Question

A composite wall is formed of a 2.5 cm copper plate (k=385 W/m-K). a 3.2 cm layer of

asbestos (k=0.2 W/m-K), and a 5 cm layer of fibreglass (k=0.045 W/m-K). The wall is subjected to an overall temperature drop of 560°C. Calculate the heat flux through the composite wall and also determine the temperature drop across the asbestos layer?

Answer 1

Thickness of copper plate = 2.5cm

k_c = 385 W/m.K

thickness of absestos = 3.2cm

k_a = 0.2 W/m.K

thickness of fireglass = 5cm

k_f = 0.045 W/m.K

∆T = 560°

rate of heat loss (Q) =

R_thC = $\dfrac{L_c}{k_cA} = \dfrac{2.5×10^{-2}}{385×1}$

$= 6.5× 10^{-5}°C/W$

R_thA = $\dfrac{L_A}{k_AA} = \dfrac{3.2×10^{-2}}{0.2×1}$

$=0.16°C/W$

R_thF = $\dfrac{L_F}{k_FA} = \dfrac{5×10^{-2}}{0.045×1}=1.11°C/W$

$Q = \dfrac{∆T}{R_{thC}+R_{thA}+R_{thF}}=$

$\dfrac{560}{6.5×10^{-5}+0.16+1.11}= 440.92\ W$