heat transfer

Question 1

Air is best heated with steam in a heat exchanger of

Answer 1

double pipe type with fin on air side

Question 2

Black liquor generated during paper manufacture is concentrated in a

Answer 2

multiple effect evaporator.

Question 3

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

Answer 3

nucleate boiling

Question 4

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

Answer 4

C/E

Question 5

Reynold’s analogy states that

Answer 5

Nst α f

Question 6

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

Answer 6

increases

Question 7

Fourier’s law applies to the heat transfer by

Answer 7

conduction

Question 8

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

Answer 8

none of these.

Question 9

In the equation Q = UAΔt; Δt is

Answer 9

logarithmic mean temperature difference.

Question 10

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

Answer 10

decreases

Question 11

For an ideal black body

Answer 11

absorptivity = 1

Question 12

Fouling factor

Answer 12

accounts for additional resistances to heat flow.

Question 13

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

Answer 13

decreases

Question 14

The steam ejector is used to

Answer 14

create vacuum.

Question 15

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

Answer 15

passes through a maxima

Question 16

In a heat exchanger, floating head is provided to

Answer 16

relieve stresses caused by thermal expansion.

Question 17

Leidenfrost point is a term concerned with the

Answer 17

boiling of a liquid on a hot surface.

Question 18

A process stream of dilute aqueous solution flowing at the rate of10 Kg.s-1 is to be heated. Steam condensate at 95°C is available for heating purpose, also at a rate of 10 Kg.s-1. A 1 - 1 shell and tube heat exchanger is available. The best arrangement is

Answer 18

counter flow with process stream on shell side.

Question 19

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

Answer 19

I2R

Question 20

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 20

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

Question 21

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.

Answer 21

highly turbulent

Question 22

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

Answer 22

mercury

Question 23

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

Answer 23

increases.

Question 24

In case of evaporators, liquid entrainment results primarily due to

Answer 24

foaming of the solution.

Question 25

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

Answer 25

increase the shell side heat transfer co-efficient.

Question 26

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

Answer 26

finned tube heat exchanger with air outside and steam inside.

Question 27

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

Answer 27

W/m2°K

Question 28

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

Answer 28

economy

Question 29

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 29

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

Question 30

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

Answer 30

boiling point elevations

Question 31

Increasing the liquor level in the evaporator results in the

Answer 31

decreased capacity.

Question 32

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

Answer 32

smaller than

Question 33

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

Answer 33

the largest

Question 34

Pick out the wrong statement. The controlling resistance in case of heating of air by condensing steam is in the air film. The log mean temperature difference (LMTD) for counter flow and parallel flow can be theoretically same when any one of the fluids (hot or cold fluid) passes through the heat exchanger at constant temperature. In case of a 1 - 2 shell and tube heat exchanger, the LMTD correction factor value increases sharply, when a temperature cross occurs. Phase change in case of a pure fluid at a given pressure from liquid to vapor or vice-versa occurs at saturation temperature.

Answer 34

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

Question 35

The average heat transfer co-efficient for laminar film condensation on vertical surface is inversely proportional to (where, ΔT = Temperature drop across condensate film )

Answer 35

(ΔT)1/4

Question 36

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

Answer 36

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

Question 37

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

Answer 37

higher economy

Question 38

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

Answer 38

less than that of the uninsulated steam pipe.

Question 39

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

Answer 39

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

Question 40

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

Answer 40

between two bodies not in physical contact with each other.

Question 41

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

Answer 41

Schmidt number.

Question 42

Finned tube heat exchangers

Answer 42

give larger area per tube.

Question 43

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

Answer 43

may increase or decrease ; depends on the material

Question 44

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

Answer 44

Planck's law

Question 45

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

Answer 45

1

Question 46

In a shell and tube heat exchanger,

Answer 46

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

Question 47

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

Answer 47

for fluids in tubulent flow.

Question 48

Out of the following four assumptions used in the derivation of the equation for LMTD , which one is subject to the largest deviation in practice ?

Answer 48

Constant rate of fluid flow.

Question 49

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

Answer 49

radiation becomes important

Question 50

Temperature profile in steady state heat transfer is

Answer 50

linear

Question 51

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

Answer 51

increases the overall heat transfer coefficient

Question 52

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

Answer 52

film

Question 53

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

Answer 53

0.2 D

Question 54

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 )

Answer 54

T4

Question 55

What is the unit of thermal conductivity ?

Answer 55

Kcal/hr.m.°C

Question 56

. 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

Answer 56

all bodies

Question 57

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

Answer 57

low film co-efficient

Question 58

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

Answer 58

3.06 x l05

Question 59

LMTD for counterflow and prallel flow heat exchanger will be the same, when the

Answer 59

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

Question 60

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)

Answer 60

NPr1/3

Question 61

Economy of a multiple effect evaporator depends upon the

Answer 61

heat balance consideration

Question 62

The Graetz number is concerned with the

Answer 62

heat transfer in laminar flow.

Question 63

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

Answer 63

called the angle of vision.

Question 64

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 64

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

Question 65

In forced convection, the heat transfer depends on

Answer 65

Re, Pr

Question 66

With increase in temperature, the thermal conductivity of most liquids

Answer 66

decreases

Question 67

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

Answer 67

radiation

Question 68

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 68

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

Question 69

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

Answer 69

in high range of temperature differences.

Question 70

Stefan-Boltzman law applies to __________ body.

Answer 70

black

Question 71

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

Answer 71

some (whut?)

Question 72

In case of parallel flow heat exchanger, the lowest temperature theoretically attainable by the hot fluid is __________ the outlet temperature of the cold fluid.

Answer 72

equal to

Question 73

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

Answer 73

heat exchanger

Question 74

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

Answer 74

Grashoff number

Question 75

ick out the wrong statement: With change in temperature, the radiant energy emitted by a black body remains unchanged. Absorptivity of a body approaches unity in case of diffuse reflection. Absorptivity of a perfectly black body is unity. Value of Stefan-Boltzman constant is 4.876 x 10-8 KCal/m2.hr.°K4.

Answer 75

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

Question 76

Planck's distribution law is valid for __________ bodies.

Answer 76

coloured

Question 77

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

Answer 77

increased

Question 78

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

Answer 78

0°K

Question 79

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

Answer 79

εx+εy/(εx+εy-εx*εy)