E-PolyLearning

46. Bulk of the convective heat transfer resistance from a hot tube surface to the fluid flowing in it, is
a. in the central core of the fluid.
b. uniformly distributed throughout the fluid.
c. mainly confined to a thin film of fluid near the surface.
d. none of these.
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Answer: (c).mainly confined to a thin film of fluid near the surface.

47. Which of the following is concerned with both heat and mass transfer ?
a. Lewis relationship
b. Nusselt number
c. Kutateladze number
d. Froude number
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Answer: (a).Lewis relationship

48. A backward feed multiple effect evaporator is better than forward feed for concentrating cold feed, because it provides
a. higher economy
b. lower capacity
c. both (a) & (b)
d. lower economy
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Answer: (a).higher economy

49. A 10 cm diameter steam pipe, carrying steam at 180°C, is covered with an insulation (conductivity = 0.6 W/m.°C). It losses heat to the surroundings at 30°C. Assume a heat transfer co-efficient of 0.8 W/m².°C for heat transfer from surface to the surroundings. Neglect wall resistance of the pipe and film resistance of steam. If the insulation thickness is 2 cms, the rate of heat loss from this insulated pipe will be
a. greater than that for uninsulated steam pipe.
b. less than that of the uninsulated steam pipe.
c. equal to that of the uninsulated steam pipe.
d. less than the steam pipe with 5 cms insulation.
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Answer: (b).less than that of the uninsulated steam pipe.

50. Fouling factor
a. is a dimensionless quantity.
b. does not provide a safety factor for design.
c. accounts for additional resistances to heat flow.
d. none of these.
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Answer: (c).accounts for additional resistances to heat flow.