Minimum duct insulation thickness

[THAPHE]

For the Duct which supply 320 C heating air. I need to know the below following is the correct or not for method of finding the estimation minimum insulation thickness which need to maintain supply air temperature 320 C for the whole line and maintain the outer surface of insulation to 52 C.

Give
T hot = 320 C
T Outer surface of insulation = 52 C
T ambiant = 27 C
k = thermal conductivity of insulation
h = surface coefficient
Take Q insulation (Conduction) = Q surface (Convection)
(k/L)A(T hot - T Surface) = hA(T surface- T ambiant)
so L(Insulation thickness) = (k/h)(T hot - T Surface)/(T surface - T ambiant)

 

[chicopee]

If you get a book on heat transfer, there is a section that deals on the minimum thickness of insulation. That particular equation is based on the thermal conductivity of the insulation and the thermal convection on the outside surface of the insulation