Heat Transfer

Question 1

Crystal size in a continuous crystallizer depends upon the

rate of heat transfer.
degree of turbulence.
degree of supersaturation.
all (a), (b) and (c).

Answer 1

all (a), (b) and (c).

Question 2

Heat waves

can not pass through vacuum.
travel in straight line.
can be reflected by a mirror.
both (b) and (c).

Answer 2

both (b) and (c).

Question 3

Air is best heated with steam in a heat exchanger of

plate type.
double pipe type with fin on steam side.
double pipe type with fin on air side.
shell and tube type.

Answer 3

double pipe type with fin on air side.

Question 4

Which of the following is generally considered as opaque surface towards radiations?

Gases
Solids
Liquids
Both (b) and (c)

Answer 4

Both (b) and (c)

Question 5

Black liquor generated during paper manufacture is concentrated in a

single effect evaporator.
single effect evaporator followed by a crystalliser.
multiple effect evaporator.
multiple effect evaporators followed by a crystalliser.

Answer 5

multiple effect evaporator.

Question 6

When vaporisation takes place directly at the heating surface, it is called

film boiling
nucleate boiling
vapour binding
none of these

Answer 6

nucleate boiling

Question 7

Steam consumption in kg/hr in case of an evaporator is given by (where, C & E are capacity and the economy of the evaporator respectively)

C/E
E/C
CE
1/CE

Answer 7

C/E

Question 8

Reynold’s analogy states that

Nst α f
Nst α NRe
N/Nu α f
NRe α f

Answer 8

Nst α f

Question 9

With increase in temperature, the thermal conductivity of non-metallic amorphous solids

decreases
increases
remains constant
first decreases upto certain temperature and then increases

Answer 9

increases

Question 10

Fourier’s law applies to the heat transfer by

convection
radiation
conduction
all (a), (b) & (c)

Answer 10

conduction

Question 11

Shell side pressure drop in a shell and tube heat exchanger does not depend upon the

baffle spacing & shell diameter.
tube diameter & pitch.
viscosity, density & mass velocity of shell side fluid.
none of these.

Answer 11

none of these.

Question 12

In the equation Q = UAΔt; Δt is

geometric mean temperature difference.
arithmetic mean temperature difference.
logarithmic mean temperature difference.
the difference of average bulk temperatures of hot and cold fluids.

Answer 12

logarithmic mean temperature difference.

Question 13

With increase in porosity, the thermal conductivity of a solid substance

increases
decreases
remains unchanged
may increase or decrease; depends on the solid

Answer 13

decreases

Question 14

For an ideal black body

absorptivity = 1
reflectivity = 1
emissivity = 0
transmissivity = 1

Answer 14

absorptivity = 1

Question 15

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

40.5
34.4
26.8
25

Answer 15

34.4

Question 16

Fouling factor

is a dimensionless quantity.
does not provide a safety factor for design.
accounts for additional resistances to heat flow.
none of these.

Answer 16

accounts for additional resistances to heat flow.

Question 17

In case of vertical tube evaporator, with increase in liquor level, the overall heat transfer co-efficient

increases
decreases
is not affected
may increase or decrease; depends on the feed

Answer 17

decreases

Question 18

The steam ejector is used to

remove condensate from the steam pipelines.
create vacuum.
superheat the steam.
none of these.

Answer 18

create vacuum.

Question 19

For shell and tube heat exchanger, with increasing heat transfer area, the purchased cost per unit heat transfer area

increases
decreases
remains constant
passes through a maxima

Answer 19

passes through a maxima

Question 20

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 = (COP)HP = 0.6
(COP)R = 2.5; (COP)HP = 1.5
(COP)R = 1.5; (COP)HP = 2.5
(COP)R = (COP)HP = 2.5

Answer 20

(COP)R = 1.5; (COP)HP = 2.5

Question 21

Hot water (0.01 m^3 /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 m^3/min) of density 800 kg/m^3 and specific heat of 2 kJ/kg.K enters at 20°C. The log mean temperature difference in °C is approximately

32
37
45
50

Answer 21

32

Question 22

In a heat exchanger, floating head is provided to

facilitate cleaning of the exchanger.
increase the heat transfer area.
relieve stresses caused by thermal expansion.
increase log mean temperature gradient.

Answer 22

relieve stresses caused by thermal expansion.

Question 23

Leidenfrost point is a term concerned with the

condensation of the saturated vapor on a cold surface.
concentration of a corrosive solution by evaporation.
heat transfer between two highly viscous liquids.
boiling of a liquid on a hot surface.

Answer 23

boiling of a liquid on a hot surface.

Question 24

A process stream of dilute aqueous solution flowing at the rate of 10 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.
counter flow with process stream on tube side.
parallel flow with process stream on shell side.
parallel flow with process stream on tube side.

Answer 24

counter flow with process stream on shell side.

Question 25

Heat produced when a steady state current, I passes through an electrical conductor having resistance, ‘R’ is

IR
I^2 R
IR^2
I^2 R^2

Answer 25

I^2 R

Question 26

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.

Answer 26

Orifice baffles are never used in a shell and tube heat exchanger.

Question 27

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.

laminar
transition
both ‘a’ & ‘b’
highly turbulent

Answer 27

highly turbulent

Question 28

The actual temperature drop across the heating surface in an evaporator depends on the

feed.
depth of liquid over heating surface.
pressure difference between steam chest and vapour space.
all (a), (b) and (c).

Answer 28

all (a), (b) and (c).

Question 29

Which of the following is not used as a medium for high temperature heating ?

Dowtherm
mercury
liquid metal (e.g. molten sodium)
fused salts (e.g., an eutectic mixture of 53% KNO3, 40% NaNO2and 7% NaNO3)

Answer 29

mercury

Question 30

Presence of a non-condensing gas in a condensing vapour

increases the rate of condensation.
decreases thermal resistance.
is desirable to increase the film co-efficient.
none of these.

Answer 30

none of these.

Question 31

Kirchoff s law applies to __________ radiation.

total
monochromatic
both (a) & (b)
neither (a) nor (b)

Answer 31

both (a) & (b)

Question 32

With the increase of temperature, the Col-burn jH factor

increases.
decreases.
remains unchanged.
may increase or decrease ; depending on temperature.

Answer 32

increases

Question 33

In case of evaporators, liquid entrainment results primarily due to

high vacuum in the evaporator.
high evaporation rate.
foaming of the solution.
high heat transfer rate.

Answer 33

foaming of the solution.

Question 34

Baffles in the shell side of a shell and tube heat exchanger

increase the cross-section of the shell side liquid.
force the liquid to flow parallel to the bank.
increase the shell side heat transfer co-efficient.
decrease the shell side heat transfer co-efficient.

Answer 34

increase the shell side heat transfer co-efficient.

Question 35

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 inside and steam outside.
finned tube heat exchanger with air outside and steam inside.
shell and tube heat exchanger with air inside tubes and steam on shell side.
shell and tube heat exchanger with air on shell side and steam inside tubes.

Answer 35

finned tube heat exchanger with air outside and steam inside.

Question 36

The unit of heat transfer co-efficient in SI unit is

J/M^2°K
W/m^2°K
W/m°K
J/m°K

Answer 36

W/m^2°K

Question 37

The number of kg vaporised per kg of steam fed to the evaporator is defined as

capacity
rate of evaporation
economy
rate of vaporisation

Answer 37

economy

Question 38

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.

Answer 38

The equivalent diameter of heat transfer for a duct of square cross-section (having each side as ‘x’) is equal to 4x.

Question 39

Economy of a multiple effect evaporator is not influenced much by the

boiling point elevations
temperature of the feed
rate of heat transfer
ratio of the weight of the thin liquor to thick liquor

Answer 39

boiling point elevations

Question 40

Increasing the liquor level in the evaporator results in the

decreased capacity.
increase in liquor film co-efficient.
decreased effect of hydrostatic head.
increased true temperature drop.

Answer 40

decreased capacity.

Question 41

The equivalent diameter for pressure drop is __________ that for heat transfer.

smaller than
greater than
equal to
not related with

Answer 41

smaller than

Question 42

Controlling heat transfer film co-efficient is the one, which offers __________ resistance to heat transfer.

no
the least
the largest
lower

Answer 42

the largest

Question 43

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/m^2. The temperature of the inside wall is - 5°C. The outside wall temperature is

25°C
30°C
35°C
40°C

Answer 43

35°C

Question 44

Thermal conductivity of a conducting solid material depends upon its

temperature
porosity
both (a) & (b)
neither (a) nor (b)

Answer 44

both (a) & (b)

Question 45

Pick out the wrong statement.

O The controlling resistance in case of heating of air by condensing steam is in the air film.
O 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.
O In case of a 1 - 2 shell and tube heat exchanger, the LMTD correction factor value increases sharply, when a temperature cross occurs.
O Phase change in case of a pure fluid at a given pressure from liquid to vapor or vice-versa occurs at saturation temperature.

Answer 45

In case of a 1 - 2 shell and tube heat exchanger, the LMTD correction factor value increases sharply, when a temperature cross occurs.

Question 46

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)^2
(ΔT)^1/2
(ΔT)^1/4
(ΔT)^3/2

Answer 46

(ΔT)^1/4

Question 47

Bulk of the convective heat transfer resistance from a hot tube surface to the fluid flowing in it, is

in the central core of the fluid.
uniformly distributed throughout the fluid.
mainly confined to a thin film of fluid near the surface.
none of these.

Answer 47

mainly confined to a thin film of fluid near the surface.

Question 48

Which of the following is concerned with both heat and mass transfer ?

Lewis relationship
Nusselt number
Kutateladze number
Froude number

Answer 48

Lewis relationship

Question 49

A backward feed multiple effect evaporator is better than forward feed for concentrating cold feed, because it provides

higher economy
lower capacity
both (a) & (b)
lower economy

Answer 49

higher economy

Question 50

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

greater than that for uninsulated steam pipe.
less than that of the uninsulated steam pipe.
equal to that of the uninsulated steam pipe.
less than the steam pipe with 5 cms insulation.

Answer 50

less than that of the uninsulated steam pipe.

Question 51

Which area is used in case of heat flow by conduction through a cylinder?

Logarithmic mean area
Arithmetic mean area
Geometric mean area
None of these

Answer 51

Logarithmic mean area

Question 52

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 in physical contact with each other.
between two bodies not in physical contact with each other.
from one part of a body to the another part of the same body.
both ‘b’ & ‘c’

Answer 52

between two bodies not in physical contact with each other.

Question 53

The dimensionless group in mass transfer that is equivalent to Prandtl number in heat transfer is

Nusselt number.
Sherwood number.
Schmidt number.
Stanton number.

Answer 53

Schmidt number.

Question 54

Finned tube heat exchangers

give larger area per tube.
use metal fins of low thermal conductivity.
facilitate very large temperature drop through tube wall.
are used for smaller heat load.

Answer 54

give larger area per tube.

Question 55

The absorptivity of a grey body at a given temperature __________ with increasing wavelength of radiation.

increases
decreases
remains constant
may increase or decrease ; depends on the material

Answer 55

may increase or decrease ; depends on the material

Question 56

Which one gives the monochromatic emissive power for black body radiation ?

Planck’s law
Kirchhoffs law
Wien’s law
Stefan-Boltzman law

Answer 56

Planck’s law

Question 57

For what value of Prandtl number, St = f/2?

1.5
1
> 1
< 1

Answer 57

1

Question 58

In a shell and tube heat exchanger,

the temperature drops in the two fluids and the wall are proportional to individual resistances.
the temperature drop is inversely proportional to the resistance across which the drop occurs.
there is no relationship between temperature drop and resistance.
the relationship is not generalised.

Answer 58

the temperature drops in the two fluids and the wall are proportional to individual resistances.

Question 59

Dietus-Boelter equation used for the determination of heat transfer co-efficient is valid

for fluids in laminar flow.
for fluids in tubulent flow.
when Grashhoff number is very important.
for liquid metals.

Answer 59

for fluids in tubulent flow.

Question 60

The equation, Nst = (f/2) / [1+5 (NPr - 1) (f/2)^1/2], corrosponds to __________ analogy.

Von-Karmann
Reynolds
Colburn
Prandtl

Answer 60

Prandtl

Question 61

For large heat transfer area requirement, shell and tube heat exchanger is preferred, because it

occupies smaller space.
is more economical.
is easy to operate and maintain.
all (a), (b) and (c).

Answer 61

all (a), (b) and (c).

Question 62

Out of the following four assumptions used in the derivation of the equation for LMTD [LMTD = (Δt1 - Δt2) / ln(Δt1/Δt2)] , which one is subject to the largest deviation in practice ?

Constant overall heat transfer co-efficient.
Constant rate of fluid flow.
Constant specific heat.
No partial phase change in the system.

Answer 62

Constant rate of fluid flow.

Question 63

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

convection becomes important
conduction becomes important
radiation becomes important
sub-cooled boiling occurs

Answer 63

radiation becomes important

Question 64

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. )

t
1/ρ
1/A
all (a), (b) & (c)

Answer 64

all (a), (b) & (c)

Question 65

Temperature profile in steady state heat transfer is

asymptotic
hyperbolic
parabolic
linear

Answer 65

linear

Question 66

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

decreases the pressure drop.
decreases the outlet temperature of cooling medium.
increases the overall heat transfer coefficient.
none of these.

Answer 66

increases the overall heat transfer coefficient.

Question 67

When vaporisation takes place through a blanketting film of gas, the phenomenon is termed as __________ boiling.

pool
nucleate
transition
film

Answer 67

film

Question 68

Minimum recommended baffle spacing in a shell and tube heat exchanger is about (where, D = shell diameter)

0.2 D
0.5 D
0.66 D
0.80 D

Answer 68

0.2 D

Question 69

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 )

t^4
T^4
1/t^4
1/T^4

Answer 69

T^4

Question 70

The Nusselt number for fully developed (both thermally and hydrodynamically) laminar flow through a circular pipe, where the wall heat flux is constant, is

2.36
4.36
120.36
dependent on NRe only

Answer 70

4.36

Question 71

What is the unit of thermal conductivity ?

Kcal/hr. m^2 °C
Kcal/hr.m.°C
Kcal/hr.m
Kcal/hr. °C

Answer 71

Kcal/hr.m.°C

Question 72

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

black painted bodies only
all bodies
polished bodies only
a black body

Answer 72

all bodies

Question 73

It is not preferable to use superheated steam in evaporators, because of its very

high temperature
high pressure
low film co-efficient
none of these

Answer 73

low film co-efficient

Question 74

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 10^5
6.12 x 10^5
7.24 x 10^5
9.08 x 10^5

Answer 74

3.06 x 10^5

Question 75

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).
outlet temperature of both the hot and cold fluid are same.
outlet temperature of hot fluid is less than the outlet temperature of the cold fluid.
none of these.

Answer 75

cold fluid is heated to a certain temperature by condensing steam (isothermal fluid).

Question 76

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)

NPr
NPr^(1/3)
NPr^(-1)
NPr^(-1/3)

Answer 76

NPr^(1/3)

Question 77

The range of electromagnetic spectrum important in heat transfer by radiation is __________ microns.

0.38-0.78
0.5-50
100-1000
5-50

Answer 77

0.5-50

Question 78

Use of transverse baffles in a shell and tube heat exchanger is done to increase the

rate of heat transfer.
flow velocity.
turbulence of shell side fluid.
all (a), (b) and (c).

Answer 78

all (a), (b) and (c).

Question 79

The actual temperature drop across the heating surface of an evaporator depends on the

liquid depth over the heating surface.
solution being evaporated.
pressure difference between the steam chest and the vapor space above the boiling liquid.
all (a), (b) and (c)

Answer 79

all (a), (b) and (c)

Question 80

Economy of a multiple effect evaporator depends upon the

heat balance consideration
rate of heat transfer
both (a) and (b)
neither (a) nor (b)

Answer 80

heat balance consideration

Question 81

The Graetz number is concerned with the

mass transfer between a gas and a liquid.
absorption with chemical reaction.
heat transfer in turbulent flow.
heat transfer in laminar flow.

Answer 81

heat transfer in laminar flow.

Question 82

Solid angle subtended by the finite surface at the radiating element is

called the view factor.
called the angle of vision.
proportional to the square of the distance between surfaces.
expressed in terms of radians.

Answer 82

called the angle of vision.

Question 83

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.

Answer 83

The emissivity of a surface decreases, if it gets corroded by atmospheric environment.

Question 84

In forced convection, the heat transfer depends on

Re, Pr
Re, Gr
mainly Gr
Re only

Answer 84

Re, Pr

Question 85

With increase in temperature, the thermal conductivity of most liquids

increases
decreases
remains same
first increases upto a certain temperature and then becomes constant

Answer 85

decreases

Question 86

Heat transfer by __________ may not necessarily require the presence of a medium.

conduction
natural convection
forced convection
radiation

Answer 86

radiation

Question 87

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.

Answer 87

Logarithmic mean area is used for calculating the heat flow rate through a thick walled cylinder.

Question 88

In a shell and tube type heat exchanger, the floating tube bundle heat arrangement is used

in low range of temperature differences.
in high range of temperature differences.
because of its low cost.
to prevent corrosion of the tube bundles.

Answer 88

in high range of temperature differences.

Question 89

Stefan-Boltzman law applies to __________ body.

black
white
grey
any colour

Answer 89

black

Question 90

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

more
less
some
either more or less ; depends on the nature of vapor

Answer 90

some

Question 91

A perfect black body is a perfect __________ of radiation.

absorber
emitter
both (a) & (b)
neither (a) nor (b)

Answer 91

both (a) & (b)

Question 92

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
more than
less than
either more or less than (depending upon the fluid)

Answer 92

equal to

Question 93

An equipment which converts the latent or sensible heat of one fluid into the latent heat of vaporisation of another, is called a

boiler
heat exchanger
recuperator
regenerator

Answer 93

heat exchanger

Question 94

Which of the following is directly concerned with the heat transfer ?

Strouhal number
Sherwood number
Euler number
Grashoff number

Answer 94

Grashoff number

Question 95

Pick 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/m^2.hr.°K^4.

Answer 95

With change in temperature, the radiant energy emitted by a black body remains unchanged.

Question 96

Planck’s distribution law is valid for __________ bodies.

black
white
coloured
all (a), (b) & (c)

Answer 96

coloured

Question 97

For hot feed, forward feeding as compared to backward feeding results in __________ economy.

increased
decreased
no effect on
none of these

Answer 97

increased

Question 98

Radiation energy is emitted by all the substances, which are above

0K
0°C
100°C
room temperature

Answer 98

0K

Question 99

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
1/εx + 1/εy
εx+εy / εx+εy - εx * εy

Answer 99

εx+εy / εx+εy - εx * εy

Question 100

What is the steam economy in case of a single effect evaporator system ?

1
>1
< 1
0.1

Answer 100

< 1

Question 101

If average heat transfer co-efficient is ha and the local coefficient at the end of the plate is hl then in case of heat transfer to a fluid flowing over a flat plate, heated over its entire length

ha = hl
ha = 2hl
ha = 0.5 hl
ha = 0.75 hl

Answer 101

ha = 2hl

Question 102

jH factor for heat transfer is not a function of the __________ number.

Reynolds
Nusselt
Grashoff
both (b) & (c)

Answer 102

both (b) & (c)

Question 103

The Fourier number (defined as a.t/L^2) is used in the analysis of problem involving heat transfer by

forced convection
natural convection
transient conduction
steady state conduction

Answer 103

transient conduction

Question 104

The energy radiated from a surface Q at absolute temperature T is related as

Q ∝ T^2
Q ∝ T^4
Q ∝ T^3
none of these

Answer 104

Q ∝ T^4

Question 105

Baffles are provided in heat exchangers to increase the

fouling factor
heat transfer area
heat transfer co-efficient
heat transfer rate

Answer 105

heat transfer co-efficient

Question 106

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/m^2.s. The initial moisture content of the solid is 0.2 kg moisture/kg dry solid. The interfacial area available for drying is 4 m^2/1000 kg of dry solid. The moisture content at the end of the drying period is (in kg moisture/kg dry solid)

0.5
0.05
0.1
0.15

Answer 106

0.1

Question 107

In case of a vertical tube evaporator, with increase in the liquor level, the

capacity of the evaporator is decreased.
capacity of the evaporator is increased.
true temperature drop increases.
both (b) and (c).

Answer 107

capacity of the evaporator is decreased.

Question 108

The ratio of total radiating power to the absorptivity of the body depends upon the __________ as per Kirchoffs law.

wavelength of radiation
nature of the body
temperature of the body
none of these

Answer 108

temperature of the body

Question 109

For gases, the thermal conductivity increases with temperature rise. For liquids, with increase in concentration, its thermal conductivity generally

decreases
increases
remains unchanged
increases exponentially

Answer 109

decreases

Question 110

A 2-4 heat exchanger involves

only counter-flow of fluids.
only parallel-flow of fluids.
both counter and parallel-flow of the fluids.
smaller pressure drop compared to 1-2 exchanger.

Answer 110

both counter and parallel-flow of the fluids.

Question 111

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
> 1
1
∞

Answer 111

< 1

Question 112

With increase in temperature, the thermal conductivity of steel

increases
decreases
remains unchanged
increases exponentially

Answer 112

decreases

Question 113

Which of the following forced convection heat transfer equation accounts for the liquid viscosity effect for viscous liquids ?

Dittus-Boeltier equation
Sieder-Tate equation
Nusselt equation
None of these

Answer 113

Sieder-Tate equation

Question 114

The main function of baffles provided in a shell and tube heat exchanger is to

facilitate the cleaning of outer tube surface.
enhance turbulence.
hold the tubes in position.
all ‘a’, ‘b’ & ‘c’.

Answer 114

enhance turbulence.

Question 115

In a single effect evaporator, the economy is

1
< 1
> 1
none of these

Answer 115

< 1

Question 116

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

0
1
>1
<1

Answer 116

<1

Question 117

Which of the following is not concerned with the heat transfer?

Brinkman number
Stanton number
Schmidt number
Peclet number

Answer 117

Schmidt number

Question 118

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
970
3.97
none of these

Answer 118

180

Question 119

In counter flow compared to parallel flow,

LMTD is greater.
less surface area is required for a given heat transfer rate.
both (a) and (b).
more surface area is required for a given heat transfer rate.

Answer 119

both (a) and (b).

Question 120

The thermal radiative flux from a surface of emissivity = 0.4 is 22.68 kW/m^2. The approximate surface temperature (K) is
(Stefan-Boltzman constant = 5.67x10^-8 W/m^2.K^4)

1000
727
800
1200

Answer 120

1000

Question 121

Crystallisation of solids from a homogeneous solution is a/an __________ process.

exothermic
mildly endothermic
highly endothermic
none of these

Answer 121

exothermic

Question 122

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

6000
24000
60000
48000

Answer 122

24000

Question 123

Value of Prandtl number for water ranges from

1 to 2
5 to 10
100 to 500
1000 to 2000

Answer 123

5 to 10

Question 124

Heat transfer in the laminar sublayer in case of a liquid flowing through a pipe, is mostly by

eddies current
conduction
convection
none of these

Answer 124

conduction

Question 125

Among liquids, water has a comparatively high value of thermal conductivity, due to its

low density
high viscosity
partial ionisation
dense structure

Answer 125

partial ionisation

Question 126

Reynold’s analogy states that (where, St = Stanton number f = friction factor)

St = f/2
St = f/4
St = 4f
St = f^1/2

Answer 126

St = f/2

Question 127

For evaporation of viscous solution in a multiple effect evaporator, the prefered feeding scheme is

forward
backward
parallel
none of these

Answer 127

backward

Question 128

The sum of reflectivity and absorptivity for an opaque body is equal to

0.5
1
0
2

Answer 128

1

Question 129

The film thickness for laminar film condensation on vertical surface __________ from top to bottom.

cumulatively increases
cumulatively decreases
remains constant
and the surface conductance increase

Answer 129

cumulatively increases

Question 130

In case of a multipass shell and tube heat exchanger, providing a baffle on the shell side __________ the heat transfer rate.

increases
decreases
does not affect
may increase or decrease, depends on the type of baffle

Answer 130

increases

Question 131

The local surface conductance for laminar film condensation on vertical surface is (where, t = film thickness)

∝ t
∝ 1/t
∝ (t)^1/2
independent of ‘t’

Answer 131

∝ 1/t

Question 132

Heat transfer co-efficient (h1) for liquids increases with

increasing temperature.
decreasing temperature.
decreasing Reynolds number.
none of these.

Answer 132

increasing temperature.

Question 133

A diathermaneous substance __________ the thermal radiation completely.

absorbs
reflects
transmits
none of these

Answer 133

transmits

Question 134

A hot body will radiate heat most rapidly, if its surface is

white & rough
black & rough
white & polished
black & polished

Answer 134

black & rough

Question 135

Evaporation of 1kg of water from a solution in a single effect evaporator requires about __________ kg of steam.

0.4 - 0.6
1-1.3
1.8-2
2 - 2.4

Answer 135

1-1.3

Question 136

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/m^2.K. The Biot number is

5.6
0.025
3.5
0.0035

Answer 136

0.025

Question 137

Analogy between mass and heat transfer is not applicable in case of

same velocity profile or equal eddy diffusivities.
thermal or pressure mass diffusion.
viscous heating or chemical reaction.
both (b) and (c).

Answer 137

both (b) and (c).

Question 138

A __________ surface has the maximum thermal emissivity out of the following.

black & smooth
black & rough
white & smooth
white & rough

Answer 138

black & rough

Question 139

Radiant energy received by a body is proportional to (where, d = the distance between the object emitting radiation and that receiving it.)

d^1/2
d
d^2
d^1.5

Answer 139

d^2

Question 140

Mechanical recompression evaporation is used in the production of

alcohol
distilled water
salt
fruits jam

Answer 140

distilled water

Question 141

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.

Condensation
Grashoff
Stantan
Brinkman

Answer 141

Brinkman

Question 142

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

0.35
0.45
0.55
0.85

Answer 142

0.45

Question 143

The purpose of providing a ‘catchall’ in the vapor line of an evaporator is to

create vacuum
regulate the vapor flow
vent the non-condensible gases
arrest the entrained liquid

Answer 143

arrest the entrained liquid

Question 144

What is the thermal conductivity of a perfect heat insulator ?

zero
one
∞
between 0 and ∞

Answer 144

zero

Question 145

‘Fouling factor’ used in the design of a multipass shell and tube heat exchanger is a

non-dimensional factor.
factor of safety.
conversion factor for individual film heat transfer co-efficient to overall heat transfer co-efficient.
none of these.

Answer 145

factor of safety.

Question 146

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^0.8
d^-0.2
d^-1
d^-1.8

Answer 146

d^-0.2

Question 147

Prandtl and Reynolds analogy are same, when Prandtl number is

0.5
1
> 2
1.5

Answer 147

1

Question 148

Heat transfer occurs by natural convection because change in temperature causes difference in

viscosity
density
thermal conductivity
heat capacity

Answer 148

density

Question 149

The ratio of kinematic viscosity to thermal diffusivity is called the __________ number.

Peclet
Prandtl
Stanton
Nusselt

Answer 149

Prandtl

Question 150

The critical radius ‘r’ of insulation on a pipe is given by

r = 2k/h
r = k/h
r = k/2h
r = h/k

Answer 150

r = k/h

Question 151

As per Kirchoffs law, the ratio of the total radiating power to the absorptivity of a body depends on the

temperature of the body only.
wavelength of monochromatic radiation.
both (a) and (b).
nature of material of body.

Answer 151

temperature of the body only.

Question 152

If the baffle spacing in a shell and tube heat exchanger increases, then the Reynolds number of the shell side fluid

remains unchanged.
increases.
increases or decreases depending on number of shell passes.
decreases.

Answer 152

decreases.

Question 153

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 black.
painting the surface white (with aluminium paint).
giving the surface a mirror finish.
roughening the surface.

Answer 153

painting the surface white (with aluminium paint).

Question 154

__________ paint has the maximum absorption coefficient.

Black
Yellow
White
Grey

Answer 154

Black

Question 155

What is Nusselt number ?

Cpμ / k
hD / k
hCP / μ
Cp*μ / h

Answer 155

hD / k

Question 156

Steam traps are provided in steam carrying pipelines to

condense steam.
release excess steam pressure by bleeding steam.
remove condensate and inert gases.
none of these.

Answer 156

remove condensate and inert gases.

Question 157

The Dietus-Boelter equation for convective heat transfer [(i.e.) h=0.023 (K/D)(Re)^0.8 (Pr)^0.4]
cannot be used for

low Reynold’s number
very low Grashoff number
molten metals
all (a), (b) and (c)

Answer 157

all (a), (b) and (c)

Question 158

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

ellipse
straight line
parabola
circular arc

Answer 158

straight line

Question 159

Radiation heat transfer rates does not depend upon the

type of absorbing surface.
distance between the heat source and the object receiving the heat.
surface area and temperature of the heat source.
none of these.

Answer 159

none of these.

Question 160

Which of the following is correct ?

Rate = Driving force x Resistance
Driving force = Rate x Resistance
Resistance = Driving force x Rate
Rate = Resistance/Driving force

Answer 160

Driving force = Rate x Resistance

Question 161

Convective heat transfer, in which heat is transferred by movement of warmed matter is described by

Fourier’s law
Newton’s law of cooling
Fick’s law
none of these

Answer 161

Newton’s law of cooling

Question 162

As the difference between the wall temperature and bulk temperature increases, the boiling heat transfer co-efficient

continues to increase.
continues to decrease.
goes through a minimum.
goes through a maximum.

Answer 162

goes through a minimum.

Question 163

Heat transfer rate per unit area is called

thermal conductivity
heat flux
heat transfer co-efficient
thermal diffusivity

Answer 163

heat flux

Question 164

Log mean temperature difference (LMTD) cannot be used, if

heat transfer co-efficient over the entire heat exchanger is not constant.
there exists an unsteady state.
the heat capacity is not constant and there is a phase change.
none of these.

Answer 164

none of these.

Question 165

In a multipass shell and tube heat exchanger, tube side return pressure loss is equal to __________ the velocity head.

twice
four times
square root of
square of

Answer 165

four times

Question 166

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

decrease
increase
first decrease and then increase
first increase and then decrease

Answer 166

first increase and then decrease

Question 167

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

minimum
maximum
zero
infinity

Answer 167

zero

Question 168

For what value of Prandtl number, the Col-burn analogy is valid ?

0.06 to 120
0.6 to 120
1 to 103
l to 50

Answer 168

0.6 to 120

Question 169

In a forward feed multiple effect, the pressure build up will be

least at the inlet of the first effect.
least at the outlet of the last effect.
highest at the inlet of the last effect.
highest at the outlet of the last effect.

Answer 169

least at the outlet of the last effect.

Question 170

Film boiling is usually not desired in commercial equipments, because

the heat transfer rate is low in view of the large temperature drop.
it is difficult to maintain.
it is not economic.
none of these.

Answer 170

the heat transfer rate is low in view of the large temperature drop.

Question 171

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

3
0.33
1
1.33

Answer 171

1

Question 172

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.k
W/m
W/m.k^2
none, ‘a’ is just a number

Answer 172

W/m.k^2

Question 173

Film condensation is promoted on a/an __________ surface.

oily
coated
clean & smooth
dirty

Answer 173

clean & smooth

Question 174

Dietus-Boelter equation cannot be used for molten metals mainly due to its very low

Prandtl number
Grashoff number
thermal conductivity
viscosity

Answer 174

Prandtl number

Question 175

If Prandtl number is greater than the Schmidt number, then the

thermal boundary layer lies inside the concentration boundary layer.
concentration boundary layer lies inside the thermal boundary layer.
thermal & concentration boundary layers are of equal thickness.
hydrodynamic(i.e., momentum)boundary layer is thicker than the other two.

Answer 175

thermal boundary layer lies inside the concentration boundary layer.

Question 176

Which of the following accessories is provided in the vapor line of an evaporator for removing the entrained liquid ?

Bleed point
Vent
Catchall
Baffle

Answer 176

Catchall

Question 177

Rate of heat transfer by vaporisation from pools of water is affected by the

nature of heating surface and distribution of bubbles.
surface tension of water.
viscosity of water.
all (a), (b) and (c).

Answer 177

all (a), (b) and (c).

Question 178

Multiple effect evaporators are commonly used in the manufacture of
P. Paper
Q. Superphosphate
R. Sugar
S. Fats

P and Q
P and R
P and S
R and S

Answer 178

P and S

Question 179

Which of the following has the highest thermal conductivity ?

Brick
Air
Water
Silver

Answer 179

Silver

Question 180

Which of the following is unimportant in forced convection ?

Reynolds number
Prandtl number
Grashhoff number
none of these

Answer 180

Grashhoff number

Question 181

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 tube side.
vertical heat exchanger with steam on shell side.
horizontal heat exchanger with steam on tube side.
horizontal heat exchanger with steam on shell side.

Answer 181

vertical heat exchanger with steam on shell side.

Question 182

In a multiple effect evaporator, the effect of boiling point elevation is to

reduce the capacity
reduce the economy
increase the economy
none of these

Answer 182

reduce the capacity

Question 183

The rate of heat transfer through a pipe wall is given by, q= [2nK(Ti - T0)] / [ln(ri/r0)] . For cylinder of very thin wall, q can be approximated by

q= {2nK[(Ti - T0)/2]} / [ln(ri/r0)]
q= [2nriK(Ti - T0)] / (r0-ri)
q= [2nK(Ti - T0)] / (r0-ri)
q= [2nK(Ti - T0)] / [(r0-ri)/2]

Answer 183

q= [2nK(Ti - T0)] / [(r0-ri)/2]

Question 184

Dropwise condensation occurs on __________ surfaces.

clean and dirt free
smooth clean
contaminated cooling
polished

Answer 184

contaminated cooling

Question 185

For specified tube outside diameter, higher BWG means higher

tube thickness
cross-sectional area
weight per unit length
none of these

Answer 185

cross-sectional area

Question 186

Pick out the wrong statement.

Swenson-Walker crystalliser is a batch crystalliser.
Super saturation of the solution is the driving potential for a crystal growth.
The liquor left after the removal of crystals from a solution is called mother liquor.
The first stage of crystal formation is called nucleation.

Answer 186

Swenson-Walker crystalliser is a batch crystalliser.

Question 187

In case of a vertical tube evaporator, with increase in the liquor level, the __________ is increased.

velocity of circulation
liquor-film co-efficient
both (a) and (b)
neither (a) and (b)

Answer 187

neither (a) and (b)

Question 188

Electromagnetic radiations propagate in vacuum with a velocity of __________ metre/second.

3 x 10^5
3 x 10^8
3 x 10^10
3 x 10^12

Answer 188

3 x 10^8

Question 189

Heat flux is the time rate of heat transfer per unit

length
area
volume
none of these

Answer 189

area

Question 190

Film boiling occurs at __________ pressure.

atmospheric
sub-atmospheric
negative
very high

Answer 190

very high

Question 191

Heat flux through several resistances in series in analogous to the current flowing through several

resistances in parallel.
capacitors in series.
resistances in series.
none of these.

Answer 191

resistances in series.

Question 192

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

surrounding temperature.
average temperature of the fin.
temperature of the fin end.
constant temperature equal to that of the base

Answer 192

constant temperature equal to that of the base

Question 193

Vacuum is generally maintained in the vapour space of an evaporator mainly to

get economical temperature difference by using moderate pressure steam.
facilitate forward feeding in multiple effect evaporation.
concentrate heat sensitive materials.
achieve very high concentration of the final product.

Answer 193

get economical temperature difference by using moderate pressure steam.

Question 194

Double pipe heat exchangers are used

when heat transfer area required is very high.
when heat transfer area required is very low, i.e (100-200 ft^2).
because it occupies less floor area.
because it is less costly.

Answer 194

when heat transfer area required is very low, i.e (100-200 ft^2).

Question 195

Nusselt number (for forced convection heat transfer) is a function of the __________ number.

Prandtl
Reynolds
both (a) & (b)
neither (a) nor (b)

Answer 195

both (a) & (b)

Question 196

Pick out the wrong statement.

Heat transfer by radiation can not occur across an absolute volume.
In case of a shell and tube heat exchanger, the pressure drop through the shell is proportional to the number of times the fluid crosses the bundle between baffles.
Propagation velocity for travel of heat radiation through vacuum is equal to the velocity of the light.
The amount of heat involved in the condensation or vaporisation of 1 kg of a fluid is the same.

Answer 196

Heat transfer by radiation can not occur across an absolute volume.

Question 197

Which type of heat exchanger is preferred for heavy heat loads ?

Double pipe
Plate fine
Series and parallel set of shell and tube
None of these

Answer 197

Series and parallel set of shell and tube

Question 198

Pick out the wrong statement.

Reciprocal of the resistance to heat flow is called thermal conductance.
Unit of thermal conductance is W/°K.
Thermal conductance of a wall of thickness ‘L’, thermal conductivity ‘k’ and heat flow area ‘A’ is kL/A.
none of these.

Answer 198

Thermal conductance of a wall of thickness ‘L’, thermal conductivity ‘k’ and heat flow area ‘A’ is kL/A.

Question 199

While the total emissivity of a perfect black body is unity, the same for a real body is

0
1
> 1
between 0 and 1

Answer 199

between 0 and 1

Question 200

The Grashoff number is defined as the ratio of the

buoyancy to inertial forces.
buoyancy to viscous forces.
inertial to viscous forces.
buoyancy to surface tension forces.

Answer 200

buoyancy to viscous forces.

Question 201

According to Reynolds analogy, Stanton number is equal to (where, f = Fanning friction factor)

2f
f
f/2
f/4

Answer 201

f/2

Question 202

The absorptivity of a body is equal to its emissivity

at a particular temperature.
for circular bodies.
under thermal equilibrium.
none of these.

Answer 202

under thermal equilibrium.

Question 203

Absorptivity and refletivity of a perfect black body are respectively

1 and 0
0 and 1
1 and ∞
0 and 0.5

Answer 203

1 and 0

Question 204

In thermal radiation for a black body (where, ε is emissivity and α is absorptivity)

α = 1; ε ≠ 1
α ≠ 1, ε = 1
α ≠ 1, ε ≠ 1
α = 1; ε = 1

Answer 204

α = 1; ε = 1

Question 205

Pick out the wrong statement.

Superheated steam is preferably not used for process heating because of its low heat transfer film co-efficient.
In a shell and tube heat exchanger, the shell pressure drop is maximum for orifice baffles.
S.I. unit of fouling factor is Watt/m2.°K.
Longitudinal fins are used in extended surface heat exchangers, when the direction of fluid flow is parallel to the axis of the tube.

Answer 205

S.I. unit of fouling factor is Watt/m2.°K.

Question 206

When warm and cold liquids are mixed, the heat transfer is mainly by

conduction
convection
radiation
both(a) & (c)

Answer 206

convection

Question 207

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 ?

Cube
Plate
Sphere
All will cool at the same rate

Answer 207

Sphere

Question 208

Vent pipes are provided in a condenser to

remove non-condensable gases.
purge the condenser.
facilitate easy cleaning of tubes.
none of these.

Answer 208

remove non-condensable gases.

Question 209

Heat transfer by natural convection is enhanced in system with

high viscosity.
high co-efficient of thermal expansion.
low temperature gradients.
low density change with temperature.

Answer 209

high co-efficient of thermal expansion.

Question 210

In case of __________ boiling, the bubbles formed on a submerged hot surface get absorbed in the mass of the liquid.

nucleate
pool
low pressure
none of these

Answer 210

pool

Question 211

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
>10
<10
either (b) or (c), depends on the mass of the body

Answer 211

> 10

Question 212

In forced convection, the Nusselt number is a function of

Re and Pr
Re and Gr
Pr and Gr
Re and Sc

Answer 212

Re and Pr

Question 213

At what value of Prandtl number, the hydrodynamic and thermal boundary layers are identical ?

0.5
1
1.5
80

Answer 213

1

Question 214

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 heat transfer area of all the effects is -less than that in a single effect evaporator system.
total amount of vapor produced per Kg of feed steam in a multiple effect system is much higher than in a single effect.
boiling point elevation in a single effect system is much higher than that in any effect in a multieffect system.
heat transfer co-efficient in a single effect is much lower than that in any effect in a multieffect system.

Answer 214

total amount of vapor produced per Kg of feed steam in a multiple effect system is much higher than in a single effect.

Question 215

Heat transfer efficiency leading of energy conservation in a heat exchanger can be achieved by

keeping the heat transfer surface clean.
enhancing the fluid pumping rate.
increasing the tube length.
none of these.

Answer 215

keeping the heat transfer surface clean.

Question 216

Trap is used to remove __________ from steam pipe lines.

steam
condensate
non-condensables
none of these

Answer 216

condensate

Question 217

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

125
133
150
160

Answer 217

160

Question 218

Tube pitch is the __________ of tube diameters and the clearances.

sum
difference
ratio
none of these

Answer 218

sum

Question 219

With increase in the distance between the heat source and the object receiving the heat, the radiation heat transfer

decreases
increases
increases exponentially
remains unaffected

Answer 219

decreases

Question 220

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.

more
less
same
unpredictable

Answer 220

less

Question 221

Natural convection is characterised by

Grashhoff number
Peclet number
Reynolds number
Prandtl number

Answer 221

Grashhoff number

Question 222

Nusselt number for full developed, laminar, constant property flow in a pipe at uniform heat flux is

0.72
4.364
18
83

Answer 222

4.364

Question 223

Thickness of thermal boundary layer is more compared to that of hydrodynamic boundary layer, when the value of Prandtl number is

1
< 1
> 1
> 5

Answer 223

< 1

Question 224

The critical radius of insulation for cylindrical pipe is (where, hi = heat transfer coefficient at inside of the pipe )

K/h0
2K/h0
hi/K
2hi/K

Answer 224

K/h0

Question 225

The heat flux in the nucleate boiling regimes is proportional to (where, ΔT = excess temperature)

(ΔT)^2
(ΔT)^4
(ΔT)^3
(ΔT)

Answer 225

(ΔT)^3

Question 226

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
equal to the difference between hot and cold fluids inlet temperature.
equal to the difference between hot fluid inlet temperature and cold fluid outlet temperature.

Answer 226

0

Question 227

A body is called grey if the monochromatic emissivity of the body is

zero
unity
same for all wavelengths
different for all wavelengths

Answer 227

same for all wavelengths

Question 228

In an extended surface heat exchanger, fluid having lower co-efficient

flows through the tube.
flows outside the tubes.
can flow either inside or outside the tubes.
should not be used as it gives very high pressure drop.

Answer 228

flows outside the tubes.

Question 229

In a shell and tube heat exchanger, the tube side heat transfer co-efficient just at the entrance of the tube is

infinity.
zero.
same as average heat transfer co-efficient for tube side.
none of these.

Answer 229

infinity.

Question 230

Grashoff number, which is defined as g . β(Ts - T∞) ρ2. L3/μ2, is proportional to the ratio of buoyancy force to __________ force.

viscous
elastic
inertial
none of these

Answer 230

viscous

Question 231

Pick out the wrong statement.

By increasing the number of shell passes, the temperature cross in a shell and tube heat exchanger can be prevented.
An underdesigned steam trap will back up the condensate instead of discharging it out.
Steam condensate is corrosive because of the presence of dissolved oxygen in it.
Film boiling is desirable in commercial equipments due to high heat transfer rate at low temperature drop.

Answer 231

Film boiling is desirable in commercial equipments due to high heat transfer rate at low temperature drop.

Question 232

Graetz number is given by

mCp / kL
kL/mCp
mCp/kμ
kL/mCp

Answer 232

mCp / kL

Question 233

For a cold dilute feed to produce thick viscous liquor, backward feeding as compared to forward feeding results in

increased economy
decreased economy
lower capacity
no effect on economy

Answer 233

increased economy

Question 234

Economy of an evaporator is influenced by the

steam pressure
feed temperature
number of effect
both(b) & (c)

Answer 234

both(b) & (c)

Question 235

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.

hyperbolic
parabolic
linear
logarithmic

Answer 235

linear

Question 236

In regenerative air preheater (as practised in heating of coke ovens), the heat is transferred

through a metallic wall.
by direct contact of hot flue gas with air.
by heating an intermediate material (like chequor bricks) and then heating the air from this hot material.
none of these.

Answer 236

by heating an intermediate material (like chequor bricks) and then heating the air from this hot material.

Question 237

__________ equation relates the thermal conductivity of a solid to its temperature.

Antonie
Kopp’s
Lee’s
Kistyakowski

Answer 237

Lee’s

Question 238

For small temperature difference, the heat transfer rate as per Newton’s law of cooling is proportional to (where, Δt = excess temperature)

Δt
Δt^2
Δt^3
(Δt)^1/2

Answer 238

Δt

Question 239

Pick out the wrong statement.

The condensing film co-efficient is about 3 times lower for vertical condenser as compared to the equivalent horizontal condenser for identical situation.
Film co-efficient for vaporisation decreases as a result of vapor binding.
In industrial practice, sub-cooling of condensate is required, when the condensate is a volatile liquid and is to be transferred for storage.
Overall heat transfer co-efficient in a heat exchanger is controlled by the value of the film co-efficient, which is higher.

Answer 239

Overall heat transfer co-efficient in a heat exchanger is controlled by the value of the film co-efficient, which is higher.

Question 240

Viscous & heat sensitive liquids are concentrated in __________ evaporators.

open pan
long tube
agitated film
none of these

Answer 240

agitated film

Question 241

1 BTU/hr.ft.°F is equal to __________ kcal/hr. m.°C.

1.49
1
4.88
none of these

Answer 241

1.49

Question 242

Heat transfer by radiation between two bodies at T1 & T2 and in an ambient temperature of Ta°C depends on

T1 - T2
T1 - Ta
T2 - Ta
none of these

Answer 242

none of these

Question 243

The ratio of momentum diffusivity to thermal diffusivity is the __________ number.
Prandtl
Nusselt
Stanton
Grashoff

Answer 243

Prandtl

Question 244

For a perfectly transparent surface (like gases), the

absorptivity = 0
transmissivity = 1
reflectivity = 0
all (a), (b) & (c)

Answer 244

transmissivity = 1

Question 245

For a fluid flowing in an annulus space, the wetted perimeter for heat transfer and pressure drop are

same
different
never different
linearly related

Answer 245

different

Question 246

During crystallisation, formation of crystal can occur in __________ solution only.

saturated
supersaturated
undersaturated
all (a), (b) and (c)

Answer 246

supersaturated

Question 247

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
9000
4600
3000

Answer 247

4000

Question 248

An insulator should have

low thermal conductivity.
high thermal conductivity.
less resistance to heat flow.
a porous structure.

Answer 248

low thermal conductivity.

Question 249

In a backward feed multiple effect evaporator

feed is introduced in the first effect.
feed flows from low pressure to high pressure.
no pumps are required between successive effects.
none of these.

Answer 249

feed flows from low pressure to high pressure.

Question 250

Choose the correct equation.

Nu = (Re) (Pr) (Gz)
Nu = (Re) (Pr) (St)
Nu = (Re)(Pr)
Nu = (Pr)(St)

Answer 250

Nu = (Re) (Pr) (St)

Question 251

In a forward feed multiple effect evaporator unit

viscosity of liquid is highest in first effect.
transfer from effect to effect is done by pumps.
no pump is required to withdraw the product from the last effect.
none of these.

Answer 251

none of these.

Question 252

In case of heat flow by conduction for a cylindrical body with an internal heat source, the nature of temperature distribution is

linear
hyperbolic
parabolic
none of these

Answer 252

parabolic

Question 253

For heat flow through very thick walled cylinder, use____mean radius.

arithmetic
logarithmic
geometric
either (a) or (c)

Answer 253

logarithmic

Question 254

Indirect contact heat exchangers are preferred over direct contact heat exchangers, because

heat transfer co-efficient are high.
there is no risk of contamination.
there is no mist formation.
cost of equipment is lower.

Answer 254

there is no risk of contamination.

Question 255

Water is normally used as a coolant in the heat exchange equipments mainly because of its

abundance & high heat capacity
low density
low viscosity
high fluidity

Answer 255

abundance & high heat capacity

Question 256

The rate of emission of radiation by a body does not depend upon the

wavelength of radiation.
surface temperature of the body.
nature of the surface.
shape and porosity of the body.

Answer 256

shape and porosity of the body.

Question 257

Which of the following has maximum thermal conductivity at the same temperature?

Steel
Petrol
Air
All have the same conductivity

Answer 257

Steel

Question 258

Maximum heat transfer rate is obtained in __________ flow.

laminar
turbulent
creeping
transition region

Answer 258

turbulent

Question 259

In case of __________ boiling, the liquid temperature is below the saturation temperature and the boiling takes place in the vicinity of the heated surface.

nucleate
local
pool
saturated

Answer 259

local

Question 260

The ratio of velocity head to tube side return loss in case of a multipass shell and tube heat exchanger is

2
1/2
4
1/4

Answer 260

1/4

Question 261

Terminal point temperature differences between fluids in case of a heat exchanger is termed as

approach.
log mean temperature difference.
arithmetic mean temperature difference.
geometric mean temperature difference.

Answer 261

approach.

Question 262

LMTD can’t be used as such without a correction factor for the

multipass heat exchanger.
baffled heat exchanger.
condensation of mixed vapour in a condenser.
all (a) (b) and (c).

Answer 262

all (a) (b) and (c).

Question 263

For cold viscous feed, backward feed gives __________ than forward feed.

a higher capacity
a lower capacity
lower economy
none of these

Answer 263

a higher capacity

Question 264

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 ?

35.5
43.75
81.25
48.25

Answer 264

81.25

Question 265

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
U2 > U1
U1 > U2
U1 = dirt factor - U2

Answer 265

U1 = U2

Question 266

In SI units, thermal conductivity is expressed in

Watt/m.°K
Watt/m^2. °K
Watt/m^3. °K
Watt/m^4. °K

Answer 266

Watt/m.°K

Question 267

Boiling point of a solution according to Duhring’s rule is a linear function of the __________ of water.

boiling point (at the same pressure)
viscosity
density
thermal conductivity

Answer 267

boiling point (at the same pressure)

Question 268

The overall resistance for heat transfer through a series of flat resistance, is the __________ of the resistances.

average
geometric mean
product
sum

Answer 268

sum

Question 269

At what value of Prandtl number, conduction is negligible in the turbulent core of a fluid flowing through a heated pipe ?

0.5
< 0.5
> 0.6
< 0.1

Answer 269

> 0.6

Question 270

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

0
R2/(R1+R2)
1
(R1/R2)^2

Answer 270

R2/(R1+R2)

Question 271

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

1.26 h1
0.794 h1
0.574 h1
1.741 h1

Answer 271

0.794 h1

Question 272

The main purpose of providing fins on heat transfer surface is to increase the

temperature gradient.
mechanical strength of the equipment.
heat transfer area.
heat transfer co-efficient.

Answer 272

heat transfer area.

Question 273

A BTU/hr.ft.2 °F is equal to

1 kcal/hr. m^2°C.
4.88 kcal/hr. m^2°C.
1 kcal/hr. m^2.°K.
none of these.

Answer 273

4.88 kcal/hr. m.2°C.

Question 274

Which of the following has the lowest Prandtl number ?

Molten sodium (a coolant used in fast breeder reactor)
Water
Transformer oil
Dilute H2SO4

Answer 274

Molten sodium (a coolant used in fast breeder reactor)

Question 275

In SI units, fouling factor is expressed in

m^2°K/W
W/m^2°K
m2°K
m°K/W

Answer 275

m^2°K/W

Question 276

An ejector is used to

increase pressure
increase temperature
remove condensate
none of these

Answer 276

none of these

Question 277

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

less corrosion problems.
flexibility possible in the baffle arrangement.
low pressure drop.
high heat transfer co-efficient.

Answer 277

flexibility possible in the baffle arrangement.

Question 278

All analogy equations connecting friction factor and heat transfer co-efficient apply only to

wall or skin friction
form friction
both (a) and (b)
turbulent flow

Answer 278

wall or skin friction

Question 279

Electro-magnetic spectrum range, which is important for radiation varies from __________ microns.

1 to 100
0.5 to 50
10 to 100
100 to 1000

Answer 279

0.5 to 50

Question 280

The characteristic dimensionless groups for heat transfer to a fluid flowing through a pipe in laminar flow are

Re, Gz
Nu, Pr
Nu, Pr, Re
Nu, Gz

Answer 280

Nu, Gz

Question 281

Heat transfer rate described by Fourier’s law will decrease, if the __________ increases.

thermal conductivity
thickness
temperature difference
heat transfer area

Answer 281

thickness

Question 282

A black body does not __________ radiation.

absorb or emit
refract
reflect
both(b) & (c)

Answer 282

both(b) & (c)

Question 283

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.
evaporators operate under vacuum.
evaporators operate at atmospheric pressure.
none of these.

Answer 283

solution has an elevation of boiling point.

Question 284

Peclet number (Pe) is given by

Pe = Re.Pr
Pe = Re/Pr
Pe = Pr/Re
Pe = Nu.Re

Answer 284

Pe = Re.Pr

Question 285

Nusselt number is the ratio of the

temperature gradient of the wall to that across the entire pipe.
temperature difference to the temperature gradient at the wall.
heat flux at the wall to that across the entire pipe.
none of these

Answer 285

temperature difference to the temperature gradient at the wall.

Question 286

Prandtl number is the ratio of

momentum diffusivity to mass diffusivity.
momentum diffusivity to thermal diffusivity.
thermal diffusivity to mass diffusivity.
thermal diffusivity to momentum diffusivity.

Answer 286

momentum diffusivity to thermal diffusivity.

Question 287

Which of the following parameters is increased by use of finned tube in a multipass shell and tube heat exchanger ?

Tube side pressure drop and the heat transfer rate.
Convective heat transfer co-efficient.
Effective tube surface area for convective heat transfer.
All (a) (b) and (c).

Answer 287

All (a) (b) and (c).

Question 288

Forced circulation evaporators are useful for the concentration of viscous, salting and scale forming liquors. Which of the following is a forced circulation evaporator?

Long vertical evaporator
Horizontal tube evaporator
Agitated film evaporator
Calenderia vertical tube evaporator

Answer 288

Agitated film evaporator

Question 289

“The ratio of the total emissive power to the absorptivity for all bodies is same at ther-mal equilibrium”. This is __________ law.

Kirchoffs
Planck’s
Wien’s displacement
Stefan-Boltzman

Answer 289

Kirchoffs

Question 290

View factor is important in heat transfer by

steady state conduction
natural convection
forced convection
radiation

Answer 290

radiation

Question 291

The advantage of using a 1 - 2 shell and tube heat exchanger over a 1 - 1 shell and tube heat exchanger is

lower tube side pressure drop.
lower shell side pressure drop.
higher tube side heat transfer co-efficient.
higher shell side heat transfer co-efficient.

Answer 291

higher tube side heat transfer co-efficient.

Question 292

For a multipass shell and tube heat exchanger, the LMTD correction factor is always

1
> 1
< 1
between 1 & 2

Answer 292

< 1

Question 293

If heat transfer rate varies with the time, it is termed as

forced convection
steady state conduction
monochromatic radiation
none of these

Answer 293

none of these

Question 294

Pick out the wrong statement.

Fluid movement under the influence of buoyant forces resulting from change in density takes place in case of natural convection.
The ratio NNu/NRe * Npr is called the Stanton number.
The Peclet number is a measure of the ratio of energy transport by convection to that by conduction.
The Colbum jH factor for heat transfer is given by Nst Npr.

Answer 294

The Colbum jH factor for heat transfer is given by Nst Npr.

Question 295

Tube expansion allowances exist in __________ heat exchanger.

multipass fixed tube sheet
U-tube
single pass fixed tube sheet
none of these

Answer 295

U-tube

Question 296

Pick out the wrong statement.

In drying a solid containing moisture above the critical moisture content the number of degrees of freedom is 2.
Sherwood number in mass transfer corresponds to Nusselt number in heat transfer and Schmidt number to Prandtl number.
Forced convection is relatively more effective in increasing the rate of mass transfer, if Schmidt number is larger.
Hot gases at moderate pressure are usually in the shell side of shell and tube heat exchangers. At higher pressure, however, it is customary to put gas in the tube side.

Answer 296

Forced convection is relatively more effective in increasing the rate of mass transfer, if Schmidt number is larger.

Question 297

Agitated film evaporator is suitable for concentrating __________ liquids.

foaming
viscous
very thin
corrosive

Answer 297

viscous

Question 298

The thickness of condensate layer in filmwise condensation depends on the

condensation rate
surface configuration
liquid flow rate from the surface
all (a), (b) and (c)

Answer 298

all (a), (b) and (c)

Question 299

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
hyperbolic
linear
logarithmic

Answer 299

parabolic

Question 300

Intermittant tube cleaning is possible to be done in case of a __________ evaporator.

basket type
horizontal tube
calendria
none of these

Answer 300

basket type

Question 301

Harmonic mean temperature difference is given by

(ΔT1* ΔT2)^1/2
(2 * ΔT1* ΔT2) / (ΔT1+ ΔT2)
(2 * ΔT1* ΔT2) / (ΔT1- ΔT2)
(ΔT1- ΔT2) / (ΔT1* ΔT2)

Answer 301

(2 * ΔT1* ΔT2) / (ΔT1+ ΔT2)

Question 302

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.
thick walled.
having the value of Ao/Ai > 2.
both (b) and (c).

Answer 302

thin walled having the value of Ao Ai/< 2.

Question 303

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
cylinder
sphere
equal for all the three

Answer 303

cube

Question 304

Overall heat transfer co-efficient for cooling of hydrocarbons by water is about

50 -100 Kcal/hr.m^2.°C
50 -100 W/m^2 . °K
50 -100 BTU/hr. ft.^2°F
1000 - 1500 BTU/hr. ft.^2°F

Answer 304

50 -100 BTU/hr. ft.^2°F

Question 305

Absorptivity of a perfect black body is unity. Which of the following has maximum absorptivity ?

aluminium foil
refractory bricks
iron plate
coke breeze

Answer 305

coke breeze

Question 306

Thermal diffusivity is given by

k/ρCp
ρCp/k
Cpμ/a
μ/hCp

Answer 306

k/ρCp

Question 307

At what value of Prandtl number, the hydrodynamic and thermal boundary layers of a fluid flowing over a heated plate will be identical ?

1
< 1
> 1
none of these

Answer 307

1

Question 308

Fourier’s law of heat conduction applies to __________ surfaces.

isothermal
non-isothermal
both (a) and (b)
neither (a) and (b)

Answer 308

both (a) and (b)

Question 309

__________ chart is known as transient heat conduction chart.

Dirhing’s
Heisler’s
Mollier’s
Cox

Answer 309

Dirhing’s

Question 310

Choose the most important factor on which the heat conducted through a wall in a unit time will depend on ?

Thickness of the wall.
Area of the wall perpendicular to heat flow.
Material of the wall.
Temperature difference between the two surfaces of the wall.

Answer 310

Temperature difference between the two surfaces of the wall.

Question 311

Which of the following has the minimum thermal conductivity ?

Nitrogen
Steel
Carbon black
Tar

Answer 311

Nitrogen

Question 312

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.
the size of the exchanger in which heat transfer rate is 1 kcal/hr.
both (a) and (b)
none of these

Answer 312

a section of the exchanger in which change in temperature of one stream equals the average driving force in the section.

Question 313

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
hf > hd
hf < hd
hf could be greater or smaller than hd

Answer 313

hf < hd

Question 314

Pick out the wrong statement:

The capacity of an evaporator is reduced by the boiling point elevation.
Corrosive liquid is normally passed through the tubes in a shell and tube heat exchanger.
Steam jet ejector is used for vapor compression in a thermal recompression evaporator.
Heat sensitive materials should be concentrated in high pressure evaporators.

Answer 314

Heat sensitive materials should be concentrated in high pressure evaporators.

Question 315

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.

high pressure
vacuum
backward feed
none of these

Answer 315

vacuum

Question 316

Circulation pump is located below the evaporater to

avoid cavitation.
avoid frequent priming.
create more suction head.
none of these.

Answer 316

create more suction head.

Question 317

The statement that “maximum wavelength of radiation is inversly proportional to the temperature” is __________ law.

Stefan-Boltzman’s
Planck’s
Wien’s displacement
none of these

Answer 317

Wien’s displacement

Question 318

Steam condensate is recovered by steam traps and recycled for use as boiler feed water, because of its low

hardness
dissolved solids content
suspended solids content
all (a), (b) and(c)

Answer 318

all (a), (b) and(c)

Question 319

At steady state the temperature variation in a plane wall, made of two different solids I & II is shown below :
[insert picture here]
The thermal conductivity of material I

is smaller than that of II.
is greater than that of II.
is equal to that of II.
can be greater than or smaller than that of II.

Answer 319

is smaller than that of II.

Question 320

Duhring rule is important in solving problems on

distillation
crystallisation
evaporation
humidification

Answer 320

evaporation

Question 321

In a shell and tube heat exchanger, the height of 25 percent cut baffles is equal to (where, D = inside diameter of shell)

0.25 D
0.75 D
0.50 D
none of these

Answer 321

0.75 D

Question 322

Multiple effect evaporation is generally recommended, when the

large scale evaporation of liquor is needed.
corrosive liquids are to be concentrated.
fuel is cheaply available.
evaporation on small scale is to be done.

Answer 322

large scale evaporation of liquor is needed.

Question 323

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.

2^0.8
4^0.8
4^0.4
2^0.4

Answer 323

4^0.8

Question 324

__________ heat exchanger is the most suitable, when the temperature of shell side fluid is much higher than that of tube side.

Single pass, fixed tube sheet
U-tube
Three pass, fixed tube sheet
none of these

Answer 324

U-tube

Question 325

In which mode of heat transfer, the Biot number is important ?

Transient heat conduction
Natural convection
Forced convection
Radiation

Answer 325

Transient heat conduction

Question 326

__________ paint has the minimum absorption co-efficient.

Black
White lead
Grey
Light cream

Answer 326

White lead

Question 327

Asymptotic conditions is reached, when for a fluid flowing in laminar flow through a long tube

exit-fluid temperature > wall temperature.
exit fluid temperature < wall temperature.
exit fluid temperature = wall temperature.
Graetz number > 100.

Answer 327

exit fluid temperature = wall temperature.

Question 328

1000 kg of wet solids are to be dried from 60% to 20% moisture (by weight). The mass of moisture removed in kg is

520
200
400
500

Answer 328

400

Question 329

Which is the most suitable for the con-certration of highly concentrated solution?

Open pan evaporation
Long tube vertical evaporator
Agitated film evaporator
None of these

Answer 329

Open pan evaporation

Question 330

Unsteady state heat conduction occurs, when

temperature distribution is independent of time.
temperature distribution is dependent on time.
heat flows in one direction only.
three dimensional heat flow is concerned.

Answer 330

temperature distribution is dependent on time.

Question 331

What is the absorptivity of a black body ?

1
0
0.78
0.95

Answer 331

1

Question 332

Mode of heat transfer in which the fluid moves under the influence of changes in fluid pressure produced by external work is called

radiation
natural convection
forced convection
conduction

Answer 332

forced convection

Question 333

Baffle spacing

is not the same as baffle pitch.
should be less than one fifth the diameter of the shell.
should be less than the inside diameter of the shell.
none of these.

Answer 333

should be less than the inside diameter of the shell.

Question 334

Boiling point elevation of a solution of NaOH

increases rapidly with temperature rise.
is almost independent of temperature.
is almost independent of pressure.
both (b) and (c).

Answer 334

both (b) and (c).

Question 335

The advantage of backward feed multiple effect evaporators over forward feed units is that

heat sensitive material can be handled.
there is no additional cost of pumping.
most concentrated liquid is at highest temperature.
equal heat transfer co-efficients exist in various effects.

Answer 335

most concentrated liquid is at highest temperature.

Question 336

Which of the following is the most controlling factor for the rate of bubble detachment from the hot solid surface ?

Liquid density
Liquid viscosity
Hot surface temperature
Interfacial tension

Answer 336

Interfacial tension

Question 337

Which characteristic of a fluid is not important in deciding its route in a shell and tube heat exchanger ?

Corrosiveness
Fouling characteristic
Viscosity
None of these

Answer 337

None of these

Question 338

Thermal conductivities of most of the liquids __________ with rise in temperature.

increases
decreases
remains unchanged
may increase or decrease; depends on the liquid

Answer 338

decreases

Question 339

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

555
55.5
0.55
0.15

Answer 339

0.15

Question 340

The purpose of floating head in a heat exchanger is to

avoid buckling of tubes.
provide support for tubes.
decrease the pressure drop.
facilitate its lengthening, if needed.

Answer 340

avoid buckling of tubes.

Question 341

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

15 min
22 min
44 min
51 min

Answer 341

51 min

Question 342

Overall thermal resistance for conductive heat transfer through a series of flat resistances is equal to the

maximum resistance in the series.
sum of all resistances.
average of all resistances.
minimum resistance presents in the series.

Answer 342

sum of all resistances.

Question 343

Double pipe heat exchangers are preferably useful, when

high viscosity liquid is to be cooled.
requirement of heat transfer area is low.
overall heat transfer co-efficient is very high.
a corrosive liquid is to be heated.

Answer 343

requirement of heat transfer area is low.

Question 344

Removal of __________ heat is involved in the condensation of a vapor under saturated conditions.

super
sensible
latent
both(b) & (c)

Answer 344

latent

Question 345

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)^1/4
ΔT^1/2
(ΔT)^5/4
(ΔT)^3/4

Answer 345

(ΔT)^3/4

Question 346

If air (a non-condensing gas) is present in a condensing vapor stream, it will __________ the condensation rate of vapor.

increase
decrease
not affect
increase the condensing film co-efficient as well as

Answer 346

decrease

Question 347

Prandtl and Reynold’s analogies are identical for Prandtl number value of

0
0.5
1
5

Answer 347

1

Question 348

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

its evaporation loss
excessive corrosion
uneconomic LMTD
decrease in heat exchanger efficiency

Answer 348

excessive corrosion

Question 349

In evaporators, lowering the feed temperature

increases the heating area required.
reduces the economy.
both (a) and (b).
decreases the heating area required.

Answer 349

both (a) and (b).

Question 350

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
density
thermal conductivity
corrosive nature

Answer 350

viscosity

Question 351

In case of surface condensers, a straight line is obtained on plotting __________ on an ordinary graph paper.

1/V^(-0.8)
V^(-0.8)
V^(-2)
1/V^(-2)

Answer 351

1/V^(-0.8)

Question 352

Thermal diffusivity is the most important in heat transfer by

conduction
radiation
condensation
natural convection

Answer 352

conduction

Question 353

(NGr x NPr) is called the __________ number.

Graetz
Reyleigh
Nusselt
Stanton

Answer 353

Reyleigh

Question 354

A dephlegmator is a

total condenser
vacuum evaporator
partial condenser
double pipe heat exchanger

Answer 354

partial condenser

Question 355

A tank painted with which of the following coloured paints, would heat up maximum by radiation from sun ?

Yellow paint
White paint
Black paint
Grey paint

Answer 355

Black paint

Question 356

The separation of liquid droplets from the vapor is done by a/an __________ , in the evaporators.

steam ejector
entrainment separator
compressor
vacuum pump

Answer 356

entrainment separator

Question 357

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)

U1 - U2
1/U1 - 1/U2
1/U2 - 1/U1
U2 - U1

Answer 357

1/U1 - 1/U2

Question 358

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 1.8 times
decrease by 2^2
increase by 2^16
remain unchanged

Answer 358

increase by 2^16

Question 359

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
10 minutes
20 minutes
40 minutes

Answer 359

5 minutes

Question 360

Fluid motion in the natural convection heat transfer between a solid surface and a fluid in contact with it, results from the

existence of thermal boundary layer.
temperature gradient produced due to density difference
buoyancy of the bubbles produced at active nucleation site.
none of these.

Answer 360

none of these.

Question 361

Evaporator tubes are generally

horizontal
vertical
inclined
random

Answer 361

vertical

Question 362

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
d^2T / dZ^2 = 0
d^2T / dZ^2 < 0
dT / dZ =0

Answer 362

d^2T / dZ^2 = 0

Question 363

In a boiling curve, the peak heat flux is called the __________ point.

Nusselt
Leidenfrost
boiling
burnout

Answer 363

burnout

Question 364

Pick out the wrong statement.

Heat transfer from a hot body to cold body by the emission of heat waves is called radiation.
Filmwise condensation takes place on non-wettable surfaces.
The boiling point of a solution is affected by liquid head as well as boiling point elevation.
none of these.

Answer 364

Filmwise condensation takes place on non-wettable surfaces.

Question 365

h.D/K is called the __________ number.

Nusselt
Peclet
Rayleigh
Grashoff

Answer 365

Nusselt

Question 366

Prandtl number is the ratio of

mass diffusivity to thermal diffusivity.
momentum diffusivity to thermal diffusivity.
thermal diffusivity to mass diffusivity.
thermal diffusivity to momentum diffusivity.

Answer 366

momentum diffusivity to thermal diffusivity.

Question 367

Conductance is given by(where, x = thickness, A = heat flow area, K = thermal conductivity.)
x/KA
KA/x
K/Ax
A/Kx

Answer 367

KA/x

Question 368

Heat flux, as defined in heat flow is analogous to __________ in electricity flow.

current
voltage
resistance
none of these

Answer 368

current

Question 369

The wavelength at which the maximum monochromatic emissive power occurs for a black body, is (where, T = absolute temperature of the black body)

αT
α 1/T
α T^4
independent of T

Answer 369

α 1/T

Question 370

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.

once
twice
thrice
four times

Answer 370

twice

Question 371

When the ratio of the Grashoff number and to the square of Reynolds number is one, the dominant mechanism of heat transfer is

free convection.
entry length problem in laminar forced conduction (developing thermal boundary layer).
mixed convection (both free and forced).
forced convection.

Answer 371

mixed convection (both free and forced)

Question 372

The purpose of providing bleed points in the evaporator is to

admit the feed
remove the product
facilitate removal of non-condensable gases
create vacuum

Answer 372

facilitate removal of non-condensable gases

Question 373

Water always boils when its

temperature reaches 100° C.
vapour pressure equals 76 cm of Hg.
saturated vapour pressure equals the external pressure on its surface.
saturated vapour pressure is less than the atmospheric pressure.

Answer 373

saturated vapour pressure equals the external pressure on its surface.

Question 374

A multiple effect evaporator as compared to a single effect evaporator of the same capacity has

lower heat transfer area.
lower steam economy.
higher steam economy.
higher solute concentration in the product.

Answer 374

higher steam economy.

Question 375

Which is the most suitable for cold viscous feed?

Forward feed
Backward feed
Mixed feed
Parallel feed

Answer 375

Backward feed

Question 376

Dropwise condensation is promoted on a/an __________ surface.

glazed
oily
coated
smooth

Answer 376

oily

Question 377

At Pr > 1, conduction in an ordinary fluid flowing through a heated pipe is limited to the

buffer zone
turbulent core
both (a) and (b)
viscous sub-layer

Answer 377

viscous sub-layer

Question 378

In a single evaporator system, the steam economy __________ by creating vacuum in the evaporator.

increases
decreases
remains constant
may increase or decrease, depends on the vacuum

Answer 378

increases

Question 379

(NRe * NPr) is called the __________ number

Peclet
Stanton
Graetz
none of these

Answer 379

Graetz

Question 380

Nucleate boiling is promoted

on polished surfaces
on rough surfaces
in the absence of agitation
none of these

Answer 380

on rough surfaces

Question 381

In forced circulation, the heating element is injected

internally
externally
both (a) and (a)
neither (a) nor (b)

Answer 381

internally

Question 382

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

1
2
1/2
4

Answer 382

2

Question 383

The equation, Nst = f/2, is the __________ analogy.

Colburn
Reynolds
Prandtl
none of these

Answer 383

Reynolds

Question 384

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
dynamic viscosity
kinematic viscosity
density

Answer 384

boiling point

Question 385

Rate of crystallisation does not depend upon the

extent of supersaturation.
turbulence within the solution.
number and active surface area of the crystals.
none of these.

Answer 385

none of these.

Question 386

Stefen’s block body radiation law can also be derived from __________ law.

Kirchoffs
Planck’s
Fourier’s
none of these

Answer 386

Planck’s

Question 387

Multipass heat exchangers are used

because of simplicity of fabrication.
for low heat load.
to obtain higher heat transfer co-efficient and shorter tube.
to reduce the pressure drop.

Answer 387

to obtain higher heat transfer co-efficient and shorter tube.

Question 388

A hollow sphere and a solid sphere of the same material and equal radii are heated to the same temperature. In this case,

the cooling rate will be the same for the two spheres and hence the two spheres will have equal temperatures at any instant.
both the spheres will emit equal amount of radiation per unit time in the beginning.
both will absorb equal amount of radiation from the surrounding in the beginning.
both(b) & (c).

Answer 388

both(b) & (c).

Question 389

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

higher condensing film co-efficient is desired.
condensate is corrosive in nature.
lower pressure drop through the exchanger is desired.
temperature of the incoming vapor is very high.

Answer 389

condensate is corrosive in nature.

Question 390

When does the heat generated by fluid friction becomes appreciable compared to the heat transferred between the fluids ?

At high fluid velocity
At low velocity
When fluid flows past a smooth surface
None of these

Answer 390

At high fluid velocity

Question 391

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)^-1.8
(D)^-0.2
(D)^0.2
(D)^1.8

Answer 391

(D)^-0.2

Question 392

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 = 283 s
t = 356 s
t = 400 s
t = 800 s

Answer 392

t = 800 s

Question 393

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.

parallel
mixed
counterflow
same in either ‘a’, b’ or ‘c’

Answer 393

same in either ‘a’, b’ or ‘c’

Question 394

Thermal diffusivity of a material

has the unit m2 /sec.
is defined as K/ρ . Cp.
is the ratio of thermal conductivity to thermal capacity.
all (a), (b) and (c).

Answer 394

all (a), (b) and (c).

Question 395

Viscosity of gases __________ with increase in temperature.

increase very rapidly
increase slowly
decrease slowly
remain unaffected

Answer 395

increase slowly

Question 396

What is the emissivity of a black body?

1
0
0.90
0.5

Answer 396

1

Question 397

Steam is routed through the tube in case of a __________ evaporator.

basket type
horizontal tube
short tube vertical
long tube vertical

Answer 397

horizontal tube

Question 398

Dropwise condensation of steam on cooling surface is promoted

an oily surface.
when both the steam and the tube are clean.
only in presence of air.
none of these.

Answer 398

an oily surface.

Question 399

Conduction occurs in the buffer zone for a fluid flowing through a heated pipe, only when Prandtl number is

0.1
>1
< 1
1

Answer 399

0.1

Question 400

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.
directly proportional to LMTD and inversely proportional to the area.
varies as square of the area.
none of these.

Answer 400

varies directly as the area and the LMTD.

Question 401

With increase in temperature, the total emissivity of conductors

increases
decreases
remains same
decreases linearly

Answer 401

increases

Question 402

Loss of heat from untagged steam pipe to the ambient air is by

conduction
convection
radiation
all (a), (b) & (c)

Answer 402

all (a), (b) & (c)

Question 403

Fruit juice (a heat sensitive material) can be concentrated in a __________ evaporator.

long tube
falling film
high pressure
none of these

Answer 403

falling film

Question 404

Tubes are held between top and bottom tube sheets in Calenderia type evaporator by keeping

both the tube sheets fixed.
both the tube sheets floating.
the top tube sheet floating and bottom tube sheet fixed.
the top tube sheet fixed and the bottom tube-sheet floating.

Answer 404

both the tube sheets fixed.

Question 405

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
2^0.8
2^0.2
2^0.5

Answer 405

2^0.8

Question 406

Heat sensitive materials can be concentrated in an evaporator employing

vacuum
high pressure
high residence time
none of these

Answer 406

vacuum

Question 407

A concentric double pipe heat exchanger as compared to the shell and tube heat exchanger for the same heat load requires

less heating surface.
more space.
lower maintenance cost.
none of these.

Answer 407

more space.

Question 408

In a shell and tube heat exchanger, floating head is used for

large temperature differentials.
high heat transfer co-efficient.
low pressure drop.
less corrosion of tubes.

Answer 408

large temperature differentials.

Question 409

Steam trap is used to

condense the steam flowing in the pipeline.
remove water resulting from partial condensation of steam.
stop the supply of steam.
none of these.

Answer 409

remove water resulting from partial condensation of steam.

Question 410

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 = 0.8hL
ha = 2hL
ha = hL
ha = 5hL

Answer 410

ha = 2hL

Question 411

What is the logarithmic mean of r1 and r2 ?
r1- r2 / ln(r1/r2)
r1- r2 / ln(r2/r1)
r2- r1 / ln(r1/r2)
r1- r2 / -ln(r1/r2)

Answer 411

r1- r2 / ln(r1/r2)

Question 412

Maximum heat transfer rate is achieved in __________ flow.

co-current
counter-current
turbulent
laminar

Answer 412

turbulent

Question 413

Wavelength corresponding to the maximum energy is inversely proportional to the absolute temperature. This is __________ law.

Stefan’s
Dalton’s
Wien’s
Kirchoffs

Answer 413

Wien’s

Question 414

Three material A, B and C of equal thickness 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.

40
95
70
50

Answer 414

70

Question 415

Which of the following parameters of the fluid is not very important, while deciding its route in a shell and tube heat exchanger?

Corrosiveness & fouling characteristics
pressure
viscosity
density

Answer 415

density

Question 416

Corrosiveness of steam condensate is due to the presence of

CO2
dissolved O2
both (a) & (b)
neither (a) nor (b)

Answer 416

both (a) & (b)

Question 417

The equation, (NSt x (NPr)^2/3) = f/2, is the __________ analogy.

Colburn
Reynolds
Prandtl
none of these.

Answer 417

Colburn

Question 418

The driving potential for the crystal growth during crystallisation is the __________ of the solution.

concentration
viscosity
super-saturation
density

Answer 418

super-saturation

Question 419

jH factor for heat transfer depends upon the __________ number.

Biot
Nusselt
Reynolds
Prandtl

Answer 419

Reynolds

Question 420

Kg of liquid evaporated per hour in an evaporator is defined as its

capacity
economy
steam load
none of these

Answer 420

capacity

Question 421

Pick out the correct statement.

1 kcal/hr.m.°C is equal to 1 BTU/hr. ft.°F.
In steady state heat conduction, the only property of the substance which determines the temperature distribution, is the thermal conductivity.
In unsteady state heat conduction, heat flows in the direction of temperature rise.
In heat transfer by forced convection, Grashoff number is very important.

Answer 421

In steady state heat conduction, the only property of the substance which determines the temperature distribution, is the thermal conductivity.

Question 422

Open pan evaporators are preferred to be used, when the solution to be concentrated is

scaling
highly viscous
corrosive
salty

Answer 422

highly viscous

Question 423

Cp*μ / K is termed as the __________ number.

Grashoff
Nusselt
Prandtl
Stanton

Answer 423

Prandtl

Question 424

The maximum heat transfer co-efficient from steam heating will be attained when the steam is

supersaturated
saturated
wet
none of these

Answer 424

saturated

Question 425

Kirchoff’s law is applicable to

monochromatic radiation only
total radiation only
both (a) and (b)
only volumes and not to surfaces

Answer 425

both (a) and (b)

Question 426

In a forward feed multiple effect evaporator, the pressure is

highest in last effect
lowest in last effect
same in all effects
dependent on the number of effects

Answer 426

lowest in last effect

Question 427

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

increase
decrease
remain unchanged
either increase or decrease; depends on the coolant flow rate

Answer 427

remain unchanged

Question 428

Which of the following situations can be approximated to a steady state heat transfer system ?

A red hot steel slab (having outside surface temperature as 1300°C) exposed to the atmosheric air at 35°C.
10 kg of dry saturated steam at 8 kgf/cm^2 flowing through a short length of stainless steel pipe exposed to atmospheric air at 35°C.
Boiling brine kept in open vessel when the bottom surface temperature of the vessel is maintained constant at 180°C.
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.

Answer 428

10 kg of dry saturated steam at 8 kgf/cm^2 flowing through a short length of stainless steel pipe exposed to atmospheric air at 35°C.

Question 429

Sensible heat absorbed by 1 lb of water when it is heated from 32 to 212°F may be around __________ BTU.

180
970
3.97
data insufficient, can’t be predicted

Answer 429

180

Question 430

The unit of conductance in SI unit is

W/m
W/m2
W/°K
W/m°K

Answer 430

W/°K

Question 431

In a shell and tube heat exchanger, square pitch compared to triangular pitch

gives a higher shell side pressure drop.
gives a lower shell side pressure drop.
can pack more surface area into a shell of given diameter.
none of these.

Answer 431

gives a lower shell side pressure drop.

Question 432

Which of the following has maximum thermal conductivity ?

Iron
Coal
Nitrogen
Tar

Answer 432

Iron

Question 433

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.

6
12
18
24

Answer 433

12

Question 434

The film co-efficient between condensing vapour and metal wall increases with

increasing temperature of the vapour.
decreasing temperature of the vapour.
increasing viscosity of the film of condensate.
increasing temperature drop.

Answer 434

increasing temperature of the vapour.

Question 435

Critical value of the __________ number governs the transition from laminar to turbulent flow in free convection heat transfer.

Grashoff
Reynolds
both ‘a’ & ‘b’
Prandtl & Grashoff

Answer 435

Prandtl & Grashoff

Question 436

The thermal boundary layer at NPr > 1

is thicker than hydrodynamic boundary layer.
is thinner than hydrodynamic boundary layer.
and the hydrodynamic boundary layer are identical.
disappears.

Answer 436

is thinner than hydrodynamic boundary layer.

Question 437

In the free convection regime of pool boiling, the heat flux is proportional to

Δt^1/2
Δt^2
Δt^5/4
Δt

Answer 437

Δt^5/4

Question 438

With increase in temperature, the thermal conductivity of fresh lubricating oil

increases
decreases
remains unchanged
may increase or decrease; depends on its composition

Answer 438

decreases

Question 439

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

0.4
0.5
0.6
0.7

Answer 439

0.7

Question 440

Nusselt number is the ratio of the temperature gradient at the wall to

temperature difference
heat flux
that across the entire pipe
none of these

Answer 440

that across the entire pipe

Question 441

The Stefan-Boltzman constant depends on the

medium
temperature
surface
none of these

Answer 441

none of these

Question 442

The units of resistance to heat transfer is

J.m^-2.K^-1
J.m^-1.K^-1
W.m^-2.K^-1
W^-1m^2K

Answer 442

W^-1m^2K

Question 443

Thermal conductivity of a gas at low density, __________ with increase in temperature.

decreases
increases
remains unchanged
may increase or decrease; depends on the gas

Answer 443

increases

Question 444

Nusselt number is a function of Prandtl number and __________ number of fluid in natural convection heat transfer.

Grashoff
Biot
Stantan
Reynolds

Answer 444

Grashoff

Question 445

__________ heat exchanger is used for chilling oil to be dewaxed.

U-tube
Double pipe
Fixed tube
Floating head

Answer 445

Double pipe

Question 446

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
< 0.75
< 0.50
< 0.25

Answer 446

< 0.75

Question 447

Pick out the wrong statement.

Economy of a multiple effect evaporator is not influenced by the boiling point elevation.
Two identical cubes of iron and copper will have the same heat content under the same conditions of temperature.
Double pipe heat exchangers are mostly used in the field of refrigeration.
Finned tube heat exchangers are suitable for heating air by steam.

Answer 447

Two identical cubes of iron and copper will have the same heat content under the same conditions of temperature.

Question 448

In sub-cooled boiling,

temperature of the heating surface is less than the boiling point of the liquid.
temperature of the heating surface is more than the boiling point of the liquid.
bubbles from heating surface are absorbed by the mass of the liquid.
very large vapour space is necessary.

Answer 448

temperature of the heating surface is more than the boiling point of the liquid.

Question 449

With increase in temperature, the thermal conductivity of a gas

increases.
decreases.
remains same.
may increase or decrease depending on the type of gas.

Answer 449

increases.

Question 450

A __________ evaporator employs an annular downtake.

basket type
horizontal
long tube vertical
none of these

Answer 450

basket type

Question 451

Radiation heat losses from satisfactorily insulated high pressure boilar may be about __________ percent.

1
7
18
26

Answer 451

7

Question 452

In natural convection heat transfer, the correlating parameter is the

Graetz number
Eckert number
Grashoff number
Bond number

Answer 452

Grashoff number

Question 453

Boiling of milk in an open vessel is an example of __________ boiling.

film
sub-cooled
saturated nucleate
none of these

Answer 453

film

Question 454

Pick out the wrong statement.

• In process heat exchangers, saturated steam is preferred over the superheated steam.
• 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.
• 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.
• A body at 925 K emits an energy of 1.42 x 10^11σW/m^2 (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.

Answer 454

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.

Question 455

Mode of heat transfer involved in the cooling of air cooled internal combustion engine is

conduction
natural convection
forced convection
none of these

Answer 455

natural convection

Question 456

Steady state one dimensional heat flow by conduction as given by Fourier’s low does not assume that

there is no internal heat generation.
boundary surfaces are isothermal.
material is anisotropic.
constant temperature gradient exists.

Answer 456

material is anisotropic.

Question 457

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

77.2
71.2
63.8
48.7

Answer 457

48.7

Question 458

In a shell and tube heat exchanger, the shell side fluid velocity can not be changed by changing the

tube layout
tube diameter
tube pitch
no. of baffles

Answer 458

tube diameter

Question 459

To reduce the tube side pressure drop for the same flow rate, the heat exchanger recomended is

1-2 heat exchanger
1-1 heat exchanger
3-2 heat exchanger
2-4 heat exchanger

Answer 459

1-1 heat exchanger

Question 460

Heat transfer co-efficient (h) for a fluid flowing inside a clean pipe is given by
h=0.023K/D (DVρ/μ)^0.8 (Cp.μ/k)^0.4.
This is valid for the value of NRe equal to

< 2100
2100-4000
> 4000
> 10000

Answer 460

> 10000

Question 461

Steam side heat transfer co-efficient in an evaporator is in the range of __________ kcal/hr.m^2°C.

10-50
100-500
1000-1500
5000-15000

Answer 461

5000-15000

Question 462

Correction is applied to LMTD for __________ flow.

parallel
counter
cross
none of these

Answer 462

cross

Question 463

If h1 = inner film co-efficient and /h2 = outer film co-efficient, then the overall heat transfer co-efficient is

always less than h1
always between h1 and h2
always higher than h2
dependent on metal resistance

Answer 463

always between h1 and h2

Question 464

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

heat the water from 0°C to 100°C.
evaporate the water.
to superheat the steam.
data insufficient, can’t be predicted

Answer 464

evaporate the water.

Question 465

For __________ Prandtl number values, the heat conduction will be negligible in the buffer zone.

extremely low
low
high
no

Answer 465

high

Question 466

The radiation heat flux from a heating element at a temperature of 800°C, in a furnace maintained at 300°C is 8 kW/m^2. The flux, when the element temperature is increased to 1000°C for the same furnace temperature is

11.2 kW/m^2
12.0 kW/m^2
14.6 kW/m^2
16.5 kW/m^2

Answer 466

16.5 kW/m^2

Question 467

Heat exchangers operating, when the asymptotic range is reached,

provide very large heat transfer co-efficient.
results in making part of the heating surface inactive.
results in abruptly increased velocity.
none of these.

Answer 467

results in making part of the heating surface inactive.

Question 468

In case of a shell and tube heat exchanger, the minimum and maximum baffle spacing is respectively (where, D = inside diameter of the shell)

D/5 and D
D/2 and 2 D
D/4 and 2 D
D and 2 D

Answer 468

D/5 and D

Question 469

In case of heat transfer by conduction in a hollow cylinder, __________ mean area is used to calculate the heat transfer rate.

geometric
arithmetic
logarithmic
either (a), (b) or (c)

Answer 469

logarithmic

Question 470

In a gas-liquid shell and tube heat exchanger, the

presence of a non-condensible gas decreases the condensing film co-efficient.
gases under high pressure are routed through the tube side, because high pressure gases are corrosive in nature.
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.
all ‘a’, ‘b’ & ‘c’

Answer 470

all ‘a’, ‘b’ & ‘c’

Question 471

Which is the most suitable for the concentration of foamy & frothy liquors ?

Agitated film evaporator
Long tube vertical evaporator
Open pan evaporator
None of these

Answer 471

Long tube vertical evaporator

Question 472

Boiling point elevation of an ideal solution

increases rapidly with temperature rise.
decreases rapidly with temperature rise
in independent of pressure.
both (b) and (c).

Answer 472

increases rapidly with temperature rise.

Question 473

The rate of heat transfer is a product of overall heat transfer co-efficient, the difference in temperature and the

heating volume
heat transfer area
Nusselt number
none of these

Answer 473

heat transfer area

Question 474

Which of the following has the lowest overall heat transfer co-efficient ?

Dowtherm
Molten sodium
Water
Air

Answer 474

Air

Question 475

Which of the following has the minimum absorptivity ?

Aluminium foil
Coal dust
Refractory bricks
Iron plates

Answer 475

Aluminium foil

Question 476

A black body when hot, emits heat radiation of __________ wavelengths.

small
large
all
one fixed

Answer 476

all

Question 477

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

CPA > CPB
CPA < CPB
kA < 0.5kB
kA>2 kB

Answer 477

kA < 0.5kB

Question 478

Vibrations in the tubes of a shell and tube heat exchanger is induced due to the

flow of fluid on the tube and shell sides.
oscillations in the flow of shell/tube sides fluid.
vibrations transmitted through piping and/or supports due to external reasons.
all (a), (b) and (c)

Answer 478

all (a), (b) and (c)

Question 479

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

0.2
0.4
0.6
0.8

Answer 479

0.6

Question 480

The Nusselt number for fully developed (both thermally and hydrodynamically) laminar flow through a circular pipe whose surface temperature remains constant is

1.66
88.66
3.66
dependent on NRe only

Answer 480

3.66

Question 481

Sensible heat of hot industrial flue gases can not be recovered by a/an

economiser
regenerator
ceramic recuperator
none of these

Answer 481

none of these

Question 482

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)

K/h0
2K/h0
h0/K
h0/2K

Answer 482

2K/h0

Question 483

Radiator of an automobile engine is a __________ type of heat exchanger.

co-current
cross-current
counter-current
direct contact

Answer 483

direct contact

Question 484

In pipe flow, heat is transferred from hot wall to the liquid by

conduction only.
forced convection only.
forced convection and conduction.
free and forced convection.

Answer 484

forced convection and conduction.

Question 485

The value of Stefan-Boltazman constant in SI unit is

5.6697 x 10^-8 W/m^2.°K^4
0.1714 x 10^-8 W/m^2.°K^4
5.6697 x 10^-8 kcal/m^2 . °K^4
0.1714 x 10^-8 kcal/m^2. °K^4

Answer 485

5.6697 x 10^-8 W/m^2.°K^4

Question 486

The unit of heat transfer co-efficient is

BTU/hr. ft^2°F
BTU/hr. °F. ft.
BTU/hr. °F
BTU/hr. ft

Answer 486

BTU/hr. ft^2°F

Question 487

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

1
1.74
6.1
37

Answer 487

6.1

Question 488

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^-2 is

165.4
167.5
172.5
175

Answer 488

165.4

Question 489

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

0.5
2
10
100

Answer 489

2

Question 490

Low thermal conductivity of heat insulating materials is due to its

dense structure.
high proportion of air space.
high specific heat.
none of these.

Answer 490

high proportion of air space.

Question 491

The most conducive surface for dropwise condensation to occur is the __________ surface.

coated
oily
glazed & polished
smooth

Answer 491

oily

Question 492

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

concentrations
chemical potentials
activity co-efficients
mass transfer co-efficients

Answer 492

concentrations

Question 493

Calburn analogy is applicable for the value of Prandtl number from

0.001 to 1
0.6 to 120
0.5 to 5
120 to 400

Answer 493

0.6 to 120

Question 494

Pick out the correct equation.

jH = (St)(Pr)^2/3 = f/2
jH = (St)(Pr)^1/3 = f/2
jH = (St)^2/3(Pr) = f/2
jH = (St)^1/3(Pr) = f/2

Answer 494

jH = (St)(Pr)^2/3 = f/2

Question 495

Multiple effect evaporators ar used to

increase the steam economy & decrease the capacity.
increase the steam economy & the capacity.
decrease the steam economy & the capacity.
decrease the steam economy & increase the capacity.

Answer 495

increase the steam economy & the capacity.

Question 496

Prandtl number is given by

Cp.μ/a
hD/k
Cp.μ/k
μ/h.Cp

Answer 496

Cp.μ/k

Question 497

Small scale evaporation is done in a

heat exchanger
condenser
multiple effect evaporator
steam jacketed kettle

Answer 497

steam jacketed kettle

Question 498

Heat exchanger tubes are never made of

plain carbon steel
stainless steel
lead
copper

Answer 498

lead

Question 499

At constant temperature, the thermal conductivities of gases __________ with rise in pressure.

decrease
increase
remain unchanged
may increase or decrease; depends on the pressure

Answer 499

decrease

Question 500

Reason for operating an evaporator in multiple effect is to secure

increased steam economy.
decreased steam consumption.
both (a) and (b).
increased capacity.

Answer 500

both (a) and (b).

Question 501

In a cooling tower, water becomes cool by

loosing sensible heat.
heat transfer to surroundings.
vaporisation due to heat loss to air.
loosing latent heat.

Answer 501

loosing sensible heat.

Question 502

In Fourier’s law, the proportionality constant is called the

heat transfer co-efficient
thermal diffusivity
thermal conductivity
Stefan-Boltzman constant

Answer 502

thermal conductivity

Question 503

The heat transfer co-efficient in film type condensation is __________ that for dropwise condensation.

greater than
lower than
is same as
half

Answer 503

lower than

Question 504

Pick out the wrong statement.

Bubble size increases with the dynamic viscosity of the liquid in case of nucleate pool boiling.
Thermal conductivity of a dry material is more than that of the damp material.
Ratio of its capacity to economy equals the steam consumption in kg/hr in an evaporator.
Vaporisation of organic substances in evaporator mostly causes foam formation.

Answer 504

Thermal conductivity of a dry material is more than that of the damp material.

Question 505

‘Duhring’s plot’ is of use in

extractive distillation
evaporation
leaching
absorption

Answer 505

evaporation

Question 506

Which has the lowest Prandtl number?

Liquid metal
Aqueous solution
Water
Lube oil

Answer 506

Liquid metal

Question 507

Pick out the wrong statement.

The value of hydrostatic head increases with increase in vacuum in the effect in a multiple effect evaporator system.
Entering velocity of the liquid in the tubes of natural circulation evaporators is in the range of 0.3 to 0.9 metre/second.
Duhring’s plot is used for calculating the concentration of solution.
In a multiple effect evaporation system, the number of effects is limited by the total boiling point rise.

Answer 507

Duhring’s plot is used for calculating the concentration of solution.

Question 508

Prandtl number for most of dry gases is about

0.001
0.72
70
150

Answer 508

0.72

Question 509

Prandtl number is the reciprocal of

Thermal diffusivity/Momentum diffusivity
thermal diffusivity x Momentum
Thermal diffusivity x Mass diffusivity
Mass diffusivity x Momentum diffusivity

Answer 509

Thermal diffusivity/Momentum diffusivity

Question 510

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

less than 45°C
more than 45°C
equal to 45°C
initially decreases and remains constant thereafter.

Answer 510

more than 45°C

Question 511

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

Nu = 0.023 Re^0.8 . Pr^0.4
Nu = (π/2) Gz
Nu = (2/π) Gz
Nu = 2Gz^0.5

Answer 511

Nu = (2/π) Gz

Question 512

Multiple effect evaporation accounts for

steam economy
lower operating costs
investment economy
none of these

Answer 512

steam economy

Question 513

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

more
less
same
more or less; depends on the nature of solution

Answer 513

more

Question 514

Maximum water velocity in tubes of a 1-2 shell and tube heat exchanger may be around __________ metres/second.

1
10
20
30

Answer 514

1

Question 515

In Biot number, the characteristic length used is the ratio of the __________ of the solid.

volume to surface area
perimeter to surface area
surface area to volume
surface area to perimeter

Answer 515

volume to surface area

Question 516

Which of the following is the most widely used heat insulating material for pipelines carrying steam ?

Tar dolomite bricks followed by asbestos.
Fireclay refractory followed by aluminium sheet.
Cotton followed by aluminium foil.
85% magnesia cement and glass wool.

Answer 516

85% magnesia cement and glass wool.

Question 517

Heat transfer co-efficient equation for forced convection, Nu = 0.023 Re0.8 . Prn, is not valid, if the value of

n = 0.4 is used for heating.
n = 0.3 is used for cooling.
Reynolds number for the flow involved is > 10000.
Reynolds number for the flow involved is < 2100.

Answer 517

Reynolds number for the flow involved is < 2100.

Question 518

Which is the best tube arrangement (in a shell and tube heat exchanger) if the fluids are clean and non-fouling ?

Square pitch
Triangular pitch
Diagonal square pitch
None of these

Answer 518

Triangular pitch

Question 519

In case of a multipass shell and tube heat exchanger, the temperature drop in the fluid

is inversely proportional to the resistance across which the drop occurs.
and the wall are proportional to individual resistances.
and the wall is not related.
none of these.

Answer 519

and the wall are proportional to individual resistances.

Question 520

The purpose of providing expansion bellows in the shell of tubular exchanger is to

increase the heating load.
impart structural strength.
account for the uneven expansion of shell and tube bundles.
facilitate increase of shell length, if needed.

Answer 520

account for the uneven expansion of shell and tube bundles.

Question 521

A graph between __________ is called Wilson plot.

1/U vs 1/v^0.8
1/v^0.8 vs U
v^0.8 Vs U
1/U vs 1/V

Answer 521

1/U vs 1/v^0.8

Question 522

Grashhoff number is given by

gD^3.β.Δtρ^2/μ^2
gD^2.β.Δtρ/μ^2
gD^2.β.ΔtP^2μ
gD^3.β.ΔtP^2/μ

Answer 522

gD^3.β.Δtρ^2/μ^2

Question 523

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

1200 W/m^2.K
894 W/m^2.K
333 W/m^2.K
287 W/m^2.K

Answer 523

333W/m^2.K

Question 524

In a laboratory test run, the rate of drying was found to be 0.5 x 10^-3 kg/m^2.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 m^2/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

2000
4000
5000
6000

Answer 524

5000

Question 525

Pick out the wrong statement.

‘Solvates’ are chemical compounds formed by solute with their solvents. When water is the solvent, then it is called a ‘hydrate’.
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.
Heat transfer co-efficient during nucleate boiling is not influenced by the agitation imparted.
In case of short tube vertical evaporators, area of central downtake is about 50 to 100% of the total tube cross-sectional area.

Answer 525

Heat transfer co-efficient during nucleate boiling is not influenced by the agitation imparted.

Question 526

What is the geometric mean of two heat transfer areas A1 and A2 ?

(A1 . A2)^1/2
(A1 + A2)^1/2
(1/2)(A1 . A2)^1/2
2(A1 . A2)^1/2

Answer 526

(A1 . A2)^1/2

Question 527

Evaporation by thermocompression results in the

saving of steam.
realisation of multiple effect economy in a single effect.
both (a) and (b).
none of these.

Answer 527

both (a) and (b).

Question 528

Forced circulation evaporators are normally used for concentrating liquids having

scaling characteristics
high viscosity
both(a) & (b)
neither (a) nor (b)

Answer 528

both(a) & (b)

Question 529

Steam economy in case of a triple effect evaporator will be

1
< 1
> 1
between 0 and 1

Answer 529

> 1