A 4-m high and 6-m-wide wall consists of long 15-cm x 20 cm cross section horizontal bricks (k of 0.69 W/m K) separated by 2-cm thick plaster layers (k of 0.2 W/m K). There are also 2-cm-thck plaster layers on each side of the brick and a 3-cm-thick rigid foam (k of 0.025 W/m K) on the inner side of the wall. See figure below. The indoor and the outdoor air temperatures are 18 and -8°C, respectively. Convective heat transfer coefficient for inside and outer surfaces are 8 and 22 W/m²K, respectively. Assume 1-D heat transfer. Neglect radiation. a. assume any plane wall normal to the x-direction is isothermal, draw the thermal circuit and calculate the heat transfer rate through the wall assume any plane parallel to the x-axis is adiabatic,, draw the thermal circuit and calculate the heat transfer rate through the wall Compare the results from a and b, discuss the reason for any difference there might be. b. c.

An industrial cold room has four 200 mm thick walls made of concrete. The walls are insulated on the outside with a layer of foam 60 mm thick. Cladding with a thickness of 15 mm protects the foam on the outside from the elements. The composite wall surface temperatures are –3 °C on the inside and 18 °C on the outside of the room respectively. The thermal conductivities of concrete, foam and cladding are 0.75, 0.35 and 0.5 W/m K respectively.   Assuming perfect thermal contact between the layers of the composite walls, draw the typical temperature distribution across the layers and determine the heat energy gained per hour through all 4 walls of the room with a total surface area of 20 m2. What does this heat energy represent in terms of the refrigeration system of the cold room?

3-52 A 4-m-high and 6-m-wide wall consists of a long 18-cm X 30-cm cross section of horizontal bricks (k = 0.72 W/m . °C) separated by 3-cm-thick plaster layers (k = 0.22 W/m . °C). There are also 2-cm-thick plaster layers on each side of the wall, and a 2-cm-thick rigid foam (k = 0.026 W/m - °C) on the inner side of the wall. The indoor and the outdoor temperatures are 22°C and −4°C, and the convec- tion heat transfer coefficients on the inner and the outer sides are h₁ = 10 W/m² . °℃ and h₂ = 20 W/m². °C, respectively. Assuming one-dimensional heat transfer and disregarding radi- ation, determine the rate of heat transfer through the wall. Foam Plaster Brick 1+₂+18 2 FIGURE P3-52 18 cm N 1.5 cm 30 cm 1.5 cm