3-68 A 5-m-wide, 4-m-high, and 40-m-long kiln used to cure concrete pipes is made of 20-cm-thick concrete walls and ceiling (k = 0.9 W/m-K). The kiln is maintained at 40°C by injecting hot steam into it.The two ends of the kiln, 4 m x 5 m in size, are made of a 3-mm-thick sheet metal covered with 2-cm-thick Styrofoam (k = 0.033 W/m K). The convection heat transfer coefficients on the inner and the outersurfaces of the kiln are 3000 W/m2-K and 25 W/m2-K, respectively. Disregarding any heat loss through the floor, determine the rate of heat loss from the kiln when the ambient air is at -4°C.FIGURE P3-68Tout-4°C40 mTin=40°C4 m20 cm5 m

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Asked Oct 1, 2019
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3-68 A 5-m-wide, 4-m-high, and 40-m-long kiln used to cure concrete pipes is made of 20-cm-thick concrete walls and ceiling (k = 0.9 W/m-K). The kiln is maintained at 40°C by injecting hot steam into it.
The two ends of the kiln, 4 m x 5 m in size, are made of a 3-mm-thick sheet metal covered with 2-cm-thick Styrofoam (k = 0.033 W/m K). The convection heat transfer coefficients on the inner and the outer
surfaces of the kiln are 3000 W/m2-K and 25 W/m2-K, respectively. Disregarding any heat loss through the floor, determine the rate of heat loss from the kiln when the ambient air is at -4°C.
FIGURE P3-68
Tout-4°C
40 m
Tin=40°C
4 m
20 cm
5 m
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3-68 A 5-m-wide, 4-m-high, and 40-m-long kiln used to cure concrete pipes is made of 20-cm-thick concrete walls and ceiling (k = 0.9 W/m-K). The kiln is maintained at 40°C by injecting hot steam into it. The two ends of the kiln, 4 m x 5 m in size, are made of a 3-mm-thick sheet metal covered with 2-cm-thick Styrofoam (k = 0.033 W/m K). The convection heat transfer coefficients on the inner and the outer surfaces of the kiln are 3000 W/m2-K and 25 W/m2-K, respectively. Disregarding any heat loss through the floor, determine the rate of heat loss from the kiln when the ambient air is at -4°C. FIGURE P3-68 Tout-4°C 40 m Tin=40°C 4 m 20 cm 5 m

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Expert Answer

Step 1

The following data have been given to us.

The length of the kiln, L = 40 m.

The breadth of the kiln, B = 5 m.

The height of the kiln, H = 4 m.

The thickness of the concrete wall, t = 20 cm.

The thermal conductivity of the ceiling, kc = 0.9 W/mK.

The temperature of the kiln, Tin = 40 0C.

The thickness of the kiln sheet, t1 = 3 mm.

The thickness of the Styrofoam, t2 = 2 cm.

The thermal conductivity of the Styrofoam, ks = 0.033 W/mK.

The thickness of the fin, t = 3 mm.

The temperature of ambient air, Tout = -4 0C.

The convective coefficient on the inner surface, hi = 3000 W/m2K.

The convective coefficient on the outer surface, ho = 25 W/m2K.

Step 2

The area of heat transfer corresponding to the inner and outer dimension is calculate as,

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4, L(2x (H-21)+1x (B-21) - 40 2x (4-2(0.2)1x(5-2(0.2) 472 m2 A, 2(LxH)+LxB 2(40x4)40x5 = 520 m2

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Step 3

The area of heat transfer through concrete w...

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Aue -L(2x(Н-)+1x(В-1) - 40{2x (4-0.2)+1x(5-0.2) conc = 496 m2 - (H -1) x(В-1) end = (4-0.2)x(5-0.2) 18.24 m2

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