schnorkel vs air duct

schnorkel and air duct both are nouns.

Word	Noun	Adjective	Verb	Adverb
schnorkel	Yes	No	No	No
air duct	Yes	No	No	No

As nouns, air duct is a hypernym of schnorkel; that is, air duct is a word with a broader meaning than schnorkel:

schnorkel: air passage provided by a retractable device containing intake and exhaust pipes; permits a submarine to stay submerged for extended periods of time
air duct: a duct that provides ventilation (as in mines)

Other hypernyms of schnorkel include air passage, airway.

schnorkel (noun)	air duct (noun)
air passage provided by a retractable device containing intake and exhaust pipes; permits a submarine to stay submerged for extended periods of time	a duct that provides ventilation (as in mines)