Unformatted text preview: ME 114 Homework Set 9-Fall 2008 Due November 6 iii: ‘ Water ﬂowing at 2 kg/s through a 40-mm diameter 1&2 Water ﬂows at 2 kg/s through a 40-mm diameter ,
tube is to be heated from 25 to 75°C by maintain- tube 4 m long. The water enters the tube at 25°C,
mg the tube surface temperature at 100°C. and the surface temperature is 90°C.
What is the required tube length for these con- What is the outlet temperature of the water? ditions? What is the rate of heat transfer to the water? fit/t9: L : )OJQW AME: Twin" 47:5”C/ According to experimental results for parallel In a fire—tube boiler, hot products of combustion
ti? 3 airﬂow over a uniform temperature, heated ver- M ﬂowing through an array of thin-walled tubes are
tical plate, the effect 0f free convection on the heat used to boil water ﬂowing over the tubes. At the
transfer convection coefficient Will bf? 5% when time of installation, the overall heat transfer coefﬁ-
GT’L/REL = 0-08- Consider a heated vertical plate 2'3 cient was 400 W/mz - K. After 1 year of use, the in-
m long, maintained at a surface temperature _Of60 C ner‘and outer tube surfaces are fouled, with corre-
in atmospheric air at 25°C. What is the numrnum spending fouling factors of R51. ,- = 0.0015 and R; a
vertical velocity required of the airﬂow such that = 0.0005 In2 - K/W, respectively. Should the boiler
free convection effects will be less than 5% of the _ be scheduled for cleaning of the tube surfaces?
heat transfer rate? Avg: V: 249MB :i’L/ S4 A type—302 stainless steel tube of inner and outer
diameters D = 22 mm and DD = 27 mm, respec- A : ‘ R
tively, is used in a cross-ﬂow heat exchanger..The Vig - 211W 0 l g i—/— I W
fouling factors, Rgi, for the inner and outer Osurfaces
are estimated to be 0.0004 and 0.0002 m“ - K/W, ( [A \
respectively. Fouling Rga factors Tube, 35302 N i O 4 (,d il/VL L) R115, 7 ﬂ ’ ‘9 ﬂ Tm. I: 7500
um: 0.5 m/s _, (a) Determine the overall heat transfer coefficient
based on the outside area of the tube. U A an- pare the thermal resistances due to convection,
tube wall conduction, and fouling. Mar Wt eta—cm M3 WW: m
WT Wﬂ [4‘5 WM ii 5 ...
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