A furnace wall is of thickness 0.25 m (L) and it is exposed to gas at its inner surface and to air at itsouter surface. The gas and air are maintained at 400°C (T) and 50°C (T). The heat transfer coeffi-cients of hot gas (h_) and cold air (h_) sides are 60 and 10 W/m'K respectively. If the surface area (A)of the wall is 3 m', find (i) the overall heat transfer coefficient; (ii) the heat transfer rate 'Q' and (ii)the surface temperature of the wall (T and T). Assume that the thermal conductivity (K) of the wallmaterial is 1.5 W/mK.

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Answered: A furnace wall is of thickness 0.25 m…

Elements Of Electromagnetics

7th Edition

ISBN:9780190698614

Author:Sadiku, Matthew N. O.

Publisher:Sadiku, Matthew N. O.

ChapterMA: Math Assessment

Section: Chapter Questions

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An industrial freezer is designed to operate with an internal air temperature of -20°C when the external air temperature is 27°C, and the internal and external heattransfer coefficients are 8 W/m2 -K and 12 W/m2 -K, respectively. The walls of thefreezer are composite construction, comprising an inner layer of plastic (k = 0.33 W/m-K, and thickness of 10 mm), and an outer layer of stainless steel (k = 15 W/m-K, and thickness of 3 mm). Sandwiched between these two layers is a layer of insulation material with k = 0.07 W/m-K. Find the width (mm) of the insulation thatis required to reduce the convective heat loss to 60 W/m2.
The wall of a furnace consists of a 0.85 cm-thick layer of steel (ks = 15.1 W/m/K) and an outer layer of brick (kb = .72 W/m/K). The outer surface temperature of the brick, To = 50oC, and is exposed to convection with air at 20oC providing a heat transfer coefficient of 150 W/m^2/K. Determine the steady temperature at the steel brick interface, Tmid, and on the exposed steel surface, Ti, if the brick layer has thickness Lb = 5 cm.
Q) An apple has been put into a freezer at (-18°C) to be cooled. Initially, the apple is at a uniform temperature of 17°C. The convection heat transfer coefficient on its surface is 6 W/m2°C. Assuming the apple as a sphere having a diameter of 10-cm and taking its properties as p = 750 kg/m³, p = 2850 J/kg°c, and k = 0.450 W/m°C, determine; (a) the center temperature of the apple in 45 minutes, (b) the actual amount of heat transfer from the apple, and (c) the final equilibrium temperature of the apple. Hint: consider the temperature is a function of location and time.
A pipe carrying hot gasses is insulated with a material having a thermal conductivity of 0.15 W/mK. The average local heat transfer coefficient by convection to the ambient air is 3 W/m K. The critical diameter of the pipe under 35. these conditions is (a) 100 mm (c) 160 mm (b) 130 mm (d) 200 mm
A thin board of 0.5 cm thickness is exposed to a fluid of T, = 22°C and h = 30 W/m².K from one side with an emissivity of 0.7 where it faces a surrounding whose temperature is Tsur = 30°C. A heat flux of q" = 3500 W/m? is applied to the other side. Each side has an area of 12cm×12cm. Tsur (a) If the board has the following properties: k = 15.1 W/m.K, c, = 480 J/kg.K and p = 8055 kg/m², express an equation for the variation of temperature with respect to time. (b) Find the initial time rate of change of the board temperature if the initial board temperature is q" h 50°C.
A steam pipe of 120-mm outside diameter is covered with a 20-mm-thick layer of calciumsilicate insulation (k = 0.089 W/m-K). The pipe surface temperature is 800 K, and theambient air and surroundings temperatures are 300 K. The convection and radiationcoefficients for the outer surface of the insulation are estimated as 5.5 W/m2-K and 10W/m2-K, respectively. Determine the heat rate per unit length from the pipe (W/m) andthe insulation outer surface temperature.
The upper surface of a 50-cm-thick solid brass plate (k=109 W/mK) is being cooled by water with temperature of 10°C. The upper and lower surfaces of the solid plate maintained at constant temperatures of 60°C and 190°C, respectively. Determine the water convection heat transfer coefficient and the water temperature gradient at the upper plate surface. Water Ts.l · Ts.,2
A large window glass 0.5 cm thick (k= 0.78 W/m K) is exposed to warm air at 25oC, over its inner surface with convection coefficient of 15 W/m2K. The outside air is at 258 K with convection coefficient of 50 W/m2K. Determine the heat transfer rate and temperature at the inner and outer surface of the glass.
The heat flux from a wall to air in motion having a heat transfer coefficient of 15 W/m². K and exposed to a temperature difference of 20 °C is www.bilibilitetit O 225 W O 300 W O 250 W O 375 W wwwwww
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The thermal conductivity of a solid material can be determined using asetup similar to the one shown in the accompanying figure. The thermocouples are placed at 2.5 cm intervals in the known material (copper alloy, k - 52 w/m.k) and the unknown sample, as shown. The known material is heated on the top by a heating element, and the bottom surface of the sample is cooled by running water through the heat sink shown. Determine the thermal conductivity of the unknown sample for the set of data given in the accompanying table. Assume no heat loss to the surroundings and perfect thermal contact at the common interface of the sample and the copper.
A furnace wall comprises three layers of thickness 250 mm, 100 mm, and 150 mm with thermal conductivities of 1.65, k and 9. W/m K respectively. The inside is exposed to gases at 1250 ˚C with convection of 25 W/m2K, and the inside surface is at 1100 ˚C, the outside surface air at 25 ˚C with convection of 12 W/m2K. Determine the temperatures of the mid-inner surface in 0C to 1 d.p.?

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