Drier - dryer

Drier and dryer are permissible variant spellings of the same words, derived from the adjective and adverb dry. Neither is wrong for either meaning, but usage is hardening in the direction here, which AWE advises you to follow.

• Drier is the preferred spelling for the comparative form of the adjective dry. The superlative, likewise, should be driest. The past forms of the verb 'to dry' are always dried.
• Dryer is the spelling for the noun, whether naming a machine, a chemical, or a person. Hence hair-dryer, clothes-dryer, hand-dryer and tumble-dryer are preferable spellings.

• • This is not universal advice - this is a topic on which development is occurring, and unanimity has not been achieved. On a related spelling, the Concise Dictionary of Modern English Usage (ed. Butterfield, 2016) observes, s.v. 'dryly', "As the adverb formed from dry, this spelling is nowadays more common than drily throughout the English-speaking world. The spelling drily conforms to the rule that words ending in –y change it to –i before a suffix. Some people prefer it for that reason, especially in Br[itish] E[nglish], where it is still slightly more frequent than dryly."