The chimney caps are built with galvanized steel sheet, adaptable to galvanized or stainless helical duct. 
They are designed as a part to be added to the end of the ventilation duct. It is also possible to use them as a fresh air intake.

There are different models according to their functionality:
Inverted Chinese hat: Technical Data Sheet
Anti-return hat: Technical Data Sheet

Others:

Jet hat.
Intake hat
Nival hat