This question is often asked, but if you realize what you have been doing for years, the lightbulb will light up!

Examples:

R-12 . . . R-twelve

R-134a . . . R-one thirty-four 'a'

R-744 . . . R-seven forty-four

R-502 . . . R-five 'O'-two ('O' = ˈ... Login to read more.