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
Which of the following is unimportant in forced convection ?
Grashhoff 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
vertical heat exchanger with steam on shell side.
In a multiple effect evaporator, the effect of boiling point elevation is to
reduce the capacity
Dropwise condensation occurs on __________ surfaces.
contaminated cooling
For specified tube outside diameter, higher BWG means higher
cross-sectional area
Pick out the wrong statement.
Swenson-Walker crystalliser is a batch crystalliser.
In case of a vertical tube evaporator, with increase in the liquor level, the __________ is increased.
neither (a) and (b)
Electromagnetic radiations propagate in vacuum with a velocity of __________ metre/second.
3 x l0^8
Heat flux is the time rate of heat transfer per unit
area
Film boiling occurs at __________ pressure.
very high
Heat flux through several resistances in series in analogous to the current flowing through several
resistances 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
constant temperature equal to that of the base
Vacuum is generally maintained in the vapour space of an evaporator mainly to
get economical temperature difference by using moderate pressure steam.
Double pipe heat exchangers are used
when heat transfer area required is very low, i.e (100-200 ft2).
Nusselt number (for forced convection heat transfer) is a function of the __________ number.
both (a) & (b)
Pick out the wrong statement.
Heat transfer by radiation can not occur across an absolute volume.
Which type of heat exchanger is preferred for heavy heat loads ?
Series and parallel set of shell and tube
Pick out the wrong statement.
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
between 0 and 1
The Grashoff number is defined as the ratio of the
buoyancy to viscous forces.
According to Reynolds analogy, Stanton number is equal to (where, f = Fanning friction factor)
f/2
The absorptivity of a body is equal to its emissivity
under thermal equilibrium.
Absorptivity and refletivity of a perfect black body are respectively
1 and 0
In thermal radiation for a black body (where, ε is emissivity and α is absorptivity)
α = 1; ε = 1
Pick out the wrong statement.
S.I. unit of fouling factor is Watt/m2.°K.
When warm and cold liquids are mixed, the heat transfer is mainly by
convection
A cube, sphere & a thin circular plate (all having same mass and made of same material) are all heated to 300°C and allowed to cool in natural air. Which one will cool the slowest ?
Sphere
Vent pipes are provided in a condenser to
remove non-condensable gases.
Heat transfer by natural convection is enhanced in system with
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.
pool
A body cools down from 75°C to 70°C in 10 minutes. It will cool down from 70° C to 65° C in __________ minutes.
> 10
In forced convection, the Nusselt number is a function of
Re and Pr
At what value of Prandtl number, the hydrodynamic and thermal boundary layers are identical ?
1
A dilute aqueous solution is to be concentrated in an evaporator system. High pressure steam is available. Multiple effect evaporator system is employed, because
total amount of vapor produced per Kg of feed steam in a multiple effect system is much higher than in a single effect.
Heat transfer efficiency leading of energy conservation in a heat exchanger can be achieved by
keeping the heat transfer surface clean.
Trap is used to remove __________ from steam pipe lines.
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
160
Tube pitch is the __________ of tube diameters and the clearances.
sum
With increase in the distance between the heat source and the object receiving the heat, the radiation heat transfer
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.
less
Natural convection is characterised by
Grashhoff number
Nusselt number for full developed, laminar, constant property flow in a pipe at uniform heat flux is
4.364
Thickness of thermal boundary layer is more compared to that of hydrodynamic boundary layer, when the value of Prandtl number is
< 1
The critical radius of insulation for cylindrical pipe is (where, hi = heat transfer coefficient at inside of the pipe )
K/h0
The heat flux in the nucleate boiling regimes is proportional to (where, ΔT = excess temperature)
(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
0
A body is called grey if the monochromatic emissivity of the body is
same for all wavelengths
In an extended surface heat exchanger, fluid having lower co-efficient
flows outside the tubes.
In a shell and tube heat exchanger, the tube side heat transfer co-efficient just at the entrance of the tube is
infinity
Grashoff number, which is defined as g . β(Ts - T∞) ρ2. L3/μ2, is proportional to the ratio of buoyancy force to __________ force.
viscous
Pick out the wrong statement.
Film boiling is desirable in commercial equipments due to high heat transfer rate at low temperature drop.
Graetz number is given by
mCp/kL
For a cold dilute feed to produce thick viscous liquor, backward feeding as compared to forward feeding results in
increased economy
Economy of an evaporator is influenced by the
both(b) & (c)
If the thermal conductivity of a wall material is independent of temperature, the steady state temperature distribution in the very large thin plane wall having steady, uniform surface temperature follows __________ law.
linear
In regenerative air preheater (as practised in heating of coke ovens), the heat is transferred
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.
Lee’s
For small temperature difference, the heat transfer rate as per Newton’s law of cooling is proportional to (where, Δt = excess temperature)
Δt
Pick out the wrong statement.
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.
agitated film
1 BTU/hr.ft.°F is equal to __________ kcal/hr. m.°C.
1.49
Heat transfer by radiation between two bodies at T1 & T2 and in an ambient temperature of Ta°C depends on
none of these
The ratio of momentum diffusivity to thermal diffusivity is the __________ number.
Prandtl
For a perfectly transparent surface (like gases), the
transmissivity = 1
For a fluid flowing in an annulus space, the wetted perimeter for heat transfer and pressure drop are
different
During crystallisation, formation of crystal can occur in __________ solution only.
supersaturated
An evaporator while concentrating an aqueous solution from 10 to 40% solids evaporates 30000 kg of water. The amount of solids handled by the system in kg is
4000
An insulator should have
low thermal conductivity.
In a backward feed multiple effect evaporator
feed flows from low pressure to high pressure.
Choose the correct equation.
Nu = (Re) (Pr) (St)
In a forward feed multiple effect evaporator unit
none of these.
In case of heat flow by conduction for a cylindrical body with an internal heat source, the nature of temperature distribution is
parabolic
For heat flow through very thick walled cylinder, use____mean radius.
logarithmic
Indirect contact heat exchangers are preferred over direct contact heat exchangers, because
there is no risk of contamination.
Water is normally used as a coolant in the heat exchange equipments mainly because ofits
abundance & high heat capacity
The rate of emission of radiation by a body does not depend upon the
shape and porosity of the body.
Which of the following has maximum thermal conductivity at the same temperature?
Steel
Maximum heat transfer rate is obtained in __________ flow.
turbulent
In case of __________ boiling, the liquid temperature is below the saturation temperature and the boiling takes place in the vicinity of the heated surface.
local
The ratio of velocity head to tube side return loss in case of a multipass shell and tube heat exchanger is
1/4
Terminal point temperature differences between fluids in case of a heat exchanger is termed as
approach
LMTD can’t be used as such without a correction factor for the
all (a) (b) and (c).
Fqra cold viscous feed, backward feed gives __________ than forward feed.
a higher capacity
Walls of a cubical oven are of thickness l, and they are made of material of thermal conductivity k. The temperature inside the oven is 100°C and the inside heat transfer co-efficient is 3k/l. If the wall temperature on the outside is held at 25°C, what is the inside wall temperature in degree centigrade ?
81.25
Overall heat transfer co-efficient of a particular tube is U1. If the same tube with some dirt deposited on either side has coefficient U2, then
U1 = U2
In SI units, thermal conductivity is expressed in
Watt/m.°K
Boiling point of a solution according to Duhring’s rule is a linear function of the __________ of water.
boiling point (at the same pressure)
The overall resistance for heat transfer through a series of flat resistance, is the __________ of the resistances.
sum
At what value of Prandtl number, conduction is negligible in the turbulent core of a fluid flowing through a heated pipe ?
> 0.6
A sphere of radius ‘R1’ is enclosed in a sphere of radius ‘R2’. The view (or shape) factor for radiative heat transfer of the outer sphere with respect to the inner sphere is
R2/(R1+R2)
For a laminar flow of fluid in a circular tube, ‘h1’ is the convective heat transfer co-efficient at velocity ‘V1’. If the velocity is reduced by half and assuming the fluid properties are constant, the new convective heat transfer co-efficient is
0.794 h1
The main purpose of providing fins on heat transfer surface is to increase the
heat transfer area.
A BTU/hr.ft.2 °F is equal to
4.88 kcal/hr. m.2°C.
Which of the following has the lowest Prandtl number ?
Molten sodium (a coolant used in fast breeder reactor)
In SI units, fouling factor is expressed in
m2°K/W
An ejector is used to
none of these
Extremely large or small volumes of fluids are generally best routed through the shell side of a shell and tube heat exchanger, because of the
flexibility possible in the baffle arrangement.
All analogy equations connecting friction factor and heat transfer co-efficient apply only to
wall or skin friction
Electro-magnetic spectrum range, which is important for radiation varies from __________ microns.
0.5 to 50
The characteristic dimensionless groups for heat transfer to a fluid flowing through a pipe in laminar flow are
Nu, Gz
Heat transfer rate described by Fourier’s law will decrease, if the __________ increases.
thickness
A black body does not __________ radiation.
both(b) & (c)
The capacity of double-effect evaporator is less than half of the capacity of two single effects, each of which is operating over same terminal temperature difference, when the
solution has an elevation of boiling point.
Peclet number (Pe) is given by
Pe = Re.Pr
Nusselt number is the ratio of the
heat flux at the wall to that across the entire pipe.
Prandtl number is the ratio of
momentum diffusivity to thermal diffusivity.
Which of the following parameters is increased by use of finned tube in a multipass shell and tube heat exchanger ?
All (a) (b) and (c).
Forced circulation evaporators are useful for the concentration of viscous, salting and scale forming liquors. Which of the following is a forced circulation evaporator?
Agitated film evaporator
“The ratio of the total emissive power to the absorptivity for all bodies is same at ther-mal equilibrium”. This is __________ law.
Kirchoffs
View factor is important in heat transfer by
radiation
The advantage of using a 1 - 2 shell and tube heat exchanger over a 1 - 1 shell and tube heat exchanger is
higher tube side heat transfer co-efficient.
For a multipass shell and tube heat exchanger, the LMTD correction factor is always
< 1
If heat transfer rate varies with the time, it is termed as
none of these
Pick out the wrong statement.
The Colbum jH factor for heat transfer is given by Nst Npr.
Tube expansion allowances exist in __________ heat exchanger.
U-tube
Pick out the wrong statement.
Forced convection is relatively more effective in increasing the rate of mass transfer, if Schmidt number is larger.
Agitated film evaporator is suitable for concentrating __________ liquids.
viscous
The thickness of condensate layer in filmwise condensation depends on the
all (a), (b) and (c)
If the thermal conductivity of a wall material is independent of temperature, the steady state temperature distribution in the very large thin plane wall having steady, uniform surface tempeature follows __________ law.
parabolic
Intermittant tube cleaning is possible to be done in case of a __________ evaporator.
basket type
Arithmetic mean area can be used in heat transfer problem to calculate the heat flow by conduction through a cylinder which is
thin walled having the value of Ao Ai/< 2.
Three solid objects of the same material and of equal mass-a sphere, a cylinder (length = diameter) and a cube are at 500°C initially. These are dropped in a quenching bath containing a large volume of cooling oil each attaining the bath temperature eventually. The time required for 90% change in temperature is the smallest for
cube
Overall heat transfer co-efficient for cooling of hydrocarbons by water is about
50 -100 BTU/hr. ft.2°F
Absorptivity of a perfect black body is unity. Which of the following has maximum absorptivity ?
coke breeze
Thermal diffusivity is given by
k/pCp
At what value of Prandtl number, the hydrodynamic and thermal boundary layers of a fluid flowing over a heated plate will be identical ?
1
Fourier’s law of heat conduction applies to __________ surfaces.
both (a) and (b)
__________ chart is known as transient heat conduction chart.
Dirhing’s
Choose the most important factor on which the heat conducted through a wall in a unit time will depend on?
Temperature difference between the two surfaces of the wall.
Which of the following has the minimum thermal conductivity ?
Nitrogen
In a heat exchanger, one transfer unit means
a section of the exchanger in which change in temperature of one stream equals the average driving force in the section.
For condensation of pure vapors, if the heat transfer co-efficients in filmwise and drop-wise condensation are respectively hf and hd, then
hf < hd
Pick out the wrong statement:
Heat sensitive materials should be concentrated in high pressure evaporators.
For concentrating an aqueous solution of a material like anhydrous Na2SO4, whose solubility decreases with rise in temperature, the most suitable evaporator is a __________ evaporator.
vacuum
Circulation pump is located below the evaporater to
create more suction head.
The statement that “maximum wavelength of radiation is inversly proportional to the temperature” is __________ law.
Wien’s displacement
Steam condensate is recovered by steam traps and recycled for use as boiler feed water, because of its low
all (a), (b) and(c)
Duhring rule is important in solving problems on
evaporation
In a shell and tube heat exchanger, the height of 25 percent cut baffles is equal to (where, D = inside diameter of shell)
0.75 D
Multiple effect evaporation is generally recommended, when the
large scale evaporation of liquor is needed.
The inside heat transfer co-efficient in case of turbulent flow of liquid in the tube side in a 1-2 shell and tube heat exchanger is increased by __________ times, when the number of tube passes is increased to 8.
4^__________ heat exchanger is the most suitable, when the temperature of shell side fluid is much higher than that of tube side.
0.8
__________ heat exchanger is the most suitable, when the temperature of shell side fluid is much higher than that of tube side.
U-tube
In which mode of heat transfer, the Biot number is important ?
Transient heat conduction
__________ paint has the minimum absorption co-efficient.
White lead
Asymptotic conditions is reached, when for a fluid flowing in laminar flow through a long tube
exit fluid temperature = wall temperature.
1000 kg of wet solids are to be dried from 60% to 20% moisture (by weight). The mass of moisture removed in kg is
400
Which is the most suitable for the con-certration of highly concentrated solution?
Open pan evaporation
Unsteady state heat conduction occurs, when
temperature distribution is dependent on time.
What is the absorptivity of a black body ?
1
Mode of heat transfer in which the fluid moves under the influence of changes in fluid pressure produced by external work is called
forced convection
Baffle spacing
should be less than the inside diameter of the shell.
Boiling point elevation of a solution of NaOH
both (b) and (c).
The advantage of backward feed multiple effect evaporators over forward feed units is that
most concentrated liquid is at highest temperature.
Which of the following is the most controlling factor for the rate of bubble detachment from the hot solid surface ?
Interfacial tension
Which characteristic of a fluid is not important in deciding its route in a shell and tube heat exchanger ?
None of these
Thermal conductivities of most of the liquids __________ with rise in temperature.
decreases
A metal ball of radius 0.1 m at a uniform temperature of 90°C is left in air at 30°C. The density and the specific heat of the metal are 3000 kg/m3 and 0.4 kJ/kg.K respectively. The heat transfer co-efficient is 50 W/m2.K Neglecting the temperature gradients inside the ball, the time taken (in hours) for the ball to cool to 60°C is
0.15
The purpose of floating head in a heat exchanger is to
avoid buckling of tubes.
1000 Kg of liquid at 30°C in a well stirred vessel has to be heated to 120°C, using immersed coils carrying condensing steam at 150°C. The area of the steam coils is 1.2 m2 and the overall heat transfer co-efficient to the liquid is 1500 W/m2.°C. Assuming negligible heat loss to the surrounding and specific heat capacity of the liquid to be 4 kJ/kg.°C, the time taken for the liquid to reach desired temperature will be
51 min
Overall thermal resistance for conductive heat transfer through a series of flat resistances is equal to the
sum of all resistances.
Double pipe heat exchangers are preferably useful, when
requirement of heat transfer area is low.
Removal of __________ heat is involved in the condensation of a vapor under saturated conditions.
latent
The rate of heat transfer from a vertical plate by natural convection depends upon the temperature differences (ΔT) between wall and outside bulk. The proportionality is given as
(ΔT)^3/4
If air (a non-condensing gas) is present in a condensing vapor stream, it will __________ the condensation rate of vapor.
decrease
Prandtl and Reynold’s analogies are identical for Prandtl number value of
1
The outlet temperature of cooling water in a heat exchanger is generally not allowed to exceed above 50°C in industrial practice mainly to avoid
excessive corrosion
In evaporators, lowering the feed temperature
both (a) and (b).
The type of liquor circulation system to be ‘ employed in evaporators (viz. natural or forced circulation) is determined mainly by the __________ of the liquid.
viscosity
In case of surface condensers, a straight line is obtained on plotting __________ on an ordinary graph paper.
1/V^0.8
Thermal diffusivity is the most important in heat transfer by
conduction
(NGr x NPr) is called the __________ number.
Reyleigh
A dephlegmator is a
partial condenser
A tank painted with which of the following coloured paints, would heat up maximum by radiation from sun ?
Black paint
The separation of liquid droplets from the vapor is done by a/an __________ , in the evaporators.
entrainment separator
Fouling factor for a heat exchanger is given by (where, U1 = heat transfer co-efficient of dirty surface U2 = heat transfer co-efficient of clean surface)
1/U1 - 1/U2
If a single tube pass heat exchanger is converted to two pass, then for the same flow rate, the pressure drop per unit length in tube side will
increase by 2^16
A steel sphere of radius 0.1 m at 400°K is immersed in an oil at 300°K. If the centre of the sphere reaches 350°K in 20 minutes, how long will it take for a 0.05 m radius steel sphere to reach the same temperature (at the centre) under identical conditions ? Assume that the conductive heat transfer co-efficient is infinitely large.
5 minutes
Fluid motion in the natural convection heat transfer between a solid surface and a fluid in contact with it, results from the
none of these.
Evaporator tubes are generally
vertical
For a counter current heat exchanger with Tih = 80°C, T°c = 60°C, T°h = 50°C and Tic = 30°C, and the temperature difference between the two streams being the same everywhere along Z, the direction of flow of hot fluid. The temperature profile should satisfy
d^2T/dZ^2 = 0
In a boiling curve, the peak heat flux is called the __________ point.
burnout
Pick out the wrong statement.
Filmwise condensation takes place on non-wettable surfaces.
h.D/K is called the __________ number.
Nusselt
Prandtl number is the ratio of
momentum diffusivity to thermal diffusivity.
Conductance is given by(where, x = thickness, A = heat flow area, K = thermal conductivity.)
KA/x
Heat flux, as defined in heat flow is analogous to __________ in electricity flow.
current
The wavelength at which the maximum monochromatic emissive power occurs for a black body, is (where, T = absolute temperature of the black body)
α 1/T
In a shell and tube heat exchanger, putting a longitudinal baffle across the shell, forces the shell side fluid to pass __________ through the heat exchanger.
twice
When the ratio of the Grashoff number and to the square of Reynolds number is one, the dominant mechanism of heat transfer is
mixed convection (both free and forced)
The purpose of providing bleed points in the evaporator is to
facilitate removal of non-condensable gases
Water always boils when its
saturated vapour pressure equals the external pressure on its surface.
A multiple effect evaporator as compared to a single effect evaporator of the same capacity has
higher steam economy.
Which is the most suitable for cold viscous feed?
Backward feed
Dropwise condensation is promoted on a/an __________ surface.
oily
At Pr > 1, conduction in an ordinary fluid flowing through a heated pipe is limited to the
viscous sub-layer
In a single evaporator system, the steam economy __________ by creating vacuum in the evaporator.
increases
Nucleate boiling is promoted
on rough surfaces
In forced circulation, the heating element is injected
internally
If all the conditions and dimensions are same, then the ratio of velocity through the tubes of a double pass heat exchanger to that through the single pass heat exchanger is
2
The equation, Nst = f/2, is the __________ analogy.
Reynolds
Boiling point elevation for a strong and concentrated solution is found by Duhring’s rule, which states that at the same pressure, the boiling point of a solution is a linear function of the __________ of pure water.
boiling point
Rate of crystallisation does not depend upon the
none of these.
Stefen’s block body radiation law can also be derived from __________ law.
Planck’s
Multipass heat exchangers are used
to obtain higher heat transfer co-efficient and shorter tube.
A hollow sphere and a solid sphere of the same material and equal radii are heated to the same temperature. In this case,
both(b) & (c).
Condensing film co-efficient for steam on horizontal tubes ranges from 5000 to 15000 Kcal/hr.m2 .°C. Condensation of vapor is carried out inside the tube in a shell and tube heat exchanger, when the
condensate is corrosive in nature.
When does the heat generated by fluid friction becomes appreciable compared to the heat transferred between the fluids ?
At high fluid velocity
The Sieder-Tate correlation for heat transfer in turbulent flow in pipe gives Nu α Re0.8, where, Nu is the Nusselt number and Re is the Reynolds number for the flow. Assuming that this relation is valid, the heat transfer co-efficient varies with the pipe diameter (D) as
(D))^-0.2
A long iron rod initially at a temperature of 20°C has one end dipped in boiling water (100°C) at time, t = 0. The curved surface of the rod is insulated so that heat conduction is one dimensional in the axial direction. The temperature at a distance 100 mm from the dipped end becomes 40°C at time, t = 200 s. The same temperature is achieved at a distance of 200 mm from the dipped end at time
t = 800 s
LMTD for evaporators & condensers for a given terminal parameters & set of conditions for counterflow is equal to that for parallel flow. In such heat exchangers, with one of the fluids condensing or evaporating, the surface area required is the least in the __________ flow.
same in either ‘a’, b’ or ‘c’
Thermal diffusivity of a material
all (a), (b) and (c).
Viscosity of gases __________ with increase in temperature.
increase slowly
What is the emissivity of a black body ?
1
Steam is routed through the tube in case of a __________ evaporator.
horizontal tube
Dropwise condensation of steam on cooling surface is promoted
an oily surface.
Conduction occurs in the buffer zone for a fluid flowing through a heated pipe, only when Prandtl number is
0.1
In a heat exchanger, the rate of heat transfer from the hot fluid to the cold fluid
varies directly as the area and the LMTD.
With increase in temperature, the total emissivity of conductors
increases
Loss of heat from untagged steam pipe to the ambient air is by
all (a), (b) & (c)
Fruit juice (a heat sensitive material) can be concentrated in a __________ evaporator.
falling film
Tubes are held between top and bottom tube sheets in Calenderia type evaporator by keeping
both the tube sheets fixed.
A single pass air heater is connected to a two pass unit. For the air flow rate and other conditions remaining the same, the film heat transfer co-efficient for air will vary in the ratio of
2^Heat sensitive materials can be concentrated in an evaporator employing
0.8
Heat sensitive materials can be concentrated in an evaporator employing
vacuum
A concentric double pipe heat exchanger as compared to the shell and tube heat exchanger for the same heat load requires
more space.
In a shell and tube heat exchanger, floating head is used for
large temperature differentials.
Steam trap is used to
remove water resulting from partial condensation of steam.
The average heat transfer co-efficient over the entire length of the plate (ha) and the local heat transfer co-efficient (hL), in case of heat transfer over a flat plate in lminar zone is related as
ha = 2hL
Maximum heat transfer rate is achieved in __________ flow.
turbulent
Wavelength corresponding to the maximum energy is inversely proportional to the absolute temperature. This is __________ law.
Wien’s
Three material A, B and C of equal thick-nes and of thermal conductivity of 20, 40 & 60 kcal/hr. m. °C respectively are joined together. The temperature outside of A and C are 30°C and 100°C respectively. The interface between B and C will be at a temperature of __________ °C.
70
Which of the following parameters of the fluid is not very important, while deciding its route in a shell and tube heat exchanger?
density
Corrosiveness of steam condensate is due to the presence of
both (a) & (b)
The equation, (NSt x N2/3Pr) = f/2, is the __________ analogy.
Colburn
The driving potential for the crystal growth during crystallisation is the __________ of the solution.
super-saturation
jH factor for heat transfer depends upon the __________ number.
Reynolds
Kg of liquid evaporated per hour in an evaporator is defined as its
capacity
Pick out the correct statement.
In steady state heat conduction, the only property of the substance which determines the temperature distribution, is the thermal conductivity.
Open pan evaporators are preferred to be used, when the solution to be concentrated is
highly viscous
Cp u/K
Prandtl
The maximum heat transfer co-efficient from steam heating will be attained when the steam is
saturated
Kirchoff’s law is applicable to
both (a) and (b)
In a forward feed multiple effect evaporator, the pressure is
lowest in last effect
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
remain unchanged
Which of the following situations can be approximated to a steady state heat transfer system ?
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.
Sensible heat absorbed by 1 lb of water when it is heated from 32 to 212°F may be around __________ BTU.
180
The unit of conductance in SI unit is
W/°K
In a shell and tube heat exchanger, square pitch compared to triangular pitch
gives a lower shell side pressure drop.
Which of the following has maximum thermal conductivity ?
Iron
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.
12
The film co-efficient between condensing vapour and metal wall increases with
increasing temperature of the vapour.
Critical value of the __________ number governs the transition from laminar to turbulent flow in free convection heat transfer.
Prandtl & Grashoff
The thermal boundary layer at NPr > 1
is thinner than hydrodynamic boundary layer.
In the free convection regime of pool boiling, the heat flux is proportional to
Δt5/4
With increase in temperature, the thermal conductivity of fresh lubricating oil
decreases
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
0.7
Nusselt number is the ratio of the temperature gradient at the wall to
that across the entire pipe
The Stefan-Boltzman constant depends on the
none of these
The units of resistance to heat transfer is
W-1m2K
Thermal conductivity of a gas at low density, __________ with increase in temperature.
increases
Nusselt number is a function of Prandtl number and __________ number of fluid in natural convection heat transfer.
Grashoff
__________ heat exchanger is used for chilling oil to be dewaxed.
Double pipe
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
< 0.75
Pick out the wrong statement.
Two identical cubes of iron and copper will have the same heat content under the same conditions of temperature.
In sub-cooled boiling,
temperature of the heating surface is more than the boiling point of the liquid.
With increase in temperature, the thermal conductivity of a gas
increases
