I have a heat exchanger for a boiler flue. The flue is connected to two big (150 HP) boilers. A boiler HP = 33,000 BTUH. The are less than 80% efficient. The heat exchanger problem is this: The Flue contains gas temps above 300 F. The CFM is problematic but apparently the actual CFM could be increased by a factor of 4 or more due to the oxygen and moisture content of the flue gases. In any case the ambient temp in the flue is a near 300 F when the boiler(s) are operating. The heat exchanger is made of 304 Stainless. It's a 1" tube with 7 fins per inch and the fins are .5" high. The length of the tubing is 360" and it is configured in a coil 30" in diameter. There are 95 square feet of surface area on the heat exchanger due to the fins. The design entering water into the tube is 80 F and leaving 90 F. So I have a 300 F heat source (steady) and I want a 10 F dT with a flow rate of 15 GPM. ( I can't get much more flow into a 1" pipe.) I want to pick up 250,000 BTUH from this configuration and I am not adept at the Q= U*A*LMTD calcs to put this together. Further, if it makes sense to do so, I would like to increase the Pipe size to 2" and put 40 GPM through this. Any help would be greatly appreciated.