282514 Machinery and apparatus for filtering or purifying gases n.e.c.

28251410 Machinery and apparatus for filtering or purifying air (excluding intake filters for internal combustion engines) 28251430 Machinery and apparatus for filtering and purifying gases (other than air and excl. those which operate using a catalytic process, and isotope separators) 28251440 Machinery and apparatus for filtering or purifying gases by catalytic process (excluding intake air filters for internal combustion engines, machinery and apparatus for filtering or purifying air)