steam boiler vs vessel

steam boiler and vessel both are nouns.

Word	Noun	Adjective	Verb	Adverb
steam boiler	Yes	No	No	No
vessel	Yes	No	No	No

As nouns, vessel is a hypernym of steam boiler; that is, vessel is a word with a broader meaning than steam boiler:

steam boiler (noun)	vessel (noun)
sealed vessel where water is converted to steam	a craft designed for water transportation
	an object used as a container (especially for liquids)
	a tube in which a body fluid circulates