snorkel breather vs air duct

snorkel breather and air duct both are nouns.

Word	Noun	Adjective	Verb	Adverb
snorkel breather	Yes	No	No	No
air duct	Yes	No	No	No

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

snorkel breather: 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 snorkel breather include air passage, airway.

snorkel breather (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)