Deck 3: Heat Transfer and Thermal Conductivity: Part B

Question 1

Pick out the correct statement.

A)1 kcal/hr.m.°C is equal to 1 BTU/hr. ft.°F
B)In steady state heat conduction, the only property of the substance which determines the temperature distribution, is the thermal conductivity
C)In unsteady state heat conduction, heat flows in the direction of temperature rise
D)In heat transfer by forced convection, Grashoff number is very important

Answer 1

In steady state heat conduction, the only property of the substance which determines the temperature distribution, is the thermal conductivity

Question 2

Evaporation by thermo compression results in the

A)Saving of steam
B)Realisation of multiple effect economy in a single effect
C)Both (A) and (B)
D)None of these

Answer 2

Both (A) and (B)

Question 3

Fouling factor for a heat exchanger is given by (where, U1 = heat transfer co-efficient of dirty surface U2 = heat transfer co-efficient of clean surface).

A)U1 - U2
B)1/U1 - 1/U2
C)1/U2 - 1/U1
D)U2 - U1

Answer 3

1/U1 - 1/U2

Question 4

Use of transverse baffles in a shell and tube heat exchanger is done to increase the

A)Rate of heat transfer
B)Flow velocity
C)Turbulence of shell side fluid
D)All (A), (B) and (C)

Question 5

10 ⁵.All analogy equations connecting friction factor and heat transfer co-efficient apply only to

A)Wall or skin friction
B)Form friction
C)Both (A) and (B)
D)Turbulent flow

Question 6

Which of the following situations can be approximated to a steady state heat transfer system?

A)A red hot steel slab (having outside surface temperature as 1300°C) exposed to the atmospheric air at 35°C
B)10 kg of dry saturated steam at 8 kgf/cm² flowing through a short length of stainless steel pipe exposed to atmospheric air at 35°C
C)Boiling brine kept in open vessel when the bottom surface temperature of the vessel is maintained constant at 180°C
D)A sub-cooled refrigerant liquid at 8°C flowing at the rate of 6 Kg/minute through a copper pipe exposed to atmospheric air at 35°C

Question 7

Reason for operating an evaporator in multiple effects is to secure

A)Increased steam economy
B)Decreased steam consumption
C)Both (A) and (B)
D)Increased capacity

Question 8

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

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

Question 9

Fluid motion in the natural convection heat transfer between a solid surface and a fluid in contact with it, results from the

A)Existence of thermal boundary layer
B)Temperature gradient produced due to density difference
C)Buoyancy of the bubbles produced at active nucleation site
D)None of these

Question 10

Nusselt number is the ratio of the

A)Temperature gradient of the wall to that across the entire pipe
B)Temperature difference to the temperature gradient at the wall
C)Heat flux at the wall to that across the entire pipe
D)None of these

Question 11

Critical value of the __________ number governs the transition from laminar to turbulent flow in free convection heat transfer.

A)Grashoff
B)Reynolds
C)Both 'a' & 'b'
D)Prandtl & Grashoff

Question 12

Prandtl number is the reciprocal of

A)Thermal diffusivity/Momentum diffusivity
B)Thermal diffusivity × Momentum
C)Thermal diffusivity × Mass diffusivity
D)Mass diffusivity × Momentum diffusivity

Question 13

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

A)Equal to
B)More than
C)Less than
D)Either more or less than (depending upon the fluid)

Question 14

For a multipass shell and tube heat exchanger, the LMTD correction factor is always

A)1
B)> 1
C)< 1
D)Between 1 & 2

Question 15

Evaporator tubes are generally

A)Horizontal
B)Vertical
C)Inclined
D)Random

Question 16

Thermal conductivity of a gas at low density, __________ with increase in temperature.

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

Question 17

Which of the following is the most widely used heat insulating material for pipelines carrying steam?

A)Tar dolomite bricks followed by asbestos
B)Fireclay refractory followed by aluminium sheet
C)Cotton followed by aluminium foil
D)85% magnesia cement and glass wool

Question 18

The interchange factor for radiation heat transfer from surface 'x' to surface 'y' in case of an infinite parallel planes with emissivities ?x & ?y is given by

A)?x + ?y
B)?x . ?y
C)1/?x + 1/?y
D)(?x + ?y)/( ?x + ?y - ?x . ?y)

Question 19

If the thermal conductivity of a wall material is independent of temperature, the steady state temperature distribution in the very large thin plane wall having steady, uniform surface temperature follows __________ law.

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

Question 20

With increase in temperature, the thermal conductivity of a gas

A)Increases
B)Decreases
C)Remain same
D)May increase or decrease depending on the type of gas

Question 21

Grashoff number is given by

A)gD³.?.?t?²/?²
B)gD²??t?/?²
C)gD²??tP²?
D)gD³??tP²/?

Question 22

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

A)Logarithmic mean area
B)Arithmetic mean area
C)Geometric mean area
D)None of these

Question 23

Pick out the wrong statement.

A)Heat transfer from a hot body to cold body by the emission of heat waves is called radiation
B)Filmwise condensation takes place on non-wettable surfaces
C)The boiling point of a solution is affected by liquid head as well as boiling point elevation
D)None of these

Question 24

Choose the correct equation.

A)Nu = (Re) (Pr) (Gz)
B)Nu = (Re) (Pr) (St)
C)Nu = (Re) (Pr)
D)Nu = (Pr) (St)

Question 25

In a heat exchanger, the rate of heat transfer from the hot fluid to the cold fluid

A)Varies directly as the area and the LMTD
B)Directly proportional to LMTD and inversely proportional to the area
C)Varies as square of the area
D)None of these