Insulated Pipe Heat Loss Calculator
Estimate the heat lost from a hot pipe per metre of length, with and without insulation, and the outer surface temperature of the insulation.
- Rins = ln(r₂/r₁) ÷ (2πk) = 2.223 m·K/W
- Rsurface = 1 ÷ (2πr₂h) = 0.1485 m·K/W
- q = ΔT ÷ (Rins + Rsurface) = 65.36 W/m
Pipe Heat Loss Formulas
Tsurface = Ta + q ÷ (2πr₂h)
Bare pipe: q = 2πr₁h × (Ti − Ta)
q is heat loss per metre, r₁ the pipe outer radius, r₂ the insulation outer radius, k the insulation conductivity at mean temperature and h the combined outside coefficient. The inside film and steel wall resistances are small and are neglected, which gives a slightly conservative (high) loss. The bare pipe estimate uses the same h; real bare pipe loss at high temperature is larger because radiation grows.
Worked example
A 4" pipe (114.3 mm OD) at 180 °C with 50 mm of insulation (k 0.045) in 25 °C air loses about 65 W/m, with the cladding at about 35 °C. Bare, it would lose about 557 W/m.
Frequently Asked Questions
What k value should I use?
Use the manufacturer value at the mean insulation temperature. Mineral wool is about 0.04 to 0.05 W/m·K at moderate temperatures.
What is the surface coefficient h?
The combined convection and radiation coefficient at the outer surface, around 8 to 12 W/m²·K in still indoor air and higher outdoors in wind.
Why is surface temperature important?
For personnel protection and for hazardous areas, where the surface must stay below the gas temperature class.
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