23.51.00 Breechings, Chimneys, and Stacks
23.51.43.16 Particulate Filtration
23.51.13 Draft Control Devices 23.51.13.11 Draft Control Fans 23.51.13.13 Draft-Induction Fans 23.51.13.16 Vent Dampers 23.51.13.19 Barometric Dampers 23.51.16 Fabricated Breechings and Accessories 23.51.19 Fabricated Stacks 23.51.23 Gas Vents 23.51.33 Insulated Sectional Chimneys 23.51.43 Flue-Gas Filtration Equipment 23.51.43.13 Gaseous Filtration 23.51.43.16 Particulate Filtration