Detail this asset from Unity Store: Original Link
This is a paid asset, but now you can download Air Duct Pack Free.
Air Duct Pack
Construct ventilation systems with this Air Duct Pack!
Pack Includes 2 types of air duct, large and small. Each air duct size has 2 support models available to use, along with details such as various vent models. 40 individual models in total, with 2 different textures.
Source files to the air ducts are available in .max format, there is also a demo scene included!