How should heat-transfer area be specified for industrial heating elements?

Size the heat-transfer area from the actual duty: flow rate, inlet-to-outlet temperature rise, specific heat and the permissible sheath temperature, then cross-check against tube diameter, material and surface loading. Excessive watt density drives up sheath temperature and accelerates oxidation or scaling; too low wastes tube length and space. Ask the supplier for the design basis, and verify velocity, mounting, protection class and control response under real conditions, allowing margin for the worst case. Accept on measured sheath temperature, electrical readings and heat balance, not catalogue watt-density figures alone.