Insulation Heat Loss Calculator

Formula overview

SI: temperatures in °C, radii in m, conductivity k in W/m·K, heat loss in W per metre of pipe, film coefficients in W/m²·K.

ASTM C680 screening methodology — conduction through the layers is
balanced against the surface heat transfer:

  Q/L = 2π·(T1 − Ts) / Σ[ ln(ro/ri) / ki ]
      = 2π·r2 · [ hc·(Ts − Ta) + hr·(Ts − Tr) ] − q_solar

The two expressions are solved together for the unknown surface
temperature Ts, since hc and hr both depend on it.

Worked example

6" line at 200 °C, ambient 30 °C, 50 mm mineral wool at
k = 0.045 W/m·K, surface film coefficient about 10 W/m²·K

r_i = 0.0842 m,  r_o = 0.1342 m,  ln(ro/ri) = 0.466
R_insulation = 0.466 / (2π × 0.045) = 1.650 K·m/W
R_surface    = 1 / (10 × 2π × 0.1342) = 0.119 K·m/W
Q/L = (200 − 30) / 1.769 = 96 W/m
Ts  = 30 + 96 × 0.119 = 41 °C

Bare pipe under the same film coefficient would lose around 900 W/m,
so the insulation cuts the loss by roughly a factor of nine and brings
the touchable surface below the usual 60 °C personnel limit.

Common mistakes

• Assuming a surface temperature instead of solving for it. Surface temperature and heat loss are coupled through the film coefficients — fixing one arbitrarily gives an answer that does not satisfy the heat balance.
• Neglecting the cladding emissivity. Bright new aluminium radiates poorly, around 0.1, so it runs hotter than a weathered or painted surface at the same heat flux. Personnel-protection checks on shiny cladding are the ones most likely to be optimistic.
• Ignoring wind. Forced convection raises the surface coefficient several-fold, increasing heat loss while lowering surface temperature — so a design that just passes a personnel check in still air may fail its heat-loss budget on a windy day, and vice versa.

FAQ