Deck 4: Heat Transfer and Thermal Conductivity: Part C

Question 1

Pick out the wrong statement.

A)In process heat exchangers, saturated steam is preferred over the superheated steam
B)The maximum is the emissive power of a surface at a temperature T1 occurs at a wavelength of ?1. If the surface temperature is halved, the maximum in the emissive power would occur at a wavelength of 0.5 ?1
C)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
D)A body at 925 K emits an energy of 1.42 x 1011?W/m² (? is the Stefan-Boltzmann constant) in the wavelength band between 3 ?m to 4 ?m. The fraction of this energy in the total energy emitted over the entire wavelength range is equal to emissivity

Answer 1

The maximum is the emissive power of a surface at a temperature T1 occurs at a wavelength of ?1. If the surface temperature is halved, the maximum in the emissive power would occur at a wavelength of 0.5 ?1

Question 2

The Sieder-Tate correlation for heat transfer in turbulent flow in pipe gives Nu ? 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

A)D-1.8
B)D-0.2
C)D0.2
D)D1.8

Answer 2

D-0.2

Question 3

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

A)0.2 D
B)0.5 D
C)0.66 D
D)0.80 D

Answer 3

0.2 D

Question 4

For heat flow through very thick walled cylinder, use _________mean radius.

A)Arithmetic
B)Logarithmic
C)Geometric
D)Either (A) or (C)

Question 5

Corrosiveness of steam condensate is due to the presence of

A)CO₂
B)Dissolved O₂
C)Both (A) & (B)
D)Neither (A) nor (B)

Question 6

In a shell and tube heat exchanger, the shell side fluid velocity can?t be changed by changing the

A)Tube layout
B)Tube diameter
C)Tube pitch
D)Number of baffles

Question 7

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

A)Called the view factor
B)Called the angle of vision
C)Proportional to the square of the distance between surfaces
D)Expressed in terms of radians

Question 8

LMTD for evaporators & condensers for a given terminal parameters & set of conditions for counter-flow 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.

A)Parallel
B)Mixed
C)Counter flow
D)Same in either 'a', b' or 'c'

Question 9

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

A)0
B)R² /(R1+R² )
C)1
D)(R1/R² ) ²

Question 10

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

A)Supersaturated
B)Saturated
C)Wet
D)None of these

Question 11

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

A)Heat the water from 0°C to 100°C
B)Evaporate the water
C)To superheat the steam
D)Data insufficient, can't be predicted

Question 12

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

A)More
B)Less
C)Some
D)Either more or less; depends on the nature of vapor

Question 13

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

A)Re.Gz
B)Nu, Pr
C)Nu, Pr, Re
D)Nu, Gz

Question 14

Which of the following has maximum thermal conductivity?

A)Iron
B)Coal
C)Nitrogen
D)Tar

Question 15

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

A)Geometric
B)Arithmetic
C)Logarithmic
D)Either (A), (B) or (C)

Question 16

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

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

Question 17

Thermal diffusivity of a material

A)Has the unit m²/sec
B)Is defined as K/? . Cp
C)Is the ratio of thermal conductivity to thermal capacity
D)All (A), (B) and (C)

Question 18

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

A)Long vertical evaporator
B)Horizontal tube evaporator
C)Agitated film evaporator
D)Calandria vertical tube evaporator

Question 19

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

A)Temperature difference
B)Heat flux
C)That across the entire pipe
D)None of these

Question 20

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

A)Dowtherm
B)Molten sodium
C)Water
D)Air

Question 21

What is the unit of thermal conductivity?

A)Kcal/hr. m²°C
B)Kcal/hr.m.°C
C)Kcal/hr.m
D)Kcal/hr. °C

Question 22

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

A)Heat transfer co-efficient are high
B)There is no risk of contamination
C)There is no mist formation
D)Cost of equipment is lower

Question 23

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

A)Both the tube sheets fixed
B)Both the tube sheets floating
C)The top tube sheet floating and bottom tube sheet fixed
D)The top tube sheet fixed and the bottom tube-sheet floating

Question 24

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

A)0.2
B)0.4
C)0.6
D)0.8

Question 25

What is the emissivity of a black body?

A)0
B)1
C)0.5
D)0.90