This heading covers only the generating apparatus — the equipment that produces gas through a chemical or water-process reaction. Compressed gas cylinders and storage vessels are classified under separate headings in the tariff schedule based on their material and pressure rating.

Parts specifically designed for the generators covered by this heading — including acetylene, producer gas, and water gas generators — are classified under 840590. However, general-purpose valves, fittings, or pressure regulators that are not dedicated to these generators may fall under their own specific headings, so the degree of specificity to the generator is the deciding factor.