A diffuse, gray radiation shield of 60−mm diameter and emissivities of ε 2 , i = 0.01 and ε 2 , o = 0.1 on the inner and outer surfaces, respectively, is concentric with a long tube transporting a hot process fluid. The tube surface is black with a diameter of 20 mm. The region interior to the shield is evacuated. The exterior surface of the shield is exposed to a large room whose walls are at 17°C and experiences con vection with air at 27°C and a convection heat transfer coefficient of 10 W/m 2 ⋅ K . Determine the operating temperature for the inner tube if the shield temperature is maintained at 42°C.
A diffuse, gray radiation shield of 60−mm diameter and emissivities of ε 2 , i = 0.01 and ε 2 , o = 0.1 on the inner and outer surfaces, respectively, is concentric with a long tube transporting a hot process fluid. The tube surface is black with a diameter of 20 mm. The region interior to the shield is evacuated. The exterior surface of the shield is exposed to a large room whose walls are at 17°C and experiences con vection with air at 27°C and a convection heat transfer coefficient of 10 W/m 2 ⋅ K . Determine the operating temperature for the inner tube if the shield temperature is maintained at 42°C.
Solution Summary: The author compares the operating temperature for the inner tube and the temperature of the heated tube.
A diffuse, gray radiation shield of 60−mm diameter and emissivities of
ε
2
,
i
=
0.01
and
ε
2
,
o
=
0.1
on the inner and outer surfaces, respectively, is concentric with a long tube transporting a hot process fluid. The tube surface is black with a diameter of 20 mm. The region interior to the shield is evacuated. The exterior surface of the shield is exposed to a large room whose walls are at 17°C and experiences con vection with air at 27°C and a convection heat transfer coefficient of
10
W/m
2
⋅
K
.
Determine the operating temperature for the inner tube if the shield temperature is maintained at 42°C.
Determine the net heat transfer by radiation between two gray surfaces, A (εA= 0.90) andB (εB= 0.25) at temperatures 500°C and 200°C, respectively if a. surfaces are infinite parallel planes b. surface A is a spherical shell 3 m in diameter and surface B is a similar shell concentric with A and 0.3 m in diameter c. surfaces A and B concentric cylindrical tubes with diameters of 300 mm and 275 mm, respectively d. both surfaces are squares 2 m × 2
Two large parallel plates are at temperatures T1 = 500 K, and T2 = 300 K. Their emissivities are ε1 = 0.85 and ε2 = 0.90. (a) What is the radiant flux between the plates? (b) If a polished aluminum sheet (ε = 0.10) is placed between the plates as a radiation shield, how much is the reduction in radiant heat transfer?
Consider two rectangular surfaces perpendicular to each other with a common edge which is 1.6 m
long. The horizontal surface is 0.8 m wide and the vertical surface is 1.2 m high. The horizontal surface has an
emissivity of 0.75 and is maintained at 400 K. The vertical surface is black and is maintained at 550 K. The back
sides of the surfaces are insulated. The surrounding surfaces are at 290 K, and can be considered to have an
emissivity of 0.85. Determine the net rate of radiation heat transfers between the two surfaces, and between
the horizontal surface and the surroundings.
A. 2245 W, 725 W
T2 = 275 K
E2 = 1
T; = 300 K
Ez = 1
%3D
B. 1245 W, 725 W
C. 1245 W, 825 W
D. 2245 W, 825 W
D
Tj = 425 K
Ej = 1
-)
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