Greetings,
Some very rough calculations:
5 gallons liquid = 42 pounds at 212 degrees cooled to 68 degrees in a single pass would require 6,048 BTU's (212 - 68 = 144 * 42). Okay, now what?
Many additional unknown factors are required in order to design and engineer a heat exchanger for your purpose.
1) inner tube diameter, wall thickness, material composition, etc.
2) shell (outer tube) diameter, material composition, etc.
3) incoming water temperature range, average pressure, mineral content, etc.
4) distance between the outlet of the boil kettle and the inlet of the heat exchanger.*
5) ambient temperature range and insulation method.
* = The physical size of your heat exchanger MAY (depending on the above factors) prohibit gravity as the means of moving wort necessitating the need for a pump which adds a multitude of additional challenges and calculations.
Again, yes, your idea will work, but......
My opinion is to simply purchase a plate cooler for around $125 and a pump for around $100. With these two items you can cool your wort under nearly every condition imaginable. The engineering has already been completed for you. Or, for even less money, use an immersion cooler.
Good luck!