Question

As part of your summer job as an engineering intern at an insulation manufacturer, you are asked to determine the R factor of insulating material. This particular material comes in 1/2 in sheets. Using this material, you construct a hollow cube that has 15 in long edges. You place a thermometer and a 85 W heater inside the box. After thermal equilibrium has been attained, the temperature inside the box is 90°C when the temperature outside the box is 20°C. Determine the R factor of the material. F°· ft2· h/Btu

Answer 1

Given, Power of the heater or heat supplied by heater, Q = 85 W = 290.032 Btu/hr

The hollow cube has side length, l = 15 in = 1.25 ft

Total surface area of the cube, S = 6l² = 6 (1.25)² = 9.375 ft²

Temperature difference, = (90 - 20) ^oC = 126 ^oF

Let R factor of material be denoted by "R"

At thermal equilibrium,

Thus, the R factor of the material is 4.073 ^oF ft² h/Btu