How pipe heat loss is calculated
Heat flows from the fluid through the insulation by conduction, then from the outer surface to the room by convection and radiation. The two resistances add in series. Insulation resistance grows with the logarithm of the radius ratio, so the first few centimetres save the most.
- Heat flow per metre:
q = 2π·(Tf − Ta) / [ ln(r₂/r₁)/k + 1/(h·r₂) ] - Surface temperature:
Ts = Ta + q / (2π·r₂·h)
Worked example: steam line
A 2-inch steam line (60.3 mm OD) at 150 °C with 50 mm of mineral wool, indoors at 20 °C, loses 31.6 W per metre against 222 W/m bare in this model, a reduction of 86%. The jacket surface sits at 27 °C. Over 50 m of pipe that's 1.6 kW.
Good to know
- On chilled-water and refrigerant lines the risk is condensation, not energy. The outer surface must stay above the dew point, behind a vapour-tight jacket.
- Insulation conductivity rises with temperature and sharply with moisture. Wet insulation can lose most of its value.
- Bare valves and flanges have a lot of surface area and are often the biggest remaining loss on an insulated line.
- Pro adds yearly energy cost and savings, a dew-point check and heat loss across insulation thickness.