Heat-Loss Calculator › Reference › DN125 (5″) @ 350°C
Heat loss from a DN125 (5″) pipe at 350 °C
A bare DN125 (5″) pipe running at 350 °C in 20 °C still air loses about 1448 W per metre. Wrap it in 50 mm of removable wired mat 80 and loss drops to about 215 W/m — a 85% cut — while the outer surface falls to about 49 °C (touch-safe). Figures use the ASTM C680 steady-state method.
These conditions are typical of high-pressure or superheated steam headers and hot-oil supply lines. Bare metal at this temperature is a severe burn hazard and, near combustible residues or leaks, an ignition risk — so insulation is as much a safety as an energy measure.
Per-metre heat loss
| Quantity | Bare | Insulated (50 mm) |
|---|---|---|
| Heat loss | 1448 W/m | 215 W/m |
| Heat loss (imperial) | 1506.7 BTU/hr·ft | 223.7 BTU/hr·ft |
| Reduction | — | 85% |
| Outer surface temp | ~350°C | 49°C |
| Conductivity k (at 185°C mean) | — | 0.061 W/m·K |
How much insulation thickness?
The same DN125 (5″) line at 350 °C, with different removable-insulation thicknesses (wired mat 80, k≈0.061 W/m·K). Heat loss and surface temperature both fall as thickness increases — with diminishing returns past 50–75 mm.
| Thickness | Heat loss | Reduction | Surface temp |
|---|---|---|---|
| 25 mm | 343 W/m | 76% | 78°C |
| 40 mm | 250 W/m | 83% | 56°C |
| 50 mm | 215 W/m | 85% | 49°C |
| 75 mm | 165 W/m | 89% | 38°C |
| 100 mm | 137 W/m | 91% | 33°C |
Above 250 °C the calculator uses a wired mineral-wool mat (higher-temperature binder); confirm the hot-face material rating for superheated duty.
Example: a 50 m line with 4 valves
| Result | Value |
|---|---|
| Heat loss, bare | 75.9 kW |
| Heat saved by insulation | 64.6 kW |
| Fuel energy saved | 608.2 MWh/yr |
| Money saved | €42,575/yr |
| CO₂ avoided | 121.6 t/yr (≈ 26.4 cars off the road, 5,793 trees, 25.4 homes’ power, 282 barrels of oil, 498,127 km of driving or 122 transatlantic flights) |
| Payback (removable insulation) | 2.8 months |
Assumptions. 50 m pipe + 4 valves (each ≈0.6 m bare pipe), 8000 h/yr, 85% boiler efficiency, €0.07/kWh fuel, 0.20 kg CO₂/kWh, 50 mm insulation at €250/m². CO₂ equivalences use US EPA / DEFRA conversion factors. Change any of these in the live calculator. Estimates for guidance — confirm with a site survey.
Run your own numbers
Adjust size, temperature, thickness, hours and energy price live — or get an exact heat-loss study and fixed insulation price for your equipment list.
Open the calculator →Get an exact quoteHow hot is a bare DN125 (5″) line at 350 °C, and is it dangerous?
The bare metal sits near the process temperature on a DN125 (5″) line at 350 °C — far above the ~60 °C burn threshold and a serious safety risk. It also loses about 1448 W/m, so insulation cuts the loss and returns the surface to safe-to-touch.