Ventilation apparatus and ventilation systems; exhaust air systems and air intake systems for industrial purposes; gravity roof ventilators; smoke and heat extraction systems; ventilation systems; silencers for ventilation apparatus, exhaust air systems and air intake systems; air intake louvres and weather protection grilles for ventilation apparatus and ventilation systems; flues and installations for conveying exhaust gases; exhaust air filters for extractors [parts of commercial ventilati...