A rectangular forced air healing duct is suspended fromthe ceiling of a basement whose air and walls are at atemperature of T ∞ = T s u r = 5 °C . The duct is 15 m long, and its cross section is 350 mm × 200 mm . (a) For an uninsulated duct whose average surface temperature is 50°C, estimate the rate of heat loss from the duct. The surface emissivity and convectioncoefficient are approximately 0.5 and 4 W/m 2 ⋅ K, respectively. (b) If heated air enters the duct at 58°C and a velocity of4 m/s and the heat loss corresponds to the result ofpart (a), what is the outlet temperature? The densityand specific heat of the air may be assumed to be ρ = 1 .10 kg/m 3 and c p = 1008 J/kg ⋅ K , respectively.
A rectangular forced air healing duct is suspended fromthe ceiling of a basement whose air and walls are at atemperature of
T
∞
=
T
s
u
r
=
5
°C
. The duct is 15 m long, and its cross section is
350
mm
×
200
mm
. (a) For an uninsulated duct whose average surface temperature is 50°C, estimate the rate of heat loss from the duct. The surface emissivity and convectioncoefficient are approximately 0.5 and
4
W/m
2
⋅
K,
respectively. (b) If heated air enters the duct at 58°C and a velocity of4 m/s and the heat loss corresponds to the result ofpart (a), what is the outlet temperature? The densityand specific heat of the air may be assumed to be
ρ
=
1
.10
kg/m
3
and
c
p
=
1008
J/kg
⋅
K
, respectively.
Air (1 atm) enters into a 5-cm-diameter circular tube at 20°C with an average velocity of 5 m/s. The tube wall is maintained at a constant surface temperature of 160°C, and the outlet mean temperature is 80°C. Estimate the length of the tube.
Example 4: Hot air at atmospheric pressure and 80°C enters an 8–m-long uninsulated square duct of cross section 0.2 m 0.2 m that passes through
the attic of a house at a rate of 0.15 m3/s (Figure). The duct is observed to be nearly isothermal at 60°C. Determine the exit temperature of the air and the rate of heat loss from the duct to the attic space.
Cold air at 5°C enters a l2-cm-diameter, 20-m-long isothermal pipe at a velocity of 2.5 m/s and leaves at 19°C. Estimate the surface temperature of the pipe.
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