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KNOWLEDGE · K10

Duct heater and fan interlocks: air first, heat second

A duct heater dumps its heat into air. Stop the air and you have dry firing. The interlock has one job: no proven airflow, no power to the elements.

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  • Airflow switch as permissive
  • Loss of air cuts heat
  • Fan runs on after a trip
  • No standalone heat start button

Duct heaterPublished 2026-10-08中文版 ↗

01Four interlock rules

Start permissive

A differential pressure switch across the section, or the drive running feedback, must be closed before the heat contactor can pick up.

Stop cuts heat

Fan stop or airflow decay opens the heat circuit at once while the fan keeps running to carry residual heat out of the shell.

High limit

Outlet temperature probe trips heat only, never the fan. Keep the alarm 5~10 °C away from the process setpoint.

Group switching

Banks stage in on temperature deviation, reducing inrush and avoiding a high surface load at low duty.

02Cross-checking airflow against temperature rise

For atmospheric air use P(kW) ≈ 0.335 × airflow (m³/h) × rise (°C) ÷ 1000, which already assumes 1.2 kg/m³ density and 1.005 kJ/(kg·°C) specific heat. 5000 m³/h with a 60 °C rise needs about 100 kW; add 10%~20% margin and specify 110~120 kW.

Then work backwards to watt density: spread that duty over the heated surface area and compare with the 3~5 W/cm² range for forced air. Outside the range means the envelope is too small — lengthen the heated section or add elements rather than raising power per tube. Verify both numbers once with measured current and outlet temperature.

03Common wiring problems

Separate heat and fan buttons

An operator can energise heat before air. Heat should have a permissive, never a start of its own.

Probe on the inlet side

It reads cold air, so the controller keeps driving output while the real outlet runs far above setpoint.

Relying on controller limiting

A controller stuck at full output has no backstop; an independent over-temperature trip is required.

Unrepaired duct leaks

Leaked air takes no part in heat exchange, so measured airflow and temperature rise no longer agree and the heater looks underpowered.

04One calculation before wiring

Commissioning order: clamp-meter the three phases and compare with nameplate current (clearly low means a bank is not switching in); record the delay between fan start and heat permissive (it must be greater than zero); then back-calculate the actual temperature rise from airflow and outlet temperature against setpoint. When all three agree, the interlock and the duty are both proven.

↗Related guide

Related guide: SCR vs. Contactor Control for Electric Heaters: How to Choose

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FAQ

Common questions

01Can a timer replace the airflow switch?+

No. A timer cannot prove airflow; with a blocked duct or slipping belt the elements still energise — that is dry firing.

02Should the fan run on after a trip?+

The fan should, the heaters must not. The direction is air on first, heat cut fast, fan delayed to clear stored heat.

03Is drive running feedback enough?+

It helps, but a differential pressure switch across the section is stronger evidence because it proves air movement, not motor status.

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