hair dryer vs drier

hair dryer and drier both are nouns.

Word	Noun	Adjective	Verb	Adverb
hair dryer	Yes	No	No	No
drier	Yes	No	No	No

As nouns, drier is a hypernym of hair dryer; that is, drier is a word with a broader meaning than hair dryer:

hair dryer: a hand-held electric blower that can blow warm air onto the hair; used for styling hair
drier: an appliance that removes moisture

Other hypernyms of hair dryer include blower, dryer.

hair dryer (noun)	drier (noun)
a hand-held electric blower that can blow warm air onto the hair; used for styling hair	an appliance that removes moisture
	a substance that promotes drying (e.g., calcium oxide absorbs water and is used to remove moisture)