Deck 20: Understanding the Dominant Mechanism of Heat Transfer

Question 1

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

A)Free convection
B)Entry length problem in laminar forced conduction (developing thermal boundary layer)
C)Mixed convection (both free and forced)
D)Forced convection

Answer 1

Mixed convection (both free and forced)

Question 2

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

A)Kirchoff's
B)Planck's
C)Wien's displacement
D)Stefan-Boltzmann

Answer 2

Kirchoff's

Question 3

The units of resistance to heat transfer is

A)J.m ^-2.K ^-1
B)J.m^-1.K ^-1
C)W.m ^-2.K ^-1
D)W^-1m²K

Answer 3

W^-1m²K

Question 4

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

A)Nu = 0.023 Re^0.8. Pr^0.4
B)Nu = (?/2) Gz
C)Nu = (2/?) Gz
D)Nu = 2Gz^0.5

Question 5

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

A)Increased
B)Decreased
C)No effect on
D)None of these

Question 6

Pick out the wrong statement.

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

Question 7

In sub-cooled boiling,

A)Temperature of the heating surface is less than the boiling point of the liquid
B)Temperature of the heating surface is more than the boiling point of the liquid
C)Bubbles from heating surface are absorbed by the mass of the liquid
D)Very large vapour space is necessary

Question 8

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

A)Square pitch
B)Triangular pitch
C)Diagonal square pitch
D)None of these

Question 9

Heat transfer by conduction results due to the transfer of free electrons, kinetic energy & vibrational energy from one molecule to another. Conduction heat transfer cannot take place

A)Between two bodies in physical contact with each other
B)Between two bodies not in physical contact with each other
C)From one part of a body to the another part of the same body
D)Both 'b' & 'c'

Question 10

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

A)Lower heat transfer area
B)Lower steam economy
C)Higher steam economy
D)Higher solute concentration in the product

Question 11

The thickness of condensate layer in filmwise condensation depends on the

A)Condensation rate
B)Surface configuration
C)Liquid flow rate from the surface
D)All (A), (B) and (C)

Question 12

With increase in temperature, the total emissivity of conductors

A)Increases
B)Decreases
C)Remain same
D)Decreases linearly

Question 13

Steam economy in case of a triple effect evaporator will be

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

Question 14

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

A)For fluids in laminar flow
B)For fluids in turbulent flow
C)When Grashoff number is very important
D)For liquid metals

Question 15

In a forward feed multiple effect evaporator unit

A)Viscosity of liquid is highest in first effect
B)Transfer from effect to effect is done by pumps
C)No pump is required to withdraw the product from the last effect
D)None of these

Question 16

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

A)Large temperature differentials
B)High heat transfer co-efficient
C)Low pressure drop
D)Less corrosion of tubes

Question 17

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

A)Greater than
B)Lower than
C)Is same as
D)Half

Question 18

Resistance to heat flow by conduction is proportional to (where, t & ? are thickness & density of the material respectively and A = area normal to the direction of heat flow.)

A)t
B)1/?
C)1/A
D)All (A), (B) & (C)

Question 19

Dropwise condensation is promoted on a/an __________ surface.

A)Glazed
B)Oily
C)Coated
D)Smooth

Question 20

Maximum heat transfer rate is obtained in __________ flow.

A)Laminar
B)Turbulent
C)Creeping
D)Transition region

Question 21

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

A)Corrosiveness & fouling characteristics
B)Pressure
C)Viscosity
D)Density

Question 22

In case of a super-cooled solution, which is on the verge of crystallisation, the free energy of the solution as compared to that of the solid is

A)More
B)Less
C)Same
D)More or less; depends on the nature of solution

Question 23

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

A)3.06 × 10 ⁵
B)6.12 × 10 ⁵
C)7.24 × 10 ⁵
D)9.08 × 10 ⁵

Question 24

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

A)Increases
B)Decreases
C)Remain constant
D)May increase or decrease, depends on the vacuum

Question 25

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

A)A higher capacity
B)A lower capacity
C)Lower economy
D)None of these