Deck 9: Heat Transfer and Thermal Properties: Part A

Question 1

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

A)In the central core of the fluid
B)Uniformly distributed throughout the fluid
C)Mainly confined to a thin film of fluid near the surface
D)None of these

Answer 1

Mainly confined to a thin film of fluid near the surface

Question 2

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 × 10^-3 kg/m² .s. The initial moisture content of the solid is 0.2 kg moisture/kg dry solid. The interfacial area available for drying is 4 m² /1000 kg of dry solid. The moisture content at the end of the drying period is (in kg moisture/kg dry solid).

A)0.5
B)0.05
C)0.1
D)0.15

Answer 2

0.1

Question 3

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

A)Thermal boundary layer lies inside the concentration boundary layer
B)Concentration boundary layer lies inside the thermal boundary layer
C)Thermal & concentration boundary layers are of equal thickness
D)Hydrodynamic (i.e., momentum) boundary layer is thicker than the other two

Answer 3

Thermal boundary layer lies inside the concentration boundary layer

Question 4

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

A)Isothermal
B)Non-isothermal
C)Both (A) and (B)
D)Neither (A) and (B)

Question 5

Thermal diffusivity is the most important in heat transfer by

A)Conduction
B)Radiation
C)Condensation
D)Natural convection

Question 6

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

A)Gases
B)Solids
C)Liquids
D)Both (B) and (C)

Question 7

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

A)0
B)1
C)> 1
D)< 1

Question 8

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

A)Vertical heat exchanger with steam on tube side
B)Vertical heat exchanger with steam on shell side
C)Horizontal heat exchanger with steam on tube side
D)Horizontal heat exchanger with steam on shell side

Question 9

The statement that "maximum wavelength of radiation is inversely proportional to the temperature" is __________ law.

A)Stefan-Boltzmann's
B)Planck's
C)Wien's displacement
D)None of these

Question 10

The steam ejector is used to

A)Remove condensate from the steam pipelines
B)Create vacuum
C)Superheat the steam
D)None of these

Question 11

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

A)Low density
B)High viscosity
C)Partial ionisation
D)Dense structure

Question 12

Heat flux is the time rate of heat transfer per unit

A)Length
B)Area
C)Volume
D)None of these

Question 13

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

A)Transient heat conduction
B)Natural convection
C)Forced convection
D)Radiation

Question 14

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

A)Feed
B)Depth of liquid over heating surface
C)Pressure difference between steam chest and vapour space
D)All (A), (B) and (C)

Question 15

A diathermanous substance __________ the thermal radiation completely.

A)Absorbs
B)Reflects
C)Transmits
D)None of these

Question 16

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

A)Surrounding temperature
B)Average temperature of the fin
C)Temperature of the fin end
D)Constant temperature equal to that of the base

Question 17

Boiling point elevation of a solution of NaOH

A)Increases rapidly with temperature rise
B)Is almost independent of temperature
C)Is almost independent of pressure
D)Both (B) and (C)

Question 18

Thermal conductivity of a conducting solid material depends upon its

A)Temperature
B)Porosity
C)Both (A) & (B)
D)Neither (A) nor (B)

Question 19

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

A)?d
B)d
C)D²
D)d1.5

Question 20

Pick out the wrong statement.

A)Reciprocal of the resistance to heat flow is called thermal conductance
B)Unit of thermal conductance is W/°K
C)Thermal conductance of a wall of thickness 'L', thermal conductivity 'k' and heat flow area 'A' is kL/A
D)None of these

Question 21

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

A)Maximum resistance in the series
B)Sum of all resistances
C)Average of all resistances
D)Minimum resistance presents in the series

Question 22

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

A)Film boiling
B)Nucleate boiling
C)Vapour binding
D)None of these

Question 23

Prandtl and Reynolds analogy are same, when Prandtl number is

A)0.5
B)1
C)> 2
D)1.5

Question 24

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

A)Temperature of the body only
B)Wavelength of monochromatic radiation
C)Both (A) and (B)
D)Nature of material of body

Question 25

Arithmetic mean area can be used in heat transfer problem to calculate the heat flow by conduction through a cylinder which is

A)Thin walled having the value of Ao Ai /< 2
B)Thick walled
C)Having the value of Ao/Ai > 2
D)Both (B) and (C)

Question 26

Fourier's law applies to the heat transfer by

A)Convection
B)Radiation
C)Conduction
D)All (A), (B) & (C)

Question 27

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

A)Reynolds
B)Nusselt
C)Grashoff
D)Both (B) & (C)

Question 28

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

A)Painting the surface black
B)Painting the surface white (with aluminium paint)
C)Giving the surface a mirror finish
D)Roughening the surface

Question 29

In a heat exchanger, one transfer unit means

A)A section of the exchanger in which change in temperature of one stream equals the average driving force in the section
B)The size of the exchanger in which heat transfer rate is 1 kcal/hr
C)Both (A) and (B)
D)None of these

Question 30

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

A)IR
B)I² R
C)IR²
D)I² R²

Question 31

A 2-4 heat exchanger involves

A)Only counter-flow of fluids
B)Only parallel-flow of fluids
C)Both counter and parallel-flow of the fluids
D)Smaller pressure drop compared to 1-2 exchanger

Question 32

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

A)Twice
B)Four times
C)Square root of
D)Square of

Question 33

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

A)0.25 D
B)0.50 D
C)0.75 D
D)None of these

Question 34

The heat flux (from outside to inside) across an insulating wall with thermal conductivity, K = 0.04 W/m.°K and thickness 0.16m is 10 W/m² . The temperature of the inside wall is - 5°C. The outside wall temperature is

A)25°C
B)30°C
C)35°C
D)40°C

Question 35

Latent heat absorbed by 1 lb of water at 212°F, when it is changed to steam at 212°F, may be around __________ BTU.

A)180
B)970
C)3.97
D)None of these

Question 36

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

A)Prandtl number
B)Grashoff number
C)Thermal conductivity
D)Viscosity

Question 37

Baffle spacing

A)Is not the same as baffle pitch
B)Should be less than one fifth the diameter of the shell
C)Should be less than the inside diameter of the shell
D)None of these

Question 38

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

A)C/E
B)E/C
C)CE
D)1/CE

Question 39

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

A)? t
B)? 1/t
C)? ?t
D)Independent of 't'

Question 40

Which of the following is unimportant in forced convection?

A)Reynolds number
B)Prandtl number
C)Grashoff number
D)None of these

Question 41

The purpose of floating head in a heat exchanger is to

A)Avoid buckling of tubes
B)Provide support for tubes
C)Decrease the pressure drop
D)Facilitate its lengthening, if needed

Question 42

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.

A)Laminar
B)Transition
C)Both 'a' & 'b'
D)Highly turbulent

Question 43

Mechanical recompression evaporation is used in the production of

A)Alcohol
B)Distilled water
C)Salt
D)Fruits jam

Question 44

Pick out the wrong statement.

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

Question 45

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

A)Its evaporation loss
B)Excessive corrosion
C)Uneconomic LMTD
D)Decrease in heat exchanger efficiency

Question 46

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

A)Capacity
B)Rate of evaporation
C)Economy
D)Rate of vaporisation

Question 47

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

A)Get economical temperature difference by using moderate pressure steam
B)Facilitate forward feeding in multiple effect evaporation
C)Concentrate heat sensitive materials
D)Achieve very high concentration of the final product

Question 48

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

A)Cube
B)Cylinder
C)Sphere
D)Equal for all the three

Question 49

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

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

Question 50

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

A)Dittus-Boelter equation
B)Sieder-Tate equation
C)Nusselt equation
D)None of these