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KNOWLEDGE · K11

Ex heater temperature class and surface limits

Explosion protection is not only shell category — the maximum surface temperature decides. Control accuracy and probe position set whether the margin holds.

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  • Medium and zone first
  • Class against peak surface temp
  • Include control swing in margin
  • Re-check flame path and glands

Ex heaterPublished 2026-10-08中文版 ↗

01The six classes

T1 450 °C / T2 300 °C

For media with very high ignition temperatures, natural gas among them; equipment surface stays below the value.

T3 200 °C

Many solvent vapours sit near this class, and it is the one most electric heating equipment is built to.

T4 135 °C / T5 100 °C / T6 85 °C

Low-ignition media are stricter: the heater needs much lower watt density, or indirect heat transfer instead.

How to verify

Take the ignition temperature from the process data sheet rather than inferring it from the medium name; confirm with the safety assessment when data is thin.

02Where the margin comes from

A T3 nameplate means maximum surface temperature below 200 °C, and that maximum includes fault conditions, not just the control value. A ±5 °C control band, a probe reading a few degrees off, and one hotter element in an unbalanced three-phase group all push local shell temperature above what the display shows. Allow 15~20 °C, then choose the control strategy and bank switching.

If the medium demands T4 and direct electric heating cannot hold the surface below the limit, take one of two routes: reduce watt density (longer heated section, more elements), or move to indirect heat transfer with a thermal oil or hot water loop, which decouples shell temperature from process temperature.

03Four things to inspect at install

Flame paths

Scratches or rust on flange or thread flame paths need reassessment; grease against corrosion and keep the full engagement length.

Cable glands

Seals and compression must suit the actual outer diameter; spare entries get a certified blanking plug, never left open.

Earthing

Continuity of the body earth, with terminal boxes inside the hazardous area on the same checklist.

Probing and high limit

The over-temperature trip must act before peak surface temperature reaches its limit, and probe position decides whether it measures anything.

04What to record at handover

Three records make later audits simple: the measured peak surface temperature under hot conditions (scan shell, flange and terminal box with an IR pyrometer and keep the highest), the over-temperature trip value and its action sequence, and the assembly state of glands and flame paths. Re-audit all three whenever the process ceiling changes.

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FAQ

Common questions

01Is a higher protection category always better?+

No. Category has to match zone and medium. Over-specifying brings watt-density limits, bigger envelopes and cost, and is more easily voided by site modifications.

02Does replacing the element affect Ex approval?+

Yes. Element dimensions, watt density and the fit to the flame path were part of the original design; substitutes must match and be re-confirmed.

03Can the control panel sit in the hazardous area?+

It depends on zone classification and enclosure type. The common layout keeps control and over-temperature isolation outside the area, leaving only the heater body and terminal box inside.

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