Deck 6: Heat Transfer and Heat Exchangers: Part A

Question 1

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

A)Increases
B)Decreases
C)Remains unchanged
D)May increase or decrease; depends on its composition

Answer 1

Decreases

Question 2

Vent pipes are provided in a condenser to

A)Remove non-condensable gases
B)Purge the condenser
C)Facilitate easy cleaning of tubes
D)None of these

Answer 2

Remove non-condensable gases

Question 3

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

A)1-2 heat exchanger
B)1-1 heat exchanger
C)3-2 heat exchanger
D)2-4 heat exchanger

Answer 3

1-1 heat exchanger

Question 4

LMTD for counter-flow and parallel flow heat exchanger will be the same, when the

A)Cold fluid is heated to a certain temperature by condensing steam (isothermal fluid)
B)Outlet temperature of both the hot and cold fluid are same
C)Outlet temperature of hot fluid is less than the outlet temperature of the cold fluid
D)None of these

Question 5

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

A)Nucleate
B)Local
C)Pool
D)Saturated

Question 6

Steam trap is used to

A)Condense the steam flowing in the pipeline
B)Remove water resulting from partial condensation of steam
C)Stop the supply of steam
D)None of these

Question 7

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

A)Extremely low
B)Low
C)High
D)No

Question 8

Planck's distribution law is valid for __________ bodies.

A)Black
B)White
C)Coloured
D)All (A), (B) & (C)

Question 9

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

A)1 to 100
B)0.5 to 50
C)10 to 100
D)100 to 1000

Question 10

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

A)Gives a higher shell side pressure drop
B)Gives a lower shell side pressure drop
C)Can pack more surface area into a shell of given diameter
D)None of these

Question 11

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.

A)10
B)> 10
C)< 10
D)Either (B) or (C), depends on the mass of the body

Question 12

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

A)Presence of a non-condensable gas decreases the condensing film co-efficient
B)Gases under high pressure are routed through the tube side, because high pressure gases are corrosive in nature
C)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
D)All 'a', 'b' & 'c'

Question 13

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

A)Occupies smaller space
B)Is more economical
C)Is easy to operate and maintain
D)All (A), (B) and (C)

Question 14

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

A)Approach
B)Log mean temperature difference
C)Arithmetic mean temperature difference
D)Geometric mean temperature difference

Question 15

The Stefan-Boltzmann constant depends on the

A)Medium
B)Temperature
C)Surface
D)None of these

Question 16

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

A)CPA > CPB
B)CPA < CPB
C)kA < 0.5 kB
D)kA >2 kB

Question 17

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

A)In low range of temperature differences
B)In high range of temperature differences
C)Because of its low cost
D)To prevent corrosion of the tube bundles

Question 18

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

A)U1 = U2
B)U2 > U1
C)U1 > U2
D)U1 = dirt factor - U2

Question 19

In forced convection, the Nusselt number is a function of

A)Re and Pr
B)Re and Gr
C)Pr and Gr
D)Re and Sc

Question 20

Maximum heat transfer rate is achieved in __________ flow.

A)Co-current
B)Counter-current
C)Turbulent
D)Laminar

Question 21

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

A)Co-current
B)Cross-current
C)Counter-current
D)Direct contact

Question 22

Temperature profile in steady state heat transfer is

A)Asymptotic
B)Hyperbolic
C)Parabolic
D)Linear

Question 23

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

A)Tube side pressure drop and the heat transfer rate
B)Convective heat transfer co-efficient
C)Effective tube surface area for convective heat transfer
D)All (A) (B) and (C)

Question 24

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 material A 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.

A)6
B)12
C)18
D)24

Question 25

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

A)0.5
B)2
C)10
D)100

Question 26

Economy of a multiple effect evaporator depends upon the

A)Heat balance consideration
B)Rate of heat transfer
C)Both (A) and (B)
D)Neither (A) nor (B)

Question 27

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

A)Temperature gradient
B)Mechanical strength of the equipment
C)Heat transfer area
D)Heat transfer co-efficient

Question 28

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

A)Stefan's
B)Dalton's
C)Wien's
D)Kirchoff's

Question 29

Trap is used to remove __________ from steam pipe lines.

A)Steam
B)Condensate
C)Non-condensable
D)None of these

Question 30

Prandtl number is given by

A)CP µ/a
B)hD/k
C)CP µ/k
D)µ/h CP

Question 31

Pick out the wrong statement:

A)With change in temperature, the radiant energy emitted by a black body remains unchanged
B)Absorptivity of a body approaches unity in case of diffuse reflection
C)Absorptivity of a perfectly black body is unity
D)Value of Stefan-Boltzmann constant is 4.876 × 10^-8 KCal/m² .hr.°K⁴

Question 32

Prandtl number is the ratio of

A)Momentum diffusivity to mass diffusivity
B)Momentum diffusivity to thermal diffusivity
C)Thermal diffusivity to mass diffusivity
D)Thermal diffusivity to momentum diffusivity

Question 33

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

A)?t1/2
B)?t2
C)?t5/4
D)?t

Question 34

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

A)1
B)7
C)18
D)26

Question 35

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

A)Pool
B)Nucleate
C)Transition
D)Film

Question 36

In SI units, fouling factor is expressed in

A)m²°K/W
B)W/m²°K
C)m²°K
D)m°K/W

Question 37

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

A)Capacity
B)Economy
C)Steam load
D)None of these

Question 38

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

A)77.2
B)71.2
C)63.8
D)48.7

Question 39

In forced convection, the heat transfer depends on

A)Re, Pr
B)Re, Gr
C)Mainly Gr
D)Re only

Question 40

Agitated film evaporator is suitable for concentrating __________ liquids.

A)Foaming
B)Viscous
C)Very thin
D)Corrosive

Question 41

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

A)125
B)133
C)150
D)160

Question 42

Kirchoff's law is applicable to

A)Monochromatic radiation only
B)Total radiation only
C)Both (A) and (B)
D)Only volumes and not to surfaces

Question 43

The radiation heat flux from a heating element at a temperature of 800°C, in a furnace maintained at 300°C is 8 kW/m² . The flux, when the element temperature is increased to 1000°C for the same furnace temperature is

A)11.2 kW/m²
B)12.0 kW/m²
C)14.6 kW/m²
D)16.5 kW/m²

Question 44

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

A)Decreases
B)Increases
C)Remain constant
D)First decreases upto certain temperature and then increases

Question 45

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

A)Low Reynold's number
B)Very low Grashoff number
C)Molten metals
D)All (A), (B) and (C)

Question 46

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

A)Small
B)Large
C)All
D)One fixed

Question 47

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

A)Sum
B)Difference
C)Ratio
D)None of these

Question 48

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

A)40.5
B)34.4
C)26.8
D)25

Question 49

Heat transfer rate per unit area is called

A)Thermal conductivity
B)Heat flux
C)Heat transfer co-efficient
D)Thermal diffusivity

Question 50

The unit of heat transfer co-efficient is

A)BTU/hr. ft2°F
B)BTU/hr. °F. ft
C)BTU/hr. °F
D)BTU/hr. ft

Question 51

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

A)(COP)R = (COP)HP = 0.6
B)(COP)R = 2.5; (COP)HP = 1.5
C)(COP)R = 1.5; (COP)HP = 2.5
D)(COP)R = (COP)HP = 2.5

Question 52

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

A)0.06 to 120
B)0.6 to 120
C)1 to 103
D)1 to 50

Question 53

Natural convection is characterised by

A)Grashoff number
B)Peclet number
C)Reynolds number
D)Prandtl number

Question 54

Colburn analogy is applicable for the value of Prandtl number from

A)0.001 to 1
B)0.6 to 120
C)0.5 to 5
D)120 to 400

Question 55

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

A)Counter flow with process stream on shell side
B)Counter flow with process stream on tube side
C)Parallel flow with process stream on shell side
D)Parallel flow with process stream on tube side

Question 56

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

A)W/m.k
B)W/m
C)W/m.K²
D)None, 'a' is just a number

Question 57

Heat exchanger tubes are never made of

A)Plain carbon steel
B)Stainless steel
C)Lead
D)Copper

Question 58

Presence of a non-condensing gas in a condensing vapour

A)Increases the rate of condensation
B)Decreases thermal resistance
C)Is desirable to increase the film co-efficient
D)None of these

Question 59

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

A)K/h₀
B)2K/h₀
C)hi/K
D)2hi/K

Question 60

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

A)P and Q
B)P and R
C)P and S
D)R and S

Question 61

In natural convection heat transfer, the correlating parameter is the

A)Graetz number
B)Eckert number
C)Grashoff number
D)Bond number

Question 62

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

A)Finned tube heat exchanger with air inside and steam outside
B)Finned tube heat exchanger with air outside and steam inside
C)Shell and tube heat exchanger with air inside tubes and steam on shell side
D)Shell and tube heat exchanger with air on shell side and steam inside tubes

Question 63

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

A)Reduce the capacity
B)Reduce the economy
C)Increase the economy
D)None of these

Question 64

Correction is applied to LMTD for __________ flow.

A)Parallel
B)Counter
C)Cross
D)None of these

Question 65

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

A)(?T)²
B)(?T)⁴
C)(?T)³
D)?(?T)

Question 66

Increasing the liquor level in the evaporator results in the

A)Decreased capacity
B)Increase in liquor film co-efficient
C)Decreased effect of hydrostatic head
D)Increased true temperature drop

Question 67

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

A)Velocity of circulation
B)Liquor-film co-efficient
C)Both (A) and (B)
D)Neither (A) and (B)

Question 68

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

A)D/5 and D
B)D/2 and ² D
C)D/4 and ² D
D)D and ² D

Question 69

Pick out the wrong statement.

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

Question 70

Double pipe heat exchangers are used

A)When heat transfer area required is very high
B)When heat transfer area required is very low, i.e. (100-200 ft2).
C)Because it occupies less floor area
D)Because it is less costly

Question 71

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

A)Economiser
B)Regenerator
C)Ceramic recuperator
D)None of these

Question 72

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

A)Zero
B)Unity
C)Same for all wavelengths
D)Different for all wavelengths

Question 73

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

A)Greater than that for un-insulated steam pipe
B)Less than that of the un-insulated steam pipe
C)Equal to that of the un-insulated steam pipe
D)Less than the steam pipe with 5 cms insulation

Question 74

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

A)0
B)1
C)> 1
D)Between 0 and 1

Question 75

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

A)Concentrations
B)Chemical potentials
C)Activity co-efficients
D)Mass transfer co-efficients