Camp fire water heater

Has anyone created a water heater that uses the camp fire in such a
way the you can still use the fire for cooking?

I've seen one that's a converted beer barrel that sits over the fire
on a frame and one that sits over the end of a fire, but not one that
works on the same principle as a domestic hot water boiler - i.e. a
heat exchanger over the fire (attached underneath the grate that the
pans go on) and using simple convection currents heats the water in a
header tank.

I'm trying to create one over the next couple of weeks with my
Explorers. I will try and create a diagram to post and/or a picture of
the finished product.