The Combined Nomenclature (CN), v1:2018
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=97dd4f6e-1b47-43b5-8532-380d3d1b7c2f