Great build, well done!
What was the SS tube originally from? Did you build it specifically for the HEX?
Do you think you'll add a pump to recirc the water?
Will this gadget be used for other heat exchanges aside from the mash? (cooling at knockout, sparge heating, etc.)
Did you have any trouble overshooting the water temp / relief valve pops? I think I saw that the tube is also vented at the top.