Question

there is a vertical air space and heat is transferred horizontally. Surface temperature on the two sides of the air space are -10 and 10F respectively. The thikness and effective emmitanceof the air space is 1.5'' and 0.35 respectively. Find U value in IP unit

Answer 1

Surface temperature of two sides of air space is T₁ and T_{2 ,} T₁ = -10F and T₂ = 10F

thickness of air space is t, t = 1.5" and effective emittance , = 0.35

U value in IP unit =?

U = 1/R where R = t / k = thickness/ thermal conductivity

U= _A /  T = heat flux or rate of heat energy per unit area / temp difference

[   = 1.714 10^-9 BTU/ hr ft² R⁴ ]

U= _A / T = E =T⁴ / T = 0.351.71410^-9{10⁴-(-10)⁴} / [10-(-10)]

= 1.201910^-5 / 20

= 610^-7 BTU/ hr ft² F