Question

A panel is made from an air gap between inner and outer panel surfaces along n necessary plastic support as shown in the tollowing schematic. The air gap accounts for 90% of the cross-sectional area while the plastic supports. take 10% of the cross-sectional area. ED 1 " thick insulated space z 18" steel plates. - 1" plastic supports air, steel, plastic It is known the following conductivities for Kair = 0.01 Btu/hift of Ksteet = 26.2 Btu/hift. F K plastic = 0,2 Btu/hift. F 1 What is overall R value of the panel? 2) If the surface temperature diff across the panel is 30°F, what is the heat transfer rate per unit area across this panel?

Answer 1

Draw the thermal circuit diagram. k,(0.1A) mo M® k.(0.9A) -B k,(0.1A) Calculate the overall R value of the panel.

R=R1+ 1 1 1 - + - +- (RRR ik, (0.14 0.2k, 022 0.9k. Consider unit cross section area 0.2(0.2) 0.9(0.01) liż li - 126.2)(4x12) + *12) ((0.2(0.2)x12)+(0.9(0.01)x12) =1.7015_F W Btu/h

Calculate the heat transfer rate per unit area across this panel. 0=47-6301. = 17.6315 Btu/h 217.63 Btu/h