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KNOWLEDGE · K7

Watt density for electric heater tubes: ranges by medium

Watt density is tube output divided by heated-surface area. It is the first variable in service life: same power, shorter heated length means higher density and hotter sheath.

Read the article ↓Get a sizing check →
  • Calculate heated area first
  • Pick the range by medium
  • Allow for scaling and voltage
  • Never run duct spec in water

Immersion heaterPublished 2026-10-08中文版 ↗

01Ranges by medium

Flowing water

6~10 W/cm². Water carries heat away well; take the upper bound with good velocity, the lower bound for still or slow water.

Static oil

1.5~3 W/cm². Oil transfers heat poorly, so the sheath skin runs hot and coking becomes the limiting factor.

Still air

1~2 W/cm². Air is the weakest carrier; with no flow all the heat stays at the surface, so be most conservative here.

Forced air in ducts

3~5 W/cm². Acceptable when airflow is proven and stable, but only with a fan-to-heater interlock in place.

02How to work out the area, and why one tube length matters

Take a Ø12 mm tube with a 1000 mm heated section: surface area is about π×1.2×100 ≈ 377 cm². At 3 kW per tube the watt density is roughly 8 W/cm² — fine for flowing water, clearly too high for static oil. For oil, extend the heated section to 1500~2000 mm, or drop to 1~1.5 kW per tube and use more tubes.

For the same total duty, more tubes with longer heated sections give lower density. The extra cost shows up in flange size and terminations, while the difference in life is usually several-fold. Prefer a lower per-tube load over a smaller envelope.

03Four common mistakes

Reading total power only

3 kW over 1 m and 3 kW over 0.5 m differ by a factor of two in watt density. State the heated length on the order.

Fitting a duct spec into a water tank

A low-density duct element is bulky and underpowered in water; a high-density water element fails fast in air.

Ignoring scaling

Scale acts like insulation on the sheath, raising wall temperature at the same load. Re-check density with the descaling cycle.

Assuming rated voltage

Output falls with the square of supply voltage. Compensating by shortening the heated length squeezes life twice over.

04What to do at order stage

Start at the lower end of the range for the medium, then check three things: total heated length the envelope can hold, flange or thread size, and how many lead sets the terminal box takes. If they do not fit, reduce per-tube power and add tubes rather than raising watt density.

↗Related guide

Related guide: Flanged vs. Screw Plug Immersion Heaters: How to Choose

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FAQ

Common questions

01Should I cut the load or remove scale?+

Do both. Reduce watt density or shorten energised time until the descaling cycle is met, and treat the water on the source side. Raising power just accelerates the next scale layer.

02Can I raise density to heat faster?+

Ramp speed depends on total power, not density. Add power and lengthen the heated section at the same time to keep density inside its range.

03What if I have no medium data?+

Design for the worst case (lower density) and keep 10%~20% power margin. A slightly larger envelope is cheaper than a rebuild.

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