In a heat exchanger, as shown in accompanying figure, air flows over brass tubes of 1.8- cm-ID and 2.1-cm-OD that contain steam. The convection heat-transfer coefficients on the air and steam sides of the tubes are 70 W/(m2 K) and 210 W/(m2 K), respectively. Calculate the overall heat transfer coefficient for the heat exchanger (a) based on the inner tube area, (b) based on the outer tube area.
GIVEN
• Air flow over brass tubes containing steam
• Tube diameters
? Inside (Di) = 1.8 cm = 0.018 m
? Outside (Do) = 2.1 cm = 0.021 m
• Convective heat transfer coefficients
? Air side h c= 70 W/(m2 K)
? Steam side h i= 210 W/(m2 K)
FIND
• The overall heat transfer coefficient for the heat exchanger based on
(a) the inner tube area (Ui) and
(b) the outer tube area (Uo)
ASSUMPTIONS
• The heat transfer coefficients are uniform over the transfer surfaces
SKETCH
PROPERTIES AND CONSTANTS
the thermal conductivity of brass at 20°C (kb) = 111 W/(m K)
(a) The overall heat transfer coefficient based on the inner area is
(b) The overall heat transfer coefficient based on the outer area is
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