a surface increases with increase in surface roughness.
The emissivity of a polished surface is quite low.
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
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.
Higher is the temperature of the radiating body, higher is the wavelength of radiation.
Logarithmic mean area is used for calculating the heat flow rate through a thick walled cylinder.
The wavelength corresponding to maximum mono-chromatic emissive power increases with rise in temperature.
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 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
some (whut?)
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
heat exchanger
Which of the following is directly concerned with the heat transfer ?
Grashoff number
ick out the wrong statement:
With change in temperature, the radiant energy emitted by a black body remains unchanged.
Absorptivity of a body approaches unity in case of diffuse reflection.
Absorptivity of a perfectly black body is unity.
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.
coloured
For hot feed, forward feeding as compared to backward feeding results in __________ economy.
increased
Radiation energy is emitted by all the substances, which are above
0°K
The interchange factor for radiation heat transfer from surface ‘x’ to surface ‘y’ in case of an infinite parallel planes with emis-sivities εx & εy is given by
εx+εy/(εx+εy-εx*εy)
