The Combined Nomenclature (CN), v1:2023
8404: Auxiliary plant for use with boilers of heading 8402 or 8403, e.g. economizers, superheaters, soot removers and gas recoverers; condensers for steam
https://www.dst.dk/en/Statistik/dokumentation/nomenklaturer/kn?id=72c00acf-45ba-43e2-9d08-63bdac4ce8a8