in series vs serial

in series is not a noun while serial is a noun.

Word	Noun	Adjective	Verb	Adverb
in series	No	Yes	No	No
serial	Yes	Yes	No	No

As adjectives, in series and serial are synonyms defined as:

in series and serial: of or relating to the sequential performance of multiple operations

Other synonyms of in series include nonparallel.

in series (noun)	serial (noun)
	a serialized set of programs
	a periodical that appears at scheduled times

in series (adjective)	serial (adjective)
of or relating to the sequential performance of multiple operations	pertaining to or occurring in or producing a series
	pertaining to or composed in serial technique
	of or relating to the sequential performance of multiple operations
	in regular succession without gaps