fryer vs donut fryer

fryer and donut fryer both are nouns.

Word	Noun	Adjective	Verb	Adverb
fryer	Yes	No	No	No
donut fryer	Yes	No	No	No

As nouns, donut fryer is a hyponym of fryer; that is, donut fryer is a word with a more specific, narrower meaning than fryer:

fryer: A kitchen appliance for deep frying; its size depends on intended use.
donut fryer: a machine used to fry large quantities of doughnuts at once.

Other hyponyms of fryer include donut machine, doughnut fryer, doughnut machine.

fryer (noun)	donut fryer (noun)
A kitchen appliance for deep frying; its size depends on intended use.	a machine used to fry large quantities of doughnuts at once.
flesh of a medium-sized young chicken suitable for frying