Maybe you should read this --
TUBE: A hollow body, usually cylindrical, and long in proportion to its diameter, of wood, metal, glass, or other material, used to convey or contain a liquid or fluid, or for other purposes; a pipe.
A more recent and more generic term than pipe, in which the form of the thing is chiefly considered, and thus used in reference to many things to which pipe is not applied, pipe being an older term retained for tubes used for the passage of liquids, smoke, air, or gas, while tube is applied to most recent inventions; but the distinction is often arbitrary, depending on the custom of the workshops. (OED)
***So there you have it. If something looks like a pipe but is not a pipe, it's a tube :) Simple really.