Heat flows radially through the insulation (a logarithmic conduction resistance) and then from the jacket surface to ambient air (a film resistance). The two resistances in series give the loss per metre, and the split between them fixes the outer surface temperature — the number that matters for burn protection.
Worked example
A 4-inch pipe at 150 °C with 50 mm of mineral wool in still 25 °C air. ≈ 47 W/m, a ~90% reduction versus the bare pipe, with the jacket surface at a touch-safe ≈ 32 °C.
Inputs
Pipe outside diameter (mm) — 114.3 mm = 4-inch NPS.
Insulation thickness (mm)
Insulation conductivity k (W/m·K) — Mineral wool ≈ 0.04, calcium silicate ≈ 0.06 (at moderate temperature).
Pipe (process) temperature (°C)
Ambient temperature (°C)
Outside film coefficient (W/m²·K) — Still air ≈ 5–10; add wind and it rises fast.
Pipe length (m)
Frequently asked questions
How thick should insulation be?
It's an economic and safety optimum: each added centimetre saves less energy than the one before. Typical drivers are a target heat loss, a maximum surface temperature (≈ 60 °C for personnel protection per ASTM C1055), or process needs like freeze protection.
What outside film coefficient should I use?
Still indoor air is around 5–10 W/m²·K including radiation. Wind raises it sharply — 20–30 W/m²·K at a few m/s. A higher h₀ increases heat loss slightly but lowers the surface temperature.
Why is the pipe wall resistance ignored?
Steel conducts ~1000× better than mineral wool, so the wall contributes a negligible fraction of the total resistance. The insulation and the outside film dominate.
Assumptions
Steady state; pipe-wall and inside-film resistances neglected (pipe OD surface at process temperature).
h₀ lumps convection + radiation into a single outside film coefficient.
k taken constant at the mean insulation temperature.
References
Cylindrical-conduction resistance ln(r₂/r₁)/2πk — Incropera & DeWitt, Fundamentals of Heat and Mass Transfer.
ASTM C1055 — Standard Guide for Heated System Surface Conditions that Produce Contact Burn Injuries.
Related Heat Transfer tools
Heat Exchanger LMTD — Log-mean temperature difference for a two-stream exchanger, plus duty Q if you supply U and area.
Heat Exchanger Duty & Area — Sensible heat duty from flow, specific heat, and temperature change — plus the required exchanger area from U and the LMTD.