How should heated-section length be specified for commercial heating elements?

Size the heated section to the wetted working band: start above the minimum liquid level, end below the maximum, and keep both ends clear of seals, flanges, baffles and any dry or vapour zone. Then check the resulting surface load against sheath diameter, material, medium properties and IP protection so hot spots and scaling are avoided. Give the supplier the level fluctuation range, tank clearances, mounting orientation, tilt angle and control point location, and agree cold length, tolerances and acceptance criteria together, with low-level alarm or interlock where needed for safe operation.