>does anyone know if you can put one of those bags on direct flame? coals?
I doubt it. I'm no chemist, but the only reason this works is because the water inside continuously absorbs the heat energy from the fire. In addition, any material that is not touching water, i.e. the upper part of the bag, will melt.
You can boil water in a paper cup by setting it in the fire, but every bit of the cup that isn't touching the water burns off.
I think it'd be just a messy, fussy, hassle.