National Institute of Technology 2007 B.E Chemical Engineering Final 28TH. - Question Paper

FOURTH] SEMESTER B.Tech CHEMICAL ENGINEERING
HEAT TRANSFER
Time : three Hours Max. Marks : 100

1 A large window glass of 0.5 cm thick with thermal conductivity k = 0.78 08
W/m °C, is exposed to warm air at Ti = 25 °C over its inner surface, and
the heat transfer coefficient for the inside air is hi = 15 W/m2 °C. The outside air is at To = - 15 °C, and the heat transfer coefficient associated with the outside surface is ho = 50 W/m2 °C. elaborate the temperatures of the inner and outer surfaces of the glass?
2 An electric resistance wire of radius a = 1X10^-3 m, with thermal 08
conductivity k = 25 W/m °C is heated by the passage of current which
generates heat within the wire at a constant rate of 2X109 W/m3.
Determine the centerline temperature rise above the surface temperature
of the wire if the surface is maintained at a constant temperature.
3 A copper rod of diameter 5mm is heated by the passage of an electric 12
current. The surface of the rod is maintained at a temperature of 175°C
while it is dissipating heat by convection into an ambient at 25°C with a
heat transfer coefficient of 150 W/m2 °C. If the rod is to be covered with a lmm thick coating of thermal conductivity 0.6 W/m °C, will the heat loss from the surface increase or decrease? Derive the formula used.
4 For a fin with convection at the fin tip, the temperature distribution at the 12
fin tip is provided by
Using this result, develop an expression for fin effectiveness and fin efficiency.
5 Derive Nusselt's formula for film wise condensation on vertical surfaces. 15
State your assumptions clearly.
6 By doing dimensional analysis for free convection, find an empirical 15
relation for heat transfer coefficient.
7 2.4 kg/s of fluid having a specific heat of 0.81 kJ/kg K enters a counter 15
flow heat exchanger at 0°C and is heated to 400°C by a two kg/s of a fluid
having a specific heat of 0.96 kJ/kg K entering the unit at 700°C.
Determine that to heat the cooler fluid to 500°C, all other conditions
remaining unchanged, by what percentage should the heat transfer area be
increased/decreased?

8 What is a sizing and rating problem? How do you solve these issues by
the heat exchanger effectiveness method?
9 An 8% solution of water soluble polymer is concentrated to 40% in a
single effect evaporator at the rate of 2000 kg of feed per hour. The feed
enters at 30°C and a vacuum of 660 mm Hg is maintained in the
evaporator. Saturated steam is available at a pressure of 2.15 bar absolute.
compute the heat transfer rate needed and the steam requirement. Water
boils at 100 mm Hg pressure at 325 K and enthalpy is 2607 kJ/kg. Heat of
vaporization of steam at 2.15 bar is 2188 kJ/kg.


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Earning:   Approval pending.