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 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?.

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.

Chapter2: Steady Heat Conduction

Section: Chapter Questions

Problem 2.1DP

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1.1 On a cold winter day, the outer surface of a 0.2-m-thick concrete wall of a warehouse is exposed to temperature of –5°C, while the inner surface is kept at 20°C. The thermal conductivity of the concrete is 1.2 W/m K. Determine the heat loss through the wall, which is 10-m long and 3-m high. Problem 1.1
2.30 An electrical heater capable of generating 10,000 W is to be designed. The heating element is to be a stainless steel wire having an electrical resistivity of ohm-centimeter. The operating temperature of the stainless steel is to be no more than 1260°C. The heat transfer coefficient at the outer surface is expected to be no less than in a medium whose maximum temperature is 93°C. A transformer capable of delivering current at 9 and 12 V is available. Determine a suitable size for the wire, the current required, and discuss what effect a reduction in the heat transfer coefficient would have. (Hint: Demonstrate first that the temperature drop between the center and the surface of the wire is independent of the wire diameter, and determine its value.)
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A dormitory at a large university, built 50 years ago, has exterior walls constructed of Ls = 32-mm-thick sheathing with a thermal conductivity of ks = 0.2 W/m · K. To reduce heat losses in the winter, the university decides to encapsulate the entire dormitory by applying an Li = 35-mm-thick layer of extruded insulation characterized by ki = 0.021 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 = 1.1 W/m · K. Determine the heat flux through the original and retrofitted walls when the interior and exterior air temperatures are T∞,i = 20°C and T∞,o = -5°C, respectively. The inner and outer convection heat transfer coefficients are hi = 6 W/m2 · K and ho = 27 W/m2 · K, respectively
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 Li= 25-mm-thick layer of extruded insulation characterized by ki= 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 T∞,i= 22°C and T∞,o= -2.5°C, respectively. The inner and outer convection heat transfer coefficients are hi= 5 W/m2·K and ho= 25 W/m2·K, respectively. a. what is the heat flux through the original walls in W / m^2 b. what is the heat flux through the retrofitted wall in W / m^2