An industrial freezer is designed to operate with internal air temperature of -2
ID: 1690342 • Letter: A
Question
An industrial freezer is designed to operate with internal air temperature of -20 °C when the external air temperature is 25 °C and the internal and external heat transfer coefficients are 12 W/m2K and 8 W/m2K respectively. The walls of the freezer are composite construction, comprising of an inner layer of plastic (0.35 W/mK, thickness 3 mm), and an outer layer of Cr-Al stainless steel (thickness 1 mm). Sandwiched between the two layers is a layer of insulation material with 0.07 W/mK thermal conductivity. Find the width of the insulation that is required so that the heat flux through the wall is reduced to 15 W/m2.Explanation / Answer
U=q/delta T
U=15/(25-(-20)) = 0.333
U=((1/hi)+(Lp/kp)+(Li/i)+(Ls/ks)+(1/ho))^-1
Find Li
Li= 0.07(1/0.333-((1/12)+(0.003/1)+(0.001/16)+(1/8)))
Li=0.195m=195mm.