EML4147C HW2 - h o = 200 W/ m 2 K, respectively. a)...

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11.1 In a fire-tube boiler, hot products of combustion flowing through an array of thin-walled tubes are used to boil water flowing over the tubes. At the time of installation, the overall heat transfer coefficient was 400 W/ m 2 ∙K. After one year of use, the inner and outer tube surfaces are fouled, with corresponding fouling factors of R" f,i = 0.0015 and R" f,o = 0.0005 m 2 ∙K/W, respectively. Should the boiler be scheduled for cleaning of the tube surfaces? Explain.
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11.3 A shell-and-tube heat exchanger is to heat an acidic liquid that flows in unfinned tubes of inside and outside diameters D i = 10 mm and D o = 11 mm, respectively. A hot gas flows on the shell side. To avoid corrosion of the tube material, the engineer may specify either a Ni-Cr-Mo corrosion-resistant metal alloy ( ρ m = 8900 kg/m 3 , k m = 8 W/ m∙K) or a polyvinylidene fluoride (PVDF) plastic ( ρ p = 1780 kg/m 3 , k p = 0.17 W/ m∙K). The inner and outer heat transfer coefficient are h i = 1500 W/ m 2 ∙K and
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Unformatted text preview: h o = 200 W/ m 2 K, respectively. a) Determine the ratio of plastic to metal tube surface areas needed to transfer the same amount of heat. b) Determine the ratio of plastic to metal mass associated with the two heat exchanger designs. c) The cost of the metal alloy per unit mass is three times that of the plastic. Determine which tube material should be specified on the basis of cost. 11.9. A finned-tube, cross-flow heat exchanger is to use the exhaust of a gas turbine to heat pressurized water. Laboratory measurements are performed on a prototype version of the exchanger, which has a surface area of 10 m 2 , to determine the overall heat transfer coefficient as a function of operating conditions. Measurements made under particular conditions, for which = 2 kg/s, T h,i = 325 C, = 0.5 kg/s, and T c,i = 25 C, reveal a water outlet temperature of T c,o = 150 C. What is the overall heat transfer coefficient of the exchanger?...
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EML4147C HW2 - h o = 200 W/ m 2 K, respectively. a)...

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