air duct vs upcast

air duct and upcast both are nouns.

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

As nouns, upcast is a hyponym of air duct; that is, upcast is a word with a more specific, narrower meaning than air duct:

air duct: a duct that provides ventilation (as in mines)
upcast: air passage consisting of a ventilation shaft through which air leaves a mine

Other hyponyms of air duct include register, breather, schnorchel, schnorkel, snorkel, snorkel breather, ventilation shaft.

air duct (noun)	upcast (noun)
a duct that provides ventilation (as in mines)	air passage consisting of a ventilation shaft through which air leaves a mine