Atmospheric pressure air is heated in a long annulus (25-cm-ID, 38-cm-OD) by steam condensing at 149°C on the inner surface. If the velocity of the air is 6 m/s and its bulk temperature is 38°C, calculate the heat transfer coefficient.
GIVEN
Atmospheric flow through an annulus with steam condensing in inner tube
Inside diameter (Di) = 25 cm = 0.25 m
Outside diameter (Do) = 38 cm = 0.38 m
Steam temperature (Ts) = 149°C
Air velocity (V) = 6 m/s
Air bulk temperature (Tb) = 38°C
FIND
The heat transfer coefficient (hc)
ASSUMPTIONS
Steady state
Steam temperature is constant and uniform
Heat transfer to the outer surface is negligible
Air temperature given is the average air temperature
Thermal resistance of inner tube wall and condensing steam is negligible (Inner tube wall surface
temperature = Ts)
SKETCH
PROPERTIES AND CONSTANTS
for dry air at 38°C
the hydraulic diameter of the annulus is given by
The Reynolds number based on this diameter is
Applying the Sieder-Tale correlation
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