Question

It is proposed to coat a 1 mm diameter wire with enamel paint ( k=0.1 W/mK) to increase heat transfer with air. If the air side heat transfer coefficient is $100W/m^{2}K$, the optimum thickness of enamel paint should be

• A. 0.25 mm
• B. 0.5 mm
• C. 1 mm
• D. 2 mm

Answer 1

The correct option is B 0.5 mm

Given data:
d=1 mm
$\therefore r=\frac{d}{2}=\frac{1}{2}=0.5mm$

k=0.1 W/mK

$h_o=100W/m^{2}K$

Critical thickness,
$r_c=\frac{k}{h_o}=\frac{0.1}{100}$
= 0.001 m = 1 mm
Optimum thickness of enamel paint,
$t_{opt}=r_c-r=1-0.5$
= 0.5 mm