lowryd,
I did build a Steamaway, (Water Reduction Unit= WRU, thread on the build on years ago) it is not exactly like you described. Mine has 20 gallons of sap bulling over the hot tubes with air causing the bubbles. It works pretty well to make 2% sap into 3% sap using the steam from the rear pan. They are not cheap to buy. I would recommend that you look hard at R.O.'s first.
Regards,
Chris