Heat Loss From Pipes: Convection, Radiation and What It Costs
A hot pipe loses heat to the air around it by natural convection and by radiation. The loss grows with pipe size and, faster than linearly, with temperature. It is calculable, and the calculation is the basis for every insulation decision.
You'll learn
- Explain how a bare pipe loses heat
- Read heat loss by pipe size and temperature
- Convert heat loss into condensate
- Know why ambient temperature matters to radiation
Before this: Conduction, convection and radiation, Expansion, supports and insulation
How a hot pipe loses heat
Air touching a hot pipe warms, rises, and is replaced by cooler air: natural convection. At the same time the pipe surface radiates heat to everything cooler around it. For a steam pipe at ordinary pressures the two are of similar size, and together they cost a steady stream of condensed steam.
Bigger pipes have more surface, so lose more per meter. Hotter pipes lose more per square meter, and radiation grows especially fast with temperature.
Loss by size and temperature
| Pipe | ΔT 50 °C | ΔT 100 °C | ΔT 150 °C | ΔT 200 °C |
|---|---|---|---|---|
| DN25 | 83 | 204 | 361 | 558 |
| DN50 | 140 | 343 | 609 | 947 |
| DN100 | 246 | 605 | 1,080 | 1,689 |
| DN150 | 347 | 856 | 1,532 | 2,404 |
Computed by GyanTosh’s pipe code (natural convection plus radiation, emissivity 0.95), which reproduces published emission tables within 4%.
- Steam at 169.8 °C; still air at 30 °C.
- Loss: 973 W/m × 50 m = 48.6 kW.
- Condensate made: 85.4 kg/h.
Details that change the answer
- Ambient matters to radiation: radiation depends on absolute temperatures, so the same ΔT above a hot boiler house radiates more than above cool outdoor air. Tables are quoted at a stated ambient for this reason.
- Wind multiplies convection: an outdoor pipe in a breeze loses considerably more than still-air tables show.
- Surface condition: bright, new aluminum cladding radiates far less than rusty steel; painted surfaces radiate like steel.
- Fittings: a bare valve, flange pair or strainer has a large, irregular surface and loses roughly what a length of bare pipe does. A plant with dozens of bare valves can lose as much through them as through a bare main.
Key takeaways
- Bare pipes lose heat by natural convection and radiation, in similar measure.
- Loss rises with pipe size and faster than linearly with temperature.
- 50 m of bare DN100 at 7 kg/cm²(g) condenses about 85 kg/h of steam.
- Ambient, wind, surface and bare fittings all change the real loss.