Vizag Steel Plant (VSP)

Sample Questions

Chemical Engineering  Heat Transfer

1.Loss of heat from unlagged steam pipe to the ambient air is by

• conduction
• convection
• radiation
• all of above

2.Fourier's law applies to the heat transfer by

• convection
• radiation
• conduction
• all of above

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

• isothermal
• non-isothermal
• both above
• none of above

4.Unsteady state heat conduction occurs, when

• temperature distribution is independent of time
• temperature distribution is dependent on time
• heat flows in one direction only
• three dimensional heat flow is concerned

5.What is the unit of thermal conductivity ?

• Kcal/hr.m2 0C
• Kcal/hr.m. 0C
• Kcal/hr.m
• Kcal/hr.0C

6.The unit of heat transfer co-efficient is

• BTU/hr.ft2 0F   (Ans)
• BTU/hr.0F.ft.
• BTU/hr.0F
• BTU/hr.ft

7.Which of the following is correct ?

• Rate = Driving force * Resistance
• Driving force = Rate * Resistance    (Ans)
• Resistance = Driving force * Rate
• Rate = Resistance/Driving force

8.An insulator should have

• low thermal conductivity    (Ans)
• high thermal conductivity
• less resistance to heat flow
• a porous structure

9.Heat flux through several resistance in series in analogous to the current flowing through several

• resistances in parallel
• capacitors in series
• resistances in series     (Ans)
• none of these

10.The overall resistance for heat transfer through a series of flat resistance, is the ..... of the resistances.

• average
• geometric mean
• product
• sum     (Ans)

11.Which of the following has the highest thermal conductivity ?

• Brick
• Air
• Water
• Silver     (Ans)

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

• Logarithmic mean area     (Ans)
• Arithmetic mean area
• Geometric mean area
• None of these

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

• arithmetic
• logarithmic     (Ans)
• geometric
• either or (c)

14.What is the logarithmic mean of r1 and r2 ?

•  r1 - r2/ln r1/r2     (Ans)
•  r1 - r2/ln r2/r1
•  r2 - r1/ln r1/r2
•  r1 - r2/ - ln r1/r2

15.What is Nusselt number ?

• Cp.µ/k
• hD/k    (Ans)
• h.Cp/µ
• (Cp.µ/h

16.Prandtl number is given by

• Cp.µ/a
• hD/k
• Cp.µ/k    (Ans)
• µ/h.Cp

17.Thermal diffusivity is given by

• k/r.Cp    (Ans)
• r.Cp/k
• Cp.µ/a
• µ/hCp

18.Grashhoff number is given by

• g D3. ß .? tr2/µ2    (Ans)
• g D2ß?tr/µ2
• g D2ß?tr2µ
• g D3ß?tr2/µ

19.If h1 = inner film co-efficient and h2 = outer film co-efficient, then the overall heat transfer co-efficient is

• always less than h1
• always between h1 and h2    (Ans)
• always higher than h2
• dependent on metal resistance

20.Gratez number is give by

• mCp/kL   (Ans)
• kL/mCp
• mCp/kµ
• kL/mCp

21.In the equation Q= UA ?t ; ?t is

• geometric mean temperature difference.
• arithmetic mean temperature difference.
• logarithmic mean temperature difference.    (Ans)
• the difference of average bulk temperatures of hot and cold fluids.

22. Peclet number (Pe) is given by

• Pe = Re.Pr       (Ans)
• Pe = Re/Pr
• Pe = Pr/Re
• Pe = Nu.Re

23.Prandtl number is the reciprocal of

• Thermal diffusivity/ Momentum diffusivity       (Ans)
• thermal diffusivity x Momentum
• Thermal diffusivity x Mass diffusivity
• Mass diffusivity x Momentum diffusivity

24.Maximum heat transfer rate is obtained in .......... flow.

• laminar
• turbulent       (Ans)
• creeping
• transition region

25.Heat flux is the time rate of heat transfer per unit

• length
• area        (Ans)
• volume
• none of these

26.Nusselt number is the ratio of the

• temperature gradient of the wall to that across the entire pipe.        (Ans)
• temperature difference to the temperature gradient at the wall.
• heat flux at the wall to that across the entire pipe.
• none of these

27.Fouling factor

• is a dimensionless quantity.
• does not provide a safety factor for design.
• accounts for additional resistances to heat flow.        (Ans)
• none of these.

28.Viscosity of gases ....... with increase in temperature.

• increase very rapidly
• increase slowly        (Ans)
• decrease slowly
• remain unaffected

29.Heat transfer co-efficient (h1) for liquids increases with

• increasing temperature       (Ans)
• decreasing temperature
• decreasing Reynolds number
• none of these

30.At what value of Prandtl number, conduction is negligible in the turbulent core of a fluid flowing through a heated pipe ?

• 0.5
• < 0.5
• > 0.6       (Ans)
• < 0.1

31.At Pr > 1, conduction in an ordinary fluid flowing through a heated pipe is limited to the

• buffer zone
• turbulent core        (Ans)
• both and (b)
• viscous sub-layer

32.Conduction occurs in the buffer zone for a fluid flowing through a heated pipe, only when Prandtl number is

• 0.1
• > 1
• < 1       (Ans)
• 1

33.Natural convection is characterized by

• Grashhoff number        (Ans)
• Peclet number
• Reynolds number
• Prandtl number

34.Which of the following is unimportant in forced convection ?

• Reynolds number
• Prandtl number
• Grashhoff number        (Ans)
• none of these

35.In fored convection, the heat transfer depends on

• Re, Pr        (Ans)
• Re, Gr
• mainly Gr
• Re only

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

• Re. Gz
• Nu, Pr
• Nu, Pr, Re
• Nu, Gz        (Ans)

37.At what value of Prandtl number, the hydrodynamic and thermal boundary layers of a fluid flowing over a heated plate will be identical ?

• 1       (Ans)
• < 1
• > 1
• none of these

38.Reynold's analogy states that

• St = f/2     (Ans)
• St = f/4
• St = 4f
• St = f1/2

39.For what value of Prandtl number, St = f/2 ?

• 1.5
• 1      (Ans)
• > 1
• < 1

40.Which has the lowest Prandtl number ?

• Liquid metal      (Ans)
• Aqueous solution
• Water
• Lube oil

41.Dietus-Boelter equation used for the determination of heat transfer co-efficient is valid

• for fluids in laminar flow
• for fluids in tubulent flow       (Ans)
• when Grashhoff number is very important
• for liquid metals

42.For what value of Prandtl number, the Colburn analogy is valid ?

• 0.06 to 120
• 0.6 to 120      (Ans)
• 1 to 103
• 1 to 50

43.Pick out the correct equation.

• jH = (St) (Pr)2/3 = f/2     (Ans)
• jH = (St) (Pr)1/3 = f/2
• jH = (St)2/3 (Pr) = f/2 
• jH = (St)1/3 (Pr) = f/2

44.Choose the correct equation.

• Nu = (Re) (Pr) (Gz)
• Nu = (Re) (Pr) (St)     (Ans)
• Nu = (Re) (Pr)
• Nu = (Pr) (St)

45.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

• Nu = 0.023 Re0.8 . Pr0.4
• Nu = p/2 Gz
• Nu = 2/p Gz    (Ans)
• Nu = 2Gz0.5

46. 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

• ha = hl
• ha = 2 hl     (Ans)
• ha = 0.5 hl
• ha = 0.75 hl

47.When does the heat generated by fluid friction becomes appreciable compared to the heat transferred between the fluids ?

• At high fluid velocity     (Ans)
• At low velocity
• When fluid flows past a smooth surface
• None of these

48.The heat transfer co-efficient in film type condensation is ....... that for drop wise condensation.

• greater than
• lower than      (Ans)
• is same as
• half

49.Prandtl number is the ratio of

• momentum diffusivity to mass diffusivity
• momentum diffusivity to thermal diffusivity      (Ans)
• thermal diffusivity to mass diffusivity
• thermal diffusivity to momentum diffusivity

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

• wall or skin friction       (Ans)
• form friction
• both and (b)
• turbulent flow

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