Deck 7: Heat Transfer and Heat Exchangers: Part B

Question 1

Heat waves

A)Cannot pass through vacuum
B)Travel in straight line
C)Can be reflected by a mirror
D)Both (B) and (C)

Answer 1

Both (B) and (C)

Question 2

Out of 100 kcal/second of incident radiant energy on the surface of a thermally transparent body, 300 kcal/second is reflected back. If the transmissivity of the body is 0.25, the emissivity of the surface will be

A)0.35
B)0.45
C)0.55
D)0.85

Answer 2

0.45

Question 3

If air (a non-condensing gas) is present in a condensing vapor stream, it will __________ the condensation rate of vapor.

A)Increase
B)Decrease
C)Not affect
D)Increase the condensing film co-efficient as well as

Answer 3

Decrease

Question 4

The most conducive surface for dropwise condensation to occur is the __________ surface.

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

Question 5

For a perfectly transparent surface (like gases), the

A)Absorptivity = 0
B)Transmissivity = 1
C)Reflectivity = 0
D)All (A), (B) & (C)

Question 6

Shell side pressure drop in a shell and tube heat exchanger does not depend upon the

A)Baffle spacing & shell diameter
B)Tube diameter & pitch
C)Viscosity, density & mass velocity of shell side fluid
D)None of these

Question 7

A fluid is flowing inside the inner tube of a double pipe heat exchanger with diameter 'd'. For a fixed mass flow rate, the tube side heat transfer co-efficient for turbulent flow conditions is proportional to

A)d^0.8
B)d^-0.2
C)d^-1
D)d^-1.8

Question 8

In evaporators, lowering the feed temperature

A)Increases the heating area required
B)Reduces the economy
C)Both (A) and (B)
D)Decreases the heating area required

Question 9

Heat transfer co-efficient (h) for a fluid flowing inside a clean pipe is given by h = 0.023 (K/D) (DV?/µ) 0.8 (CP.µ/k) 0.4. This is valid for the value of NRe equal to

A)< 2100
B)2100-4000
C)> 4000
D)> 10000

Question 10

In case of vertical tube evaporator, with increase in liquor level, the overall heat transfer co-efficient

A)Increases
B)Decreases
C)Is not affected
D)May increase or decrease; depends on the feed

Question 11

The ratio of kinematic viscosity to thermal diffusivity is called the __________ number.

A)Peclet
B)Prandtl
C)Stanton
D)Nusselt

Question 12

Intermittent tube cleaning is possible to be done in case of a __________ evaporator.

A)Basket type
B)Horizontal tube
C)Calandria
D)None of these

Question 13

Vibrations in the tubes of a shell and tube heat exchanger is induced due to the

A)Flow of fluid on the tube and shell sides
B)Oscillations in the flow of shell/tube sides fluid
C)Vibrations transmitted through piping and/or supports due to external reasons
D)All (A), (B) and (C)

Question 14

Pick out the wrong statement.

A)Orifice baffles are never used in a shell and tube heat exchanger
B)Pressure drop on the shell side of a heat exchanger depends upon tube pitch also
C)In a horizontal tube evaporator, surface blanketing by air is avoided
D)Split ring type and pull through type floating heads are two commonly used floating heads is heat exchangers

Question 15

What is the steam economy in case of a single effect evaporator system?

A)1
B)> 1
C)< 1
D)0.1

Question 16

Overall heat transfer co-efficient for cooling of hydrocarbons by water is about

A)50 -100 Kcal/hr.m².°C
B)50 -100 W/m².°K
C)50 -100 BTU/hr. ft.2°F
D)1000 - 1500 BTU/hr. ft.2°F

Question 17

For turbulent flow in a tube, the heat transfer co-efficient is obtained from the DittusBoelter correlation. If the tube diameter is halved and the flow rate is doubled, then the heat transfer co-efficient will change by a factor of

A)1
B)1.74
C)6.1
D)37

Question 18

For small temperature difference, the heat transfer rate as per Newton's law of cooling is proportional to (where, ?t = excess temperature)

A)?t
B)?t2
C)?t3
D)?(?t)

Question 19

Baffles in the shell side of a shell and tube heat exchanger

A)Increase the cross-section of the shell side liquid
B)Force the liquid to flow parallel to the bank
C)Increase the shell side heat transfer co-efficient
D)Decrease the shell side heat transfer co-efficient

Question 20

The ratio of total radiating power to the absorptivity of the body depends upon the __________ as per Kirchoff?s law.

A)Wavelength of radiation
B)Nature of the body
C)Temperature of the body
D)None of these

Question 21

__________ chart is known as transient heat conduction chart.

A)Dirhing's
B)Heisler's
C)Mollier's
D)Cox

Question 22

Steam side heat transfer co-efficient in an evaporator is in the range of __________ kcal/hr.m² °C.

A)10-50
B)100-500
C)1000-1500
D)5000-15000

Question 23

Controlling heat transfer film co-efficient is the one, which offers __________ resistance to heat transfer.

A)No
B)The least
C)The largest
D)Lower

Question 24

The main function of baffles provided in a shell and tube heat exchanger is to

A)Facilitate the cleaning of outer tube surface
B)Enhance turbulence
C)Hold the tubes in position
D)All 'a', 'b' & 'c'

Question 25

Pick out the wrong statement:

A)The capacity of an evaporator is reduced by the boiling point elevation
B)Corrosive liquid is normally passed through the tubes in a shell and tube heat exchanger
C)Steam jet ejector is used for vapor compression in a thermal recompression evaporator
D)Heat sensitive materials should be concentrated in high pressure evaporators

Question 26

Boiling point elevation of an ideal solution

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

Question 27

Which of the following is concerned with both heat and mass transfer?

A)Lewis relationship
B)Nusselt number
C)Kutateladze number
D)Froude number

Question 28

The thermal radiative flux from a surface of emissivity = 0.4 is 22.68 kW/m² . The approximate surface temperature (K) is (Stefan-Boltzmann constant = 5.67 × 10^-8 W/m² .K⁴ )

A)1000
B)727
C)800
D)1200

Question 29

Duhring?s rule is important in solving problems on

A)Distillation
B)Crystallisation
C)Evaporation
D)Humidification

Question 30

Which is the most suitable for the concentration of foamy & frothy liquors?

A)Agitated film evaporator
B)Long tube vertical evaporator
C)Open pan evaporator
D)None of these

Question 31

Heat transfer by radiation between two bodies at T1 & T2 and in an ambient temperature of Ta°C depends on

A)T1 - T2
B)T1 - Ta
C)T2 - Ta
D)None of these

Question 32

Air is best heated with steam in a heat exchanger of

A)Plate type
B)Double pipe type with fin on steam side
C)Double pipe type with fin on air side
D)Shell and tube type

Question 33

The film thickness for laminar film condensation on vertical surface __________ from top to bottom.

A)Cumulatively increases
B)Cumulatively decreases
C)Remain constant
D)And the surface conductance increase

Question 34

A metal wire of 0.01 m dia and thermal conductivity 200 W/m.K is exposed to a fluid stream with a convective heat transfer coefficient of 100 W/m² .K. The Biot number is

A)5.6
B)0.025
C)3.5
D)0.0035

Question 35

__________ paint has the minimum absorption co-efficient.

A)Black
B)White lead
C)Grey
D)Light cream

Question 36

The value of Stefan-Boltzmann constant in SI unit is

A)5.6697 × 10^-8W/m².°K⁴
B)0.1714 × 10^-8W/m².°K⁴
C)5.6697 × 10^-8kcal/m². °K⁴
D)0.1714 × 10^-8kcal/m². °K⁴

Question 37

For shell and tube heat exchanger, with increasing heat transfer area, the purchased cost per unit heat transfer area

A)Increases
B)Decreases
C)Remain constant
D)Passes through a maxima

Question 38

What is the thermal conductivity of a perfect heat insulator?

A)Zero
B)One
C)?
D)Between 0 and ?

Question 39

Mode of heat transfer in which the fluid moves under the influence of changes in fluid pressure produced by external work is called

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

Question 40

The ratio of momentum diffusivity to thermal diffusivity is the __________ number.

A)Prandtl
B)Nusselt
C)Stanton
D)Grashoff

Question 41

Steam traps are provided in steam carrying pipelines to

A)Condense steam
B)Release excess steam pressure by bleeding steam
C)Remove condensate and inert gases
D)None of these

Question 42

A metal ball of radius 0.1 m at a uniform temperature of 90°C is left in air at 30°C. The density and the specific heat of the metal are 3000 kg/m³ and 0.4 kJ/kg.K respectively. The heat transfer co-efficient is 50 W/m².K Neglecting the temperature gradients inside the ball, the time taken (in hours) for the ball to cool to 60°C is

A)555
B)55.5
C)0.55
D)0.15

Question 43

Kirchoff?s law applies to __________ radiation.

A)Total
B)Monochromatic
C)Both (A) & (B)
D)Neither (A) nor (B)

Question 44

Heat transfer occurs by natural convection because change in temperature causes difference in

A)Viscosity
B)Density
C)Thermal conductivity
D)Heat capacity

Question 45

Log mean temperature difference (LMTD) cannot be used, if

A)Heat transfer co-efficient over the entire heat exchanger is not constant
B)There exists an unsteady state
C)The heat capacity is not constant and there is a phase change
D)None of these

Question 46

The rate of heat transfer from a vertical plate by natural convection depends upon the temperature differences (?T) between wall and outside bulk. The proportionality is given as

A)(?T) ^1/4
B)(?T) ^1/2
C)(?T) ^5/4
D)(?T) ^3/4

Question 47

Pick out the wrong statement.

A)In case of heat transfer by purely forced convection, GR/Re 2 ? 1
B)The equivalent diameter of heat transfer for a duct of square cross-section (having each side as'x') is equal to 4x
C)Distillation process is not the same as evaporation
D)The effectiveness of nucleate boiling depends basically on the ease with which the bubbles are formed and detached from the heating surface

Question 48

If average heat transfer co-efficient is ha and the local coefficient at the end of the plate is hl then in case of heat transfer to a fluid flowing over a flat plate, heated over its entire length

A)ha = hl
B)ha = 2hl
C)ha = 0.5 hl
D)ha = 0.75 hl

Question 49

In a parallel flow heat exchanger, if the outlet temperature of hot and cold fluids are the same, then the log mean temperature difference (LMTD) is

A)Minimum
B)Maximum
C)Zero
D)Infinity

Question 50

Harmonic mean temperature difference is given by

A)?(?T1 . ?T2)
B)2 (?T1 . ?T2)/(?T1 + ?T2)
C)2 (?T1 . ?T2)/(?T1 - ?T2)
D)(?T1 - ?T2)/(?T1 . ?T2)