Question

An industrial freezer is designed to operate with an 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/m2.K respectively The walls of the freezer have a composite construction, comprising of an inner layer of plastic (k = 1 W/m•K, and thickness of 3 mm), and an outer layer of stainless steel (k = 16 W/mok, and thickness of 1 mm). Sandwiched between these two layers is a layer of insulation material with k = 007 W/m.k 31 Draw a diagram to represent the situation described above and label your diagram (5) 32 Determine the width of the insulation that is required to reduce the convective heat loss to 15 W/m² (15) Hint Calculate overall heat transfer coefficient and use it to calculate width of insulation as a function of resistances in series

Answer 1

Ti = -20°c hi= 12W/mo To = 25€ t o = 8w/280k his 12 freeser K=1 | Koot K16 | Air h ow look 3mm lo Imm R KA R3 FA - Ret Ret Ret Ryths 25-1-20) 45 _ 0.003 0 g+ + de 10.00) +0.007+ 16 + us 0.0833+0.003+ le +6.95x10s +0.125 0-007

0.2114 tle = 3 0.007 207866 0.007 = de = 0.0195 m Tlg = 19.5mm Required thickness of insulation (k=0007w/mik) is 19.5 mm.