A dormitory at a large university, built 50 years ago, has exterior walls constructed of Ls=25-mm-thick sheathing with a thermal conductivity of ks=0.1 W/m-K. To reduce heat losses in the winter, the university decides to encapsulate the entire dormitory by applying a Li=25-mm-thck layer of extruded insulation with a thermal conductivity of ki=0.029 W/m-K. Determine the ratio of the heat flux through the retrofitted and original walls (q"retro/q"original) when the interior and exterior air temperatures are 22 C and -20 C, respectively. Assume that the convection coefficients on the interior and exterior surfaces of the walls are extremely large (i.e., h ).

A dormitory at a large university, built 50 years ago, has exterior walls constructed of Ls=25-mm-thick sheathing with a thermal conductivity of ks=0.1 W/m-K. To reduce heat losses in the winter, the university decides to encapsulate the entire dormitory by applying a Li=25-mm-thck layer of extruded insulation with a thermal conductivity of ki=0.029 W/m-K. Determine the ratio of the heat flux through the retrofitted and original walls (q"retro/q"original) when the interior and exterior air temperatures are 22 C and -20 C, respectively. Assume that the convection coefficients on the interior and exterior surfaces of the walls are extremely large (i.e., h ).

Principles of Heat Transfer (Activate Learning with these NEW titles from Engineering!)

8th Edition

ISBN:9781305387102

Author:Kreith, Frank; Manglik, Raj M.

Publisher:Kreith, Frank; Manglik, Raj M.

Chapter1: Basic Modes Of Heat Transfer

Section: Chapter Questions

Problem 1.10P: 1.10 A heat flux meter at the outer (cold) wall of a concrete building indicates that the heat loss...

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A dormitory at a large university, built 50 years ago, has exterior walls constructed of Ls = 25-mm-thick sheathing with a thermal conductivity of ks = 0.1 W/m-K. To reduce heat losses in the winter, the university decides to encapsulate the entire dormitory by applying an L; = 25-mm-thick layer of extruded insulation characterized by k; = 0.029 W/m-K to the exterior of the original sheathing. The extruded insulation is, in turn, covered with an Lg = 5-mm-thick architectural glass with kg the heat flux through the original and retrofitted walls when the interior and exterior air temperatures are Ti = 22°C and T.o -2.5°C, respectively. The inner and outer convection heat transfer coefficients are h; = 5W/m2-K and ho = 1.4 W/m-K. Determine 25 W/m2-K, respectively. The heat flux through the original walls is i W/m?. The heat flux through the retrofitted walls is i W/m?.
A dormitory at a large university, built 50 years ago, has exterior walls constructed of L, = 25-mm-thick sheathing with a thermal conductivity of ks = 0.1 W/m-K. To reduce heat losses in the winter, the university decides to encapsulate the entire dormitory by applying an L = 25-mm-thick layer of extruded insulation characterized by k; = 0.029 W/mK to the exterior of the original sheathing. The extruded insulation is, in turn, covered with an Lg = 5-mm-thick architectural glass with kg = 1.4 W/m-K. Determine the heat flux through the original and retrofitted walls when the interior and exterior air temperatures are Too₁ = 22°C and T∞,0 = -20°C, respectively. The inner and outer convection heat transfer coefficients are h; = 5 W/m²-K and h, = 25 W/m²-K, respectively. The heat flux through the original walls is i The heat flux through the retrofitted walls is i W/m². W/m².
20-m pipe has an outside diameter of 50 mm. Pipe is insulated with a layer of asbestos, then followed by a layer of cork. Inside and outside diameter of the cork is 77 mm and 80 mm, respectively. If the temperature drop from pipe to cork is 1165°C, calculate the inside diameter of the pipe (mm). The rate of the heat transfer is 8778 W. The thermal conductivity of steam pipe, asbestos and cork are 0.045 kW/m-K, 0.058 W/m-K and 0.043 W/m-K respectively.
A dormitory at a large university, built 50 years ago, has exterior walls constructed of L, = 25-mm-thick sheathing with a thermal conductivity of k, = 0.1 W/m-K. To reduce heat losses in the winter, the university decides to encapsulate the entire dormitory by applying an L; = 25-mm-thick layer of extruded insulation characterized by k; = 0.029 W/m-K to the exterior of the original sheathing. The extruded insulation is, in turn, covered with an Lg = 5-mm-thick architectural glass with kg = 1.4 W/m-K. Determine the heat flux through the original and retrofitted walls when the interior and exterior air temperatures are Ti = 22°C and T.0 -17.5°C, respectively. The inner and outer convection heat transfer coefficients are h; = 5W/m?-K and h, = 25 W/m²-K, respectively. The heat flux through the original walls is i W/m?. The heat flux through the retrofitted walls is i W/m?.
A dormitory at a large university, built 50 years ago, has exterior walls constructed of L, = 25-mm-thick sheathing with a thermal conductivity of k, = 0.1 W/m-K. To reduce heat losses in the winter, the university decides to encapsulate the entire dormitory by applying an L; = 25-mm-thick layer of extruded insulation characterized by k; = 0.029 W/m-K to the exterior of the original sheathing. The extruded insulation is, in turn, covered with an L, = 5-mm-thick architectural glass with kg = 1.4 W/m-K. Determine the heat flux through the original and retrofitted walls when the interior and exterior air temperatures are Ti = 22°C and T0 -15°C, respectively. The inner and outer convection heat transfer coefficients are h; = 5 W/m?-K and h, = 25 W/m²-K, respectively. The heat flux through the original walls is i W/m?. The heat flux through the retrofitted walls is i W/m?.
QUESTION 2 The wall of a refrigerated truck consists of 1.5mm sheet steel outer surface, 10mm plywood at the inner surface and 20mm of glass wool in between. The inside temperature is -15°C and outside temperature is 24°C. Take the thermal conductivities of the materials as follows: - =0,052W/mK = 23,2W/mK k for glass-wool = 0,14W/mK k for plywood k for steel %3D Calculate: 2.1. the rate of heat flow per unit area; 2.2. the interface temperature. If the glass-wool is replaced by a 5mm cork board with a thermal conductivity of 0.043W/mK; 2.3. What percentage change in heat flow is obtained? 2.4. What must be the thickness of the cork board be, to achieve the same heat flow as in (2.1.).
Calculate the rate of heat loss through the vertical walls of a boiler furnace of size 4 m by 3 m by 3 m high. The walls are constructed from an inner fire brick wall of 20 cm thick with thermal conductivity 0.4 W/mK, a layer of ceramic blanket insulation with thermal conductivity of 0.2 W/mK and 10 cm thick, and a steel protective layer with thermal conductivity of 55 W/mK and 3 mm thick. The inside temperature of the fire brick layer was measured at 600°C and the temperature of the outside of the insulation was 60°C. Also find the interface temperature of layers.
A steel pipe (outside diameter 100 mm) is covered with two layers of insulation. The inside layer, 40 mm thick, has a thermal conductivity of 0.07 W/(m K). The outside layer, 20 mm thick, has a thermal conductivity of 0.15 W/(m K). The pipe is used to convey steam at a pressure of 600 kPa. The outside temperature of insulation is 24°C. If the pipe is 10 m long, determine the following, assuming the resistance to conductive heat transfer in steel pipe and convective resistance on the steam side are negligible: a. The heat loss per hour. b. The interface temperature of insulation.
It is necessary to insulate a flat surface so that the rate of heat loss per unit area of this surface does not exceed 450 W/m2 . The temperature difference across the insulating layers is 400K. Evaluate the thickness of insulation if (i) The insulation is made of asbestos cement having thermal conductivity of 0.11 W/m-K and (ii) The insulation is made of fire clay having thermal conductivity of 0.84 W/m-K.
A hot water pipe is used for domestic applications is insulated with a layer of calcium silicate. If the insulation is 25 mm thick and its inner and outer surfaces are maintained at Ts,1 = 800 K and Ts,2 = 400 K, respectively. The outside diameter is 0.12 m. Calculate the heat loss per unit length of the pipe. Given the thermal conductivity of calcium silicate insulation equals to 0.09 W/m.K.
A furnace of 0.3m by 0.3m by 0.6m inside dimensions is constructed of a material having a thermal conductivity of 0.894 W/m.K. The wall thickness is 10cm. The inner and outer surface temperatures are 600 degree Celcius and 100 degree Celcius respectively. Calculate the heat loss through the furnace wall by using (i) Conduction shape factor. (ii) Thermal resistance.
The oven of a stove must have a sufficient insulation so that the surface temperature of the stove is not greater than 50 oC. To accomplish this, insulation, k = 0.11 W/m-K, is used between the inside and outside metal surfaces. The room temperature is 20 oC, and the outside unit convective coefficient is 8 W/m2-K. Neglecting the resistance of the metal, what is the minimum thickness of insulation required, if the outside temperature reaches 315 oC?