Heat Transfer Flashcards
Crystal size in a continuous crystalliser depends upon the
all (a), (b) and (c).
Heat waves
both (b) and (c).
Air is best heated with steam in a heat exchanger of
double pipe type with fin on air side.
Which of the following is generally considered as opaque surface towards radiations?
Both (b) and (c)
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
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
34.4
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
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
(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
32
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.
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
all (a), (b) and (c).
Which of the following is not used as a medium for high temperature heating ?
mercury
Presence of a non-condensing gas in a condensing vapour
none of these.
Kirchoff s law applies to __________ radiation.
both (a) & (b)
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.
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
The 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
35°C
Thermal conductivity of a conducting solid material depends upon its
both (a) & (b)
Pick out the wrong statement.
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.
Which of the following is concerned with both heat and mass transfer ?
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
less than that of the uninsulated steam pipe.
Which area is used in case of heat flow by conduction through a cylinder ?
Logarithmic mean area
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.
The equation, , corrosponds to __________ analogy.
Prandtl
For large heat transfer area requirement, shell and tube heat exchanger is preferred, because it
all (a), (b) and (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 ?
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
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. )
all (a), (b) & (c)
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
The Nusselt number for fully developed (both thermally and hydrodynamically) laminar flow through a circular pipe, where the wall heat flux is constant, is
4.36
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 l0^5
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
The range of electromagnetic spectrum important in heat transfer by radiation is __________ microns.
0.5-50
Use of transverse baffles in a shell and tube heat exchanger is done to increase the
all (a), (b) and (c).
The actual temperature drop across the heating surface of an evaporator depends on the
all (a), (b) and (c)
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.
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.
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
A perfect black body is a perfect __________ of radiation.
both (a) & (b)
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
Pick out the wrong statement:
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
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 = 2hl
jH factor for heat transfer is not a function of the __________ number.
both (b) & (c)
The Fourier number (defined as a.t/L2) is used in the analysis of problem involving heat transfer by
transient conduction
The energy radiated from a surface Q at absolute temperature T is related as
Q ∝ T4
Baffles are provided in heat exchangers to increase the
heat transfer co-efficient
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)
0.1
In case of a vertical tube evaporator, with increase in the liquor level, the
capacity of the evaporator is decreased.
The ratio of total radiating power to the absorptivity of the body depends upon the __________ as per Kirchoffs law.
temperature of the body
For gases, the thermal conductivity increases with temperature rise. For liquids, with increase in concentration, its thermal conductivity generally
decreases
A 2-4 heat exchanger involves
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
decreases
Which of the following forced convection heat transfer equation accounts for the liquid viscosity effect for viscous liquids ?
Sieder-Tate equation
The main function of baffles provided in a shell and tube heat exchanger is to
enhance turbulence.
In a single effect evaporator, the economy is
< 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
<1
Which of the following is not concerned with the heat transfer?
Schmidt number
Latent heat absorbed by 1 lb of water at 212°F, when it is changed to steam at 212°F, may be around __________ BTU.
180
In counter flow compared to parallel flow,
both (a) and (b).
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)
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
24000
Value of Prandtl number for water ranges from
5 to 10
Heat transfer in the laminar sublayer in case of a liquid flowing through a pipe, is mostly by
conduction
Among liquids, water has a comparatively high value of thermal conductivity, due to its
partial ionisation
Reynold’s analogy states that (where, St = Stanton number f = friction factor)
St = f/2
For evaporation of viscous solution in a multiple effect evaporator, the prefered feeding scheme is
backward
The sum of reflectivity and absorptivity for an opaque body is equal to
1
The film thickness for laminar film condensation on vertical surface __________ from top to bottom.
cummulatively increases
In case of a multipass shell and tube heat exchanger, providing a baffle on the shell side __________ the heat transfer rate.
increases
The local surface conductance for laminar film condensation on vertical surface is (where, t = film thickness)
∝ 1/t
Heat transfer co-efficient (h1) for liquids increases with
increasing temperature.
A diathermaneous substance __________ the thermal radiation completely.
transmits
A hot body will radiate heat most rapidly, if its surface is
black & rough
Evaporation of 1kg of water from a solution in a single effect evaporator requires about __________ kg of steam.
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
0.025
Analogy between mass and heat transfer is not applicable in case of
both (b) and (c).
A __________ surface has the maximum thermal emissivity out of the following.
black & rough
Radiant energy received by a body is proportional to (where, d = the distance between the object emitting radiation and that receiving it.)
d2
Mechanical recompression evaporation is used in the production of
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 __________ number.
Brinkman
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
0.45
The purpose of providing a ‘catchall’ in the vapor line of an evaporator is to
arrest the entrained liquid
What is the thermal conductivity of a perfect heat insulator ?
zero
‘Fouling factor’ used in the design of a multipass shell and tube heat exchanger is a
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
d^-0.2
Prandtl and Reynolds analogy are same, when Prandtl number is
1
Heat transfer occurs by natural convection because change in temperature causes difference in
density
The ratio of kinematic viscosity to thermal diffusivity is called the __________ number.
Prandtl
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
painting the surface white (with aluminium paint).
__________ paint has the maximum absorption coefficient.
Black
What is Nusselt number ?
hD/k
Steam traps are provided in steam carrying pipelines to
remove condensate and inert gases.
The Dietus-Boelter equation for convective leat transfer
cannot be used for
all (a), (b) and (c)
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
straight line
Radiation heat transfer rates does not depend upon the
none of these.
Which of the following is correct ?
Driving force = Rate x Resistance
Convective heat transfer, in which heat is transferred by movement of warmed matter is described by
Newton’s law of cooling
As the difference between the wall temperature and bulk temperature increases, the boiling heat transfer co-efficient
goes through a minimum.
Heat transfer rate per unit area is called
heat flux
Log mean temperature difference (LMTD) cannot be used, if
none of these.
In a multipass shell and tube heat exchanger, tube side return pressure loss is equal to __________ the velocity head.
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
first increase and then decrease
In a parallel flow heat exchanger, if the outlet temperature of hot and cold fluids are the same, then the log mean temperature difference (LMTD) is
zero
For what value of Prandtl number, the Col-burn analogy is valid ?
0.6 to 120
In a forward feed multiple effect, the pressure build up will be
least at the outlet of the last effect.
Film boiling is usually not desired in commercial equipments, because
the heat transfer rate is low in view of the large temperature drop.
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
1
The variation of thermal conductivity of a metal with temperature is often correlated using an expression of the form K = K0 + at. where, K is the thermal conductivity and T is the temperature (in °K). The units of ‘a’ in SI system will be
W/m.k2
Film condensation is promoted on a/an __________ surface.
clean & smooth
Dietus-Boelter equation cannot be used for molten metals mainly due to its very low
Prandtl number
If Prandtl number is greater than the Schmidt number, then the
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 ?
Catchall
Rate of heat transfer by vaporisation from pools of water is affected by the
all (a), (b) and (c).
Multiple effect evaporators are commonly used in the manufacture of
P. Paper Q. Superphosphate
R. Sugar S. Fats
P and S
Which of the following has the highest thermal conductivity ?
Silver
